Studying zinc coatings with EQCM-D and EIS

Importance of the topic

Zinc electrodeposition is a widely used industrial approach to protect steel against corrosion and is increasingly important in biomedical and energy-related applications. Control of deposit morphology, thickness and mechanical properties is essential to achieve durable, biocompatible and functional coatings. Additives such as polyethylene glycol (PEG) are commonly used to tailor surface nucleation, growth kinetics and final roughness. Combining electrochemical measurements with mass and viscoelastic monitoring using EQCM-D (electrochemical quartz crystal microbalance with dissipation) delivers time-resolved insight into both the amount of material deposited and the evolving mechanical state of the growing film, enabling mechanistic understanding and optimization of plating recipes.

Objectives and overview of the study

The study applied an eSorptionProbe-equipped EQCM-D integrated with a potentiostat to investigate zinc electrodeposition from ZnCl2/KCl electrolytes, comparing processes in the absence and presence of a common additive (PEG, mw ~6000). The goals were to: (i) correlate electrochemical signatures (CV, chronoamperometry, EIS) with mass and dissipation changes during deposition, (ii) elucidate how PEG adsorption/desorption alters nucleation and growth pathways, and (iii) demonstrate how EQCM-D augments conventional electrochemical analysis for coating development.

Methodology

• Electrolytes: two solutions were investigated: A) 0.01 M ZnCl2 + 2.8 M KCl; B) same composition with 1×10^-3 M PEG (mw ~6000).
• Cell configuration: three-electrode beaker cell with a 10 MHz Au EQCM crystal as working electrode (mounted in a 3T analytik eSorptionProbe OS), Pt counter electrode and Ag/AgCl reference. Electrolytes were degassed with N2 for ≥45 min and headspace kept under N2.
• Electrochemical techniques: cyclic voltammetry (CV) between +0.5 V and -1.8 V at 30 mV/s; potentiostatic deposition (chronoamperometry, CA) at -1.35 V (90 s) for both solutions and additionally at -1.8 V (90 s) for solution B; electrochemical impedance spectroscopy (EIS) from 100 kHz to 0.1 Hz, 10 mV RMS.
• EQCM-D monitoring: simultaneous measurement of resonance frequency shifts and dissipation (damping) across the fundamental and overtones to obtain mass uptake (via the Sauerbrey relation for rigid films) and viscoelastic behavior for non-rigid deposits.

Used instrumentation

• 3T analytik eSorptionProbe OS with a 10 MHz Au EQCM crystal (probe format compatible with standard beaker cells).
• Metrohm/Autolab potentiostat/galvanostat equipped with FRA/EIS module (document references AUT204 and PGSTAT204 platforms with FRA32M module for EIS recording and NOVA/qGraph software for data integration).

Main results and discussion

• CV behavior without PEG (solution A): a single cathodic deposition peak near -1.23 V and a stripping peak around -0.8 V were observed. Frequency shifts measured by EQCM-D during deposition were large (~-44,000 Hz reported), corresponding to substantial mass loading (on the order of 1.9×10^5 ng/cm2) and an estimated film thickness in the few-hundred-nanometer range when treated as a rigid Zn layer by the Sauerbrey relation. Dissipation changes remained low (<10% of the frequency response), consistent with predominately rigid metallic deposit behavior.
• CV behavior with PEG (solution B): the first cathodic peak amplitude was reduced by about half and slightly shifted, and a second cathodic peak appeared at ~-1.63 V. These changes are attributed to PEG adsorption at the electrode that blocks active nucleation sites, slowing initial Zn deposition. At more negative potentials PEG desorbs (or is displaced), enabling further deposition reflected by the second cathodic feature and by a renewed increase in mass.
• Frequency (ΔF) and dissipation signals: the ΔF vs. potential slope in B was shallower than in A, confirming slower mass accumulation with PEG present. Dissipation traces differed qualitatively: A showed a double-peak damping pattern consistent with an initial rough island growth followed by coalescence into layers, while B exhibited a triple-peak pattern. The intermediate dissipation peak in B matches the potential where PEG desorption exposes previously blocked sites, transiently increasing roughness before layer completion. In both electrolytes the final dissipation peak corresponds to stripping/cleaning during the reverse sweep.
• Chronoamperometry (potentiostatic deposition): CA combined with EQCM-D showed that at -1.35 V the initial deposition rate in solution B is markedly slower than in A due to PEG suppression of active sites; over time the deposition rates converge as either PEG is displaced or bulk deposition dominates. When deposition in B was performed at -1.8 V the mass uptake curve overlapped the A curve at -1.35 V, indicating that more negative potential overcomes PEG adsorption and allows unrestricted Zn growth.
• EIS after deposition: Nyquist spectra indicated that deposits formed in B at -1.35 V produced electrochemical signatures consistent with incomplete coverage and increased charge-transfer impedances relative to A and B at -1.8 V. Spectra for A at -1.35 V and B at -1.8 V were similar, supporting the interpretation that PEG blocking is removed at the more negative potential.

Benefits and practical applications of the method

• EQCM-D coupled with conventional electrochemical techniques provides simultaneous, time-resolved mass and viscoelastic information, enabling discrimination between rigid metallic growth and softer/rough deposits that are not apparent from current alone.
• This combined approach allows direct observation of additive effects (adsorption/desorption), nucleation regimes and transition points between island growth and layer formation—data that are highly relevant for optimizing plating baths, additive concentrations and operating potentials in industrial coating processes.
• Useful for development of biocompatible coatings in medical devices, corrosion-protection layers for infrastructure, and tailored metal films for energy-storage or sensor applications.

Future trends and potential applications

• Extension to multi-additive and complex bath chemistries: using EQCM-D to screen combinatorial additive blends and concentrations for controlled morphology and mechanical properties.
• High-resolution in situ diagnostics: integrating imaging (optical/AFM) or operando spectroscopies with EQCM-D and electrochemistry to correlate structural, chemical and mechanical evolution during electrodeposition.
• Flow-cell and industrial-relevant geometries: adapting the eSorptionProbe approach to realistic flow or substrate configurations to translate lab-scale mechanistic understanding to production lines.
• Data-driven optimization: combining EQCM-D-derived kinetic and viscoelastic metrics with machine learning to predict optimal plating recipes and operating windows.

Conclusion

This Application Note demonstrates that EQCM-D integrated with standard electrochemical techniques is a powerful tool to dissect zinc electrodeposition mechanisms and the role of additives such as PEG. PEG adsorption reduces initial nucleation and growth rates, produces distinct electrochemical and dissipation signatures, and can be overcome at sufficiently negative potentials. The combined mass, viscoelastic and electrochemical data allow clear identification of nucleation regimes, additive desorption potentials, and the point at which bulk deposition dominates—information that directly supports optimization of coating quality and process control.

References

1. Tamurejo-Alonso P., González-Martín M. L., Pacha-Olivenza M. Á. Electrodeposited Zinc Coatings for Biomedical Application: Morphology, Corrosion and Biological Behaviour. Materials (Basel) 2023, 16(17), 5985. DOI:10.3390/ma16175985
2. Ballesteros J. C., Díaz-Arista P., Meas Y., et al. Zinc Electrodeposition in the Presence of Polyethylene Glycol 20000. Electrochimica Acta 2007, 52(11), 3686–3696. DOI:10.1016/j.electacta.2006.10.042
3. Vanoppen V., Johannsmann D., Hou X., et al. Exploring Metal Electroplating for Energy Storage by Quartz Crystal Microbalance: A Review. Advanced Sensor Research 2024, 3(9), 2400025. DOI:10.1002/adsr.202400025