Cladding Modification Of Zinc-aluminium Powder And Its Application In Chromium-free Dacromet Coatings

Traditional Dacromet coatings are non-hydrogen brittle,corrosion resistant and high temperature resistant,and are a green and environmentally friendly coating used in automotive,appliance,marine and military applications.However,the carcinogen hexavalent chromium in it has limited its development and attention has subsequently been paid to the research of chromium-free Dacromet coatings.The corrosion resistance of the new chromium-free Dacromet coatings is not yet comparable to that of traditional Dacromet coatings,and the zinc and aluminium powders are prone to react and are less stable in the coatings,so there is a need to improve the corrosion resistance of the zinc and aluminium powders and their dispersion in the coatings.In this paper,the zinc powders were firstly modified by coating with KH570,to which acrylic monomers were added to form a composite modified zinc powder.TEOS was used as the silicon source and VIPOS was added to modify the surface of the aluminium powder to obtain TEOS-modified aluminium powder and TEOS/VIPOS-modified aluminium powder respectively.The results of scanning electron microscopy showed that the surfaces of the modified zinc and aluminium powders were rougher than those of the raw zinc and aluminium powders,and that the modified powders had a good coating effect with the addition of Si elements and uniform distribution.The initial formulation of the chromium-free Dacromet coating was chosen as follows: 20% polyethylene glycol,25%total metal powder mass（6:1 zinc and aluminium powder mass ratio）,20% KH560 and 1%NP-9.The modified zinc powder was added to the coating instead of the raw zinc powder to form the modified zinc-aluminium coating and the corresponding coating was prepared.By comparing the overall performance and electrochemical corrosion rate of the coating,the results showed that the appearance of the coating with KH570 modified zinc powder remained flat and its corrosion resistance was improved by 12.5% compared to the raw zinc-aluminium coating.In addition,when the modified aluminium powder was added to the Dacromet coating instead of the raw aluminium powder,the results of the kinetic potential polarisation curve showed that the corrosion current density of the coating with the modified aluminium powder was lower and showed a passivation trend around 10^-5A/cm².Finally,modified zinc powder and modified aluminium powder were added to the coating at the same time.After a comprehensive comparison of the overall coating performance and electrochemical test results,the modified zinc and aluminium coating formulations were obtained as follows: KH570 modified zinc powder,TEOS/VIPOS modified aluminium powder,polyethylene glycol,KH560 and NP-9.The suitable materials for each component of the modified coating formulation were confirmed in terms of coating appearance,adhesion and corrosion resistance,and the optimized coating formulation was obtained using orthogonal experiments: 20% of the total mass of modified zinc powder and modified aluminium powder(m _Zn:m _Al=6:1),15% of the wetting agent ethylene glycol,20% of the binder KH560 and 2% of the dispersant NP-9.Single-factor experiments were used to investigate the effects of the mass ratio of modified zinc and aluminium powder,the p H value of KH560 hydrolysate,the addition of sodium molybdate and the addition of hydroxyethyl cellulose on the anticorrosive properties of the coating: too much or too little modified zinc powder will lead to a reduction in the protective properties of the coating,and the mass ratio of modified zinc and aluminium powder between 6 and 8 has excellent anticorrosive properties;the p H value of KH560 hydrolysate directly affects the The p H value of KH560 directly affects the degree of hydrolysis of KH560,thus indirectly affecting the bonding of the coating.Infrared spectroscopy and coating kinetic potential polarisation curves show that the hydrolysate is almost completely hydrolysed at p H=7 and that the coating exhibits excellent protective properties.The initial viscosity of the modified coating was about 18 s.When the addition of hydroxyethyl cellulose exceeded 8%,the viscosity of the coating increased rapidly and could not be coated normally.The viscosity of the coating prepared using the best modified coating formula was20.23 s,the surface of the coating was flat and silvery white,the thickness of the modified zinc aluminium coating was 41.8 μm,and the adhesion of the coating was grade 1 and the hardness was 8H.Using SEM to observe the surface of the modified coating,it was found that the corrosion process of the modified coating immersed in 3.5% Na Cl solution occurred as follows: the destruction of the passivation film on the surface of the coating,the disappearance of the protective film on the surface of the modified Zinc and aluminium powder surface protection film disappearance and powder activation,corrosion product generation and shedding,the modified coating surface after 45 days of immersion to produce spherical corrosion products and small cracks.The impedance spectrum and kinetic potential polarisation curve of the coating in 3.5% Na Cl solution were measured by electrochemical tests: at the beginning of the immersion in 3.5% Na Cl solution,the coating showed a high impedance value,and then the impedance value gradually decreased and finally stabilised;after Tafel fitting the kinetic potential polarisation curve,the corrosion rate of the modified coating was 0.00976 mm/a,and finally the corrosion resistance of the modified coating was good.The modified Zn-Al coating with good corrosion resistance was obtained.