| With the continuous development and progress of the times,environmental awareness has gradually been valued by people.The traditional Dacromet coatings have gradually been abandoned due to the inclusion of elemental chromium,which is extremely contrary to environmental protection.An emerging and truly environmentally friendly water-based chromium-free Dacromet preparation technology is slowly becoming a research focus.The chrome-free coating has the advantages of high corrosion resistance and no hydrogen embrittlement,but some of its properties are still incomparable with conventional coatings.Due to the late start of related research in China,it has become extremely urgent to develop a chromium-free Dacromet formulation that can be put into industrial production with superior protection performance.In this paper,through the preliminary literature review and a large number of experimental exploration,we selected suitable experimental raw materials:A suitable metal powder was selected as the primary filler,KH560 as the coupling agent,propylene glycol polyether as the wetting and dispersing agent,and NP-9 as the emulsifier.Based on the evaluation indexes of coating appearance,adhesion and corrosion resistance time immersed in 20%ammonium nitrate solution,orthogonal experiment was used to optimize the coating composition.The optimum composition of the coating was 25%of Zn-Al(6:1),20%of silane coupling agent,20%of propylene glycol polyether.Then,the coating preparation process and coating process were optimized and the most suitable process was selected:According to the weight percentage,the KH560 was weighed into a three-necked flask,and Zn powder was added while stirring.After stirring,the A1 power was added,keep stirring for 30-60 minutes,then the propylene glycol poly ether was added,and stirring was continued.After 3-4 h,the A component was done;the ethanol and water were weighed and stirred,the silica was added,and the NP-9 was added dropwise at a rate of 50-60 drops/min while stirring,and stirring was continued for 6-8 hours.The component B was done;the two components A and B were mixed and stirred for 3 to 4 hours to obtain a coating.Through the single factor test method,the influence of zinc-aluminum powder mass ratio,propylene glycol polyether,passivation agent and pH value and viscosity of coating on the protective performance of the coating was discoverrd:the greater the aluminum powder content,the more white the coating color,the greater the viscosity of the coating,and the thicker the coating thickness;the higher the content of propylene glycol polyether,the smaller the viscosity of the coating.When the coating thickness got thinner,the surface of the coating becomes smooth firstly and then difficult to be coated;the greater the amount of passivating agent added,the greater the viscosity of the coating and the longer the corrosion resistance of the coating.When the passivation agent was increased to a certain extent,increasing its content had little significance for improving the corrosion resistance of the coating,but affecting the viscosity of the coating,making it difficult to coat.The optimum addition amount was 1 g,which was 4%of the metal powder;the pH value affects the dissolution rate of the anode.When the pH value of the coating was 7,the anodic polarization of the coating was higher,which was similar to the passivation tendency.At this time,the protective performance of the coating was better;with the increase of the amount of thickener added,the viscosity of the coating and the thickness of the coating showed a phenomenon of slowly increasing and then rapidly increasing.When the thickener was added in an amount of 6.75%,the viscosity of the coating was 21.2 s,which provided the most effective protection for the substrate.The resulting coating had good stability and can be left at room temperature for more than 15 days.The surface of the coating was smooth without leakage coating;the thickness was 38 ?m;the adhesion was grade land the hardness was 8 H.The protective properties of the coating were tested by the length of time the coating was immersed in an ammonium nitrate solution.The coating properties before and after immersion were investigated by observing the morphology and chemical composition of the coating.After soaking for 300 min in 20%ammonium nitrate solution,the coating still showed no obvious cracks and the substrate was not corroded.The surface morphology of the coating in different immersion time in 3.5%(mass fraction)NaCl solution was studied.The results showed that:the coating before immersion was formed by overlapping layers of flaky zinc powder and flake aluminum powder,and the surface was compact and flat;after soaking for 7 h,the dense passivation film formed on the surface showed fine cracks;after soaking for 72 h,the corrosion products were observed.After immersion for 480 h,the surface coating began to corrode severely and the coating began to split into spherical particles.Using the electrochemical method,the open circuit potential of the coating in the sodium chloride brine soaking process was recorded,and the change of the open circuit potential was obtained:as the immersion time increases,the open circuit potential first moves rapidly and negatively,then rapidly moves forward,and finally steadily shifts positively.Under the same experimental conditions,the coating protection properties were investigated by comparing the polarization curves and impedance spectra of the coated Q235 steel working electrode and the uncoated Q235 steel working electrode.The coating's self-corrosion potential was 332 mV lower than that of Q235 steel,the self-corrosion current density was two orders of magnitude smaller than that of Q235 steel,and the blunt current density was three orders of magnitude smaller than that of Q235 steel.Through the change of the impedance spectrum of the coating during the immersion process and the variation of the fitting parameters,the failure law of the coating is obtained:metal powder activation,passivation film formation and destruction,corrosion product formation and shedding. |
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