High-Temperature Corrosion Behaviour of Aluminized-Coated and Uncoated Alloy 718 Under Cyclic Oxidation and Corrosion in NaCl Vapour at 750 °C

Corrosion behaviour of hot-dip aluminized-coated and uncoated Alloy 718 was examined in dry air and with NaCl salt. Tests were done at 750 °C for 10 h and cooled to room temperature for 30 min per thermal cycle. An oxide layer of Cr2O3 formed on the surface of uncoated material oxidized in air without NaCl. Some Fe and Ni were also detected on the surface. However, porous oxide scale of Fe2O3 formed on the surface of uncoated substrate exposed to thermal cycle in NaCl with extensive corrosion attack and scale spallation. Oxide of Fe2O3 formed on top of Cr2O3 oxide layer due to high volatilization of iron chloride compared to chromium chloride. The hot-dip coating highly protected and improved oxidation resistance of the Alloy 718. Nevertheless, scale spallation and formation of voids in the aluminium layer increased with thermal cycle. The coating also improved corrosion resistance of the alloy exposed to NaCl. However, degradation of the aluminized layer due to formation of cracks and voids allowed NaCl to diffuse into the substrate causing extensive corrosion. Existence of voids in the coating is attributed to outward diffusion of aluminium to form oxide scale, inward diffusion for NiAl layer formation, and oxychloridation process.