Abstract
Purpose - Zn-Al alloys are widely used as coatings for corrosion protection of steel. These alloys provide long-term protection to steel in several aqueous media; however, little attention has been paid to their behaviour in acidic media. The aim of this investigation is to study the corrosion and inhibition of 90 per cent Zn-10 per cent Al alloy in hydrochloric acid.Design methodology approach - Pyridine and a number of its methyl-containing derivatives were applied in controlling the corrosion of 90 per cent Zn-10 per cent Al alloy. The inhibitive action and mechanism of these compounds were investigated by weight loss tests, linear polarisation resistance measurements, galvanostatic polarisation tests and electrochemical impedance spectroscopy.Findings - It was found that the corrosion rate of this alloy was much higher than that of either Zn or Al alone. Pyridine and its derivatives exhibited good inhibition for the corrosion of 90 per cent Zn-10 per cent Al alloy, their inhibition efficiency increased in the order: I < II < III < IV. Potentiodynamic polarisation studies revealed that the investigated pyridine derivatives were mixed-type inhibitors and functioned by adsorption on reactive sites on the alloy surface through the influence of the nucleophilic nitrogen atom forming a good physical barrier to prevent access of aggressive ions to the alloy surface. This was supported by the impedance measurements which showed a change in the charge transfer resistance and double layer capacitance indicating adsorption on the alloy surface. It was shown that the introduction of the methyl group(s) into the pyridine ring enhanced the effectiveness of pyridine inhibition. The improvement was attributable to the electron-donating tendency of this group that gives rise to the increase in the electron density at the adsorption site. All the investigated substituents showed negative values of the Hammett constant (σ), indicating their electron-donating property. The negative value of this constant increased according to the order: I < II < III < IV, the greater the negative value of σ being consistent with the greater degree of corrosion inhibition. It also was revealed that the presence of these inhibitors increased the value of activation energy for corrosion with a ranking order that was consistent with that of their inhibition effectiveness.Research limitations implications - The evaluated inhibitors assured significant corrosion inhibition of the Zn-Al alloy in HCl, however, their behaviour in different acid media was ambiguous.Originality value - This paper makes significant contibution to understanding the corrosion and inhibition of one of the widely apply coatings for steel protection in one of the most widely used aqueous mineral acids in industry.