Corrosion protection by zinc‐magnesium coatings on steel studied by electrochemical methods

The mechanism of corrosion protection of zinc‐magnesium coatings on steel is investigated in this work in order to understand if each layer of the metallic coating is able to provide galvanic protection to the underlying layer and to the steel substrate. Thus, the electrochemical behavior of the metallic coating is studied as a function of the in‐depth structure and composition. The microstructure of each layer is analyzed using the scanning electron microscopy in combination with energy dispersive X‐ray spectroscopy (SEM/EDXS). The electrochemical characterization is carried out by means of electrochemical micro‐cell and scanning vibrating electrode technique (SVET). The corrosion protection mechanism of Zn‐Mg metallic coatings is based on the galvanic protection provided by the Zn‐Mg and Zn layers to the steel substrate. Preferential Mg dissolution in the metallic coating plays an important role in the protection mechanism. Local alkalinization at cathodic sites favors the precipitation of protective Mg‐rich oxides/hydroxides that reinforce the protective corrosion products and therefore could inhibit corrosion processes on the metallic coating and steel substrate.