51312-01331-Shot peening of SS 316L hydrometallurgy tanks to reduce chloride stress corrosion cracking susceptib

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Product Number: 51312-01331-SG

ISBN: 01331 2012 CP

Author: Joel Fortin

Publication Date: 2012

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The stainless steel tanks of a zinc hydrometallurgy plant have experienced localized corrosion since they were built 20 years ago. The diluted sulphuric acid solution with 300 ppm chloride ions is known to be particulary aggressive. In order to reduce the susceptibility to pitting and stress corrosion cracking the repair procedures including welding and passivation were optimized. Since it is sometimes suggested in the litterature that compressive surface residual stresses could reduce the susceptibility to localized corrosion this hypothesis was tested in the laboratory by shot peening 316L welded and stressed samples that were immersed in the hydrometallurgy solution. The technique was also tested in the field by shot peening a 316L tank that had undergone repairs. The 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 occurence of pitting. Since the bottom of the pits eventually reached a depth where the tensile stress was high stress corrosion cracks were initiated and propagated more rapidly than in the case where no shot peening was done.