Preparation And Performance Of Composite Conversion Coating On Iron And Steel

Steel is widely used in various aspects of modern life, but its corrosion has causedtremendous losses. Phosphating coating is one of the most common methods forprotecting steels against corrosion. Phosphating coating formed on steel byelectrochemical and chemical reaction is a way to increase the corrosion resistance ofsteels or as pretreatment coating to improve the adhesion of fnal coating on steels whichimprove the anti-corrosion performance of steel effectively. However, traditionalphosphating and organic coating techniques have shortcoming of complex process.Moreover, wastewater containing phosphate and heavy metal ions pollutes surface water,which especially causes water eutrophication problem.On the basis of successful preparation of fluoride ferrite coatings, the effect ofdifferent inorganic substances for conversion coating were studied by single factorexperiments of potassium ferrocyanide, zirconium sulfate, cerium sulfate, chromiumsulfate and chromium nitrate. The corrosion resistance of conversion coating wasinvestigated by neutral salt spray test. The results showed that chromium nitrate couldimprove the morphology and corrosion resistance of conversion coating effectively. Thesalt spray test reached5hours.On the basis of successful preparation of fluoride ferrite coatings, the effect ofdifferent organic substances for conversion coating were studied by single factorexperiments of tannin, gallic acid and acrylic acid. The corrosion resistance ofconversion coating was investigated by neutral salt spray test. The results showed thattannin could improve the morphology and corrosion resistance of conversion coatingeffectively. Black blue conversion coating was preparated by chemical reaction on steel.The salt spray test reached5hours. The morphology, performance and composition of the conversion coating were investigated by scanning electron microscopy(SEM), energydisperse spectroscopy(EDS) and fourier transformed infrared spectra.Considering the performance of organic conversion coating and inorganicconversion coating, the optimal formula and parameters of composite conversion coatingwere obtained by experiments as follows: KF125g/L, HNO3（65%）75mL/L,Cr(NO3)35g/L，accelerator1.0g/L, tannin4g/L, a little OP10,60℃, pH2~3, reactiontime30min. The performance of the composite conversion coating were investigated byelectrochemical measurement, neutral salt spray test, fourier transformed infraredspectra, scanning electron microscopy(SEM) and energy disperse spectroscopy(EDS).The results showed that primary components of the composite conversion coatings onsteel were tannin-Fe3+, tannin-Cr3+complexes and potassium fluoferrite. The compositeconversion conversion coating improved the anti-corrosion performance of steeleffectively and the salt spray test reached9hours.