51312-01202-Selective Dissolution and Stress Corrosion Cracking Susceptibility of Austenitic and Duplex Stainles

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Product Number: 51312-01202-SG

ISBN: 01202 2012 CP

Author: Pekka Pohjanne

Publication Date: 2012

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ABSTRACTThe formation of nickel-rich layers on austenitic stainless steels in strong caustic solutions was reported in 1979. Recently a more detailed study has shown the nature of the de-alloying process and established firm links with the other metal-environment systems that show de-alloying and associated stress corrosion cracking (SCC) in strong caustic environments. In this study the role of de-alloying in SCC of the austenitic EN1.4301 (AISI 304) and duplex stainless steels EN1.4362 (UNS S32304) and EN1.4462 (UNS S32205) was evaluated with U-bend specimens in NaOH and NaOH + Na2S containing caustic environments at 190ºC. In both environments SCC and general corrosion was detected. Selective dissolution of Fe Cr and Mo and/or enrichment of Ni as well as Mo took place in the reaction layer of the studied steels depending on steel and the test solution. In pure 50% NaOH in the austenitic stainless steel selective dissolution of Fe and Cr and enrichment of Ni in the reaction layer was observed and the cracks grew to the unaffected material. In the duplex stainless steels selective dissolution of Fe Cr and Mo took place both at the ferrite and austenite phases. Ni was enriched in the reaction layer in the former ferrite and austenite phases but cracks were found only in the reaction layer. The addition of Na2S to NaOH resulted in through thickness cracking in EN1.4301. Again selective dissolution of Fe and Cr and enrichment of Ni S and Na in smaller amounts in the reaction layer and the surfaces of the larger cracks was observed. In the EN 1.4362 duplex stainless steel the selective dissolution of Fe took place in the reaction layer and Ni was enriched in the reaction layer whereas the Cr content remained unaffected. Sulphur was enriched in the reaction layer. Cracks were found only in the reaction layer. In the Mo alloyed 1.4462 duplex stainless steel dissolution of Fe took place in the reaction layer and Ni and Mo were enriched in the reaction layer whereas the Cr content remained unaffected. Sulphur was enriched in the reaction layer. Again cracks were found only in the reaction layer. The results are in accordance to the field experience according to which duplex stainless steels experience less SCC as compared with austenitic grades. The results suggest that selective dissolution plays an important role in SCC of stainless steels in alkaline environments.