Effect of coating degradation on the hot corrosion behaviour of directionally solidified superalloy

The effect of coating degradation on the hot corrosion performance of coated alloys was investigated in Na2SO4/ NaCl (75:25, wt./wt.) salt at 900 degrees C. Degraded coatings were obtained by thermal exposure before the hot corrosion test. The results indicate that when the beta-NiAl phases within the coating were completely depleted, the hot corrosion process was delayed due to the Al-rich interdiffusion zone (IDZ) formed after the 900 degrees C thermal exposure to provide Al for the healing of the Al2O3 layer. Nevertheless, the Cr-rich IDZ formed after 1000 degrees C thermal exposure failed to repair the Al2O3 layer, causing the alloy to corrode aggressively even though Cr has been substituted for Al to participate in the corrosion process.