The effect of oxygen source on the reaction mechanism of potassium chloride-induced high-temperature corrosion

The role of two oxygen sources, air and water vapor, in the initiation of KCl-induced high-temperature corrosion was addressed with three different commercial alloys typically used in power plants. The focus was on the initiation of the corrosion reaction, so an exposure time of 120 minutes at 540°C was used under flowing conditions. The possible selectivity of oxygen in the corrosion reaction was studied by using two different stable oxygen isotopes, ¹⁶O in air and ¹⁸O in water vapor, and identifying the isotopes after the reaction with ToF-SIMS. In addition, the surface morphologies were imaged with SEM-EDX and the depth profiles mapped with XPS. Despite the chemical composition of the tested alloys, the alloy surface appears to favor a direct reaction with oxygen from water vapor, when available. On the contrary, the oxygen from air is more involved in reaction(s) forming potassium chromate (K₂CrO₄). The formation of K₂CrO₄ provides further evidence of the role of potassium in the initiation of KCl-induced high-temperature corrosion.