| The Ni60/WC as-sprayed coatings were prefabricated by the flame spraying,and then the coatings were remelted and treated by forced co oling to form Ni60/WC directional structure composite coatings reinforced by WC particles in this study.The spraying powders were Ni60 self-fluxing alloy powders and WC powders mixed with mass ratios of 100:0,19:1,9:1,8:2 and 7:3,respectively.First of all,the corrosion resistance and corrosion mechanism of directional structure coatings with different WC contents in a 10%H2SO4 solution were investigated by electrochemical test and immersion corrosion test,and the WC addition content of composite coating which had the best corrosion resistance was determined.Secondly,the corrosion resistance and corrosion behavior of the coating with the optimal WC addition were studied in different concentrations of H2SO4 solution.Finally,the corrosion behavior of coatings was evaluated in a 3.5%NaCl solution.The results showed that the corrosion current density of Ni60/10%WC directional structure composite coatings in a 10%H2SO4 solution was 1.34×10-6A/cm2,which was 1/21,1/11,1/2 and 1/3 of that of Ni60,Ni60/5%WC,Ni60/20%WC and Ni60/30%WC coatings,respectively.The polarization resistance of Ni60/10%WC coatings was 9710.8?,which was 13 times,5 times,1.5 times and 1.7 times higher than that of Ni60,Ni60/5%WC,Ni60/20%WC and Ni60/30%WC coatings,respectively.That showed that Ni60/10%WC coatings presented the smallest corrosion current density,the highest polarization resistance and the best corrosion resistance.After immersion for 168 h,the corrosion mechanism of Ni60 coating was mainly the microgalvance corrosion and the pitting corrosion.Ni60/5%WC and Ni60/30%WC coatings were mainly pitting corrosion.Ni60/20%WC coating presented the slight corrosion on the intracrystalline location s without WC.However,there was no obvious corrosion phenomenon in Ni60/10%WC coating surface.The corrosion behaviors of Ni60 and Ni60/10%WC directional structure coatings in 5%H2SO4,20%H2SO4,40%H2SO4,60%H2SO4 and 80%H2SO4 solutions were studied.The results indicated that the two kinds of coatings all had good corrosion resistance in different concentrations of sulfur ic acid solution.But electrochemical tests showed that the corrosion resistance of Ni60 coatings was not as good as that of Ni60/WC composite coatings at the same concentration.With the increase o f the concentrations of sulfuric acid solution,the corrosion current density of two coatings increased first and then decreased,while the polarization resistance decreased first and then increased.Hence,the corrosion resistance of coatings decreased fi rst and then increased.The two kinds of coatings both displayed the worst corrosion resistance when the concentration of sulfuric acid solution was 40%.The two kinds of coatings both formed the protective film of NiO in oxidizing acids?60%and 80%H2SO4?,which can effectively protect the coatings from corrosion and improve the corrosion resistance of coatings.Moreover,WC addition made this effect more obvious.In a 3.5%NaCl solution,the corrosion potentials of Ni60 coatings and Ni60/10%WC composite coatings were-0.399 V and-0.344 V,respectively.The corrosion current density of Ni60/10%WC composite coatings was 5.567×10-7 A/cm2,which was l/2 of that of Ni60 coatings.The polarization resistance of Ni60/10%WC composite coatings was 74227.2?,which was 5.6 times of that of Ni60 coatings.It indicated that the anti-corrosion ability of Ni60/10%WC composite coatings was better than that of Ni60 coatings and WC addition significantly improved corrosion resistance of Ni60 coatings.The corrosion morphologies of coating surface showed corrosion pits after electrochemical test and immersion corrosion test.It meant that the mechanisms of two coatings were mainly pitting corrosion in NaCl solution. |
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