Pitting resistance of stainless steels studied by PPR and CPT tests

The effects of alloying elements Cr, Mo and N on the stability of passive film were investigated by measuring Pit Propagation Rate (PPR) and Critical Pitting Temperature(CPT) in aqueous 0.1N H2SO4+0.1N NaCl solution. CPT tests were carried out by measuring the temperature representing the critical current density obtained by increasing temperature at the specific applied potential within passive potential range while PPR tests were conducted by measuring the pit propagation rate obtained by the current density produced due to the pit growth divided by the pit area. The effects of alloying elements Cr, Mo, N on the stability of passive film were investigated by the comparison between CPT and PPR test results in terms of pitting resistance equivalent. PPR test results showed that pit propagation rate was decreased with the increase in Cr, Mo and N contents, indicating the synergistic effect of Mo and N. CPT test results exhibited that the higher contents of Cr, Mo and N, the higher critical pitting temperatures. The synergistic effects of Mo and N on the resistance to pitting corrosion were also observed. As the pitting resistance equivalent increases the passive potential range is extended due to the increased pitting potential. The increase in pitting resistance equivalent also results in the reduction of pit propagation rates, showing the increased critical pitting temperatures. Pitting resistance equivalent may be the good parameter representing the extent of the resistance to pitting corrosion regardless of the kinds of stainless steels