Synergistic corrosion inhibition study of Armco iron in sodium chloride by piperidin-1-yl-phosphonic acid–Zn2+ system

Electrochemical techniques, weight loss method and surface analysis were used to study the synergistic inhibition offered by Zn2+ and piperidin-1-yl-phosphonic acid (PPA) to the corrosion of Armco iron in 3% chloride solution. It is observed that the combination between PPA and Zn2+ shows excellent inhibition efficiency. The potentiodynamic polarization curves reveal that 5 × 10−3 mol l−1 of PPA has only 76.7% inhibition efficiency whereas the mixture containing 5 × 10−3 mol l−1 PPA -20%Zn2+ has 90.2% inhibition efficiency. This suggests that a synergistic effect exists between Zn2+ and PPA. The Fourier transform infrared (FTIR) spectrum of the film formed on iron indicates phosphonates zinc salt formation. A suitable mechanism of corrosion inhibition is proposed based on the results obtained. The surface film analysis showed that in the absence of Zn2+, the protective film consists of Fe2+–PPA complex formed on the anodic sites of the metal surface, whereas in the presence of Zn2+, the protective film consists of Fe2+–PPA complex and Zn(OH)2.