Corrosion Inhibition of Delta 37 Steel in Acid Pickling Solution by Heterocyclic Thiourea Derivatives

Abstract

Galvanostatic cathodic behaviour of Delta-37 Steel in 1 N-H₂SO₄ pickling bath solutions containing different concentrations of the heterocyclic thiourea derivative. 1-(3-Amino-4-phenylazopyrazol-5-yl)-3-benzoyl-thiourea [A.P.T.U.] was studied. The study shows that this compound acts as a good corrosion inhibitor for this steel in H²SO⁴ bath and with an efficiency which reaches 94% at 50ºC. The inhibitory action of this compound seems to be due to its adsorption on the metal surface, hence blocking the available area for hydrogen evolution reaction without any change in the mechanism. At low concentration less than 5 X 10^–5 M, the additive acts as corrosion accelerator because its concentration is insufficient to block all the anodic sites. The heat of adsorption, Q, was found to be 30.51 K.J./deg. mole. The adsorption of the additive molecule on the metal surface was appeared to be through the sulphur atom. The adsorption process follow the simplified Temkin isotherm. The results show that the surface coverage, θ and the inhibition efficiency increases with the rise of temperature.