Beneficial effects of yttrium on the mechanical failure and chemical stability of the passive film of 304 stainless steel

Recent studies demonstrated that yttrium improved the resistance of stainless steel to corrosive wear. However, the mechanism responsible for the beneficial effect of yttrium was not well understood. In this work, the breakdown of passive films of Y-free and Y-containing 304 stainless steel specimens during indentation and scratch testing was studied using a micro-mechanical probe. The failure of the passive films during indentation and scratch was detected by measuring changes in the electrical contact resistance during the processes. The passive films of 304 steel formed in air and in a 35% nitric acid solution were also tested. It was demonstrated that the critical load that caused failure of the passive film on Y-containing 304 stainless steel was higher than that of Y-free 304 steel. Morphologies of indents and scratch tracks were observed using SEM. The surface mechanical properties of the materials were studied using a nano-mechanical probe. A scanning Kelvin probe was used to evaluate the electron stability of the passive films by measuring their electron work functions. All the experiments demonstrated that yttrium enhanced the mechanical failure resistance and chemical stability of the passive film on 304 stainless steel.