In-Service Degradation of Corrosion Resistant Coatings

The rainbow test was performed to evaluate the effects of cooling hole design modifications and the MCrAlY coating deterioration with and without top aluminide. The cooling design modifications considered in this program include a) camberline cooling hole design, b) peripheral cooling hole design with a turbulated leading edge hole and c) peripheral cooling hole design with all turbulated holes. Following approximately 16,700 hours of operation, one blade from each category was removed for metallurgical evaluation. The MCrAlY coating on the camberline cooled blade without the top aluminide and coat had degraded the most at the blade tip region and along a portion of the leading edge. In some regions, the coating on the leading edge of the blade was oxidized through thickness. However, no oxidation of coating was noted on the leading edge of the blade with the MCrAlY with top aluminide coating. The results show that the top aluminide coating and the cooling hole design modifications extend service life of the MCrAlY coating. The coating deterioration results on these blades are correlated with the β NiAl depleted zone in the coating and the oxide thickness at the cooling hole surfaces. Variations of coating degradation and cooling hole surface oxidation among the blades are discussed in terms of estimated service temperatures measured from γ′ particle size. The cooling hole design modifications significantly lower metal temperature and extend service life of the MCrAlY coating.