Effect of Co, Ni, Mo and W on the corrosion properties of amorphous electrospark coatings

Coatings based on amorphous metals with a different number of elements: nickel, cobalt, molybdenum and tungsten, were obtained by electrospark deposition in a mixture of crystalline granules. The thickness of the deposited coatings was 20 to 33 μm. X-ray diffraction analysis showed a wide halo in the range of angles 2θ ~ 43 °, which indicates the predominance of the amorphous phase in the coating composition. The content of the amorphous phase, depending on the coatings composition, varied from 81 to 99 vol.%. The smallest fraction of the amorphous phase was observed in the coating without molybdenum. According to the EDS analysis, it was shown that the concentration of elements along the cross-section of coatings was constant, which indicates the homogeneity of the composition of the deposited coatings. A study by the SEM showed that the coatings have a homogeneous structure and do not have a precise boundary with the substrate which indicates good adhesion of the coatings to the substrate. Potentiodynamic polarization tests in 3.5% NaCl solution showed that amorphous coatings can improve of the corrosion resistance of the steel 35 surface up to 5 times. Tungsten reduced the rate of electrochemical corrosion of coatings, while cobalt worsen the corrosive properties of FeCrCoNiMoWCB coatings. A high-temperature corrosion test for 100 hours at temperature of 700°C showed that the samples with coatings were oxidized 3.4 to 7.9 times less than steel 1035. Barrier properties, under high-temperature gas corrosion, coatings without tungsten were ~ 2.3 times lower than coatings without cobalt.