The phenomena of hydrogen induced cracking (HIC) in 304 stainless steels was considered in a hydrogen pressure and stress corrosive atmosphere. Microstructures with chloride pits and stress corrosion cracks around the HIC were analyzed by SEM/EDS. Abnormal phase transformations induced by the hydrogen were analyzed using TEM and diffraction. In the hydrogen pressure atmosphere, pits and pores were observed on the surface of the 304 stainless steels. In addition, it was determined that Cl, an etchant component, was concentrated at a high concentration in the pits. SCC (stress corrosion cracking) was induced in the Cl atmosphere by stress caused by the abrasive embedded in the pits. It was assumed that the SCC mechanism is similar to HIC in that it occurs in the surface tensile stress and Cl atmosphere and is accompanied by grain boundary cracks similar to IGSCC (inter-granular SCC). The deformation induced phase transformation accompanied by planar slip should be related to the main cause of HIC in the hydrogen pressured atmosphere. Abnormal forbidden spots between the main diffraction spots were induced by the HIC in the hydrogen attacked area, where the microstructure was hardened. Understanding the HIC mechanism related to chloride corrosion can be used to assess the fitness of austenitic stainless steels for uses where there is a possibility of various susceptible cracking in hydrogen and chloride atmospheres.
Austenitic stainless steel SS304 is vulnerable to Cl atmosphere SCC (stress corrosion crack). In this study, SCC phenomena related to stress and corrosion composition were analyzed to identify the mechanism for SCC initiation and propagation in SS304. The microstructure and mechanical properties resulting from crack propagation were analyzed by OM, SEM/EDS and micro Vickers hardness tests. The abnormal phase transformation induced by the SCC was analyzed by TEM and diffraction. As a result of these analyses, the shape of SCC was observed to form a branched type crack, which was related to etch pit patterns on the etched surface due to the austenitic fcc (face centered cubic) lattice slip. In addition, the high concentration accumulation of Cl and S components at the SCC site, observed by SEM/EDS, indicated that the SCC was affected by the corrosive atmosphere. The SCC crack propagation was accompanied by hardening, which is believed to be associated with the mechanism of hydrogen embrittlement. High resolution TEM analysis found abnormal satellite diffraction points in the SCC high hardness region. This means that a superlattice phase with high hardness values is formed near the SCC region. And the HIC (hydrogen induced crack) effect, a kind of hydrogen embrittlement, was also influenced by the hardened superlattice phase. It is assumed that the SCC and HIC are similar phenomena produced in the same stress and corrosive atmosphere by superlattice phase transformation.
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