Authors: Ana Kraš | Yulong Wu | Antoine Seyeux | Sviatlana V. Lamaka | Ingrid Milošev | Philippe Marcus
Source: ECS Meeting Abstracts (article, 2025-11-24)
Field: Materials Science — Materials Chemistry

<i>(Invited)</i> Zirconium Conversion Coatings: Understanding and Optimization through Speciation and Multimodal Characterization With increasing environmental and regulatory pressure to replace toxic chromate- and phosphate-based coatings, zirconium conversion coatings (ZrCCs) have emerged as a sustainable and versatile alternative for the corrosion protection of light metals and steels. Despite their industrial potential, the chemical mechanisms underlying ZrCC formation remain only partially understood—primarily due to Zr complex aqueous speciation, which results in a mixture of mono- and polymeric species and exhibits strong sensitivity to bath composition. Also, a persistent challenge in ZrCC application lies in the broad, often vague operating windows prescribed in commercial bath formulations. These lack substrate-specific optimization and are typically based on proprietary industrial data. In an attempt to elucidate this, we combined corrosion performance assessment based on thermodynamic modelling, statistical modelling - response surface methodology (RSM), advanced surface characterization, and electrochemical analysis to investigate the mechanisms of ZrCC formation on cold-rolled steel (CRS), zinc, and the AA5754 aluminum alloy. Electrochemical impedance spectroscopy (EIS)-derived RSM models (Figure 1) produced highly predictive performance maps and proved a universally applicable tool for ZrCC optimization. This multidimensional approach outperforms traditional one-variable-at-a-time studies by capturing nonlinear and interactive effects. The coating performance in dilute Harrison’s solution was assessed using EIS across various Zr concentrations, pH, immersion times, and substrates. The results demonstrated the need for substrate-specific optimization strategies. CRS (Figure 1a-c) and Zn (Figure 1d-f) required higher Zr concentrations (825–1226 ppm), longer immersion times (430–480 s), and pH 4.0–4.6 1 . In contrast, optimal coatings on AA5754 (Figure 1g,h) formed under lower Zr levels (~150 ppm), shorter immersion times (~230 s), and higher pH (~4.6) due to localized growth on intermetallic particles and associated cracking phenomena 2 . EIS effectively characterized coating behaviour, revealing charge transfer control on CRS and AA5754 and diffusion-controlled corrosion on Zn. Revised Zr–OH and Zr–F thermodynamic predominance diagrams were developed using Specific Ion Interaction Theory to understand the mechanism of ZrCC deposition as a function of pH. These diagrams identified the polymeric tetrameric ion Zr 4 (OH) 8 8+ —an already known building block of amorphous zirconia—as a key species in coating precipitation. The formation of polymeric species strongly supports a sol-gel-like mechanism for ZrCC formation rather than a purely electrochemical one 3 . The thermodynamic postulates were further corroborated using the advanced surface method of Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS). The analysis confirmed the presence of polymerized tetrameric Zr-species in coatings on CRS deposited at pH 4.0, previously established as optimal for high-quality films. In contrast, coatings formed on AA5754 aluminum alloy showed only partial polymerization, with monomeric Zr species dominating the bulk and dimeric/trimeric species near the surface 4 . These substrate-dependent differences correlated with local pH measurements, indicating that a smaller interfacial pH rise on aluminum suppresses further polymerization —findings align with our previous speciation predictions 5 . Further, the effects of halide additives (F⁻ and Cl⁻) were examined in NH 4 HCO 3 -buffered electrolytes. Fluoride is strongly complexed with Zr on both CRS and AA5754, narrowing the passive region on aluminum—a critical consideration for bath formulation. On the other hand, chloride-induced pitting corrosion on AA5754 at elevated concentrations reinforces the need to minimize aggressive anions in ZrCC formulations. For zinc substrates, thermodynamic modelling showed that corrosion behaviour is driven primarily by ionic strength and complexation with bicarbonate at higher pH rather than by halide chemistry directly 6 . These findings underscore the importance of substrate-specific approaches in ZrCC optimization, accounting for distinct interfacial behaviours and coating mechanisms. This, we hope, provides both a deeper mechanistic understanding and a practical roadmap for fine-tuning ZrCC deposition parameters—paving the way for robust, multi-metal conversion coatings. In conclusion, by bridging Zr aqueous and solid speciation, thermodynamic modelling, surface analytics, and electrochemical evaluation, this work lays the foundation for the rational design of next-generation, environmentally responsible Zr-based conversion coatings tailored to the needs of diverse engineering alloys. References Kraš, A., D. Kramar, and I. Milošev, Corros. Sci. 242 (2024): p. 112551, https://linkinghub.elsevier.com/retrieve/pii/S0010938X24007467. Kraš, A., D. Kramar, and I. Milošev, Corros. Sci. 249 (2025): p. 112824, https://linkinghub.elsevier.com/retrieve/pii/S0010938X25001519. Kraš, A., and I. Milošev, J. Electrochem. Soc. 170 (2023): p. 21508, https://dx.doi.org/10.1149/1945-7111/acb9c2. Kraš, A., I. Milošev, A. Seyeux, and P. Marcus, npj Mater. Degrad. 8 (2024): p. 65, https://www.nature.com/articles/s41529-024-00485-3. Kraš, A., “Deposition and Protection Mechanisms of Zirconium-Based Conversion Coatings on Aluminium Alloy, Steel, and Zinc Substrates,” Jožef Stefan International Postgraduate School, 2024. Kraš, A., and I. Milošev, Electrochim. Acta 502 (2024): p. 144819, https://linkinghub.elsevier.com/retrieve/pii/S0013468624010594. Figure 1 Zirconium ; Corrosion ; Dielectric spectroscopy ; Coating ; Immersion (mathematics) ; Conversion coating ; Characterization (materials science) ; Electrochemistry Corrosion Behavior and Inhibition ; Electrodeposition and Electroless Coatings ; Hydrogen embrittlement and corrosion behaviors in metals