The corrosion mechanism initiation of a 75Sn-25Pb coating on a low-carbon steel sample in HCl environments

The corrosion of a 75Sn-25Pb coating on low-carbon steel exposed to a vapor/condensate phase over a 1 mol/L HCl solution has been studied. The corrosion mechanism initiation was based on several steps. The presence of CO₂ and lead involves the precipitation of cerussite PbCO₃ crystals. Overtime, the cerussite evolved and transformed into PbCl₂. This transformation was not direct and involved the creation of reaction intermediates whose chemical composition was variable. In parallel, tin formed oxides that were not stable in the HCl solution. These oxides dissolved and led to the formation of an electrolyte concentrated in Sn²⁺. When tin ions' saturation was reached, tin precipitated to form abhurite, Sn₂₁Cl₁₆(OH)₁₄O₆, or hydrated tin chloride, SnCl₂.2H₂O. This study highlighted the particularity of the corrosion mechanism. It also showed that the presence of tin promoted the oxidation of lead and consequently the formation of cerussite. (authors)