Preparation And Properties Of Environment-friendly Conversion Coating On Steel Surface

Reinforced concrete structures are widely used in various construction facilities,it will cause huge economic losses,even life safety problems once the failure occurs.One of the major causes of failure of reinforced concrete structures is the corrosion of steel in reinforced concrete.Therefore,it has great importance to improve the corrosion resistance of the steel.Chemical conversion coating technology,which have the benefits of low cost,easy to cooperate with other techniques,improve the corrosion resistance of steel dramatically.Since conventional chromate conversion films have a carcinogenic effect,development of new efficient and environmentally friendly corrosion resistance conversion coatings.Based on the ability of phytic acid to chelate metal ions,Zn²⁺ions and sodium mono-fluorophosphate were used in the converting solution to modified the phytic acid conversion membrane.Deferent Zn²⁺concentration modified converting film were made by changing the quantity of zinc nitrate.Electrochemical measurements which contain Electrochemical impedance spectroscopy,polarization curves,the potential of zero charge,the MS curve were used to get a best concentration of Zn²⁺.And SEM,XPS were applied to analysis the morphology and elemental composition of the film,compared to based one.The result indicated that Zn²⁺phytic modified film shows an effectively improvement on the corrosion resistance,Zn²⁺played a role of"bridge"in the film forming process.When Zn（NO₃）₂ was added at the optimal value（1g/L）,the film pitting potential is greatly increased,the passive current density and carrier density were reduced,the surface of the film was smoother for fewer defects,and the corrosion resistance was best.The zinc ion-modified phytic acid conversion film isolates the substrate from the environment to improve corrosion resistance.The synergistic effect of mono-fluorophosphate and phytic acid was used to prepare mono-fluorophosphate/phytic acid film.The optimal conversion temperature and pH range of the film-forming solution were obtained through single-factor experiments.Different processes were prepared by orthogonal test,with the pitting potential as an indicator,passive current density and the charge transfer resistance was the criterion of selecting the best preparation process.When the conversion temperature is 25℃,pH=6,the film formation time 15min,the pitting potential is the highest,the passive current density is also lowest,and the film layer has the best corrosion resistance.The effect of sodium mono-fluorophosphate content on the film layer was studied under the conditions of the optimal preparation process.When the addition amount of sodium mono-fluorophosphate is 2g/L,the film has the best quality.The mono-fluorophosphate/phytic acid membrane isolates the substrate from the environment to improve corrosion resistance.Using Zn²⁺ions to modify the mono-fluorophosphate/phytic acid film,it was found that the pitting potential was reduced and the passive current density and carrier density were enhanced.The coexistence of Zn²⁺and mono-fluorophosphate produces ZnF₂,which is a slightly soluble precipitate,precipitated at the interface between the membrane and the substrate,destroying the integrity of the membrane.