Investigating the Influence of Cr and Mo Additions to Commercial Ni-Based Alloys Exposed to Neutral and Acidic Chloride Solutions

The corrosion behaviour of four commercially available Ni-based alloys is presented for both natural and potential-controlled corrosion in chloride solutions. Electrochemical evidence suggests a balance of Cr and Mo is essential to maintaining passive film stability in chloride solutions, especially those in which acidic conditions may develop. In near-neutral solutions, increased Cr content results in lower corrosion rates and improved passive properties; however, an increase in Cr content above 15 wt.% provided only minor additional benefits. In acidic solutions, Mo content is essential to corrosion resistance and imparts two major benefits: increased film stability and rapid repassivation of breakdown events. Since localized corrosion and the critical chemistry which accompanies these processes are of concern in many chloride-containing applications, a delicate balance of Cr and Mo must be considered; although optimal concentrations have yet to be determined.