Corrosion fatigue behavior of hyper-quenched X6CrNiMoCu25-6 steel in synthetic seawater at 70 °C

The aim of this study is to investigate the corrosion fatigue behavior of X6CrNiMoCu25–6 steel in a hyper-quenched state in synthetic seawater at 70°C. Cracking tests were performed on standardized CT10 test specimens in environments representative of the operating conditions of pump components (synthetic seawater according to ASTM 1141–86). Microfractographic examinations using SEM, coupled with metallographic analyses of the test specimen surfaces, were used to elucidate the role of microstructure, the physicochemical characteristics of the environment (nature, pH, temperature) and loading frequencies (Hz) in the metal-environment interaction mode, as well as their consequences on local propagation rates. The propagation rate of fatigue cracks in synthetic seawater at 70°C is significantly accelerated at a loading frequency of around 0.1 Hz. At this frequency of maximum susceptibility, the predominant propagation mechanism is ferrite cleavage, initiated at values of ΔK ≥ 60 MPa√m, corresponding to a maximum stress value of around 240 MPa. This is likely to exceed the critical cleavage stress of hydrogen-embrittled ferrite. Localized anodic dissolution, evident on fatigue striations, is the source of hydrogen adsorption by the two phases constituting the steel.