The Modification Of Fluoferrite Conversion Coating On Iron And Steel

Iron and steel is widely used but corrodes easily，which brings about huge lost toindustrial and agricultural production and human’s life. Conversion coating technologyis widely applied due to its simple operation and low cost. Traditional treatments（phosphating and chromate treatment） are being limited to use because of theirenvironmental pollution problem. Carcinogenic Cr（VI） was limited to use by theEuropean Union RoHS Directive in2006, and “Heavy metal pollution comprehensiveprevention and control12thfive year plan”, the first12thfive year plan,was putforward in2011, giving strict control requirements over the five heavy mentalsincluding hexavalent Cr. The research on non-chromate and green conversiontreatment technology is imperative.In order to further improve the anti-corrosion properties of the conversion coatingand reduce F-concentration in conversion solution, on the basis of fluoferriteconversion coating developed by our research group, moderate F-concentrationfluoferrite conversion solution and its modification by ferrous salts and surfactants areresearched in this paper.The research results showed that5⁸hours salt spray time of the sample treatedby KF50g/L，HNO₃25ml/L，（NH₄）₂S₂O₈15g/L at room temperature for30minutes isclose to salt spray time treated by125mg/L KF conversion solution. When the sampletreated by the conversion solution adding with ferrous salt at room temperature for1.5hours, its salt spray time increased to50hours. With the addition of cationic surfactantDTAB （Dodecyl Trimethyl Ammonium Bromide），the treated sample salt spray timeincreased to40hours. The results from SEM and EDS showed that, the conversioncoating modified by ferrous salt was denser columnar particles and consisted of 39.36%F,27.81%K,21.09%Fe and11.73%O; the polarization curves showed thatcorrosion potential was about100mV more positive, corrosion current density wasnearly7.2times lower, and the corrosion resistance of the coating was greatlyimproved. After modified by DTAB, the composition of the coating was almost thesame as the one treated by the solution with ferrous addition, but the conversioncoating was more homogeneous.