Investigation of Pitting on Stainless Steel Using Scanning Electrochemical Microscopy

Metallic materials are usually employed in a state of passivation with an oxide layer formed on their surface acting as a barrier to ionic migration effectively separating the underlying metal from its environment. As a result, the release of metal ions from the substrate into the electrolytic phase is greatly hindered, and the material is regarded to be stable for sufficiently long service periods. Unfortunately, passivity breakdown and localized pitting corrosion are common phenomena to many materials in chloride solutions. In that event, though most of the material remains passive, effectively experiencing very slow corrosion rates, pits propagate very rapidly penetrating into the bulk of the material. Failure and even catastrophic destruction of the material often occurs as result of such penetration, which is the motivation for the significant research efforts investigating the causes and mechanisms of pitting. Though pit nucleation can often be correlated to heterogeneities in the material, pits can also be nucleated on homogeneous metallic surfaces, producing random distributions as a result. Significant progress in the knowledge of the origins of corrosion has been gathered from electrochemical measurements in the micrometer range, in order to greatly reduce the number of pitting events simultaneously occurring on the investigated section of the substrate. In this way, the electrochemical signal obtained from the sample could eventually arise from individual pits.

The inherently localized nature of the pitting process requires methods capable to gain information about the generation and evolution of the pit formation in the micrometer range with high spatial resolution, preferably in real time. In situ measurements during the corrosion process performed in the actual corroding media under study are mandatory, especially for the analysis of the initial stages of the process. In this regard, scanning electrochemical microscopy (SECM) is highly attractive for corrosion studies, and various experimental procedures have been developed for studying corrosion phenomena.