Alkali sulphates induced hot corrosion of a β-NiAl coating on a second-generation single-crystal Ni-based superalloy
Wang, Yaping; Spatscheck, Robert (Thesis advisor); Zander, Brita Daniela (Thesis advisor)
Aachen : RWTH Aachen University (2022)
Dissertation / PhD Thesis
Dissertation, RWTH Aachen University, 2022
High-temperature components in gas turbines are frequently exposed to sulphur-containing gases and alkali sulphate deposits during service. The deposit-induced accelerated corrosion attack termed hot corrosion may result in rapid degradation and failure of the turbine components. Hot corrosion studies of the involved materials (Ni-based alloys and coatings) are thus of significant importance for understanding its mechanisms, finding the way to mitigate the attack, and extending the life span of the components. In the present study, alkali sulphates induced hot corrosion of a β-NiAl coating on a single crystal Ni-based superalloy was investigated. Thermodynamics of the Na2SO4-K2SO4-NiSO4 system is essential for understanding the reactions during the hot corrosion process. In the present work, the phase equilibria in the system NiSO4-K2SO4 were experimentally determined for the first time using differential thermal analysis (DTA), dilatometry (DiL), and X-Ray diffraction method (XRD). Based on the obtained experimental data, the NiSO4-K2SO4 system was thermodynamically assessed using the CALPHAD methodology. This dataset extends the previously developed general sulphate database including alkali- and alkaline-earth sulphates (CaSO4-MgSO4-Na2SO4-K2SO4) with the addition of NiSO4. Using the optimized dataset, the stability diagrams and the liquid content of the Na2SO4-K2SO4-NiO-SO3 system and sub-systems were calculated, to provide an insight into the corrosion mechanisms. The aggressiveness of Na2SO4, K2SO4, and binary Na2SO4-K2SO4 deposits was compared by performing a series of hot corrosion tests using a β-NiAl coating on a single crystal Ni-based superalloy. Apart from the hot corrosion experiments, various mixtures of alkali sulphates and Ni-oxide/sulphate powders were prepared and exposed under the same conditions as the coated specimens. Comparison of the composition and morphologies of reaction products between the coated superalloy and mixed powders allowed a better understanding of the involved corrosion phenomena. The corrosion attack rates and morphologies could be correlated with the deposit compositions and its different reactions with oxides. K2SO4 and Na2SO4-K2SO4 deposits are shown to be more aggressive than Na2SO4. This is attributed to the reduction of melting temperature of the deposit and the formation of an intermediate compound K2Ni2(SO4)3 in the presence of K2SO4.The hot corrosion kinetics of the aluminised Ni-based superalloy was studied in a wide temperature range of 600-1000 °C under the Na2SO4-20% K2SO4 deposit. The corrosion rate-temperature dependence for the studied system reveals two maxima at around 700 °C and 950 °C, corresponding to type II and type I hot corrosion, respectively. Typical hot corrosion morphology was found at 600 °C, where the liquid formation is thermodynamically unlikely, indicating the importance of solid-state reactions for the component lifetime in the low-temperature range. At 800 °C, a localized corrosion attack (pitting) was identified at the coating grain boundaries, correlated with Cr-rich precipitates, that might promote the attack by reacting with the deposit. At longer exposure times localized pitting leads to coating penetration and corrosion attack of the base material. At 1000 °C, a relatively dense protective alumina scale formed, and the corrosion rate dramatically decreased compared to that at 850-950 °C.
- Division of Materials Science and Engineering 
- Chair of Corrosion and Corrosion Protection