Influence of soil chemical characteristics on corrosion behaviour of galvanized steel

Hot-dip galvanized steel is widely used for the construction of underground structures, thanks to the presence of a zinc coating that is able to protect the carbon steel substrate through the formation of a corrosion products layer. Soil corrosion is a very complex process and hence it is difficult to predict the corrosion resistance of a specific component because many factors can directly influence the behavior of material immerged in soil. This work focused on the effect of chlorides and sulphates concentrations on the corrosion of hot-dip galvanized steel wires buried in soil. In particular, the wires were monitored by means of electrochemical techniques, as open circuit potential, electrochemical impedance spectroscopy and potentiodynamic polarization. Moreover, at the end of the exposure, the samples were extracted and observed by optical and electron microscopy, to examine the corroded surface and the corrosion morphology. The analysis evidenced that the effect of chlorides and sulphates is very severe, producing a great degradation of the zinc coating. In particular, the synergistic effect performed by the combined action of these elements caused, in the most critical situation, the protection loss of the external layer, putting steel substrate in direct contact with soil, and therefore obtaining a substantial reduction in material corrosion resistance.