Microstructural evolution of germanium modified AlSi-slurry coatings on alloy 600 at 620 °C in metal dusting environment

Surface and Coatings Technology 315 (2017), 335-341, DOI: 10.1016/j.surfcoat.2017.02.061

Germanium-modified AlSi-slurry coatings were produced via airbrush spraying of two different suspensions on the conventional Ni-base alloy 600, followed by a heat-treatment in Ar at 800 °C. The resulting coatings exhibit a thickness of approximately 130 μm and mainly consist of outer Al-rich β-Ni(Al,Ge) phase followed by inner Ni-rich β-Ni(Al,Ge). In the interdiffusion zone, a continuous layer of (Cr,Ni)₃Ge phase formed. Long term exposure tests in carbonaceous atmosphere at 620 °C showed excellent metal dusting protection. Furthermore, the exposure tests revealed that the (Cr,Ni)₃Ge layer acts as a diffusion barrier, preventing the diffusion of the major coating element aluminum into the substrate. Depletion of aluminum would be detrimental for the lifetime of the coating and consequentially for that of the structural component. Microstructural evolution of the coating during exposure showed inward diffusion of nickel into the coating, resulting in fine Ge₃Ni₅ precipitates, whose formation will be discussed based on the ternary phase diagram of Al-Ge-Ni.