Recent Progress on Corrosion Behavior, Mechanism, and Protection Strategies of Bronze Artefacts

Abstract

With their rich historical, artistic, and scientific value, bronze artefacts form a significant part of our cultural heritage. These items, often found in museums around the world, offer a glimpse into past civilizations and their technological advancements. However, due to their prolonged burial and subsequent exposure to varying environmental conditions, these artefacts are prone to corrosion, necessitating meticulous preservation efforts. This review discusses the cultural significance and preservation challenges of bronze artefacts, which are emblematic of human civilization’s progression. This text highlights the historical and artistic value of ancient bronze artefacts, especially those from China, underscoring their intricate casting techniques and aesthetic richness. Despite their cultural importance, these bronze artefacts confront severe preservation issues, particularly the pervasive threat of corrosion, commonly referred to as “bronze disease”. This text also reviews the complex interplay between alloy composition, microstructure, and environmental factors that influence corrosion mechanisms. It requires an enhanced understanding of these factors to develop effective preservation strategies. This paper also emphasizes the need for innovative, eco-friendly technologies to prevent further degradation while maintaining the integrity of these precious artefacts. The applications of corrosion inhibitions, organic/inorganic coatings, as well as the newly developed strategies like the photo-induced passivation technique, 3D scanning and 3D printing techniques, and holographic projection/real and virtual technique for the direct or indirect protection and cultural transmission of the bronze artefacts were also introduced. This review concludes by underscoring the urgency of these research and development efforts to safeguard our cultural heritage for future generations.

1. Introduction

1.1. The Value Embodiment of Archaeological Bronze Artefacts

Bronze artefacts represent a significant milestone in the advancement of civilization and history, holding a crucial place in humankind’s cultural heritage [1,2]. Over 4000 years ago, our ancestors began to master the bronze casting technique. Throughout the extended Bronze Age, they created numerous exquisite bronze artefacts with substantial technological value [3]. Ancient Chinese bronze artefacts are renowned for their remarkable diversity and exceptional aesthetic appeal. Their exceptional smelting and casting techniques, rich cultural significance, varied yet unified forms, and refined artistic taste vividly and authentically reflect the enduring and splendid culture of the Chinese nation, serving as vital material carriers of its esteemed traditional culture. The Bronze Age marks the period when metal objects first played a crucial role in human production and daily life, representing a pivotal stage in the evolution of human civilization. Ancient bronze artefacts, distinguished by their unique shapes, intricate ornamentation, and advanced casting technology, are one of the finest metallic artefacts in the world. They possess high artistic, ornamental, historical, cultural, and scientific research value. The burial site of Marquis Yi of Zeng, situated in Suizhou, Hubei Province, dates to the early years of the Warring States period and is dedicated to Marquis Yi of Zeng. The excavation of the tomb has yielded an extensive collection of bronze artefacts, including vessels such as the Ding and Gui, as well as musical instruments like chime bells and cymbals. Notably, the chime bells unearthed from the Zenghou Yi’s tomb represent the most comprehensive and extensive ensemble of bronze chime bells discovered to date [4]. The Sanxingdui Site in Guanghan, Sichuan Province, which was first excavated in 1986, is known as “the source of Yangtze River civilization” and one of the most significant archaeological discoveries of the 20th century [5]. Among them, the excavated bronze masks, particularly those represented by the longitudinal masks unearthed, are one of the most distinctive and spiritual cultural artefacts in Sanxingdui [6]. Other ancient civilizations similar to China also had splendid bronze cultures and left behind a considerable number of bronze artefacts. For instance, there are the naval amphora of Maria Theresia and the statue of Dardanikos in ancient Greece; the water taps, statues of gods, and pots and vessels in ancient Rome; and the statues of gods, stove bases, pendants, ornaments, and tools in ancient Egypt. However, due to complex factors such as history, the environment, and resources, the bronze artefacts unearthed in Western countries are generally fewer in number. But there is no doubt that these bronze artefacts carry essential historical information, providing a robust foundation for studying human history and ancient civilization.

1.2. The Challenges to Bronze Artefacts Protection

Most of the ancient bronze artefacts were unearthed from archaeological sites and buried underground for extensive periods. Exposure to soil moisture, oxygen, soluble salts, and biological factors make them susceptible to various forms of corrosion, resulting in different degrees of corrosion damage. After excavation, the corrosion behavior and rate are also affected to a certain extent due to changes in light, oxygen, and water conditions, etc. The most severe issue of unearthed bronze artefacts is the bronze disease [7]. Once it sets in or spreads, it becomes exceedingly difficult to control, potentially destroying significant historical and archaeological artefacts [8]. For instance, when patterns or inscriptions on the surface of bronze artefacts are eroded, the original text or designs can be damaged, leading to pitting corrosion. The information contained in these inscriptions is of great value in determining the identity of the tomb owner and understanding their lives. It can also directly reveal the social history and culture of the time, the significance of which is self-evident.

The State Administration of Cultural Heritage (SACH) of China conducted a national survey on the corrosion loss of cultural archaeological bronze artefacts in collections in 2002, revealing that many valuable bronze artefacts were under severe threat of corrosion [9]. Therefore, the corrosion and maintenance of bronze artefacts have become essential tasks for many museums. However, due to the limitations of the materials and the impact of the preservation environment, bronzes are prone to corrosion and damage. Similar to other metal materials, such as steel [10,11,12], magnesium alloy [13,14], titanium alloy [15,16,17], and aluminum alloy [18,19], copper alloys (e.g., bronze) [20,21,22,23,24] also face corrosion issues.

The corrosion process of bronze is intricate, influenced by its composition and closely related to the burial and collection environments. A comprehensive understanding of the impacts of alloy composition, microstructure, and environmental factors on the corrosion damage of bronzes is crucial for exploring the corrosion mechanisms and assessing the degree of corrosion in specific exposure environments [25,26]. Additionally, to prevent further corrosion and festering of unearthed bronzes due to air exposure, staff must implement temporary or long-term sealing measures. Developing new protection technologies that are environmentally friendly, efficient, and do not alter the surface state of bronze has always been a significant challenge in bronze preservation [27].

1.3. The Main Content of This Review

Since the 19th century, bronze corrosion mechanisms and preservation strategies have been a focus for scholars worldwide [28,29]. However, there are limited and systematic reviews on the types, occurrence, and development of powdery corrosion products, particularly on variations in corrosion products across different environmental contexts. This research gap hinders the practical application of preservation measures, leading to inadequate protection treatments for bronze artefacts. Consequently, a detailed examination of powdery bronze diseases and their causes is crucial for the protection of bronzes.

This review comprehensively analyses the significance of bronzes to cultural heritage and technological advancement. Ancient bronze artefacts were used in a wide range of ceremonial, musical, and warfare applications, as well as in everyday life, reflecting the rich cultural heritage and traditional values of humankind. This review highlights the multifaceted roles of bronzes, encompassing ritual vessels and weapons, tools, musical instruments, currency, household items, and ornaments. The central challenge discussed is the corrosion and deterioration of these artefacts, influenced by both internal factors (such as alloy composition and microstructure) and external environmental conditions. In delving into the intricate corrosion mechanisms, this review highlights the roles of tin and lead elements in enhancing bronze’s durability and resistance to corrosion. It also examines the influence of various environmental factors (soil, water, and atmosphere) on the corrosion behavior of bronze. This review provides a systematic classification of bronze powdery diseases, analyzes the contributing factors to their emergence and progression, and stresses the need for environmentally friendly and efficient preservation technologies that preserve the artefacts’ surface integrity.

Moreover, this review examines the formation of corrosion products on bronzes, the material exchange with the environment, the role of microorganisms in corrosion, and strategies to prevent microbial-induced corrosion. It underscores the importance of maintaining optimal environmental conditions to reduce corrosion risks during the storage and display of bronzes. The preservation of bronze artefacts necessitates a multifaceted approach, including controlled environments, protective treatments, and regular monitoring to ensure that these historical treasures are preserved for future generations.

The problems faced with the protection of bronze artefacts are mainly caused by the changes in the environment after their excavation. Generally speaking, the environment after being unearthed is rich in oxygen and has more abundant light, which, to some extent, will accelerate their corrosion. Although the extremely harsh preservation environment can minimize its corrosion to the greatest extent, it also causes inconvenience to the exhibition and display of cultural artefacts. Therefore, appropriate protective measures become particularly important. In the section on the protection strategies for unearthed bronze wares, there is focus on reviewing the currently mainstream corrosion inhibitor strategies, coating strategies, as well as newly developed technologies such as photo-induced passivation, 3D scanning and 3D printing, holographic imaging, and virtual reality technology. These technologies have directly or indirectly protected the bronze artefacts and preserved their cultural dissemination value as historical witnesses to the greatest extent possible.

During the preparation of this review, the references were collected from Web of Science with the keywords “bronze artefacts”, “bronze artifacts”, “patina”, and “bronze disease”. Non-classical references published before 2000, or those with topics more related to the nondestructive examination, lead isotope analysis, and casting techniques, were excluded. The specific cases on recent protection strategies were obtained from articles published after 2010.

2. Overview of Bronze Artefacts

2.1. Chemical Composition of Bronzes

Bronze is a copper-based alloy, and other metallic elements or non-metallic elements are added in the smelting process to form an alloy [30]. Traditional copper alloys can be divided into brass, white copper, and bronze. Brass is a copper–zinc alloy, and white copper is a copper–nickel alloy. Except for these two alloys, other copper alloys are collectively referred to as bronze. Bronze is mainly composed of copper (Cu), tin (Sn), and lead (Pb), with a small amount of elements such as nickel (Ni), manganese (Mn), iron (Fe), zinc (Zn), and some mineral impurities [31]. Bronze has several advantages, including a low melting point, high hardness, good castability, and excellent mechanical properties [32].

2.2. Types of Bronze Artefacts

There are many types of bronze, which are primarily classified based on their uses, shapes, and functions. The common types of bronze artefacts mainly include (1) ritual vessels, (2) weapons, (3) tools, (4) musical instruments, (5) currencies, (6) household items, and (7) ornaments. These artefacts reflect various aspects of society at that time, like production, daily life, history, culture, military, and economy, and possess immeasurable research value. Due to space limitations, this article only provides a detailed introduction to ritual vessels and weapons made of bronze, showcasing some of the more typical cultural artefacts that have been passed down through the ages. The following are some typical bronze artefacts, the images of which are illustrated in Figure 1.

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Ritual Vessels

Ding: A cooking vessel traditionally used for sacrifices and banquets, typically featuring three or four legs. One of the most renowned examples is the Simuwu Ding, the image of which is shown in Figure 1a. This ancient Chinese bronze artefact is a quintessential piece from the late Shang Dynasty and stands as the largest of its kind [33]. With a height of 1.33 m and a weight of 875 kg, the Simuwu Ding is one of the heaviest bronze artefacts in China (the heaviest bell, the Great Bell of Yongle in Beijing weighs many tons). Its imposing shape, intricate and exquisite ornamentation, and solemn atmosphere make it a pinnacle of Chinese bronze craftsmanship.

Li: A cooking vessel similar in shape to a tripod but without handles. As shown in Figure 1b, the “Ox-Head Patterned Covered Bo Gui”, an early Western Zhou dynasty bronze artefact, features a flat lid with a knob composed of two opposed small ox heads, each adorned with relief ox head patterns on the lid [34]. The rim flares outward, with a square lip, upright ears, a neck, and pouch-shaped feet. The neck is decorated with six short flanges, each spaced with kui patterns, and the feet are all decorated with ox head patterns, with the ox horns rising above the surface. Its decorative patterns are exquisitely crafted, with the main motifs being high-relief ox heads, giving an impression of grandeur and might. The artistic design and casting techniques are exceptionally skilled, marking it as a masterpiece of early Western Zhou bronze artefacts. Unearthed in 1975 from Tomb No. 251 at the Liulihe Site in Fangshan District, Beijing, it is now housed in the Capital Museum of Beijing.

Gu: A tall and slender wine vessel. The Taotie Patterned Bronze Gu is a masterpiece of Shang dynasty bronze, meticulously crafted and beautifully decorated, as shown in Figure 1c. Dated to the period of King Wu Ding, it was unearthed in January 1973 in Wangwan Village, Yinzhuang Town, Lingbao City. The vessel measures 28.2 cm in height, with a caliber of 16.1 cm, and a base diameter of 8.6 cm. It features a trumpet-shaped mouth, an elongated neck and body, a high foot that flares outward and a rounded base support. The overall shape is exquisite, resembling a trumpet flower in full bloom, with intricate and mysterious decorations that exude a solemn and dignified presence. Originally collected in the Anhui Provincial Museum, it was stolen on 13 December 1988.

Jue: A wine vessel with a spout and a tail used for warming and drinking wine. As significant cultural and archaeological artefacts, bronze jues had an essential place in ancient Chinese culture and were commonly used in rituals and banquets. The jue not only showcases the exquisite craftsmanship of antiquity but also carries a wealth of historical and cultural connotations. The “Ruiming Patterned Bronze Jue” was unearthed in 1975, measuring 22.5 cm in height, 31.5 cm in length from the base to the tail, and 0.1 cm in thickness (Figure 1d). It features a narrow and long stream, a pointed and elongated tail, a waisted body, a flat bottom, three legs, a handle on one side, and five horizontal ruiming patterns on the belly, hence the name. The jue’s shape is graceful, like a lithe and elegant lady standing in the wind, exuding a delicate and fresh aura, and is hailed as the “First Jue of China [35]”. It is now preserved in the Erlitou Xia Capital Site Museum.

Zun: A large wine vessel used for holding wine or for sacrificial purposes. The Four Sheep Square Zun, a Shang Dynasty bronze, is the largest surviving square Zun from the Shang dynasty and is a national treasure due to its unique shape and exquisite craftsmanship (Figure 1e). It is now in the National Museum of China.

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Weapons

Ge: A long-handled weapon like a spear. The “Liang Bo Ge”, from the State of Wei, is 17.5 cm long and weighs 0.28 kg, featuring an angled tip and a rectangular interior with a long hole (Figure 1f). Adorned with animal head decorations and inscribed with 14 characters, the palace guards used it for defense. The weapon’s design, with its gu-shaped blade and decorative elements, dates back to the early Spring and Autumn period, and it is now housed in the Palace Museum.

Mao: A pointed, long-handled weapon used for stabbing. The spear of King Fuchai of Wu, unearthed in 1983 from Tomb No. 5 at Mashan in Jiangling, Jingzhou, Hubei, is a bronze spear used by King Fuchai of the Wu State during the Spring and Autumn period (Figure 1g). It measures 29.5 cm in length. The body of the spear is like that of a sword, but shorter, with a ridge on the center line, blood grooves on both sides of the ridge, and an animal head cast at the end of each of the blood grooves. The socket (also known as “Shangzhu Jiao” for attaching the handle) is hollow. The mouth of the socket is flattened and rounded, with an inwardly concave edge. The spear body is covered with geometric diamond patterns. At the base, there are two lines of eight characters inlaid with gold: “King Fuchai of Wu made it for his use”. This spear is well-cast and well-preserved, and it is currently housed in the Hubei Provincial Museum.

Jian: A short double-edged weapon used for close combat. The Sword of King Goujian of Yue is a cultural artefact of the late Spring and Autumn period, unearthed in 1965 from Tomb No. 1 at Wangshan in Jingzhou, Hubei, and now housed in the Hubei Provincial Museum. As shown in Figure 1h, this first-class cultural archaeological bronze artefact is 55.7 cm long, with a handle measuring 8.4 cm in length and 4.6 cm in width, weighing 875 g. The pommel is circular, featuring 11 concentric circles at 0.2 mm intervals, and the blade is adorned with black diamond patterns. An inscription in the bird seal script on the obverse translates as “King Goujian of Yue made this sword for his use”. The guard is adorned with blue glaze and turquoise. Recognized as the “Best Sword in the World”, it exemplifies the pinnacle of bronze weapon craftsmanship and is invaluable for studying the history of the Yue state and ancient Chinese bronze casting and script.

Dao: A single-edged weapon used for slashing. The sheep-headed bronze knife was a common tool among the northern nomadic tribes during the late Shang Dynasty (c. 1600 BC to 1300 BC). As shown in Figure 1i, this knife features a slightly curved handle shaped like a sheep’s head, with a long snout surrounding the eyes and long ears curving back to form a larger ring, and a smaller ring on the inner side of the sheep’s head. The handle is decorated with a twisted thread pattern, separated from the blade by a partition, which widens at the guard and tapers sharply towards the front.

2.3. Distribution of Bronze Artefacts

Bronze artefacts are widely distributed across the world, with each region showcasing unique characteristics of its bronze culture. The following are some of the major areas where bronze artefacts are found, along with their distinctive features:

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China

China is one of the critical origins of bronze culture, and bronze artefacts hold significant importance in ancient Chinese society. There are many kinds of Chinese bronzes, including ritual vessels, weapons, tools, musical instruments, currencies, household items, and ornaments. Early Chinese bronzes were primarily unearthed in the northwestern regions, such as Gansu and Shaanxi.

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West Asia

West Asia is one of the birthplaces of bronze smelting technology. Archaeological discoveries indicate that the use of bronze in West Asia dates back to approximately 3000 BC. West Asian bronzes were primarily used for religious rituals and daily life, characterized by unique artistic styles.

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Europe

European bronze culture is mainly distributed in Central and Eastern Europe. Archaeological findings indicate that the European Bronze Age culture had exchanges with Asia. For example, bronze artefacts found in the Volga River region of Russia bear similarities with those from the Shang and Zhou periods in China.

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South Asia

The Bronze Age culture in South Asia is primarily concentrated in the Indus Valley Civilization. Bronze artefacts from the Indus Valley include tools, weapons, and ornaments, demonstrating highly developed metallurgical techniques.

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Southeast Asia

Southeast Asian bronze culture is influenced by both China and South Asia, and is primarily found in countries such as Vietnam, Thailand, Cambodia, and Myanmar. The bronze drums of Southeast Asia are particularly famous, and they are used not only as musical instruments but also in rituals and other ceremonies.

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Americas

Bronze culture in the Americas is primarily found in the Andean region, including Peru and Bolivia. The Andean civilization’s bronze artefacts comprise tools, weapons, and religious items, reflecting a distinct cultural identity.

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Africa

African bronze culture is primarily concentrated in sub-Saharan West Africa, such as the Ife civilization and the Benin Kingdom in Nigeria. West African bronzes are renowned for their sculptures and masks, showcasing advanced casting techniques and artistic levels.

The distribution and characteristics of these bronze artefacts not only demonstrate the metallurgical skills of ancient civilizations but also reflect the cultural exchanges and influences among different regions.

2.4. Significance of Cultural Bronze Artefacts

Bronze artefacts are historically, culturally, and artistically significant. Their significance is mainly reflected in the following aspects:

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Historical Value

Bronze artefacts are crucial physical materials for studying ancient societies, politics, economies, and cultures. Through the study of bronze artefacts, archaeologists and historians can gain insight into the development processes and social structures of ancient civilizations. For example, the use of bronze artefacts in ancient China reflects a strict hierarchical system and ritual culture.

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Cultural Symbolism

In many ancient civilizations, bronze artefacts were seen as symbols of power and status. For instance, ancient Chinese bronze ritual vessels, such as the ding, symbolized royal authority and national stability. As ritual vessels, bronze artefacts were used not only for sacrifices but also for noble weddings, banquets, and alliances, making them a crucial tool for maintaining social hierarchy.

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Artistic Value

The production of bronze artefacts requires complex craftsmanship and exquisite decoration, resulting in high artistic value. The shapes, patterns, and inscriptions of ancient Chinese bronze artefacts showcase the aesthetic concepts and artistic achievements of the time. For example, the bronze artefacts from the Shang Dynasty are renowned for their sophisticated ceramic mold-casting techniques and intricate decorative motifs.

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Technical Achievements

The production of bronze artefacts requires advanced metallurgical techniques and casting processes. The techniques for making bronze artefacts vary across different regions. For example, China used ceramic mold casting, whereas ancient Egypt and Mesopotamia primarily employed forging and lost-wax casting.

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Evidence of Cultural Exchange

Bronze artefacts also reflect the exchanges and influences between different ancient civilizations. For example, specific shapes and decorations of Chinese bronze artefacts were influenced by cultures from West Asia and Central Asia, providing evidence of early cultural exchanges.

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Modern Influence

Bronze artefacts continue to have a significant influence on modern society. They are not only precious exhibits in museums but also promote international cultural exchange and public understanding of ancient civilizations through exhibitions and research.

In summary, bronze artefacts hold significant importance in history, culture, art, and technology, making them indispensable physical materials for studying and understanding ancient civilizations.

3. Factors Affecting the Corrosion Behavior of Bronze Artefacts

Several studies have highlighted factors that influence the corrosion behavior of bronze artefacts [36]. Internal and external factors are the two primary causes that affect the erosion of bronze artefacts. Internal causes include alloy composition and microstructure [35]. The external factors encompass the variable differences in the preservation cycles of bronze artefacts.

3.1. Influence of Alloy Itself

3.1.1. Chemical Composition

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The effects of tin and lead in bronze

Bronzes are outstanding representatives of ancient metal craftsmanship, with their primary materials being copper, tin, and sometimes lead. These elements play a crucial role in the manufacture of bronzes:

The role of tin:

• Enhance hardness and wear resistance: The addition of tin significantly increases the hardness of bronze, making it an ideal material for making tools, weapons, and vessels.
• Improve casting performance: Tin lowers the melting point of copper, enhancing the metal’s fluidity and making casting easier while reducing iron inclusions and improving the quality of castings.
• Enhance corrosion resistance: Bronze with a high tin content exhibits better corrosion resistance, enabling bronzes to maintain their good condition over an extended period.
• Adjustment of color: Variations in tin content can affect the color of the bronze, ranging from pale yellow to dark brown and even to a golden luster.

So et al. [37] investigated the impact of Sn content on the microstructure and mechanical properties of Cu–Sn alloys, as well as the influence of post-deformation processing at high temperatures. As shown in Figure 2, the as-cast alloy exhibits dendritic and eutectoid phases, accompanied by shrinkage-related defects (Figure 2a–d). High-pressure die (HPD) treatment at 750 °C enhances the alloy’s properties through recrystallization and phase transformation, thereby eliminating defects and refining the microstructure (Figure 2e,f). It is concluded that the work hardening mechanisms, such as precipitation and twin gliding, further increase the tensile strength. Rapid solidification following high-temperature processing results in a diffusionless transformation into a martensitic phase, thereby enhancing both the strength and ductility. The RT tensile stress–strain curves of the specimens are illustrated in Figure 3.

The role of Pb:

• Decrease hardness: The addition of lead can decrease the hardness, which is ascribed to the low hardness itself, as well as its weakening effect on the binding strength of the bronze grain boundaries
• Helpful for manufacturing parts with complex shapes or those that require close fitting.
• Decrease melting point: Lead can also decrease the melting point, thereby improving its fluidity and mold-filling capacity, which is conducive to pouring large pieces and bronze components with uneven thickness.
• Reduce oxidation and deterioration: Lead helps decrease oxidation and spoilage of bronze during the melting process, thereby reducing surface tension and promoting the fluidity of the melt.

In summary, tin and lead play an essential role in the production of bronzes. They determine not only the physical properties of the bronzes but also their aesthetic appeal, making them an integral part of ancient bronze civilization.

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Influence of Sn on the corrosion resistance of bronze

Tin is the primary alloying element in bronze. The content of Sn in ancient bronzes varied due to differences in era, region, purpose, and production techniques. The content of Sn in ancient bronzes was not fixed, but it can be roughly summarized based on historical records and archaeological findings. Specifically, in some ancient bronzes, the content of Sn could be as high as 20% or more, especially when high hardness and corrosion resistance were required. However, there were also bronzes with relatively lower Sn content, perhaps a few percentage points or less. Several researchers have indicated that the Sn concentration in bronze alloys typically ranges from 3% to 14%. They have observed that the resistance of bronze to corrosion improves as the tin content rises. However, concentrations of tin that are either excessively high or low are detrimental to the safeguarding and maintenance of bronze artefacts [38,39]. For example, the Sn content in Sanxingdui bronzes ranges from 3% to 12% [40].

Muller et al. [41] studied the effect of Sn content (7%, 11%, and 14%) on the electrochemical corrosion behavior of bronze alloy in a sulfate solution. They found that corrosion is controlled by mass transfer and charge transfer. A colloidal-like passivation layer was rapidly formed on the surface of high-Sn-content bronze, reducing the diffusion coefficient of copper ions from the alloy to the electrolyte to 10⁻¹⁰ cm²/s. Figure 4 shows the relationship between the RMS of different sample scanning rates and current density. As can be seen from the plot, the current density (J_p) of each group increases with the increase of the scanning rate. However, the relative size of the current density of different samples at the same scanning rate remained unchanged, that is, J_p(Cu7Sn) < J_p(Cu11Sn) < J_p(Cu14Sn). That is to say, the corrosion resistance of the Cu-Sn alloy in a sulfate medium improved with the increase in Sn content from 7% to 14%. Studies on the corrosion behavior of Cu-Sn bronze alloys with different Sn contents (0%, 4%, and 6%) for long-term atmospheric exposure showed that in urban environments, tin oxide enriched the patina corrosion products and the interface between the film and substrate, making the corrosion products thinner and improving adhesion to the substrate, thereby enhancing protection. However, in seashore environments, Sn did not improve corrosion resistance and could even thicken the patina corrosion products, weakening the adsorption of corrosion products to the outer layer. Research by Sun et al. [42] indicated that tin bronze containing 6% to 7% Sn in the deep-sea environment of the South China Sea produces copper compounds such as Cu₂O, CuCl₂, and Cu₂(OH)₃Cl, and also undergoes Sn removal corrosion.

Liang and colleagues [25] prepared Cu-Sn-Pb alloys with various compositions and investigated their corrosion behavior in an archaeological soil medium. Sn content in Bronze-I, II, III, and IV gradually increases, with Bronze-IV having 14.15 wt.% Sn. They found that the corrosion resistance of the ternary alloy is positively correlated with the Sn content; the higher the Sn content, the better the corrosion resistance of the lead bronze alloy (as shown in Figure 5). Additionally, the patina on the surface of leaded bronze typically comprises two primary layers: an inner layer rich in tin compounds and an outer layer that includes copper, lead oxides, as well as metal chlorides and sulfates.

Wang et al. [43] investigated the relationship between the composition of bronze alloys and powdery corrosion products. They found that the formation rate and state of powdery corrosion products varied with different proportions of Cu, Sn, and Pb. Within a specific range, high Sn content made powdery corrosion products less likely to form, whereas lower Sn content facilitated their formation. Bronze blocks with a Sn mass fraction higher than 20% were less likely to form powdery corrosion products, while those with about 10% Sn were most susceptible. Zhou [44] initially investigated the relationship between bronze corrosion and burial environment, revealing significant differences in corrosion resistance among bronze alloys with varying compositions. High Sn bronze alloy exhibited the most robust corrosion resistance.

Sn can enrich the oxide film on the bronze alloy surface, thus affecting the corrosion behavior. The tin oxide forms due to the oxidative hydrolysis of mobilized tin chloride complexes, which diffuse to the interface after the selective corrosion of the tin-rich phases under low oxygenation conditions.

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Effect of Pb on the corrosion resistance of bronze

Pb is typically a secondary and auxiliary element in bronze. In Cu-Sn-Pb ternary alloys, Pb and Cu-Sn binary solid solutions are completely immiscible but dispersed in the Cu-Sn solid solution matrix as free particles, resembling “hollow bubbles”. The distribution of Pb particles improves solidification properties, hinders coarse crystalline structure, and enhances casting properties. Adding Pb reduces the alloy’s melting point, increases fluidity and formability, improves filling ability during pouring, and reduces bubble formation [45].

Scholars have also investigated the specific behavior of Pb during the process of bronze corrosion. Wang et al. [43] found that the Pb content, within a specific range, did not significantly affect the amount or rate of formation of powdery corrosion products. Zhou and co-authors [44] found that the high Pb copper alloy had the weakest corrosion resistance, and pure copper had intermediate corrosion resistance. Quaranta et al. [46] studied the corrosion mechanism of spherical Pb in high Pb bronzes unearthed in Shaanxi, China. They found that metallic Pb on the surface of spherical Pb first corroded into Pb compounds (oxides, sulfates, or carbonates). These unstable, soluble particles diffused outward through pores and channels. Meanwhile, Cu ions dissolved around spherical Pb deposited as chalcopyrite in the pores left by Pb corrosion products, eventually replacing spherical Pb with chalcopyrite. Fan et al. [47] showed that PbCO₃ could also transform into Pb₅(PO₄)₃Cl in an acidic environment rich in phosphorus and chlorine. Preliminary analysis of Pb’s forms and corrosion products in bronze alloys has been conducted; however, further studies are needed to investigate the impact of Pb corrosion products on the corrosion behavior of bronze artefacts. Wang [40] discovered a blue and white corrosion product on the surface of bronzes unearthed in Sanxingdui, named “crisp powdery corrosion products”, primarily composed of PbCO₃ with small amounts of Cu₃(OH)₂(CO₃)₂ and Cu₂(OH)₂CO₃. These corrosion products formed due to Pb’s reactivity, forming a dense PbO film at normal temperature, which protected Pb from oxidation. High temperatures during sacrifice promoted Pb oxidation to PbO [25,48]. After cooling, PbO absorbed crystal water, transforming into white amorphous colloidal PbO·H₂O. In an acidic, water-soluble environment with CO₃²⁻ and O₂, PbO·H₂O was converted to PbCO₃, which was mixed with copper corrosion products to form blue-white corrosion products.

3.1.2. Microstructure

The binary Cu-Sn phase diagram is shown in Figure 6a [49]. Although the Sn content varies in different periods, regions, and specific bronzes, it is generally not more than 30% in bronze artefacts. However, alloys of various compositions vary considerably in their metallurgical structure. Bronze alloys can have a single-phase α solid solution microstructure when the tin content is less than 6%, and a multiphase microstructure with α solid solution and (α + δ) eutectoids (Figure 6b) when tin content is higher than 6%, whereas lead often exists in a free state within these phases (Figure 6c) [50]. Bronze alloys have numerous grain boundaries and phase boundaries, as well as differences in alloy composition in different phase regions. These differences in composition and potential differences caused by grain boundaries are internal factors leading to electrochemical corrosion, which induces and develops powdery corrosion products [51]. For example, (α + δ) eutectoid structure and free Pb are found in the α solid solution matrix in bronzes unearthed in Sanxingdui [40]. Xu et al. [52] showed that crack propagation paths are at the interface between hard/brittle second-phase grains and the matrix. Wang et al. [53] found that dendrites in cast bronze α solid solution matrix are more severely corroded than interdendritic regions and Sn-rich eutectoid structures. This phenomenon could be explained by the higher potential of the δ phase than the α phase, which can facilitate the galvanic corrosion between these two phases during corrosion, and thus accelerate α phase corrosion. Limited research on the influence of microstructure on the corrosion behavior of bronze alloys indicates that segregation phases or inclusions can exacerbate corrosion.

The typical distribution pattern of Pb in leaded bronze is described in a study conducted by Li and co-authors [56]. They examined an ancient Chinese bronze sword from the Warring States Period and found that the black patina on the sword revealed the typical casting microstructures of the sword body. It was predominantly made up of intricate α dendritic structures interspersed with a uniform distribution of (α + δ) eutectoid formations and isolated lead particles. Numerous minute black porosity spots, a byproduct of the casting process, were also present. The optical images of the sword body are shown in Figure 7.

Generally, current research on the influence of composition and microstructure on the corrosion behavior of bronze alloys lacks sufficient depth and systematicity. There is a need to accurately characterize the growth of corrosion product films and explain the mechanisms underlying the influence of alloy elements and multi-element coupling, as well as the corrosion mechanisms of bronzes. Researchers tend to overlook the impact of segregation phases or inclusions on the corrosion behavior of bronze alloys, while the micro-galvanic corrosion between segregation phases or inclusions and the bronze matrix phase is a significant factor that promotes bronze alloy corrosion.

3.2. Natural Environment

3.2.1. Water Environment

The corrosion conditions of effluent bronzes differ from those of unearthed bronzes [57]. Due to the characteristics of the environment, the corrosion mechanisms and products of effluent bronzes in the water have distinct features. The pH value, salt content, water quality, and even the velocity of the water can significantly impact the corrosion of bronze. However, current research is limited mainly to the effect of salt in water on the corrosion of bronze. Based on the geographical environment of bronze storage, they are typically categorized into freshwater and seawater environments. Bronzes stored in different water environments will develop various types of corrosion products. The most significant factor influencing bronze in seawater and freshwater environments is the concentration of Cl⁻ ions. Generally, the upper limit of chloride ion concentration is 200 ppm in freshwater and 19,000 ppm in seawater. The influence of chloride ions on the corrosion of bronzes will be discussed in Chapter 4. This chapter only introduces the corrosion of bronzes in freshwater and seawater environments from the perspective of buried media.

(1)

Influence of freshwater environment on corrosion of bronze artefacts

The degree of bronze corrosion caused by freshwater is relatively mild. However, it is essential to consider factors such as different water flow velocities, floating sand and bubbles carried by freshwater, and erosion caused by the impact of the water flow itself. Due to long-term scouring, the surface of bronze often becomes uneven, with trachoma and sandstone. Some bronzes are directly exposed without corroded skin; others have black skin on their surfaces due to copper sulfides of the general form Cu₂S or Cu_xS. In still water or slow-flowing environments, white calcium carbonate may adhere to the surface of bronze, but it is less likely to form in fast-flowing water. Bright blue-green corrosion product crystals on the surface of some effluent bronzes result from the comprehensive chemical reaction between bronzes, humid air, and underground acids and alkalis. In this case, the surface corrosion type is similar to that of the excavated bronzes, primarily due to the change in the river channel that caused the Shengkeng bronzes to be re-entered into the water. Furthermore, if the bronze artefacts are directly immersed in water, dissolved CO₂, NH₃, and other gases will also directly participate in the corrosion of the bronzes, accelerating the corrosion process and increasing the degree of corrosion [58]. In contrast to soil erosion, metallic artefacts buried in aquatic environments undergo simpler erosion processes. It has been observed that protective oxygenation membranes tend to form on bronze surfaces in oxygen-rich conditions, attributed to an oxygen cathode polarization effect, which inhibits further erosion of the metallic artefacts.

(2)

Influence of seawater environment on corrosion of bronze artefacts

The composition of seawater is relatively complex, and the dissolved substances are primarily inorganic salts, such as sodium chloride (NaCl), potassium chloride (KCl), magnesium chloride (MgCl₂), magnesium sulfate (MgSO₄), and sodium bicarbonate (NaHCO₃). The corrosion products of marine effluent bronze differ from those found in freshwater and soil environments. Analyses of marine effluent bronzes show that corrosion products include cuprous oxide (Cu₂O), copper sulfide (CuS), tin dioxide (SnO₂), cuprous chloride (CuCl), lead sulfate (PbSO₄), lead carbonate (PbCO₃), and copper oxychloride (Cu₂(OH)₃Cl).

Zhang et al. [59] examined the exfoliation mechanism of corrosion products on bare copper sheets and three copper-based alloys through a seawater immersion test. The results indicated that the volume expansion of CuCl during its conversion to Cu₂(OH)₃Cl was the primary cause of exfoliation of the corrosion product. Chang et al. [60] investigated the corrosion mechanism of tin bronze in the marine environment. They discovered that the layered corrosion products formed in a chlorine-rich environment mainly consisted of Cu₂O and Cu₂(OH)₃Cl mixed with tin oxide, which was mainly composed of SnO₂. The stratification phenomenon on the surface of tin bronze is the main reason for increased peeling and accelerated corrosion. Tang et al. [61] investigated the corrosion behavior of bronze in sodium chloride and sodium sulfate environments, analyzing the relationship between the dissolution factor of copper and electrification time. They pointed out that the selective corrosion order of each element in bronze is Pb > Cu > Sn. Veleva et al. [62] used X-ray diffraction to analyze the composition of surface corrosion products on copper samples exposed to different seawater zones (splash zone, tidal zone, and complete immersion zone) and marine atmosphere. The findings show that chlorination pollution and specific areas have a significant influence on the composition of verdigris.

Nuñez et al. [63] characterized in depth the corrosion behavior of copper in the seawater environment in the tropical area of the Cuba archipelago. The results indicated that under complete immersion conditions, the influence of offshore and onshore regions on copper corrosion is not significant. The main component of the patina formed in the marine splash area is Cu₂(OH)₃Cl, and the patina’s adhesion is poor. Yang et al. [64] quantitatively investigated the transformation process of cuprous chloride in aqueous environments by placing powdered corrosion products in simulated neutral and weakly acidic environments to explore its dissolution behavior and characteristics in humid environments. The results showed that under neutral conditions, chlorocopperite can dissolve and ionize more chloride ions than parachlorocopperite. At the same time, a weak acidic environment does not increase the solubility of chlorocuperite but significantly increases the solubility of parachlorocopperite.

Researchers analyzed the corrosion products of copper coins from the effluent of Nanhai I. They found that they mainly include cuprous oxide (Cu₂O), copper oxychloride [Cu₂(OH)₃Cl] and [Cu(OH)Cl], and copper hydroxide [Cu(OH)₂]. Among them, copper chloride is the primary corrosion product, such as copper oxychloride [Cu(OH)Cl] and [Cu₂(OH)₃Cl]. Chloride is a critical factor in inducing bronze disease, which can easily cause continuous corrosion of bronze.

In seawater environments, copper corrosion products exhibit distinct characteristics under both anaerobic and aerobic conditions. In oxygen-free seawater, chloride ions react with copper to form a layer of cuprous chloride (CuCl):

Cu + Cl⁻ → CuCl

(1)

CuCl layer reacts with water to form cuprous oxide (Cu₂O):

2CuCl + H₂O → Cu₂O + 2H⁺ + 2Cl⁻

(2)

No further corrosion occurs without oxygen. However, in an oxygenated environment, chloride ion (Cl⁻) reacts directly with cuprous oxide (Cu₂O) in the presence of oxygen and water to form cuprous oxychloride [Cu₂(OH)₃Cl], which is a relatively slow process:

Cu₂O + 2H₂O + O₂ + Cl⁻ → Cu₂(OH)₃Cl

(3)

As the cuprous oxide (Cu₂O) transforms, the cuprous chloride (CuCl) layer also undergoes transformation and expansion. Cuprous chloride (CuCl) is converted to cuprous oxide (Cu₂O) along the equilibrium reaction direction and gradually transforms into cuprous oxychloride [Cu₂(OH)₃Cl]. Additionally, corrosion can be exacerbated if oxygen is dissolved in chloride-rich environments. If Cl⁻ encounters a eutectoid structure or lead (Pb), it can still extend in the pitting direction, and this oxidation process is rapid.

3.2.2. Soil Environment

According to incomplete statistics, more than 80% of bronzes are excavated from the field, and the colorful corrosion on their surfaces results from soil environmental corrosion. Soil is a multiphase porous colloidal system (composed of gas, liquid, and solid) with ionic conductivity, which is influenced by natural factors such as climate, biology, topography, and human activities. Imbalances in the soil environment can lead to primary cell corrosion and chemical corrosion of bronzes. Factors influencing soil environment corrosion on bronze include humidity, salt content, pH, soil type, temperature, oxygen content, coverings or nearby materials, the presence and variety of microorganisms, etc. [25,65]. Zhou [44] comparatively studied the corrosion behavior of bronze alloy samples in various buried environments and analyzed the effects of four corrosion factors—ion concentration, pH value, water content, and porosity—on the corrosion process of bronze samples. The results showed that the pH value and ion concentration significantly influenced the corrosion of the bronze alloy, while water content and porosity had a minimal impact. Wang and others [66] analyzed the corrosion status of bronzes in a cellar environment that was not subject to soil erosion. They found that a weakly alkaline environment was beneficial for protecting bronze. He et al. [67] analyzed the influence of pH on the corrosion behavior of bronze and concluded that the initial pH affects the hydrolysis process of CuCl and the formation of the final corrosion products. Scott [68] studied the corrosion penetration rate and depth of copper fragments collected in the Getty Conservation Museum. He concluded that the growth of red copper deposits played a protective role for copper in benign soil environments. Sun and others [69] concluded that carboxyl and phenolic hydroxyl groups in soils can be complexed with copper metal ions, which is essential for the formation of oxides on bronze surfaces and the loss of copper.

Due to casting, bronzes can develop shrinkage cavities, cracks, and other defects. When buried in the soil environment for an extended period, the surface of the matrix cracks due to physical friction caused by natural machinery, and the compression from changes in soil weight and geological structure also cause the bronze artefacts to break. The commonly known process of bronze corrosion occurs when the soil does not contain chloride ions. First, oxidation reactions easily occur at holes, cracks, and fractures, forming red cuprous oxide (Cu₂O):

4Cu + O₂ → 2Cu₂O

(4)

Or CuO:

2Cu + O₂ → 2CuO

(5)

As oxidation increases, Cu₂O disseminates and fills the fissures, forming a red oxidized corrosion product closest to the body as oxidation intensifies. Under certain conditions, thermodynamically unstable cuprous oxide (Cu₂O) can undergo a disproportionation reaction:

Cu₂O → Cu + CuO

(6)

Typically, the red cuprous oxide (Cu₂O) is covered with green, blue, and black corrosion products. This is because cuprous compounds are unstable and continue to react in complex soil environments. In humid environments, malachite [Cu₂(OH)₂CO₃] forms due to oxidative hydrolysis in a carbonate-rich environment, as primary copper (I) ions migrate out through the cuprite layer.

2Cu₂O + 2H₂O + 2CO₂ + O₂ → 2Cu₂(OH)₂CO₃

(7)

Simultaneously, salt, water, oxygen, and CO₂ in the soil continuously infiltrate into cuprous oxide (Cu₂O) and basic copper carbonate [Cu₂(OH)₂CO₃] through the pores, causing continuous corrosion. During the corrosion process, copper is lost more easily than tin and is almost completely lost through continuous oxidation and leaching, leaving a white-grey SnO₂ layer after corrosion. Some of the dissolved Pb is deposited in situ to form insoluble yellow lead oxide or white lead carbonate (PbCO₃). In contrast, others migrate through pores to form lead carbonate (PbCO₃) deposition in the outer layers.

When chloride ions are present in the soil, cuprous chloride (CuCl) is usually formed, which exists underneath the dense shell of the bronze surface and is relatively stable. Under high environmental humidity, if a particular surface area is damaged or defective, water and oxygen can easily enter the shell layer and react with cuprous chloride (CuCl) to form basic cuprous chloride Cu₂(OH)₃Cl. Due to its volume expansion, it will emerge from the damaged area and defects, burst, and form what is generally considered to be a bronze disease. Powdery corrosion products seep out in large quantities from the inside to the outside, causing continuous damage to the bronze substrate. Tie [70] suggested that there is a material exchange between bronzes and the external corrosive environment. On the one hand, the generation of chloride-containing powdery corrosion products results from the activation of chloride in the external environment. On the other hand, chloride from the external environment enters the bronze artefacts through the exchange of environmental media, and then it is activated in a suitable environment to produce powdery corrosion products.

3.2.3. Atmospheric Environment

The environment in which bronzes are interred is usually characterized by a sustained lack of oxygen. Upon excavation, the time of exposure to air is significantly increased, and the possibility of corrosion and oxidation of the bronze artefacts increases due to the abundant oxygen in the environment. The corrosion of bronzes in an atmospheric environment is primarily caused by chemical and electrochemical reactions involving various corrosive ions in a humid atmosphere. The high levels of water vapor, carbon dioxide, oxygen, ozone, and sulfur dioxide in the atmosphere, coupled with environmental temperatures and humidity levels, can be absorbed by the porous corrosion products on the surface of recently excavated bronze artefacts. This absorption actively participates in the corrosion process, promoting the development of powdery corrosion products. In humid conditions, the proliferation of corrosive microorganisms is also more likely, further accelerating the corrosion of bronzes [71]. Various investigations into the deterioration of bronze have examined the decay of unearthed artefacts after conservation, with a focus on the effects of atmospheric exposure. Moreover, benign byproducts of corrosion, including malachite and azurite, are prone to developing under conditions of elevated CO₂ levels and surrounding moisture. Conversely, detrimental powdery corrosion products arise in the presence of chloride within the corrosion byproducts. If preservation conditions are not optimal, excessive corrosion of bronze can occur even within museum environments. For instance, Li and colleagues [72] have reported the case of a bronze wine vessel with a loop handle, which is now housed in the Xiaogan Museum in Hubei Province, China. Figure 8 shows the SEM and Raman spectra of the stalactite. They attributed the corrosion to the overly sealed museum display case, where, aside from the vessel’s lid, the high concentration of H₂O and CO₂ gases inside the container over time led to the formation of ultra-pure, high-purity malachite. The growth of malachite continuously consumes the bronze material, shortening its lifespan.

The interaction of cuprous chloride with atmospheric water vapor and oxygen results in the formation of greenish, powdery corrosion products, copper (II) chloride dihydrate [CuCl·3Cu(OH)₂], which exacerbates the deterioration of the artefacts. The presence of acidic gases, such as nitrogen oxides and sulfur dioxide, in the air, which readily dissolve and turn acidic, can exacerbate the corrosion of bronzes. Fan et al. [73] investigated the development and changes of powdery corrosion products on bronzes before and after excavation. They concluded that bronzes oxidized by oxygen in the presence of chloride ions before excavation would continue to corrode at an accelerated rate after exhumation. This acceleration is due to the lack of O₂ deep underground, where Cu is fully oxidized, but in the form of CuCl or Cu₂O. After excavation, sufficient oxygen allows for copper to continue oxidizing in a humid environment, changing from univalent to bivalent corrosion products. The corrosion products gradually break down, loosen, and swell, leading to overall damage to the bronzes.

In particular, sulfur dioxide (SO₂) can adhere to the surface film of bronzes and form bisulfite ions and hydrogen ions, which have the potential to be further oxidized to sulfate ions to produce sulfates such as copper sulfate [CuSO₄], thereby hastening the corrosion process. Additionally, a dark layer of copper (I) sulfide [Cu₂S] is much more likely to form from the biological reduction of sulfate to sulfide and the precipitation of a range of copper sulfides. Studies by Sasaki et al. [74] indicated that the Cu₂O and Cu₂S corrosion product layers were particularly susceptible to pitting corrosion. Rice et al. [75] measured the indoor corrosion rates of copper and other alloys at eight locations in the United States and found that sulfur dioxide and ozone had a significant impact on their corrosion rates. Eriksson et al. [76] investigated the corrosion behavior of a Cu-Zn-Sn-Pb alloy exposed to environments containing sulfur dioxide, nitrogen dioxide, and sodium chloride. They found that the nitrogen dioxide environment is more corrosive than the sulfur dioxide environment. Strandberg et al. [77] found that black corrosion appeared on the surface of copper samples after 12 h of testing when the relative humidity exceeded 75% and the mass concentration of SO₂ was 4–69 μg/L. The results showed that the corrosion rate of bronze materials increases with the increase in ozone and sulfur dioxide concentration, and the increase in ambient temperature accelerated the corrosion of bronze materials.

Zhao et al. [78] summarized the mechanistic process of copper corrosion by formic acid and high relative humidity under laboratory conditions (Figure 9). The sequence began with a pristine copper surface (a) and then formed an enlarged Cu₂O layer via solid-state crystallization (b). Subsequently, the Cu₂O layer underwent either a partial dissolution (pathway 1) or a complete dissolution (pathway 2), both of which were triggered by the presence of formic acid (c). Finally, Cu(OH)(HCOO) was produced regardless of the dissolution extent, and an enhanced Cu₂O layer was detected in the vicinity of the Cu(OH)(HCOO) formation zone because of the occurrence of a cathodic reaction (d).

Fang [79] studied the synergistic effect of NO and NO₂ on the corrosion of simulated cultural archaeological bronzes. The results showed that increasing the content of NO and NO₂ gases and reaction temperature would accelerate the corrosion of simulated materials. Rainwater and surface orientation also significantly affect the composition and morphology of verdigris [80,81]. In the collection environment, volatile, harmful gases and organic acids in display cabinets can also pose corrosion hazards to bronzes [82].

Light exposure can cause the Cu₂O present on bronze artefacts to emit photoelectrons, which transform into oxidative photoholes that can snatch electrons from Cu atoms, thereby causing the artefacts to corrode. Additionally, light can facilitate the absorption of O₂ by Cu₂O, leading to further corrosion of the alloy components and a continuous thickening of the surface corrosion products [83]. Wu et al. [53] conducted a thorough examination of how ultraviolet (UV) and visible light impact the corrosion behavior of bronze alloys. They observed that the bronze samples in the control group exhibited significant localized corrosion in the dendritic areas. However, under visible light, the surfaces of the bronze samples were covered with numerous porous corrosion products, indicating a more severe degree of corrosion than in the dark environment. Conversely, under UV light, only minor surface depressions were observed on the bronze samples, suggesting that UV light significantly reduced the corrosion rate of the bronze substrate. This reduction is attributed to the fact that the UV-induced increase in Sn concentration within the oxide film fortified and stabilized the film, thereby enhancing its protective capabilities. Considering the effect of light on the corrosion of bronze, an upper limit of illumination of 300 lux is recommended for bronze collections.

Consequently, museums must comply with stringent air quality standards in the preservation and display of their collections. It is imperative that construction materials are free of toxicity, pollution, and harmful gas emissions; the same criteria apply to the materials used in the fabrication of display cases. Moreover, the museum’s illumination must be maintained at an appropriate level to minimize the corrosion of bronze artefacts and thus fulfil preservation goals.

3.2.4. Microorganisms

Microorganisms play a crucial role in the corrosion of metals. Such corrosion can occur in various environments, including soil, water, and air. The primary microorganisms responsible for this effect are bacteria, fungi, and algae, with sulfate-reducing bacteria being particularly notorious in such processes [84,85]. Research has shown that all buried or poorly preserved materials, including metal cultural artefacts, are susceptible to microbial corrosion. During their metabolic activities, these microorganisms generate substances that can erode metals, such as acidic and basic compounds, sulfides, and other detrimental materials. When artefacts made of metal are exposed to moist conditions, the interplay between biological and chemical corrosion can lead to significant damage [86]. Numerous microorganisms and harmful substances in the soil adhere to the surface of bronze, causing it to corrode.

Microbial corrosion of bronzes is primarily caused by certain fungi [87,88], and the extent of corrosion is influenced by three key factors: the type of bronzes, the microbial species, and the environment [89]. Microbial corrosion and electrochemical corrosion are interconnected and mutually reinforcing, with the fundamental process being the metal losing electrons (i.e., Cu becomes Cu⁺ or Cu²⁺). Concurrently, microorganisms grow and reproduce on bronzes, producing various metabolic organic acids [88]. These organic acids can also directly combine with electrons released from metal corrosion to produce H₂ [89], ultimately accelerating the corrosion of bronzes. It is noteworthy that, in addition to the H⁺ produced by microorganisms to combine with electrons, some anaerobic reducing microorganisms can also accept electrons. For instance, denitrifying bacteria can reduce nitrate to nitrite by accepting electrons, sulfate-reducing bacteria can reduce sulfate to sulfur anion after accepting electrons, and methanogenic bacteria can convert CO₃²⁻ to carbon anion. These microorganisms can cause or exacerbate the corrosion of bronze in soil and other suitable environments. Additionally, Pseudomonas aeruginosa can form a biofilm on the alloy surface, which reduces the open-circuit potential and increases the corrosion current density and the depth of pitting pits [90]. However, copper itself possesses specific antibacterial properties, and copper ions can disrupt the cell wall and cell membrane of bacteria, thereby achieving an antibacterial effect that slows down microbial corrosion [91,92].

Research conducted by Luo et al. [88] suggested that microorganisms can expedite the corrosion process in bronzes. Under favorable environmental conditions, these microorganisms release metabolites that accumulate on the copper surface over time, resulting in corrosion and the formation of corrosion products. These metabolites are slightly acidic. Microorganisms generally exist in a dormant, spore-like state in dry conditions but become active and proliferate in large numbers as the environment becomes more humid. A decrease in pH within the corroding areas has been observed, likely due to the formation of corrosive organic acids that create galvanic cells. Zhao et al. [93] explored the effect of fungi on copper, finding that copper is relatively non-toxic to fungi. The Aspergillus Niger species can degrade copper through proton and ionic action, leading to significant mass loss and etching on the copper surface. Videla et al. [94] suggested that microbial corrosion occurs when microorganisms adhere to metal surfaces or are encapsulated in biofilms, a gelatinous organic matrix. This adherence causes notable changes at the metal/solution interface, influencing various corrosion-related factors such as ion species and concentrations, pH levels, and oxygen availability in the immediate liquid microenvironment or interface. In research conducted by G. Ghiara et al. [95], the corrosion properties of bronze with 12 wt.% Sn were investigated when exposed to a pseudomonas fluorescens culture for 100 h. The SEM images of samples after the impedance test are shown in Figure 10. Micro-Raman spectroscopy was used to determine the composition of the featured regions. As shown, in abiotic conditions, a relatively uniform surface with protective cuprite (Cu₂O), Sn(OH)_4, and hydrated cassiterite (SnO₂·2H₂O) was obtained (Figure 10a). After testing for 100 h, hydroxy-chloride clinoatacamite (Cu₂Cl(OH)₃) was detected over the passive layer (Figure 10b). After immersion in biotic conditions for 1 h, the adhesion and colonization of the biofilm on the sample surface can be observed (Figure 10c). Areas underneath the removed biofilm presented a preferential localized attack (Figure 10d). After 100 h, corrosion products and biofilms growing from isolated microcolonies can be observed. Biofilms were identified by a monolayer of cells with dried extracellular material (Figure 10e). Once the biofilm is removed, corrosion products featured with narrow but deeper pits (1 μm diameter and 5 μm depth) can be clearly observed (Figure 10f).

To prevent microbial corrosion of bronze artefacts, the following combined strategies can be implemented: (1) Ensure proper storage in a dry, controlled environment. (2) Utilize copper’s natural antimicrobial properties. (3) Apply protective coatings or patinas as barriers. (4) Conduct regular cleaning with gentle, non-abrasive cleaning solutions to remove biofilms and accumulated corrosive substances. (5) Monitor pH levels to maintain neutrality and reduce the potential for microorganism-induced corrosion.

3.3. Artificial Environment

The standard storage environment for bronze artefacts should be between 18 °C and 24 °C, with humidity below 40%. Currently, most museum exhibition halls can only achieve partial temperature control and lack effective humidity control for bronze cultural artefacts. The warehouses and restoration rooms of some heritage conservation units lack effective and consistent temperature and humidity control systems, resulting in significant fluctuations in temperature and humidity due to seasonal and weather changes. Archaeological site storerooms suffer from problems, such as the mixing of different types of cultural artefacts and the lack of timely and effective protection of these artefacts. In the absence of environmental control, the existing corrosion products on bronze artefacts will further develop, and the phenomenon of harmful patina spreading and infection will occur. The monitoring and control of air pollutants is one of the shortcomings in the preservation of all types of cultural artefacts. In environments such as cultural artefact exhibition halls, storage rooms, and restoration rooms, pollutants like sulfur dioxide, hydrogen sulfide, nitrogen dioxide, and dust in the air are released due to the release of decorative materials and the input of outdoor air pollutants. Bronze artefacts exposed to these air pollutants for an extended period, combined with the synergistic effects of temperature and humidity, are highly prone to oxidation and corrosion.

4. Corrosion Products and Their Formation Mechanism

4.1. Causes of Bronze Corrosion

From a thermodynamic perspective, bronze is a high-energy metal that can transform into a low-energy compound under specific conditions, demonstrating its inherent potential for corrosion [96]. As shown in Figure 11 [72], the bronze wine vessel from the Shang Dynasty (1700 B.C.–1100 B.C.) is covered with a specific stalactitic product. It is foreseeable that, without targeted protection, this corrosion will continue until it is completely consumed. Typically, bronze remains relatively stable in low-temperature and low-humidity conditions without sunlight. This implies that the environment where bronzes were buried lacked oxygen. However, once the external environment changes, bronze develops powdery corrosion products due to variations in temperature and humidity. Bronze is a ternary alloy, and typically, small amounts of free metallic lead (Pb) are distributed around the α-solid solution and the (α + δ)-eutectoid, resulting in chemical composition inhomogeneity due to variations in raw materials and manufacturing techniques [97]. This inhomogeneity is an internal factor that leads to bronze corrosion. In a corrosive environment, bronze undergoes complex chemical corrosion.

With advancements in science and technology, we have a more in-depth understanding of the corrosion mechanism of bronzes, and the views of experts are more unified. Since bronzes have been buried underground for extended periods, they come into contact with chlorides. Chloride ions (Cl⁻) have a tiny radius (0.181 nm) and can react with copper through a water film to produce cuprous chloride (CuCl). CuCl is inherently unstable and reacts with water to form cuprous oxide (CuO), hydrogen ions (H⁺), and chloride ions (Cl⁻). In the presence of air, water, and carbon dioxide, CuO transforms into basic copper carbonate (Cu₂(OH)₂CO₃), which further converts into basic cuprous chloride (Cu₂(OH)₃Cl) in an environment containing water, oxygen, chloride, and hydrogen ions. In summary, while copper is a relatively stable metal, the corrosion of its bronze alloy is a complex process.

The primary cause of corrosion in excavated bronze artefacts is external factors, and the corrosion products often exhibit a cascading shape, a phenomenon confirmed by archaeologists and related experts [98]. The transformation of cuprous chloride into basic copper chloride results in a loose, porous, and powdery surface, commonly referred to as powdery corrosion products [99]. As oxygen and water gradually penetrate the bronze, basic copper chloride converts into basic copper oxide, generating chloride ions (Cl⁻) in the process, which then react with copper to form cuprous chloride. This cycle of corrosion continues, exacerbating the surface corrosion of bronzes, extending the corroded area, and eventually causing perforations—a common issue known as bronze disease in bronze artefacts.

Given the intricate corrosion mechanisms of bronze, discussions often focus on the corrosion mechanisms of copper. If the bronze does not contain chloride, copper will react with oxygen to produce cuprous oxide (Cu₂O), as shown in Reaction (4). In a dry environment, cuprous oxide (Cu₂O) can prevent further oxidation of bronzes. However, in the presence of water vapor and carbon dioxide, they will react to produce Cu₂(OH)₂CO₃, as shown in Reaction (7). In the presence of chloride, bronzes undergo further corrosion to form new corrosion products, such as cuprous chloride (CuCl). Cuprous chloride reacts with water in a humid environment and produces cuprous oxide (Cu₂O), chloride and hydrogen ions, as shown in Reaction (2). The resulting hydrogen ions and chloride ions react with copper corrosion products:

CuCO₃·Cu(OH)₂ + 4H⁺ + 4Cl⁻ → 2CuCl₂ + 3H₂O + CO₂

(8)

Cu₂O + 2H⁺ + 4Cl⁻ − 2e⁻ → 2CuCl₂ + H₂O

(9)

If copper chloride (CuCl₂) is already present in the corrosion of bronze, it reacts with water and oxygen in the air to produce basic copper chloride, which is loose:

4CuCl₂ + 6H₂O + O₂ → 2Cu₂(OH)₃Cl+ 6H⁺ + 6Cl⁻

(10)

4.2. Material Exchange Between Bronzes and the Surrounding Environment

Tie and others [70] used modern analytical instruments and experimental methods to analyze the corrosion process of bronzes. They found that anions in the corrosion products of bronzes mainly include H, N, C, S, O, and Cl. When the corrosive environment was soil, the main influencing factors were H₂O, O₂, SO₄²⁻, NO₃⁻, CO₃²⁻, Cl⁻, and HCO₃⁻. When the corrosive environment was atmospheric, the main influencing factors were H₂O, O₂, SO₂, H₂S, NO_X, HCl, CO_2, and so on.

These results showed that the elements corresponding to the anions of bronze corrosion products are consistent with those found in soil and the atmosphere. The only difference is that, apart from water and oxygen, the other factors are acid radicals in water and acid gases in the atmosphere. Therefore, it can be concluded that there is a material exchange between the corrosion of bronze and the external corrosion environment. According to the thermodynamic analysis of bronze corrosion products, it is found that basic copper chloride is the most stable substance among all copper corrosion products, indicating that, from a thermodynamic perspective, any copper corrosion product will eventually become basic copper chloride. Basic copper chloride is also a component of powdery corrosion products, and the mineral name is atacamite. Basic cupric chloride belongs to the orthorhombic crystalline system, with isomers that form a blue-green or light-green crystalline powder. It is stable under sunlight, air, water, or CO₂ conditions.

However, this stability is only relative because its loose structure is insufficient to block the invasion of water, air, and oxygen. At the same time, it exhibits hygroscopicity due to its extremely fine powder form, allowing for it to transfer the absorbed water to the inner layer. Harsh gases in the air can also diffuse through it to the inner layer. Moreover, corrosion products often form hybrid corrosion product bodies, in which numerous physical pores and defects serve as channels for the transmission and diffusion of harmful substances. High chlorine levels can often be found in such holes and channels, as observed in the analysis of corroded samples.

Although harmful powdery corrosion products have no chemical activity, they exhibit physical activity due to their physical adsorption and diffusion properties. The corrosion reaction is exchanged through holes and defects in the corrosion products by means of the physical activity of the corrosion products. Harmful chlorine elements are highly distributed in the inner layer of corrosion products, and there are crisscrossed corrosion grooves at a certain depth of the matrix metal adjacent to the innermost corrosion products, in which chlorine elements are highly distributed. This reveals why ions harmful to bronzes are difficult to remove entirely. Furthermore, it is suggested that the protection of corroded bronze should start with controlling the corrosive environment.

Pure crystals can only be free of lattice defects, such as vacancies or interstitial atoms, at absolute zero degrees [100]. Starting from 0 K, point defects will emerge as the temperature increases according to thermodynamics. At room temperature, crystals also contain holes and lattice defects, which are not stationary but continuously move within the crystal. The vacancy mechanism is the most prevalent diffusion mechanism in both metal and ionic compound systems. Oxide films formed on copper and copper alloys primarily grow at the film–gas interface through cation diffusion. Due to the concentration of vacancies at the metal film interface, a significant number of vacancies often appear within the metal film.

When cations diffuse outward through these holes, anions must migrate in the opposite direction. These holes facilitate the invasion of anions. Naturally, the migration of anions varies greatly depending on differences in electronegativity and ion radius. Among the harmful components, the Cl⁻ ion has the smallest radius, making it the most easily invasive and the fastest to migrate. Consequently, Cl⁻ ions can easily penetrate and pass through the corrosion products, accumulating in the inner layer of the copper corrosion products and providing the necessary chloride ions for further corrosion.

4.3. Corrosion Mechanism of Bronze Artefacts

It is widely recognized that the bronze surface losses due to erosion mechanisms can be categorized into chemical etching and electrochemical etching [36,101]. Corrosion morphology can further distinguish between comprehensive erosion and localized erosion. The mechanisms of localized erosion include pitting corrosion, crevice corrosion, intergranular or grain boundary corrosion, and selective ingredient corrosion [36,102,103].

4.3.1. Uniform Corrosion

Uniform corrosion refers to corrosion that occurs across the entire surface of the bronze, typically caused by micro-galvanic corrosion due to differences in the material composition of the bronze itself [104]. A humid environment can further exacerbate micro-galvanic corrosion, leading to more severe overall corrosion of the bronze. The primary corrosion product of uniform corrosion is copper oxide (Cu₂O). The overall reaction mechanism is as follows:

Cathode: 2H₂O + O₂ + 4e⁻ → 4OH⁻

(11)

Anode: Cu − e⁻ → Cu⁺

(12)

If the microscopic cathode is close to the anode area, Cu⁺ produced in the anode reaction process will react with OH⁻ produced in the cathode reaction process, and Cu₂O will be formed on the bronze surface. The chemical equation of the reaction is as follows:

2Cu⁺ + 2OH⁻ → Cu₂O + H₂O

(13)

In a dry environment, the Cu₂O corrosion product film can protect the bronze and prevent further corrosion. In this case, even if O₂ and HCl are present in the environment, Cu₂O will not convert. In humid environment, Cu₂O will react with water vapor and CO₂ in the air to form malachite (Cu₂(OH)₂CO₃) (Reaction (7)), and even in a strongly basic environment (pH > 9), Cu₂(OH)₂CO₃ and Cu₂O will react with O₂ and CO₂ in the air to form azurite (Cu₃(OH)₂(CO₃)₂) (Reaction (14)) [105]. As Cu₂O transforms into Cu₂(OH)₂CO₃ and Cu₃(OH)₂(CO₃)₂, the protective quality of the corrosion product film decreases, accelerating the corrosion of the bronze.

4Cu₂(OH)₂CO₃ + 2Cu₂O + 4CO₂ + O₂ → 4Cu₃(OH)₂(CO₃)₂

(14)

In a humid environment with sufficient oxygen, chloride, and hydrogen ions, Cu₂O will eventually convert into atacamite or paratacamite (Cu₂(OH)₃Cl), with the following reaction equation:

2Cu₂O + 2H⁺ + 2Cl⁻ + 2H₂O + O₂ → 2Cu₂(OH)₃Cl

(15)

4.3.2. Pitting Corrosion

Pitting corrosion is the primary corrosion phenomenon that leads to the destruction of bronze [104]. It occurs at specific points on the bronze surface and extends into depths, potentially causing perforation. This is generally caused by small numbers of impurities, casting cracks, or holes in the bronze. Typically, the corrosion products on the bronze surface are divided into three layers (Figure 12 [83]): the innermost layer (first layer) is white CuCl, the middle layer (second layer) is reddish-brown Cu₂O, and the outermost layer (third layer) is green or blue CuCO₃·Cu(OH)₂.

The corrosion products will eventually form green CuCl₂·3Cu(OH)₂ at the pitting corrosion site, transforming the corrosion products in this area into harmful corrosion products (powdery corrosion products) mainly containing CuCl and CuCl₂·3Cu(OH)₂. Lucey et al. [102] proposed the “membrane cell” theory of pitting corrosion. They suggested that the cathode and anode processes occur in the outer and inner layers of the Cu₂O film, respectively. The cathode reaction, Cu²⁺ + e⁻ → Cu⁺, occurs in the outer layer, and the anode reaction occurs in the inner layer (Reaction (11)). The Cu⁺ in the reaction partly diffuses from the corrosion pits through the pores in the Cu₂O film, while the remainder is produced by cathodic reduction on the surface of the Cu₂O film.

4.4. Corrosion Products Formed on the Surface of Bronze

The main components of ancient bronzes, according to the literature, are copper (Cu), tin (Sn), and lead (Pb). Consequently, three types of compounds are typically formed during natural mineralization processes, as illustrated in

Table 1. Corrosion product types formed on bronze surfaces [36].

	Oxides	Sulfate	Carbonate	Chloride	Sulfide
Cu	Tenorite [CuO] Cuprite [Cu2O]	Antlerite [Cu3(SO4)(OH)4] Chalcanthite [CuSO4·5H2O] Brochantite [Cu4SO4(OH)6]	Azurite [Cu3(OH)2(CO3)2] Malachite [Cu2(OH)2(CO3)]	Paratacamite [Cu2(OH3)Cl] Atacamite [Cu2(OH)3Cl] Botallackite [Cu2(OH3)Cl] Melanothallite [Cu2(OH3)Cl] [CuCl]	Covelite [CuS] Chalcocite [Cu2S]
Sn	Cassiterite [SnO2]
Pb	Massicot [PbO] Plattnerite [Pb2O]	Lanarkite [PbO·PbSO4] Linarite [PbCuSO4(OH)2] Anglesite [PbSO4]	Cerusite [PbCO3] [Pb3(CO3)2(OH)2]	Cotunnite [PbCl2] Pyromorphite [Pb3(PO4)3Cl]

.

Experience indicates that structural defects and ever-changing external environments significantly influence the value of these metal artefacts due to the formation of diverse types of surface corrosion products [106]. In general [36], corrosion products can be classified in several ways: (1) by color differences, as the bronze matrix is covered with coatings or patinas of different colors; (2) by the microstructure of the corrosion products, which include the matrix layer, the transition layer, and the corrosion products; (3) by the cations or anions of the corrosion compounds or minerals [31]; and (4) by the effect of the corrosion products on the preservation of metallic artefacts, categorizing them as either harmful or harmless.

The formation and development of surface corrosion products pose a significant threat to the integrity of ancient bronzes. Our most urgent task is to preserve the artistic value and scientific knowledge embedded in these ancient cultural artefacts. Modern techniques such as X-ray fluorescence spectroscopy (XRF) [107], Raman spectroscopy (Raman) [108,109], neutron diffraction [110,111], and electron-backscattered diffraction (EBSD) [112] have been successfully employed for in-depth analysis of corrosion products formed in various environments. Moreover, effective protective measures should be implemented to ensure continuous monitoring of the condition of ancient bronzes.

Most of the corrosion of bronze is complex, and the corrosion of different colors causes the bronze surface to exhibit a variety of colors. The red one is cuprous oxide. The black one is copper oxide. Lead grey is cuprous sulfide (Cu₂S). Indigo blue is copper sulfide (CuS). The blue color is copper sulfate (CuSO₄·5H₂O). The dark green color is basic copper carbonate [Cu₂(OH)₂CO₃]. From green to dark green, alpha basic copper chloride [Cu₂Cl(OH)₃]. Pale green is β basic copper chloride. The white ones are cuprous chloride (CuCl).

4.4.1. Harmless Corrosion Products

Bronze artefacts, being buried underground for extended periods, often accumulate various corrosion products on their surfaces, which can be broadly categorized into harmless corrosion products (stable corrosion products) and harmful corrosion products [113]. Generally, stable corrosion products are harmless to bronzes and can even add a sense of historical significance, thus requiring no special treatment in the protection of cultural artefacts. Harmless corrosion products can significantly enhance the artistic value of bronze artefacts and provide some protection to the internal metal materials. The high Sn corrosion product layer is a typical harmless corrosion product. The optical (Figure 13a) and SEM (Figure 13b) images, as well as the structure diagram of the high Sn corrosion products layer (Figure 13c), are illustrated in Figure 13 [114,115]. It has a dense and flat surface, bright and clean; jade texture; and good corrosion resistance. The colors can include green, black, yellow-green, and blue-green, and this type is more common in the humid and rainy regions of southern China, such as Hubei, Hunan, and Jiangxi [115].

Black patina is a special film with a glossy and smooth crystal jade texture that generally has excellent corrosion resistance. However, it is not clear whether the tin-rich corrosion layer is formed after artificial tinning treatment or selective corrosion of copper or lead in natural environments. Li et al. [56] investigated an ancient Chinese bronze sword of the Warring States Period, which is covered with a blank patina. The physical photo of the copper sword is shown in Figure 14a. A metallographic microscope, scanning electron microscope (SEM), energy-dispersive spectrometer (EDS), Raman spectrometer (Raman), and X-ray diffraction (XRD) were used to characterize this sword. In the cross-section optical image of the sword (Figure 14c–e), it can be found that the black patina consists of a completely mineralized outside layer (with a thickness of approximately 100–150 μm) and a partially mineralized intermediate transition layer (with a thickness of approximately 60–80 μm). EDS results indicate that the sword body contains high Sn with an average content of 22.92 wt.%, while in the intermediate transition layer and the complete mineralized layer, the average Sn concentration was 36.82 wt.% and 49.01 wt.%, respectively. The elemental profiles of Cu, Sn, and Pb can be found in Figure 14b. In the end, the authors claimed that the formation of the black patina was ascribed to the natural corrosion mechanism, higher content of Sn, as well as the damp, wet, and acidic burial environment, which is responsible for the formation of this black patina.

Bronze with high Sn content forms a dense protective layer on its surface after prolonged exposure to water, soil, and air, preventing further environmental damage. The primary chemical composition of these corrosion products is Sn, followed by Cu, with minor amounts of Pb, S, Fe, Cl, Si, Al, and other elements. The specific relative content of elements within the harmless corrosion products is related to alloy composition, excavated environment, and corrosion degree [116]. Its chemical composition mainly includes white tin oxide, black copper oxide, red cuprous oxide, and green or blue basic copper carbonate. Occasionally, basic copper carbonate may be mixed with small amounts of black copper sulfide, white tin oxide, etc. These corrosion products are colorful and relatively stable, forming exquisite ancient patinas. The main phase of this corrosion product is nanocrystalline or amorphous tin dioxide (SnO₂), which may be doped with other atoms [117]. A study conducted by He and co-authors [114] indicated that the Sn content (wt.%) in the outer layer, second layer, and substrate is 63.93%, 25.36%, and 17.85%, respectively. Generally, it consists of a non-metallic layer and a transition layer (Figure 13c). The natural corrosion theory and a selective corrosion mechanism primarily explain the formation of high tin corrosion products on the bronze surface. Simply put, the Cu and Pb in the bronze alloy are lost to the environment due to corrosion, while the Sn element is left behind, and its oxides fill the vacancies formed by Cu and Pb.

Regarding the corrosion resistance mechanism of high Sn corrosion products, most scholars believe that a SnO₂ passivation film forms following tin oxidation [115]. This film impedes the transmission of reactants and corrosion products between the copper substrate and the external environment, thereby slowing down the electrochemical reaction process and significantly decelerating the corrosion.

Bronzes with high Sn corrosion products are predominantly found in regions with frequent water activity and low chloride ion content, such as the humid “puddle” environments in southern China and inland riparian areas [31,118,119]. The proportion of intergranular corrosion in the high-tin corrosion products is only 22%, indicating that this type of corrosion product offers better protection, with fewer ions causing localized corrosion, and a slower metal oxidation reaction predominating in the corrosion process. A few scholars have also explored the corrosion resistance mechanism of high-Sn corrosion products through valence electron structure analysis. They have identified a lattice structure of overlapping positions at the phase boundary between the δ phase Cu-Sn alloy and Sn_1−xCu_xO₂ in the transition layer. This structure lowers the energy of copper atoms at the phase interface, stabilizing this part of the δ-phase grains, which in turn imparts corrosion resistance to the entire high tin corrosion products. Gettens et al. [120] and Robbiola [31] suggested that copper is lost and tin oxides are deposited in situ during natural corrosion, resulting in the formation of high tin corrosion products. Since copper and tin oxides are insoluble in each other, an oxide film composed of these oxides forms on the metal surface at the initial stage of oxidation, as shown in Reactions (16) and (17). However, because the copper content is much higher than the tin content, the oxide film is primarily cuprous oxide. As the reaction progresses, tin oxide, being insoluble and having limited migration ability, is deposited in situ to form a tin dioxide layer with a low Cu/Sn ratio. This results in a composition distribution of decreasing Cu/Sn in the inner corrosion products. Corrosion product analysis reveals that samples with high tin corrosion products contain cuprous oxide layers and secondary corrosion product layers, which are related to the position of copper ion diffusion and deposition. Copper ions that do not move out of the tin dioxide layer in time mix with the tin dioxide layer to form cuprous oxide, creating a “transition layer”.

2Cu + 2OH⁻ → Cu₂O + H₂O + 2e⁻

(16)

Sn(s) + 4OH⁻(aq) → SnO₂(s) + 2H₂O(l) + 4e⁻

(17)

Copper ions diffuse outward along the corrosion product layer, forming either a cuprous oxide layer outside the tin dioxide layer or secondary corrosion products, such as basic copper carbonate and hydrated copper sulfate, upon contact with water, oxygen, and various anions in the environment. When copper ion loss is rapid, the outer layer cannot form a corrosion product layer, resulting in different corrosion product structures. Suppose the loss rate is less than the equilibrium rate of corrosion products accumulation, the structure of the inner layer–cuprous oxide layer. In that case, the secondary corrosion product layer will form when water, oxygen, and various anions are insufficient, or the corrosion products of the inner secondary corrosion product layer will form when sufficient, as shown in Reaction (17).

4.4.2. Harmful Corrosion Products

Harmful corrosion products can negatively impact the long-term preservation of bronzes, especially powdery corrosion products (a type of harmful corrosion product), which are in a pulverized state [121]. Harmful corrosion products can make bronze artefacts brittle, damage them, and shorten their lifespan [122]. In severe cases, it can cause the artefacts to crumble, pulverize, or even be destroyed. Powdery corrosion products are somewhat diffusive and are referred to as “bronze disease” in the industry. This type of corrosion product can also spread to surrounding bronzes through the soil, infecting the entire bronze and those nearby. Long-term and in-depth research on the causes of powdery corrosion products in bronzes reveals that corrosion is primarily due to direct chemical reactions and indirect electrochemical corrosion, with these two types of corrosion often occurring in alternation [123]. Under suitable environmental conditions, powdery corrosion products exacerbate the corrosion of bronzes, severely damaging their artistic value and threatening the long-term preservation of bronze cultural artefacts [109,124,125]. The primary chemical components of the harmful corrosion products are cuprous chloride and basic cuprous chloride. Basic copper chloride is a significant harmful component in the corrosion of bronze artefacts [122,126]. It develops rapidly, can cause severe expansion, and plays a crucial role in the deterioration of bronze artefacts.

The composition and type of powdery corrosion products are closely linked to the bronze alloy’s composition, the corrosive medium in the environment, and other factors. There are two common distribution patterns of powdery corrosion products on the surface of bronzes: one is the open “point” or localized “dispersion” distribution (open “powdery corrosion products”) [127]; the other is a closed “skin-shell” distribution [68]. Despite their different distribution modes, both types of powdery corrosion products exhibit strong diffusion characteristics. They can be activated in a suitable environment, causing the bronzes to turn into powder during the erosion process, ultimately leading to the collapse and disintegration of the bronzes. Powdery corrosion products typically appear as punctate corrosion products, presenting as a light green, loose powder. The surface often features a tumor-like corrosion accumulation layer, covered with cuprous oxide (Cu₂O), with cuprous chloride (CuCl) at the base, which is a typical disease of bronzes. Additionally, there are corrosion products of alloy elements, such as tin, lead, or iron, including white tin dioxide (SnO₂), lead carbonate (PbCO₃), lead chloride (PbCl₂), and yellow lead oxide (PbO), which cause the surface corrosion of bronzes to display various colors.

The occurrence and development of powdery corrosion products mainly exist in the underground burial stage and the preservation stage after excavation. During burial, the corrosion environment of bronzes is primarily humid soil environments and saturated water environments, including groundwater and marine water environments. The corrosion environment of unearthed bronzes is mainly atmospheric. Typically, under favorable conditions that are moderately conducive (including cooler temperatures, minimal humidity, the absence of ultraviolet radiation, and the absence of detrimental gases, among others), the corrosion products of bronze can maintain their stability over an extended period. However, should the environment undergo alterations, particularly regarding fluctuations in temperature and humidity, the bronze surface, when exposed to a damp atmosphere, becomes susceptible to the formation of numerous micro-batteries through the interaction of chloride ions, water, and oxygen. This, in turn, triggers electrochemical corrosion, resulting in the formation of a powdery layer of corrosion products. Additionally, bronze artefacts enter an oxygen-enriched environment after excavation from an anoxic environment, a critical factor that leads to corrosion. Environmental aspects of powdery corrosion products include soil, water, atmospheric, and biological environments [128].

(1)

Open “Powder corrosion products”

Research [39,129] indicates that the punctate powdery corrosion products on bronzes follow the general pattern of metal pitting corrosion, initially forming preferentially at scars, inclusions, and grain boundaries on the metal surface. Typically, once etching holes appear on the bronze surface, they continue to expand. In the early stages, the distribution of powdery corrosion products on bronze is typically punctate, and under suitable conditions, it gradually expands both inward and outward, developing into a dispersion. In some cases, punctate and dispersed powdery corrosion products can also protrude from the surface of artefacts in the form of flakes, accompanied by surface cracking, expansion, thickening, and a loose structure, as shown in Figure 15a,b [121].

(2)

Skin-type “powdery corrosion products”

Skin-type powdery corrosion products are characterized by a layered “skin” appearance, with multiple interspersed corrosion products that conceal the corrosion products beneath [68,119]. These corrosion products often manifest as either smooth, “tumor-like” skin or rough, mottled skin. Macroscopically, these layers vary in thickness, creating an uneven surface, yet they maintain a relatively intact outer appearance. Despite complete mineralization, the layers are tightly bound together, as shown in Figure 15c.

Microscopic examination reveals that the corrosion products exhibit structural expansion, cracking, and a loose, fragile texture. Similar to the glossy tumor-like skin corrosion products, the mottled variety possesses a layered structure. However, unlike the glossy type, the mottled skin corrosion products have the potential to expand under the right conditions. Over time, as the corrosion products develop and change, they can break through the outer layers, inflicting severe damage to the bronze artefacts.

The concealment provided by the skin-type corrosion products is notable; it integrates various colored corrosion products, including white, light green, green, bright green, blue, red, and black, forming belts of green and white corrosives. This complex layering not only affects the aesthetic integrity of the bronzes but also poses a threat to their structural stability, emphasizing the need for careful monitoring and preservation efforts.

5. Protection Strategies for Exhumed Bronze Artefacts

Suppose the shape and size of the bronze artifacts remain relatively intact after excavation, or only have minor damage. In that case, they can be directly collected and preserved after the surface corrosion product is appropriately cleaned. If there is damage to the shape or structure, then restoration is required before taking protective measures. The cleaning of corrosion products and restoration work have been reported in much of the literature, so they are not covered in this article. For related work, please refer to References [130,131,132]. This chapter briefly introduces the protective methods for this type of bronze artefact that do not require the removal of corrosion products and proposes some constructive suggestions.

5.1. Surface Corrosion Inhibitors

Corrosion inhibitors have been used to conserve metallic artifacts, such as copper, silver, and iron, since the 1960s. Their protection mechanisms differ depending on the specific type. For example, some inhibitors delay the corrosion by adsorbing onto the metallic surface and forming a thin film that is not visible to the naked eye, by forming precipitates that cover the metal surfaces and isolate them from the electrolyte atmosphere, or adsorbing directly and forming a passive corrosion product layer on the metal surface [133]. Benzotriazole (BTA) and its derivatives are effective corrosion inhibitors for copper and its alloys, having been utilized in industrial applications since the 1960s. But due to their toxicity, more environmentally friendly inhibitors need to be developed. A classic and detailed discussion of the development history of corrosion inhibitors can be found in Cano’s work [134]. The following statement focuses on recent cases of new emerging inhibitor systems used in bronze protection.

Balbo et al. [135] studied the protection effect of 3-mercapto-propyl-trimetoxy-silane (PropS-SH) and 2-amino-5-mercapto-1,3,4-thiadiazole (AMT) on the corrosion resistance in both concentrated acid rain and in 3.5 wt.% NaCl solution, aiming to reveal their capability as good candidates to replace BTA in bronze and gilded corrosion inhibition. They found that the best inhibition effect is obtained with PropS-SH, which inhibits galvanic corrosion. In a study conducted by Lago and co-authors [126], they investigated the effects of ethanol content and immersion time on the corrosion inhibition of AMT on bronze in synthetic rainwater. The results indicate that the most defensive film was obtained with 0.06 M AMT, 90 vol. ethanol, and an immersion time of 90 min.

Wang et al. [136] developed a coating system consisting of a mixed water-based acrylic emulsion with BTA as the primer coating and a mixed water-based acrylic emulsion with TiO₂ and SiO₂ nanoparticles as the top coating. The results indicate that with 3% BTA in 5.5% emulsion as the primer coating and 0.5% TiO₂ plus 2.5% SiO₂ nanoparticles in 5.5% emulsion as the top coating, the corrosion resistance in sulfuric acid (0.05 M H₂SO₄), sodium hydroxide (0.1 M NaOH), alkaline, and chlorine-containing (3.5% NaCl) solutions were improved. Additionally, the glossiness, UV durability, hydrophobicity, and oleophobicity of this coating system were also enhanced.

Zhou et al. [137] investigated the corrosion behavior of bronze in NaCl solution by using a hybrid corrosion inhibitor composed of BTA and sodium formate (SFA). The electrochemical impedance spectroscopy shows the improved corrosion resistance of SFA containing an inhibitor, as evidenced by increased film resistance and interfacial electron transfer resistance on bronze. XPS results indicate that the oxide formed on the bronze surface has a significant impact on the formation of the Cu-BTA complex. With the incorporation of SFA, Cu₂O becomes the primary corrosion product, which is more conducive to the adsorption of BTA and the formation of a Cu(I)-BTA complex, thereby enhancing the corrosion resistance of bronze.

In conclusion, although a variety of environmentally friendly corrosion inhibitors have been developed in current research, there is still considerable room for improvement in their durability. Therefore, a significant amount of work is needed for a more in-depth study.

5.2. Organic or Inorganic Coating Strategies

Appropriate coating can also be used to hinder the corrosion processes of bronze artefacts by isolating them from the corrosive atmosphere. Kosec and co-authors have conducted numerous studies on the corrosion behavior of fluoropolymer (FA)-based coatings on bronze surfaces, primarily due to their good protective properties. For example, FA coatings with an appropriate adhesion promoter, either with or without a silane-modified BTA inhibitor, were developed to endow the bronzes with hydrophobicity, improve adhesion, and enhance corrosion resistance [138]. The simulated acid rain was used as the testing solution. It was shown that all the designed coatings exhibit a significant decrease in corrosion resistance. The aged coating remained transparent and did not change by UV exposure and/or thermal cycling. The patinated bronze can be successfully covered by the self-assembled single-layer coating with high hydrophobicity and possesses good color stability either when applied or after ageing in a UV/climatic chamber. They also [139] prepared three types of simulated patina (brown, green sulfate, and green persulfate) and subsequently studied the protection effects of the fluoropolymer-based coating (FA-MS) and FA-MS coating with mercaptopropyl group addition (FA-MS-SH). Their results show that the FA-MS coatings present the most efficient protection to bare bronze and the sulfate patina, while the FA-MS-SH coating provided excellent protection to bare and brown patinated bronze. In another work of Kosec [140], they investigated the effects of the single-component (fluoroacrylate, FA or methacryloxypropyl-trimethoxysilane, MS) coating, as well as the multi-component (a mixture of FA and MS, FA-MS) coating on the protection capability of artificially formed oxidized bronze patinas. The results show that the FA-MS coating has high hydrophobicity, but no noticeable improvement in proportion efficiency can be detected in comparison to coating 10%. This unexpected result was explained by the fact that the FA component in the FA-MS coating exists throughout the coating rather than being present only on the uppermost surface of the coating.

In a work conducted by Zhao et al. [141], they developed a net-like menthol coating by a spraying method. Their results indicate that the protective efficiency shows a linearly increasing trend with the thickness of the coating increase and a menthol coating with 500 μm thickness exhibits effective protection on bronzes from water and Cl⁻ up to 120 days. They also attributed this protection capability to the labyrinth effect of the dense microstructure with disordered needle-like crystals that block the corrosive medium.

5.3. New Strategies

(1)

Photo-induced passivation

Previous research has shown that visible light can accelerate the corrosion of bronze alloy, whereas ultraviolet light can inhibit it [53]. The accelerating effect of visible light on corrosion can also be observed on Zn, Ag, and Fe and their alloys, with the underlayer enhanced anodic dissolution of metals driven by photo-generated holes. The inhibiting effect of ultraviolet light on metal corrosion behavior can be explained by the photo-generated cathodic protection. In a recent study conducted by Wu et al. [142], photo-induced passivation treatment was used to explore the effects of tin content on corrosion resistance of four ternary Cu-Sn-Pb alloys, and was is evidenced that even with relatively low tin content, the inhibiting effect is still significant. They also applied this method to two types of simulated patinated bronzes, and inhibition efficiencies of up to 82% and 93% were obtained. The study highlights the potential application of photo-induced passivation for the protection of bronze artefacts.

(2)

Three-dimensional scanning and printing techniques

Due to the unsatisfactory preservation conditions of the exhibition hall, especially the unqualified humidity, temperature, illumination, and gas atmosphere, it is better to have a replica to replace the real bronze artefact to meet the visiting needs of the public, and the noumenon can be preserved more rigidly and reliably. Thanks to the evolution of 3D scanning and 3D printing techniques, the damage of the traditional dewaxing casting method to the noumenon can be avoided. In a study conducted by Tang et al. [143], they applied a 3D scanning technique, a point cloud data processing technique, a 3D modelling technique, 3D printing, and other key techniques to produce a Dasheng Chime replica.

(3)

Holographic projection technology/real and virtual technology

As previously mentioned, in the absence of sufficient external interference, the corrosion of metals is spontaneous and irreversible, and the final end of the corrosion process is that all the atoms of the metal artefacts are transformed into their stable compounds. That is also the reason why corrosion is called the reverse process of metallurgy. That is to say, strictly speaking, as long as the time is long enough, even the most complete protective technology may fail and eventually cause complete corrosion of the metal. Therefore, how to achieve the long-term preservation of other metal and non-metal cultural relics, including bronze wares, especially the conservation across their actual carriers and the transmission of the cultural information they contain, is the problem we ultimately have to face.

Holographic projection technology and real and virtual technology give us an opportunity. Based on these techniques, the information that the artefacts carry can be stored digitally. What is more, the audience can easily access information about the exhibits they are interested in via the interactive interfaces. In a study conducted by Zhu et al. [144], they systematically introduced the research methodology and technical implementation of digital interactive display of patterns of bronze artefacts from the western Zhou dynasty via holographic projection technology, including 3D scanning of cultural artefacts, 3D modelling, projection equipment, and design of interactive interfaces. A user research was also conducted to collect feedback and demands from the audience. Their results showed that the holographic projection technology is more helpful for students to remember than traditional textbooks. Xiong et al. [145] reported their work on the real and virtual technologies for the replication and display of a Chinese light-transmitting bronze mirror. They aim to combine the real technique (3D printing) and virtual technique (holographic interactivity) into a teaching system. These researches will play a positive role in promoting the subsequent use of this technology for the protection of bronze artefacts, stone artefacts, ancient calligraphy, and paintings, as well as the dissemination of cultural heritage.

6. Summary and Prospects

Bronzes, as significant markers of cultural heritage, embody the advancement of human civilization with their exquisite craftsmanship and historical depth. This article delves into the multifaceted value of bronzes, the challenges they face in preservation, and the complex corrosion processes influenced by alloy composition and environmental factors. The roles of tin and lead in enhancing the physical and aesthetic qualities of bronzes are highlighted, underscoring the importance of these elements in ancient metallurgy. This article categorizes bronze artefacts into various types based on their use and cultural significance, reflecting the global distribution and unique characteristics of bronze cultures across different regions.

The protection of bronze presents significant challenges, primarily due to corrosion issues that can compromise their structural integrity and artistic value. This article examines the influence of internal factors, including alloy composition and microstructure, as well as external factors such as environmental conditions, on the corrosion behavior of bronzes. It provides an in-depth analysis of the types of corrosion products formed on bronze surfaces, distinguishing between harmless and harmful corrosion products, and emphasizes the need for proper preservation strategies to mitigate these effects.

We cannot deny that the corrosion and protection of bronze artefacts are contradictory and opposite processes. At present, with current technology, we can only try our best to extend the lifespan of bronze artefacts, and the length of their lifespan will eventually be tested by time. Human civilization with clear records has only been around for a few thousand years and will continue to advance in the future, which brings tremendous pressure to the protection of cultural archaeological artefacts. Therefore, new technologies will continue to emerge. Corrosion inhibitors, coating technologies, as well as newly developed photo-induced passivation, 3D scanning and 3D printing, holographic imaging, and virtual reality technologies are all practical measures to directly or indirectly protect and pass on our cultural heritage. The development of these technologies helps preserve the memory of human civilization and is also a favorable witness to our understanding and transformation of the world.

Looking forward, this article underscores the necessity for further research into the mechanisms of bronze corrosion and the development of effective protection strategies. With the advancement of scientific techniques such as XRF, Raman spectroscopy, and EBSD, there is an opportunity to gain a deeper understanding of the corrosion processes and to implement more precise preservation measures. Future research should focus on developing environmentally friendly and efficient protection technologies that do not alter the surface state of bronzes, thereby ensuring their longevity and preserving their cultural significance.

Museums and cultural heritage preservation units are encouraged to adopt a multifaceted approach to bronze conservation. This includes maintaining controlled environmental conditions, leveraging the natural antimicrobial properties of copper, applying protective coatings, conducting regular non-abrasive cleaning, and monitoring pH levels to ensure optimal conditions. By integrating these strategies, the preservation community can better protect bronze artefacts from the ravages of corrosion and other harmful influences, ensuring that these treasures of human history remain accessible for future generations.