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 min 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 of
time-of-flight secondary ion mass spectrometry (ToF-SIMS). In addition,
the surface morphologies were imaged with scanning electron microscopy
and energy dispersive x-ray spectroscopy (SEM-EDX) and the depth
profiles mapped with x-ray photoelectron spectroscopy (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.