Current Challenges in Corrosion Research

1. Introduction

Corrosion is a degradation phenomenon with huge economic consequences and severe security implications [1,2]; thus, issues such as accurate assessment, prevention and prediction are fundamental to maintaining the integrity of many critical components and installations [3,4,5,6,7,8,9]. Definition of the mechanisms of the corrosion processes over the past decades has enabled significant advancement in terms of the understanding of degradation events [10,11,12,13], but still further knowledge is required in crucial industries with increasingly demanding exposure conditions [14,15,16,17,18]. In addition, current climate change mitigation plans and policies involve, among other things, reductions in raw material use as a priority goal. Therefore, seeking sustainable strategies to prolong the service life of new and existing metallic structures has become a concern among the research community, see [19,20,21,22,23] for some examples.

2. Overview and Scope

Long-lasting materials have become a priority to reduce carbon footprints and mitigate climate change. Raw materials are increasingly scarce and product manufacturing costs are reaching unprecedented levels. In addition to new technologies with, in most cases, hazardous exposure conditions, the development of new design strategies, prevention methods, and monitoring procedures is required to fight against corrosion. Thus, corrosion control is essential for the goal of creating a sustainable society.

This Special Issue is intended to compile the most recent advances in corrosion research and contributions from traditional fields such as transport, the chemical industry, or civil engineering are welcome, but innovative improvements in the areas of additive manufacturing or biomaterials are also expected.

Methods for corrosion mitigation, including environmentally friendly solutions, such as high-performance coatings, cost-effective advanced materials, inhibitors, or novel solutions, as well as innovative testing procedures—including sensors or remote monitoring—to reliably assess corrosion behaviour and to predict corrosion damage, are encouraged.

3. Contributions

Ten articles have been published in this Special Issue. A variety of topics have been discussed here, focused on the development of surface treatments and advanced corrosion testing methodologies, among others. Yoo et al. showed the effects of laser shock peening on the stress corrosion cracking of AISI 304L [Contribution 1]. The induced compressive residual stresses and the grain refinement were responsible for both extending the time of crack initiation and reducing the crack propagation rate. An increased pitting potential was also defined after this treatment, but no strong correlation was found between the pitting potential of the cross section and the crack propagation rate. Laser shock peening decreases the intergranular corrosion rate but a detrimental effect on the stress corrosion cracking resistance was also recorded. An analogous correlation was observed for the degree of sensitization. The biofilms created on steel samples by two aerobic marine bacteria, Tenacibaculum mesophilum D-6 and the Bacillus sp. Y-6, were studied by Ruan et al. [Contribution 2]. Surface and electrochemical characterisations are compiled in this article and increased polarisation resistance and reduced corrosion rate were found for the samples treated in the bacterial medium. Reduced weight loss, along with the formation of narrower and less deep pits, was also measured in the bacteria-treated specimens. A thicker and denser biofilm, providing the best corrosion properties, was measured in the treatment with T. mesophilum D-6. Thus, the corrosion inhibition effect was directly correlated to the ability of the biofilm to form in the bacterial medium. Composite coatings prepared with polyaniline, doped with acetic acid, citric acid, succinic acid, and sulfamic acid, and commercial alkyd-based paints, were investigated by Grgur et al. [Contribution 3]. The corrosion rates, measured as the amount of Fe²⁺ cations released into the corrosive solution, revealed a direct correspondence with the doping degree; that is, the average number of doped anions per polymer unit in the polymer chain. The appearance of the corroded surfaces, after immersion in 3% NaCl for 150 h, was analysed. In the specimen treated with the base coating surface, about 20% of the surface rusted, whereas only approximately 1% of the sample coated with the composite coating prepared with polyaniline–sulfamic acid was covered with rust. It was concluded therefore that the degree of polyaniline doping in the composite coatings greatly influences the corrosion protection behaviour of mild steel. The study by Fedorischeva et al. discussed the corrosion resistance of AISI 321 specimens treated with the combined effects of aluminium, boron, and oxygen implantation [Contribution 4]. A study of the chemical changes on the surface induced by the implantation process is also included. The investigation revealed the formation of chromium and aluminium oxides as well as mixed Cr-Ni oxides (located to a depth of 250 nm), whereas B (located at a depth of no more than 50 nm) remained in the non-oxidised state. These modifications produced an improvement in the material’s corrosion resistance. The paper showed that the corrosion current density of the implanted specimen is reduced to almost half the value obtained for the untreated specimen. Severe corrosion damage, along with significant weight loss (1.5 mg after 800 h), was measured for the non-implanted specimen. Conversely, the implanted surface showed barely 0.2 mg of weight loss after testing in the salt spray chamber. Lanzutti et al. studied the use of ALD (Atomic Layer Deposition) (TiO₂) and/or PVD (Physical Vapour Deposition) (CrN) thin films to seal the discontinuities of thermal-sprayed alumina coatings on AISI 1040 steel specimens [Contribution 5]. Improved corrosion resistance performance was achieved after recording both the OCP (Open Circuit Potential) values and the corrosion current density. Even though the OCP measurements show that the corrosive media reached the substrate for all the samples, a sealant effect was evident since the corrosive medium took a longer period to reach the substrate, which was even more evident for the combined ALD/PVD film. A lower corrosion rate was obtained from the potentiodynamic polarisation measurements performed after 15 min and 24 h of immersion. Again, the lowest current density was recorded for the sample sealed with the combined ALD/PVD film. The PVD-only film behaved slightly better that the ALD-only film. The improved performance of the sealed specimens was also observed via the detection of a reduced amount of corrosion products. Corrosion behaviour in the physiological conditions of several CoCr-based alloys was investigated by Preda et al. [Contribution 6]. The presence of Cr₂O₃, Cr(OH)₃, WO₃, and MoO₃ on the surface of the alloy with 21% Cr–9% W provided excellent corrosion resistance performance. The alloy with 29% Cr also displayed reasonably good corrosion resistance, but the unique presence of Cr species and tungsten oxide in the passive layer resulted in poorer performance. In addition, the ion release measurements confirmed a very low concentration of chromium ions for the alloy Co21Cr8Mo7W, whereas the alloy Co29Cr7W released a higher concentration of chromium over the medically acceptable value. This study concluded that the use of the Co21Cr8Mo7W alloy for biomedical applications is feasible since, as well as being a cost-effective material, it enables the production of devices where the quantity of released chromium ions needs to be reduced. The influence of the Mg/Al ratio on Zn-Al-Mg hot-dip coatings was investigated in a paper by Zhang et al. [Contribution 7]. The microstructure of several coatings with Mg/Al ratios between 0.63 and 1.63 was studied. An increase in the proportion of the MgZn₂ phase and a reduction in HCP-Zn was revealed as the Mg/Al ratio increased. The binary eutectic phase (HCP-Zn/MgZn₂) also increased with an increase in the Mg/Al ratio. The ternary eutectic phase (HCP-Zn/MgZn₂/FCC-Al) did not show a clear correlation with the Mg/Al ratio, but the amount of Al influences its proportions. The benefit of increasing the MgZn₂ phase was proved via corrosion tests. A lower current density along with a lower weight loss was measured for the coating with the biggest Mg/Al ratio. Souto et al. [Contribution 8] used a scanning electrochemical microscope to monitor the degradation of a Cu electrode. This made it possible to obtain local information and to distinguish zones separated by only a few micrometres. When used in combination with voltametric methods, the amount of Cu²⁺ released can be quantified. Freire et al. designed and validated a corrosion sensor to monitor the corrosion damage produced in a geothermal pipeline, a complex system with harsh conditions where the development of scaling is an issue [Contribution 9]. The new device recorded electrochemical impedance spectroscopy measurements over a period of 90 days with a variety of working electrodes. The simulation of these data made it possible to conclude that the highest corrosion rate was measured for API 5CT K55 steel, whereas the most corrosion resistant material was SMO 254 stainless steel. Finally, a review by Wang et al. compiles descriptions of the influence of composition and microstructure on the corrosion performance of high entropy alloys (HEAs) [Contribution 10]. Cr, Ti, Mo, N, and Si favour the formation of protective films, but elements such as Al and Mn are not able to form stable and dense films. A dendritic structure develops in the presence of Cu where the interdendritic zones, enriched with Cu, suffer corrosion. The formation of second phases, such as those developed with the incorporation of Al and Cr with Mo, reduce the corrosion resistance of the alloy. The problem of galvanic corrosion was identified as the main inconvenience from the point of view of the corrosion of HEAs. The authors also suggested some future research directions that would make it possible to continue improving the performance of HEAs.

4. Conclusions and Outlook

As the Guest Editors of this Special Issue, we are proud of the quality of the papers included here. We hope these publications can help to advance developments in the field of corrosion resistance. We would like to thank all of the authors for their contributions and to acknowledge the support of the reviewers and the Editorial Assistants in the preparation of this Special Issue.