Stress corrosion cracking (SCC) is a critical failure mechanism in numerous metal systems, characterized by the synergistic action of corrosion and tensile stress, leading to crack propagation and reduced fracture resistance. The unpredictable nature of SCC, influenced by subtle environmental changes and microstructural variations, makes failure prediction challenging, resulting in numerous unexpected failures across various industries (e.g., gas, oil, nuclear). Existing characterization techniques, while offering various length and time scales, often lack the high resolution or in situ capabilities needed for detailed mechanistic understanding. This study employs high-speed atomic force microscopy (HS-AFM), a technique capable of high temporal and spatial resolution imaging in various environments, to observe SCC in real-time. Specifically, the researchers investigated intergranular stress corrosion cracking (IGSCC) in thermally sensitized AISI Type 304 austenitic stainless steel exposed to a 395 mg L⁻¹ aqueous sodium thiosulfate solution—a model system known to reliably induce IGSCC. The study aimed to provide dynamic in situ observations of the cracking process and high-resolution ex situ analysis via complementary techniques such as APT, EDX, FIB, and SEM.

Existing literature on IGSCC in sensitized Type 304 stainless steel in thiosulfate solutions presents two primary theories: film rupture and anodic dissolution, and hydrogen-induced fracture. Newman et al. proposed anodic dissolution accelerated by strain-induced martensite fracture at grain boundaries. However, the effects of martensite on crack velocity remain debated. Hydrogen-induced fracture has also been suggested to explain brittle microcracks along multiple grain boundaries. Despite these conflicting models, the presence of elemental sulfur at the crack tip is considered crucial. Adsorbed sulfur inhibits repassivation, enhancing dissolution of Fe and Ni at sensitized grain boundaries and potentially catalyzing hydrogen entry, leading to embrittlement. Prior studies primarily relied on reaction-sensing techniques (e.g., electrochemical noise, scanning vibrating electrode technique) or post-corrosion SEM analysis; high-resolution in situ visualization of the mechanisms was lacking.

The study employed a multi-faceted approach. Initially, the individual contributions of microstructure, environment, and stress to IGSCC were investigated separately using HS-AFM. AISI Type 304 stainless steel was thermally sensitized at 600 °C for 70 h, creating a microstructure representative of certain nuclear applications. Microstructural characterization was performed using EBSD, revealing a grain size ranging from 1 to 43 μm. HS-AFM revealed grain boundaries (GBs) and chromium-rich carbide precipitates. The effects of stress were studied by deflecting a specimen in a three-point strain rig, observing slip bands indicative of plastic deformation. The effect of the corrosive environment (395 mg L⁻¹ aqueous sodium thiosulfate) was examined independently, revealing preferential dissolution at carbide precipitates. The in situ SCC experiments combined all three factors: thermally sensitized samples were stressed in the thiosulfate solution while being monitored by HS-AFM. The pre-exposure time to the solution was optimized (6 days) to minimize the time to SCC initiation. High-speed AFM captured crack initiation and propagation. Complementary ex situ analyses were conducted: atom probe tomography (APT) analyzed crack tip chemistry, focused ion beam (FIB) milling and scanning electron microscopy (SEM) examined subsurface cracking, and energy-dispersive X-ray spectroscopy (EDX) provided elemental analysis. Sample preparation involved careful polishing to achieve a mirror finish.

In situ HS-AFM observations revealed grain boundary uplift ahead of the visible crack tip, a phenomenon not previously reported. This uplift, on the order of tens of nanometers, suggested subsurface processes contributed significantly to the cracking mechanism. In situ HS-AFM also captured the crack propagation, revealing a smooth, continuous process, contrasting with previous reports of stepwise crack growth associated with hydrogen-induced fracture. The crack propagation speed was significantly slower than values reported in other studies. Ex situ HS-AFM imaging of cracked grain boundaries showed widened boundaries with an oxide layer, indicating localized corrosion. APT analysis of the crack tip revealed a layered oxide composition (Cr-rich inner layer, followed by a region with varying Cr, Fe, Na, Ni, and O concentrations). The oxide layer did not completely fill the crack, consistent with a stress-driven cracking process. FIB milling revealed a network of intergranular cracks below the surface, with a porous oxide layer along their walls. This oxide layer had a thickness comparable to the observed grain boundary uplift, suggesting a potential mechanistic link. The analysis also showed Cr depletion and Ni enrichment at the grain boundary, consistent with sensitization. The observation of the oxide layer, grain boundary uplift, and the continuous crack propagation process contribute novel insights into the mechanisms of IGSCC.

The findings highlight the importance of subsurface processes in IGSCC. The observed grain boundary uplift and the presence of a porous oxide layer along the subsurface cracks suggest a complex interplay between stress, corrosion, and oxide formation. The layered oxide composition indicates that the corrosion process is not uniform but rather occurs in stages. The limited diffusion within the occluded crack interior allows a buildup of aggressive electrolyte chemistry. While the results may support a film rupture mechanism, other mechanisms such as hydrogen-induced fracture cannot be entirely ruled out. The smooth crack propagation observed suggests that the cracking process is more complex than previously reported, possibly involving pre-oxidation that weakens the grain boundaries. This work demonstrates the significant contribution of in situ HS-AFM observation in characterizing the initial stages of cracking.

This study used high-resolution techniques to provide critical insights into IGSCC in a model system. The in situ HS-AFM observations, combined with ex situ APT, FIB, SEM, and EDX analyses, revealed previously unreported phenomena such as grain boundary uplift and a layered oxide structure at the crack tip. The findings suggest a complex mechanism involving subsurface crack propagation, oxide formation, and stress-driven crack growth. Future work should focus on further investigating the role of sulfur and resolving the relative contributions of film rupture and hydrogen-induced fracture.

The HS-AFM's interaction with the sample surface might influence the corrosion rate. The APT results for sulfur were inconclusive due to challenges in deconvoluted sulfur and oxygen signals. The study focused on a specific model system, and the generalizability of the findings to other materials and environments requires further investigation. The interpretation of the high-speed AFM images in the later stages of crack propagation was hampered by imaging artifacts due to tip convolution.