By Ryan Rennie, from the Spada-Rennie Group
One Eighty was appointed by an air-conditioning contracting supplier and installer to investigate the failure of a heat exchanger unit. Part two of a two-part article.
Ryan Rennie, from the Spada Rennie Group
From the microstructural images, it was clearly evident that the copper material had suffered corrosive attack from the exterior side of the tube, which presented itself as pitting. Several of the welded joints also showed to have completely dislodged themselves from the copper material, which further indicated poor welding/brazing techniques were implemented in the fabrication. during the paint application.
Microstructural image – sample A location 3, 50x magnification. Images supplied by One Eighty
Microstructural image – sample A location 3(a), 100x magnification.
SEM and EDS analysis confirmed the presence of chlorides near the external surface of the copper tubing, which are typical constituents of paint coatings and/or may have originated from external elements within the environment. The iron that was suspended between the paint coating and material substrate suggested rust contamination was present during the paint application. Elevated levels of sulphur were detected in the paint coating. Sulphur is more harmful to copper than chlorine and would have reacted with the copper to form copper sulphides, which would have likely initiated or accelerated the corrosion of the copper tubes. It is more likely that the chlorides were introduced from the salts used to cure the meat in the processing plant where the heat exchanger is located. These chlorides would react with the copper, resulting in corrosion by forming copper chlorides.
Spectrographic analysis confirmed that the steel frame plate was AISI 1005, a typical low-carbon steel with low corrosion resistance. It was clear that the current surface coating system was not appropriate due to the lack of corrosion protection noted on the frame. The copper was confirmed to be UNS C18150, a high-copper alloy with good corrosion resistance properties. However, high-copper alloys are susceptible to pitting corrosion when exposed to the wrong environmental conditions. The most corrosive elements to copper are typically chlorides, sulphur compounds and other compounds with heavy-metal salts.
Vickers hardness testing showed that the hardness values of the copper tubing in the samples were similar, suggesting that the tubes underwent the same production process. The low hardness values also suggest that the copper tubes did not undergo any hardening treatment/process.
CONCLUSION
Based on the findings and discussion, the following conclusions were made:
- The failure of the heat exchanger was as a result of environmental corrosive attack, most likely from an external source.
- The overall construction of the heat exchanger suggests inappropriate material selection and/or fabrication techniques were utilised. A combination of improper material selection, welding and inappropriate paint selection/application increased the exposure of the copper tubes to the corrosive environment in which it operates. This allowed for corrosion to initiate and propagate throughout the whole heat exchanger, resulting in multiple leaks to occur over time.
RECOMMENDATIONS
Based on the outcome of our investigation, One Eighty provided recommendations with regards to the failed heat exchanger.
These include:
- Review for appropriate material (material selection)
- Review weld procedures
- Review paint coating chemical composition for compatibility
- Review surface preparation and coating application
Successful implantation of the above-listed recommendations would minimise the overall risk for this type of failure from occurring in the future. The utilisation of the appropriate codes and/or standards should always be adhered to in any design in order to prevent unwanted failures due to lack of following the principle guidelines as set forth by the appropriate standardisation organisation.
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