Corrosion resistance of Ni-Cr-Mo and Ni-Mo-Cr alloys in different metallurgical conditions

Ni Cr Mo alloys offer an outstanding corrosion resistance in a wide variety of highly-corrosive environments. This versatility is due to the excellent performance of nickel in hot alkaline solutions and the beneficial effect of chromium and molybdenum in oxidizing and reducing conditions, respectively. Alloy C-22 (22%Cr-13%Mo-3%W) is a well known versatile member of this family. Recently, alloys C-22HS (21%Cr-17%Mo-1%W) and HYBRID-BC1 (17%Cr-22%Mo) have been introduced. Alloy C-22HS is age-hardenable. It exhibits corrosion resistance comparable to other Ni Cr Mo alloys and it can be strengthened to obtain approximately double the yield strength. Alloy HYBRID-BC1 is Ni-Mo-Cr and it was developed to deal with reducing acids with some oxidant content. Aging of Ni-Cr-Mo alloys lead to different microstructure changes depending on the temperature range. A Long Range Ordering (LRO) reaction occur in the range of 350 to 650ºC, producing an ordered Ni2(Cr,Mo) phase. Tetrahedral Close Packed (TCP) phases, precipitate at grain boundaries in the range of 600 to 1100ºC. The aim of the present paper was to compare the corrosion resistance of these alloys in the mill annealed and in different thermally-aged conditions. General and localized corrosion resistance in chloride solutions were assessed.