Cyclodextrins in Corrosion Protection of Zinc
Authors: Altin Abdulrahman , Krzywiecki Maciej , Rohwerder Michael , Erbe Andreas 
Published: February 2020

In corrosion protection, coatings separate a metal from the environment. Once a coating is mechanically damaged and the metal is exposed to a corrosive environment, electrochemical dissolution of the metal occurs in the defect. Consequently, cathodic oxygen reduction takes place at the metal/polymer interface, resulting in the cathodic disbondment of the coating.1 This cathodic delamination can be slowed down by changing the chemistry of the metal coating interface.2–4However, for a complete suppression of the cathodic delamination, the healing of the defect is required. Healing can be realized by adding corrosion inhibitors into the coating. To passivate the metal, such inhibitors must be released from the coating and transported to the defect.
Organic corrosion inhibitors are widely used for a variety of metals. Because many of these inhibitors are poorly soluble, a sufficient release from the coating is rather difficult. This work will show that β-cyclodextrin (CD) can be used to facilitate the release of organic corrosion inhibitors, e.g. 2-mercaptobenzothiazole (MBT). CDs are cyclic oligosaccharides which allow the encapsulation of hydrophobic organic compounds, while their hydrophilic shell ensures sufficient solubility in aqueous phases. CDs are used e.g. in the pharmaceutical industry for drug delivery, including the controlled release of pharmaceutics in the body.
In this work, model coatings were filled with organic corrosion inhibitors in the presence and absence of CD. The released inhibitor concentration from the coating was examined with UV-Vis spectroscopy as function of pH. An up to 5-fold increase of the released inhibitor concentration was observed in the presence of CD. The highest increase was observed at alkaline pH. In the presence of CD, electrochemical impedance spectroscopy (EIS) showed improved barrier properties of the coatings. By Scanning Kelvin Probe (SKP) experiments, the delamination velocity in the presence of CD was found to show a significant decrease compared to the control experiment. The potential in the mechanical defect rose to the passive potential under certain experimental conditions. After exposure of the mechanical defect to 0.1M KOH electrolyte, the delamination slowed down with time and stopped after ~10 h, accompanied by a rise of the defect potential.5

SKP control experiments with pure CD without corrosion inhibitor suggested furthermore that CD itself acts as corrosion inhibitor. Therefore electrochemical experiments were conducted in a classical three electrode setup. An inhibition efficiency of 95% for zinc was found by EIS at low chloride concentrations. Inspection of the zinc surfaces after electrochemical testing with scanning electron microscopy showed differences in the morphologies of the corrosion products. The presence of CD on the zinc surface was detected by Raman spectroscopy. In situ ellipsometric experiments showed no formation of a thick adsorption layer of CD, and no increase in the thickness of corrosion products. For an in depth understanding of the interaction mechanism of CD with zinc surfaces, angular dependent x-ray photoelectron spectroscopy (ADXPS) in combination ultraviolet photoelectron spectroscopy (UPS) was performed ex situ. It was found that CD changes the electronic structure of the surface oxide on zinc. In the presence of CD, the oxide contained less defects, i.e. became closer to an intrinsic semiconductor. The formation of such a defect-poor oxide suppresses charge transfer processes in electrochemical reactions. Therefore, CD adsorption inhibits corrosion.
Oligosaccharides such as CD are environmentally friendly agents. This work shows the potential for a wider use of these agents in corrosion protection.

References

1. C. D. Fernández-Solis et al., in Soft Matter at Aqueous Interfaces, Lecture Notes in Physics. P. R. Lang and Y. Liu, Editors, p. 29–70, Springer International Publishing (2016)
2. D. Iqbal et al., ACS Appl. Mater. Interfaces, 6, 18112–18121 (2014).
3. D. Vijayshankar et al., J. Electrochem. Soc., 163, C778–C783 (2016).
4. J. S. Mondragón Ochoa, A. Altin, and A. Erbe, Mater. Corros., (2017). DOI:10.1002/maco.201609289
5. A. Altin, M. Rohwerder, and A. Erbe, J. Electrochem. Soc., 164, C128–C134 (2017).