Selective Dissolution and Stress Corrosion Cracking Susceptibility of Austenitic and Duplex Stainless Steels in Sodium Sulfide Containing Alkaline Solutions

摘要

The role of de-alloying in stress corrosion cracking (SCC) of the austenitic EN1.4301 (UNS S30400) and duplex stainless steels EN 1.4362 (UNS S32304) and EN 1.4462 (UNS S32205) was evaluated in simulated hot liquor environments. The samples were tested at 190°C using U-bend specimens in 100 g NaOH + 55 g Na_2S and 15 g NaOH + 150 g Na_2S caustic environments simulating hot white (HWL) and hot black (HBL) liquors, respectively. SCC and general corrosion was detected in the steel surface layers. In austenitic stainless steels, severe cracking was observed throughout the samples in both solutions. In duplex stainless steels, cracking was observed only in the affected surface layer in both solutions. In EN 1.4301 steel, selective dissolution of Fe and Cr and enrichment of Cr, Ni, S, O and Na was observed in the affected surface layer and on the wakes of the large SCC cracks depending on the solution. In HWL, Na was clearly enriched and Cr was both selectively dissolved and enriched at different parts of the affected surface layers. In HBL, Na enrichment was lower and Cr was selectively dissolved in the affected surface layers. In the duplex stainless steels EN 1.4362 and EN 1.4462, dissolution of Fe, Cr and Mo and/or enrichment of Ni, Cr, Mo, Na, S and 0 took place depending on the steel and test solution. Cr content of both steels remained unaffected in HBL. In HWL, Cr was enriched in the surface layer in the Mo-alloyed steel (EN1.4462) and remained unaffected in the non-Mo-alloyed steel (EN 1.4362). In HWL, Mo dissolved selectively from the affected surface layer, whereas in HBL Mo enriched in the surface layer in the Mo-alloyed steel. Na was enriched in the affected surface layer of both duplex stainless steels immersed in HWL. Immersion in HBL resulted in a lower Na enrichment in the affected surface layer in the Mo-alloyed steel as compared with that in the non-Mo-alloyed steel. The results suggest that selective dissolution plays an important role in SCC of stainless steels in alkaline environments.