| From the economic and operational considerations, zinc alloy plating andpassivation membrane treatment process is still the best choice to enhance the corrosionresistance of iron base, in addition to the requirements of the subject, the finalappearance is black. So the goal of this project is to obtain zinc alloy plating of highcorrosion resistance on steel base, and its black passivation process.Alkaline zinc-nickel alloy electroplating with better dispersing power, ability touniform electroplating, corrosion resistance and other advantages than acidic zinc-nickelalloy electroplating, has great commercial market. This paper optimizes bathcomposition according to the nickel content, coating thickness, current efficiency,makes the alkaline zinc-nickel alloy electroplating process easy to maintain, when bathis ZnO14g·L-1, NaOH120g·L-1, organic amine A40mL·L-1, triethanolamine16mL·L-1, NiSO4·6H2O14g·L-1, brightener8mL·L-1, Dk2.5A·dm-2, temperature20℃(room temperature), can obtain13mass%nickel content of Zn-Ni alloy coating.Through systematic study of the various process conditions’ influence on the propertiesof the coating, such as the concentration of each component, the cathodic currentdensity and temperature, learned that in order to get bright Zn-Ni alloy plating with13mass%nickel content, and which has a certain thickness and current efficiency,maintain the concentration of each component and process conditions in theelectroplating process: NiSO4·6H2O12~40g·L-1, ZnO10~20g·L-1，triethanolamine0~20mL·L-1, Dk2~3A·dm-2, temperature20~35℃, brightener4~10mL·L-1. On thisbasis, we design the bath maintenance plan accordingly: the use of stainless steelinsoluble anode, dissolve ZnSO4·7H2O in plating solution to add Zn2+, takeNiSO4·6H2O complexed with organic amine A added to the bath in order to add Ni2+.This method introduces a number of SO42-, after long time production, taking freezingto remove Na2SO4from the plating solution is necessary.Although zinc alloy coating enhanced the corrosion resistance of iron base, but thegeneral use will cover a layer of passivation film on zinc alloy plating. This film notonly greatly enhances the corrosion resistance of zinc alloy, also has a decorative role,this topic requires a black passivating film. In traditional way, people use blackhexavalent chromium passivation treatment to obtain the black film of zinc and zincalloy with high corrosion resistance. But hexavalent chromium is toxic and carcinogenic. Black trivalent chromium passivation process is a promising alternative routes. In thispaper, through the primary and orthogonal experiment with copper sulfate drop, Tafel,EIS, SEM, NSS, we obtained the black trivalent chromium passivation film on theZn-Ni alloy coating, the film is uniform, black and good adhesion. The specific process:chromium acetate12g·L-1, chromium chloride8g·L-1, organic carboxylic acid B32g·L-1, sodium nitrate20g·L-1, cobalt chloride4g·L-1, nickel chloride2g·L-1, pH1.8,room temperature(20±5℃), time60s. Because of mud crack on the layer surface, itscorrosion resistance is poor, the neutral salt spray test (NSS) white rust time of only24h. In order to improve the corrosion resistance of the film, we use copper sulfate spot,tafel, EIS electrochemical test to study the effects of additives on the corrosionresistance of the film. Then we know cerium chloride, zinc chloride, nano SiO2, sodiummolybdate, sodium silicate addition and Ni content of the coating can enhance thecorrosion resistance of passive film to a certain extent. Especially nano SiO2, neutralsalt spray test white rust time can be up to48h. Any how, the mud crack defect on thepassivation film still exists, a additional finishing layer is needed to improve corrosionresistance of the passive film. |
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