Detais: Two-comp nent ep x co ting, ty icaly used for maritime st eel cons ructions, a o e an below the wat er level.. A ve tical thr e-ele trode setup, with a saturat ed Ag Ag l r fe en
Basic considerations
A basic introduction to electrochemical impedance spectroscopy, especially in connection with corrosion, is given in ASTM G106.
The interpretation of EIS measurements is not confined to the models presented; alternative interpretations may also be valid Selecting the appropriate model necessitates consideration of additional experimental and theoretical factors.
Examples of models
Purely capacitive coating
A metal covered with an undamaged coating generally has a very high impedance The equivalent circuit for such a situation is shown in Figure 1.
Figure 1 — Equivalent circuit for a purely capacitive coating
The model includes a resistor representing the resistance R s , of the solution and, connected in series with it, a capacitor representing the capacitance C c , of the coating.
In practical applications, the resistance of an ideal coating may not be observable within the specified frequency range Any discrepancies from the Bode plot in Figure 2 suggest either a modified model or the limitations of the impedance device, as outlined in ISO 16773-2:2016, Annex A.
Randles equivalent circuit
The Randles equivalent circuit includes the resistance of the solution Rs, the capacitance of the coating
C c and the ohmic resistance of the coating R c , as shown in Figure 3.
Figure 3 — Randles equivalent circuit The Bode plot for a Randles equivalent circuit is shown in Figure 4.
Figure 4 — Bode plot for a Randles equivalent circuit
Extended Randles equivalent circuit
Fitting experimental data to the model in Figure 3 frequently leads to systematic errors However, literature indicates that utilizing the model in Figure 5 can yield a more accurate fit.
Figure 5 — Extended Randles equivalent circuit
NOTE This model is not necessarily the most appropriate and other models are not excluded.
In high-impedance coatings, the charge-transfer resistance (Rct) and double-layer capacitance (Cdl) are key parameters in the extended Randles circuit, reflecting the characteristics of the coating rather than the corrosion processes affecting the underlying metal.
The Bode plot in Figure 6 illustrates the significant impact of the two additional elements Although the plot does not extend to high frequencies to assess the solution resistance, this limitation is not a concern in practice, as the solution resistance is determined by the characteristics of the test solution and the geometry of the test cell, rather than the coating itself.
This annex contains a collection of spectra obtained from materials described briefly in the relevant clause The examples were obtained from various laboratories using a range of different equipment and materials.
This collection of spectra does not guarantee that all mentioned materials will exhibit similar spectra, nor does it ensure that the provided spectra are devoid of experimental errors Additionally, it does not encompass the entire spectrum of coating materials available.
This example shows how a smaller than usual thickness of a high-build coating material can be used to investigate the influence of immersion time on EIS measurements (see Figure A.1).
Two-component epoxy coatings are commonly utilized for maritime steel constructions, both above and below the waterline These coatings are applied using airless spray techniques, with a recommended dry film thickness (DFT) ranging from 1,000 µm to 3,000 µm as specified by the manufacturer.
Measurements were conducted on a steel coat with a dry film thickness (DFT) of 200 µm over an area of 10 cm² at 21 °C, utilizing concentrated artificial rainwater A vertical three-electrode configuration, featuring a saturated Ag/AgCl reference electrode, was employed, and spectra were captured after specified immersion durations.
Y1 modulus of the impedance, |Z|, in Ω⋅cm 2
Y2 modulus of the phase angle, |φ|, in degrees t = 0 h t = 2 h t = 24 h t = 168 h t = 504 h Figure A.1 — Bode plot for a high-build coating material under immersion conditions
This example highlights a surface-tolerant coating material that requires less surface pretreatment compared to Example 1 (refer to Figure A.2) Typically, mechanical tools are employed for de-rusting instead of grit blasting.
This surface-tolerant two-component epoxy coating is designed for maritime steel constructions, suitable for application both above and below water level It can be effectively applied on corroded steel, grit-blasted steel, and undamaged old paint coatings The coating can be applied using various methods, including airless spray, conventional spray, brushing, or rolling The manufacturer recommends a dry film thickness (DFT) of 100 µm to 200 µm for optimal performance.
Measurements were conducted on a steel coat with a dry film thickness (DFT) of 250 µm over an area of 10 cm² at a temperature of 21 °C, utilizing concentrated artificial rainwater A vertical three-electrode configuration, featuring a saturated Ag/AgCl reference electrode, was employed for the experiment Spectra were captured following specific immersion durations.
Y1 modulus of the impedance, |Z|, in Ω⋅cm 2
Y2 modulus of the phase angle, |φ| , in degrees t = 0 h t = 2 h t = 24 h t = 168 h t = 504 h Figure A.2 — Bode plot for a surface-tolerant coating material under immersion conditions
This example represents a high-build, solvent-free coating material with high abrasion resistance, applied as a single coat (see Figure A.3).
This solvent-free two-component epoxy coating is designed for grit-blasted metals, concrete, and fiberglass in aggressive environments It offers high abrasion resistance and excellent corrosion protection The coating can be applied using airless spray or brush methods, with a recommended dry film thickness (DFT) of 500 µm to 1,000 µm for optimal performance in a single coat.
Measurements were conducted on a steel coat with a dry film thickness (DFT) of 230 µm over an area of 10 cm² at a temperature of 21 °C, utilizing concentrated artificial rainwater A vertical three-electrode configuration, featuring a saturated Ag/AgCl reference electrode, was employed, and spectra were captured after specified immersion durations.
Y1 modulus of the impedance, |Z|, in Ω⋅cm 2
Y2 modulus of the phase angle, |φ|, in degrees t = 0 h t = 2 h t = 24 h t = 168 h t = 504 h Figure A.3 — Bode plot for a solvent-free coating material under immersion conditions
This example illustrates a representative powder coating applied via spray on aluminum, utilizing a measurement area of 16.5 cm² with a three-electrode setup However, the open-circuit potential was not provided alongside the spectra The observed discontinuities in the phase-angle plot result from changes in the potentiostat current range, coupled with the low capacitance of the examined system, suggesting an incorrect configuration of the measurement device.
Details: Polyester powder coating material sprayed on chromatized aluminium frames as a single coat with a DFT of (93 ± 3) àm No ageing.
Measurements were performed at 25 °C in 3 g/l Na2SO4 solution on an area of 16,5 cm 2 A three- electrode setup, with an Ag/AgCl reference electrode, in a vertical plastic tube was used.
Y1 modulus of the impedance, |Z|, in Ω⋅cm 2
Y2 modulus of the phase angle, |φ|, in degrees
Figure A.4 — Bode plot for a powder coating before ageing
The spectra shown in Figure A.5 were obtained after ageing through eight thermal cycles, the coating remaining continuously in contact with the electrolyte.
A complete cycle involves heating from 25 °C to 75 °C over the course of 1 hour, maintaining the temperature at 75 °C for 4 hours, and then cooling back to room temperature Each cycle is separated by an interval of approximately 24 hours, with measurements taken at a consistent temperature of 25 °C.
Y1 modulus of the impedance, |Z|, in Ω⋅cm 2
Y2 modulus of the phase angle, |φ|, in degrees
Figure A.5 — Bode plot for a powder coating after ageing
Packaging materials often feature thin, unpigmented "clear coats" that were analyzed for their spectral properties following exposure to citric acid and sorbic acid While these coatings do not exhibit high impedance values, they do demonstrate relatively high capacitance values Additionally, the phase angle plot reveals measurement anomalies in the high-frequency range, which may result from non-steady state conditions, inadequate shielding (such as Faraday cages or cables), or interference from the reference electrode.
Epoxy-phenolic lacquer coating, commonly utilized for packaging, is applied in two coats on tin-plated steel using a roller The coating is then stoved at 220 °C for 20 minutes, achieving a total dry film thickness (DFT) of 7 to 8 micrometers.