Effect of cobalt content on corrosion resistance of electroplated Ni-W-Co alloy coating
[Introduction]The effect of cobalt content in Ni-W-Co alloy coating on its corrosion resistance needs to be studied.[Method]Ni-W-Co alloy coatings with different Co contents were electrodeposited on N80 steel at direct current by adjusting the mass concentrations of cobalt sulfate(CoSO4·7H2O)in the electrolyte.The effect of the dosage of cobalt sulfate on the deposition rate,and the composition,morphology,and corrosion resistance of Ni-W-Co alloy coating were studied by coating thickness measurement,composition analysis,morphology observation,immersion corrosion test,and potentiodynamic polarization measurement.[Result]With the increasing of cobalt sulfate concentration in the electrolyte,the deposition rate and the mass fraction of cobalt in Ni-W-Co alloy coating were increased,the coating surface became rough,and its corrosion resistance was initially improved and then deteriorated.The Ni-W-Co alloy coating electrodeposited with 2.0 g/L of CoSO4·7H2O in the electrolyte had the best corrosion resistance,as shown by its lowest corrosion rate in both 5wt%NaCl solution and mud acid solution comprising 10vol%hydrochloric acid and 5vol%hydrofluoric acid.[Conclusion]Corrosion-resistant Ni-W-Co alloy coatings suitable for oilfield environment can be electrodeposited by adjusting the amount of cobalt salt in plating solution.
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