Use of Artificial Pits to Investigate Conditions for the Growth and Repassivation of Open Macroscopic Corrosion Pits in Stainless Steels

Abstract

Two different artificial pit configurations were developed to investigate the conditions required for continued growth of open macroscopic corrosion pits in stainless steel sheets. To reproduce the conditions in growing real pits in oxidising environments, the pit growth was activated under anodic polarisation either by injecting concentrated chloride solution into the pit or by initially filling the pit by chromic chloride crystals (CrCl 3 .6H 2 O). Experiments were made on sheet specimens of stainless steels of type UNS S31603 or UNS S31254 in bulk solutions containing chloride and/or sulphate ions. Various aspects of the test arrangements and pitting of stainless steels are discussed, including the electrolyte composition within the pits and the IR-drops both inside the pits and in the bulk solution. It is demonstrated that using the artificial pit configurations the effects of the main variables affecting the conditions for growth and repassivation of open corrosion pits can be investigated quantitatively, including electrode potentials and composition of the bulk solution. It is concluded that for continued growth of corrosion pits with a large opening to the bulk solution, a strongly oxidising environment is required, and that sulphate ions are fairly inefficient in stopping the growth of propagating pits.