Product: Heat exchanger of the indoor unit of an air conditioning installation
Material: Copper tube bundle in aluminium lamellae cooling fins
Sector: Manufacturing industry, Construction, Air conditioning, Food production
First observation: Leakage of the tube bundle
Analysis procedure: Leakage detection, visual and stereomicroscopic analysis, light optical microscopy, scanning electron microscopy with spectral analysis (SEM-EDS)
Main Conclusions
Through leak detection and stereomicroscopy, several very small leaks were found in the copper tube bundle. Laboratory analysis prevailed that the leakages initiated at the outer wall of the copper tubing and are due to so-called ‘ant nest corrosion’, probably induced by the presence of organic acids.
The attack was doubled by the crevice corrosion effect in the narrow gap between the aluminium fins and the copper tube bundle.
Furthermore spectral analysis of some external wall deposits indicated for the presence of sulfur and chlorine. Sulfur has been detected in all analysed deposits, where in 5 out of 6 cases calcium was detected as well; in many cases this combination indicates for the presence of gypsum (calcium sulphate). The presence of chlorine is limited, yet significantly detected at one position. Chemical compounds that contain sulfur and/or chlorine might have had an enhancing effect on corrosion processes at the specific location where these compounds were present.
As to the client’s question whether formaldehyde could have played a role, it can be mentioned that formaldehyde in contact with atmospheric oxygen readily oxidizes to formic acid, member of the organic acid family. The environmental analysis also showed that tartaric acid, another organic acid, may have contributed to the damage. In particular, the module was placed at a wine producer; tartaric acid is present in grapes, among other things.
Recommendations for repair
Since several leaks were found at different positions in the heat exchanger and the configuration of the bundle does not allow for local repairs, it was necessary to replace the entire heat exchanger package.
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Cross-section images of ant nest corrosion of the outer wall of a copper heat exchanger tube (to the left: after polishing, to the right: after etching)