Corrosion behavior of austenitic stainless steel UNS S30403, duplex stainless steels UNS S32101, UNS S32304, UNS S32205 and UNS S32507 as well as ferritic stainless steels UNS S43932 and ASTM 444 was evaluated with U-bend specimens in alkaline solutions containing 500 g/l NaOH, 100 g/l NaOH + 55 g/l Na2S (hot white liquor [HWL]) and 15 g/l NaOH + 150 g/l Na2S (hot black liquor [HBL]) at temperatures of 140 and 190 °C. Corrosion rates were determined from weight losses of the samples. The susceptibility to stress corrosion cracking (SCC) was characterized with optical microscopy and field emission gun scanning electron microscope (FEG-SEM). In addition, the corrosion products were analyzed with energy dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD).
Based on the results the highest corrosion rates were obtained in 500 g/l NaOH solution, whereas in hot black and hot white liquors the corrosion rates were significantly lower. Increase in temperature resulted in higher corrosion rates, especially in pure NaOH solutions. Molybdenum alloying enhanced corrosion particularly at 190 °C and in the presence of Na2S, whereas chromium seems to improve the corrosion resistance. Severe SCC was observed in the austenitic and ferritic stainless steels. The EDS and XRD analyses showed that different corrosion products form in sulfide containing solutions than in pure NaOH solutions. In pure NaOH solutions various oxides are formed, but in the presence of Na2S also sulfide containing corrosion products, such as iron-nickel sulfides, are formed.
