Comparative corrosion behavior of Ni-Mo and Ni-Mo-Cr alloy for applications in reducing environments

The corrosive behaviors of Ni-Mo and Ni-Mo-Cr alloy for applications in reducing environments were analyzed. Alloys included in the study had nominal chemical compositions of Ni-27Mo, Ni-27Mo-Cr, Ni-27Mo-3Cr, and Ni-27Mo-4Cr in weight percent. All alloys were processed to sheets 1 mm in thickness. Specimens were solution annealed at 1065°C and then water quenched. Corrosion properties were determined from aqueous corrosion tests in boiling 20% Hcl. Corrosion rates were determined from weight loss measurements made on specimens after 24-h immersion in Hcl and extrapolated to mm/year. Light optical metallography, analytical electron microscopy and Auger electron spectroscopy were used to characterize the microstructure. The results show that critical additions of Cr to a Ni-27Mo alloy could be effective in decelerating the kinetics of long-range ordering to Ni₄Mo, which was found to have detrimental effects on corrosion resistance as well as mechanical strength.