Duplex stainless steels (DSS) have generally performed very well in the pulp mill environment. However, some corrosion and stress corrosion cracking (SCC) of duplex stainless steels has been reported in different pulping liquors. Studies have shown that corrosion and SCC susceptibility of duplex stainless steels depend on alloy composition, microstructure and temperature. In this study the effect of sulfide containing caustic environment (pulping liquor) and material properties (DSS alloy composition and microstructure) on the corrosion and SCC of DSS in pulping liquors (synthetic white liquor) was evaluated. During metal fabrication processes, localized areas of DSS may be exposed to different cooling rates or aging temperatures, which may lead to a change in the microstructure or produce precipitates in these regions. This change in microstructure may in turn affect the general and localized corrosion or SCC susceptibility of the affected area as compared to the rest of the metal. Hence the effect of different annealing and aging temperatures as well as cooling rates on the microstructure and corrosion behavior of 2101 and 2205 DSS in pulping liquors was evaluated. 2205 and 2304 welded DSS specimens were tested for SCC susceptibility in pulping liquor. Reduction in molybdenum content in welded 2304 DSS specimens did not seem to provide any protection against SCC as compared to 2205 DSS.
Duplex stainless steels (DSS) are characterized by a two phase microstructure consisting of a mixture of austenite and ferrite phases. These phases are present in almost equal volume fractions but may vary depending upon the composition and thermo-mechanical treatments. Owing to the balanced microstructure, these steels have excellent corrosion resistance and higher mechanical strength as compared to austenitic stainless steels. DSS are increasingly being used in the chemical and petrochemical industries, in construction of oil platforms, in nuclear power plants and in the pulp and paper industry.
However, deterioration in mechanical properties and corrosion resistance may occur if the duplex stainless steels are exposed to high temperatures during fabrication and thermo-mechanical processes. Exposure of DSS to temperatures above ~350 °C can result in precipitation of undesirable intermetallic phases such as carbides, nitrides and sigma phases, depending upon the temperature and time of exposure[1]-[4]. Presence of these precipitates may adversely affect the corrosion resistance of DSS and may make them more susceptible to localized corrosion. One of the detrimental effects of the formation of these precipitates is chromium depletion in the matrix, which may affect the passive film and make these sites prone to localized corrosion like pitting. If other conditions are conducive, localized corrosion areas may act as precursors to stress corrosion crack initiation. Pitting may also start due to the galvanic effects between the matrix and the precipitate, depending upon the environment.
Two hardening and embrittlement processes that have the most pronounced effect on the mechanical and corrosion properties of DSS are the formation of sigma (s) phase at temperatures in the range of 700 °C to 900 °C and the precipitation of Cr-rich (a') phase at temperatures in the range of 300 °C to 600 °C, known as "475 °C embrittlement"[5][6].
