Evaluation of a commercial graphene‐additive for boosting the corrosion performance of a thermal curing primer

Graphene has emerged as a promising additive for improving the corrosion resistance of organic coatings due to its planar structure and low permeability. Multiple publications have reported positive results, primarily because of its barrier qualities. However, the incorporation of graphene into commercial coatings has not been studied in great detail, as the limited availability of commercial‐grade graphene has hindered its widespread use. This study investigates the addition of a commercially available graphene additive to a commercial primer, using industry‐standard corrosion tests and advanced characterization techniques to evaluate the coating's performance. The results show that while the addition does improve the barrier properties of the coating, as shown in salt spray tests, it also reduces its functionality due to factors such as oxygen reduction on graphene and accelerated diffusion pathways. Overall, the cost of incorporating graphene may outweigh the limited benefits observed in this study. These findings underscore the need for further research to explore the potential of graphene and the importance of utilizing multiple testing methods for performance evaluation.


| INTRODUCTION
Metals and their alloys are widely used in outdoor applications but are susceptible to corrosion, which can compromise their performance and lifespan.To protect metals from corrosive environments, coatings are applied, which have the ability to separate the underlying metal from the corrosive media, reducing the danger of corrosion and making them an affordable and efficient anticorrosion protection method.For industrial applications, organic coatings are often used, as one or multilayer systems, based on various polymeric matrices such as alkyds, polyurethanes, polyesters, melamine resins, or epoxides.However, they by themselves are often insufficient to provide adequate protection against the environment, prompting the incorporation of additional components into coatings.3][4][5][6][7] Another material that has received considerable attention in recent years, owing to its characteristics, is graphene.Graphene is regarded as a constitutional isomer of carbon-based substances such as fullerenes, carbon nanotubes, and graphite.Unlike the other modifications, it is a two-dimensional, atomically thin carbon film that consists of a hexagonal honeycomb structure with sp 2 binding orbitals and therefore possesses special properties. [8]Notable characteristics include very high thermal conductivity and strength, a theoretically predicted specific surface area of more than 2500 m 2 g −1 , and low gas permeability. [9]As a result, numerous research groups try to utilize different graphene-based technologies for various applications.12][13][14] Attempts have been made to use pure graphene coatings for corrosion protection, for example, by chemical vapor deposition (CVD), but this is associated with some difficulties.Although these coatings provide very good short-term protection, they can even have the opposite effect over the longer term.A possible reason for this could be that it is almost impossible to produce coatings without defects using CVD and these can lead to microgalvanic corrosion.An electrochemical connection between graphene and the metal occurs as soon as the corrosive liquid penetrates the graphene layer and reaches the metal interface. [15,16]Moreover, the graphene coating starts to peel off and split owing to the formation of corrosion products on the metal surface. [17]Consequently, some scientists believe that graphene can only be used against corrosion for short-term protection and not to prevent corrosion over an extended period of time.
Corrosion protection provided by graphene particles is mainly due to the barrier protection caused by its large surface area as well as its hydrophobic nature.By acting as a barrier inside the coating, it prevents crack propagation and penetration of the corrosion medium to the substrate.Additionally, graphene acts as an electron flow route in the coating between the anode particles and substrate, enhancing the protective properties of the sacrificial anode.The high adhesion characteristics of graphene and the film-forming properties of the coating matrix are combined in graphene composite coatings, thereby improving the overall performance of the setup.Another benefit of employing graphene as a filler is that coatings can be produced using conventional preparation methods, compared to the elaborate and costly process required for pure graphene coatings. [18]s an anticorrosion pigment, graphene is hardly used commercially.The combination of graphene dispersions and resins is not ideal, which is the fundamental reason why several important scientific problems have not yet been resolved.The main obstacle to the application of graphene in composite coatings is its poor dispersibility in solvents because it lacks functional groups.Additionally, owing to its large aspect ratio and van der Waals forces, graphene agglomerates very quickly.Therefore, coatings used in practical applications have complex formulations and include a variety of auxiliary materials, including levelling agents, antisettling agents, and antifoaming agents.Furthermore, contemporary studies have concentrated on short-term corrosion behavior in laboratory settings.21] Despite these limitations, some suppliers now offer ready-made graphene particles that can be used directly for coating applications.Therefore, the goal of this study was to investigate whether the addition of graphene platelets to an existing primer base would improve its anticorrosion properties without compromising the coating's overall performance.[24][25] The samples were further characterized using scanning electron microscopy (SEM) in combination with energy dispersive X-ray spectroscopy (EDX) as well as Fourier-transform infrared (FT-IR), Raman spectroscopy, thermomechanical analysis (TMA), adhesion tests, surface tension tests, tribometry tests, and blister tests.This comprehensive range of tests was employed to evaluate all effects of graphene addition, yielding a broad range of outcomes.Such a comprehensive evaluation of the properties of a commercially available graphene product incorporated into a commercial primer has received limited attention in the literature to date.Our objective in doing so was to provide novel insights into the potential of graphene-based coatings as a cost-effective and practical solution for anticorrosion coatings.

| Materials
Hot-dip galvanized steel strip samples with a zinc coating of 275 g cm −2 supplied by voestalpine Stahl GmbH were used as substrates (thickness = 0.52 mm).The primers used in this study were derived from a commercially available polyester polyol binder-based primer, incorporating blocked trimeric isocyanate crosslinkers.The primer also contained calcium-exchanged silica pigments with an average particle size of 5 μm, which are widely used ion-exchange pigments.In addition, the primer incorporated layered silicate pigments to improve corrosion protection.This primer base was either spiked with 20 wt.% xylene or with 20 wt.% of a suspension containing graphene nanoparticles in xylene, which was supplied by Applied Graphene Materials (AGM).This method of addition, recommended by the manufacturer, was chosen because its simplicity would facilitate potential industrial use.The addition yielded a final graphene concentration of 0.1 wt.% in the primer.A concentration of 0.1% was selected for the addition, as according to the literature and the manufacturer, an addition as low as that is already sufficient for positive effects and because the aim was to minimize the color change of the primer to enable an industrial application.In addition to the graphene nanoparticles, the suspension also contained ethylbenzene, toluene, polyurethane, 2-methoxy-1-methylethyl acetate, and 2-methoxypropyl acetate.A commercially available topcoat based on polyester polyol binders was used for some tests.

| Coating application
To get rid of contaminants and oil residues, the substrates were cleansed using an alkaline washing procedure.The cleaning solution, which contained 10 g/L Bonderite C-AK C 72 (Henkel AG & Co. KGaA) which adjusted the pH to 10, was heated to 40°C during the process.The clean substrates were then treated with a chromium-free pretreatment solution (Bonderite 1455; Henkel AG & Co. KGaA) to create a conversion layer that offers extra corrosion protection and helps the organic coating adhere.Using spiral bar coaters (RD Specialties, Inc.), the substrates were coated with primers of 6 ± 1 μm thickness and subsequently dried at the metal peak temperature of 235°C (PMT; temperature needed to completely crosslink the coating) of the corresponding material in an air-circulating oven (Hofmann Wärmetechnik GmbH).Samples that were only coated with primer were subjected to a second cure at the PMT of the primer to mimic the curing behavior of a primer topcoat system.For the primer topcoat systems, a topcoat based on polyester polyol binders and hexa(methoxymethyl)melamine crosslinkers with a thickness of 20 ± 1 μm was applied on top of the primer, and the systems were then cured at the appropriate topcoat PMT of 241°C.The dry film thicknesses (DFT) of the top coat and primer were measured using the magnetoinductive method (Deltascope FMP30, Fischer) and the beta backscatter method (Fischerscope MMS, Helmut Fischer GmbH), respectively.
Furthermore, free coating films were produced by coating degreased steel substrates covered with selfadhesive PTFE films with primer using spiral bar coaters, heating the setup to the PMT, and afterward peeling the coating films off.The free film thickness was determined magnetoconductively (DFT ~11 μm).

| Characterization
According to the SDS, the graphene particles had a particle size of D50 5-6 μm and D90 6-28 μm.To determine the presence of functional groups on the graphene surface, Fourier-transform infrared spectroscopy (Transit Platinum ATR, Bruker) measurements were conducted.Raman spectra of graphene from 100 to 3400 cm −1 were recorded using a confocal Raman spectrometer (DXR SmartRaman Spectrometer, Thermo Fisher Scientific) to assess the various flaws in the particles.For the purpose of determining the surface morphology of the produced composite coatings, crosssectional images were acquired using SEM (Zeiss/Ultra 55).Gold was sputtered onto the samples before embedding them in an epoxy matrix.An energy dispersion X-ray analyzer was used to identify the components and provide quantitative compositional data (EDX.Tescan Clara, 15 keV).An argon ion beam was used to prepare the samples via cross-section polishing (CSP).

| Physical/mechanical tests
Various tests were performed to determine the effects of graphene nanoparticle addition on the physical and mechanical properties of the coatings.Color measurements were performed to determine the changes in the CIELAB color space (MA68II, X-Rite).Tribological measurements were carried out to determine the respective friction coefficients (Pin-On-Disk Tribometer: TRB³, Anton Paar).The wettability was calculated by calculating the surface energy by measuring the respective contact angles of applied liquids (Mobile Surface Analyzer [MSA] One-Click SFE; Krüss) and pull-off tests in accordance with DIN EN ISO 4624 were used to determine the dry adhesion.The influence of graphene nanoparticles on the glass transition temperature (Tg) of the primer was also determined using thermomechanical analysis (TMA; Q400, TA Instruments).
In accordance with DIN EN ISO 9227, neutral SST were carried out.Plate shears were used to trim the specimens to proportions of 150 × 100 mm.Both the barrier protection of the specimen, for which samples without a scribe were used, and protection against anodic delamination, for which two scribes were applied to the sample surface, were tested.One scribe was down to the zinc coating, and the other down to the steel substrate beneath.Adhesive tape was attached to the edges of all the samples to prevent the occurrence of inconclusive corrosion phenomena at the specimen edges.During the first few weeks, the samples were imaged every day, subsequently once per week, to monitor the corrosion's development.All samples were removed from the chamber after a specified amount of time (3000 h for the samples without scribes, 1000 h for the samples with scribes), and in the case of samples with scribes, the delaminated area was removed using a blunt scalpel.SMART, a commercial image analysis tool, was used to analyze each sample to determine the size of the corroded or delaminated area (http://www.quantiz.tech/).By enabling the detection of all color variations, this software enables users to distinguish between intact and delaminated portions of primer in captured images.
To simulate and expedite the effects of rain and dew on the coated materials, a blister tester was employed with 100% condensing humidity as depicted in Figure 1a.Sample setups consisting of primers and a top coat with a side length of 90 mm were placed above a water bath.Owing to the ambient temperature of approximately 23°C and the constant temperature of 70°C in the water bath, the backsides of the samples had a thermal gradient and a surface temperature of approximately 55°C.Every week, the coatings were inspected for defects and the level of blistering was assessed visually in accordance with ISO 4628-2.The bubble evaluation matrix, which was used to convert the bubble size (g) and bubble quantity (m) into numerical values, is shown in Table 1.The samples were assessed using reference images, with 0 expressing no bubble formation and 5 expressing substantial bubble formation.

| Electrochemical tests
Home-made measuring cells were used for all electrochemical measurements.PMMA tubes were glued to the sample surface using a two-component adhesive (UHU 2-Komponentenkleber Plus, UHU GmbH & Co KG), leaving an exposed area of approximately 12.5 cm².A 1 M NaCl solution was used as the electrolyte, the steel substrate was employed as the working electrode, and a platinum mesh served as the counter/reference electrode.We chose to use a 2-electrode setup despite being aware of its limitations.Our decision was based on the fact that the results obtained with the 2-electrode setup were the same as those obtained with a 3-electrode setup, while the 2-electrode setup offered some practical advantages over the 3-electrode setup that is, it being simpler to assemble and requiring fewer parts.This minimized potential sources of error and increased the reliability of our results.In addition, the measured values contained less noise and were subject to fewer fluctuations.Figure 1b shows a schematic illustration of the setup.A potentiostat (Gamry Reference 600; Gamry Reference Inc) was attached to the sample setup and the parameters were adjusted according to the measurements.
After the sample had been in contact with the electrolyte solution for 24 h, the equilibrium OCP value was recorded, and this value served as the starting point for impedance measurements.The excitation amplitude was 10 mV, and the frequency range that was measured was 10 −2 to 10 5 Hz.For 24 h, EIS measurements were taken every 120 min to monitor how the coatings' impedance altered.The analysis was performed using Echem Analyst™ software (Gamry Instruments Inc).
The CD was measured by applying a steady current flow of 0.5 A at 36°C for a predetermined amount of time simultaneously to up to eight samples.A 0.5 M KOH solution was used as electrolyte.All samples were sliced to a size of 30 mm × 150 mm and scribed in the center with a 55 mm long scribe down to the zinc coating.The specimens' edges were taped to prevent delamination starting from there.Each test was followed by the removal of the delaminated primer using sticky tape in accordance with ISO 2409:2013.

| Water uptake
The water absorption was determined by analyzing the Nyquist plots obtained from impedance measurements using the Brasher-Kingsbury equation on the one hand and gravimetrically using a sorption balance (Aquadyne DVS™, Quantachrome Instruments) on the other.Free coating films were used for the sorption balance, and a program was employed in which the temperature was maintained at 30°C and the relative humidity was increased in increments of 10% from 0% to 100% and then stepwise decreased back to 0%.The change in mass of the sample material was measured permanently.

| Characterization
The primer samples with and without graphene nanoparticles show a comparable microstructure with a similar distribution on the crosscut SEM images.Thus, the addition of graphene did not affect the orientation or distribution of primer components.Figure 2a,b shows crosscut images of primer on hot-dip galvanized steel.The presence of more pigments in Figure 2a is coincidental.In addition, the graphene particles obtained from the graphene-xylene suspension were characterized.Figure 3 shows a combined FTIR and Raman spectrum.FTIR spectroscopy was used to study the modification of the obtained graphene and to detect the presence of graphene oxide (GO).One indication for the presence of GO is the existence of the peak at position 3000 cm −1 denoting Note: The bubble quantity (m) and bubble size (g), where 0 represents no bubble formation and 5 represents substantial bubble formation, were assessed using reference images.The blister image rating was consequently used for a numerical evaluation.
F I G U R E 2 (a) A cross-section polishing (CSP) cross-section of a primer to which xylene has been added and (b) CSP cross-section of a primer to which xylene and graphene nanoparticles have been added.The average dry-film coating thickness was always approximately 6 μm.
hydroxyl groups.Further evidence for the presence of GO are the bands at 1055 cm −1 and 1150 cm −1 , indicating C-O stretching vibrations of alkoxy and epoxy groups as well as the bands at 1370 cm −1 (stretching of tertiary C-OH groups) and 1730 cm −1 (C═OH stretching of COOH groups).Additionally, two more bands are visible in the FTIR spectrum: one at 1220 cm −1 due to C═C skeletal vibration and one at 1640 cm −1 because of a stretch of C═C groups. [26,27]he Raman spectrum of the graphene particles is shown in Figure 3 as well.The D band was found at 1360 cm −1 , the G band at 1560 cm −1 , the D' band at 1580 cm −1 , and the 2D band at 2840 cm −1 in the spectrum.The G band exists because of the first-order dispersion of the E2g mode, whereas the D band is present because of defects in the material, such as bond-angle disorder, bond-length disorder, vacancies, and edge defects.The structural parameters of the orientation of the c-axis are evaluated using the 2D band.Graphene and graphite both feature G and 2D peaks; however, the 2D peak in graphene is very intense and sharp, whereas the 2D peak in graphite is broad and less intense, and it also splits.For this reason, it can be assumed that graphite was present in the suspension in addition to graphene oxide. [28,29]ased on the analysis of the spectra, it can be concluded that the sample analyzed was not pure graphene but contained a high proportion of graphene oxide and graphite.

| Physical/mechanical tests
For this study, color measurements were performed on primer samples with and without graphene nanoparticles.
The results were evaluated using the CIELAB color space, a widely accepted color model that characterizes colors based on their lightness (L*), red-greenness (a*), and yellow-blueness (b*).The results showed that the addition of graphene nanoparticles had a significant effect on the color of the primer.Specifically, the graphene particles caused a reduction in the L* value, indicating that the color of the primer became darker.In addition, the a* and b* values were increased, indicating that the green and blue hues of the primer became more red and yellow, respectively.These changes in the color properties of the primer samples were quantified by calculating the color difference (dE) between the samples with and without graphene particles.Table 2 summarizes the average L*, a*, b*, and dE values measured for the samples.The results suggest that the addition of graphene particles alters the color of the primer, potentially impacting the aesthetics of coated surfaces.To provide a visual representation of the color changes, Figure 4 shows images of the primer samples with and without graphene nanoparticles.The images highlight the differences in color between the two samples and demonstrate the effects of graphene on the appearance of the primer.Higher graphene concentration would have even larger effects, which would limit its industrial applicability.The addition of graphene particles to the primer lead to a decrease in the measured pull-off adhesion, as shown in Figure 5, which is likely due to the presence of graphene particles at the interface, allowing only nonchemical bonds or van der Waals bonds to form.In addition, graphene particles tend to form agglomerates due to their high surface energy and tendency to stick together.The presence of agglomerates in the primer can cause poor adhesion and a rough surface finish.
The evaluation of the maximum measured coefficients of friction showed that adding graphene led to an increase, as also shown in Figure 5. Graphene is known for its excellent lubrication properties, and its addition to a primer can reduce the coefficient of friction of the surface, making it more slippery and reducing wear and tear on the coated surface.However, if graphene pigments agglomerate, this can lead to an uneven dispersion and coverage, leading to areas of higher and lower concentrations of graphene and therefore affecting the coating's performance.In some cases, agglomeration can cause the coating to have a rough or uneven surface, which can increase the coefficient of friction.
The coefficient of friction can also be affected by the orientation and distribution of graphene nanoparticles within the primer matrix.As graphene is a highly anisotropic material, its properties can vary significantly depending on the orientation of the flakes.The observed increase in the coefficient of friction with the addition of graphene pigments to the primer may therefore be attributed to both the agglomeration of graphene particles and the anisotropic nature of graphene.The uneven distribution of graphene particles and increased surface roughness resulting from agglomeration could lead to local increases in the coefficient of friction, while the anisotropic nature of graphene could create directional dependence of the coefficient of friction.The right choice of polymer and its interaction with graphene could play an important role, since although the majority of publications speak of a reduction in the coefficient of friction, there are also publications that report an increase. [30,31]he surface tension of the coated samples was determined using contact angle analysis of two different liquids, water and formamide, on the primers with and without graphene nanoparticles.The contact angle values obtained were used to calculate surface energies, and the outcomes are summarized in Table 3.The surface tension values for the primers with and without graphene were similar, but the primer with graphene particles exhibited a slightly higher proportion of disperse energy.This increase is likely due to changes in the surface energy of the primer caused by altering the interactions between the primer and the test liquids.It is worth noting that the addition of graphene particles to the primer was only 0.1%, which may explain why the influence on the surface tension was minor.Figure 5 displays the measured contact angle values and the surface energies that were calculated employing them.TMA measurements showed that the addition of graphene had no effect on the glass transition temperature of the primer system, with both systems having a Tg of 68 ± 1°C.Since the Tg can be influenced not only by the molecular weight of the polymer chains and the spatial structure of the polymer but also by the addition of additives, this information is of great importance for possible applications, as the Tg is a key parameter in coating technology and polymer processing. [32,33]Since it is known that the addition of graphene can increase the Tg, 0.1% graphene may not be enough to have a significant impact on the glass transition temperature of the polyester primer.Additionally, if the graphene is not dispersed well enough in the polyester primer, it may not be able to interact with the polymer matrix and affect its properties.

| Samples in SST without scribes
Analysis of the long-term corrosion resistance of primer samples with and without graphene particles was conducted using a SST over 3000 h.The samples were observed periodically, and images of their conditions were recorded using Figure 6a.After 1500 h, no visible changes were observed on the sample surface.Subsequently, the corrosion phenomena gradually migrated from the taped edges toward the center of the specimen.After 3000 h, both the samples with and without graphene particles exhibited a considerable amount of corrosion products on the surface, which had migrated from the edges to the center.Despite the addition of graphene particles potentially improving the barrier protection, the protective effect of the coating without graphene was already sufficient to withstand the entire test period.Therefore, the influence of graphene platelets on the barrier properties as indicated in the literature could not be verified in this test. [18,34]

| Samples in SST with scribes
The samples with scribes were evaluated after 1000 h in the salt spray chamber, and the delamination front was analyzed using the SMART image analysis software.Figure 6b,c shows typical images of the samples after the delaminated primer had been removed.The evaluation of the delamination using the SMART software is shown in Figure 6d.The primer samples containing graphene particles showed decreased delamination in both the scribes down to the zinc coating and the scribes down to the underlying steel substrate.One reason for this could be the high potential of the graphene particles, which shifts the potential of the entire system to a more noble region.In addition, the increased barrier protection results in less diffusion of oxygen and other ions through the coating to the interface, which inhibits redox reactions from occurring.This leads to a reduction in the delamination reactions taking place. [18,35]

| Blistering tests
Samples coated with primer and topcoat were subjected to a homemade blister testing apparatus and monitored for a period of 14 weeks.During the first 7 weeks, all samples, regardless of the presence of graphene in the primer, remained unchanged.However, at the 8th week, small bubbles emerged on the primer samples that contained graphene, while the nongraphene primer samples remained undamaged.Subsequently, the number and size of the blisters on the graphene-containing samples increased progressively, peaking at the 12th week, after which no significant changes were observed.The experiment was concluded after 14 weeks, at which point the surfaces of the nongraphene primer samples had still remained entirely intact.Figure 7b shows the evaluation of bubble formation according to Table 1.
Our findings reveal a marked decrease in blistering resistance, potentially resulting from several factors, such as oxygen reduction on graphene or accelerated diffusion pathways.The test involved exposing the coating system to high humidity conditions, which can cause moisture to infiltrate the coating layers, creating blisters and bubbles.
Given that the adhesion of the graphene-containing primer is weaker than its nongraphene counterpart, moisture infiltration may have been more pronounced, resulting in more severe blistering.These results differ significantly from the SST results because the processes of delamination are different.This demonstrates the importance of applying different tests to determine the performance of the graphene coating.

| CD tests
Samples coated with primer and topcoat were evaluated.The samples containing graphene nanoplatelets showed no improvement in comparison to the primer samples without graphene but in contrast a small increase in CD.Therefore, the addition of the particles had no positive effect on the performance of the samples.However, the change was only slight.Figure 7c shows the average CD.An evaluation of the delamination using the SMART image analysis software yielded the following results: A delamination of 1.08 ± 0.06 mm for primer samples with graphene and a delamination of 0.98 ± 0.06 mm for primer samples without graphene.

| Electrochemical tests
Since graphene is a noble material, its addition shifts the OCP into more positive areas.And as the OCP is an indicator of the susceptibility of coatings to corrosion, this shift would confirm that graphene enhances the corrosion protection of coatings. [36]Impedance measurements of all primer samples showed comparable values, with slightly higher results for the samples containing graphene particles.This would also indicate increased barrier protection.However, all the measured |Z| values were <10 6 Ohm cm², indicating that the coating was no longer able to provide sufficient barrier protection. [37]his is not unexpected as the primers are designed to be used in combination with a top coat and do not provide adequate protection on their own.The measured phase angles showed a similar progression over the measured frequency range; however, the graph was shifted to the left in the primer samples with graphene.The breakthrough frequency, that is, the frequency at which there is a change from capacitive to ohmic behavior (phase angle = 45°), thus occurs at somewhat lower frequencies in the graphene samples.Consequently, the primer samples that contained graphene performed slightly better in this test.Breakpoint frequencies greater than 10 Hz signify the breakdown of an organic coating's protective function. [38]This threshold, however, should only be used as a guideline because it can change depending on the coating system.Figure 8 shows the averaged measured Bode plots of the primer samples with and without graphene.The slightly better performance in the electrochemical tests and the better barrier protection derived from this are in agreement with the results of the SST with a scribe, in which slightly better barrier properties can also be attributed to the coatings containing graphene.The fluctuations at the beginning of the impedance measurements, as shown in Figure 8b, are due to the water absorption at the beginning of the measurements.As soon as these are for the most part completed, the measured impedance values stabilize.

| Water uptake
Two distinct techniques were employed to assess the impact of graphene incorporation on the water sorption behavior of the coating, namely sorption balance measurements and evaluation of EIS data using the Brasher-Kingsbury equation.The results of both methods are presented in Figure 9.
The results demonstrate that the introduction of graphene as a pigment has a negligible effect on the water vapor sorption, which is primarily driven by the ion exchange pigments present in the coating.Specifically, the water absorption variation caused by the addition of graphene was found to be less than 0.2 percentage points in the sorption balance measurements, indicating that the graphene pigment does not significantly alter the coating's water sorption behavior.
However, the results should be taken with caution, as the Brasher-Kingsbury equation is dependent on several assumptions, including low water content in the coating and uniform distribution of water throughout the coating.While the first assumption often restricts the validity of the equation to the early phases of absorption, the second assumption is rarely met in a process of transitory mass transfer. [39]These findings suggest that graphene-containing coating systems could be used in moisture-sensitive applications without compromising their water-sorption properties.Further studies could focus on examining the coating's other properties, such as its mechanical performance, to assess its suitability for practical applications.In this study, we tested if the addition of graphene nanoparticles to an existing primer base would improve its anticorrosion properties without compromising the coating's overall performance.Both the graphene suspension and the primer are commercially available and were tested as additives at 0.1 w%.Thorough characterization of the graphene particles using Raman and FTIR revealed the presence of both graphene oxide as well as graphite.While some industrial tests showed slight improvements in performance, others exhibited significant decreases, as summarized in Table 4.
In particular, blistering resistance was found to be dramatically decreased, possibly due to oxygen reduction on graphene or accelerated diffusion pathways.Furthermore, the presence of graphene at the surface was found to lower adhesion, likely due to weaker binding of the coatings to the base material, as graphene cannot bond specifically, or via any acid/base mechanism.However, the addition of graphene did improve SST performance, possibly due to an increased barrier effect or electrochemical modification of the surface.
Overall, the mixed results of the various tests suggest that further research and optimization are required before graphene can be considered a viable additive for industrial corrosion-resistant coatings.For example, reducing interfacial graphene adsorption may provide a pathway to better performance for all test protocols.Additionally, the limited beneficial properties of graphene, combined with its cost, suggest that its incorporation into commercial coatings may not be justified at this time.
Schematic representation of the blister test.The samples positioned above a water bath experienced hot vapor condensation, raising their surface temperature to >50°C.The samples begin to blister after a few weeks of exposure.(b) Schematic illustration of the two-electrode setup used for the electrochemical measurements.[Color figure can be viewed at wileyonlinelibrary.com]

T A B L E 1
An evaluation matrix for blisters according to ISO 4628-2 standard.

F I G U R E 4 F 6 +G
Coated samples: (a) primer without graphene and (b) primer with graphene.I G U R E 5 (a) Adhesion, determined by means of pull-off tests, (b) the maximum measured coefficient of friction, measured with a tribometer, (c) surface energies, calculated from contact angle measurements with formamide and water.All measurements were conducted for primed samples with (+G) and without (−G) graphene.[Color figure can be viewed at wileyonlinelibrary.com]T A B L E 3 Measured contact angles for primers with (+G) and without (−G) graphene nanoplatelets.Contact anglewater/°C ontact angleformamide/°− G 81.4 ± 0.9 66.1 ± 0.

F
I G U R E 6 (a) Comparison of the conditions of the primer samples without scribes with and without graphene in the salt spray chamber over the course of 3000 h.(b) Image of a sample without graphene with a scribe to the zinc coating on the left and on the right down to the steel substrate below after 1000 h in the salt spray chamber.The delaminated primer was removed using a blunt scalpel.(c) Image of a primer with graphene nanoparticles.(d) The evaluation of delamination using the commercial software SMART is presented.[Color figure can be viewed at wileyonlinelibrary.com]

F
I G U R E 7 (a) Development of the sample surface in the blister test over the weeks.The development of blisters on the samples containing graphene started at Week 8 (8 weeks).(b) Evaluation of blister formation according to Table 1 over the course of 14 weeks.G+ indicates primer samples with graphene, and G− primer samples without.All samples were additionally coated with a top coat.(c) Samples with primer and topcoat after a 30-min cathodic delamination test.The samples were subjected to a current of 0.5 A at 36°C in a 0.5 M KOH solution.The primer of the sample on the left-hand side contained graphene nanoparticles, and the primer of the sample on the right-hand side contained no graphene.[Color figure can be viewed at wileyonlinelibrary.com]

F
I G U R E 8 (a) The Bode plots of electrochemical impedance spectroscopy (EIS) measurements performed on primer samples with (+G) and without (−G) graphene pigments over the frequency range 10 5 -10 −1 Hz after 24 h in 1 M NaCl solution.The lines show the measured impedance value, and the dots the phase angle.(b) Progression of the impedance magnitude at 0.1 Hz over the course of a 24-h EIS measurement.[Color figure can be viewed at wileyonlinelibrary.com](b) (a) F I G U R E 9 (a) Change in the mass of primers with (+G) and without (−G) graphene pigments with changes in relative humidity.The measurements were carried out using a sorption balance at a constant temperature of 30°C.The relative humidity was increased in increments of 10% from 0% to 100% and then stepwise decreased back to 0%.(b) Diffusion coefficients calculated from electrochemical impedance spectroscopy (EIS) measurements using the Brasher-Kingsbury equation.[Color figure can be viewed at wileyonlinelibrary.com] Fourier transform infrared and Raman spectrum of the graphene nanoplatelets.[Color figure can be viewed at wileyonlinelibrary.com]