Investigation of the corrosion behaviors of continuously hot-dip galvanizing Zn–Mg coating

Corrosion behaviors of continuously hot-dip galvanizing Zn–Mg (ZM) coating with 0.2 wt.% Al containing 1–3 wt.% Mg and Zn (GI) coating with 0.2 wt.% Al in an aerated and quiescent 5 wt.% NaCl solution were investigated. The linear polarization method was used to study corrosion path of GI and ZM coatings. The experimental results show that the grain corrosion in the ZM coatings can be significantly inhibited by Mg after 24 hour immersion especially for the Zn coating containing 2 wt.% Mg. Localized corrosion occurred within Zn grains or Mg-rich regions when the Mg content is not 2 wt.%. The results of electrochemical impedance spectroscopy (EIS) also proved the results mentioned above. It was also found that Mg can significantly improve the adherence of the corrosion product layer and reduce surface cracks especially for a content of 2 wt.% Mg in the coating after 11 days of linear polarization treatment. The corrosion products of GI coating are as same as ZM coatings after the linear polarization treatment. The enhanced corrosion resistance of ZM coatings is mainly due to grain corrosion inhibition and strong adherence of corrosion products.

► The addition of Mg to the Zn coating inhibits grain boundary corrosion.
► With Mg content increasing, the preferential corrosion regions are different.
► Mg in the coating can improve the adherence of the corrosion product layer.
► 2 wt.% Mg in the coating obtains the best corrosion resistance.