Mechanism of Cracking of High Strength Martensltic Stainless Steels in Sodium Chloride Solutions

Abstract

Abstract This paper reviews previously proposed mechanisms for the environmentally induced cracking of high strength steels in sodium chloride. The electrochemical basis for the differentiation between hydrogen embrittlement and active path corrosion is examined along with the consequences of the recently demonstrated acidification that occurs in areas of localized geometry such as precracks, pits, and crevices. From electrochemical studies on a modified 12% chromium martensitic stainless steel, data are presented which indicate that hydrogen absorption occurs under conditions of exposure previously thought to preclude this possibility; that is, at applied bulk potentials noble to the reversible hydrogen potential. Data are also presented to show that the activation energy for crack growth at the corrosion potential and at cathodic and anodic applied potential is 9.5 Kcal/mole (±1); a value close to that reported previously for the cracking of cathodically embrittled steel. The experimental data are explained on the basis that an embrittlement process associated with hydrogen absorption is responsible for crack growth under all conditions of applied potential. With polarization at potentials active to the hydrogen reversible potential, hydrogen absorption results directly from the bulk cathodic discharge of protons. At the corrosion potential and more noble potentials, hydrogen absorption takes place from the cathodic discharge of protons occurring in the low pH conditions resulting from the hydrolysis of anodic dissolution products in pits. In the noble potential region, although active path anodic dissolution appears not to be the mechanism of crack growth, anodic dissolution is a prerequisite to crack growth by a hydrogen embrittlement mechanism. It is suggested that a similar mechanism can explain the cracking behavior of other high strength steels in sodium chloride solutions where conditions suitable for hydrolysis arise either by pitting or by crevice corrosion occurring in the presence of a mechanical precrack.