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Abstract

This thesis focuses on the development and optimisation of zirconium conversion coatings (ZrCCs) on cold-rolled steel (CRS), zinc (Zn), and aluminium alloy AA5754 substrates through a multidimensional approach.
The utilization of statistical tools, specifically response surface methodology (RSM), enabled the assessment of the individual and mutual effects of various parameters in the zirconium conversion bath (H2ZrF6 concentration, pH, and conversion time) on corrosion resistance responses of ZrCCs obtained by different evaluation techniques. Specifically, CRS and Zn favoured mid to higher concentrations (825−1226 ppm), longer conversion times (430−480 s), and mid to higher pH levels (4.0−4.6), while AA5754 favoured significantly lower concentrations (150 ppm), shorter immersion times (230 s), and a higher pH (4.6). This is attributed to the preferential, thicker zirconium conversion coating deposition on intermetallic particles, accompanied by cracking. RSM models derived from Electrochemical Impedance Spectroscopy were the most effective for characterising and optimising ZrCCs on all substrates, suggesting it as the most universal ZrCC evaluation technique.
Updating equilibrium predominance diagrams for Zr OH and Zr−F aqueous speciation implied the tetramer Zr4(OH)88+ as a fundamental building block in the solid phase of ZrCCs. This also suggested that the ZrCC process is of a sol-gel nature and not only an electrochemical one. Time-of-Flight Secondary Ion Mass Spectrometry further confirmed the presence of polymerized tetrameric structures in ZrCCs on CRS formed under conditions previously shown to be vital for adequate coating formation (pH = 4.0).
The proposed sol-gel chemistry of ZrCCs was further examined using SiO2 and ZrO2 colloidal pretreatments. SiO2 pretreatment significantly improved the uniformity and reduced the cracking of ZrCCs on AA5754, unlike ZrO2, which showed a limited impact on coating morphology and corrosion protection, implying the need to form a Si−Zr network to enhance corrosion resistance.
Additionally, the electrochemical behaviour of F− and Cl− in NH4HCO3 buffer at different concentrations was examined to assess their suitability as additives in the H2ZrF6 bath. It was revealed that F− induced significant complex formation on CRS and AA5754, narrowing the passive region of aluminium, while Cl− mainly led to pitting on AA5754 at higher concentrations. In contrast, equilibrium calculations further revealed the hat corrosion of zinc was mainly influenced by the overall ionic strength and further complexation with the buffering agent at higher pH levels and not with halide ions.
This dissertation lays the groundwork for future research aiming to refine the methodologies applied and extend the findings to industrial applications.