On the mechanism of formation of zinc pack coatings

The mechanism of the formation of zinc (Zn) pack coatings is studied with scanning electron microscopy (SEM), X-ray diffraction (XRD) and differential scanning calorimetry (DSC). DSC showed that the coatings formation takes place in three steps. The initial step (at 193.9 °C) is endothermic and involves the transformation of α-NH4Cl to β-NH4Cl and the NH4Cl decomposition to NH3 and HCl. During the second step (at 248.6 °C), which is exothermic, Zn2+ salts are formed and especially ZnCl2. Finally at 264.1 °C Zn is deposited by an endothermic reaction on the ferrous substrate through the decomposition of ZnCl2. The as-cast Zn diffuses into the iron lattice forming the gamma (Γ-Fe11Zn40) and delta (δ-FeZn10) phases. Al2O3 is not involved in the above-mentioned mechanism and acts only as filler.