The Synthesis Of Submicron-sheet Zinc Phosphate And Its Influence On Anti-corrosion Performance Of Epoxy Coating

Zinc phosphate is widely used in heavy-duty coatings as non-toxic anticorrosive pigment,which is an effective pigment to replace the traditional toxic anticorrosive pigments,such as the heavy metal pigments including lead and chromium.But the zinc phosphate preparated by traditional process have some defects as large particle sizes,incomplete conversion,and other on.The defects restrict the application of traditional zinc phosphate in coatings.Submicron-sheet zinc phosphate?SZP?has been synthesised by sol–gel method,and has been added into epoxy resin to make the protection coatings in this paper.Then,the protection performances and mechanismes of SZP coatings have been investigated.According to the refining method of zinc oxide ore by ammoniacal leaching,the colorless and transparent solution of Zn?NH₃?₄²⁺was preparated.Through ethanol diluting phosphoric acid solution and Zn?OH?₂ Gel suspension,submicron-sheet zinc phosphate with the thinkness about 200nm was quickly prepared at room temperature.Then the composition and structure of SZP pigment was characterised by X-Ray Diffractometer?XRD?,Fourier Transform Infrared Spectrometer?FT-IR?,Thermogravimetric Analysis?TG?,and Scanning Electron Microscope?SEM?:the prepared SZP pigment was a single crystal phase of?-Zn₃?PO₄?₂·4H₂O with uniform shape and thichness about 200nm,and it contained 1.5%adsorbed water;the purchased CZP pigment was main Zn₃?PO₄?₂·2H₂O and small amount of Zn₃?PO₄?₂·4H₂O,and its shape was not uniform and the thichness was about 2?m.The high-speed impeller mixing,planetary ball milling and simple bead milling were used to disperse CZP and SZP into epoxy resins.The dispersion results showed that:CZP pigment with larger particle could be dispersed by the three modes and got good dispersion status in the coatings.However,SZP pigment was only dispersed by simple bead milling,effectively.CZP coating and SZP coating with 10 wt.%pigment addition were preparated by simple bead milling,then the protection performances of coatings were researched and analysed through Electrochemical Impedance Spectroscopy?EIS?test,adhesion test,and SEM observation.The submicron-sheet structure of SZP pigment could reduce defects in the coating at a certain extent;reduce the diffusion coefficient of electrolyte in SZP coating and resist corrosion electrolyte reaching the coating/metal interface,to improve the barrier performance of coatings in the early immersion time.SZP pigment had more contact area with the electrolyte solution to promote the dissolution of znic phosphate,and to release more PO₄^3-to inhibit corrosion reaction in the coating/metal intersurface,effectively.The protection performances of CZP coatings and SZP coatings with different pigment concentrations?5,10,20,30 wt.%?were researched.We found that:the protection performance was closely related to dispersion status of pigment in coating.The factors of value of volume concentration of pigment?PVC?/critical pigment volume concentration?CPVC?,dissolvation of zinc phosphate on the pigment surface,releasion of stress in pigment/resin interface,and aggregation of pigments in coatings affected the evolutions of defects in coatings:there were not fatal defects forming in 5%,10%CZP coatings and 5%SZP coating during the 3000h immresions,but the lower pigment addtions made these coatings had the general barrier performances;the protection performance of 20%and 30%SZP coatings lost the main protective performance during the initial 500h immersions;the protection performance of 20%and 30%CZP coatings lost the main protective performance during the 1500-2000h immersions;Because of submicron-sheet structure and proper value of PVC/CPVC,10%SZP coating??=0.35?still has excellent protection performance after 3000 hours immersion.