Unusual corrosion behaviour of carbon steel in hydrochloric acid driven by surface polymerization of propargyl alcohol

Propargyl alcohol (PA) is a widely used acetylenic inhibitor for protecting carbon steel in acidic environments, particularly hydrochloric acid (HCl). Its performance is commonly assumed to improve as the acidity of the medium decreases. In this work, the corrosion behaviour of carbon steel in HCl was investigated over a broad concentration range (0.036–6.8 mol/L). The results reveal a pronounced non‑monotonic corrosion behaviour, with the corrosion rate passing through a minimum at intermediate HCl concentrations and increasing both at higher acid concentrations and, counterintuitively, upon dilution of the acid.To elucidate the origin of this behaviour, surface processes were examined using iron powder combined with time‑resolved ¹H NMR spectroscopy to quantify PA consumption under corrosion conditions. Kinetic analysis reveals a two‑stage transformation of PA at the iron surface, comprising an initial stage associated with rapid adsorption and surface activation, followed by a slower stage governed by surface‑initiated polymerization and growth of a protective polymer film. The kinetics of this polymerization‑controlled stage show a strong dependence on acid concentration. Spectroscopic analyses demonstrate that corrosion protection is associated with the formation of a surface‑bound polymeric film composed of partially unsaturated poly(propargyl alcohol) and predominantly saturated poly(allyl alcohol) fragments. Quantitative estimates based on PA consumption indicate that dilution of HCl leads to the formation of markedly thinner polymeric film, correlating directly with the observed increase in corrosion rate at lower acid concentrations. Continuous consumption of PA provides direct evidence for a dynamic protective film maintained by surface‑controlled polymerization processes in corrosion.