Modification Of Fluoroaluminate Conversion Coating On Al2024

Because of having superior performances such as good ductibility, high strength,low density and great heat resistance, Al2024is widely used in the aerospace andelectronic industries. Nevertheless, Al2024is particularly susceptible to corrosion dueto the strengthening phases CuAl2and Al2CuMg, thus it is often passivated by theeconomic and effective chromate technology in the industry. However, because of thetoxicity and carcinogenicity of hexavalent chromium, RoHS Directive （the limitationof the use of certain hazardous substances in the electrical and electronic equipments）released by the European Union imposed strict restrictions on the use of hxavalentchromate.“Heavy metal pollution comprehensive prevention and control12thfive-yearplan”, the first12thfive-year special plan promulgated by our country recently, clearlyput forward chrome as the main total amount control metal. Therefore, it is extremelyurgent to study and develop effective, low cost and environmental friendlychromium-free conversion coating technology.Based on the previously successful preparation of fluoroaluminate conversioncoating on Al3003by our research group, this paper researched the modificationeffects of fluosilicates, complexing agents and corrosion inhibitor on fluoroaluminateconversion coating on Al2024by using the single factor experiment. The resultsshowed that the optimal conversion solution and converting conditions were as follows:NaF5g/L,（NH4）2SiF65g/L,（NaPO3）60.9g/L and inorganic accelerator0.5g/L, pH4.7,treatment time20min and room temperature. The scanning electron microscope（SEM）indicated that modified fluoroaluminate conversion coating was composed ofcontinuous and compact crystal particles，and covered with colloidal species. The energy disperse spectroscopy （EDS） analysis showed that the primary elements ofconversion coating and their contents were O7.53%，F48.87%，Na19.11%，Al20.78%，Si0.79%，P1.66%，Cu1.26%. SEM photos and EDS analysis manifested thecoating was made of sodium fluoroaluminate skeleton and SiO2gel and CuNa₄P₆O₁₈precipitation. The result of polarization curve analysis demonstrated thatfluoroaluminate conversion coating made the corrosion potential of Al2024increasing58mV and the corrosion current density decreasing approximate9times. Neutral saltspray time was up to285hours, more than the industry standard requirements of96～120h to chromium-free conversion coating on average aluminum alloy, showing thatthe corrosion resistance of Al2024was evidently improved.