Effects of alloy and solution chemistry on the fracture of anodic films formed at metastable pitting potentials

The Taguchi analysis method along with a nanoindentation technique were used to study the effects of alloy chemistry, pH and halide ion concentrations on the mechanical fracture of electrochemically grown passive films formed at a metastable pitting potential. Three austenitic stainless steels, 304L, 316L and 904L, were potentiostatically polarised in hydrochloric acid solutions. The load at film fracture during nanoindentation was primarily dominated by both alloy chemistry and the salt concentration. Passive films mechanically weakened as the ratio of iron to the total metallic atoms increased in the film; an estimate of the applied tensile stress that caused fracture increased from 0.8 GPa to 1.18 GPa as the alloy was changed from 304L to 904L. Increasing the salt concentration present during film growth from 0.01M to 0.2M reduced the applied stress for fracture by a factor of two. Prolonged anodic aging times at 40 degrees C decreased the fracture load of these passive films.