Proceedings of the Korean Institute of Surface Engineering Conference (한국표면공학회:학술대회논문집)

The Korean Institute of Surface Engineering (한국표면공학회)
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Effects of phosphating bath compositions on the formation and structure of zinc phosphate conversion coatings on magnesium alloy AZ31

  • Phuong, Nguyen Van (Materials Processing Division, Korea Institute of Materials Science) ;
  • Lee, Kyuhwan (Materials Processing Division, Korea Institute of Materials Science) ;
  • Chang, Doyon (Materials Processing Division, Korea Institute of Materials Science) ;
  • Moon, Sungmo (Materials Processing Division, Korea Institute of Materials Science)
  • Published : 2012.05.31
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Abstract

This study discussed the formation of phosphate conversion coatings on AZ31 Mg alloy (AZ31) from the zinc phosphating bath with various concentrations of sodium fluoride (NaF). The effects of NaF on the formation, structure, composition and electrochemical behavior of the phosphate coatings were examined using scanning electron microscopy (SEM), X-ray diffraction (XRD) weight balances, open circuit potential (OCP) transients, potentiodynamic polarization curves and immersion test. The coatings were composed of two layers: an outer $Zn_2(PO_4)_3.4H_2O$ (hopeite) crystal layer and an inner amorphous of $MgZn_2(PO_4)_2$. NaF concentration is emphasized to be highly effective in the formation of the hopeite crystal and etching and coating rates. Potentiodynamic polarization and immersion test showed that the coatings formed in the zinc phosphating bath with addition of NaF have much higher corrosion resistance than bare AZ31.

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