Corros. Mater. Degrad., Volume 1, Issue 1 (June 2020) – 10 articles

This article presents a synthesis of recent studies focused on the corrosion product layers forming on carbon steel in natural seawater and the link between the composition of these layers and the corrosion mechanisms. Additional new experimental results are also presented to enlighten some important points. First, the composition and stratification of the layers produced by uniform corrosion are described. A focus is made on the mechanism of formation of the sulfate green rust because this compound is the first solid phase to precipitate from the dissolved species produced by the corrosion of the steel surface. Secondly, localized corrosion processes are discussed. In any case, they involve galvanic couplings between anodic and cathodic zones of the metal surface and are often associated with heterogeneous corrosion product layers. The variations of the composition of these layers with the anodic/cathodic character of the underlying metal surface, and in particular the changes in magnetite content, are thoroughly described and analyzed to enlighten the self-sustaining ability of the process. Finally, corrosion product layers formed on permanently immersed steel surfaces were exposed to air. Their drying and oxidation induced the formation of akaganeite, a common product of marine atmospheric corrosion that was, however, not detected on the steel surface after the permanent immersion period. Full article

Hydrogen in combination with mechanical stress can lead to rapid degradation of high-strength steels through environmentally assisted cracking mechanisms. The scanning Kelvin probe (SKP) was applied to automotive martensitic steel grade MS1500 in order to detect local reactivity of the surface after hydrogen uptake and tensile deformation. Hydrogen and stress distribution in microstructures can be characterized by SKP indirectly measuring the potential drop in the surface oxide. Thus, the links between electron work function, oxide condition, and subsurface accumulation of hydrogen and stress have to be investigated. It was shown that plastic strain can mechanically break down the oxide film creating active (low potential) locations. Hydrogen effusion from the steel bulk, after cathodic charging in aqueous electrolyte, reduced the surface oxide and also decreased potential. It was shown that surface re-oxidation was delayed as a function of the current density and duration of cathodic hydrogen pre-charging. Thus, potential evolution during exposure in air can characterize the relative amount of subsurface hydrogen. SKP mapping of martensitic microstructure with locally developed residual stress and accumulated hydrogen displayed the lowest potential. Full article

Atmospheric corrosion seriously affects the working life of construction metals. The quantitative knowledge of the corrosion effects helps the maintenance and the materials’ logistics. In this work, in a military airport located by the sea, the corrosion damage equations of carbon steel, aluminum, zinc and copper are determined after outdoor exposure for four years. Exposure started in 2014, at two different periods of the year, in summer and in winter, for all cases. Weight loss measurements were performed as well as characterization of the exposed metal coupon surfaces by microscopy: optical, electron and atomic force, by X-ray diffraction and Fourier Transform Infrared Spectroscopy. Atmospheric conditions and pollutants were also evaluated. The derived corrosion equations of all tested metals for the exposure have been employed for the 30-year projection of expected corrosion. A parallel 12-month exposure of steel and aluminum—the most common airport metals—have been evaluated for five consequent years to designate the rating of the airport, according to ISO and ASTM Standards. The results showed that there is not a good correlation between the predictions of the Corrosion Damage Algorithm and the Europe Corrosion Map and the actual measurements on steel and aluminum at the site. Full article

Magnesium alloys have shown great potential for applications as both structural and biomedical materials due to their high strength-to-weight ratio and good biodegradability and biocompatibility, respectively. Among them, Mg-Zn based alloys are attracting increasing interest for both applications. As such, this article provides a review of the corrosion performance and mechanical properties of Mg-Zn based alloys, including the influence of environment and processing on both of them. The strategies for tailoring corrosion resistance and/or mechanical properties by microstructure adjustment and surface treatment are discussed. Full article

Martensitic stainless steels are widely used materials. Their mechanical and corrosion properties are strongly influenced by their microstructure and thereby can be affected by heat treatment. In the present study, the effect of different austenitizing temperatures on the passive film growth kinetics of martensitic stainless steel is studied by electrochemical impedance spectroscopy. The data was further fitted by the point defect model to determine kinetic parameters. We show that an increasing austenitizing temperature leads to a more protective passive film and slows down passive film dissolution in sulfuric acid. Full article

Microbiologically influenced corrosion (MIC) is a major concern in a wide range of industries, with claims that it contributes 20% of the total annual corrosion cost. The focus of this present work is to review critically the most recent proposals for MIC mechanisms, with particular emphasis on whether or not these make sense in terms of their electrochemistry. It is determined that, despite the long history of investigating MIC, we are still a long way from really understanding its fundamental mechanisms, especially in relation to non-sulphate reducing bacterial (SRB) anaerobes. Nevertheless, we do know that both the cathodic polarization theory and direct electron transfer from the metal into the cell are incorrect. Electrically conducting pili also do not appear to play a role in direct electron transfer, although these could still play a role in aiding the mass transport of redox mediators. However, it is not clear if the microorganisms are just altering the local chemistry or if they are participating directly in the electrochemical corrosion process, albeit via the generation of redox mediators. The review finishes with suggestions on what needs to be done to further our understanding of MIC. Full article

For infrastructure applications in marine environments, the eventual initiation of corrosion (and pitting) of steels (and other metals and alloys) often is assumed an inescapable fact, and practical interest then centres on the rate at which corrosion damage is likely to occur in the future. This demands models with a reasonable degree of accuracy, preferably anchored in corrosion theory and calibrated to actual observations under realistic exposure conditions. Recent developments in the understanding of the development of corrosion loss and of maximum pit depth in particular are reviewed in light of modern techniques that permit much closer examination of pitted and corroded surfaces. From these observations, and from sometimes forgotten or ignored observations in the literature, it is proposed that pitting (and crevice corrosion) plays an important role in the overall corrosion process, but that longer term pitting behaviour is considerably more complex than usually considered. In turn, this explains much of the, often high, variability in maximum depths of pits observed at any point in time. The practical implications are outlined. Full article

Common concretes use considerable amounts of fresh water and river sand, and their excessive use is already seriously implicating the environment. In this respect, seawater and sea sand concrete (SWSSC) is a very attractive alternative, since it addresses the increasing shortage of fresh water and dredging of river sand. A major concern with reinforced SWSSC is the severe corrosion of the steel reinforcements by seawater (that has a very high content of chloride which is very corrosive), thereby seriously impairing the strength of such concrete. Fibre reinforced polymer (FRP) can be a suitable alternative to replace steels as reinforcement. However, there has been little systematic work to understand the degradation kinetics and mechanisms of FRP in the chloride-containing alkaline SWSSC environment. This review first provides an overview of the degradation of FRP composites in normal concrete and chloride-containing alkaline SWSSC environments, and then presents an example of a recent study using scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR) that may provide a pathway to systematic experimental approach to understanding such degradation. The review also makes a comprehensive assessment of the influence of environment-assisted degradation on mechanical properties of FRPs. Full article

The influence of hydrogen on the mechanical properties of four, medium-strength, commercial, quenched-and-temped steels has been studied using the linearly increasing stress test (LIST) combined with cathodic hydrogen charging. The relationship was established between the equivalent hydrogen pressure and the hydrogen charging overpotential during cathodic hydrogen charging, though the use of electrochemical permeation experiments and thermal desorption spectroscopy. The cathodic hydrogen charging conditions were equivalent to testing in gaseous hydrogen at hydrogen fugacities of over a thousand bar. Under these hydrogen-charging conditions, there was no effect of hydrogen up to the yield stress. There was an influence of hydrogen on the final fracture, which occurred at the same stress as for the steels tested in air. The influence of hydrogen was on the details of the final fracture. In some cases, brittle fractures initiated by hydrogen, or DHF: Decohesive hydrogen fracture, initiated the final fracture of the specimen, which was largely by ductile micro-void coalescence (MVC), but did include some brittle fisheye fractures. Each fisheye was surrounded by MVC. This corresponds to MF: Mixed fracture, wherein a hydrogen microfracture mechanism (i.e., that producing the fisheyes) competed with the ductile MVC fracture. The fisheyes were associated with alumina oxide inclusion, which indicated that these features would be less for a cleaner steel. There was no subcritical crack growth. There was essentially no influence of hydrogen on ductility for the hydrogen conditions studied. At applied stress amplitudes above the threshold stress, fatigue initiation, for low cycle fatigue, occurred at a lower number of cycles with increasing hydrogen fugacity and increasing stress amplitude. This was caused by a decrease in the fatigue initiation period, and by an increase in the crack growth rate. In the presence of hydrogen, there was flat transgranular fracture with vague striations with some intergranular fracture at lower stresses. Mechanical overload occurred when the fatigue crack reached the critical length. There was no significant influence of hydrogen on the final fracture. Full article