Observation of stress corrosion cracking using real-time in situ high-speed atomic force microscopy and correlative techniques

S Moore,T L Martin,P E J Flewitt,D Kumar,R Burrows,M B Kloucek,L Picco,A D Warren,O D Payton

doi:10.1038/s41529-020-00149-y

Abstract

Contact-mode high-speed atomic force microscopy (HS-AFM) has been utilised to measure in situ stress corrosion cracking (SCC) 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. During SCC measurements, 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 layered oxide composition at the surface of the crack walls. Oxide formation is posited to be mechanistically linked to grain boundary uplift. This study shows how in situ HS-AFM observations in combination with complementary techniques can give important insights into the mechanisms of SCC.

Highlights

Conjoint corrosion and stressing of a metal or alloy can result in the development of cracks which propagate through the material, causing failure due to the reduced fracture resistance[1,2]
Measurements of individual factors leading to stress corrosion cracking (SCC)
The microstructure of this material was measured by electron backscattered diffraction (EBSD) as shown in Supplementary Fig. 1

Summary

Introduction

Conjoint corrosion and stressing of a metal or alloy can result in the development of cracks which propagate through the material, causing failure due to the reduced fracture resistance[1,2]. The observations made in this study show that thiosulfate does have a corrosive effect on the sample surface, and this occurs at the GB carbide precipitates positioned on sensitised GBs. This dissolution process may occur at the crack tip during IGSCC, resulting in stress accumulation at the resultant micro-pits.

Results

Conclusion