The Effect of Ca-Rich Deposits on the High Temperature Degradation of Coated and Uncoated Superalloys

Abstract

When gas turbines use alternate fuels, such as syngas derived from coal, ash from the fuels can deposit on turbine hardware. These deposits can cause substantial corrosion of the hardware which may have significantly different characteristics than Type I and Type II hot corrosion. The composition of the ash is determined by the mineral matter in coals, which often have kaolinite (Al2O3·2SiO2·2H2O), pyrites (FeS2), and calcites (CaCO3) as major components. This study was directed at degradation produced by CaO and CaSO4 and comparing it with the attack induced by Na2SO4 deposits. The alloys GTD 111, IN 738, and René N5, as well as these alloys coated with CoNiCrAlY and platinum aluminide, were exposed to conditions relevant to corrosion induced using alternative fuels. The initial test conditions involved a number of deposits including Na2SO4, CaO, and CaSO4 in dry and wet (pH2O = 10.1 kPa, 0.1 atm) air at 950oC. The most severe degradation occurred with CaO deposits. Specimens of the three alloys were subsequently exposed to cyclic oxidation conditions at 950oC with deposits of CaO in dry and wet air. All three alloys were attacked more severely when CaO deposits were present and this attack became even more severe in wet compared to dry air. However, the increase in attack caused by the presence of water vapor was small compared to the attack caused by the CaO deposits. The degradation induced via CaO deposits caused more severe degradation of René N5 compared to GTD 111 and IN 738. Tests using CaO deposits and cyclic oxidation conditions at 950oC in dry and wet air were also performed for the two coatings on the three alloy substrates. Both coatings were significantly degraded by attack induced by the CaO deposits. No effect of the alloy substrates on coating performance was apparent. Mechanisms for the effects of Ca-rich deposits on superalloy and coating degradation are discussed.