Corrosion of mild steel in acid solutions containing thiourea and thiosemicarbazide

Cathodic polarization curves for mild steel in 1.0 N H2SO4 and 1.0N HCl, containing various concentrations of thiourea (TU) (1–25 mM) and thiosemicarbazide (TSC) (0.2–25 mM), were measured in the temperature range 20–50 °C. The apparent activation energy for corrosion (ΔE*) was calculated from Icor vs 1/T and Rp/T vs 1/T relations. ΔE* is virtually constant by changing TU concentrations (55–57 KJ/mole in H2SO4). An interpretation for the loss of inhibition efficiency of TU in acid solution is given. This is based on the decomposition of TU to ammonium thiocyanate.