Preparation And Corrosion Resistance Of Phosphate Coating On Q235 Steel Surface

Low carbon steel is widely used in the industrial field due to its high plastic toughness,good cold formability,and excellent welding performance.However,low carbon steel has poor corrosion resistance and reduces its service life in practical applications,so it is necessary to prepare protective coatings on its surface.In this paper,a direct coating method was used to prepare a phosphate-based corrosion-resistant coating on the surface of Q235 low carbon steel.By exploring the influence of the preparation process parameters and the amount of modified filler on the corrosion resistance of phosphate coatings,the formation process of the coating was analyzed and the related corrosion resistance mechanism was clarified.The research results show that the La（NO₃）₃ solution is first used to pretreat the Q235 substrate with chemical conversion.The chemical conversion film formed effectively blocked the reaction between the substrate and the acidic binder and reduced the formation of pores in the coating.Compounding it with the subsequent phosphate coating can further improve the corrosion resistance of the coating.With MgO as the curing agent,when the curing temperature is 150℃,with the increase of the H₃PO₄/Al（OH）3 molar ratio,the bubbles on the surface of the coating increase and the corrosion resistance decreases;when the H₃PO₄/Al（OH）3 molar ratio is 3:1 can prepare the aluminum hydrogen phosphate binder with the best viscosity,and promote the transformation of amorphous materials into crystalline state.They are coated on the surface of Q235 substrate after conversion treatment to obtain a network phosphate macromolecular structure with AlPO₄ as the main film-forming substance,which makes the coating have certain corrosion resistance.Adding Al₂O₃ to the above aluminum dihydrogen phosphate binder can improve the density of the coating,effectively reduce the porosity,and thereby improve the corrosion resistance of the phosphate coating.With the increase of Al₂O₃ content,effectively reduce internal defects such as cracks,the corrosion current of the coating containing 20 wt.% Al₂O₃ filler is reduced to 10-6A/cm2,the impedance value increases in a short time,and the corrosion resistance improves,but the surface of the coating will still be damaged and corroded for a long time.After modification with silane,metakaolin has certain hydrophobic properties,and its use as a filler can also improve the corrosion resistance of the coating.As the modified filler increases,the hydrophobic angle of the coating shows an increasing trend.When the metakaolin filler is 12 wt.%,the hydrophobic angle of the coating can reach up to 149℃.And the coating surface is evenly and densely distributed,which affects the corrosion medium to a good barrier.Corrosion behavior research shows that the polarization resistance of the coating containing 8wt.% modified metakaolin is higher,and the corrosion resistance is most significantly improved.Soaking in NaCl solution for 20 d can still resist the corrosion medium.