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.

► The current study demonstrates a comprehensive study for Cysteine + Cu(II) ions as an efficient inhibitor as demonstrated by EIS, XPS and potentiodynamic polarization measurements, in addition to traditional weight loss measurements.
► The novelty of the current work originates from the combined use of an eco-friendly compound (i.e., cysteine) with a minute amount of copper ions (in the micro molar range) as a corrosion inhibitor for low carbon steel in acidic medium. To this end, cysteine shows only moderate inhibition ca. 60% for iron which jumps up to more than 95% in the presence of micro molar range of Cu(II) ions.
► Cysteine–Cu(II) blends are found superior to benzotriazole (BTAH)–Cu(II) blends in terms of their long-term stability in addition to the avoidance of the use of the well-reported highly toxic BTAH.