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| DEGRADATION OF ALUMINIDE COATINGS ON Ni-BASE ALLOYS IN SULFUR-CONTAINING ATOMSPHERE |
| Du Hailiang Zhou Peide and Shih Shengtai(Shanghai Institute of Metallurgy Academia Sinica) |
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Abstract In the present paper, a study of degradation of aluminide coatings on Ni, Ni-20％Cr and K3 superalloy(Cr10～12％, Co4.5～6.0％, Al5.3～5.9％, Mo3.8～4.5％, W 4.8～5.5％, Ti 2.3～2.9％, Ni Bal.) has been conducted by two test methods at 900℃, one is the burner rig test with 20ppm NaCl injected,and the other is the modified crucible test with evaporation of a mixture of 25％ NaCl+75％Na_2SO_4. Oxidation test at the same temperature was carried out for comparison. The tested samples were analyzed by SEM, X-ray diffraction,and EPMA. It was found that alumina whiskers appeared on the surface of the samples in the burner rig test. The zones having the alumina whiskers became the initiation of pits. The number and size of pits increased with time. When the pits reached the interface between coating andsubstrate,the coating cracked and became unprotective. In the modified crucible test,β-NiAl phase disappeared earlier than in the burner rig test due to stripping of oxide scale and depletion of Al. Then pits appeared and the coatings cracked. The results indicated that Cr greatly increased the resistance of coating to hot corrosion. However, the extent of the improvement is different in both tests. The effect is more pronounced in the burner rig test than in the modified crucible test. This is probably because the higher NaCl concentration in the modified crucible test offsets the beneficial effect of Cr.
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Received: 25 June 1987
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