Stress Corrosion Cracking Susceptibility of Ni- and Cr-Plated 304 Stainless Steel in H<SUB>2</SUB>SO<SUB>4</SUB>-NaCl Solution

Stress corrosion cracking tests were done on surface modified 304 stainless steels in a 2.5 kmol/m3 H2SO4-0.4 kmol/m3 NaCl sloution in order to know the effect of treatments by diffusion coating (Si, Ti and Al), plating (Cr and Ni) and the combination of these methods on the SCC susceptibility in the solution.The time to failures of Cr-plated specimens and Cr plated specimens after Si diffusion coating were prolonged by 3 to 4 times compared with that of 304 stainless steel under an applied stress of 90% of 0.2% proof stress. The time to failure of the Cr-plated specimen after Ni plating increased with increasing diffusion time at 1373 K and the SCC susceptibility of the specimen diffusion-treated for 14.4 ks was improved by 1 order compared with that of 304 stainless steel under an applied stress over the proof stress. The corrosion rate of the Ni-plated specimen was remarkably suppressed but the initiation of intergranular corrosion under an applied stress accelerated the SCC. Such intergranular corrosion of Ni-plated specimen was controlled by the combination of Cr plating and the diffusion treatment at 1373 K. The composition of the specimen in the middle of the diffusion layer corresponded to that of the high Ni stable austenitic phase.It is thought that the surface modification of 304 stainless steel for the inhibition of SCC in a H2SO4-NaCl solution depends largely on the improvement in corrosion resistance and the composition of a stable austenitic phase.