Mechanism of corrosion protection of zinc-magnesium coatings on steel studied by electrochemical depth profiling

摘要

In automotive industry, zinc coated steel is largely used thanks to its high corrosion resistance and sacrificial protection. However, zinc layer must be thick to assure a good protection. As a consequence, consumption of zinc is very intensive and coating prices are increasing. A large number of new coated steel formulations have consequently been studied for many years in order to reduce the consumption of zinc and its cost in coatings. Several studies have shown that the addition of magnesium into the metallic coating allows to improve corrosion resistance without increasing coating thickness.The mechanism of corrosion protection is based on a coupling galvanic protection between magnesium which is less noble that zinc, which is itself less noble than iron. Nevertheless, protection mechanism understanding is not entirely complete. The aim of this work is to understand if each layer of the metallic coating is able to cathodically protect the underlying layer. Thus, the electrochemical behavior of zinc-magnesium coating on steel as a function of the in-depth structure and composition was studied. To achieve this objective, progressive glow discharge optical emission spectroscopy (GDOES) sputtering was used to create craters reaching the different sublayers. The microstructure structure of each layer was analysed using the scanning electron microscopy in combination with energy dispersive X-ray spectroscopy (SEM/EDX) .The electrochemical analyses were carried out in NaCl 0.02 M using a micro-capillary three-electrode microcell and the scanning vibrating electrode technique (SVET).rnAnodic and cathodic polarization curves at different depths using microcell allow to understand the corrosion behaviour of each layer. SVET measurements were carried out in order to understand the formation of galvanic coupling between each sublayer of the zinc-magnesium coating.