Corrosion of High-Temperature Alloys in Chloride-Containing Supercritical Water Oxidation Systems

Supercritical water oxidation (SCWO) is a promising and effective method for the oxidation of organic wastes. Its high solubility for organic compounds and its special physical properties (i.e., density and viscosity) result in a key advantage over standard processes such as incineration. During oxidation of chlorine organics, severe corrosion attack of most of the materials has been observed as a result of the formation of hydrochloric acid (HCl). An investigation was conducted of corrosion of high-temperature alloys such as alloy 214 (UNS N07214), alloy 602CA (UNS N06602), alloy G-30 (UNS N06030), alloy 625 (UNS N06625), and alloy 686 (UNS N06686), exposed at 400 bar (40 MPa) and 420°C for 24 h in a dichloromethane (CH₂Cl₂)/hydrogen peroxide (H₂O₂) mixture with ~ 2,000 wppm chloride (Cl₂). Weight losses of all the samples varied between 1.3 mm/y for alloy G-30 and ~ 31.6 mm/y for alloy 686. Alloy 214 showed increased corrosion for the preoxidized material, whereas the corrosion rate of alloy 602CA was reduced by a factor of three as a result of preoxidation at 900°C prior to SCWO exposure. Analysis revealed that alloy 686 lost most of its nickel down to a depth of ~ 150 μm. Alloy G-30 showed good corrosion behavior as a result of formation of a protective oxide layer during the exposure. The layer was homogeneous, with a thickness of ~ 12 μm, but slight nickel leaching was observed.