Relationship between Corrosion Form and Elution Behavior of Copper Tubes Surfaces with Different Residual Carbon Amounts ()
1. Introduction
Leakage of water due to pitting corrosion was a problem in heat-transfer copper tubes for air handling units used in building facilities. The pitting corrosion process of copper was affected by the amount of residual carbon on the inner surface of the material [1] [2] [3]. Environmental factors, such as water quality, pH [4] [5] [6], chloride ions [7] [8] [9] [10], sulfate ions [11] [12], and dissolved oxygen [13], also had effects on this process. It was reported that pitting corrosion tended to occur when the amount of residual carbon (hereafter referred to as “residual carbon amount”) was 2 mg/m2 or higher [14] [15], but no standardized test method for pitting corrosion resistance of copper tubes had been established, and electrochemical tests and water flow tests were generally used [16] [17] [18] [19] [20]. However, these tests were difficult to apply for quality analysis because it took time to obtain the results of pitting corrosion. Therefore, we evaluated pitting corrosion resistance of copper tubes by a 1 h filling test with a test solution consisting of a mixture of corrosion promoting factors (hydrogen peroxide, chloride ions, sulfate ions) and a corrosion inhibiting factor (benzotriazole). As a result, the optimal solution concentration for the test and the measurement range of micromounds were reported. A relationship between the number of micro-mounds and the amount of residual carbon was observed, and it was concluded that this rapid test would be useful to evaluate the pitting corrosion resistance of copper tubes [21]. In this study, immersion tests were conducted using a rapid evaluation test solution to approximate the actual environment. The relationship between the corrosion form of copper tubes and the amount of copper eluted was examined by cross-sectional observation of corrosion products.
2. Experimental Methods
2.1. Test Materials
Unfortunately, it was not possible to produce copper tubes with a determined amount of residual carbon. Therefore, JIS H3300 C1220 phosphorus-deoxidized soft copper tubes with 5 different levels of residual carbon amount in the market were used as the test material. Residual carbon amount on the inner surfaces of 0.5 mg/m2 (hereafter referred to as “C 0.5”), 2.5 mg/m2 (hereafter referred to as “C 2.5”), 5.3 mg/m2 (hereafter referred to as “C 5.3”), 6.6 mg/m2 (hereafter referred to as “C 6.6”), and 13 mg/m2 (hereafter referred to as “C 13”). The C 0.5 specimen had an outer diameter of 15.2 mm and a wall thickness of 0.4 mm, and the C 2.5, C 5.3, C 6.6, and C 13 specimens had an outer diameter of 15.88 mm and a wall thickness of 0.8 mm. The residual carbon amount of this test material was measured by the conventional method [15]. The specimens were cut to a length of 10 cm, and each test material was cut in half. The amount of residual carbon was determined by the conventional method. The test material was coated with silicone resin except for a 3 cm2 test area on the inner surface and to ensure testing under the same conditions as experienced in actual machines, no treatments, such as degreasing, were performed.
2.2. Test Solutions
The test solutions used consisted of pure water to which hydrogen peroxide (H2O2: special grade reagent; Santoku Chemical Industries Co., Ltd., Tokyo, Japan), chloride ions (Cl−), sulfate ions (
), and benzotriazole (BTA: special grade reagent; Kanto Chemical Co., Inc. Tokyo, Japan) had been added. The concentrations of H2O2 and BTA were adjusted to 10 mg/L, while Cl− and
were adjusted to 100 mg/L. For Cl− and
, sodium salts (special grade reagent; Kanto Chemical Co., Inc. Tokyo, Japan) were used.
2.3. Immersion Tests
The test specimens were immersed in 1 L of test solutions for 30 days, after which the appearance of the specimen was observed. In addition, the amount of copper eluted and pH was measured every 7 days after the start of the test. The tests were conducted at room temperature, with the specimens open to the atmosphere and the solution stirred at 300 rpm with a magnetic stirrer. Copper elution was measured by inductively coupled plasma optical emission spectroscopy (ICP-OES) (ULTIMA2; Horiba Jovin Ybon, Ltd., Kyoto, Japan) at a wavelength of 324.75 nm. Measurements were made for C 0.5, C 6.6, and C 13, and the results of copper elution analysis are shown as maximum, minimum, and average values. The results of copper dissolution analysis are shown as the maximum, minimum, and average values. After the immersion test, the surface of the test specimen was observed with a digital microscope (DMV5000; Leica Microsystems, Wetzlar, Germany) and by scanning electron microscopy (SEM) (S-4300; Hitachi High-Technologies Co., Tokyo, Japan) equipped with energy dispersive X-ray analysis (EDX) (EX-220; Horiba, Ltd., Kyoto, Japan) performed under high-vacuum conditions with an acceleration voltage of 15 kV.
3. Results and Discussion
3.1. Progressive Observation of Copper Corrosion Form
Figure 1 shows the results of corrosion after immersion tests. The surfaces of the test specimens after the immersion test are shown in the upper row of Figure 1. Corrosion products were observed throughout the specimens regardless of the residual carbon amount. Cross-sectional observations at the locations where corrosion products were detected are shown in the second row of Figure 1. C 0.5 and C 2.5 showed corrosion with a wide front, while C 5.3, C 6.6, and C 13 showed corrosion with a narrow front that progressed in the depth direction. In SEM observation of the cross-sections shown in the third row of Figure 1, differences in the form of corrosion products were observed according to the residual carbon amount, similar to the cross-section observations shown in the second row. In addition, EDX mapping analysis of copper and chlorine (hereafter referred to as Cu and Cl) shown in the fourth and fifth rows of Figure 1 indicated that Cl was concentrated in the upper layer of the sample where Cu was not detected at C 0.5 and C 2.5, while Cl was concentrated under part of the micromounds (including the bottom of the pits) in C 5.3, C 6.6, and C 13. From the above results, it was inferred that the form of the corrosion products and the progression of corrosion were affected by differences in residual carbon amount.
Figure 2 shows the area per corrosion product. At C 0.5, the area of corrosion products is approximately 200,000 μm2. As the residual carbon amount increases, the area becomes smaller, and the area per corrosion product is about 50,000 μm2. It seems that the relationship between the width of the frontage and the corrosion form, as shown in the cross-section observation in Figure 1, is related to the area per corrosion product.
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Figure 1. Observations of the inner surfaces and results of EDX analysis for the cross sections of the copper tubes.
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Figure 2. Relationship between area of corrosion products and amount of residual carbon.
3.2. Evaluation of Corrosion Resistance
Caption: Fig. 2 Relationship between area of corrosion products and amount of residual carbon.
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Figure 3. Copper concentration eluted from copper tube and pH conditions of immersion time.
these conditions. Therefore, it was inferred that C 0.5 with low residual carbon amount, in which Cl was concentrated in the upper layer of the specimen, had good corrosion resistance.
4. Conclusions
The results of this study can be summarized as follows.
1) Cross-sections at the location where corrosion products occurred showed full corrosion for C 0.5 and C 2.5, while C 5.3, C 6.6, and C 13 showed pitting corrosion.
2) Cross-sectional EDX mapping analysis showed that Cl was concentrated in the upper layers of C 0.5 and C 2.5, and Cl was concentrated under part of the micromounds (including the bottom of the pits) in C 5.3, C 6.6, and C 13.
3) We speculated that the difference in the corrosion form of copper tubes was related to the amount of carbon film that acts as an accelerator of the anodic reaction, and we considered the amount of leaching elution with the pitting corrosion form.
Acknowledgements
This research was supported by a 2019 research grant from the Japan Institute of Copper. We are grateful to Dr. H. Tamagawa (UACJ Copper Co., Toyokawa-shi, Aichi, Japan) who provided specimens for the test. In addition, we are grateful to Mr. K. Kawano (UACJ Co., Chiyoda-ku, Tokyo, Japan) for assistance with the experiments.