High Temperature Oxidation and Corrosion Behavior of Chromized Rene 80 – by Pack Cementation and Slurry

Ni-based superalloys are commonly used in gas turbines because of their outstanding high temperature strength and creep properties. To also ensure sufficient oxidation and corrosion resistance, protective diffusion coatings are widely applied, such as Cr-based coatings which are advantageous for hot corrosion and oxidation resistance between 650°C-950°C.

Cr-based diffusion coatings are primarily formed using pack cementation where the parts are fully embedded into a powder mixture, which is energy- and labor-intensive. Alternatively, slurry-based coatings would be significantly more economical. For Al, the slurry process is well established as a liquid phase is easily achievable at the substrate/particle interface, which is necessary for sufficient diffusion rates during heat treatment. Because of the high melting point of Cr, Cr-based diffusion coatings by the slurry technique have been unavailable to date. Recently, Cr-Si slurry coatings have been successfully developed which form a liquid at the substrate/particle interface. Higher Cr-activities, due to the partial liquid state, enable ~10X higher coating thicknesses when compared to similar coatings applied by pack cementation.

The high temperature oxidation and corrosion performance of these novel Cr-Si slurry coatings on the alloy Rene 80 were investigated and compared to pack cementation and uncoated samples. Isothermal and cyclic (1h/cycle) exposures in lab air at 900°C for 1000 h were carried out. The hot corrosion behavior was investigated by the deposition of 2.5 mg/cm² Na₂SO₄ salt. Afterwards, the samples were exposed at 700°C and 900° C for 300 h in a gas mixture of synthetic air containing 0.1 % SO₂. The slurry-coated samples showed significantly reduced attack by oxidation and corrosion during the exposures due to the formation of a slowly growing Cr₂O₃ scale and a Si-rich subscale.