(Invited) Surface Analytical Study of Adsorption Mechanisms of 2-Mercaptobenzothiazole and 2-Mercaptobenzimidazole Corrosion Inhibitors on Cu
Authors: WU Xiaocui , Wiame Frédéric , Maurice Vincent , Marcus Philippe 
Published: December 2020

The corrosion of metals and alloys is an important issue in industrial applications with an annual global cost estimated to be about 3% of the global GDP (2013). In order to mitigate corrosion, inhibitors are widely used and developed including on copper, a metal which is widely used for its high thermal and electrical conductivity but not totally immune against corrosion.
In this work, 2-mercaptobenzothiazole (2-MBT) and 2-mercaptobenzimidazole (2-MBI) [1-2], two structurally related molecules already known for their inhibitory action against corrosion of copper, were studied. The objective was to understand the interaction mechanisms of these two corrosion inhibitors with copper in a well-controlled environment and their influence on the reactivity of copper.
To this end, 2-MBT and 2-MBI were dosed at room temperature on a model copper surface, Cu(111), by ultra-low pressure (~10-9 mbar) gas evaporation in ultra-high vacuum conditions. The surfaces were characterized by X-ray Photoelectron Spectroscopy (XPS) and Scanning Tunneling Microscopy (STM) at various growth stages of the inhibitor layer. Thermal stability of the adsorbed molecular layers was also investigated.
Growth of molecular layers for 2-MBT and 2-MBI was evidenced on Cu(111) in metallic state with the formation of a monolayer at 5–10 L (1 L = 1×10-6 Torr·s) and multilayers at higher exposure. An excess of sulfur was observed by XPS, with a S 2p component at the same binding energy as atomic S bonded to Cu, indicating a partial decomposition of the molecules at the initial stages of adsorption. STM characterization at low exposure (3–4 L) revealed locally ordered triangular structures for 2-MBT on Cu(111), which can be attributed to atomic S and S of the molecule bonded to Cu, and flat-lying molecules in the multilayer. While adsorption of 2-MBI leads to the formation of an ordered monolayer with different organizations including a (√7×√7)R19.1° structure, which may be attributed to the adsorption of atomic S. Molecules in the monolayer and multilayer are also lying flat. The thickness of the molecular layers was calculated from XPS attenuation to be of the order of several nanometers.
The pre-adsorbed molecular layers were observed to markedly decrease the reactivity of copper towards oxygen. Annealing of the molecular layers to above 100°C leads to a partial decomposition and desorption of the inhibitors, and the formation of (√7×√7)R19.1° structure.
This study brings new insight into the corrosion inhibition mechanisms of 2-MBT and 2-MBI on copper, which contributes to the knowledge-based development of more efficient corrosion inhibitors for the corrosion protection of copper and its alloys.

This project has received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation program (ERC Advanced Grant CIMNAS no. 741123).

[1] M. Petrovic Mihajlovic, M. Antonijevic, International journal of electrochemical science 10 (2015) 1027-1053.
[2] M. Finšgar, D. K. Merl, Corrosion Science 83 (2014) 64-175.