Preparation And Coating Performance Analysis Of High Anticorrosive Water-based Chromium-free Dacromet Coatings

Metal corrosion often significantly reduce the strength,plasticity,toughness and other mechanical properties of materials,destroy the geometric shape of metal components,deteriorate the electrical and optical physical properties,shorten the service life of equipment,and even cause major catastrophic accidents,causing huge economic losses to the national economy.Improving the corrosion resistance of metal parts plays an important role in economic development,resource recycling and environmental protection.Coating protection is an important technical means of metal corrosion prevention.Among many coating technologies,Dacromet coating is one of the coating protection technologies with excellent comprehensive performance,wide application and rapid development.However,it uses hexavalent chromium compounds as film forming materials,which has huge environmental risks in the process of production,transportation and construction,and its application is subject to more and more restrictions.Water-based chromium-free technology came into being.At present,there are many problems such as poor corrosion resistance,high production cost and short storage time of waterborne chromium-free coatings.In this thesis,the processing and coating properties of coating were improved by optimizing the composition of coating liquid,refining the processing process,using flake zinc aluminum alloy powder,adding rare earth compounds as corrosion inhibitors and other measures.Combined with modern analysis and testing technology,a kind of water-based chromium-free dacromet coating with stable storage,excellent performance and environmental friendliness was developed.The specific research content is as follows:Firstly,with alcohols such as dipropylene glycol（DPG）as co-solvent,flake zinc-aluminum alloy powder as disperse pigment,and silane coupling agent KH560 as film forming agent,the main components of the coating solution were optimized by L9（3³）orthogonal experiment in this thesis.The initial formula of the coating solution was determined based on the appearance,adhesion and corrosion resistance of the coating as evaluation indexes.The initial formula of the coating solution was obtained as follows:Flake zinc-aluminum powder 20-25g,DPG 10-15g,KH560 10-12g,non-ionic wetting dispersant2.5-3.5g.Secondly,the corrosion resistance of the coating was enhanced by adding rare earth oxides MoO₃ and Y₂O₃ in this thesis.It can be seen from the SEM image that the addition of MoO₃ and Y₂O₃ can fill the pores of the coating,making the film layer more dense and enhancing the shield protection effect of the coating on corrosive media.The results of rapid corrosion resistance to NH₄NO₃ show that the samples prepared by adding MoO₃ and Y₂O₃have the longest corrosion resistance to NH₄NO₃,up to 320min,and the best corrosion resistance.The potentiodynamic polarization curve test method shows that the addition of MoO₃ and Y₂O₃ can make the polarization curve of the coating move towards the direction of lower corrosion current density without changing the dynamic behavior of electrode reaction.The results show that the addition of MoO₃ and Y₂O₃ slows down the corrosion rate and improves the corrosion resistance of chromium-free coating.Thirdly,the effects of the proportion of fillers to film forming substances,the amount of cosolvent DPG in this thesis,the pH of different aqueous phase,the amount of corrosion inhibitor,the compound coupling agent and other components on the corrosion resistance and mechanical properties of the coating were investigated by single factor experiment.According to the results of mechanical properties test,ammonium nitrate corrosion test and electrochemical test,the optimal composition of coating is as follows:flake zinc-aluminum powder 25g,DPG 13.5g,KH560 9.5g,（CH₃）₂Si（CH₃O）₂ 1g,wetting dispersant 3g,rare earth oxide 3.8g and aqueous pH 4-5.The Tafel curve and EIS curve analysis show that the scaly layered structure of metal powder can effectively prevent the diffusion of corrosive medium to the metal substrate surface.Meanwhile,the passivation of zinc and aluminum powder occurs during the corrosion process of the coating,which can delay the corrosion loss of the coating itself.This physical shielding effect,in cooperation with the passivation of zinc and aluminum powder,can significantly improve the coating performance.