A Study of Water Treatment Chemical Effects on Type I” Pitting Corrosion of Copper Tubes

Takashi Iyasu,Yutaka Yamada,Itaru Ikeda,Noriyuki Tanaka,Motoki Kuratani,Osamu Sakurada

doi:10.4236/msa.2020.117034

Abstract

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/m2 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/m2, 0.5 mg/m2, and 6.1 mg/m2. 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/m2 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.

Highlights

Copper has a high thermal conductivity and is easy to process, so it is used for heat exchangers in refrigerators and air conditioning units
It was confirmed that type I” pitting corrosion of the copper tube is suppressed by keeping the residual carbon amount at 2 mg/m2 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
Under the current water treatment chemical conditions, only the copper tube with a residual carbon amount of 6.1 mg/m2 showed a significant increase in the natural corrosion potential after two weeks, and pitting corrosion occurred

Summary

Introduction

Copper has a high thermal conductivity and is easy to process, so it is used for heat exchangers in refrigerators and air conditioning units. It has been reported that one of the causes of this pitting corrosion is the effect of the residual carbon on the inner surface of the copper tube [1] [2] [3] [4]. One is type I’ pitting corrosion when using groundwater, and the other is type I” pitting corrosion when using heat storage or cooling water [5] To prevent these types of corrosion, it has been proposed that in the case of type I’ corrosion, the concentration of free carbonic acid be reduced to less than 15 ppm or the amount of mean residual carbon on the copper tube inner surface (hereafter referred to as “residual carbon amount”) be reduced to 5 mg/m2 or less [6]. Due to the production cost of copper tubes as industrial products, the development of a water treatment chemical capable of suppressing pitting corrosion even in the presence of residual carbon is desired, rather than removing the residual carbon [8]

Methods

Results

Conclusion