The remarkable passivity of austenitic stainless steel in sulphuric acid solution and the effect of repetitive temperature cycling

The passivation of 316L austenitic stainless steel in sulphuric acid solution has been measured at a single temperature, and then further, after a series of temperature sweeps. After 15 h at 20 °C and constant electrode potential, the passive current density has fallen to 16 nA cm−2 and still continues to decline, with no evidence of an approach to the steady state. After subjecting the metal to a series of temperature sweeps the passive current density has decayed much further to ca. 0.5 nA cm−2 and probably still decaying slowly. The metal is thereby rendered extremely passive. It is argued that the passive current density as measured by conventional potential sweep voltammetry is not in fact the steady state current density, because the true approach to a steady state is far too slow.