Corrosion Resistance of Composite Coatings Based on the Alloys of Cobalt With Refractory Metals

We study the corrosion behaviors of electrolytic composite coatings based on ternary cobalt alloys under different conditions and analyze the distribution of metals over the thickness of the coating and the parameters of their surface roughness. The accumulated results show that, in alkaline and neutral media, the corrosion potentials shift in the negative direction as compared with the potentials measured in an acid media but corrosion resistance does not undergo any significant changes. This can be explained by the fact that tungsten and molybdenum oxides formed on the surface are unstable in alkaline media. In view of the fact that in neutral solutions, the subsurface layer becomes alkaline due to the reduction of oxygen and chlorides provoke local fractures of the protective film, we can assert that alloying metals exert a synergistic effect on the rate of corrosion of the composites. According to the computed values of the depth indicator of corrosion rate, the analyzed coatings can be referred to the group of highly resistant, whereas the composites based on Co64Mo33Zr3 alloy in neutral and alkaline media belong to the group of very stable materials. This opens wide possibilities for their application as protective coatings on metals in aggressive media.