Tannic acid compound as a corrosion inhibitor: film-forming characteristics and corrosion resistance

Tannic acid (TA) is widely used to protect metals from corrosion because it is environmentally friendly and inexpensive. However, the single effect of TA used as corrosion inhibitor has been widely investigated and studies focusing on the corrosion inhibition effect have been limited. Some studies have proven that the addition of a compound corrosion inhibitor can considerably improve the corrosion inhibition efficiency of an inhibitor, and this method can be applied to TA. The corrosion inhibition effect of the combination of two compounds, FeCl₃ and Na₂MoO₄, with TA was analyzed on carbon steel Q235. Copper sulfate drip test, soaking test, and electrochemical test were used to compare the film-forming characteristics and corrosion inhibition effect of the combination of FeCl₃ and Na₂MoO₄ with TA on carbon steel surface. The discoloration time of copper sulfate droplets initially increases and subsequently decreases with the increase in the concentrations of FeCl₃ and Na₂MoO₄ in TA. At the end of the soaking test, fewer pits are observed on the surface of carbon steel following the addition of FeCl₃ and Na₂MoO₄ to the TA inhibitor. Based on the results of the electrochemical tests, the corrosion inhibition effects of the TA inhibitor on carbon steel before and after the addition of FeCl₃ were compared. The results reveal that the charge transfer resistance of the two inhibitors increases from 2698 to 3711 Ω·cm², and the corrosion current density decreases from 2.734 to 1.902 μA·cm⁻². A clear increase and decrease in the charge transfer resistance and corrosion current density, respectively, are observed once Na₂MoO₄ is added. Further, the charge transfer resistance increases from 2698 to 5100 Ω·cm², and the corrosion current density decreases from 2.734 to 0.714 μA·cm⁻². The following conclusions can be drawn from these results: the addition of FeCl₃ or Na₂MoO₄ to TA can both improve the corrosion inhibition effect of TA; the compound system of TA and Na₂MoO₄ exhibits a better corrosion inhibition effect compared with the compound system of TA and FeCl₃.