The formation of nickel-rich layers on austenitic stainless steels in strong caustic solutions was reported
in 1970’s. 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 and duplex stainless steel AISI 304 (EN1.4301) and S322205 (EN1.4462) is
evaluated in 50% NaOH and also in NaCl/Na2S containing caustic environments (pulping liquors) at
413 K (140ºC) and 463 K (190ºC).
The results show that selective dissolution of alloying elements occurs on surface of the specimens and
at the crack tip and strain is necessary for selective dissolution. The proposal that the rate determining
step in SCC is generation of vacancies by selective dissolution at the crack tip will be confirmed in the
further detailed examinations. Chromium, iron and molybdenum are dissolving selectively and/or
precipitating during the process which affects the general corrosion and SCC susceptibility depending
on the alloy composition and microstructure of the steel. FEG-SEM examination shows that the dealloyed
layer exhibits extremely fine nanoporosity.
Key words: selective dissolution, de-alloying, duplex stainless steel, austenitic stainless steel, stress
corrosion cracking, caustic solution