It is known that one of the causes of pitting corrosion of copper tubes is residual carbon on the inner surface. It was confirmed that type I" pitting corrosion of the copper tube is suppressed by keeping the residual carbon amount at 2 mg/m 2 or less, which is lower than that of the type I' pitting corrosion, or by removing the fine particles that are the corrosion product of galvanized steel pipes. The developed water treatment chemical was evaluated using three types of copper tubes with residual carbon amounts of 0 mg/m 2 , 0.5 mg/m 2 , and 6.1 mg/m 2. The evaluation was conducted for three months in an open-circulation cooling water system and compared with the current water treatment chemical. Under the current water treatment chemical conditions, only the copper tube with a residual carbon amount of 6.1 mg/m 2 showed a significant increase in the natural corrosion potential after two weeks, and pitting corrosion occurred. No pitting corrosion and no increase in the natural corrosion potential were observed in any of the copper tubes that were treated with the developed water treatment chemical. In addition, the polarization curve was measured using the cooling water from this field test, and the anodic polarization of two cooling waters was compared. For copper tubes with a large amount of residual carbon, the current density near 0 mV vs. Ag/AgCl electrode (SSE) increased when the developed water treatment chemical was added.

The copper tubes of heat exchangers used in air conditioning and sanitation equipment have been reported to show type I pitting corrosion due to the synergistic effects of carbon film and water quality. To quantify the carbon film, the inner surfaces of the tubes after degreasing with acetone are dissolved with a mixture of nitric and hydrochloric acid to allow collection and analysis of the attached carbon. However, there are concerns about the difficulty and danger of the conventional procedure. Therefore, a simple method for quantifying carbon film is required. We reported a method for measuring the difference between the corrosion potentials on the inside with attached carbon film and outside with removed carbon film of copper tubes. However, data variation is observed in medium residual carbon, which is thought to be due to uneven carbon film. In this study, we examined the correlation between residual carbon and composition of surface for X-ray photoelectron spectroscopy (XPS) measurement. The results confirmed that the peak of C 1s increased and Cu 2p 3/2 decreased with increasing residual carbon. As the residual carbon and the integrated intensity at C 1s in the same way showed a linear relation, the residual carbon can be determined by the XPS measurement.

It was well known that residual carbon on the inner surface of copper tubes was a cause of pitting corrosion, and copper tube surfaces with different residual carbon amount were different corrosion forms. In this study, the relation between corrosion form and elution of copper in immersion tests by accelerated corrosion test solutions was examined. Generally, pitting corrosion that occurs in actual machines was reported to observe chloride ion at the bottom of the pitting corrosion of copper tubes. Energy dispersive X-ray (EDX) mapping analysis indicated that chlorine was concentrated in the upper layer of the samples where copper was not detected under conditions of low residual carbon amount, while chlorine was concentrated under part of the micromounds (including the bottom of the pits) under conditions of high residual carbon amount. It was also suggested that the amount of copper eluted was related to the amount of residual carbon.

Residual carbon on the inner surface of copper tubes is known to be a cause of pitting corrosion. We showed previously that the rapid filling test was useful to evaluate the pitting corrosion resistance of copper tubes. Immersion tests using the rapid evaluation test solution showed that corrosion occurs on the entire surface of copper tubes with low residual carbon amounts, while those with high residual carbon amounts show pitting corrosion. Therefore, it is necessary to improve the corrosion resistance of copper tubes with high residual carbon amount, which are expected to undergo pitting corrosion. As pitting corrosion occurs when anodes are locally concentrated on part of the metal surface, it has been suggested that anodes be dispersed over the entire surface by the processing of the metal surface. Metal processing methods have various purposes, including changing the shape and properties of metals, and in this case, leading to desirable surface properties (such as expansion and drawing processes). Here, we focused on the expansion process and its effects on corrosion resistance of copper tubes. The results showed that hydraulic expansion has a significant effect on the inner copper surface by improving corrosion resistance as the anode area increases.

Copper tubes are used in refrigerator and air conditioning unit heat exchangers. However, in some cases, the areas in copper tubes subjected to mechanical processing experience pitting corrosion. Therefore, a test scheme to rapidly evaluate the corrosion resistance of copper tubes in these sections and the effectiveness of the initial treatment on corrosion resistance were investigated. The initial treatment is a chemical treatment that improves the corrosion resistance of copper tubes. The corrosion of copper tubes was investigated by observing the surface after immersing the copper tube in various solutions containing hydrogen peroxide, chloride ions, sulfate ions, benzotriazole and hydrogen carbonate ions for one day. The test solution that contained a combination of 10 mg/L hydrogen peroxide (H 2 O 2 ), 300 mg/L chloride ions (Cl ¹ ), 300 mg/L sulfate ions (SO 4 2¹ ) and 10 mg/L benzotriazole was found to replicate actual corrosion processes on the copper tubes over 1 day. The results indicated a high tendency for corrosion to occur in the mechanically processed sections. The improved corrosion resistance in these sections from the initial treatment was also confirmed in the test solution. Additionally, the corrosion resistance in the mechanically processed sections of copper tubes was weaker as the residual carbon amount increased.