Mechanical effects on the corrosion resistance of ferritic stainless steels during microabrasion-corrosion

Abstract

Mechanical events affect the corrosion behaviour of metals in a wide range of industrial applications, but the synergy between corrosion and mechanical events do not allow them to be studied individually. For ferritic stainless steels in H2SO4 corrosive media, a negative synergy between microabrasion and corrosion has been reported, where low friction passive layers reduce wear rates. However, for the abrasion-corrosion of ferritic stainless steels, little attention has been paid to mechanical effects on their corrosion resistance. To investigate this, potentiodynamic curves were obtained for ferritic stainless steels during standard corrosion tests, which were compared to potentiodynamic curves obtained in two other conditions: i) aerated corrosion tests (non-static electrolyte); and ii) microabrasion-corrosion tests using silica + H2SO4 solution slurries. The specimens were ferritic stainless steels with different chemical compositions (11 wt%Cr with and without Ti stabilization; 16 wt%Cr with and without Nb stabilization, 17.5 wt%Cr with Nb + Ti stabilization) and, for comparison, one austenitic stainless steel (18 wt%Cr-8 wt%Ni) and one carbon steel (0.2 wt%C). The results showed that the use of aerated conditions increased passivation current, which was further increased for microabrasion-corrosion conditions. Moreover, although the potentiodynamic curves for all the stainless steel specimens presented a clear passive region under standard corrosion test conditions, only the austenitic stainless steel showed a classic passive region when mechanical effects were present. For the low Cr ferritic stainless steel under microabrasion conditions, the potentiodynamic curves showed a region where the current increased little with potential increased, but this was called a pseudopassivation, since current increased steadily, although little, with potential increase. For higher Cr contents, after the pseudopassive region, a real passive region was present at higher potentials. The potentials for achieving real passive regions were higher under microabrasion conditions than under non-static condition, evidencing a contribution of both mechanical effects (fluid flow and mechanical wear) on their corrosion resistance.