| Ni-Co alloy coatings have excellent electromagnetic properties and corrosionresistance, which have been widely applied in mechanical, electronics andaerospace. With the development of the material surface engineering technology,binary and multicomponent alloy deposits have aroused increasing attention.Trivalent chromium and its alloy coatings have high temperature resistance,corrosion resistance and low toxicity, and have become a research hotspot.Electrodeposition Ni-Co-Cr alloy plating on Q235substrate has not beeninvestigated yet. Ni-Co-Cr alloy coatings were prepared, the composition of bathsolution and process parameters were based on the electrochemical characteristicsof bath solution, which have excellent corrosion resistance.Electrochemical regularity of alloy codeposition was studied by means ofsteady-state polarization and A.C. impedance. It showed that codeposition ofNi-Co alloy was anomalous codeposition, the codeposition of Ni-Co-Cr alloy waseasier than trivalent chromium deposit, and it was an irreversible step; Saccharinincreases the cathodic polarization and promotes the trivalent chromium depositwithin certain realms; when the pH value was2.5, it can enhance the cathodepolarization effect and which was favourable for the deposition of Cr in thecathode; the polarization effect was weakened and hindered the chromiumdeposition with the increasing of temperature; aging was advantageous to theprecipitation of chromium and improved the stability of the plating solution.Under the open circuit potential, alloy codeposition process was controlled by theelectrochemical and concentration polarization; it was controlled by theelectrochemical reaction completely when the electrode potential negative movedto-0.6V, and the adsorption characteristics can be strengthened in low frequencyfield with potential negative shift.The appropriate electrolyte composition and process conditions ofelectrodeposition Ni-Co-Cr alloy were as follows,90g/L Cr2(SO4)3.6H2O,10g/L NiSO4.6H2O,1.5g/L CoCl2,20g/L H3BO3,10g/L citric acid,10g/L sodiumcitrate,8g/L additives,1g/L saccharin, respectively. The bath pH was2.5,temperature was25°C, current density was6A/dm2. The prepared alloy coatingswith bright surface and coatings composition were Cr10.25%, Co21.2%, Ni68.55%, repectively. The average grain size was8.6nm. The chromium contentwas15.7%when the concentration of Cr2(SO4).36H2O was100g/L, and it can beobserved that coatings structure was transfered from nanocrystalline intoamorphous.The grain size of Ni-Co alloy coatings was about10.3nm, hardness was605.5HV2N, and grain was gradually coarsening after heat treatment, grainboundary gradually relaxed after400℃heat treatment, after500℃heat treatmentneedle oxide appeared on coatings surface, and microhardness decreased. Thecorrosion resistance of as-deposited coatings in3.5%NaCl solution was decreasedwith increasing time of immersion, however pitting hole has not appeared after720h. Polarization curve and impedance spectroscopy test showed that the Ni-Cocoatings corrosion resistance declines in3.5%NaCl solution after heat treatment,but coatings corrosion resistance has improved after500℃heat treatment sincesurface oxide film was formed, the corrosion resistance of coatings in10%NaOHsolution reduced with the increase of annealing temperature, coatings corrosionresistance in10%H2SO4solution decreased by the increase of heat treatmenttemperature. Soaking in3.5%NaCl solution at240h corrosion experimentsshowed that as-deposited and after heat treatment Ni-Co alloy coatings haveexcellent corrosion resistance.The amorphous Ni-Co-Cr alloy coatings were partial crystallization with theincrease of heat treatment temperature. Polarization curve showed that coatingscorrosion resistance increased with the increase of Cr content. Impedance testresults showed that the corrosion resistance of coatings in3.5%NaCl solutiongradually reduced with the corrosion time prolonging. Ni-Co-Cr platings have astrong corrosion resistance in10%NaOH solution. The corrosion resistance ofas-deposited alloy coatings in3.5%NaCl solution were the best, after500℃heattreatment the corrosion resistance fell sharply because of crystallization occurred. In10%H2SO4solution, it can be observed by EIS that plating coatings in the pitcorrosion induction period, the coatings corrosion resistance decreased with theincrease of heat treatment temperature. Coatings surface was still brightness withno corrosion phenomena after neutral salt spray test for100h.Ni-Co/Ni-Co-Cr gradient coatings were prepared with smooth and shiningsurface. As-deposited coatings have the largest polarization resistance in3.5%NaCl solution, corrosion resistance of coatings after400℃heat treatment wasbetter than300℃, and it was the worst after500℃heat treatment. After400℃heat treatment gradient coatings showed better corrosion resistance than platingcoatings in10%H2SO4solution. Gradient coatings corrosion resistance werewrose than Ni-Co-Cr platings in3.5%NaCl solution, but it is better than Ni-Co-Crplatings in10%H2SO4solution. |
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