Fast Discovery Methods for Green Corrosion Inhibitors to Galvanised Steel

The use of organic molecules as corrosion inhibitors has attracted increasing attention to mitigate metal corrosion. The development of new inhibitors is crucial for various industrial sectors dependent on corrosion prevention, e.g., civil infrastructure, energy storage, and manufacturing. However, challenges persist in identifying desired inhibitor molecules in a rational manner due to the chemical diversity and structural complexity over a wide range of inhibitor clusters. In this regard, the inverse molecular design offers promise for rapid inhibitor discovery of desired properties, which requires the establishment of predictive model(s) based on the determined structure–property relationship. As such, this PhD work contributes to revealing the structure-performance correlation of inhibitor molecules, in attempts to elaborate on the critical molecular features in relation to the presented inhibition behaviour. A deepened mechanistic understanding of the underlying inhibition actions by different chemical functionalities was provided by a diverse array of electrochemical and analytical techniques to facilitate rational inhibitor design and discovery.

First, an inhibitor database of different chemical subclusters for electro-galvanised steel (ZE), i.e., thiazole, imidazole, thiazolidine, oxazole, aminothiazole, and benzothiazole was established using standardised protocols. It demonstrates that the inhibition efficacy varies regardless of the chemical subclusters. However, chemical subclusters with the presence of endocyclic S (thiazole, aminothiazole and benzothiazole) statistically hold greater potential to develop high-efficient inhibitor molecules. An in-depth investigation was then carried out using 2-amino-4-methylthiazole as an exemplar to reveal the impact of inhibitor molecular structure on inhibition action (Chapter 2). Electrochemical characterisation illustrated a mixed corrosion inhibition characteristic. The underlying inhibitor film formation and interfacial reaction were revolved by means of spectroscopic characterisations (FTIR and XPS). The critical role of the presence of endocyclic S and amino substituent in the adsorption process was highlighted. Mechanistic insights into the inhibitory performance in perspectives of electrochemical behaviour, interaction action and surface chemistry yielded in this chapter facilitated further understanding of thiazole derivatives and their structure–property correlation. To further explicate the substituent effect of thiazole-based inhibitor molecules, the second Result Chapter (Chapter 3) focused on aminothiazole derivatives regarding their corrosion-inhibiting effect. Different alkyl resides, i.e., methyl (Me), propan-2-yl (iPr) and tert-butyl (tBu), were replaced to the 2-position of 2-aminothiaozle (2AT-H), respectively. Density function theory (DFT) and conductor-like screening model for real solvents (COSMO-RS) calculations were performed to characterise the electronic contribution of different alkyl replacements towards the parent molecule. It reveals that all the compounds possess nearly identical reactivity, yet dissimilar inhibitory behaviours were observed suggesting the major spatial impact of the attached alkyl groups and the underlying intermolecular interaction. Whist bulky alkyl groups, i.e., iPr and tBu, spatially improve the surface coverage of adsorbed molecules at low concentrations and thus the inhibition efficacy, the increasing intermolecular interaction casts an adverse effect at high concentrations, destabilising the adsorption process of bulkier molecules upon ZE. The research findings of this chapter provide further insights to correlate the inhibitory performance of thiazole derivatives with their electronic and structural features.

Furthermore, the third Result Chapter (Chapter 4) revealed a different inhibition action of imidazole derivative 2-methylthio-imidazole (I-2MT), which favours the product precipitation by complexing with dissolved Zn2+ in the solution in a slow response. Nevertheless, the generated product layer failed to provide long-term protection due to the defective structure. Herein, we proposed a synergistic inhibition strategy using inhibitor mixture of 2AT-Me and I-2MT to achieve sustainable corrosion protection of ZE. Reinforced corrosion durability was acquired by combining both inhibitor molecules in the corrosion media. Time-dependent enhancement of corrosion resistance (up to 7 days) was determined with an increased immersion time, where the polarisation resistance (Rp) stabilises after 3-day immersion, the value more than 40 times higher as compared to the uninhibited counterparts. Surface and solution chemistry characterisations suggest a two-step process reflecting the sequential response of each inhibitor’s characteristic mode-of-action. The synergistic inhibition strategy was further verified using 5 independent inhibitor mixtures with similar inhibition actions based on the proposed principle.

Finally, the Result Chapter 5 revisited the established inhibitor database containing structurally related molecules for ZE, in an attempt to determine the structure-property relationship. In light of the previous research findings, Chapter 5 discussed the electronic and spatial effects of changing different chemical functionalities on the inhibitive activity of the molecule. Critical molecular features were successfully identified, and their predominant role was specified for ZE corrosion inhibition.

Overall, with the aim of advancing corrosion inhibitor technology, this research provides new knowledge on determining the structure–property relationship of the established comprehensive inhibitor database for ZE. The sophisticated structural impact dominating inhibitory activity was emphasised and carefully examined through the efforts resolving the surface–molecule and intermolecular interactions in the process of corrosion inhibition. The delivered research findings are anticipated to contribute to further developing the principle of rational inhibitor discovery and molecular design from both experimental and computational efforts.