Corrosion of alloy 625 in high-temperature, high-pressure sulfate solutions

Corrosion phenomena of alloy 625 (UNS N06625) were investigated in oxygenated aqueous solutions containing sulfuric acid (H2SO4), sodium hydrogen sulfate (NaHSO4), or sodium sulfate (Na2SO4) and in deaerated H2SO4 solution. Tests were conducted at temperatures and pressures up to 500 degrees C and 38 MPa, respectively. Corrosion in the oxygenated acidic solution started similar to 150 degrees C with intergranular attack (IGA). Above 250 degrees C, the whole surface of the alloy was attacked, and shallow pits and deep IGA appeared. This behavior was explained by transpassive dissolution of the protecting Cr(III) oxide layer. Severe material loss occurred between 300 degrees C and 390 degrees C. At higher temperatures, only weak corrosion was detected, probably because of the lower density of the solution. Corrosion phenomena in oxygenated NaHSO4 solution were comparable but less severe than in H2SO4. In oxygenated Na2SO4 solutions, no corrosion was observed up to the maximum test temperature of 350 degrees C. In oxygen-free solutions, severe material loss occurred between 135 degrees C and 220 degrees C, The cathodic reaction was the reduction of sulfate, and the resulting potential was in the active region of the alloy. Above 230 degrees C, the alloy passivated, and corrosion rates were low.