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Abstract: An approach for the time dependent evolution of the local chemical environment during the initial stage of localised corrosion for aluminium alloys has been developed. The approach was applied to study anodic dissolution of S phase in sodium chloride, where both metallic species, Al3+ and Mg2+, were taken into consideration. The developed model includes hydrolysis products of aluminium and species obtained as a result of homogeneous reactions between chloride and Al3+ and Mg2+ ions and Al hydrolysis products. The model does not assume the equilibrium state in solution: all terms in the homogeneous reactions are treated explicitly using kinetic constants either taken from the literature or derived using the Eigen mechanism for complex formation. It was shown that for an active dissolution process, the time needed for chemical species to achieve their steady state concentrations ranges from minutes (for solution potential and the pH value) to hours or even days. Thus, modelling predictions obtained with the steady state approximation can give inaccurate values for species concentrations. Two modelling examples are discussed: the dissolution of a large S phase and small isolated S phase particle in Al alloys. Input data for the models are supplied by microcapillary measurements. [Copyright &y& Elsevier]

Abstract: The corrosion properties of electrodeposited pure cobalt were investigated in deaerated 0.5M Na2SO4 solutions at pH 5, 7, 10 and 13 by using polarization techniques. The effect of chloride concentration (0.01 and 0.1M NaCl) was also studied. The results showed that increasing pH shifted the anodic polarization curves to more negative potentials. At pH 5 and 7, cobalt exhibited active dissolution without any distinctive transition to passivation. However, at pH 10, the metal surface was partially passivated and anodic current increased sharply at approximately −0.2VSCE and the corroded surface revealed several well-defined pits. On the other hand, at pH 13, a clear passivation region was observed within the potential range of −0.5 to 0.6VSCE. With increasing chloride concentrations of 0, 0.01, and 0.1M at pH 10, both the anodic current and the number of pits on the cobalt surface were gradually increased. X-ray photoelectron spectroscopic investigations showed that the cobalt surface was covered mostly with cobalt hydroxide after anodic polarization at pH 10 in 0.1M NaCl. [Copyright &y& Elsevier]

Alloy 22 (UNS N06022) is a Ni-Cr-Mo alloy, highly resistant to localized corrosion. It has a wide range of industrial applications. Because of its versatility, Alloy 22 has been proposed as a corrosion-resistant barrier for high-level nuclear waste containers. Crevice corrosion is expected to occur in hot chloride solutions of the corrosion potential is equal to or greater than the crevice corrosion repassivation potential For less-aggressive environments, transpassive dissolution limits the alloy performance at high potentials. The goal of the present work was to establish the effect of potential on the kinetics of chloride-induced crevice corrosion of Alloy 22 at 90°C. Electrochemical tests carried out included cyclic potentiodynamic polarization (CPP) curves, polarizations at constant applied potentials followed by repassivation, and electrochemical impedance spectroscopy (EIS) measurements at anodic applied potentials. The attack profiles were measured using a roughness checker. Crevice corrosion susceptibility of Alloy 22 was independent of bulk solution pH, but it was a strong function of potentiaL Likelihood of transpassive dissolution was found to increase for high pH solutions. Two different regions were distinguished in polarization tests at a constant potentiaL A first region was associated with crevice corrosion incubation and a second one was associated with crevice corrosion initiation and propagation. The crevice corrosion current density was estimated as a function of potential. It increased with potential up to a maximum value of approximately 20 mA/cm2. Absence of pitting corrosion was explained in terms of such a small current density, which was insufficient to satisfy a critical value of the product xi, necessary for pitting corrosion to occur. A diagram of crevice corrosion susceptibility and transpassive dissolution for Alloy 22 in 1 M chloride solutions at 90°C was presented. If the corrosion potential exceeds the crevice corrosion repassivation potential, crevice corrosion will initiate, provided that the diffusion path associated with the crevice is long enough. [ABSTRACT FROM AUTHOR]

Crevice corrosion of titanium is activated in concentrated chloride media at 100 °C. This was possible only with the tightest gap (0.005 cm) between Ti-Ti surfaces. No crevice corrosion was observed with greater gap dimensions. The design of the crevice led to the occurrence of two concentric circular rings of corroded areas, with many pits on them. After potentiostating in the passive region for 5 h in 25% NaCl (pH = 4.7)--where hydrogen evolution is thermodynamically prohibited--hydrogen gas bubbles were observed to egress out of the crevice mouth during ongoing crevice corrosion. This indicates that hydrogen evolution occurs within the crevice. The results are compatible with the occurrence of gradually increasing ohmic potential shift and localized acidification in the crevice electrolyte as judged by the measured gradual increase of the crevice corrosion current. The high acidity of the bulk electrolyte does not seem to be sufficient or even a necessary condition for crevice corrosion to occur. [ABSTRACT FROM AUTHOR]