The threshold chloride concentration for solid Type 316LN (UNS S31653) stainless steel, Type 316L (UNS S31603) stainless steel clad, 2101 (UNS S32101), Fe-9%Cr, and carbon steel rebar (ordinary ASTM A 615M) was investigated using potentiodynamic and potentiostatic current monitoring techniques in saturated calcium hydroxide (Ca[OH]2) + sodium chloride (NaCl) solutions. There is general consensus in this study and the literature that the chloride threshold for carbon steel is less than a chloride to hydroxl (Cl−/OH−) molar ratio of 1. Solid Type 316LN stainless steel rebar was found to have a much higher chloride threshold (i.e., threshold Cl−/OH− ratio > 20) than carbon steel (0.25 < Cl−/OH−< 0.34). Type 316L stainless steel clad rebar possessed a chloride threshold expressed as a Cl−/OH− ratio of 4.9 when cladding was intact. However, surface preparation, test method, duration of period exposed to a passivating condition prior to the introduction of chloride, and the presence of cladding defects all affected the threshold chloride concentration obtained. For instance, the presence of mill scale on any of the more corrosion-resistant materials reduced the chloride threshold to approximately that of carbon steel. The chloride threshold for Type 316L clad rebar was highly dependent on any defects that exposed the carbon steel core. At best, it was similar to that of solid stainless steel. However, when defective, it was equal to that of carbon steel rebar in the potentiostatic method used here. A model was implemented to predict the extension of the Cl− diffusion time period until corrosion initiation would be expected using rebar materials with a higher chloride threshold concentration than carbon steel. Model results confirmed that corrosion-resistant rebar materials in a pickled condition may increase time until chloride-induced breakdown of passivity and onset of corrosion to 100 years or more.

Iron (Fe) is an unintentional impurity present in pure magnesium (Mg) and Mg alloys, albeit nominally in low and innocuous concentrations (< 100 ppmw). Since Fe, like most metals, is more noble than Mg, the presence of Fe impurities can serve as cathodic sites within the Mg matrix. During anodic polarization of Mg, incongruent dissolution can lead to undissolved Fe impurities accumulating upon the Mg surface, permitting an increase in the overall rate of hydrogen evolution. The experimental manifestation of the incongruent dissolution of Mg, has not yet been clarified, wherein, the extent and efficiency of Fe enrichment during anodic polarization is not known, and also the increase in the hydrogen evolution rate due to Fe enrichment has not been quantified. In this work, Mg specimens with Fe concentration between 40 to 13,000 ppmw were examined in 0.1 M NaCl to obtain a quantitative relation between the Fe concentration and the rate of cathodic hydrogen evolution. These base-line alloys were then anodically polarized to facilitate surface Fe enrichment, and subsequently again cathodically polarized to determine the impact of prior dissolution and Fe enrichment on the subsequent hydrogen evolution. A simple model to predict Fe enrichment was used to analyze the electrochemical data and predict the extent and efficiency of Fe enrichment.

The relative electrochemical properties of second phases compared to the surrounding matrix gives rise to localization of corrosion on magnesium (Mg) alloys. Localized corrosion and its subsequent propagation in Mg alloys is largely driven by so-called ‘microgalvanic coupling’ of microstructural constituents within the alloy microstructure. In the present work, atomic force microscopy (AFM) imaging coupled with scanning Kelvin probe force microscopy (SKPFM) were used to generate surface Volta potential maps of a range of Mg alloys. In this manner, the relative Volta potential difference(s) between the respective alloy matrix phase and the microconstituent phase(s) of each sample were determined. Correlations between relative Volta potentials and phase composition were then inferred based on comparison of AFM optical and topographical images with corresponding scanning electron microscopy (SEM) images and energy dispersive x-ray spectroscopy (EDS) maps of the same or similar features. Sample preparation technique, testing conditions, and proper calibration of the SKPFM were all seen to influence the Volta potentials acquired. Because the relative Volta potential difference is known to serve as an index for local corrosion—particularly under thin electrolyte layers and in chloride solutions—a review of published SKPFM data was conducted to provide a critical assessment of the surface Volta potential differences between different microconstituent phases in a variety of Mg alloys to aid in understanding and in the future improvement of the atmospheric corrosion of Mg alloys

The evolution of corrosion morphology and kinetics for magnesium (Mg) have been demonstrated to be influenced by cathodic activation, which implies that the rate of the cathodic partial reaction is enhanced as a result of anodic dissolution. This phenomenon was recently demonstrated to be moderated by the use of arsenic (As) alloying as a poison for the cathodic reaction, leading to significantly improved corrosion resistance. The pursuit of alternatives to toxic As is important as a means to imparting a technologically safe and effective corrosion control method for Mg (and its alloys). In this work, Mg was microalloyed with germanium (Ge), with the aim of improving corrosion resistance by retarding cathodic activation. Based on a combined analysis herein, we report that Ge is potent in supressing the cathodic hydrogen evolution reaction (reduction of water) upon Mg, improving corrosion resistance. With the addition of Ge, cathodic activation of Mg subject to cyclic polarisation was also hindered, with beneficial implications for future Mg electrodes.

The use of multi-coupled electrode arrays in various corrosion applications is discussed with the main goal of advancing the understanding of various corrosion phenomena. Both close packed and far spaced electrode configurations are discussed. Far spaced electrode arrays are optimized for high throughput experiments capable of elucidating the effects of various variables on corrosion properties. For instance the effects of a statistical distribution of flaws on corrosion properties can be examined. Close packed arrays enable unprecedented spatial and temporal information on the behavior of local anodes and cathodes. Interactions between corrosion sites can trigger or inhibit corrosion phenomena and affect corrosion damage evolution.