Abstract
The stainless steel tanks of a zinc hydrometallurgy plant have experienced stress corrosion cracking (SCC) since they were built 20 years ago. The diluted sulphuric acid solution with 300 ppm chloride ions is known to be particularly aggressive. In order to reduce the susceptibility to stress corrosion cracking, the repair procedures, including welding and passivation, were optimized.
Since it is sometimes suggested in the literature that compressive surface residual stresses could reduce the susceptibility to stress corrosion cracking,1 this hypothesis was first tested in the laboratory by shot peening 316L welded and stressed samples that were later immersed in boiling MgCl2 according to the ASTM G362 standard. Following these tests, some samples were immersed for 3 months in a small tank in a laboratory in conditions as close as possible to the service conditions. Finally, the technique was tested in the field by shot peening (with glass beads) a 316L tank that had undergone repairs. The shot peening parameters were carefully selected so that the depth of the compressive layer was sufficient, without inducing exceedingly high tensile stress deeper in the shell.
The laboratory tests clearly showed that shot peening was effective at preventing stress corrosion cracking. The field test showed that, while shot peening prevented the initiation of stress corrosion cracks on the inside of the tank shell, it did not reduce the occurrence of pitting. Since the bottom of the pits eventually reached a depth (below the shot-peened compressive stress layer) where the tensile stress was high and also acted as a stress concentrator, stress corrosion cracks were initiated at the bottom of the pits and propagated through the shell of the tank.
