Please cite this article as: M.Curioni, F.Scenini, T.Monetta, F.Bellucci, Correlation between electrochemical impedance measurements and corrosion rate of magnesium investigated by real-time hydrogen measurement and optical imaging, Electrochimica Acta http://dx. AbstractThe corrosion behaviour of magnesium in chloride-containing aqueous environment was investigated by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) performed simultaneously with real-time hydrogen evolution measurements and optical imaging of the corroding surface. The potentiodynamic investigation revealed substantial deviations from linearity in close proximity of the corrosion potential. In particular, differences in the slope of the current/potential curves were observed for small polarizations above or below the corrosion potential. These observations, suggest that the usual method based on the use of the Stern-Geary equation to convert a value of resistance into a value of corrosion current is inadequate.Nonetheless, a very good correlation between values of resistances estimated by EIS and corrosion currents obtained from real-time hydrogen measurement was found. Real-time hydrogen measurement also enabled, for the first time, direct measurement of an 'apparent' Stern-Geary coefficient for magnesium. In order to rationalize the complex behaviours experimentally observed, an electrical model for the corroding magnesium surface is presented.

Degradation of reinforced concrete (RC) structures is a serious safety problem affecting all industrialized countries, and the economical aspect of this problem cannot be neglected. One of the main reasons for the degradation of RC is the corrosion of steel reinforcing bars as a result of concrete cover cracking and reduction in bar cross section. As a consequence, the structural capacity of RC elements decreases progressively as degradation develops. Nondestructive testing and/or analytical formulation represent high-quality rapid methods for evaluating the corrosion penetration of bars and provide useful parameters for designing retrofits. This paper presents an additional tool that can be used to evaluate and assess the vulnerability of existing structures in terms of the determination of bar cross section lost. Using simple charts and formulas, this can be accomplished by calculating reinforcing bar cross section loss due to corrosion by measuring external crack widths in the concrete cover. Predictions were obtained by using two analytical models developed. These predictions have been satisfactorily compared with both the literature and new experimental results, as well as with previous empirical models available in the scientific literature. The experimental results were obtained by corroding real-scale concrete specimens reinforced with smooth and ribbed bars (according to old and modern building codes) using 3.5 wt% sodium chloride solution and an applied current

Water reduction, which leads to the evolution of hydrogen, is a key cathodic process for corrosion of many metals of technological interest such as magnesium, aluminium, and zinc; and its understanding is critical for design of new alloys or protective treatments. In this work, real-time hydrogen evolution measurement was coupled with potentiodynamic measurements on high-purity aluminium and AA2024-T3 aluminium alloy. The results show that both materials exhibit superfluous hydrogen evolution during anodic polarisation and that the presence of cathodically active alloying elements enhances hydrogen evolution. Furthermore, it was observed for the first time that superfluous hydrogen evolution also occurs during cathodic polarisation. Both the anodic and cathodic behaviours can be rationalised by a model assuming that superfluous hydrogen evolution occurs locally where the naturally formed oxide is disrupted. Specifically, during anodic polarisation, oxide disruption is due to the combined presence of chloride ions and acidification, whereas during cathodic polarisation, such disruption is due to alkalinisation. Furthermore, the presence of cathodically active alloying elements enhances superfluous hydrogen evolution in response to either anodic or cathodic polarisation, and results in 'cathodic activation' of the dissolved regions.

Chromic acid anodizing has been used for almost a century to enhance corrosion protection of aerospace alloys. For some applications, hydrothermal sealing in hexavalent chromium-containing solution is required to enhance further the corrosion resistance but, due to environmental concerns, the use of hexavalent chromium must be discontinued. Good progress has been made to replace chromates during anodizing but comparatively less effort has focused on the sealing process. In this work, for the first time, electrochemical impedance spectroscopy (EIS) has been used to characterize in-situ the sealing processes occurring during hot water sealing, sodium chromate sealing and cerium sealing. The results suggest that the processes occurring during sodium chromate sealing are significantly different compared to hot water and cerium sealing. In particular, during chromate sealing, the porous skeleton is significantly attacked, suggesting that the anticorrosion performance is likely to arise from the residuals of chromate rather than from the improvement of the barrier properties. In contrast, during hot water sealing, little attack occurs on the porous skeleton, and the improved corrosion protection is due to the enhanced barrier effect. During cerium sealing, precipitation of cerium products occurs, providing an inhibitor reservoir, and little, if any, attack occurs on the pre-existing oxide. Chromic acid anodizing (CAA) is widely used in the aeronautic industry to improve corrosion resistance of aluminum alloys.1 Since the beginning of the 1990s, however, the high toxicity associated with Cr (VI) has imposed restrictions on their use in industrial applications. As a consequence, numerous attempts have been made to find less toxic alternatives. 2,3Anodizing with dilute sulfuric acid (DSA) has been used to obtain thin anodic films (1-5 μm) that provide some protection without excessive deterioration of the fatigue life for specific aerospace alloys. Although the fatigue performance of DSA is acceptable, the corrosion resistance is lower than that of parts anodized in chromic acid (CAA). More recently, a new anodizing procedure, involving the addition of tartaric acid in dilute sulfuric acid electrolyte and called tartaric-sulfuric acid anodizing (TSA), was introduced.4-6 The addition of tartaric acid to sulfuric acid baths improves significantly the anticorrosive properties of the anodic layers compared to those obtained by sulfuric acid anodizing.7 Recent work, 7 however, indicates that the mechanism of porous film growth is not significantly affected by tartaric acid additions and that tartaric acid is not incorporated in significant amounts into the oxide material. Thus the corrosion resistance provided by TSA is likely to be associated with residuals of tartaric acid adsorbed on the porous skeleton. Tartaric acid concentration in the order of ppm, has been proved to be effective in reducing both the oxide dissolution rate in acidic environments and the anodic reaction rate. The effect of tartaric acid on the anodic fi...