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The present work reports the results of a comparative evaluation of three commercial coatings for turbine blades: (i) low activity pack cementation aluminide; (ii) high activity pack cementation Pt modified aluminide; and (iii) slurry deposited Si modified aluminide. For a laboratory corrosion test, bare substrate (Udimet 520) and coated samples were subjected to two different salts baths, 25% NaCl-75% Na2SO4 and 100% Na2SO4, at 750 °C in an inert atmosphere (Ar) and a gas mixture of SO3-SO2-O2. For a rainbow test, nine coated blades were mounted in a gas turbine for 10 000 h. The test results showed that for the NaCl-Na2SO4 bath the damage mode of bare samples was Type I hot corrosion, while for the 100% Na2SO4 bath the damage was Type II; this effect was independent of the test atmosphere. Coated samples showed an incipient corrosion for the same tests. In accordance with the damage intensity, the coatings were rated (from worst to best) as: Al-Pt, Al, Al-Si. The rainbow test showed the same tendency; however, the corrosion damage was less intensive in all cases. |
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