Synergistic corrosion inhibition by the sodium dodecylsulphate-Zn²⁺ system

Abstract

The inhibition efficiency (IE) of sodium dodecylsulphate (SDS) in controlling corrosion of carbon steel immersed in the environment containing 60 ppm of Cl^-, in the absence and presence of Zn²⁺ has been evaluated by weight-loss method. It is observed that SDS and Zn²⁺ individually are not good inhibitors. But their combination shows excellent IE. For example, 100 ppm of SDS has only 10% inhibition efficiency whereas 75 ppm of Zn²⁺ has 45% IE. Interestingly their combination shows 93% IE. This suggests that a synergistic effect exits between Zn²⁺ and SDS. The influence of pH on the IE of the SDS-Zn²⁺ system has been evaluated. The protective film has been analysed by Fourier transform infrared (FTIR) and fluorescence spectra. A suitable mechanism of corrosion inhibition is proposed based on the results obtained from weight-loss method, and FTIR and fluorescence spectra. It is found that in the absence of Zn²⁺, the protective film consists of Fe²⁺-SDS complex formed on the anodic sites of the metal surface. In the presence of Zn²⁺, the protective film consists of Fe²⁺-SDS complex and Zn(OH)₂. The protective film is found to be UV-fluorescent.