Effect of coatings on microstructure and hot corrosion resistance of Ni₃ Al base alloy IC6AE

Al-Si and NiCr-CrAl coatings for an equiaxed Ni₃Al base alloy IC6AE were prepared respectively by the slurry and powder pack cementation methods. The hot corrosion resistance tests for the three kinds of IC6AE specimens with the above two coatings and without coating were carried out by coating salt (5wt% NaCl + 95wt% Na₂SO₄) corrosion at 900°C for 20 h, 40 h, 60 h, 80 h and 100 h. The experimental results showed that NiCr-CrAl coated specimens had the best hot corrosion resistance compared with the specimens of Al-Si coating and without coating, and Al-Si coating had no evident hot corrosion resistance improvement compared with the specimens without coating. Microstructure analysis showed that the specimens of alloy IC6AE without coating were corroded seriously after coating salt corrosive test at 900°C for few hours. After 100 h test, a porous loose corroded layer which was composed of oxide of Mo and Ni formed, and it was obvious that this kind of layer had no protective properties. While in the case of Al-Si coated specimens, the corroded layer on the specimens' surface composed of not only large amount of Mo and Ni oxide but also small amount of Al₂O₃ and NiAl₃ phases, which may have some protective effect for the specimens. For the situation of NiCr-CrAl coated specimens, the continuous and compact layer composed mainly of a large volume fraction of Al₂O₃ and NiAl₂O₄ phase and a small amount of oxide of Mo and Ni, which had better protective effect for the alloy, and therefore the hot corrosion resistance properties were improved greatly.