Influence Of Impurity On The Micromorphology And Corrosion Resistance Of High-speed Zinc Coating

Zinc sulphate electrolytes are used for continuous high speed plating on account of their simple ingredient, high current efficiency and quick deposition rate. When galvanized sheet steel is stored for a long time, a kind of black or brown film is formed on the surface. The phenomenon is went by the name of black patina. The black film seriously affects the appearance of surface. In this paper, high speed electrogalvanizing process is optimized and the influence of metallic impurities on the performances of coating, including black patina resistance, micromorphology and degree of orientation of crystal face, is researched by humid-hot-test, microscopicexamination and XRD analyses in static state and dynamic state respectively. It is to decrease degree of black patina and investigate the formation mechanism of black patina film. The influence of environmental factors on coating is researched and the measures which relieve black patina are proposed.In static state, black patina resistance of coating is the best when current density is 15A/dm2. impurities affect the performances of coating, including appearance of surface, black patina resistance and micromorphology, negatively. The tolerant scopes of electrolyte to impurities are as follows: copper ion 0~1.0mg/L; nickel ion 0~400mg/L; iron ion 0~800mg/L; influence of aluminium ion on zinc electrowinning is neglectable.When electrolyte is flowing, the crystal size of coating is smaller than coating's in static state and black patina resistance is enhanced. Concentration polarization is decreasing and electrochemical polarization is increasing with increasing flow rate of solution and current density, at the same time, crystal size become smaller and black patina resistance is enhanced; degree of orientation of crystal face (002), (004) is decreased and other's is increased gradually. The effect of other technological parameters on coating is less relatively. The best process condition is: current density (50A/dm2), flow rate (>3.2m/s), concentration of zinc ion (120g/L), temperature (40℃), pH(2.2). The tolerant scopes of electrolyte to impurities are wider than the scopes in static state.Besides, nickel electroplating can enhance black patina resistance before zinc electrowinning, and when the quantity of nickel electroplating is 100mg/m2, degree of orientation of crystal face (002), (004) is least, black patina resistance is best. Painting pickling oil can also enhance black patina resistance.