Pitting corrosion initiation in AISI 316 austenitic stainless steel

Abstract

The initiation of pitting corrosion on AISI 316 stainless steel has been examined from a phenomenological viewpoint with emphasis on the role of the metal in this complex, interaction. A modified potentiodynamic technique was used to prepare specimens corresponding with a series of different points on the anodic polarisation curve for the material in 0.05 M sulphuric acid alone and with additions of 0.1 M sodium chloride solutions-. The specimens were subsequently examined using standard metallographic, techniques. It was found that suitable pit nuclei, called 'pit sites', are manifest as a result of the initial interaction of the metal with the solution at the rest potential, (ER), i.e., at potentials far below the potential range in which catastrophic pitting processes normally occur. It was further found that these pit sites were manifest even in the absence of chloride ions for which there is no subsequent catastrophic pitting process, Estimation of pit site density (NA) for the different stages of the E-i curve and the use of a simple stereological model permit a statistical interpretation of the localisation of the phenomenon to particular areas of the metal surface. The statistical argument is extended to show that the breakdown potential for chloride media is associated with the development of a catastrophic condition which does not apply if chloride ions are absent and it is deduced that the breakdown potential is essentially indeterminate.