High-Rate Dissolution of Corrosion Resistant Alloys in Nonaqueous Solutions at Room Temperature
Authors: Atz Dick Pedro , Beekwilder Michael , Kelly Robert G. 
Published: February 2020

Syntheses of pharmaceuticals often require a series of intermediate steps involving reactions in 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 UNS N06022, N06035 and N10362 are employed. Such materials are expensive, but their use is required to prevent contamination of the product by metal ions. Among the reagents commonly present in pharmaceutical processes is 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ), a strong oxidant that is widely used in the dehydrogenation of organic compounds. Understanding the interaction of DDQ with the alloys that constitute the reaction vessels is essential.

Ni-Cr-Mo alloys are known to exhibit 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 water and/or oxygen for passivation; ii) different behavior of aggressive anions and acids as a result of the solvent’s physical properties. Nevertheless, the literature on corrosion of CRAs is focused on behavior in aqueous solutions and little is known about the influence of quinones in corrosion in general. The aim of this work is to investigate the corrosion of Ni-Cr-Mo alloys in organic solutions containing DDQ.

Preliminary work was carried out with anhydrous solutions containing acetonitrile, ethanol, DDQ and HCl (or EtAlCl2) at room temperature. Mass loss tests have shown that alloy N06022 exhibits high-rate dissolution and crystallographic pits in such conditions. The corrosion rates observed after 24 h of immersion are greater than 10 mm/year, which is remarkably high when compared with the existing literature on corrosion of Ni-Cr-Mo alloys. The dissolution process appears to be caused by a synergistic effect between DDQ and HCl, in which corrosion rate, total mass loss, and corrosion morphology are strongly dependent on acidity and Cl- anion concentration.

In order to understand the electrochemical reactions that govern the dissolution observed, potentiodynamic polarizations and electrochemical impedance spectroscopy were performed, followed by surface characterization. Various solution compositions were tested in order to characterize the dependence of the corrosion of N06022, N06035 and N10362 on the variables of importance, namely time, water content, chloride concentration, acidity, and acid type. The highest corrosion current density is observed in the presence of both HCl and DDQ, where DDQ acts as oxidant and HCl enables active dissolution at anodic potentials. The influence of acidity and Cl- on the electrochemistry of DDQ were studied by performing cyclic voltammetries with a platinum working electrode in different solutions.

With support of results from exposure and electrochemical tests for different alloys, the relation between corrosion behavior and alloy composition was assessed. Ultimately, this investigation will provide understanding of the passivity of different Ni-based alloys in organic solvents, assisting the selection of CRAs for nonaqueous applications.