Preparation And Passivation Treatment Of Nanocrystalline Zinc Coating And Its Formation Mechanism

Electrodeposited zinc coatings have been extensively used in manufacturing,automotive,and construction sectors to protect steel components from atmospheric corrosion with low costs and merit of easy fabrication.Zinc may serve as a sacrificial anodic coating to prevent corrosion of the steel beneath even the zinc coating is locally scratched or damaged.According to the International Zinc Association,more than 5 million tons of zinc per year is used to protect steel against corrosion all over the world,and about 2.2 trillion USD is retrieved annually from the capital lost for repair and replacement of corroded steel components.The utilization of zinc coatings continuously increases with increasing in global demand for steel.The electrodeposition of coarse-grained zinc coatings only retains the structural,mechanical,tribological and corrosion resistance properties of zinc,thereby the coating must be thick enough to endure the attack of friction and wear or a corrosive environment.However,thick coatings can lead to a waste of zinc,as well as a poor weldability,and difficulty in the specular finish after painting.It is therefore necessary to develop thinner electrodeposited coatings with excellent wear and corrosion resistance.In order to increase the resistance of conventional coarse-grained zinc coating to corrosive and wear,we developed a novel electrodeposition and passivation technologies of nanocrystalline zinc coating.Nanocrystalline zinc coating was electrodeposited onto the steel substrate using a sulfate bath with polyacrylamide as grain refiner.In the meantime,the influence of polyacrylamide on electrodeposition of nanocrystalline zinc coating are investigated.Microstructure,mechanical,tribological and electrochemical behaviors were studied to evaluate nanocrystalline zinc coating with its grain size around 40 nm,in comparison wi th that of coarse-grained zinc coating(grain size ~ 6 μm).In addition,this article investigates the beneficial effect of combined passivation treatment and nano-electrodeposition on surface morphology,mechanical,scratch and corrosion resistance of electrogalvanized steel.Specially,the performance improvement of zinc coatings and its passive film with the reduction of grain sizes from micro to nano-scale was deciphered from the electron work function point of view.It was demonstrated that the formation of nanocrystalline zinc is attributed to the synergistic adsorption of the polymer chain and amine group of polyacrylamide,which is beneficial for increasing the free energy to form new nuclei,thereby resulting in a higher nucleation rate and inhibiting the crystal grain growth.The nanohardness increased about 2 times when the grain size of coarse-grained zinc coatings was reduced from 6 μm to 40 nm,and the elastic modulus of the nanocrystalline zinc coating with mirror-shining is almost 2 times that of the coarse-grained counterpart.The average wear rate of the nanocrystalline zinc coating is approximately 1/24 of that of the coarse-grained zinc coating.The corrosion rate of the nanocrystalline zinc coating is less than 1/2 of that of the coarse-grained zinc coating in simulated seawater.The combined treatment has a significantly advantage in improving the mechanical properties(hardness,adhesive force and elastic modulus)of coarse-grained zinc coating than that of passivation treatment and nano-electrodeposition.The surface of electrogalvanized steel appears mirror brightness with light blue color.In particular,the combined treatment rendered the scratch resistance of the nanocrystalline zinc coating to 3 orders of magnitude higher than that of the conventional electrogalvanized steel.And the corrosion resistance of coarse-grained zinc coating is heightened almost 4 times due to this combination effect.The mechanical property variation of zinc coatings with the reduction of grain size has been explained by Hall-Petch law.The high corrosion resistance of the nanocrystalline zinc coating before and after passivation treatment,as well as improved mechanical and scratch resistance of passive film on nanocrystalline zinc coating largely benefited from its higher surface activity than that of conventional coarse-grained zinc coating.The higher surface activity of nanocrystalline zinc coating than that of coarse-grained zinc coating improved the passivation capability of conventional zinc coating and the stability of corrosion product layer formed on the coating.With the reduction of grain size from micro to nano-scale,the electrons in nanocrystalline zinc coating are less localized and the atomic bonds in nanocrystalline zinc coating are weaker than those in coarse-grained zinc coating,the surface free energy of nanocrystalline zinc coating is thus higher than that of coarse-grained zinc coating.