Electrochemical Behavior of Steel Covered with Corrosion Products of Zinc

Corrosion protection mechanism of galvanized steel is mainly attributed to galvanic action of zinc and barrier effect of corrosion products of zinc.^1)-2)  Especially, it is said that corrosion products of zinc offer some corrosion protection to steel even after sacrificial dissolution of metallic zinc.³⁾  In this study, electrochemical behavior of steel covered with zinc corrosion products was investigated to clarify protection mechanism for steel corrosion by zinc corrosion products.

A galvanic couple of Zn/steel, consisting of a steel substrate and a zinc layer, was used to prepare steel covered with zinc corrosion products.  The zinc layer with 2 mm in width and 2 um in thickness was prepared by electrodeposition of zinc on the center of the steel substrate.  The surface ratio of steel and zinc was 6:1.  The Zn/steel couple was immersed in 1 M NaCl or 1 M MgCl₂until the zinc layer dissolved completely.  The zinc corrosion products formed on the steel substrate were observed and analyzed by SEM-EDX and Raman spectroscopy.  Micro-electrochemical technique was employed to measure anodic and cathodic polarization curves for the steels covered with zinc corrosion products.  The test solution for the polarization measurements was 0.1 M NaCl.

From the results of surface observation, precipitation morphology of zinc corrosion products changed depending on the types of the electrolyte solutions and the distance from the zinc layer.  In case of 1 M NaCl, zinc corrosion products precipitated mainly around the zinc layer and the amount of the products decreased with increasing the distance from the zinc layer^4)-6).  The zinc corrosion products were confirmed to be zinc hydroxide and simonkolleite.  It was found that cathodic reaction for steel covered with these corrosion products was inhibited.  On the other hand, in case of 1 M MgCl₂, precipitation morphology of zinc corrosion products was different from that in 1 M NaCl and the corrosion products containing Mg were formed on the steel a certain distance away from the zinc layer.  The results of electrochemical polarization tests showed that anodic kinetics was suppressed on the steel covered with the corrosion products containing Mg.

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