Corros. Mater. Degrad., Volume 2, Issue 1 (March 2021) – 7 articles

Electrochemical Noise Measurement (ENM) and DC electrolytic resistance measurement (ERM) can be used to assess the level of protectiveness provided by an organic coating (paint or varnish) to the underlying metal. These techniques also have applicability to the thinner, transparent type of coatings used to protect archaeological artefacts. Two studies are presented here demonstrating how ERM and ENM techniques can be applied in artefact preservation. The similarity of the techniques, both of which are a measure of resistance, means results can be considered to be analogous. The first study investigated the use of ERM to determine the protection levels provided by typical coatings in order to develop a database of coating type and application for objects, for specific environments. The second study used ENM to evaluate coatings which had been applied to historic artefacts recovered from shipwrecks in the Baltic Sea and displayed inside the museum or kept in the museum store area. The studies showed the usefulness of both techniques for determining the level of protection of a coating and how a better performing coating can be specified if a pre-existing coating on an artefact has been found to be unsuitable. Full article

The long-term corrosion progression of steel reinforcement is important for estimating the life of reinforced concrete infrastructure. Reviews of field experience and results from recent controlled long-term experiments show that the development of reinforcement corrosion is much more complex than the classical empirical Tuutti model. A new, comprehensive model is proposed, referencing observations and inferences from many field and laboratory observations and built on the bi-modal model for the corrosion of steel. It includes the critical roles of air-voids in the concrete at the concrete-steel interface and the effect of long-term alkali leaching as accelerated by the presence of chlorides. Both are affected by compaction and concrete permeability. The role of chlorides in the early stages is confined to pitting within air-voids. These are critical for allowing initiation to occur, while their size influences the severity of early corrosion. Empirical data show that for seawater with an average water temperature in the range of 10–20 °C, the corresponding rate of long-term corrosion r_a is in the range of 0.012–0.015 mm/y. Full article

In this work, the ZnAl-NO₂ LDH (layered double hydroxide) is investigated as a possible additive for mitigating the chloride-induced corrosion of steel in reinforced concrete. The investigation focused on the stability and chloride binding capacity of this LDH in the pH range typical of cementitious materials. Until pH = 12.5 the material was stable and effective in capturing chloride ions from the surrounding aqueous environment. For higher pH, precisely that of hydrated cement, the LDH was partially dissolved and OH⁻ preferentially entrapped instead of Cl⁻. These results suggested that ZnAl-NO₂ has excellent chloride entrapping capability at neutral pH, but this is reduced with increasing pH. However, when the LDH was incorporated into mortars, the chloride ingress was delayed, signifying that the dissolution of LDH leads to a secondary mechanism responsible for chloride capture. Full article

In this work, the corrosion behavior and surface reactivity of as-cast and heat-treated nickel aluminum bronze casting alloy (UNS C95800) in 3.5 wt% NaCl solution is investigated under stagnant and flow conditions. Increasing flow rate conditions are simulated using a rotating disk electrode from 0 to 9000 revolutions per minute (rpm). Optical micrographs confirm the decrease in the phase fraction of corrosion-sensitive β phase in the microstructure of C95800 after annealing, which, in turn, enhances the corrosion resistance of the alloy. Electrochemical studies including open circuit potentiometry, potentiodynamic polarization, and electrochemical impedance spectroscopy are performed to assess the effect of flow rate and heat treatment on the corrosion of samples at 25 and 40 °C in 3.5 wt% NaCl solution. For both as-cast and heat-treated samples, increasing the flow rate (i.e., electrode rotating rate) linearly reduces the corrosion resistance, indicating that the metal dissolution rate is significantly affected by hydrodynamic flow. Increasing the solution temperature negatively impacts the corrosion behavior of the as-cast and heat-treated samples at all flow conditions. Full article

Green rust compounds (GR), i.e., Fe(II-III) layered double hydroxides, are important transient compounds resulting from the corrosion of steel in seawater. The sulfated variety, GR(SO₄²⁻), was reported as one of the main components of the corrosion product layer, while the chloride variety, GR(Cl⁻), was more rarely observed. The carbonate variety, GR(CO₃²⁻), is favored by an increase in pH and forms preferentially in the cathodic areas of the metal surface. Since Mg(II) is abundant in seawater, it may have a strong influence on the formation of GR compounds, in particular as it can be incorporated in the hydroxide sheets of the GR crystal structure. In the present work, the influence of Mg²⁺ on the precipitation reaction of GR(SO₄²⁻) was investigated. For that purpose, Mg²⁺ was substituted, partially or entirely, for Fe²⁺. The GR was then prepared by mixing a solution of FeCl₃·6H₂O, Na₂SO₄·10H₂O, NaCl, FeCl₂·4H₂O and/or MgCl₂·4H₂O with a solution of NaOH. The precipitation of the GR was followed or not by a 1-week aging period. The obtained precipitate was characterized by X-ray diffraction. It was observed that Mg(II) favored the formation of chloride green rust GR(Cl⁻) and magnetite Fe₃O₄ at the detriment of GR(SO₄²⁻). The proportion of GR(Cl⁻) and Fe₃O₄ increased with the Mg(II):Fe(II) substitution ratio. Without Fe(II), the precipitation reaction led to iowaite, i.e., the Mg(II)-Fe(III) compound structurally similar to GR(Cl⁻). It is forwarded that the presence of Mg²⁺ cations in the hydroxide sheets of the GR crystal structure is detrimental for the stability of the crystal structure of GR(SO₄²⁻) and favors the formation of other mixed valence Fe(II,III) compounds. Full article

An experimental study was conducted to assess the nature and extent of the biodeterioration of the limestone in the Batalha Monastery in Portugal. Stone fragments covered with microbial biofilms and lichenous crusts were investigated using Optical Microscopy (OM), Low Vacuum Scanning Electron Microscopy with Energy Dispersive Spectroscopy (LV-SEM + EDS), and X-ray micro-Diffractometry (μ-XRD). Microbial samples were collected from the stone surface, cultured, and analyzed with NGS metagenomic DNA test to classify the bacterial communities associated with the formation of the biofilms. Particulate air pollutants collected on Pall GN-6 paper filters using a cascade impactor were characterized by SEM-EDS + NGS. The results showed that lichens play a major role in biodeterioration by promoting both physical and chemical attack on the limestone substrate via hyphae mechanical penetration along calcite inter-crystalline spaces, the dissolution/leaching of calcite minerals, and the precipitation of secondary minerals such as Ca-oxalates within the stone porosity framework. DNA analyses identified the bacterial communities within the biofilms and their relative abundances. Air quality monitoring results suggest that the microbial population colonizing the monastery limestone could at least partially be derived from the dry and wet deposition of airborne biological particles on the stone surfaces and that S, N, and P-rich air pollutants may have provided nutrients and energy for the bacteria communities, thus indirectly facilitating biofilm formation, the growth of a lichenous crusts, and limestone biodeterioration effects. Full article

The complex nature of the damage evolution in stress corrosion cracking (SCC) leads to explore for new investigation technologies in order to better identify the mechanisms that supervise the initiation and evolution of the damage as well to provide an improvement of knowledge on this critical localized corrosion form during time. Research activities concerning the use of acoustic emission (AE) technique to assess SCC has acquiring considerably relevance in recent decades. The non-invasiveness and the possibility to provide a continuous in situ monitoring of structures and components make this non-destructive technique clearly promising in the field of structural health monitoring. In this concern, this paper aims to be a focused overview on the evaluation of SCC phenomena by AE technique. The main topic of this review is centered on the approaches that can be used in elaborating AE data to better discriminate the mechanisms that contribute to damage propagation in SCC conditions. Based on available literature, investigation approaches assessing AE waveform parameters were classified, evidencing, furthermore, the identified mechanisms that synergistically take place during the material degradation. Eventually, a brief summary and a future trend evaluation was also reported. Full article