Mechanistic interpretation of corrosion fatigue of low alloy steels in deoxygenated water

There is increasing evidence in the literature that the cyclic crackgrowth rate (da/dN) for A533B pressure vessel steel under fatigue loading in deoxygenated water at 2880C is a strong function of the sulfur content of the steel. The objective of this investigation was to propose a mechanism which explained quantitatively this compositional effect on the corrosion fatigue behavior of low alloy steels. Measurements of oxidation rates on bared surfaces in aqueous environments expected at the tip of advancing cracks have indicated that these rates are strongly dependent on the anion present. Provided sulfur-rich anions can be created in the crack tip enclave (by, for instance, dissolution of MnS precipitates), and provided that these can be retained (by, for instance, stagnantflow conditions adjacent to the specimen), then the observed crack propagation rates can be predicted quantitatively by an oxidation-related model. The model leads to implications relating to a critical strain rate at the crack tip (controlled by frequency, stress intensity amplitude, etc.) which are necessary for environmentally-enhanced cracking in this alloy/environment system