Corrosion inhibition of 6xxx aluminium alloys and mechanisms of film formation

This Ph.D. project aims to understand the in-situ and ex-situ growth and stability of organic inhibitor film on AA6xxx alloys in saline solution through AFM, TEM, FIB-SEM, XPS, SKPFM, ATR-FTIR, RAMAN spectroscopy, and electrochemical techniques. Exposure of AA6xxx to 2-mercaptobenzimidazole (an organic inhibitor to corrosion) containing saline solution results in a stiff and elastic film of ~15 nm thickness with a Young’s modulus of 30 GPa at 90 min. High-resolution AFM imaging reveals the growth of a stiffer and porosity-free elastic film of ~15 nm thickness forming over AA6061 exposed to 2-MBI treated saline solution at 90 min. The film starts to grow from 30 min with a Young’s modulus of ~2000 MPa that has increased up to ~30000 MPa after 90 min indicating strong cohesion of 2-MBI molecules within the film. Elastic moduli were reduced in later measurements from 3 to 24 h, indicating the transformation of film from hard to soft and subsequently nano to micro-sized pores seem to existent in the film structure. This film was predominantly cathodic with a moderate inhibition efficiency of 95% that could reduce the galvanic activity of Fe, Si-rich intermetallic particles up to 24 h by providing a barrier effect. S and N atoms from the 2-MBI molecule principally reacted with the AA6xxx surface with gradual changes in S, O, and N bonds towards the top surface. At longer durations (up to 14 d), pores formed within the degraded areas of the inhibitor layer could have facilitated the ingress of aggressive ions. Time-resolved mass spectrometry, XPS, and FTIR analysis of inhibited electrolytes have indicated oxidation of thiols from 2-MBI molecule and formation of dimers, polymers, and Cl complexes. The findings are expected to provide new insights into the design and develop corrosion inhibitors to mitigate corrosion of structural aluminium alloys prone to corrosion.