Studies of the anti-corrosive effect of Raphia hookeri exudate gum-halide mixtures for aluminium corrosion in acidic medium

Purpose - The purpose of this paper is to investigate the effect of Raphia hookeri exudate gum and halide ions on the corrosion inhibition of aluminiurn in HCl solutions at 30-60 degrees C and to study the mechanism of action. Design/methodology/approach - The corrosion rates were determined using the gravimetric (weight loss), gasometric (hydrogen evolution) and thermometric techniques. The results obtained in the absence and presence of Raphia hookeri, halides and Raphia hookeri - halides combination were used to calculate the inhibition efficiency (%l), degree of surface coverage and to propose the mechanism of inhibition and type of adsorption. Findings - Results obtained showed that the Raphia hooked exudates gum acted as an inhibitor for aluminium corrosion in acidic environment. Inhibition efficiency (%l) increased with increase in concentration of the Raphia hookeri exudates gum and synergistically increased to a considerable extent on the addition of halide ions. The increase in inhibition efficiency (%l) and surface coverage (theta) in the presence of the halides was found to be in the order I- > Br- > Cl- which indicates that the radii as well as electronegativity of the halide ions play a significant role in the adsorption process. Raphia hookeri exudates gum obeys Freundlich, Langmuir and Temkin adsorption isotherms. Phenomenon of physical adsorption is proposed from the values of kinetic/thermodynamic parameters obtained. The values of synergism parameter (S-l) obtained for the halides are greater than unity suggesting that the enhanced inhibition efficiency of the Raphia hookeri caused by the addition of the halide ions is only due to synergistic effect. Research limitations/implications - Electrochemical studies such as polarization and AC impedance spectra will throw more light on the mechanistic aspects of the corrosion inhibition and more exudate gums need to be evaluated as corrosion inhibitors. Practical implications - Raphia hooked exudate gum can be used as corrosion inhibitor and the addition of halides to it improves the inhibition efficiency considerably. Originality/value - This paper provides new information on the use of exudate gums as corrosion inhibitors and addition of halides in acidic medium. This environmentally friendly inhibitor could find possible applications in metal surface anodizing and surface coatings.