The Role of Passive Film Growth Kinetics and Properties in Stress Corrosion Susceptibility

A new technique has been developed for studying the rate of repassivation of a stressed metal surface exposed by film rupture. This technique, which removes the film on a metal by abrasion and then follows film regrowth and metal dissolution by transient ellipsometry and current transients, was applied to low carbon steel in a nitrate solution where it stress cracks and a nitrite where it does not. The rate of repassivation was greater in the non- susceptible solution while the ratio of current going into film formation to that producing metal dissolution was less. Another new technique was developed which measures the ductility of films on metal surfaces by determining by ellipsometry the amount of thinning the film undergoes upon straining the metal. The technique gives values for maximum ductility of the film rather than the fracture strain that other techniques give. Measurements were made on anodic films on Ta, Al and Al-4% Cu. See also Technical summary rept. no. 1, AD704882.