Microstructure and corrosion resistance of Ni-based alloy laser coatings with nanosize CeO₂ addition

Micron-size Ni-base alloy (NBA) powders were mixed with both 1.5 wt.% (hereinafter %) micron-size CeO₂ (m-CeO₂) and also 1.5% and 3.0% nano-size CeO₂ (n- CeO₂) powders. These mixtures were coated on low-carbon steel (Q235) by 2.0 kW CO₂ laser cladding. The effects on the microstructures, phases and electrochemical corrosion of the coatings upon the addition of m- and n- CeO₂ powders to NBA (m- and n- CeO₂ /NBA) have been investigated. The results showed that a smooth coating was prepared under suitable processing parameters (P= 2.0 kW, V= 180 mm min^{- 1}) by adding 1.5% n- CeO₂. In addition to the primary phases of γ-Ni, Cr₂₃C₆ and Ni₃B in the Ni-base alloy coating, CeNi₃ was formed in Ni-base alloy coatings with both n- CeO₂ and m-CeO₂ particles, and CeNi₅ appeared in the coating upon decreasing the size of CeO₂ particles. Well-developed dendrites were observed in the Ni-base alloy coating; directional dendrites grew at the interface in the coating upon the addition of m-CeO₂, whereas fine and multioriented dendrites grew upon decreasing the size of CeO₂ particles to the nanoscale. Actinomorphic dendrites and compact equiaxed dendrites grew from the interface to near the surface upon increasing the content of n- CeO₂ from 1.5 to 3.0%. In strongly acidic HNO₃ solution, the severe corrosion of dendrites occurred and there were many corrosion pits in the Ni-base alloy coating; intercrystalline corrosion also has a dominant role upon the addition of m-CeO₂, whereas uniform corrosion occurs in the coating as the size of CeO₂ particles is decreased to nanoscale.