Influence of post - deposition heat treatment on the properties of electrodeposited Ni-Mo alloy coatings

Ni-26 wt% Mo alloy coatings were annealed for 1 h in an air atmosphere at temperatures of 200, 400 and 600 degrees C. The influence of heat treatment on the structure and the mechanical and corrosion properties was investigated. It was found that the coating microhardness increased significantly after annealing at 200 and 400 degrees C. The results of the scratch test revealed that. Ni-Mo alloys heated at higher temperatures (400 and 600 degrees C) were characterized by lower adhesion and more brittle nature in comparison to the as-deposited coating and the coating heated at 200 degrees C. Annealing at 400 degrees C led to a significant improvement of corrosion resistance of the coating. This was the consequence of an increase in the thickness of an oxide layer formed on the coating surface. When exposed to a corrosive solution, the protective ability of the oxide layer deteriorated, which was followed by a systematic drop of polarization, charge transfer, and pore resistance. XPS analysis revealed that the surface layer of the as-deposited coating and the coating heated at 200 degrees C was mainly composed of NiOOH and Ni(OH)(2) and contained higher amounts of Mo in comparison to the deposit bulk. The surface layer of coatings heated at higher temperatures was composed of NiO and contained only small amounts of Mo.