Observation of Stress Corrosion Cracking of Stainless Steel Using Real-Time In-Situ High-Speed Atomic Force Microscopy and Correlative Techniques

A novel approach has been designed to observe stress corrosion cracking (SCC) as it occurs in-situ, in real time. State-of-the-art contact mode high-speed atomic force microscopy (HS-AFM) has been utilised to measure in-situ SCC propagation with nanometre resolution on AISI Type 304 stainless steel in an aggressive salt solution. SCC is an important failure mode in many metal systems but has a complicated mechanism that makes failure difficult to predict. Prior to the in-situ experiments, the contributions of microstructure, environment and stress to SCC were independently studied using HS-AFM. Uplift of grain boundaries before cracking was observed, indicating a subsurface contribution to the cracking mechanism. Focussed ion beam milling revealed a network of intergranular
cracks below the surface lined with a thin oxide, indicating that the SCC process is dominated by local stress at oxide-weakened boundaries. Subsequent analysis by atom probe tomography of a crack tip showed a thin Cr-rich oxide at the surface of the open crack. This study shows how in-situ HS-AFM observations in combination with complementary techniques can give new insight into the mechanisms of SCC.