The test materials were JIS H 3100 C1020 oxygen-free copper sheets 15 mm wide, 0.3 mm thick, and 55 mm long degreased with acetone as pretreatment.

The test solutions consisted of pure water to which acetic acid (CH₃COOH, guaranteed reagent, Nacalai Tesque, Inc., Japan) and potassium dihydrogen phosphate (KH₂PO₄, extra pure reagent, Nacalai Tesque, Inc., Japan) as phosphate were added. The concentration of acetic acid was adjusted to 0.2 mol∙L⁻¹, and the concentrations of phosphate were adjusted to various levels. The acetic acid concentration was the same as in the previous report at 1 vol.% [12] .

Figure 1 shows a schematic illustration of the immersion test and image of the test specimen before the test. As in our previous report [12] , test specimens were placed in 100 mL lidded polypropylene containers containing 20 mL of the test solution, and the bottom 2 cm of the specimens were immersed in the test solution. The following heat cycle was conducted 5 days/week. After the upper volume inside the containers was replaced with oxygen gas at 25 L∙min⁻¹ for 5 s, the containers were placed in a constant temperature and humidity apparatus (SANYO Electric Co.: MTH-4400) and kept at a temperature of 60˚C for 1 hour. The containers were then removed from the apparatus and kept at room temperature for 23 hours. For 2 days/week they were kept at room temperature. The test period was 3 weeks.

The immersion tests were conducted under the following three conditions.

1) Effect of changes in phosphate concentration on copper corrosion in acetic acid solution

To investigate the effect of changes in phosphate concentration on copper

corrosion in acetic acid solution, immersion tests were conducted using mixed solutions of 0.2 mol∙L⁻¹ acetic acid and various concentrations of phosphate.

2) Respective effects of acetic acid and phosphate on copper corrosion

To investigate the respective effects of acetic acid and phosphate on copper corrosion, immersion tests were conducted in three different test waters: mixed solution of 0.2 mol∙L⁻¹ acetic acid and 0.02 mol∙L⁻¹ phosphate, 0.2 mol∙L⁻¹ acetic acid only, and 0.02 mol∙L⁻¹ phosphate only.

3) Effect of adding of acetic acid and phosphate on copper corrosion

To investigate the effects of adding acetic acid and phosphate on copper corrosion, immersion tests were conducted under the following two test conditions.

a) The specimens were immersed in 0.2 mol∙L⁻¹ acetic acid for 1 week, then phosphate was added to the test water, and the specimens were immersed in the mixed solution of 0.2 mol∙L⁻¹ acetic acid and 0.02 mol∙L⁻¹ phosphate for 2 weeks.

b) The specimens were immersed in 0.02 mol∙L⁻¹ phosphate for 1 week, then acetic acid was added to the test water, and the specimens were immersed in the mixed solution of 0.2 mol∙L⁻¹ acetic acid and 0.02 mol∙L⁻¹ phosphate for 2 weeks.

After the tests, the specimens were evaluated by surface and cross-section observations. A digital microscope (Keyence Corp.: VHX-5000) was used for these observations. The composition of corrosion products after the test was analyzed by X-ray diffraction (Rigaku: Miniflex 600, Cu-Kα ray, tube voltage 40 kV, tube current 10 mA).

Figure 2 shows the results of surface and cross-section observations after immersion tests in mixed solutions of 0.2 mol∙L⁻¹ acetic acid and various

concentrations of phosphate. The surface observations showed that at 0.01 mol∙L⁻¹ phosphate, blackish brown discoloration was observed throughout the specimen, and a small amount of greenish blue corrosion products were observed at the gas/solution interface and gas phase. At 0.02 mol∙L⁻¹ phosphate, blackish brown discoloration was observed at the gas/solution interface and gas phase, and greenish blue corrosion products were formed throughout. In particular, the solution phase was covered with the corrosion products. At 0.2 mol∙L⁻¹ phosphate, blackish brown discoloration was observed at the gas phase, and was covered with greenish blue corrosion products at the solution phase and gas/solution interface. Figure 3 shows the results of X-ray diffraction of the corrosion products at the gas/solution interface at 0.2 mol∙L⁻¹ phosphate, where the greenish blue corrosion products were most abundant. As a result, the peak of Cu₃(PO₄)₂∙3H₂O (PCPDF #00-022-0548) was detected in the greenish blue corrosion products. Therefore, it was inferred that the greenish blue corrosion products on the copper were hydrates of copper phosphate.

Figure 4 shows a summary of the cross-section observations in Figure 2. At 0.01 mol∙L⁻¹ phosphate, light corrosion was observed throughout. At 0.02 mol∙L⁻¹ phosphate, hemispherical corrosion was observed. At 0.2 mol∙L⁻¹ phosphate, light corrosion was observed throughout. From the results, it was inferred that oxygen-free copper corrosion progressed differently in 0.2 mol∙L⁻¹ acetic acid depending on the phosphate concentration, and hemispherical corrosion progressed the most at 0.02 mol∙L⁻¹ phosphate.

Figure 5 shows the results of surface and cross-section observations after

immersion tests in the mixed solution of 0.2 mol∙L⁻¹ acetic acid and 0.02 mol∙L⁻¹ phosphate, 0.2 mol∙L⁻¹ acetic acid only, and 0.02 mol∙L⁻¹ phosphate only. The surface observations showed that in the mixed solution of 0.2 mol∙L⁻¹ acetic acid and 0.02 mol∙L⁻¹ phosphate, blackish brown discoloration was observed at the gas/solution interface and gas phase, and greenish blue corrosion products were formed throughout. In particular, the solution phase was covered with corrosion products. At 0.2 mol∙L⁻¹ acetic acid only, blackish brown discoloration was observed throughout, and a small amount of greenish blue corrosion products were observed at the gas phase. At 0.02 mol∙L⁻¹ phosphate only, dark brown discoloration was partially observed at the gas/solution interface and gas phase, and greenish blue corrosion products were observed at the solution phase and gas/solution interface.

Figure 6 shows a summary of the cross-section observations in Figure 5. In mixed solution of 0.2 mol∙L⁻¹ acetic acid and 0.02 mol∙L⁻¹ phosphate, hemispherical corrosion was observed throughout. At 0.2 mol∙L⁻¹ acetic acid only, light corrosion was observed throughout. At 0.02 mol∙L⁻¹ phosphate only, light corrosion was observed at the solution phase, and almost no corrosion was observed at the gas/solution interface and gas phase. From the results, it was inferred that hemispherical corrosion progressed more in the presence of both acetic acid and phosphate than in the presence of only one of them.

Figure 7 shows the results of surface and cross-section observations after immersion tests under the following test conditions: a) the specimens were immersed in 0.2 mol∙L⁻¹ acetic acid for 1 week, then phosphate was added to the test water, and the specimen was immersed in the mixed solution of 0.2 mol∙L⁻¹ acetic acid and 0.02 mol∙L⁻¹ phosphate for 2 weeks, and; b) the specimens were immersed in 0.02 mol∙L⁻¹ phosphate for 1 week, then acetic acid was added to the test water, and the specimen was immersed in the mixed solution of 0.2 mol∙L⁻¹ acetic acid and 0.02 mol∙L⁻¹ phosphate for 2 weeks. The surface observations showed that for a), blackish brown discoloration was observed at the gas/solution interface and gas phase, and greenish blue corrosion products were observed throughout. The corrosion products were especially observed at the solution phase. For b), blackish brown discoloration was observed throughout. Green corrosion products were observed at the solution phase. Greenish blue corrosion products were observed at the gas/solution interface and gas phase, especially at the gas/solution interface.

Figure 8 shows a summary of the cross-section observations in Figure 7. For a), near ant-nest corrosion was observed at the solution phase. At the gas/solution interface, light corrosion was observed. At the gas phase, hemispherical corrosion was observed. The corrosion forms differed depending on the phases. For b), light corrosion was observed throughout. From the results, it was inferred that the corrosion forms and corrosion progress of oxygen-free copper were affected by the test water at the initial stage of immersion.

From the above results, it was assumed that copper corrosion was affected by the difference in phosphate concentration in acetic acid, and the presence or absence of acetic acid and phosphate, resulting in differences in corrosion forms and corrosion progress.