Steel Corrosion Mechanisms

Abstract

This paper summarizes what is known regarding the reaction of steel in high‐temperature water as affected by sodium hydroxide additions and presents results of a detailed study of the reaction of 15% sodium hydroxide at 316°C by the hydrogen effusion method. The corrosion rates are correlated with microscopic study of the growth and breakdown of the protective magnetite films. It is shown that two different corrosion‐rate‐governing mechanisms are involved prior to the onset of pitting: a large decrease of the initial rate controlled by the build‐up of a protective film on the metal surface and a subsequent smaller decrease in corrosion rate possibly controlled by penetration of the magnetite film by the corrosive solution. The pitting is apparently due to the genesis of cracks in the magnetite film after it reaches a critical thickness.