The Localized Corrosion of Ni-Cr-Mo-W Alloy UNS N06022 in Aprotic Solvents Containing Chlorides

Abstract

Syntheses of pharmaceuticals often require a series of intermediate steps involving reactions in aprotic organic solvents. Some of these solutions can be aggressive towards the metallic materials that constitute the reaction vessels used, so often corrosion-resistant alloys (CRA) such as the Ni-Cr-Mo-W UNS N06022 are employed. Ni-Cr-Mo-W alloys are known for their outstanding corrosion resistance in reducing and oxidizing aqueous solutions as a result of their passive oxide film. However, the application of passive alloys in nonaqueous solvents raises additional concerns due to: i) limited availability of passivating species; ii) different behavior of aggressive halides and acids as a result of the solvent’s physicochemical properties.The aim of this work is to investigate the corrosion of Ni-Cr-Mo-W alloys in aprotic organic solvents and to understand the influence of properties of the solvent on the susceptibility to pitting and to uniform corrosion, at room temperature. A series of experiments was carried out in acetonitrile, dimethylacetamide (DMA) and dimethylformamide (DMF) solutions containing HCl or tetrabutylammonium chloride (Bu4NCl), and with the oxidant 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ). A combination of electrochemical tests and subsequent microscopy has shown that N06022 corrodes uniformly in acetonitrile-HCl solutions at much lower HCl concentrations than in aqueous solutions. In DMA- or DMF-HCl solutions, however, N06022 is passive, but undergoes pitting upon anodic polarization. Furthermore, N06022 undergoes pitting in chloride concentrations as low as a few mM in neutral solutions of all aprotic solvents tested. Depending on solvent and concentration, residual water can inhibit or promote corrosion, influencing anodic kinetics and changing corrosion morphology.Ultimately, this study aims to understand why Ni-based CRAs are much more susceptible to localized corrosion in aprotic organic solvents than in water, and explain how the properties of the solvent – or mixture of solvents – can impact corrosion behavior. This work will provide fundamental understanding of the passivity of Ni-based alloys in organic solvents, assisting the selection of materials for nonaqueous applications.