Effect of Benzotriazole on Oxidation Behavior in Pure Copper

Abstract

In this study, we investigated the effects of the corrosion inhibitor benzotriazole (BTA), with the chemical formula C₆H₅N₃, on the oxidation behavior of copper through microstructure analysis. Our results demonstrate that BTA coating on copper decreased its oxidation rate and altered the morphological characteristics of the resulting copper oxide. The biocidal ability of copper was found to be unaffected by the presence of BTA. Our findings suggest that the stability of the interface between copper and its oxide, Cu₂O, is primarily determined by atmospheric pressure rather than the presence of BTA during the oxidation process. This study provides valuable insights into the role of BTA in the oxidation behavior of copper, which can have implications on various applications.

 

This Paper was Originally Published in Japanese in J. Japan Inst. Copper 62 (2023) 242–246. Figures 1, 3, 5, and 7, as well as all of Figure captions were marginally modified.

Cu₂O layer formation at 980°C for 24 hours under a vacuum degree of 13 Pa.

1. Introduction

Recently, there has been growing interest in the inherent bactericidal and antibacterial properties of copper, with a particular focus on the fields of healthcare and hygiene.^1,2) Because the bactericidal effectiveness of copper is closely tied to its surface properties, it should be important to elucidate the impact of its surface oxidation behavior, and the morphology and structure of the resulting oxide layer on the bactericidal properties of copper. For instance, it is recognized that the bactericidal properties of copper diminish as its surface undergoes oxidation (typically resulting in the formation of copper oxide (II) (CuO) upon exposure to normal atmospheric conditions). Therefore, a critical strategy to maintain the bactericidal efficacy of copper is to suppress surface oxidation. In this study, benzotriazole (BTA) demonstrates its effectiveness as a corrosion inhibitor, preventing contamination and oxidation of copper surfaces, and is widely employed as a rust-preventative spray in roll sheets. Hence, it is hypothesized that BTA-coated copper would effectively inhibit oxidation, thereby contributing to the long-term preservation of its bactericidal properties.

An alternative strategy to preserving the bactericidal properties of copper is to stably generate not a CuO layer but another copper oxide layer that exhibits superior bactericidal characteristics. Oxide film layers formed on copper substrates after prolonged atmospheric exposure lack good adhesion (interfacial strength) and can be relatively easily detached. Therefore, the discovery of a technology that can generate a copper oxide layer with excellent adhesion properties and sufficient bactericidal efficacy can enhance the value of copper products.

This study first investigated the impact of BTA coating on the oxidation behavior and bactericidal properties of intentionally accelerated oxidized pure copper at intermediate temperatures ranging from 250 to 350°C. Second, the adhesion and bactericidal properties of the copper oxide layers generated on BTA-coated copper substrates under controlled oxidation conditions, such as temperature and oxygen partial pressure, were analyzed.

2. Experimental Procedure

Pure copper (phosphorus deoxidized copper, C1220, >99.9% in purity) plates, hereafter referred to simply as ‘copper’, were prepared. Copper samples with and without BTA spray coatings were subjected to oxidation experiments after surface polishing to remove pre-existing oxide films. Oxidation experiments were conducted under atmospheric air conditions (1.0 × 10⁵ Pa (760 torr)) and low-pressure conditions (13 Pa (9.6 × 10⁻² torr)). In atmospheric oxidation experiments, the samples were oxidized at temperatures ranging from 250 to 350°C for 3 to 15 minutes to accelerate their oxidation behavior. Under low-pressure conditions, the samples were oxidized at 980°C for 24 hours. Due to the lower oxygen partial pressure under low-pressure conditions, it was expected that the rate of oxide formation would be slower.

The bactericidal properties of the as-prepared samples surfaces were analyzed using Staphylococcus aureus (gram-positive bacteria) and Escherichia coli (gram-negative bacteria) in accordance with the JIS Z2801 standards. Additionally, the adhesion between the copper substrate and the oxide layer was evaluated through a rub test employing a rubber with a 500 g load.

3. Results and Discussion

3.1 Bactericidal efficacy of BTA-coated copper

The bactericidal effects of surface-polished copper (copper without BTA) and BTA-coated copper were analyzed following the JIS Z 2801 standards, as shown in Fig. 1. In surface-polished copper (Fig. 1(a)), both Staphylococcus aureus and Escherichia coli bacterial counts decreased rapidly, reaching below 99.99% of their initial bacterial count within approximately 15 minutes of exposure, and remaining at this level after 1 hour. On the other hand, BTA-coated copper demonstrated a different pattern, shown in Fig. 1(b), with gram-positive Staphylococcus aureus showing only a 90% reduction in the initial bacterial count after 15 minutes of exposure, whereas gram-negative Escherichia coli experienced a 99.99% reduction. After exposure for more than 1 hour, BTA-coated copper exhibited bactericidal efficacy comparable to that of surface-polished copper.

Fig. 1

Sterilization tests of (a) polished and (b) BTA-coated phosphorus deoxidized copper (C1220) with increasing exposure times (JIS Z2801).

Based on the abovementioned results, we concluded that BTA coating neither significantly enhances nor detrimentally affects the bactericidal properties of copper. This suggests that the BTA coating does not impede the movement of copper ions on the sample surface, which is known to influence bactericidal efficacy.

3.2 Oxides on BTA-coated copper and their bactericidal properties

Figure 2 shows the surface changes of polished copper (copper without BTA) and BTA-coated copper when oxidized under varying temperature and time conditions in atmospheric air. Regardless of the presence of the BTA coating, as temperature and time increased, the color of the samples transitioned from orange to gray, indicating the formation of surface oxides. According to X-ray diffraction (XRD) analysis, the aforementioned oxide film of copper oxidized in air atmosphere at intermediate temperatures (250 to 350°C) for 3 to 15 minutes was identified as cuprous oxide (Cu₂O); the detailed XRD patterns are omitted in this study. This finding is consistent with the Ellingham Diagram for copper–copper oxides.³⁾ The shape of the Cu₂O particles was spherical and approximately 50 nm in size, irrespective of BTA coating, as shown in Fig. 2(g). However, in BTA-coated copper, the formation of the Cu₂O film was observed to be less uniform in certain areas (Fig. 2(h)). This suggests that BTA coating has an inhibitory effect on the formation of Cu₂O in atmospheric air.

Fig. 2

Surface of oxidized copper at (a) 250°C, 3 mins; (b) 300°C, 5 mins; and (c) 350°C, 10 mins. Surface of oxidized Cu with BTA at (d) 250°C, 3 mins; (e) 300°C, 5 mins; and (f) 350°C, 10 mins. (g) and (h) show the enlarged surfaces of non-coated and BTA-coated oxidized copper at 250°C for 15 mins, respectively.

Next, the bactericidal properties of surface-polished copper (copper without BTA) and BTA-coated copper oxidized under atmospheric air were evaluated in accordance with the JIS Z2801 standard, results are shown in Fig. 3(a). The bactericidal efficacy after 15 minutes of exposure to post-oxidation conditions for BTA-coated copper decreased marginally compared to non-coated copper. However, after exposure times longer than 1 hour, the bactericidal properties became essentially equivalent, regardless of BTA coating.

Fig. 3

Sterilization tests of (a) Non-coated and BTA-coated oxidized pure copper. (b) After long exposure times to atmospheric air.

After subjecting both polished copper (copper without BTA) and BTA-coated copper to various oxidizing conditions and exposing the samples to atmospheric air for 5 weeks, bactericidal tests involving a 15 minutes exposure were conducted; results are shown in Fig. 3(b). Interestingly, regardless of BTA coating, even the samples exposed to atmospheric air for 5 weeks after oxidation at intermediate temperatures still exhibited bactericidal properties. In particular, Escherichia coli experienced a 99.99% reduction in its initial bacterial count with just 15 minutes of exposure time, which is equivalent to or even exceeds the bactericidal efficacy of copper immediately after surface polishing, as shown in Fig. 1(a). This suggests the possibility of improved bactericidal activity due to the Cu₂O film, irrespective of BTA coating. The reason for this phenomenon remains unclear and warrants further investigation.

3.3 Adhesion of oxide layers formed by atmospheric pressure oxidation

The adhesion between the copper substrate and the Cu₂O layer was evaluated using a rub test; the results are shown in Fig. 4. Non-coated copper exhibited an attractive appearance (Fig. 2(a)) and demonstrated minor bactericidal efficacy (Fig. 3(a), (b)). However, the Cu₂O layer was easily detached in areas subjected to rubbing with a force of 500 g, as shown in Fig. 4(a) and (b). On the other hand, for BTA-coated copper, oxidation at 350°C for 15 minutes in atmospheric air yielded a Cu₂O layer that was easily peeled off even with a light touch, as depicted in Fig. 4(c) and (d). This indicates that, similar to non-coated copper, BTA-coated copper also exhibited poor adhesion properties.

Fig. 4

Surface after performing rubbing test with a 500 g load, oxidized at (a) 250°C for 3 mins and (b) 300°C for 10 mins. (c) Surface of BTA-coated copper oxidized at 350°C for 15 mins and (d) Surface of copper after minor impact on specimen.

Carbon, which is the principal component in BTA, was not detected through SEM-EDX analyses of the sample surfaces after atmospheric air oxidation (details of the results are omitted). This suggests that the BTA coating evaporated or disappeared when exposed to atmospheric air at intermediate temperatures. In other words, when oxidation is carried out at intermediate temperatures under atmospheric air, the behavior of oxide layer formation becomes similar for both BTA-coated and uncoated copper. Therefore, the adhesion properties of the oxide layer remain poor in the two copper samples.

3.4 Adhesion of oxide layers formed by low-pressure oxidation

Regardless of whether BTA was spay-coated to copper or not, all samples exhibited insufficient adhesion between the generated Cu₂O layer and the copper substrate, making it relatively easy to detach. Based on the results shown in Fig. 2(g) and (h), it is desirable for the generated Cu₂O to be in a layered form rather than in particle form to enhance adhesion at the Cu₂O-copper substrate interface. One approach to generating Cu₂O layerss is to control the oxidation rate at very low-oxygen partial pressures. Therefore, in this study, both BTA-coated and uncoated copper were oxidized at 980°C for 24 hours under low-pressure conditions (low vacuum) of 13 Pa (9.6 × 10⁻² torr); the results are shown in Fig. 5. Regardless of BTA coating, the samples oxidized under low-pressure conditions did not exhibit detachment of the Cu₂O layer during handling, as shown in Fig. 5(b) and (d), in contrast to copper oxidized under atmospheric air pressure (Fig. 4(c) and (d)). On the other hand, for BTA-coated copper, the generation of an oxide layer primarily composed of Cu₂O was observed even under low-pressure conditions, similar to that observed under atmospheric air pressure (Fig. 5(e)). Therefore, we speculate that the bactericidal properties of the Cu₂O layer generated under low-pressure conditions are similar to those generated under atmospheric air pressure (shown in Fig. 3).

Fig. 5

Copper oxidized at (a) 300°C for 15 mins in ambient atmosphere and (b) 980°C for 24 hours at a low air pressure of 13 Pa (9.6 × 10⁻² torr). BTA-coated copper oxidized at (c) 300°C for 15 mins in an ambient atmosphere and (d) 980°C for 24 hours at a low air pressure of 13 Pa (9.6 × 10⁻² torr). (e) XRD results of the samples shown in (b) and (d).

To assess the adhesion strength of the Cu₂O layer generated under low-pressure conditions, a friction test was conducted; the results are shown in Fig. 6. Interestingly, under low-pressure conditions, irrespective of BTA coating, there was no noticeable wear of the Cu₂O layer during the rubbing test. To investigate the reason for this, the microstructure of non-coated and BTA-coated copper surfaces oxidized under low-pressure conditions (13 Pa (9.6 × 10⁻² torr)) was observed; results are shown in Fig. 7. A Cu₂O layer with a relatively coarse grain structure (grain size: approximately 100 µm) was formed in both non-coated and BTA-coated samples. Under low-pressure oxidation conditions, due to the limited oxygen supply, the nucleation of Cu₂O was inhibited and Cu₂O grew preferentially in a manner that minimized the interfacial energy with the copper substrate. As a result, the adhesion between the copper substrate and Cu₂O was improved.

Fig. 6

Surface of (a) Cu and (b) BTA-coated Cu oxidized at 980°C for 24 h and 13 Pa (9.6 × 10⁻² torr) after performing the rubbing test.

Fig. 7

Microstructure of (a) Cu and (b) BTA-coated Cu oxidized at 980°C for 24 h and 13 Pa (9.6 × 10⁻² torr). (c) and (d) show the side views of (a) and (b), respectively.

The Cu₂O layer on the non-coated copper was approximately 100 µm thick, whereas it exhibited an island-like structure approximately 10 µm thick in the BTA-coated copper. These results demonstrate that BTA coating effectively suppresses copper oxidation under low-pressure conditions. Under low-pressure conditions, unlike under atmospheric air pressure conditions, it is speculated that the BTA coating layer reacts first, leading to the suppression of nucleation and growth of Cu₂O on copper.

The abovementioned results indicate that even when BTA is coated on the copper surface, it has a minimal impact on the bactericidal properties of copper. Furthermore, it has been demonstrated that BTA coating effectively suppresses the oxidation process under low-pressure conditions and leads to changes in the shape of the resulting copper oxide (Cu₂O). The adhesion between the Cu₂O layer and the copper substrate primarily depends on the oxygen partial pressure during the oxidation process. Therefore, identifying the optimal conditions for producing a robust, bactericidal Cu₂O layer on the copper surface during the oxidation process remains a future challenge.

4. Conclusions

In this study, the influence of BTA on the oxidation behavior and bactericidal properties of copper was investigated through microstructure analysis and bactericidal testing. The obtained results are as follows:

1. (1)    When copper coated with BTA was oxidized at atmospheric air pressure, there was an uneven generation of particulate Cu₂O. This suggests that BTA coating inhibits the formation of oxides.
2. (2)    The bactericidal characteristics of copper were not affected by BTA coating. Therefore, the bactericidal effect was due to the presence of Cu₂O and was similar to that of pure copper.
3. (3)    Irrespective of BTA coating, Cu₂O formed on the copper substrate under low-pressure oxidation conditions. This oxide layer became less prone to detachment.
4. (4)    Under low atmospheric pressure oxidation conditions, BTA coating reduced the thickness of the resulting copper oxide and altered its morphology to an island-like structure.
5. (5)    The adhesion between the Cu₂O layer and the copper substrate is primarily influenced by the oxygen partial pressure during the oxidation process, with the BTA coating being a minor factor.

Acknowledgments

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIP) [NRF-2020M3D1A2098962, NRF-2022M3C1C8093916] and the Fundamental Research Program of the Korea Institute of Materials Science (grant number PNK9100). This work was also supported by a cooperative program of the Collaborative Research and Development Center for Advanced Materials (CRDAM) at the Institute for Materials Research (IMR), Tohoku University (No. 202212-CRKEQ-0403).

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