EVALUATION OF HOT CORROSION-RESISTANCE OF NI-BASE ALLOYS USING IMMERSION TEST, COATING TEST AND EMBEDDING TEST

The hot corrosion resistance of twelve Ni-base alloys was evaluated at 873 K by three different laboratory tests; immersion test, salt coati ng test and embedding test. In these tests, three different corrosion ashes were employed, a sulfate (Na2SO4-K2SO4-ZnSO4), a chloride (NaCl- KCl-LiCl-CaCl2) and a chloride/sulfate mixture (NaCl-KCl-Na2SO4-K2SO4- ZnSO4). The correlations between the ranking of the corrosion resistan ce of the alloys in the immersion test, coating test and embedding tes t were investigated using spearman's correlation coefficient. The resu lts of the three tests were highly correlated in the sulfate and chlor ide/sulfate environments, whereas they were relatively poorly correlat ed in the chloride environment. It was found from the anodic polarizat ion measurement and X-ray difraction analysis of the corrosion product s that the relatively poor correlation is attributed to the difference s in the rate-determining steps of the corrosion. In the sulfate envir onment, there was small difference of the corrosion mass loss among th e three tests, but in both the chloride and chloride/sulfate environme nts, the mass losses in the coating test and embedding test were much larger than that in the immersion test. The corrosion mechnisms under the respective corrosion test conditions were discussed.