CYCLIC HOT CORROSION OF AL DIFFUSION-COATED NI3AL AND NI3SI IN NA2SO4-NACL MIXTURE

Cyclic hot corrosion behaviours of Al diffusion coated Ni3Al and Ni3Si Were examined by using fused Na2SO4-NaCl mixtures; Specimens were immersed in fused salts at 1173 K for 10.8 ks and then surface scales were removed. This process was repeated 5 cycles. Ni3Al showed high corrosion loss under first cycle hot corrosion. X-ray diffraction analysis showed that Al2O3 was not detected in the scales. High quantity of Al and high pH value were detected in the dissolved solution of mixtures after corrosion test. It is thought that Al2O3 dissolves in the mixtures as the sodium aluminate and so this alloy dose not indicate the hot corrosion resistance. Hot corrosion resistance of NiAl which was formed as the outside layer of Ni3Al during Al diffusion coating was improved about 3 orders of magnitude for comparison with Ni3Al under first cycle hot corrosion. But corrosion loss of this alloy under the cyclic hot corrosion was increased with increasing the number of cycles. On the other hand, Al diffusion coated Ni3Si and NiAl+0.6 mass%Si alloys indicated excellent corrosion resistance under the cyclic corrosion. Al2O3 layer was observed on the surface after the cyclic corrosion. It is clear that the addition of slight amount of Si to NiAl accelerates the formation of Al2O3 layer and improves the cyclic corrosion resistance in fused Na2SO4-NaCl mixtures.