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Article

Application of AC-DC-AC Accelerated Aging to Assess the Galvanic Corrosion Risk of Mild Steel Coated with Graphene-Embedded Epoxy Coatings

by
Kazem Sabet-Bokati
* and
Kevin Paul Plucknett
*
Department of Mechanical Engineering, Dalhousie University, 1360 Barrington St., Halifax, NS B3H 4R2, Canada
*
Authors to whom correspondence should be addressed.
Coatings 2025, 15(5), 501; https://doi.org/10.3390/coatings15050501
Submission received: 28 March 2025 / Revised: 19 April 2025 / Accepted: 20 April 2025 / Published: 23 April 2025
(This article belongs to the Section Corrosion, Wear and Erosion)
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Abstract

:
This study presents a novel approach to evaluate the galvanic corrosion risk of mild steel coated with graphene-embedded epoxy coatings. The potential for graphene platelets to promote anodic dissolution of the underlying steel substrate via galvanic corrosion mechanisms was systematically assessed through the accelerated alternating current-direct current-alternating current (AC-DC-AC) technique and cathodic disbondment testing. The possible risk of displacing cathodic reactions from the coating–steel interface to the dispersed graphene platelets within the epoxy matrix was investigated by evaluating the degradation trend of the graphene-containing coating under the AC-DC-AC test. The degradation behaviour of both pure epoxy and graphene-embedded epoxy coatings during accelerated aging was characterized using electrochemical impedance spectroscopy (EIS) measurements. The finding highlighted the negligible catalytic effect of incorporated graphene platelets on the anodic dissolution of steel substrate. On the other hand, as an inert filler, graphene platelets contributed to the enhancement of the structural integrity of the epoxy matrix during the AC-DC-AC test and natural immersion in NaCl 3.5 wt % solution by enhancing the barrier performance of the coating. Despite their spectacular barrier performance, damaged graphene-containing coatings performed inferiorly against corrosion-induced delamination compared to pure epoxy. Samples underwent the cathodic disbondment test to eliminate the effect of substrate anodic dissolution from corrosion-induced delamination. The accelerated delamination of graphene-embedded epoxy coatings was attributed to the destructive impact of graphene platelets on the interfacial adhesion of the epoxy matrix to the steel substrate.

1. Introduction

The increasing demand for metallic components and infrastructure across various industries necessitates the development of effective failure mitigation strategies to maintain asset integrity. Among the various protective approaches, organic coatings have gained prominence as a widely utilized, economically viable, and durable solution for reducing the incidence of corrosion-related failures in metallic infrastructures [1,2,3]. However, the infiltration of corrosive agents into these coatings eventually compromises the integrity of the underlying metal substrate [4,5]. To tackle this challenge, extensive research has focused on incorporating functional fillers into polymeric coatings to enhance their protective capabilities. These fillers can either obstruct the diffusion of corrosive species or inhibit electrochemical reactions at the metal/coating interface [4,6,7,8,9]. Graphene platelets have emerged as promising candidates among the various barrier-enhancing fillers. Their exceptional impermeability and mechanical robustness make them potential candidates for improving the protective performance of organic coatings [6,10,11,12,13,14].
The integration of graphene-based materials with various polymers, including epoxy resin [15], polyurethane [16], polyurea [17], and polyvinyl butyral [14], has attracted considerable attention in corrosion protection applications. This trend is attributed to the synergistic interaction between the polymer matrix with strong adhesion to metallic substrates and graphene-based materials with their exceptional impermeability and chemical inertness [13]. The presence of graphene nanoparticles within the polymer matrix increases the tortuosity of diffusion pathways, thereby delaying the ingress of corrosive species at the coating–substrate interface [18,19]. Wang et al. [15] demonstrated that the incorporation of graphene nanoplatelets at concentrations ranging from 0.1 to 1 wt % into an epoxy matrix significantly improved the coating’s barrier properties. Similarly, polyurethane coatings reinforced with 1 wt % graphene nanoplatelets exhibited enhanced corrosion resistance in a 3.5 wt % NaCl solution, with performance improvements directly correlating to a reduction in platelet size [16]. Abakah et al. [20] validated the superior barrier properties of epoxy coatings incorporating smaller graphene nanoplatelets. In addition to enhancing barrier characteristics, graphene reinforcement improves the toughness of coatings, mitigating crack propagation and restricting the ingress of corrosive agents [21].
Although graphene platelets have demonstrated a remarkable ability to enhance the barrier properties of the polymer matrix, studies have reported that incorporating graphene particles may, under certain conditions, exacerbate the corrosion of the underlying substrate [13]. Kopsidas et al. [10] observed that epoxy coatings containing up to 0.5 wt % graphene nanoplatelets exhibited corrosion resistance comparable to that of unmodified epoxy. However, a notable decline in corrosion resistance was observed at elevated graphene concentrations, which was attributed to the increased electrical conductivity of the pigmented coatings. Similarly, Zhang et al. [22] reported accelerated corrosion of steel substrates that were coated with graphene-containing epoxy due to the formation of micro-galvanic cells at the coating–substrate interface. Conversely, Glover et al. [23] demonstrated that incorporating graphene nanoplatelets into polyvinyl butyral coatings substantially mitigated corrosion-induced delamination on iron and zinc substrates, reducing it by 93.5% and 99.6%, respectively. These conflicting findings highlight the need for further investigation into the potential galvanic corrosion risks associated with graphene-containing coatings applied to metallic substrates. Moreover, the delamination of the polymer matrix is a multifaceted process influenced by various factors, including the degradation of the polymer matrix, electrochemical reactions at the coating–substrate interface, and the adhesion characteristics of the coating to the substrate [4]. While the accelerated delamination of graphene-containing coatings has frequently been attributed to the galvanic corrosion of the substrate, there is no definitive evidence confirming that galvanic corrosion is the primary mechanism underlying this behaviour. A systematic investigation that independently evaluates the roles of electrochemical corrosion and adhesion loss would therefore provide enhanced clarity regarding the impact of graphene nanoparticle incorporation on the corrosion behaviour of coated metallic substrates.
The present study systematically investigates the risk of galvanic corrosion for mild steel coated with graphene-embedded epoxy coatings. Corrosion-induced delamination of pure epoxy and graphene-containing epoxy was assessed using electrochemical impedance spectroscopy (EIS) and salt spray tests. Galvanic corrosion of the substrate can be driven by the electrical connection of graphene platelets and mild steel, with the more electrochemically noble graphene particles acting as cathodic sites. Assuming that infiltration of water to the coating interface and underlying substrate leads to an electrical connection of the steel substrate and dispersed graphene platelets in an epoxy matrix, exposing these platelets to a positive potential can accelerate the coating’s degradation via alkaline hydrolysis or by forcing cations to permeate into the coating. Accordingly, a comparative analysis of the degradation behaviours of graphene-embedded epoxy coatings under natural immersion conditions versus accelerated aging conditions (e.g., AC-DC-AC testing) was performed using EIS measurements to evaluate the potential activity of graphene platelets as cathodic sites. Furthermore, by performing a cathodic disbondment test, assessing the adhesion strength of the coatings, and evaluating the spreading behaviour of liquid coating on steel substrate, the effects of interfacial adhesion strength on the delamination behaviour of these coatings was evaluated. Recognizing that interfacial coating delamination is highly dependent upon both the kinetics of electrochemical reactions at the interface and the interfacial adhesion strength of the coating to the substrate, this research provides a distinctive approach to differentiate these contributing parameters for highlighting the risk of graphene-containing coating on galvanic corrosion of steel substrates.

2. Experimental Procedures

2.1. Materials and Methods

A liquid diglycidyl ether bisphenol A (DGEBA) epoxy resin, with an epoxy equivalent weight (EEW) of 184–190 g/eq was sourced from Kukdo Chemical (Kunshan, Jiangsu, PR China), and Ancamine 2811 (Evonik, Philadelphia, PA, USA), a phenalkamine-based modified amine curing agent with an amine value 173 mg KOH/g and amine hydrogen equivalent weight (AHEW) 255 g/eq was obtained from Evonik Industries (Essen, Germany), in order to serve as the mixed organic matrix. Graphene platelets with a predominant thickness of 6–10 layers and lateral dimension of less than 2 micrometres sourced from NanoXplore (Montreal, QC, Canada) were incorporated into the epoxy matrix to improve its protective performance. The detailed composition of each coating is reported in Table 1.
After the mixing of resins and the curing agent (Table 1), the resultant coatings were applied using an air spray technique onto grit blasted mild steel 1018 substrates (15 × 10 × 1 mm3—sourced through Metal Supermarkets, Halifax, NS, Canada) using a 2 mm nozzle diameter at 140–200 kPa pressure. Using an Elcometer 224 (Elcometer Ltd., Manchester, UK) digital surface profile gauge, the average surface profile of the blasted substrates was determined to be approximately 75 µm. Before application of the coatings, the surfaces of the blasted steel samples underwent a thorough cleaning process involving degreasing and dedusting with methanol and acetone. Following a curing period of 14 days at a constant temperature of 25 °C, samples exhibiting uniform dry film thicknesses of 125 ± 10 µm were selected for subsequent analysis. The distribution of graphene platelets in the epoxy matrix evaluated using a confocal laser scanning microscope (CLSM), specifically the VK-X1100 model from Keyence Corp (Keyence, Mississauga, ON, Canada).

2.2. Corrosion-Induced Delamination

Following the standard protocol Standard Practice for Operating Salt Spray (Fog) Apparatus ASTM B117-09, the delamination propagation resistance of pure epoxy and graphene-embedded artificially scratched coatings was assessed through a controlled salt spray test. In preparation for the salt spray exposure, the edges and back faces of the samples were coated with a primer and subsequently masked to isolate specific areas. These isolated regions were subjected to a continuous salt spray environment (WTS 90 salt spray chamber, Weice Testing Instrument Co., Ltd., China). After an exposure period of 90 days, the samples were systematically evaluated for corrosion creep following the methodologies outlined in Standard Test Method for Evaluation of Painted or Coated Specimens Subjected to Corrosive Environments (ASTM D1654-24).
It has been reported that EIS spectra provide insight into the electrochemically active area at the coating–metal interface [24] representing the progress of corrosion-induced delamination of the coating. To investigate the corrosion-induced delamination behaviour of coatings, an artificial aperture with a diameter of 3 mm was created at the center of unpigmented and pigmented coatings by mechanically removing the epoxy matrix from the steel substrate. The delamination trends of coatings were assessed by EIS measurements at different immersion times. The EIS measurements were conducted with a Gamry Interface 1010E potentiostat/galvanostat (Warminster, PA, USA), configured in a three-electrode arrangement: the coated panel as the working electrode (2.85 cm2), a graphite block as the counter electrode, and a saturated calomel electrode (SCE) as the reference. EIS measurements were performed at ambient temperature (25 °C) in a 3.5 wt % NaCl solution, spanning a frequency range from 100 kHz to 10 mHz. Data acquisition was conducted at a rate of 10 points per decade, employing a signal amplitude of 10 mV at the open circuit potential (OCP).
To comprehensively understand the interfacial corrosion mechanisms, potentiodynamic polarization tests were also conducted on samples immersed in the NaCl solutions for 7 days. These polarization tests involved sweeping the potential from OCP −300 mV to OCP +1800 mV at a scan rate of 1 mV/s. All measurements were conducted in triplicate to assess the precision and reliability of the results.

2.3. AC-DC-AC Aging Test

The potential risk of graphene platelets acting as active cathodic sites was assessed using an alternating current-direct current-alternating current (AC-DC-AC) method. Thermodynamically, the electrochemical potential difference between mild steel and conductive graphene platelets means the graphene particles are a possible threat to promote steel corrosion. Given that the surface of graphene platelets can act as active cathodic sites in water-saturated penetration pathways, the cathodic reactions can displace from the metal/coating interface and into the coating [23] (shown schematically in Figure 1). In this case, the formation of highly alkaline products at the interphase of graphene and the polymer matrix leads to alkali hydrolysis of the coating [25]. Moreover, the high potential applied to the sample during cathodic polarization accelerates the diffusion of cations and exacerbates degradation of the coating [26]. Therefore, comparing the degradation trend of graphene-embedded coatings in natural immersion conditions and under AC-DC-AC aging conditions may provide valuable information about the potential risk of displacing cathodic sites from the interfacial areas toward the body of the coating. This accelerated electrochemical technique consists of DC polarization steps followed by AC EIS measurement steps (see Figure 2). This technique closely replicates delamination of coatings with the same mechanisms that occur under real-world conditions, providing valuable information about the durability and failure behaviour of these types of protective coating [27,28]. Furthermore, the AC-DC-AC test not only accelerates interfacial reactions but also has been documented to enhance the degradation of the polymer matrix by promoting the permeability for ionic species (such as H+, Na+, Cl−, etc.) from the electrolyte towards the mild steel surface subjected to alternating negative and positive potentials [26]. AC-DC-AC polarization cycles were applied on coatings for up to 120 days. The degradation trend of pure epoxy and graphene-embedded coatings was consequently evaluated using EIS measurements. The EIS data were analyzed with appropriate equivalent electrical circuits using the “ZView4” software to better interpret the electrochemical responses of the coatings.

2.4. Cathodic Disbondment Test

To eliminate the effect of anodic dissolution of the substrate from corrosion-induced delamination of coatings, pigmented and unpigmented coatings underwent cathodic disbondment tests. In this regard, an artificial aperture with a diameter of 3 mm was created at the center of pure epoxy and graphene-embedded epoxy coatings by mechanically removing the coating from the steel substrate. A constant potential of −1.5 V (vs. saturated calomel electrode) was applied on samples with artificial defects (d = 3 mm) using a Gamry Interface 1010E potentiostat/galvanostat (Gamry Instruments, Philadelphia, PA, USA). A standard calomel reference electrode and a graphite counter electrode were used to complete the cathodic disbanding set-up. The potential value was chosen based upon Standard Test Methods for Cathodic Disbonding of Pipeline Coatings (ASTM G8-96). The cathodic delamination of coatings was evaluated by EIS measurements after 1, 3, and 7 days of exposure time in an aqueous solution containing 3.5 wt % NaCl.

2.5. Adhesion Strength Evaluation

To better understand the key factors influencing the delamination behaviour of coatings, the interfacial adhesion strengths of both the pure epoxy (PE) and graphene-containing epoxy (GPE) coatings were measured utilizing the pull-off test using Elcometer 106 (Elcometer Ltd., Manchester, UK) pull-off adhesion tester in accordance with Standard Test Method for Pull-Off Strength of Coatings Using Portable Adhesion Testers (ASTM D4541-22). Furthermore, to provide additional confirmation of the adhesion performance, the cross-cut tape adhesion test was performed by a cross-cut tester with 2 mm cutter spacing (Gardco, Columbia, USA) in accordance with the Standard Test Methods for Rating Adhesion by Tape Test (ASTM D3359-22). To assess the ability of the coating to wet the substrate surface during application, the spreading behaviour of liquid pure epoxy and graphene-containing epoxy on mild steel substrate was evaluated by measuring the contact angle at the steel/liquid interface with a contact angle goniometer (Ossila Ltd., Sheffield, UK) at five locations on the steel substrate. Before the measurements, the surfaces of the blasted steel samples were thoroughly degreased with acetone.

3. Results and Discussion

3.1. Optimization of Graphene Platelets Concentration for Coating Formulation

A series of preliminary investigations assessed the influence of graphene platelet content (see Table 1) on the formulation stability and application quality of epoxy-based coatings. Although higher concentrations of graphene platelets are theoretically advantageous for enhancing barrier performance by creating a more tortuous diffusion pathway, practical limitations became increasingly apparent during formulation and application processes. Specifically, graphene platelet loadings exceeding 1 wt % resulted in a significant reduction in pot life. This phenomenon is attributed to the high oil absorption capacity of graphene platelets, which accelerated the gelation of the resin-curing agent system. Additionally, GPE2 and GPE4 coatings exhibited pronounced surface defects, including fisheyes, pinholes, and spherical voids (illustrated in Figure 3). These challenges rendered high-load graphene formulations unsuitable for further electrochemical evaluation. In contrast, both GPE0.5 and GPE1 coatings exhibited acceptable formulation behaviour and defect-free application. However, GPE1 was selected for detailed investigation due to its higher filler content, which provides a more representative condition for evaluating the potential electrochemical consequences of graphene platelet incorporation, particularly in the context of galvanic corrosion. The subsequent sections focus on the corrosion-induced delamination behaviour of pure epoxy and GPE1 coatings, utilizing various assessment methods.

3.2. Corrosion-Induced Delamination of the Coatings

To assess the effects of graphene platelet additions on corrosion-induced delamination of the epoxy matrix, the corrosion creepage of scratched PE and GPE1 coatings was evaluated using the salt spray test. The morphological assessment of these coatings, following a 90-day exposure period, is illustrated in Figure 4. Although rust creepage was observed surrounding the scratched regions across both coating systems, no blistering occurred in the intact areas of the GPE1 coating. These findings highlight the improved barrier characteristics of graphene-containing coatings, suggesting that incorporating graphene platelets significantly mitigated the ingress of moisture and corrosive ions into the polymer matrix. Figure 5 depicts the dispersion of graphene platelets in the epoxy matrix. It has been reported that graphene particles improve the protection performance of coatings by reducing permeation to oxygen and water [13,29,30,31]. The dispersed graphene platelets create a tortuous path that increases the diffusion length of corrosive agents, significantly reducing their rate of penetration to the underlying metal surface. Despite the positive effects of graphene platelets on the barrier performance of GPE1 coatings, these coatings experienced rust creepage along the entire length of the scribed areas. In intact coatings, the ingress of water, oxygen, and corrosive ions that support the undercoating corrosion is facilitated through diffusion pathways inherent within the coating matrix [32,33]. In other words, the ingress of corrosive agents from penetration paths controls the kinetics of corrosion at the interfacial area. However, defects in the coating, such as scratches and pinholes, accelerate the ingress of water and corrosive agents into the interfacial region, eliminating this diffusion-controlled stage in the corrosion process. From salt spray results, the presence of graphene platelets in the matrix cannot effectively reduce the ingress of corrosive agents from the defect area toward the interface of the coating–substrate, leading to providing insufficient resistance against corrosion-induced delamination.
To quantitatively evaluate the corrosion-induced delamination behaviour of PE and GPE1 coatings, corrosion propagation from an artificial defect was analyzed using EIS measurements. The variation in Bode and phase angle–frequency plots of damaged coatings over immersion time are presented in Figure 6a,b. Additionally, the variation in low-frequency impedance modulus at 0.01 Hz, used as a semi-quantitative measure of delamination of coatings [34], is illustrated in Figure 6c. From Figure 6c, the low-frequency impedance modulus of all coatings exhibited a progressive decline with increasing immersion time, indicative of ongoing delamination [22,24]. All coatings illustrated similar variation in the trends of their EIS spectra, suggesting their delamination originated from similar interfacial electrochemical mechanisms. To better interpret the corrosion behaviour of the samples, the EIS spectra were modelled using the equivalent circuits shown in Figure 6d. In this model, Rs represents the solution resistance, Rct corresponds to the charge transfer resistance, and Rcp is the resistance of the precipitated corrosion products on the surface of mild steel. CPEcp and CPEdl are also indicative of the constant phase elements of the corrosion product and electrical double layer. The phase angle–frequency plots exhibited two distinct response peaks within the low and medium frequency ranges for all coatings, correlating with corrosion activities and the protective effects of the resultant corrosion products, respectively [35]. These response peaks progressively shifted toward lower frequencies over time, illustrating the ongoing progression of corrosion-induced delamination. Additionally, in agreement with salt spray results, the incorporation of graphene platelets within the epoxy matrix was found to be ineffective in mitigating the corrosion-induced delamination process. On the contrary, as GPE1 samples indicated a greater shifting of response peaks toward lower frequency ranges (shown with dash lines), it can therefore be concluded that they actually experience slightly faster delamination. Moreover, the GPE1 coatings experienced a greater decline in their low-frequency impedance modulus, indicating an accelerated delamination rate for these coatings.
To better understand the delamination mechanism, the corrosion behaviour of mild steel substrate coupons at damaged areas of the coatings was assessed using potentiodynamic polarization tests (Figure 7). Regardless of a slight increase in anodic current density of the GPE1 samples, the cathodic and anodic slopes were identical for both samples, which suggests that the electrochemical processes occurring at the defect-containing areas followed identical mechanisms [36]. Furthermore, the corrosion potential of both samples was similar [37]. Theoretically, coupling mid-steel with an electrochemically noble material such as graphene shifts the corrosion potential toward more positive values and increases the anodic and cathodic current densities. The slight change of both samples’ corrosion potential and electrochemical mechanisms indicates a minimal risk of galvanic corrosion in steel samples coated with graphene-embedded coatings. However, the polarization plot of mild steel samples coated by graphene-containing coating showed higher anodic current densities than those of samples coated with pure epoxy, while their cathodic branches were identical. This could be attributed to the higher delamination of graphene-embedded coatings during 7 days of immersion in NaCl 3.5 wt % solution, which effectively exposed a larger area of the steel substrate to the aggressive corrosive environment.
The accelerated delamination of graphene-embedded coatings primarily attributed to enhanced galvanic corrosion of the underlying metallic substrate [22]. However, the delamination of the polymer matrix is a complex phenomenon influenced by several interrelated factors, including the degradation of the polymer itself, electrochemical reactions occurring at the coating–metal interface, and the inherent adhesion characteristics of the coating to the substrate [4]. The interfacial adhesion between the coating and the metallic substrate results from mechanical interlocking, molecular bonding, and thermodynamic adhesion, all of which can be impacted by the diffusion of water into the metal/coating interface region [38,39]. Accordingly, delamination processes can be evaluated from two distinct perspectives. On the one hand, the anodic dissolution of the metallic substrate and the associated growth of mechanically weak corrosion products beneath the coating leads to delamination [4]. On the other hand, the formation of highly alkaline products at cathodic regions can disrupt the bonds between the metallic substrate and the polymer matrix. Specifically, the production of hydroxide ions at these sites alters interfacial chemistry, leading to alkali hydrolysis of the coating at the interface [25]. Moreover, these alkaline products can disrupt the interface’s thermodynamic equilibrium, leading to delamination of the coating [4]. For the epoxy matrix, different oxygen concentrations at various regions of the coating–metal interface leads to the formation and localization of anodic and cathodic sites [33]. For graphene-embedded coatings, the electrical connection of electrochemically noble graphene platelets with mild steel in water-saturated pathways is likely to displace cathodic sites away from the immediate interfacial area towards the bulk of the coating, thereby enhancing the cathode-to-anode ratio [23]. Consequently, the galvanic effect induced by the presence of graphene platelets contributes to an increased rate of anodic dissolution of the mild steel substrate, thus accelerating delamination of the coating. The salt spray test and EIS measurements at open circuit potential provided valuable information about the delamination response of these coatings. However, the effect of anodic dissolution and weakening of adhesive bonds between the coating and metallic substrate on delamination cannot be distinguished clearly. Given that the formation of highly alkaline products on graphene platelets can accelerate the degradation of the polymer matrix (Figure 1), the degradation trend of graphene-embedded coatings under the AC-DC-AC aging test was assessed to evaluate the potential role of graphene platelets as cathodic sites.

3.3. Accelerated Aging of Coatings by AC/DC/AC Technique

The accelerated degradation of pure epoxy and graphene-embedded coatings under the AC-DC-AC test was assessed through EIS measurements. Figure 8 illustrates the low-frequency impedance spectra of developed coatings following a 120-day immersion in a 3.5 wt % NaCl aqueous solution. Variations in low-frequency impedance values are particularly indicative of the barrier properties of the polymeric matrix over the immersion period [40]. For pure epoxy coatings, the impedance modulus of the sample under accelerated aging significantly declined after 40 days, while for the sample naturally immersed in solution, the impedance modulus declined only after 80 days. This pronounced decrease in impedance modulus indicates substantial water ingress into the polymer matrix. Due to the inherent differences in electrical conductivity between water and the polymer matrix, the variation in the electrochemical response of coatings during exposure to the humid environment can reflect the water uptake process of organic coatings [41]. Notably, the total impedance values of pure epoxy coatings decreased to below 1.0 × 107 Ω cm2, a threshold recognized in the literature as indicative of potential microstructural damage within barrier coatings [36]. According to this criterion, epoxy coatings subjected to accelerated electrochemical aging exhibited irreversible microstructural damage after 40 days of exposure, whereas the naturally immersed samples retained their structural integrity for up to 80 days. Besides accelerating interfacial electrochemical reactions, high cathodic and anodic polarization potential applied to the sample during AC-DC-AC cycling facilitates rapid permeation of ions and water molecules into the coating matrix. This phenomenon contributes to the formation of pores and the nucleation/growth of defects within the epoxy matrix [26,28]. As a result, pure epoxy coatings undergoing accelerated aging demonstrated a markedly increased degradation rate.
Graphene-embedded coatings exhibited remarkable structural integrity following 120 days of immersion, outperforming pure epoxy coatings. The impedance modulus remained stable over the study duration, culminating in a final impedance modulus greater than 1.0 × 107 Ω cm2, which signifies the excellent structural integrity retention of these coatings. This could be attributed to the high barrier performance of the materials with graphene incorporation. Adding these graphene platelets decreases water uptake by blocking penetration pathways and reducing free volumes [4]. Given that the platelets applied in the present work are unfunctionalized graphene, they do not alter the cross-linking density of the epoxy [42]. Therefore, the enhanced barrier performance of the coating can be ascribed to forming a physical barrier against corrosive agents. Notably, accelerated aging with AC-DC-AC cycles negligibly influenced the integrity of the graphene-embedded coating.
To better understand the water uptake and degradation trends of the examined coatings, the evolution of the impedance spectra of coatings during immersion in a 3.5 wt % NaCl aqueous solution is compared in Figure 9. The EIS responses were further interpreted through simulations using electrochemical equivalent circuit models, as illustrated in Figure 10. Representative samples were selected after immersion periods of 1, 40, 80, and 120 days. During the initial stages of immersion, all coatings exhibited a linear Bode diagram, characterized by a slope of −1 and a phase angle of 90 degrees across the high and medium frequency ranges (Figure 9a). EIS responses of the coatings on the first day of immersion fitted very well with Model A, which characterizes the electrochemical response of an ideal intact coating with high structural integrity [43]. In this model, Rs signifies the solution resistance, Rc corresponds to the resistance of conduction paths through the coating, and CPEc is the constant phase element of the coating. Following 40 days of immersion, the EIS spectra of the aged pure epoxy coatings revealed a significant reduction in impedance values (Figure 9b). Notably, the emergence of a distinct response peak in the low-frequency region of the phase diagrams signifies the onset of corrosion reactions at the interface between the mild steel substrate and the pure epoxy coating [35]. Moreover, after 40 days of accelerated aging conditions, the linear behaviour represented by the −1 slope in the Bode plots almost disappeared, indicating complete delamination of the pure epoxy coating. The EIS responses of aged pure epoxy coatings across the immersion periods ranging from 1 to 40 days were more accurately represented by the equivalent circuit of Model B, wherein Rct and CPEdl represent the charge transfer resistance and constant phase element of the electrical double layer, respectively.
Upon exposing the polymer matrix to a humid environment, water molecules diffuse into the matrix through free volumes and microcavities [37,44]. The ingress of water into the interfacial area leads to the appearance of the second constant responses in EIS spectra [36]. Interaction of water molecules with polar groups of the polymer matrix at the interfacial area weakens the bonds between the coating and metallic substrate. Besides representing water infiltration into the interfacial region, the second constant response may indicate the interfacial electrochemical reactions [26]. In this regard, the appearance of a distinct response peak in the low-frequency ranges of phase diagrams distinctively represents the initiation of corrosion reactions at the interfacial area, as observed for aged pure epoxy coatings after 40 days [36]. For aged pure epoxy coatings evaluated after 80 and 120 days, the EIS spectra were modelled using an equivalent circuit designated as Model C. In this equivalent circuit, Rcp and CPEcp represent the resistance and constant phase element of the corrosion products that formed on the steel substrate beneath the fully delaminated coating. The interfacial corrosion responses were observed after 120 days for non-aged pure epoxy coatings. EIS responses of non-aged epoxy coating after 40, 80, and 120 days of immersion were fitted with Model B.
In contrast to pure epoxy coatings, both aged and non-aged graphene-embedded coatings (GPE1) preserved their structural integrity up to 120 days of immersion. Notably, no interfacial corrosion was detected in the EIS responses of these coatings. The EIS modulus slightly declined, attributed to water molecule permeation into the coating matrix. The infiltration of water into the coatings is evidenced by the emergence of a plateau in the low-frequency range of the Bode plots and a corresponding shift in the phase angle diagrams toward higher frequencies (Figure 6b). Accordingly, after the initial stages of immersion, the EIS responses of GPE1 coatings were fitted with Model B. The delamination progression can be assessed by monitoring the breakdown frequency, which is defined as the frequency at which the phase angle reaches 45 degrees. An increase in breakdown frequency indicates a greater delaminated area at the metal/coating interface [45]. Despite an initial decrease in impedance modulus and increased breakdown frequency for graphene-containing coatings, these coatings preserved their integrity throughout the 120-day immersion duration. In this case, the ultimate low-frequency impedance moduli of these coatings were about 3.0 × 109 Ω cm2 and 1.0 × 108 Ω cm2, indicating that a high barrier performance was maintained during the testing period.
When comparing EIS responses of the aged and non-aged samples, it is shown that the AC-DC-AC cycles effectively compromised the integrity of the pure epoxy coatings. At the same time, it did not change the degradation trend observed for the graphene-embedded coatings. The AC-DC-AC technique theoretically compromises the integrity of coatings via several mechanisms, including cathodic disbondment, anodic dissolution of the substrate, and the electromigration of ions into the coating during polarization cycles [26,28,36]. Besides electrochemical reactions occurring at the interface of the coating–metallic substrate, the presence of highly conductive and electrochemically noble graphene platelets in the coatings may accelerate the degradation of the coating under AC-DC-AC cycles due to possible migration of cathodic reactions from the interfacial region to the bulk of the coating, where graphene platelets in the water-saturated coating can act as active cathodic sites. As a result, the formation of alkaline byproducts on graphene platelets, coupled with the migration of cations toward negatively charged graphene particles, may undermine adhesive bonds at the interphase between these particles and the polymer matrix. However, according to EIS responses, GPE1 coatings preserved their structural integrity under destructive aging cycles despite water infiltration into the coating. This could be attributed to the considerable insulating effect of the epoxy matrix, which significantly decreases the risk of displacing cathodic reactions from the interfacial area toward the body of the coating. The insulating effect of the polymer matrix on graphene platelets has also been observed by Glover et al. [23], where they found graphene particles dispersed in a polyvinyl butyral matrix act simply as an electrochemically inert filler. Accordingly, the galvanic corrosion risk of steel substrates cannot be considered a significant factor contributing to the accelerated delamination of graphene-embedded coatings. In addition to the kinetics of electrochemical reactions at the interface, the adhesion strength of the coating to the substrate is a pivotal parameter influencing the delamination rate. The interfacial adhesion strength of pure epoxy and graphene-containing coatings was evaluated through cathodic disbondment measurements and pull-off tests in the present work.

3.4. Cathodic Disbondment

To eliminate the effects of the anodic dissolution of the substrate from the delamination process, the prepared samples underwent the cathodic disbondment test. The EIS spectra of the samples exposed to this test and the observed variations in their low-frequency impedance are presented in Figure 11. The EIS spectra were fitted very well with an equivalent electrical circuit represented in Figure 6d. However, under cathodic polarization, the EIS responses in the medium-frequency range were weaker, indicating a significant reduction in the precipitation of corrosion products on the substrate surface. Compared to samples immersed in 3.5 wt % NaCl aqueous solution at OCP (see Figure 6), pure epoxy and GPE1 coatings experienced a higher delamination rate. Remarkably, in contrast to findings from previous studies [46], graphene-embedded coatings experienced higher delamination under the cathodic disbondment test. As presented in Figure 11, GPE1 coatings experienced substantial reductions in the impedance modulus, along with a notable shift in phase angle response peaks towards lower frequency ranges. The cathodic polarization applied during the experiment inhibits the anodic dissolution of mild steel. Therefore, the higher delamination rate of graphene-containing coatings than that of pure epoxy, under cathodic polarization conditions, confirms that anodic dissolution of the substrate has a minimal impact on the delamination rate of the coatings. This increased delamination rate in graphene-embedded coatings may be attributed to their comparatively lower interfacial adhesion strength than pure epoxy coatings. Pull-off adhesion testing was conducted to quantitatively assess the adhesion properties of pure and graphene-embedded epoxy coatings to elucidate the effects of graphene platelets on the interfacial adhesion strength of the epoxy matrix adhered to the mild steel substrates.

3.5. Interfacial Adhesion Strength

Figure 12 presents the measured adhesion strength of pure epoxy and graphene-containing coatings assessed using the ASTM pull-off and cross-cut test procedures. Graphene-embedded coatings provided lower adhesion strength than those of pure epoxy. In the cross-cut test, the pure epoxy coating achieved a classification of 5B, indicating no coating removal and excellent adhesion, whereas the GPE1 coating was classified as 3B, corresponding to partial removal of the coating along the grid cuts. According to the pull-off test, the failure of response of the epoxy coatings was adhesive/cohesive in nature, while that of the graphene-containing coatings indicated adhesive failure. This observation suggests that incorporating graphene platelets improved the matrix’s mechanical integrity but concurrently reduced the interfacial adhesion between the coating and the steel substrate. In pigmented coatings, failure may initiate either at the interface between the coating and the metallic substrate (adhesive failure) or at the interphase between the pigment and the polymer matrix (cohesive failure). The adhesive failure of graphene-containing coatings confirmed that the internal strength of the coatings cannot be considered as the weak point of the coating, suggesting good interaction of graphene platelets with the polymer matrix. This interaction was also confirmed by providing high integrity against the accelerated aging process. Therefore, the failure of graphene-pigmented coatings is predominantly due to the failure of interfacial bonds rather than to those between the matrix and the pigment, which suggests that the incorporation of graphene platelets into the epoxy matrix can reduce the interfacial bonding of the coating with the steel substrate.
The coating/metal interfacial adhesion can result from molecular bonding and mechanical interlocking [4]. Polar groups of epoxy coatings (epoxide groups or oxirane rings) act as sites for strong electromagnetic bonding attractions, including hydrogen bonds, between epoxy molecules and metal surfaces [25]. Besides molecular bonding, the mechanical interlocking of the coating and substrate influences the adhesion strength. Incorporating un-functionalized graphene platelets into an epoxy matrix negligibly changes the molecular bonding. However, these platelets can affect the wettability of the coatings, which in turn influences the capability of the coating to fill all irregularities of the metal surface. The spreading behaviour of pure epoxy and graphene-containing epoxy was evaluated by measuring the variation in their droplet contact angle on the examined mild steel surfaces (as shown in Figure 13). It is apparent that the pure epoxy showed a faster decrease in contact angle, suggesting it wets the steel surface more effectively than that of graphene-embedded epoxy [47]. Accordingly, pure epoxy’s higher interfacial adhesion strength compared to graphene-embedded epoxy can be attributed to the fact that pure epoxy can create stronger mechanical interlocking with steel surfaces due to its better spreading on steel surfaces. Given the critical role of interfacial adhesion in the corrosion-induced delamination of coatings, the diminished interfacial strength of graphene-embedded coatings relative to pure epoxy is likely to lead to reduced anti-corrosion performance, as evidenced by measurements of corrosion-induced delamination and cathodic disbondment tests.

4. Conclusions

Graphene platelets are shown to increase the corrosion-induced delamination rate of the epoxy matrix from the mild steel substrate by about 20 percent. The evaluation of delamination in graphene-embedded epoxy coatings, which incorporate one weight percent of graphene platelets, was assessed from two aspects: interfacial electrochemical behaviour and interfacial adhesion strength of coatings. The possible risk of incorporating graphene platelets into the epoxy matrix in accelerating the corrosion of mild steel through a galvanic corrosion mechanism was evaluated using the AC-DC-AC accelerated aging technique. Under the AC-DC-AC aging test, the pure epoxy coatings lost their structural integrity after 40 days. However, graphene-containing coatings maintained their structural integrity under the destructive polarization cycles of the AC-DC-AC test for up to 120 days.This indicates a minimal likelihood of displacing cathodic reactions from the interfacial area onto the dispersed graphene platelets within the epoxy matrix. Delamination rates are dominantly influenced by the interfacial adhesion strength of the graphene-embedded coating on the substrate rather than the anodic dissolution of mild steel. Using cathodic disbondment and pull-off adhesion tests, it was shown that incorporating graphene platelets into the coating decreases the interfacial adhesion of the epoxy matrix to the mild steel substrate by about 15%. This was attributed to the negative effect of graphene platelets on the dispersing kinetics of the epoxy matrix on the steel surface, which accordingly decreases the ability of the matrix to establish strong mechanical interlocking.

Author Contributions

Conceptualization, K.S.-B. and K.P.P.; Software, K.S.-B.; Formal analysis, K.S.-B.; Investigation, K.S.-B.; Resources, K.P.P.; Data curation, K.S.-B.; Writing—original draft, K.S.-B.; Writing—review & editing, K.P.P.; Supervision, K.P.P.; Project administration, K.P.P.; Funding acquisition, K.S.-B. and K.P.P. All authors have read and agreed to the published version of the manuscript.

Funding

The authors would like to acknowledge funding provision through the Ocean Frontier Institute (OFI) via their Ocean Graduate Excellence Network (award no. OG-202113), Mitacs through award no. IT17056, and Graphite Innovation & Technologies, Incorporated.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data available on request due to restrictions.

Acknowledgments

The authors would like to thank the GIT Coatings, Ocean Frontier Institute (led by Dalhousie University), and Mitacs for the provision of funding for Kazem Sabet-Bokati through the Ocean Graduate Excellence Network (OGEN) program. Additionally, we extend our gratitude to the Killam Trusts and Dalhousie University for awarding the Killam Predoctoral Scholarship and the Abdul Majid Bader Graduate Scholarship to Kazem Sabet-Bokati.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Schematic illustration of the potential role of graphene platelets as cathodic sites in a water-saturated epoxy matrix.
Figure 1. Schematic illustration of the potential role of graphene platelets as cathodic sites in a water-saturated epoxy matrix.
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Figure 2. Schematic illustration of the experimental steps undertaken for the AC-DC-AC analysis.
Figure 2. Schematic illustration of the experimental steps undertaken for the AC-DC-AC analysis.
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Figure 3. Confocal laser scanning microscopy images of (a) GPE0.5, (b) GPE1, (c) GPE2, and (d) GPE4 coatings, captured using a VK-X1100 (Keyence Corp.) in confocal imaging mode.
Figure 3. Confocal laser scanning microscopy images of (a) GPE0.5, (b) GPE1, (c) GPE2, and (d) GPE4 coatings, captured using a VK-X1100 (Keyence Corp.) in confocal imaging mode.
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Figure 4. Topography of coatings (a1,b1) before and (a2,b2) after subjecting to salt spray test (following ASTM B117-09) for 90 days: (a) pure epoxy (PE) and (b) graphene-embedded epoxy (GPE1).
Figure 4. Topography of coatings (a1,b1) before and (a2,b2) after subjecting to salt spray test (following ASTM B117-09) for 90 days: (a) pure epoxy (PE) and (b) graphene-embedded epoxy (GPE1).
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Figure 5. Distribution of graphene platelets in GPE1 coating observed using a laser scanning optical microscope (VK-X1100, Keyence Corp.) in confocal imaging mode.
Figure 5. Distribution of graphene platelets in GPE1 coating observed using a laser scanning optical microscope (VK-X1100, Keyence Corp.) in confocal imaging mode.
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Figure 6. Variation in impedance modulus of (a) pure epoxy (PE) and (b) graphene-embedded epoxy (GPE1) coatings with an artificial cavity (d = 3 mm) for up to 7 days immersion in 3.5 wt % NaCl solution at 25 °C. (c) Evolution of the impedance modulus (|Z|0.01 Hz) of coatings during 7 days of immersion. (d) Equivalent electrochemical circuit model used for simulation of EIS responses.
Figure 6. Variation in impedance modulus of (a) pure epoxy (PE) and (b) graphene-embedded epoxy (GPE1) coatings with an artificial cavity (d = 3 mm) for up to 7 days immersion in 3.5 wt % NaCl solution at 25 °C. (c) Evolution of the impedance modulus (|Z|0.01 Hz) of coatings during 7 days of immersion. (d) Equivalent electrochemical circuit model used for simulation of EIS responses.
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Figure 7. Representative potentiodynamic polarization curves for the damaged pure epoxy and graphene-embedded epoxy (GPE1) coatings after 7 days immersion in flowing 3.5 wt % NaCl aqueous solution.
Figure 7. Representative potentiodynamic polarization curves for the damaged pure epoxy and graphene-embedded epoxy (GPE1) coatings after 7 days immersion in flowing 3.5 wt % NaCl aqueous solution.
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Figure 8. Variation in impedance modulus of pure epoxy (PE), aged pure epoxy (APE), graphene-embedded epoxy (GPE1), and aged graphene-containing coating (AGPE1) coatings within 120 days immersion in 3.5 wt % NaCl solution at 25 °C (aging process performed by cyclic AC-DC-AC technique).
Figure 8. Variation in impedance modulus of pure epoxy (PE), aged pure epoxy (APE), graphene-embedded epoxy (GPE1), and aged graphene-containing coating (AGPE1) coatings within 120 days immersion in 3.5 wt % NaCl solution at 25 °C (aging process performed by cyclic AC-DC-AC technique).
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Figure 9. Variation in Bode modulus (a1,b1,c1,d1), and Bode phase angle (a2,b2,c2,d2) diagrams of pure epoxy (PE), aged pure epoxy (APE), graphene-embedded epoxy (GPE1), and aged graphene-containing coating (AGPE1) after 1 (a), 40 (b), 80 (c), and 120 (d) days immersion in 3.5 wt % NaCl solution at 25 °C (aging process performed by cyclic AC-DC-AC technique).
Figure 9. Variation in Bode modulus (a1,b1,c1,d1), and Bode phase angle (a2,b2,c2,d2) diagrams of pure epoxy (PE), aged pure epoxy (APE), graphene-embedded epoxy (GPE1), and aged graphene-containing coating (AGPE1) after 1 (a), 40 (b), 80 (c), and 120 (d) days immersion in 3.5 wt % NaCl solution at 25 °C (aging process performed by cyclic AC-DC-AC technique).
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Figure 10. Schematic representation of water uptake and delamination trend of pure epoxy (PE), aged pure epoxy (APE), graphene-embedded epoxy (GPE1), and aged graphene-containing coating (AGPE1) during AC-DC-AC test (aging process performed by cyclic AC-DC-AC technique). Equivalent circuits used for numerical simulation of EIS responses.
Figure 10. Schematic representation of water uptake and delamination trend of pure epoxy (PE), aged pure epoxy (APE), graphene-embedded epoxy (GPE1), and aged graphene-containing coating (AGPE1) during AC-DC-AC test (aging process performed by cyclic AC-DC-AC technique). Equivalent circuits used for numerical simulation of EIS responses.
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Figure 11. The variation in impedance modulus of (a) pure epoxy (PE) and (b) graphene-embedded epoxy (GPE1) coatings with an artificial cavity (d = 3 mm) for up to 7 days exposing to cathodic disbondment test immersion under cathodic potential of −1.5 V (SCE). (c) Evolution of the impedance modulus (|Z|0.01 Hz) of coatings for 7 days exposing to cathodic disbondment test immersion under cathodic potential of −1.5 V (SCE).
Figure 11. The variation in impedance modulus of (a) pure epoxy (PE) and (b) graphene-embedded epoxy (GPE1) coatings with an artificial cavity (d = 3 mm) for up to 7 days exposing to cathodic disbondment test immersion under cathodic potential of −1.5 V (SCE). (c) Evolution of the impedance modulus (|Z|0.01 Hz) of coatings for 7 days exposing to cathodic disbondment test immersion under cathodic potential of −1.5 V (SCE).
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Figure 12. (a) Pull-off adhesion strength of PE and GPE1 coatings based on ASTM D4541; (b) representative images of failure surfaces after the pull-off test, illustrating adhesive and cohesive failure modes for (b1) PE and (b2) GPE1 coatings; (c) cross-cut adhesion results (ASTM D3359) for (c1) PE and (c2) GPE1 coatings.
Figure 12. (a) Pull-off adhesion strength of PE and GPE1 coatings based on ASTM D4541; (b) representative images of failure surfaces after the pull-off test, illustrating adhesive and cohesive failure modes for (b1) PE and (b2) GPE1 coatings; (c) cross-cut adhesion results (ASTM D3359) for (c1) PE and (c2) GPE1 coatings.
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Figure 13. Variation in contact angle of (a) pure epoxy and (b) graphene-embedded epoxy (GPE1) droplets on mild steel surface after (a1,b1) 2 s and (a2,b2) 5 s.
Figure 13. Variation in contact angle of (a) pure epoxy and (b) graphene-embedded epoxy (GPE1) droplets on mild steel surface after (a1,b1) 2 s and (a2,b2) 5 s.
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Table 1. The mixing ratio of pure epoxy and graphene-containing coatings.
Table 1. The mixing ratio of pure epoxy and graphene-containing coatings.
SampleResin + PigmentCuring Agent
Epoxy (g)Graphene Platelets (g)(g)
PE100074.5
GPE0.51000.574.5
GPE1100174.5
GPE2100274.5
GPE4100474.5
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Sabet-Bokati, K.; Plucknett, K.P. Application of AC-DC-AC Accelerated Aging to Assess the Galvanic Corrosion Risk of Mild Steel Coated with Graphene-Embedded Epoxy Coatings. Coatings 2025, 15, 501. https://doi.org/10.3390/coatings15050501

AMA Style

Sabet-Bokati K, Plucknett KP. Application of AC-DC-AC Accelerated Aging to Assess the Galvanic Corrosion Risk of Mild Steel Coated with Graphene-Embedded Epoxy Coatings. Coatings. 2025; 15(5):501. https://doi.org/10.3390/coatings15050501

Chicago/Turabian Style

Sabet-Bokati, Kazem, and Kevin Paul Plucknett. 2025. "Application of AC-DC-AC Accelerated Aging to Assess the Galvanic Corrosion Risk of Mild Steel Coated with Graphene-Embedded Epoxy Coatings" Coatings 15, no. 5: 501. https://doi.org/10.3390/coatings15050501

APA Style

Sabet-Bokati, K., & Plucknett, K. P. (2025). Application of AC-DC-AC Accelerated Aging to Assess the Galvanic Corrosion Risk of Mild Steel Coated with Graphene-Embedded Epoxy Coatings. Coatings, 15(5), 501. https://doi.org/10.3390/coatings15050501

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