Assessment of Charge Transport Through Barrier Membranes Before Application on Materials Surfaces

Abstract

This work aims at investigating the charge transfer resistance (R_CT) of the two membranes derived from vegetable oils which are intended to protect materials against corrosion. The membranes were characterized by thickness measurement (328±1 μm and 491± 1 μm), atomic force microscopy (AFM) to estimate of roughness (<0.5 μm), Raman spectroscopy while the R_CT values were evaluated by electrochemical impedance spectroscopy (EIS) in 0.1 mol/L KCl solution. EIS was used to measure the impedance at low frequency (Z_0.01Hz ≅ 10⁹ Ω cm²), R_CT ≅ 10⁹ Ω cm² and exchange current density (i₀ ≅ 10^-11 Ω cm²), employing simple instrumentation. The thickness of the membrane must be considered for coherent interpretation of the impedance results. The study of electrolytes permeation in membranes is important to previously estimate the lifetime offered to a substrate over the time of immersion even before being applied to the metallic surface. The impedance measurements demonstrated that the most resistant membrane to permeation presented a R_CT around 2 GΩ cm². This EIS measurements approach enables the optimization of membrane fabrication by conveniently identifying the best formulation.

Keywords:
Membrane; pristine organic film; permeation; corrosion protection

Graphical Abstract

1. Introduction

The development of protective coatings based on new materials with self-healing properties has been a route to obtain efficiency in the protection of some substrates against corrosion. Many papers were devoted to the development of coatings / films for protection materials against corrosion or degradation^1-11 however, no one of them report on studies of the evaluation of the charge transfer resistance (R_CT) during the electrolyte permeation through the pristine film.

Some studies have shown that EIS is a valuable tool for membrane characterization^12-16. EIS is a non-destructive technique¹⁷ that can provide information about electrochemical phenomena including permeation processes. These studies expanded the scope of membrane functionality and analysis and, moreover, highlight the opportunity for investigation prior to membrane utilization / application.

A few studies^18-20 have used a permeation cell to investigate the permeation of K⁺ ions through membranes measuring their R_CT values with immersion time. The study of ions permeation through the free film extends to different systems as described in the literature^21,22.

This study can also be applied as a previous investigation of film barrier properties in the development of new coatings for metallic protection against corrosion, constituting in a selective process of these materials. Therefore, it is of great interest to estimate the barrier properties, stability, and durability of barrier effect of organic membranes, and organic inorganic hybrid materials before applying on a metallic substrate to protect it against corrosion.

In this context, the purpose of this work is to develop and characterize organic free membranes, by measuring the thickness, roughness using atomic force microscopy (AFM), chemical constitution with Raman spectroscopy and to estimate the barrier property by electrochemical impedance spectroscopy during immersion in 0.1 mol/L KCl solution. From the electrochemical measurements in a permeation cell, it is possible to obtain several parameters correlated to the membrane properties, i.e., R_CT, exchange current density (i₀), and low frequency impedance (|Z|_0.01Hz) that allow accessing the barrier properties of the membranes and predicting their performance when applied as coatings. The performance of the free membranes will be compared with the corresponding coatings already applied on aluminum alloys¹.

2. Materials and Methods

2.1. Materials

Castor oil (CAO) was purchased from Asher® and crambe oil (CO) provided by Foundation Mato Grosso do Sul. Trimethylolpropane (TMP), phthalic anhydride (PhA), butylated hydroxyl toluene (BHT), propyl glycol (PG), butyl acetate (BA), ethyl glycol acetate (EGA), hexamethylene diisocyanate (HDI), lithium hydroxide (LiOH), n-butyl acetate (BA), ethyl glycol acetate (EGA) and stannous octoate (SO) were all acquired commercially and used as received.

2.2. Preparation of membranes (pristine membranes)

Two membranes, herein denominated membrane A and membrane B, were prepared as the procedure shown in Scheme 1.

Scheme 1
(a, b) Modification of vegetable oils and (c, d) formulation of membranes.

Firstly, polyester (Scheme 1) was synthesized according to our previous works^23,24 through the reaction of CO (1 mol), TMP (5 mols), PhA (3 mols), BHT (0.005 mol), LiOH (0.0104 mol) at 240 °C for 5 h, N₂ atmosphere, under stirring. Secondly, prepolymer (Scheme 1) was synthesized through the reaction of CAO (0.214 mol), TMP (0.178 mol), BHT (0.005 mol), PG (0.084 mol), BA (1.918 mols), EGA (0.725 mol), HDI (1.555 mols) at 60 °C for 6 h, N₂ atmosphere, under stirring. Then the membranes (Scheme 1) were formulated: Membrane A (Scheme 1) through the reaction of polyester (1 mol), prepolymer (3 mols), SO catalyst (3.45x10^-4 mol) at 25 °C for 10 min, under stirring and Membrane B (Scheme 1) through the reaction of polyester (1 mol), prepolymer (2 mols), SO catalyst (3.45x10^-4 mol) at 25 °C for 10 min, under stirring.

The experimental procedure for obtaining the free membranes is illustrated in Figure 1. Teflon was used as a substrate because membranes are easy removed without causing damage to the surface. The substrate was washed with deionized water and stored in a closed environment at 25 °C. After the preparation of the membranes A and B, they were applied on the substrate surface with the aid of an extensometer and maintained at 25 °C for 48 h. Then, the membranes were removed from the Teflon surface and stored in a desiccator until the characterization experiments were started.

Figure 1
Scheme to obtain the free membranes A and B.

2.3. Thickness of the membranes

The average thickness of the membranes (eight measures each sample) was determined with a MITUTOYO micrometer 0-25 mm/0.01 mm in different regions of the membrane to evaluate homogeneity of thickness throughout the film length.

2.4. Atomic Force Microscopy (AFM)

The membranes before and during 1, 4 and 24 h of immersion in 0.1 mol/L KCl solution were analyzed in situ by phase images obtained with an atomic force microscope, acquired in tapping mode with a scan size of 5 × 5 μm² and from 2 × 10⁵ Hz to 4 × 10⁵ Hz frequency range. The treatment of AFM images was made using the Gwyddion® software.

2.5. Raman spectroscopy

The membranes before and after 7 days of immersion in 0.1 mol/L KCl solution were analyzed by Raman spectroscopy using a Fourier transform Raman spectrometer (Bruker's RAMII model) at 1064 nm wavelength with a laser power of 100 mW as the radiation source with acquisition time of 6 min. The special difference is the polyester / prepolymer ratio. For each Raman spectrum 200 scans were performed in the range of 4000 - 500 cm^-1 with a resolution of 4 cm^-1.

2.6. Electrochemical impedance spectroscopy (EIS) of the membrane in the permeation cell

Figure 2 shows a 1519-6984-bjb-82-e264348-Suppl.pdf of the permeation cell used in the R_CT, i₀ and |Z|_0.01Hz measurements^18-22. The permeation cell was constructed in our laboratory²⁶ based on the Devanathan-Stachurski cell²⁵ containing two compartments to place four electrodes to the impedance measurements, operating with 50 mL of electrolyte in each compartment. Each compartment contained two electrodes: compartment 1- (a) working electrode: graphite stick; (b) reference electrode (work sense): reference electrode Ag|AgCl|KCl (3mol/L); compartment 2- (d) reference electrode: Ag|AgCl|KCl (3mol/L); (e) counter electrode: graphite stick and (c) the intersection between the two compartments of the cell, separated by the membrane to be studied, whose exposed area to the electrolyte was 1 cm².

Figure 2
Double-compartment cell²⁵ used in the permeation tests and built in our laboratory²⁶.

The measurements were carried out in a GAMRY REFERENCE 600 potentiostat-galvanostat. The aqueous solution was 0.1 mol/L KCl because both K⁺ and Cl^- ions have approximately the same mobility²⁷.

EIS measurements were performed with amplitudes between 10 and 100 mV (rms) (Figure S1, Supplementary material) and open circuit potentials were recorded between each impedance measurement. For amplitudes smaller than 50 mV (rms) dispersion of points was observed in medium and low frequency regions, since the signal to noise ratio is small, therefore, not adequate. Consequently, the measurements were performed applying 100 mV (rms), between 1 × 10⁶ Hz and 10^-2 Hz, recording 10 points per frequency decade. All impedance diagrams were validated using Kramers-Kronig Transforms (KKT). All electrochemical measurements were performed at (25 ± 2) ^oC.

3. Results and Discussion

3.1. Atomic Force Microscopy (AFM)

Figure 3 shows the topographic images of 3D AFM of the free membranes. From these images, agglomerates (or grains) are observed for all samples, although smaller agglomerates are seen in the image corresponding to the membranes before immersion in KCl.

Figure 3
Topographic AFM images of (a) membrane A and (b) membrane B, obtained before and after 1, 4 and 24 h of immersion in 0.1 mol/L KCl, respectively.

AFM images allowed obtaining the parameters given in Table 1. The roughness of the membrane A increases with the immersion time, indicating changes in the membrane surface, while the membrane B shows no significant variation in roughness during all the immersion time (24 h), indicating that the membrane surface remains almost unchanged.

Table 1
Roughness calculated from the AFM images of the membranes A and B.

Time of immersion (h)	Membrane A	Membrane B
	RRMS/µm
1	0.111	0.075
4	0.117	0.060
24	0.434	0.073

3.2. Raman spectroscopy

Figure 4 shows intensity Raman spectra for the membranes before and after immersion in 0.1 mol/L KCl for 24 h. The assignments of the bands were made according to previous studies on polyurethanes^28-33.

Figure 4
(a) Raman spectra obtained for membranes A and B before and after immersion in 0.1 mol/L KCl for 7 days and (b) zoom-in of the band for N=C=O group.

The bands detected in the Raman spectra were the same for all membranes, but different intensity can be seen to some bands (Figure 4a). The most intense bands were observed at 2928 cm^-1 and 2855 cm^-1 both attributed to ν_sim(CH₂) vibration. The other bands occurred at: 3079 cm^-1 assigned to ν_sim(CH) aromatic group; 2735 cm^-1 to ν_ass(N=C=O) (Figure 4b); 1729 cm^-1 to ν(C=O) urethane; 1659 cm^-1 to ν(C=O) ester; 1600 cm^-1 to ν(aromatic ring); 1438 cm^-1 to δ(CH₂); 1295 cm^-1 to δ(CH₂) – twist and at 1044 cm^-1 the stretching of C=O²⁸. There is no difference in the bands position before and after immersion in 0.1 mol/L KCl, but the intensity decreases, being more pronounced to the band attributed to the N=C=O group at ~2735 cm^-1 (Figure 4b). The intensity decrease of this band was higher for membrane A than for Membrane B after 24 h of immersion, which may be associated with the consumption of NCO³⁴ to form biuret as illustrated in Figure 5. The greater amount of HDI in membrane A compared to B, i.e., more reactive functional NCO groups are present in A than in B, favored the reaction with water, resulting in higher electrolyte permeation through this polymeric matrix.

Figure 5
Isocyanate chemistry with hexamethylene diisocyanate (HDI)^23,35,36.

The formation of biuret group in the reaction of HDI with water^35,36 is, therefore, an important aspect in this study to interpret the decrease in the N=C=O band after 24 h of immersion. Therefore, a greater number of biuret groups is formed in membrane A than in B.

3.3. Electrochemical impedance spectroscopy (EIS) of the membrane in the permeation cell

Before the electrochemical study, the membrane thickness was determined. The average thickness of the membrane A is (491 ± 1) μm and membrane B is (328 ± 1) μm.

Figures 6 and 7 show the impedance diagrams for the membranes A and B obtained in KCl solution at different permeation times. At 0.01 Hz, the impedance value for membrane A is ~ 1.6 × 10⁹ Ω cm² (Figure 6), while for membrane B (Figure 7) is ~ 1.2 × 10⁹ Ω cm². Considering the different permeation times, the impedance values varied more for membrane A than B, which may be related to the interaction of the isocyanate groups with the water from the electrolyte.

Figure 6
Impedance modulus |Z| Bode plots for the membrane A obtained in 0.1 mol/L KCl, with 100 mV (rms) amplitude in a permeation cell.

Figure 7
Impedance modulus |Z| Bode plots for the membrane B obtained in 0.1 mol/L KCl, with 100 mV (rms) amplitude and permeation cell.

The changes in the impedance diagrams for the membrane B with time are not significant compared to those in membrane A, suggesting that there is no appreciable permeation of the electrolyte and charge transfer through the membrane B. In case of membrane A, the |Z|_0.01Hz value decreased 22.5% from 1 h to 140 h of permeation, while only ~1% was observed for membrane B (Figure S2). For the first 16 h, Figure S2 indicates greater rate of variation in |Z|_0.01Hz values for membrane A than membrane B, suggesting that the greater amount of HDI in membrane A (functional NCO groups) favors the reaction with water, resulting in higher electrolyte permeation through this polymeric matrix. From 16 to 120 h the kinetic of permeation (variation of |Z|_0.01Hz values with time) decreases for both membranes, attributed to the consumption / depletion of the NCO groups caused by the biuret reaction in both sides of the membranes. The |Z|_0.01Hz values increased again at 140 h of immersion for both membranes A and B, however, much lower variation is observed for membrane B.

3.4. Charge transfer resistances (R_CT) and exchange current density (i₀)

Considering the complex plane response, reasonably well-defined semicircles were obtained for membranes A and B (Figure S3, Supplementary material), allowing easy determination of charge transfer resistance. The R_CT values obtained as illustrated in Figure S3 for both membranes are given in Table 2. The R_CT values systematically decrease until 16 h of permeation, then slightly increased up to 120 h and decreased again up to 140 h. As expected, the R_CT values showed the same tendency observed in Figure S2 (Supplementary material). So, the variation of the R_CT values is in line with the other analyses and causally related to the electrolyte permeation.

Table 2
Charge transfer resistances (R_CT /Ω cm²) and exchange current density (i₀ /A cm^-2) for membranes A and B.

	Membrane A		Membrane B
Time / h	RCT/109 Ω cm2	i0/10-11 A cm-2	RCT/109 Ω cm2	i0/10-11 A cm-2
1	1.75	1.46	1.20	2.12
2.5	1.38	1.85	1.19	2.14
4.5	1.33	1.92	1.18	2.16
15 h	1.20	2.12	1.08	2.36
16	1.21	2.11	1.07	2.38
24	1.36	1.87	1.07	2.38
120	1.53	1.67	1.25	2.04
140	1.27	2.01	1.24	2.05

In our case, we are considering that the resistance offered by the monolith to the electrolyte permeation can be measured as a resistance to the charge transport inside the membrane, since the ions needs to cross the solution / monolith interface and then propagates through the membrane. The determination of the R_CT and exchange current density (i₀) are relevant to characterize the barrier properties of the membrane. The i₀ reflects the rate that the salt ions can enter the membrane⁴.

The relationship between R_CT and potassium ions concentration follows the Equation 1 ¹⁹. This is theoretically expected since the i₀ at a membrane / solution interface should follow the Equation 2.

$R_{CT}\propto {(C_{K^{+}})}^{-\frac{1}{2}}$ (1)

$i_0=nF{k}^0{(C_{K^{+}})}^{\alpha }{({\overline{C}}_{K^{+}})}^{1-\alpha }$ (2)

where $C_{K^{+}}$ is the concentration of K⁺ in the aqueous phase, ${\overline{C}}_{K^{+}}$ the corresponding concentration in the membrane, $k^0$ is the heterogeneous rate constant for the charge exchange (transfer) process, whilst $\alpha$ is the transfer coefficient which is normally expected to be near 0.5^4,19. The relationship between R_CT and i₀ when$\alpha$ is near 0.5 is given by Equation 3.

$R_{CT}=\frac{RT}{nF{i}_0}$ (3)

Equation 2 considers the fact that changing $C_{K^{+}}$ changes both the interfacial potential difference according to Nernst equation and the rate of ions crossing the interface. According to Equation 3, the i₀ is inversely proportional to R_CT values as shown in Table 2.

For a direct comparison between the membranes A and B, the impedance results should be normalized for the membrane thickness, which is obtained dividing the |Z| values by the corresponding membrane thickness, now given in Ω cm.

Figure 8 shows the impedance modulus Bode plots normalized for the thickness of the membranes. The graphs are represented on the same scale to better visualize the difference between the membranes. The membrane B is much more resistant to electrolyte permeation than membrane A (Figure 8). Figure 8 evidences the importance of considering the thickness of membranes to better interpret the EIS results of electrolyte permeation or membranes performance.

Figure 8
Impedance modulus |Z| / thickness versus frequency for membrane A and membrane B.

The normalized calculated R_CT to unitary thickness is summarized in Supplementary material (Table S1, Supplementary material) and represented in Figure 9a. Note that for membrane A R_TC values decrease with permeation time, while for membrane B R_TC values increase with permeation time. These results suggest a greater mobility of potassium ions within membrane A than in membrane B. Figure 9b shows the evolution of |Z|_{0.01 Hz} / thickness vs. time obtained from EIS data.

Figure 9
Normalized (a) R_CT and (b) |Z|_0.01Hz values for membranes thickness as a function of exposure time in 0.1 mol/L KCl. Blue color corresponds to membrane A.

In the case of EIS analysis of barrier membranes or coatings, it is well known that failure is evidenced by the decrease of the low frequency impedance values³⁷. It is evident from Figure 9b that the low frequency impedance / thickness of membrane A drops from 3.58 × 10¹⁰ Ω cm to 2.75 × 10¹⁰ Ω cm during 4.5 h of permeation, while for membrane B this parameter slight varied between 3.68 × 10¹⁰ Ω cm to 3.62 × 10¹⁰ Ω cm during 4.5 h of permeation, but it does not mean that the membranes failed, since the impedance increased for longer immersion times. The initial change indicates that solution enter in the membranes and the conductivity increased, mainly for membrane A, suggesting that the surface of both sides of the membrane A may have a more open structure than membrane B, in which the changes are less evident. At the end of the experiment (140 h), the impedance values at low frequency were 2.60 × 10¹⁰ Ω cm and 3.70 × 10¹⁰ Ω cm for membranes A and B, respectively. This implies that if the coating B has the same thickness of coating A, it may present better barrier property than coating A when applied on a metallic surface, and the decrease of impedance at low frequency for coating A could also indicate higher probability of failing. The better performance of coating B against corrosion of aluminum alloy (AA 1200) in 3.5% NaCl aerated solution was already demonstrated¹. The results indicated greater corrosion resistance for coating B in the presence and absence of inhibitors in solution.

Hence, the barrier protection performance of the two membranes can be placed in the following increasing order: membrane A < membrane B. Figure 10 shows a schematic representation of the electrolyte permeation in membrane A. At the initial, the permeation is facilitated due to the reaction of the NCO group with water leading to the formation of biuret (Figure 5), therefore, membrane A that has more HDI in its formulation reacts with more water, thus the electrolyte permeation and water uptake are higher in relation to membrane B. In comparison to membrane B, the membrane A facilitates the penetration of water and consequently of K⁺ ions.

Figure 10
Schematic representation of the electrolyte permeation in membrane A.

In addition to the membrane formulation, whose barrier properties can be evaluated according to the methodology described in the present work, to have membranes with good performance against corrosion it is extremely important the surface preparation of the substrate and uniform membrane / coating deposition to avoid the formation of defects during this process, which may lead the membrane or coating to fail.

4. Conclusion

Organic-base membranes prepared with crambe and castor oils and with roughness (R_RMS) from 0.060 to 0.434 µm and thickness between 328 and 491 µm were tested for ions permeability in 0.1 mol/L KCl aqueous solution. Raman spectroscopic studies allowed identifying the functional groups, assigning the main vibrations of the free membranes, and indicating the formation of biuret groups when in contact with aqueous solution.

Both membranes have low ions permeability, but membrane B presented less variation of R_TC and i₀ values with the testing time than membrane A. The normalized R_TC and |Z|_0.01Hz values for the membrane thickness, at the same conditions, indicated that membrane B presents higher barrier properties, which can lead to a higher protection performance of metallic surfaces against corrosion compared to membrane A.

This study may serve as guide to a previous evaluation of organic resins, organic-inorganic hybrids monoliths, and some inorganic resins developed to be applied as coatings to protect metal surfaces against corrosion from aggressive solutions. The assessment of R_RMS, R_TC, i₀ and

|Z|_0.01Hz parameters and, therefore, of barrier properties and even self-healing ability before resins application on surfaces can avoid time loss and materials consume.

Supplementary material

The following online material is available for this article:

Figure S1 - Complex plane plots obtained for membrane B at different amplitudes between 10 mV(rms) and 100 mV(rms), in 0.1 mol L-1 KCl solution, at 25 oC.

Figure S2 - Evolution of the impedance modulus at 0.01 Hz for membranes A and B immersed in 0.1 mol L-1 KCl

Figure S3 - Nyquist plots for membranes A and B in 0.1 mol L-1 KCl solution

Table S1 - Normalized charge transfer resistances (RCT/Ω cm) for the thickness of membranes A and B

This material is available as part of the online article from https://doi.org/10.1590/1980-5373-MR-2022-0129

5. Acknowledgements

J.V. Nardeli author thanks the São Paulo state agency FAPESP (Process: 2015/10554-9 and 2018/09040-9) for scholarships.

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Publication Dates

• Publication in this collection
  09 Sept 2022
• Date of issue
  2022

History

• Received
  15 Mar 2022
• Reviewed
  06 July 2022
• Accepted
  21 Aug 2022