Corrosion Inhibition of Mild Steel by Coumarate-Based Ionic Liquids and Coatings

The use of ionic liquids (ILs) as corrosion inhibitors is gaining significant attention due to their attractive properties such as high inhibition efficiency and ability to absorb onto metal surfaces. In this work, six protic ILs, based on the coumarate anion in combination with the nitrogen containing ammonium, pyrrolidinium and imidazolium cations with a short or long alkyl chain attached to the nitrogen atom, have been synthesized and evaluated as corrosion inhibitors for mild steel. The anticorrosion properties of these ILs in solution as inhibitors were investigated electrochemically and the metal surface was analyzed by Scanning Electron Microscopy. Moreover, the IL prepared from the coumarate anion and N-dimethyl-N-tetradecyl ammonium ([DTA]Cou) was incorporated into an acrylic UV-cured coating formulation as an additive and by designing a similar ionic monomer which covalently links to the acrylic coating formulation. Both coatings were analyzed using impedance spectroscopy during 11 days of exposure to a solution of 0,01 M NaCl, confirming the high performance of the inhibitor in both solution and when incorporated into a coating. The synthesized ILs present efficiencies in solution exceeding 70 %, in particular the ILs [DTA]Cou and the tetradecyl imidazolium coumarate ([C14Im]Cou) showed efficiencies of 88 % and 91 % respectively. Surface analysis after 24 h confirmed that the inhibitors efficiently adsorb onto the mild steel surface to form a protective film. The obtained inhibitors showed interesting anticorrosion behaviors and demonstrated how different cations and an increase in the chain length affect the corrosion inhibition properties.