Corrosion behavior of the hot-dip galvanized steel sheet under adhesion of an alkaline mud in hot-humid environments

The corroded behavior of the hot-dip galvanized (HDG) steel sheet used in automobile under mud adhesion was investigated by the hot and humid corrosion test. The corrosion products were observed and analyzed by means of SEM and XRD. The results show that the corrosion process of the HDG steel sheet with the mud adhesion could be divided into two steps, i.e., the formation of white rust due to corrosion of the zinc coating and the formation of red-black rust due to corrosion of the steel substrate. The content of γ-FeOOH in the corrosion products of the substrate is higher than others and the aggregation of Cl^- ions in the mud accelerates the failure of the protection of the rust layer. The corrosion mass-loss increases continually with time but the corrosion mass-loss rate decreases correspondingly. The corrosion mass-loss rate at the higher temperature increases suddenly after a period of exposing time because the high temperature accelerates the interface reactions. In the corrosion process, there is a jump in the corrosion potential from a lower to a higher value, corresponding to the corrosion process of the zinc coating and the steel substrate, respectively. The potential jump will become earlier with temperature increasing.