Electrochemical evaluation of corrosion resistance of NiCrBSi coatings deposited by HVOF

The NiCrBSi alloy powder was sprayed to a steel substrate using HVOF and the corrosion resistance of the coatings was evaluated using electrochemical tests. The results show that NiCrBSi coating has excellent corrosion resistance in alkali solution, because the surface can keep in a self-passivation condition. The corrosion current density of the coating in sour solutions is bigger than that in 3.5%NaCl, and the corrosion caused by Cl− is more serious than that caused by SO42− and AC−. Porosities and inclusions may play important roles when they are dense and big enough. Inclusions facilitate the presence of corrosion micro cells on the surface of sample thus worsen the corrosion resistance of metal. If the inclusion is big enough, local corrosion will occur. The effects of porosities on the corrosion behavior of coating are a long-lasting process and the effects of porosities on the early corrosion of coating are not serious unless there are penetrating porosities. However, porosities can do harm to the persistent corrosion resistance of coatings. The presence of porosities may weaken the cohesive strength within coating, causing great deals of particles drop off from surface under the function of electrolyte. The lower the density of defects in the coating, the better the corrosion resistance. Improving the coating quality is the key to obtain coatings with high corrosion resistance. In addition, the reproducibility of the icorr obtained from Tafel polarization curves is not good enough when the coating is examined in 1 N NaOH and 3.5% NaCl, while the reproducibility can be improved in 1.0 N H2SO4, 1.0 N HCl, or 3.5% NaCl aqueous solution acidified with acetic acid. It seems that the reproducibility of the test results can be improved when a corrosive medium that keeps the tested surface in an active state is used.