Chloride-induced transgranular stress corrosion cracking of austenitic stainless steel 304L
Creators: name: Scatigno, Giuseppe Giovanni
Publisher: name: Imperial College London
Year: 2016

description: Stress corrosion cracking (SCC) of austenitic stainless steels has been a known failure mode for more than 80 years and it continues to be a major cause of concern in the nuclear industry. The so-called nuclear grades, such as 304L, contain low levels of C and are therefore hard to sensitise, which is a major problem with high C grades, and these low C grades mainly fail by transgranular SCC. The effect of cold work (CW) has long been known to have a detrimental effect on SCC performance of a stainless steel component. CW is readily introduced in engineering components, through manufacturing history, or implementation, i.e. welding and hammering during fitting. The aim of this thesis is to systematically assess the role of CW in Cl-induced atmospheric SCC in 304L grade austenitic stainless steel. 304L is widely used in the nuclear industry, for both the primary cooling system of nuclear power plants and dry casks for interim storage of spent nuclear fuel. CW was applied in uniaxial tension to levels of 0, 0.5, 1, 2, 5 10, 20, and 40%. The specimens were loaded in a jig to produce a uniform stress of 60 MPa on the top surface and corroded under atmospheric conditions at 75°C, 70% relative humidity, using MgCl2, for 20 days. The role of applied stress (from 60-180 MPa), on SCC susceptibility was investigated at a fixed level of CW (chosen as 10% CW after preliminary experiments) using indicators such as crack density. Secondary and transmission electron microscopy, electron back-scattered diffraction, focused ion beam and secondary ion spectroscopy were the main characterisation techniques used. The maximum susceptibility to SCC was observed between 0.5-5% CW, while 20 and 40% CW did not exhibit cracking. The characterisation of the samples tested provided evidence that Cl is found ahead of the crack tip, whereas oxygen is not, which was never previously observed in the literature. Secondary ion mass spectroscopy and transmission electron microscopy were both used to observe and study the presence of Cl. Simulations such as SRIM and Casino 3.2 were used to confirm that the findings were not a technique artefact. Evidence of dealloying was also observed during the characterisation. Dealloying has long been deemed unlikely in Cl-SCC of austenitic stainless steel, but recent work showed that this may also be an available mechanism for SCC as more and more of the characteristics features of dealloying are observed. The dealloying signs observed were: nanoporosity, found on fracture surfaces; severe striations, heavy dissolution of slip planes; element migration (areas of light and dark contrast in back scattered electron images, dictated by the migration of Cr); cleavage failure; Cr and Ni migration around the crack. The role of salt loading was investigated. Different levels of salt deposition were tested in order to obtain an engineering threshold for salt deposition, namely: low (&lt;5.70 x 10-3 g cm-2), medium (5.70 x 10-3–1.42 x 10-2 g cm-2) and high (&gt;1.42 x 10-2 g cm-2). A linear relationship was observed between level of salt deposited and both crack density and corrosion area. However, more work is necessary to obtain a threshold.