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)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 ZrF62– ions in Zr conversion baths and subsequently formed coatings is described. The introduction of ZrF62– 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.

Complementary corrosion protection of cast AlSi7Mg0.3 alloy using Zr-Cr conversion and polyacrylic/siloxane-silica multilayer coatings

“…2), it is reasonable to assume that IMPs containing more silicon act as a stronger cathode, readily leading to increased pH (reaction 1) and faster precipitation of Zr-and Cr-(hydr)oxides. Namely, locally increased pH on the micro-cathodic sites results in the hydrolysis of the soluble fluorometalate precursor species in the conversion bath 34 . According to thermodynamic data, the pH values at which the conditions for the precipitation of hydrated metal oxide coating (e.g., ZrO 2 × 2H 2 O) are established are still in the acidic range, i.e., between 2 and 4−4.5, depending on the concentration of Zr-bearing species 34 .…”

“…Namely, locally increased pH on the micro-cathodic sites results in the hydrolysis of the soluble fluorometalate precursor species in the conversion bath 34 . According to thermodynamic data, the pH values at which the conditions for the precipitation of hydrated metal oxide coating (e.g., ZrO 2 × 2H 2 O) are established are still in the acidic range, i.e., between 2 and 4−4.5, depending on the concentration of Zr-bearing species 34 . Using the SurTec® ST650 conversion bath with pH = 3.9, the pH conditions at which the deposition of the coating at the metal surface are readily achieved.…”

“…Using the SurTec® ST650 conversion bath with pH = 3.9, the pH conditions at which the deposition of the coating at the metal surface are readily achieved. In general, the deposition of Zr(IV) hydroxide from the hexafluorozirconic acid bath proceeds according to reaction (2) 34 .…”

“…Once deposited, Zr(IV) hydrated oxide is the most Zr-stable species at higher pHs 34 . In addition to the Zr-bearing component, the SurTec® ST650 also contains trivalent Cr salt, resulting in incorporating both Zr and Cr in the conversion coating, as shown by EDS analysis (Table 1).…”

“…Metal substrate dipped into a conversion bath containing Zr-bearing species becomes covered by Zrhydrated oxide precipitated on the metal surface once the pH reaches the value required for precipitation (usually around pH ≈ 4). This process is the most pronounced at the cathodic sites where, due to oxygen reduction reaction and concomitant OH − formation, the pH rises to the values suitable for the precipitation of Zr(IV) hydroxide (ZrO 2 × 2H 2 O), which is then stable up to highly alkaline pH values 12,21,25,30,34,35 . In the case of conversion baths containing both zirconium and trivalent chromium, the precipitation of Cr(III) hydroxide proceeds at similar pH values 35 .…”