Stress Corrosion Cracking of Type 304L Stainless Steel in Sodium Sulfide-Containing Caustic Solutions

Austenitic stainless steels (SS) are known to be susceptible to caustic stress corrosion cracking (SCC) at temperatures higher than ~121°C. However, when sulfides are present in caustic solutions the SCC of austenitic SS has been reported to occur at lower temperatures. Hot sulfide-containing caustic solutions are of interest to various industrial chemical processes, including pulp and paper as well as nuclear industry. Hot sulfide-containing caustic solution (white liquor) is used in the kraft pulping process in the pulp and paper industry. Different concentrations of sulfide and hydroxide concentrations, along with other chemicals, are encountered in different process streams of the pulp mills and therefore are of interest for further investigation. One example of such a problem is cracking of composite tubes. These tubes have an outer layer of Type 304L (UNS S30403) SS and an inner shell of c-steel and are typically used for the floor and lower furnace waterwalls in kraft recovery boilers. Field observations and laboratory data have indicated that the SCC of composite tubes occurs predominantly during the boiler shutdown. One of the corrosive environments under scrutiny is the “stagnant floor water” that is rich in sulfide and hydroxide due to their high solubility, which may form on the boiler floor during shutdown. This paper discusses results of a systematic study conducted to understand the role of sulfide in caustic solutions on SCC of austenitic SS at temperatures as low as 50°C where caustic SCC is generally not expected in hydroxide solutions alone.