Enhanced Protective Properties and Structural Order of Self-Assembled Monolayers of Aromatic Thiols on Copper in Contact with Acidic Aqueous Solution

In the present work we used different techniques to study self-assembled monolayers (SAMs) of two aromatic thiols, namely, benzenethiol (BT) and 2-naphthalenethiol (2-NT), and one alkylic thiol, 1-undecanethiol (1-UT), on polycrystalline copper, comparing their corrosion inhibition efficiency and their stability up to a week in H2SO4 0.5 M. Both electrochemical impedance spectroscopy (EIS) and linear polarization highlighted different aging trends for 1-UT on one side and aromatic thiols on the other. 1-UT was initially the best corrosion inhibitor among the three thiols, as expected from its larger thickness, but it degraded very rapidly. On the contrary, BT and 2-NT showed a noticeable increase of their protective properties during the first hours of exposure to the electrolyte leading to a superior performance over any 1-UT sample. Raman spectroscopy suggested this behavior to be related to an enhancement of the structural order of the aromatic layer. In addition, both XP spectroscopy and electrochemical measurements revealed BT and 2-NT layers to be stabler than 1-UT. In particular, BT layers exposed to H2SO4 0.5 M exhibited better protective properties, with respect to the freshly prepared samples, lasting for over 1 week of aging.