The Aqueous Chemistry of Zirconium as a Basis for Better Understanding the Formation of Zirconium Conversion Coatings: Updated Thermodynamic Data

This work tackles the aqueous chemistry of Zr, aiming to contribute to a better understanding of Zr conversion coatings as one of the important contemporary means of corrosion protection. Equilibrium predominance diagrams based on experimentally confirmed Zr–OH and Zr–F aqueous species concerning Zr amorphous solid phase, along with an updated ZrE−pH (Pourbaix) diagram, are constructed. Since ZrO²⁺existence had been conclusively disproven in both the aqueous and solid states, we chose to depict mononuclear species with ZrOH³⁺and polynuclear with Zr₄(OH)₈⁸⁺. The formation of the Zr solid phase is assumed to include Zr tetrameric species, Zr₄(OH)₈⁸⁺, as a fundamental building block thereof. The role of F^–and ZrF₆^2–ions in Zr conversion baths and subsequently formed coatings is described. The introduction of ZrF₆^2–anions keeps Zr solvated in the form of a complex, thus preventing the early onset of hydrolysis. The conversion of Zr species and the coating formation mechanism are further discussed from electrochemical and sol-gel perspectives, aiming to give a foundation for future predictions and rationalisation of Zr conversion coating formation, with the possibility of extensions to various bath additives.