Adsorption of 2-Mercaptobenzothiazole Organic Inhibitor and Its Effects on Copper Anodic Oxidation in Alkaline Environment

Abstract

2-Mercaptobenzothiazole (2-MBT) is well established as an effective organic corrosion inhibitor for the protection of copper and other metals. The presence of heteroatoms (2 sulphur and 1 nitrogen) in its structure, enables it to bond to the metallic substrate and form a protective barrier layer on the surface which restricts the pathways for the corrosive ions to the metal surface. However, despite numerous works discussing the inhibiting properties on copper, several questions remain, notably regarding the role played by the surface native oxide on the bonding mechanisms, the stability, and the inhibition efficiency of the organic film formed in various environments.In this work, the adsorption of 2-MBT on copper and its effects on anodic oxide growth in a strong alkaline medium (0.1 M NaOH solution) were investigated. Various cathodic pre-treatment methods were applied to modify and control the native oxide-covered Cu surface during the formation of the inhibitor layer. Cyclic voltammetry (CV) tests were performed to determine how the growth of anodic Cu(I) oxide is altered by the 2-MBT organic inhibitor layers pre-adsorbed in different pre-treatment conditions. Additionally, X-ray photoelectron spectroscopy (XPS) and time-of-flight – secondary ion mass spectrometry (ToF-SIMS) were used to investigate the bonding mechanisms of the 2-MBT molecule to the copper surface, including the role of surface native oxides in the formation of the interface with the inhibitor, the effect of increased exposure time to the inhibitor, the stability of the organic inhibitor layer upon anodic polarisation, and the oxide growth upon anodic polarization. The absence of chlorides eliminates their interference in the understanding the oxidation process in a 2-MBT-containing alkaline solution. Additionally, anodic oxidation is studied in the Cu(I) potential range, which allows us to evaluate the effects of 2-MBT on the oxide layer that forms directly on the bare metallic substrate.The results show that 2-MBT significantly impedes the growth of the passivating Cu(I) oxide by forming a multilayered organic film with the inner layer chemisorbed to the surface. A major factor influencing the formation and properties of the organic barrier layer is the presence and structure of the interfacial native oxides on which it forms. The 2-MBT molecules bond to the copper substrate mostly via its sulphur atoms along with a small fraction of the nitrogen atoms also bonding to the metallic substrate. Additionally, there is an interaction between the inhibitor molecules and copper released from the surface to form metal-organic complexes in the outer layers of the thicker films. Longer exposure times to the inhibitor of a surface with negligible oxides increased the efficiency of the organic barrier layer in impeding the growth of anodic oxide.Upon anodic polarisation, the 2-MBT barrier layer grows in thickness, with inclusion of Cu-2-MBT complexes. It is proposed that the thiolate form of 2-MBT, which is the dominant conformer in an alkaline environment, allows for additional bonding possibilities to the Cu atoms and Cu(I) ions of or released from the surface oxide, forming Cu-2-MBT complexes that deposit on the surface and thus increasing the thickness of the organic barrier layer. This project has received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation program (Advanced grant agreement No. 741123, CIMNAS).