Optimization of trivalent chromium electrodeposition in chloride-based bath by response surface method
[Introduction]The drastic hydrogen evolution reaction accompanies with the electrodeposition of chromium leads to the rise of local pH,which is the primary cause of dramatic decrease in deposition rate,difficulty in coating thickening,and deterioration of coating properties.[Method]Sodium formate,sodium oxalate,and urea were selected as the complexing agents for electrodeposition of trivalent chromium.The bath composition was optimized by response surface method using the deposition rate at 150 mA/cm2 within 10 min as the response,and a polynomial equation for predicting the deposition rate of trivalent chromium in a chloride-based bath was established.The effects of additive,pH,temperature,current density,and electrodeposition time on the deposition rate,throwing power of the bath,and corrosion resistance of Cr coating were studied by single-factor experiments.[Result]The bath composition and process conditions were optimized as follows:chromium trichloride 0.6 mol/L,sodium formate 0.8 mol/L,sodium oxalate 0.2 mol/L,urea 0.3 mol/L,pH 1.8,temperature 30 ℃,current density 150 mA/cm2,and time 30 min.The Cr coating electrodeposited under the optimal conditions was amorphous with a thickness higher than 12 μm and good corrosion resistance.[Conclusion]It is an effective way to maintain a high deposition rate and improve the coating properties by selecting appropriate chelating agents to inhibit the hydroxylation reaction of chromium during electrodeposition.
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