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

Abstract

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 Zr E−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)8 8+. The formation of the Zr solid phase is assumed to include Zr tetrameric species, Zr4(OH)8 8+, 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.