Ni-Cr-Mo alloy are known as one of the most versatile Ni-based alloys in resisting corrosion. Additions of Cr and Mo have been proven to be the cause of the good corrosion resistance behavior. The role of Cr and Mo in resisting localized corrosion particularly, pitting corrosion was studied. Ni-Cr-Mo alloys were fabricated with twenty four unique compositions. Polarization experiments were conducted on these alloys in 0.5M NaCl and low pH chloride solution at varying temperatures from 45 to 90°C to obtain corrosion parameters namely, pitting and repassivation potentials. Multiple linear regression analysis was performed to construct a mathematical expression that correlate the pitting and repassivation potentials to the alloying content at each environment. This expression allows us to roughly predict the corrosion behavior of the Ni-Cr-Mo alloys through Cr and Mo contents only. It appears that Cr content is more dominant than Mo content in raising the pitting potential in neutral chloride condition. The effect of both Cr and Mo are quite uniform in affecting the repassivation potential values of the alloys in neutral chloride. The effect of Mo is greater than Cr in the low pH and higher temperature solutions in affecting both pitting and repassivation potentials.
Pitting corrosion is commonly preceded by the occurrence of metastable pitting. Thus, one can better understand the pitting process of an alloy through its metastable pitting behavior. Rigorous metastable pitting study was conducted on Ni-Cr-Mo alloys with Cr content varying from 20 to 29 wt.% and Mo content varying from 12 to 25 wt.%. Potentiostatic experiments were performed in 90°C of 0.5M NaCl to promote the metastable pitting incidence. Analysis was done on the potentiostatic data to better characterize the metastable pitting behavior of the Ni-Cr-Mo alloys. It was observed that a higher Mo content in the Ni-based alloy is responsible for the lower peak current values and slowing down the growth rates of the faster growing pits. The Cr content seems to impact the repassivation rates of the metastable pits where higher Cr content increases the repassivation rates.
The repassivation behavior of Ni-Cr-Mo alloys was studied through an electrochemical testing consisted of potentiodynamic-galvanostatic-potentiodynamic polarizations. It was found that higher Mo-containing alloys were extremely corrosion resistant given that they had Cr content of at least 20 wt.%. Alloys with higher Cr and higher Ni content had higher repassivation potentials when compared to another alloy with similar Mo content. Voltage component analysis found surface overpotential and reversible potentials of the alloys determined the repassivation potential values. The crevice growth rate of Alloy C-22 was ohmically controlled. XPS analysis found that Cr(III) oxide was the main passivating element for the Ni-Cr-Mo alloys. Mo(VI) was enriched on the crevice area. Mo in 0, +4, and +6 oxidation states were detected on the transpassive dissolution layers that formed on higher Cr and Mo-containing alloys. Cr(III) hydroxide peak was seen on the crevice site and on the transpassive dissolution layers. Ni elemental peak was observed on highest Cr and highest Mo-containing alloys only.