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Purpose The purpose of this paper is to test the Natural Kermes dye (NKD) as a cheap and stable corrosion inhibitor for mild steel in 1.0 M HCl and its adsorption mechanism on the steel surface. Design/methodology/approach The inhibition action of NKD was studied using AC impedance, potentiodynamic polarization, scanning electron microscope (SEM) and UV-visible spectra techniques complemented with quantum study. Findings Here, the authors show that addition of NKD inhibits effectively the corrosion of steel in HCl solution via its adsorption on the steel surface. The inhibition efficiency of NKD increases with increase in its concentration and decreases with temperature. Potentiodynamic results revealed that NKD acts as a mixed–type inhibitor. Thermodynamic parameters for corrosion and adsorption process were obtained from the experimental data. Moreover, the experimental inhibition efficiencies were correlated with the electronic properties of NKD using density functional theory. Originality/value To the best of the authors’ knowledge, this is the first report showing the effect of NKD on the corrosion inhibition of steel.

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The electrochemical behaviour of titanium dissolution and passivation in HCl was examined with and without the SCN − anions using potentiodynamic polarisation, electrochemical impedance spectroscopy, and current transients techniques. The nature of the Ti oxide film was characterised by utilising XPS and SEM tools. The corrosion current i corr , the critical passivation current i cc , and the passive current i pass values increased significantly when the HCl concentration, scan rate, and temperature were raised. On the contrary, the addition of SCN − anions decreases the i corr value, indicating that the Ti dissolution was inhibited as a result of SCN − adsorption on the Ti surface. The inhibition efficiency increased with increasing the concentration of SCN − . However, SCN − anions lead to destabilised passivity in the passive zone. The formation of the Ti oxide film is a diffusion-controlled operation, as shown by the i-t curves. According to the XPS measurements, the passive oxide coating consists mainly of TiO 2 and less of Ti 2 O 3 .

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