The effect of temperature on the passive film properties and pitting behaviour of a Fe-Cr-Ni alloy

The effect of temperature (60-280 degrees 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(CN6-3)/Fe(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 degrees C, the loss of film protectiveness is more pronounced in chloride solutions, in which pitting occurs. Fitting morphology was found to be strongly temperature dependent: isolated and deep pits were found up to 200 degrees 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 degrees C are associated to a temperature-affected variation of the protective properties of the passive film. Copyright (C) 1996 Elsevier Science Ltd.