Ni-Cr-W cast alloy is considered one of the promising materials for high temperature air heaters in gasification and ash melting systems due to their favorable properties for both corrosion resistance and high temperature strength, as well as economical concerns over costs. The exposure test on a demonstration plant indicates that these cast alloys showed their specific temperature dependence while corroded in a high temperature exhaust gas environment.
The corrosion rate rapidly increased in a temperature range from 800°C to 900°C, and slightly decreased when the temperature went beyond that range. Chloride was formed inside the alloys and oxidized to produce chlorine. This chlorine was then involved in another chlorination. Thus, consecutive reactions of the chlorinated corrosion occurred. This chlorine recycling inside the alloys can be a cause of severe corrosion. The rapid corrosion increase in the temperature range from 800°C to 900°C was assumed due to the progress of the chlorine recycling caused by the melting of the formed CrCl2. Meanwhile, the decline of the corrosion rate at higher temperatures could have been due to the suppression of the recycling by the formed chloride becoming volatilized and dispersed for fugacity from the alloys.
