The influence of the environment factors and the surface treatment on Stress Corrosion Cracking of Type 304 Austenitic Stainless Steel

Stress Corrosion Cracking (SCC) was a common, often dramatic failure mode for many austenitic stainless steel systems. SCC occurs via a combination of three critical factors: a tensile stress (which can be applied and/or residual), a specific corrosive environment and a susceptible material [1,2]. During the past decade, a consensus had been reached about stress corrosion cracking starting from pits on the smooth samples [3,4]. The general assumption was that SCC, including Atmospheric Induced Stress Corrosion Cracking (AISCC), of austenitic stainless steels does not occur at temperature below the critical temperature, for example 50oC for type 304 stainless steel [5]. So far, it has been proven incorrect. The studies of H.E. Hannien et al. in 1979 [6], which performed SCC test using stainless steel U-bend specimens fabricated according to ASTM standard G30-97, showed that the SCC initiation was observed on the specimens deposited with simulated sea salt and exposed in the environmental test chamber at temperatures between 35 and 52oC. The amount of salt deposited on the specimen surfaces were between 0.1 and 10 g/m2 . In 2008, Tani et al. showed that there was measurable Trans-Granular Stress Corrosion Cracking (TGSCC) at the temperature of 298K but it was not clear whether this was a true threshold or whether cracking could have been seen at low temperature in longer experiments [7]. Recent published work by Phan and Cook et al. has observed AISCC at temperatures as low as 40oC of the U-bend specimens of parent material from the surface deposited salt particles exposed to a humid atmosphere [8,9]. This work described a series of test to evaluate systematically the effect of relative humidity (RH) and concentration of chloride ion deposited on the surface of 304 and 316 austenitic stainless steel samples on SCC initiation at low temperature. It was suggested that the SCC susceptibility of the stainless steel samples was both relating to the process of deliquescence and the increasing of RH. SCC cracks propagate most often by inter-granular path which is more common rather than trans-granular path in austenitic stainless steels.

This work described a series of test to systematically evaluate the effect of tensile stress, relative humidity and chloride ion of SUS 304 on SCC initiation at 40oC and 70oC. The distribution of cracks and the tree branch cracks were well presented.