Corrosion Behaviour of Heat-Treated Cold Spray Nickel Chromium/Chromium Carbides

Chromium carbide powder agglomerated with nickel/chrome was deposited using a cold spray process onto a mild steel substrate. The deposits were heat-treated at 650 °C and 950 °C in ambient conditions to reduce porosity and improve adhesion between powder particles. The corrosion behaviour of these cold-sprayed materials was studied in artificial seawater conditions using electrochemical techniques. Heat treatment at 650 °C was found to best improve corrosion resistance, while the 950 °C treatment performed better than the as-sprayed condition but lower than the 650 °C sample. Microstructural analysis revealed complex phase transformations and structural refinements with increasing heat treatment temperature. The crystallite size of both Cr₃C₂ and NiCr phases decreased, while microstrain and dislocation density increased due to heat treatment. The formation of and subsequent reduction in Cr₂₃C₆ content indicated a complex sequence of carbide dissolution, transformation, and precipitation processes. The 650 °C heat-treated sample demonstrated superior corrosion resistance, evidenced by the highest corrosion potential, lowest passive current, and largest charge transfer resistance. This enhanced performance was attributed to the formation of a more stable and protective passive film, optimal carbide dissolution, and a homogeneous microstructure. Meanwhile, the 950 °C treatment led to excessive carbide dissolution and formed increased interfaces between the carbide and matrix. Mechanical property changes were also observed, with carbide hardness significantly decreasing after corrosion testing. These findings highlight the critical role of controlled heat treatment in optimising the performance of cold-sprayed Cr₃C₂-NiCr coatings, demonstrating that achieving superior corrosion resistance requires a delicate balance between microstructural refinement, phase transformations, and preservation of coating integrity.