Search

Current transients for iron in its full passive state in acid media were studied under potentiostatic control. The experimental results were quantitatively interpreted through an analytical approximate equation relating the instationary-state current with the transient charge, based on Vetters electrode kinetics model of the passive iron interphase and the introduction of a potential distribution dependent on time across the passive layer/solution interface. Extrapolation of the passive film thickness to the Flade potential allows the prediction of a precursor film. An electric field of about 3 10[6] V cm−1 was determined, in accordance with the literature. From numerical calculations it was found that about 10% of the charge passed during the current transient corresponds to the film formation under potentiostatic step conditions.

The effect of temperature (60–280 °C) on the properties of the oxide films formed on Alloy 800 in 0.1 M NaCl and 0.1 M Na2SO4 aqueous solutions was studied by in situ AC impedance spectroscopy and polarization in the Fe(CN6−3Fe(CN)6−4 redox system. The anodic behaviour under the same experimental conditions was examined by potentiodynamic polarization techniques. In both solutions the passive film was found to become more porous, and hence less protective, with increasing temperature. However, at temperatures above 150 °C, the loss of film protectiveness is more pronounced in chloride solutions, in which pitting occurs. Pitting morphology was found to be strongly temperature dependent: isolated and deep pits were found up to 200 °C whereas at higher temperatures a broad, shallow and more generalized type of attack was detected. No effect of temperature on the defect structure of the semiconductor oxide film was found. It was concluded that the changes in the pitting behaviour of Alloy 800 at temperatures below and above 200 °C are associated to a temperature-affected variation of the protective properties of the passive film.

Repassivation kinetics of metals has been frequently related to the susceptibility to localized types of corrosion such as pitting, crevice corrosion and stress corrosion cracking (SCC). The present paper reviews recent work in our laboratory showing that repassivation kinetic measurements can be successfully used to detect passivity breakdown in metals; a step which followed by localized acidification leads to pitting and crevice corrosion. Similar experiments have confirmed that for SCC a slow repassivation rate is a necessary, but not a sufficient, condition for stress corrosion cracking.

Alloy 22 (UNS N06022) is a candidate material for the external wall of the high level nuclear waste containers for the potential repository site at Yucca Mountain. In the mill-annealed (MA) condition, Alloy 22 is a single face centered cubic phase. When exposed to temperatures on the order of 600 C and above for times higher than 1 h, this alloy may develop secondary phases that are brittle and offer a lower corrosion resistance than the MA condition. The objective of this work was to age Alloy 22 at temperatures between 482 C and 800 C for times between 0.25 h and 3,000 h and to study the corrosion performance of the resulting material. Aging was carried out using wrought specimens as well as gas tungsten arc welded (GTAW) specimens. The corrosion performance was characterized using standard immersion tests in aggressive acidic solutions and electrochemical tests in multi-component solutions. Results show that, in general, in aggressive acidic solutions the corrosion rate increased as the aging temperature and aging time increased. However, in multi ionic environments that could be relevant to the potential Yucca Mountain site, the corrosion rate of aged material was the same as the corrosion rate of the MA material.

The repassivation kinetics of AISI 304 austenitic stainless steel was studied in 0.5 M Na2SO4 solution, 1 M HCl solution and 1 M NaCl solution, at room temperature and at various electrode potentials. Under the experimental conditions used in the present work, stress corrosion cracking (SCC) of the steel was known to occur only in the HCl solutions. The repassivation kinetic results were described by relations of the type i = A·tb. In Na2SO4 solutions b = −1 was found. In NaCl solutions and in HCl solutions b = −1 was found only in the first 100 ms after straining. In the next 10 s a value of b = −0.5 was recorded, and for longer exposures values of b = 0.5 and b = 1 were observed. There is strong evidence that a dissolution-precipitation film formation mechanism is involved in the region where b = −0.5. By comparing the results in HCl solutions with those in NaCl solutions, slow repassivation, of the type b = −0.5, is concluded to be a necessary but not sufficient electrochemical condition for SCC susceptibility.

The repassivation rate kinetics of α-brass in NaNO2 solutions of various concentrations, in borate buffered Na2SO4 solutions, and in sulphide-containing Na2SO4 solutions was studied. Slow repassivation rates, with slopes of b = −0.5 in the current decay curves, were observed to be present in all the cases where SCC was found. This slow repassivation rate also proved to be a necessary, but not a sufficient condition for SCC, since the same slope was found in some cases where no SCC occurred. The use of repassivation rate techniques in the prediction of SCC susceptibility was evaluated, and the limitations of those techniques are discussed.