The Mechanism of Stress Corrosion Of Austenitic Stainless Steels In Hot Aqueous Chloride Solutions★

Abstract

Abstract An investigation into the mechanism of transcrystalline stress corrosion of austenitic stainless steels (18 Cr/8 Ni and 25 Cr/20 Ni) in hot aqueous chloride solutions has shown that this phenomenon must be described as being a process alternating between corrosion and mechanical cracking. Only sharp-edged pits formed by a specific corrosive attack initiate and propagate the mechanical cracking. These pits act as stress raisers; the conditions for notch brittleness of the materials involved are produced, so that mechanical cracking occurs, which is of a brittle nature. At high stresses small pits occurring either at the slip bands or at the grain boundaries may cause cracking, whereas at low applied stresses, only longish pits formed at the slip bands initiate it. The investigation proves that the initial corrosive attack is dependent primarily on the mechanical and chemical behavior of the passivating film already present on the steel or formed during exposure to the corrosive medium, and only to a minor extent on the structure of the metal itself. If the type of initial corrosive attack is changed by comparatively small alterations in the composition of the corrosive medium, stress corrosion is either prevented or accelerated. Tests illustrating this observation are discussed. 3.5.8