Beyond the Aqueous Corrosion Paradigm: Solvent-Dependent Dissolution of Engineering Alloys in Non-Aqueous Acidic Halide Media

Abstract

Corrosion behavior in non-aqueous acidic-halide environments remains poorly understood despite the widespread use of chlorinating reagents and chloride-containing species in pharmaceutical, agrochemical, and specialty chemical manufacturing. In the present study, we investigate the performance of engineering alloys in non-aqueous acidic-halide media using corrosion testing, inductively coupled plasma (ICP) analysis, electrochemical measurements, and flow-loop experiments. Severe corrosion was observed during an industrial chlorination process performed in a C-22 reactor, a Ni-Cr-Mo-W alloy widely used in aggressive chemical service. Follow-up testing demonstrated that the phenomenon was not unique to C-22. Addition of acetonitrile (MeCN) increased the corrosion rates of corrosion-resistant Ni-alloys C-276 and C-2000 by approximately two orders of magnitude relative to analogous MeCN-free solutions, and similar susceptibility was observed across multiple engineering-alloy families. Alloy degradation occurred in environments containing chlorinating agents, including thionyl chloride (SOCl 2 ), sulfuryl chloride (SO 2 Cl 2 ), and oxalyl chloride ((COCl) 2 ), as well as acidic chloride-containing species generated from hydrogen chloride and acidified chloride salts. ICP analysis detected substantial concentrations of major alloying elements, linking material degradation to process-stream contamination. Corrosion testing, ICP analysis, and electrochemical measurements demonstrate that severe alloy degradation occurs when acidic chloride-containing species are combined with solvent environments that support continued alloy dissolution. Increasing water content progressively promoted passivation and suppressed dissolution, whereas flow dramatically accelerated attack, increasing the corrosion rate of C-276 in a 1 wt.% SOCl 2 -MeCN solution from 12.1 to 540 mils per year (mpy) (0.31 to 13.7 mm/y) at ambient temperature. These findings support a framework involving passive-film destabilization and solvent-dependent continuation of metal dissolution and offer new guidance for materials selection beyond the traditional aqueous paradigm.

Abstract as supplied by the indexing source. © the authors / publisher.