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 de-alloyed layer exhibits extremely fine nanoporosity.
In the pulp and paper industry, chemical pulp mills utilize processes that save energy and comply with the tightening environmental standards. Structural reliability of high-performance recovery boilers and evaporation plants can be enhanced by using advanced stainless steels that have higher resistance to stress corrosion cracking (SCC) than the current solutions. The possible new biorefinery processes of the pulp and paper industry will change the conventional pulping and chemical recovery processes and have a strong effect on corrosion of stainless steels. The process changes are among others pre-extraction of hemicellulose before cooking and fraction of black liquor before or during evaporation. In addition, more detailed knowledge on how and when these plants will face an SCC hazard will improve the risk management in operating such industrially important facilities. This will enhance safety at work and helps to avoid catastrophic failures that may pose a threat to the environment. The clarification of the influence of the alloying elements of stainless steels on corrosion properties in hot caustic environments is thus very important. Additionally, the prices of the principal alloying elements of these steels are high and thus the knowledge on the optimal alloying for a specific application will become an important advantage for cost-effective production and use of stainless steels. There has been relatively little systematic work on the mechanisms of SCC in caustic environments. The most common mechanistic SCC models are based on the anodic dissolution processes and on the effects of hydrogen. The most widespread concept in the SCC research is that the crack growth occurs by a highly localized anodic dissolution. Recently, a detailed study was reported, which clarifies the nature of the de-alloying process and establishes firm links with other metal-environment systems that show de-alloying and associated SCC in strong caustic environments (Deakin et al. 2004).
