After careful testing, Titanium Grade 2 (UNS R50400) and Grade 7 (UNS R52400) were selected for the high-pressure part of the wet oxidation process (300°C, 150 bar). The definition of an operating window, which correlates pH, chlorine, phosphorus, sulfur, and related salts and acids in the process stream, was necessary for the safe operation of the equipment. Not only were the selection of the materials and the definition of the operating window carefully elaborated, but also the operating conditions and especially the passivation of the titanium surface. Problems were observed due to inadequate specification of the surface quality, the precipitation behavior of the water mix and the build up of encrustation. Furthermore, minor changes in the equipment design have lead to dramatic changes of the flow and to rapid erosion. Titanium was the best material choice for this highly oxidizing service, but some measures had to be taken after careful testing to maintain the passivity of the material. All problems found were caused by deviations from the specifications. It was clear, that small changes greatly affect the resistance of titanium. Once the oxide layer was destroyed, titanium exhibited high corrosion rates.
Keywords: titanium, temperature, pressure, corrosion, erosion, wastewater, wet oxidation