Theory Of Stainless Steel Pitting

Abstract

Abstract Pit origin and formation in stainless steels are discussed according to results of investigations from the following points of view: 1. Conditions for the Existence of Pitting Corrosion. Pitting potential range and pitting potentials. Functions dependent on the potential which are important for pitting corrosion, effect of inhibitors and results of different methods of investigation are described. 2. Kinetics of Pitting Corrosion. Analysis of the current-time relationship gives information on dependence of potential on the anodic metal dissolution process. Pit growth under potentiostatic, galvanostatic and chemical corrosion conditions can be explained theoretically. 3. Physical Forms of Pitting Corrosion. Shape of pits that can be observed in different potential ranges and under different corrosion conditions are discussed: regular etch pits, hemispherically-shaped pits, under-hollowing and elongated areal pits. 4. Theoretical Explanation of Pitting Corrosion. In connection with the “all-or-none” principle of passivity, which holds that the active and passive states should not coexist at the same potential, it is shown under which circumstances stable pitting corrosion is possible. A concentration effect and a resistance polarization effect are discussed in detail. The origin of pitting corrosion and the pitting potentials are discussed in terms of a chemisorption process and a transport and/or transference theory.