Study On The Preparation And Properties Of Dacromet Coating

Since Dacromet coating has high corrosion resistance(is galvanized 7-10 times),high heat resistance(250 ?can continue normal use),nohydrogen embrittlement,high permeability,dark painted a good ability,coatings with good adhesion and no pollution to the environment,etc.So it is caused great concern and gradually applied in many industries,aerospace,petroleum chemical industry,automobile and railway and so on.However,Dacromet coating liquid used the Cr(VI)containing highly toxic,highly carcinogenic in the preparation process.All over the world chromium has been more severe restrictions,and with people gradually raise the awareness of environmental protection,which brings great limitations in the use and development of Dacromet coating technology,for which uses chromate or chromic salt as the passivating agent.Therefore,it is imminent for us to explore and research Low Chromium or Chromium-free Dacromet to replace Cr(VI)of Dacromet.On the basis of the reference Dacromet technology,this paper studied different specifications of flake zinc powder on the properties of the coating effect by using the single factor comparison test.By using orthogonal test method,the optimal formulation of Low Chromium and Chromium-free Dacromet can be defined via partially or completely chromic anhydride.The morphology and composition of the coatings were analysed by using SEM and XRD tests.The corrosion resistance of the coatings were studied by polarization curve,EIS and salt immersion tests,etc.The followin g is the main work and results analysis:1.By using single factor experiment,under the same process conditions compared three different specifications of the flake zinc powder on the effects of Dacromet coating appearance,corrosion resistance and amount,and ultimately identified the metal zinc powder specification during the preparation of Dacromet solutions in this paper.By tests found that smaller particle size of the flake zinc powder,lower corrosion current density,and the superior corrosion resistance.Under the same conditions,the smaller the size of the flake zinc powder coating was applied to the sample corresponding to the greater amount.To obtain the same coating amount,the smaller the size of flake zinc powder,zinc powder amount required is less.2.The optimal formulation of low chromium coating was determined through orthogonal experiment by ammonium nitrate resistant to rapid corrosion test as an index.Using polarization curve,EIS,salt immersion tests studied the coating corrosion resistance of the formula,the results showed that its coating was similar to the traditional Dacromet in corrosion resistance.3.By SEM test found that Low chromium Dacromet coating surface was very dense,no poresand formed layers of overlapped structure with the substrate,whinch can effectively prevent corrosion medium directly to the substrate surface.By XRD analysis test,the coating was mainly composed of zinc,aluminum.Both the electrode potential are lower than iron.When corrosive medium penetration,the zinc and aluminum will be corroded firstly to protect the iron matrix,having pcathodic protection.In addition,complex compounds of borate and phosphate are produced and attached on the coating surface,this can play a shielding effect for corrosive media.4.Adding and not adding the Epoxy resin and silane coupling agent(KH-570)as coating binder in the formula were tested by single factor experiment.Coating adhesion was evaluated by using tape method.The results showed that two kinds of adhesive greatly improved the bonding strength of the coating and the substrate.5.In foundation of the Low Chromate Dacromet coating,the optimal formulation of Chromium-free Dacromet coating was determined through orthogonal experiment by with oxalic acid sodium,citric acid,boric acid as test factors and ammonium nitrate resistant to rapid corrosion test as an index.Compared with property of traditional Dacromet,Chromium-free Dacromet was inferior to traditional Dacromet.But Chromium-free film-forming substances was used in the preparation of Chromium-free dacromet coating,so during the test experiment there was not deposited chromium.6.By SEM observed that Chromium-free Dacromet surface formed layers of overlapped network structure,it can hinder corrosive medium and had played a good physical shielding effect,thus protecting the substrate.By XRD tests found that most of the coating components were Zn and Al.when meeting corrosive medium,the presence of these substances had palyed a sacrificial anode cathodic protection for the iron matrix.