Towards a Mechanism Based Theory for Crack Initiation

Abstract Stress corrosion cracking and corrosion fatigue commonly initiate at corrosion pits, which serve as stress raisers. Thus, the challenge in predicting the initiation of these forms of stress induced damage to a metal surface in a corrosive medium reduces to the prediction of the initiation and growth of stable pits. Because the stress intensity factor (KI) increases with increasing pit depth (for a constant stress), the transition of a pit into a crack is envisioned to occur when the stress intensity exceeds the critical value for the initiation of a crack (KISCC). In this paper, we outline a deterministic theory for the initiation of stress corrosion cracking and corrosion fatigue based on the Point Defect Model (PDM) for passivity breakdown and the Coupled Environment Pitting Model (CEPM) for pit growth within the framework of Damage Function Analysis (DFA). Once a crack initiates, the Coupled Environment Fracture Model (CEFM) and the Coupled Environment Corrosion Fatigue Model (CECFM) for stress corrosion cracking and corrosion fatigue, respectively, describe continued, stress-augmented propagation. The principals of DFA are illustrated by reference to the development of localized corrosion damage on aluminum exposed to sodium chloride solution, with emphasis on illustrating the interplay between stress-related and electrochemical effects. Practical applications in describing the failure of condensing heat exchangers and low-pressure steam turbines are also discussed.