Surface decontamination of protective duplex oxide layers on stainless steel waste using deep eutectic solvents

The decontamination capabilities of deep eutectic solvents (DESs) formed from choline chloride (ChCl) and p-toluenesulfonic acid monohydrate (PtsA), ChCl:PtsA, under different conditions (hydrated, heated, and agitated) were tested with simulant oxidized stainless steel 304 specimens. Although the leaching rates were satisfactory under all conditions, hydrated and stirred ChCl:PtsA at 60 °C showed the fastest leaching rate of 0.1647 mg/min. Oxidized specimens with an average mass gain of 1.2 ± 1 mg were leached, and their masses were reduced by 558 ± 22 mg after 26 h. These results were understood by improved physical properties of ChCl:PtsA upon hydration. Metal oxide solubility of CoO and NiO increased with water, and those of Cr₂O₃ and Fe₃O₄ decreased with hydration. Importantly, the use of choline chloride-based DESs in decontamination applications may significantly reduce the cost of decontamination because these DESs can be mass-produced and their components are both easily obtainable and economical. Also, DESs are biodegradable and eco-friendly. The different speciation of Co and Ni, which bond with Cl^-, compared with Fe and Cr, which bond with H₂O, illustrated the potential for a metal recovery for secondary liquid waste reduction.