INTRODUCTION
In an attempt to mitigate environmental impacts, there is a considerable drive towards electric vehicles (EVs), which require the adoption of rechargeable batteries. However, this effort brings the risk of overheating and potential fire risks. Automakers are heavily investing in developing methods to extend battery lifespan, focusing on thermal management. Typically, the cooling system for battery packs utilizes aluminum (Al) alloys due to their favorable strength-to-weight ratio, potentially improving fuel efficiency [1]. Moreover, Al alloys allow high thermal conductivity, facilitating fast heat transfer from the battery to the coolant. In addition, they offer excellent corrosion resistance, forming a protective layer on their surface.
The eutectic AlF3-KF mixture, known as the Nocolok®, is primarily employed in the automotive cooling system, providing a molten flux at brazing temperature to protect the Al surface [2]. A literature survey indicates that the kinetic parameters of the brazing process, such as brazing temperature, time, and the resulting properties of the base metal, have been of significant concern. It was also revealed that at brazing temperature, the elements in molten metal diffuse rapidly into the base metal, which would hurt bonding strength because of insufficient molten metal at the brazed joint.
Ethylene glycol-based coolant (EG coolant) is widely adopted owing to its low cost, high chemical stability, excellent freeze resistance, and the ability to prevent overheating [3]. However, in actual operation conditions, various side reactions occur. The passivation films made up of aluminum oxide and Al-alcohol passivation were formed in an EG cooling solution, which may affect the corrosion resistance of Al alloy [4]. Besides, EG coolant may experience oxidation to produce glycolic and other oxidizing substances, accelerating the corrosion of Al alloy in the cooling system [5]. The base coolant solution is a clear fluid, but particular colors were often applied to distinguish types of antifreeze, promote the use of a specific OEM brand, and, importantly, avoid drinking accidents in service facilities. However, during long-term operation conditions, color fading occurs occasionally and is considered a severe defect even though it does not cause any issues in the safe operation of automobile vehicles.
Despite a growing demand for effective cooling systems, the correlation between EG coolant compositions and color changes has rarely been explored. Previous studies have neglected the investigation of coolant color changes during vehicle operations. Triphenylmethane (TPM) dye is preferred for various advantages, such as water solubility, pH indication, leak detection, product identification, and temperature regulation [6]. Nevertheless, discoloration sometimes raised significant issues by compromising the coolant's identification and reliability. This study examines the fading of blue-colored TPM coolant, exploring how EG coolant and Al-flux influence this process. Through various simulated conditions, UV-Vis spectral curves of blue-coolant solutions were obtained. The coolant solutions were subjected to high temperatures for prolonged periods in the presence of Al and flux. Subsequently, residual compounds were assessed using various analytical techniques, including scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and electrochemical impedance spectroscopy (EIS). The objectives were to understand Al corrosion in the coolant system, as indicated by changes in color. The findings reveal the composition alterations in an Al-flux cooling system, offering effective visual indicators that can be readily assessed without additional instrumental analysis.
EXPERIMENTAL
Materials
Coolant, flux, and triphenylmethane dye were provided by the KD R&D Center (Korea). Aluminum powder (99.0%, 200 mesh) was purchased from Samchun Chemicals (Korea), and potassium fluoride was purchased from Daejung Chemicals (Korea). Potassium iodide was purchased from Junsei (Japan), and aluminum (III) fluoride was purchased from TCI Korea. Aluminum hydroxide (76.5%) was purchased from Alfa-Aesar (Korea), and sodium nitrite and ethylene glycol were obtained from Sigma-Aldrich Korea. All commercially available raw materials were used without further purification.
Characterization
The 1H NMR spectra were recorded on an Agilent operating at 600 MHz using D2O as the solvent. X-ray diffraction (XRD) was performed in the 2θ range of 10–80° using Cu Kα radiation (1.54 Å) on an pert Pro MPD. UV-Vis absorption spectra were obtained using a UV-1800 UV-Vis spectrophotometer (Shimadzu). Scanning Electron Microscopy (SEM) images and energy-dispersive X-ray spectroscopy (EDS) mapping were obtained on a GEMINI 500. X-ray photoelectron spectroscopy (XPS) was conducted on an AXIS Supra, and XPS spectra were referenced by the position of the C1s at 284.6 eV. Electrochemical impedance spectroscopy (EIS) was performed on the electrode materials using the three-electrode system utilizing an SP-150 potentiostat (Biologic). Nyquist impedance plots were generated by plotting Re(Z)/Ohm against Im(Z)/Ohm.
Methods
Spectroscopic analysis
Various salts and additives were added to the coolant, and it was treated with stirring at 60°C for 24 h. Each substance was added at 0.5 wt% of the coolant. After treatment, the coolant was centrifuged to separate undissolved solids, and the UV-Vis absorption spectra of clear liquid solution were obtained.
Surface characterization
Al powder in coolant with and without various salts and additives was treated at 60°C for 24 h with stirring. Al powder and each substance were added at 0.5 wt% of the coolant. After treatment, Al powder was filtered with a membrane filter and dried for 24 h in an 80°C oven. The obtained Al powder was analyzed using SEM-EDS, XRD, and XPS measurements.
Electrochemical measurements
The working electrode (Al foil) was immersed in the electrolyte (coolant) with additives at 60°C for 24 h with stirring. Each additive was added at 0.5 wt% of the coolant. Then, the working electrode was dried in a vacuum oven, and EIS was measured using an exposed work area of 1×1 cm. All electrolytes were maintained at room temperature during measurements. EG coolant was used as an electrolyte, and Al foil, platinum (Pt), and Ag/AgCl were used as the working, counter, and reference electrodes, respectively. EIS was measured in the frequency range from 5 MHz to 100 mHz.
RESULTS AND DISCUSSION
Discoloration of blue-coolants
To identify the issue related to the discoloration of TPM coolant, UV-Vis absorption spectra were recorded under various treatment conditions (Fig. 1). After treatment at 60°C for 24 h, the UV-Vis spectra displayed similar absorption patterns with a maximum absorption (lmax) at 631 nm. The initial spectra were adjusted with a maximum absorbance (Amax) of 1.0. After treating the TPM coolant with Al, flux, and a combined Al and flux, Amax values were decreased to 0.99, 0.98, and 0.59, respectively (Fig. 1a). Only small absorbance changes were obtained when separately treated with Al and flux. In comparison, the severe reduction in absorbance yielded when adding combined Al and flux, which illustrates the synergistic effect between Al and flux, inducing the discoloration of TPM coolants.
To investigate the effect of components in flux (AlF3-KF), the absorbance of TPM coolant was examined in the presence of potassium salt and fluoride salt (Fig. 1b). Adding potassium acetate (AcOK), tetrabutylammonium fluoride (TBAF), and an Al/AcOK mixture did not cause significant discoloration and led to slight pH changes from 7.02 to 7.21, 6.81 to 7.07, and 7.51 to 7.93, respectively. There was only a slight reduction in absorbance, likely due to limited interaction between metals and coolants. However, when Al/TBAF was added, there was a notable decrease in absorbance, accompanied by visible color changes and an increase in pH (6.34 to 8.29). This observation suggests that the interaction between aluminum and fluoride in flux may be the primary reason for pH elevation and TPM discoloration [7]. Fluoride ions strongly attract aluminum ions, creating stable complexes. This interaction can break down the passive oxide layers on aluminum surfaces. When these protective layers are damaged, the exposed aluminum is more prone to corrosion, subsequently increasing the pH due to the hydrogen reduction reaction. It was hypothesized that flux decomposes into AlF3 and KF, leading to an increase in the pH of the coolants.
Meanwhile, the presence of KF, AlF3, and an Al/AlF3 mixture did not significantly decrease absorbance (Fig. 1c). However, adding an Al/KF mixture drastically reduced absorbance with an apparent color change from blue to pale blue. The solution pH was increased from 7.08 to 11.16, suggesting that KF, when combined with Al, was critical in altering the color and pH of TPM coolants. Subsequent Al(OH)3/KF blend experiments confirmed this hypothesis (Fig. 1d). The blend induced a significantly lower absorbance, indicating a strong interaction between Al(OH)3 and KF, resulting in the formation of alkaline species that cause coolant color changes.
Upon observing the pH rise by introducing Al and flux into the coolant, a plausible mechanism was suggested to elucidate the pH increase. As shown in Scheme 1, when flux was introduced into the coolant, it dissociated into potassium fluoride (KF) and aluminum fluoride (AlF3). Fluoride ions, a flux component, break down the protective Al2O3 passivation layer, leading to aluminum corrosion. The resulting Al3+ and OHions from the EG coolant react to form solid aluminum hydroxide (Al(OH)3) [8]. The hydrogen reduction reaction during corrosion is the main factor contributing to the rise in pH. In addition, potassium ions can react with existing Al(OH)3, forming potassium hydroxide (KOH), which further increases the pH.
Scheme 1. Corrosion of Al and KOH generation mechanism for increasing pH.
NMR titration of triphenylmethane dye with Al powder
To understand the causes of discoloration in TPM dye, a 1H-NMR analysis was performed in D2O at 60°C (Fig. 2). Al powder (200 mg) was added to the TPM solution, and the 1H-NMR spectral peaks were recorded. Compared to the pure dye (δ = 4.68, 3.53, and 1.11 ppm), the 3-hour analysis showed an up-field shift to 4.66, 3.50, and 1.10 ppm. After 4 hours, these peaks diminished, indicating a chemical transformation within the solution over time. Notably, the aromatic region of the spectra remained stable for 2 hours, suggesting that the aromatic moieties of the dye are resistant to Al or Al(OH)3 during the period. However, significant changes were observed after 3 hours. Compared to the pristine dye (δ = 7.90, 7.57–7.49, 7.21, 7.12, 7.01, 6.91, 6.72, 6.35 ppm), the spectra of the Al-blended TPM dye solution showed merged peaks between 7.80 and 6.40 ppm.
Extended heating revealed a potential interaction between Al and the TPM dye. Prolonged exposure to heat led to the transformation of Al into Al(OH)3, causing dye discoloration. Hydroxide ions (OH–) were identified as the source of dye discoloration through nucleophilic attack [11,12], but the corresponding proton resonance was not detected individually during NMR analysis. This absence could be due to overlapping aliphatic peaks when using D2O as the solvent.
Dye discoloration mechanism
A possible mechanism concerning the TPM discoloration upon heating with Al in D2O was proposed in Scheme 2. Al powder in water produces Al(OH)3. Due to the amphoteric nature of Al(OH)3, hydroxyl ions are generated, causing a nucleophilic addition at the electrophilic site of the quinonoid group. As previously reported, OH-functionalized carbon in TPM led to carbinol formation [9], potentially affecting the absorption color of the dye.
Scheme 2. Proposed discoloration mechanism of TPM dye upon heating with Al.
Surface characterization
As evidenced by NMR analysis of the dye discoloration, the corrosion behaviors of the Al surface were observed through X-ray photoelectron spectroscopic (XPS) measurements. XPS spectra were recorded before and after Al, flux, and Al/flux treatment with TPM coolant (Fig. 3). In Fig. 3a, untreated Al exhibited the binding energy (BE) of 531 eV, indicating the composition as Al2O3. Upon treating the Al with TPM coolant at 60°C for 24 h, the peak shifted to the BE of 531 eV, suggesting that corrosion of Al did not occur (Fig. 3b). However, upon treating the Al/flux with TPM coolant, the BE of 532.1 eV was primarily observed (Fig. 3c). The shift to higher energy indicates the formation of Al(OH)3 and the corrosion of the Al surface. To confirm the formation of Al(OH)3 during the reaction of Al/flux with TPM coolant, the separately performed reaction between Al, Al/flux, and water was inspected for XPS analysis (Fig. 3d,e). Investigation of Al in water exposed the BE = 532.8 eV corresponding to the aluminum oxyhydroxide (AlOOH), revealing the Al in contact with water produces the AlOOH intermediate. Subsequently, it forms Al(OH)3, as observed in Fig. 3d (BE of Al(OH)3 = 532.1 eV). Obviously, –OOH initially forms on the Al surface, which then converts it to Al(OH)3 under continuous heating. In contrast, Al/flux in water allows direct Al(OH)3 formation, confirmed by the BE (532.1 eV) observed in Fig. 3e, supporting that the Al corrosion is accelerated when treated together with flux.
To further validate the Al corrosion, scanning electron microscope (SEM) images and corresponding energy dispersive X-ray spectroscopic (EDS) mapping were obtained after the treatment at 60°C after 24 h under various conditions (Fig. 4). No visible difference was observed in the SEM images, but significant differences were revealed by EDS analysis. After the treatment, the Al content in coolants decreased from 71 at% to 38 at% (Fig. 4b). In contrast to the Al, oxygen (O) content (observed about 2 at% before the treatment) was increased after the treatment (35 at%), showing the Al corrosion facilitated by oxidation. Similarly, the flux-treated coolants exhibited lower Al content (from 13 at% to 3 at%) and an increasing amount of O from 2 at% to 9 at%. Regarding potassium (K) and fluorine (F), it was around 15 at% and 55 at% before the treatment. After the treatment, K and F were extensively reduced to 2 at% and 5 at%, implying that the K and F in flux have contributed to the Al corrosion (Fig. 4c,d). In addition, a reduction in the amounts of Al (19 at%), F (10 at%), and K (2 at%) was observed as oxygen content increased to 37 at%, which contrasts with the initial levels of untreated Al (22 at%), F (16 at%), K (4 at%), and oxygen (2 at%). These notable alterations reflect the reactions involving aluminum and flux suggested in Scheme 1, resulting in KOH ion formation. Evidently, generating KOH has been a crucial aspect of the Al corrosion process.
Additive effect
The effect of EG, an essential component of coolant compositions, on dye discoloration was investigated (Fig. 5a). The UV-Vis spectra exhibited decreased absorbance in the Al/flux > Al/flux/EG orders compared to the pristine TPM aqueous solution. The higher intensity observed in Al/flux/EG compared to Al/flux can be attributed to the passivation of EG on the aluminum surface [10,11], which consequently delays the discoloration of the coolant. Furthermore, with the addition of Al/flux/EG, the pH shifted from 7.11 to 8.49, which is a smaller change than the Al/flux case (7.10 to 9.62). The reduced pH rise also indicates that EG passivates the aluminum surface, leading to less corrosion.
To explore the potential for delaying discoloration, sodium nitrite (NaNO2), a corrosion inhibitor, and potassium iodide (KI), known for slowing the discoloration of TPM dye, were chosen as additives. These were selected because of their solubility, minimal pH effect, and ability to maintain the coolant's color, aligning with the coolant system requirements. Halide salts like KI can cause a delay in discoloration due to the competitive reaction between OH− and halide ions for the carbocation portion, which is involved in the nucleophilic substitution reaction [12]. In the case of NaNO2, it is used as a corrosion inhibitor to delay the corrosion of aluminum. Nitrite (NO2−) is transformed into nitrous oxide (N2O), while oxygen ions (O2−) bond with aluminum ions (Al3+) to produce aluminum oxide (Al2O3). As a result, NaNO2 decreases the generation of OH−, leading to a delay in discoloration [13,14]. Absorbance values were measured (Fig. 5b). Adding Al/flux/KI induced a higher absorbance and a comparable pH value than Al/flux. The pH change was 6.79 to 8.52 and 6.82 to 8.65, respectively. KI disturbs the nucleophilic substitution reaction of TPM dye, leading to a delay in discoloration as shown in absorbance, but it does not impact the corrosion of aluminum, resulting in no significant change in pH.
Furthermore, when Al/flux/NaNO2 is added, higher absorbance and lower pH than Al/flux were observed. By adding Al/flux/NaNO2, pH was changed from 6.80 to 7.28. NaNO2 inhibits the corrosion of aluminum, reducing the generation of OH–, which causes the discoloration of TPM dye. As a result, a slight increase in pH and delayed discoloration were observed.
To further understand the corrosion inhibition of NaNO2, X-ray diffraction spectroscopy (XRD) and electrochemical impedance spectroscopy (EIS) were recorded (Fig. 5c,d). After treating Al/flux in coolant, a new peak at 2θ=18°, corresponding to Al(OH)3, appeared in the XRD spectra. Typically, the (001) XRD peak at 2θ=18° is recognized as the characteristic intense peak of Al(OH)3. However, when NaNO2 was added to Al/flux, the peak Al(OH)3 was decreased, indicating that the Al corrosion was prevented.
Furthermore, the EIS measurements were conducted to investigate the electrochemical characteristics. Fig. 5d shows the Nyquist plots for the additives studied. Rs indicates the solution resistance between the reference electrode and the aluminum foil, while Rct represents the charge transfer resistance of the aluminum foil. When comparing the EIS spectra in coolant and water, Al in the coolant showed higher Rct (104.24 Ω) and Rs (71.94 Ω) values than in water, which were 50.17 Ω and 67.89 Ω, respectively, suggesting that the coolant inhibits aluminum corrosion. The higher Rct in the coolant is due to the passivation of EG on the aluminum surface. The slight decrease in Rs of the coolant may be due to the reduction of EG, which forms a passivation layer on the aluminum. When flux was added to the coolant, Rct and Rs dropped to 34.83 Ω and 35.97 Ω, indicating that flux promotes aluminum corrosion [15]. As the passivation layer degraded during the reaction between aluminum and flux, Rct decreased. The reduction in Rs occurred during severe corrosion, likely due to the formation of salts such as aluminum hydroxide (Al(OH)3) and potassium hydroxide (KOH). In a previous scenario, NaNO2 reduced aluminum corrosion, with an Rct of 50.17 Ω and Rs of 68.98 Ω. With NaNO2 acting as a corrosion inhibitor, less damage was applied to the protective Al2O3 on the aluminum surface, leading to an increase in the Rct value. This observation emphasizes the importance of additives like NaNO₂ in controlling aluminum corrosion behavior.
CONCLUSIONS
In this study, we investigated the color changes in blue coolant used in electric vehicle cooling systems to understand the correlation between aluminum (Al) corrosion and coolant color changes. Various experiments were conducted to elucidate the interaction between Al corrosion and coolant color changes. Initially, UV-Vis spectral analysis revealed the interaction between Al and flux, suggesting their synergistic effect on coolant color changes. Through NMR analysis, we proposed the discoloration mechanism of triphenylmethane (TPM) dye in the presence of Al. Furthermore, XPS and SEM analysis were employed to observe the corrosion phenomena on the Al surface, tracking the corrosion behavior concerning the interaction with the coolant. The results findings indicated that the addition of Al and flux to the coolant leads to the corrosion of Al, resulting in the formation of Al(OH)3 and subsequent chemical interactions, which produce KOH within the coolant. Lastly, we explored methods to manage Al corrosion and TPM dye color changes by evaluating the effect of additives in the coolant. The current study on discoloration behavior offers valuable insights for improving coolant identification and long-term reliability.
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