Corrosion Failure Analysis of Copper Tube of Air Conditioner Parts

S. Kaewkumsai, N. Bunchoo, and W. Khonraeng

National Metal and Material Technology Center (MTEC),
National Science and Technology Development Agency (NSTDA),
114 Thailand Science Park, Paholyothin Rd., Klong 1, Klong Luang, Pathumthani 12120 THAILAND
Phone 66-2564-6500 ext. 4736, Fax.66-2564-6332, E-Mail: siamk@mtec.or.th

Abstract
This paper described a failure analysis of the heat exchanger copper tube of air conditioner. Ant-nest corrosion is a specific type of premature failure of copper tubes which caused the loss of refrigerant liquid and the consequent environment pollution. It is known that the attack requires a combination of moisture, oxygen and a corrodent, e.g. an organic acid, such as formic, acetic, propionic or butyric acid or other volatile organic substances. This type of corrosion usually occurs in thin-wall copper pipes and is known by several names: ant-nest corrosion, formicary corrosion, unusual corrosion, branched-pits, pinhole corrosion, etc. The results indicated that the tube underwent ant-nest corrosion. To avoid corrosion attack, periodical removal of contaminants and cleaning of copper tube surface during and after fabrication was suggested.

Keywords: Ant-nest corrosion, Copper tube, Chromic acid, Tunneling, Heat exchanger

1. Introduction
Formicary or ant-nest corrosion primarily occurrs in heating, ventilation, refrigerators, condensers, radiators, solar-systems, and air-conditioning industry [1, 2]. Ant-nest corrosion can be described a micro-tunneling attack of copper that is initiated on the copper surface. A previous review of this phenomenon describes the mechanisms where the copper base-metal oxidizes to form copper carboxylates [3]. The mechanism is considered to be a modified localized corrosion process (e.g., pitting) involving a micro-anode where copper ions combine with carboxylic acids to form an unstable cuprous complex. The complex is oxidized to cupric formate, acetate, etc., and cuprous oxide (Cu2O). The morphology of corroded surface is characterized by the development of longitudinal pits that form by interconnecting random micro-cavern channels containing porous copper oxide. Microscopic progression of tunnels usually start on the surface of the copper tubing and penetrated through the tube wall. Failures are typically characterized by leaks that form in the fin pack area of the coil [4]. During the corrosion processes, copper oxide is deposited on the inner walls of the tunnels, causing them to turn black. It has been suggested that this type of corrosion may be caused by the decomposition products of chlorinated organic solvents used in degreasing, cleaning and picking treatments of the copper tubes during manufacturing or joining processes, or by certain types of synthetic lubricant oils used during the copper tube stamping process [2].

In this paper, the leaked component was the copper tube of heat exchanger, which was used in the cooler system. It had been in service near the seashore for nearly 10 years. During service, it was found that the cooler had low performance. Then, the failed heat exchanger was subject to pressure test to locate the leaks. As-received heat exchanger for analysis is shown in Fig. 1.

Fig. 1: As-received heat exchanger for analysis

2. Investigation Methods
Visual Examination of the as-received failed components was thoroughly examined visually and with the aids of a stereo microscope. Examine general and physical appearances of the both external and internal surface of copper tube by a stereo microscope to study the characteristics of the damage that occurred. Pressure test with compressed-air was applied for identify the site of leakage. Energy Dispersive Spectrometry (EDS) and Scanning Electron Microscope (SEM) were used to determine the chemical composition of stains on copper tube and compares with normal area. Finally, cut a sample from the stained section and prepared for examination by mounting, polishing, and etching with 10% H2SO4 solution. The microstructure analysis was performed under a reflected light microscope to determine the form of corrosion attack and propagation mechanisms.

3. Results
3.1 Visual Examination
The heat exchanger tube failed by leaking. To identify the source of leakage, randomly selected tubes were pressurized with compressed-air and brushed with soap solution. The leaked tube showed bubble generation over a large area as identified by yellow circles in Fig. 1. The source of these numerous leaks was identified from subsequent cross-section analysis. Visual examination revealed that the copper tube had green stain on the u-bend positions while the white stain occurred on the aluminum fin and zinc-plated surface.

3.2 Surface Analysis
The general appearances of the external copper tube are shown in Fig. 2. Examination of the external tube surface revealed many defects such as cracks, corrosion attack, and corrosion products (Fig. 2b). Pitting defects, which were located at the crack lines, were also found in some areas. The internal surface was found to be smooth without deposits and corrosion products.

Fig. 2 a) Physical appearance of copper tube and b) macro-view of surface of tube

3.3 Chemical Composition Analysis
The chemical composition analysis of stains on external tube surface and normal area from EDS techniques are shown in Fig. 3. The predominant elements were shown to be copper, aluminum, and oxygen, with smaller quantities of carbon, sulfur, and chlorine. A section from a tunnel pitted tube was flattened and the scale picked off; in this case, significant amount of chlorine was found in the vicinity of the pit. Carbon and oxygen could have come from the organic acid.

Fig.3: EDS spectrum of deposit on tube surface

3.4 Cross-section and Microstructure Analysis
Cross-sections of the failed tube through the detected leaked area revealed sub-surface cavities and propagation direction as shown in Fig. 4. The progression of the corrosion is from the exterior of tube and penetrated through the interior tube surface. This kind of progressive pattern called “tunneling”. Finally, leakage of the tube was exhibited.

Fig.4: Corrosion attack morphologies and propagation mechanism called “tunneling” of the copper tube

4. Discussions
Visual examination of the leaked tubes revealed the corrosion products, cracks, and pits on the external surfaces. The internal tubes revealed the normal surfaces without defects. As a result, the pits were initiated from the exterior of copper tubes and propagated to the interior and eventually led to leakage. The presence of cracks on the external tube surface acts as the preferred sites for corrosion attack. The material defect only found at the external surface suggests that the leakage of copper tube must start from this site.

From this analysis, the failure of copper tube heat exchanger could be in form of localized pitting, the type of corrosion observed during this investigation is known as “ant-nest” or “formicary” corrosion due to its unique morphology. Pits created by this type of corrosion are so fine that they are not visible to the naked eye [4]. The studies of others showed that ant-nest corrosion arises because of the presence of undesirable organic species (notably carboxylic acids) on copper tube surfaces [2]. ASTM B 743 [5] calls for certain minimum cleanliness levels for refrigeration or air conditioning grades of copper tube. Typically, the inner tube surface shall be sufficiently clean so that when the ID is washed with a suitable solvent (redistilled chloroform or trichloroethylene) the remaining residue after evaporation of the solvent shall be less than 0.0035 g/ft2 (0.038 g/m2) of the interior surface.

Copper and it alloys are unique among the corrosion-resistant alloys. They do not form a truly passive corrosion product film. In aqueous environments at ambient temperature, the corrosion product that is predominantly responsible for protection is cuprous oxide (Cu2O) [6]. The copper and it alloys resist many saline solutions, alkaline solutions, and organic chemicals. However, copper is susceptible to more rapid attack in oxidizing acid, oxidizing heavy metal salts, sulfur, sulfur- dioxide, ammonia, sulfur and ammonia compounds.

From the above data and chemical composition results may indicate the cause of stains on copper tube surface, the sulfur and chlorine peaks; from EDS results on the stain indicate that it could come from corrosive media. The presence of chlorine in the area of the pit found by EDS is an indication of contamination of the antifreeze solution with chlorides, although most tap water is also chlorine treated and contains traces of chlorides. Carbon and oxygen could have come from organic acid. The aluminum peak indicates that this deposit came from aluminum fin. The analysis of these samples confirmed the presence of significant levels of formate and acetate in the service environments. The acid salts (may be in form of AlCl3), which are hydrolyzing to form acid solution may cause corrosion with combined hydrogen evolution and oxygen depolarization. The oxidizing salt could come from many sources such as seawater, fabrication process and materials handling process.

Industry research report of carrier [4] tells that formicary corrosion or ant-nest corrosion have required three conditions required for generated as follows: the presence of oxygen, the presence of a chemically corrosive agent, and the presence of moisture. In this case, this failure possesses all favorable conditions necessary for ant-nest corrosion to occur.

5. Conclusion
The presence of ant-nest corrosion patterns in the copper tube heat exchanger: a few pinholes on the surface, progressive tunneling, and a discoloration of surface could be in form of cuprous complex. One of the possible sources of organic acids is the different cleaning treatments applied to the copper tube during service and maintenance. Periodical removal of contaminants and careful cleaning of copper tube surface during and after fabrication was suggested for avoiding corrosion attack.

References
[1] O. Albert, C. Richard, 2007, “Formicary corrosion of cupronickel tubing”, Materials Selection and Design, pp.52-54.
[2] D.M. Bastidas, 2006, “Ant-nest corrosion of copper tubing in air-conditioning units”, Revista De Metalurgia, vol.45, pp.367-381.
[3] A. Richard, 2000, “Ant-nest corrosion digging the tunnels”, Corrosion2000, NACE, paper No.00646.
[4] Carrier, “Fin Pack Leaks-Formicary Corrosion”, Indoor Coil Corrosion, Industry Research Report.
[5] Annual Book of ASTM Standards, volume 02.01 (Conshohocken, PA).
[6] Corrosion of copper and copper alloys, ASM Handbook, Vol.13, Corrosion, pp. 610.