| Electroplating zinc can improve the anticorrosion longevity of steel products invarious atmospheric environments. Thus，electrogalvanized steel is used extensivelyin architectures， transportation， housewares， and other areas.However,electrogalvanized steel is often treated by cost-effective chromate conversion coatingbecause it is prone to appear white rust in humid environment. Due to carcinogenicityof hexavalent chromium，RoHS Directive released by the EU in2006restrict the useof hexavalent chomate.―Heavy metal pollution comprehensive prevention andcontrol12thfive–year plan‖，which is the first12thfive-year special plan promulgatedin2011，clearly defined chromium as main heavy metal pollutant to control. Hence,an intense research effort should be undertaken to study new chromium-freeconversion coating technology on galvanized steel．Due to tiny crack of conversion coating formed on galvanized steel treated withcerium salt solution that prepared by our research group previously,the effect ofpost-sealing with fluorosilicate D and sodium silicate on the corrosion resistance ofthe cerium conversion coating was discussed by using a single-factor method in thispaper,respectively．48h neutral salt spray test results showed that fluorosilicate Dimproved the corrosion resistance of the conversion coating, corrosion area dropedfrom28%in the one-step to8%after post-sealing with magnesium fluorosilicate.Thecorrosion resistance of cerium conversion coating sealed with sodium silicate isgreatly improved, microcracks of cerium conversion coating disappeared and acontinuous composite coating was formed on the surface of electrogalvanized steelafter sealing. The corrosion current density was decreased from4.64μA/cm2ofcerium conversion coating to1.51μA/cm2of composite conversion coating, corrosion areas decrease from52%of the cerium conversion coating to4%of the compositeconversion coating， achieving industrial applications standard requirements. |
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