Interaction of cysteine and copper ions on the surface of iron: EIS, polarization and XPS study

This study addresses the enhancing effect of copper ions on the inhibition efficiency (IE) of cysteine (an eco-friendly compound) against the corrosion of iron in 0.5 M sulphuric acid. Electrochemical impedance spectroscopy (EIS) data revealed a significant increase in the polarization resistance (Rp) of the iron/solution interface in the presence of cysteine and Cu(II) ions instead of cysteine alone. That is, IE of 95% is obtained in the presence of 5 mM cysteine and 25 μM Cu(II) ions, compared to 66% in absence of Cu(II) ions. Moreover, electrochemical polarization measurements indicate that cysteine and Cu(II) ions blends act as mixed-type inhibitors for the corrosion of iron. The formation of Cu(I)–cysteinate complex and/or cysteine SAM at Cu atop the iron surface (as evident from X-ray photoelectron spectroscopy (XPS)) blocks the underlying iron surface and imparts a pronounced protection against its corrosion. IE of cysteine–Cu(II) blend remains effectively unchanged with immersion time indicating its high stability in the used acidic medium.