The corrosion of iron and zinc by atmospheric hydrogen chloride

Laboratory-based measurements of the corrosion of iron and zinc in HCl have been carried out under a range of conditions simulating natural atmospheres. The corrosion rate of zinc did not significantly increase upon exposure to HCl at presentation rates typical of the highest found in an urban area (2.5 × 10−6 mg cm−2 s−1). This is explained by the formation of protective basic zinc chloride by reaction with pre-existing zinc hydroxide. At higher levels of pollutant (representative of episodic industrial atmospheres) reaction of HCl with zinc hydroxide proceeds further to form soluble zinc chloride. Consequently, the protective ability of the corrosion product is lost. Under these conditions, corrosion rate of zinc is controlled by the availability of HCl at the metal surface. In contrast, the corrosion rate of mild steel samples exposed to HCl presentation rates typical of the highest encountered in an urban atmosphere was 18 times that found in an unpolluted atmosphere. Increasing the HCl presentation rate further did not result in a significant increase in corrosion rate. These results are explained by the reaction of HCl at the metal surface and the subsequent formation of FeCl2, which is oxidized to FeO(OH), liberating HCl which can initiate further corrosion. The reaction sequence forms a cycle and is, therefore, apparently independent of incoming HCl.