Evolution of unpredictable behaviour of a passivated stainless steel measured electrochemically

We report here the very slow activation of a stainless steel measured by long-term constant-potential polarisation in acidic solution in a region where the net current is cathodic. This unexpected and unpredictable behaviour follows a period of time in which the metal appears to demonstrate an increasing state of passivity. The activation is observed as an increase in the cathodic current density arising from surface accumulation of electrocatalytic components, which accompanies the slow dissolution of the metal. The activation process takes a significant period of time to manifest itself, and then evolves very slowly: it does not reach a steady state even after several days of polarisation at constant potential. These unique observations introduce potential difficulties involved in long-term prediction of corrosion behaviour using short-term electrochemical measurements.