Surface corrosion behavior of β-(Ni,Pt)Al coating: α-Al2O3 decohesion-induced failure mechanism

In marine and gas turbine environments, impurities like hydrogen (H), sulfur (S) and chlorine (Cl) ions frequently impact continuity and integrity of the oxide scale of Pt-modified aluminide coating. This study systematically investigated the segregation behavior of these impurities using first-principles calculations, to reveal their weakening potency on both α-Al2O3 bulk and grain boundary (GB). The energetic properties, including separation work, surface energy and GB energy, are strongly associated with existing form of impurities. The interstitial S and Cl atoms are particularly intensive in reducing cohesion of α-Al2O3 in comparison with solute state. The single-atom S- and Cl-doping with S and Cl have similar effects on the embrittlement of α-Al2O3 bulk/GB, and the Cl-introduction exhibits extreme brittleness once the atomic concentration increases. Interestingly, single H-addition has no significant effect on the α-Al2O3 decohesion, while its weakening potency on α-Al2O3 is further enhanced upon contacting with S and Cl atoms. Therefore, we decomposed strengthening energy into mechanical contribution and chemical contribution, finding that the incorporation of H increases the chemical contribution to decohesion of α-Al2O3. This work, along with initial corrosion behavior of various (Ni,Pt)Al coatings, aims to deepen understanding of H, S and Cl on α-Al2O3 decohesion from multiple perspectives.