Tribo-Ellipsometry: A New Technique to Study the Relationship of Repassivation Kinetics to Stress Corrosion

Since the susceptibility of a material to stress corrosion cracking (SCC) may be related to the rupture of a protective film and the repassivation rate of the material thus exposed, a technique, tribo-ellipsometry, has been developed which simulates film rupture by abrading off the surface oxide. During the subsequent repassivation of the exposed surface, this technique allows simultaneous determination of film growth kinetics by ellipsometry and current transients during that time interval following removal of the oxide film. The major advantage of the technique is that the ellipsometric transient allows one to determine which part of the current transient is responsible for repassivation and which part is involved in metal dissolution. The utility of the technique is demonstrated by comparing repassivation rates for a low carbon steel in a sodium nitrate solution in which the metal is susceptible to SCC to those in a sodium nitrite solution, in which it is not. Results obtained using this method indicate that the repassivation rate is slower at elevated temperatures in the nitrate than it is in the nitrite, and that a larger proportion of the current is involved in metal dissolution in the nitrate. This may offer an insight into the reasons for observed susceptibility of mild steels to SCC in such nitrate environments.