Stress–Corrosion Cracking of AISI 316L Stainless Steel in Seawater Environments: Effect of Surface Machining

Zhe Ren,Frank Ernst

doi:10.3390/met10101324

Abstract

To understand the effect of surface machining on the resistance of AISI 316L to SCC (stress–corrosion cracking) in marine environments, we tested nuts surface-machined by different methods in a seawater-spraying chamber. Two forms of cracks were observed: on the machined surface and underneath it. On the surface, cracks connected with the pitting sites were observed to propagate perpendicular to the hoop-stress direction, identifying them as stress–corrosion cracks. Under the surface, catastrophic transgranular cracks developed, likely driven by hydrogen embrittlement caused by the chloride-concentrating level of humidity in the testing environment. Under constant testing conditions, significantly different SCC resistance was observed depending on how the nuts had been machined. Statistical evaluation of the nut surface-crack density indicates that machining by a “form” tool yields a crack density one order of magnitude lower than machining by a “single-point” tool. Microstructural analysis of form-tool-machined nuts revealed a homogeneous deformed subsurface zone with nanosized grains, leading to enhanced surface hardness. Apparently, the reduced grain size and/or the associated mechanical hardening improve resistance to SCC. The nanograin subsurface zone was not observed on nuts machined by a single-point tool. Surface roughness measurements indicate that single-point-tool-machined nuts have a rougher surface than form-tool machined nuts. Apparently, surface roughness reduces SCC resistance by increasing the susceptibility to etch attack in Cl--rich solutions. The results of X-ray diffractometry and transmission electron microscopy diffractometry indicate that machining with either tool generates a small volume fraction (< 0.01) of strain-induced martensite. However, considering the small volume fraction and absence of martensite in regions of cracking, martensite is not primarily responsible for SCC in marine environments.

Highlights

SCC, stress–corrosion cracking, a form of material degradation that is sensitive to the environment, happens under synergistic action of chemical attack from the environment and applied stress [1]
We study the micromechanisms leading to different seawater SCC susceptibility of AISI 316L tube-fitting nuts surface machined by three different tools
We focus on the test and characterization results of the annulus and chamfer

Summary

Introduction

SCC, stress–corrosion cracking, a form of material degradation that is sensitive to the environment, happens under synergistic action of chemical attack from the environment and applied stress [1]. SCC has been recognized as one of the leading failure causes of 300-series austenitic stainless steels [2,3]. Within this series, AISI 316L possesses good mechanical properties and immunity to impingement attack [4]. AISI 316L is widely used in marine applications, e.g., in compression tube-fitting and recirculation piping [2]. The corrosion resistance of AISI 316L is insufficient in many. AISI 316L in seawater undergoes localized pitting corrosion. In the presence of tensile stress, pitting corrosion can become a precursor of crevice corrosion and SCC [6]

Methods

Results

Discussion

Conclusion