Study On Chromium-free Zinc-aluminum Coatings Enhanced By Nano Materials

In order to change the situation of insufficient corrosion resistance, low hardness, scratch ofchromium-free Zn-Al coating, this paper substituted the Al-Zn alloys for traditional zinc powders andaluminium powders in Dacromet to prepare Al-Zn alloy paint. Factor-5level-4test was designed, andthe optimized formula was determined by investigating the appearance, adhesion and corrosionresistance. Furthermore, the damage mechanism of coating was discussed under the condition ofartificial damage after the preparation. In order to study the influence of nano-particles on coating,four kinds of particles like SiO2, TiO2, Al2O3, ZnO were added into the paint to prepare differentnano-composite coatings. The testing method of SEM, EDS, XRD were finally applied to analyze thetissue morphology and composition, and study the change of adhesion strength, hardness, corrosionresistance before and after the addition of nano-particles.The research results showed that the optimized formula through the orthogonal test is composedby20%of metal powder,6%of binder,1.2%of corrosion inhibitor,2.4%of dispersant,7%ofprotective agent,0.5%of thickening agent,3-4drops of defoamer and the remaining deionized water;The preparation technology is as follows: drying for10mins under110℃, sintering for25mins under290℃; The main ingredients are Al-rich phase and Zn-rich phase. The flaky alloy powders are piledup layer up layer, the thickness of the coating is12-15μm, and the adhesion is also good. Theelectrochemical corrosion potential and corrosion current density of coating in3.5%(mass) NaClsolution are-1.082V and3.105e-006A/cm2. Coating has a good resistance to water, which canwithstand corrosion for about75days namely1800h in3.5%salt water, and the time of corrosionresistance in5%(mass) salt spray test is1200h; The coating with scratch width of less than or equal to1mm has a better corrosion resistance for1800h. When the scratch width is greater than3mm, thecorrosion time reduce, and when electrochemical test was taken for broken coating, the corrosionpotential of the previous sample came out to be-1.082V, while the latter sample had a potential of-0.664V, the latter one had a similar potential with the matrix, thus it lost the cathode protection; thefour kinds of coatings containing TiO2, SiO2, Al2O3, ZnO nano-particles have no obvious change,which have perfect microstructure and similar main ingredients. Binding force are better, the hardnessof the four coatings has increased by15.3%,7.8%,25.9%,10.4%respectively. And the potential offour kinds of coatings added into nano-particles are negative than the matrix, and they are alsopositive than chromium-free Zn-Al coating. The corrosion potential of coating enhanced by Al2O3 nano-particles is the lowest, and the current density is maintained at magnitude of10-6. Each coatinghas a corrosion resistance time of1250hours in salt fog test containing5%(mass) NaCl. The maincomposition of coating enhanced by particles is determined to be more complex alkali carbonatecompounds through the detection of XRD. Nano-particles can improve the potential, increase thehardness of the coating, and increase the corrosion resistance of coating.