Spatiotemporally Resolved Corrosion Inhibition Studies of Lithium Salts for Aerospace Aluminium Alloys

Abstract

Over many decades, intensive studies have been performed towards safe and sustainable substitutes for chromate corrosion inhibitors in active corrosion protection coatings for aerospace aluminium alloys. The proposed candidates must provide similar corrosion protection as compared to that of chromate-based compounds and be cost-effective for viable introduction in industrial applications. Lithium salts have been recognised for their passivation ability on aluminium alloys through generating a lithium-based conversion layer since the 1980s. In 2010, it was discovered that organic coatings loaded with lithium salts acting as leachable inhibitor provided promising corrosion inhibition under accelerated corrosion test conditions. The formation of a protective layer with a variety of layered configurations is the key feature providing an effective physical barrier to corrosive conditions. Although extensive research has been performed to understand the inhibition mechanism as well as the layer formation process, electrochemical monitoring and local electrochemical imaging during the formation process and subsequent degradation of the lithium-based conversion layer were still lacking.

The scientific objective of this Ph.D. thesis is to gain a more thorough understanding of the formation stages and corrosion mechanism of the lithium-based conversion layer using spatiotemporally resolved electrochemistry and to link the observations to previous findings. The knowledge and understanding of local electrochemical reactions are essential for a profound understanding of the formation and inhibition mechanism and to motivate further optimization of these novel active coating techniques in the future. To this aim, Chapter 2 firstly introduces the current development of lithium-based inhibitor and coating technologies to for the corrosion protection of aluminium alloys.

The electrochemical methods used in this work are electrochemical noise (EN) and scanning electrochemical microscopy (SECM), combined with a number of different surface analytical techniques. The experimental part in this thesis can be divided into two parts: (i) characterisation of the lithium-based conversion layer and its corrosion resistance using EN (Chapters 3 and 4) and (ii) SECM investigations of the formation stages of the conversion layer and its corrosion protection mechanism (Chapters 5 and 6).

Chapter 3 is devoted to the application of EN to monitor the lithium-based conversion layer formation process in real-time and to examine its corrosion protection properties. Three different components were observed in the EN signals, associated with high-, medium- and low-frequency contributions. These could be attributed to the detachment of hydrogen bubbles from the surface, localized corrosion surrounding intermetallic particles (IMPs) and general corrosion, respectively. In addition, the conversion layer was still generally protective even though local damages occurred. In Chapter 4, the influence of ambient ageing on the corrosion resistance of lithium-based conversion layers is investigated using EN. The conversion layers with short ageing times exhibited an initially high noise resistance but a lower protectiveness when localized corrosion occurs. For the case of longer ageing treatments, the conversion layers showed a more stable and higher corrosion resistance. This phenomenon was caused by the instability of the freshly-formed lithium-based conversion layer. At wet regions, the conversion layer continued to develop locally due to the release of lithium carbonate, which contributed to the improvement of the corrosion protection with ageing time.

Chapter 5 is devoted to the application of SECM to characterise the progressive conversion layer formation stages by measuring the local surface reactivity at different formation stages. Three types of IMPs, including the S-, θ- and constituent phase, acted as cathodic areas at the beginning of immersion in a borate buffer solution. The surface reactivity initially exhibited an increasing trend and then gradually decreased until passivation. The passivation over θ- and constituent particles preceded passivation of the S-phase particles. In Chapter 6, the corrosion protection mechanism of the lithium-based conversion layer is studied. For the freshly-formed conversion layer, based on the studies in Chapter 4, further investigation revealed that the entire bilayer structure undergoes a gradual dissolution upon re-immersion in a lithium-free corrosive environment. Localized corrosion mainly occurred at and around the S- and large constituent phase particles. The trenching around the S-phase was caused by the relatively high electrochemical activity, whereas the localized corrosion around large constituent phases was caused by the lower conversion layer corrosion protection. In addition, the corrosion process developed faster around the S-phase, leading to particle undercutting at an earlier stage than for constituent particles.

In this thesis it is shown that EN and SECM are powerful tools which are able to provide corrosion information with high spatial and temporal resolution. Local characterisation at a microscale level is of pivotal importance for further elucidation of the corrosion- and inhibition mechanisms. Once this local activity is linked to the information on real-time corrosion kinetics from EN measurements successfully, this greatly increases the potential of EN for industrial corrosion monitoring.