KCl-induced corrosion of Ni-based alloys containing 35–45 wt% Cr

Ni–(35–45)Cr–4Nb alloys containing different fractions of α-Cr were exposed to potassium chloride (KCl)-induced corrosion. The corrosion exposures were carried out for 168 hr at 600°C in a 15% (vol/vol) H₂O (g) + 5% (vol/vol) O₂ (g) + N₂ (g; balance) atmosphere using KCl-free (reference) and predeposited KCl samples. To mimic the KCl deposition in real boilers, 24 hr exposures where KCl vapor condensed continuously onto samples were also performed. The corrosion attack of the studied materials increased significantly when KCl was present compared to the KCl-free samples. For the KCl exposures, the corrosion attack drastically increased when a significant α-Cr fraction was present. α-Cr was either selectively attacked or dissolved through solid-state diffusion and a layered build-up of the outer external scale of K₂CrO₄ and chromia could be observed. For the in situ condensed KCl exposure, severe corrosion was observed already within the 24 hr exposure, indicating a higher corrosion rate compared with when KCl was predeposited.