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.
Ryan Rennie, from the Spada- Rennie Group.
Introduction
The heat exchanger was operated in a butcher’s curing/drying room and was in operation for approximately 18 months before failure.
The heat exchanger allegedly failed due to leakage of the liquid coolant (glycol) during operation. An initial attempt was made to repair the leaks using epoxy putty, but the leaks persisted. The initial leak site was unidentified; however, it was suspected to have originated in the copper tube coils adjacent to the larger copper tube manifold. The unit was clearly corroded; therefore, it was imperative to determine and identify the corrosion mechanism and the root cause for the failure of the heat exchanger. One Eighty was provided with only a section of the heat exchanger for testing, as shown in the images below.
Images supplied by One Eighty
Approach and methodology
Due to the lack of identification of the failure initiation site, the heat exchanger product datasheet, the paint coating material specification and application procedure, or datasheets of any cleaning agents, we selected a set of metallurgical tests that would aid in the identification of the corrosion mechanism for this root cause failure investigation.
These tests included but were not limited to:
- Visual inspection
- Sampling selection and designation for metallurgical testing
- Macro-examination by way of stereomicroscopy
- Microstructural analysis by way of light microscopy
- Scanning electron microscopy (SEM) and Energy Dispersive Spectroscopy (EDS)
- Spectrographic Analysis by way of Optical Emission Spectrometry (OES)
- Vickers hardness testing
Results and discussion
From visual inspection, it was evident that the entire heat exchanger was subject to corrosive attack. The most affected areas were where the paint coating was damaged. The corrosion was predominantly on the outer surface.
Macro-examination through the tube cross-section showed localised pitting corrosion on the outer surface of the copper tubing, suggesting that the copper was exposed to the elements
Corrosion of the copper tubing at the interface with the enclosure plate.
Corrosion of the enclosure plates – right hand side.
Corrosion of copper pipes at welded components.
Corrosion of the enclosure plates – left hand side.
within the environment in which it was operating. It was also noted that the quality of the joints in the copper tubing was inconsistent. The copper tubes were confirmed to have corroded prior to the epoxy putty repair, with sections of the paint layer not providing sufficient corrosion protection.
Macro-image – sample A location 2, 2x magnification.
Macro-image – sample A location 2, 3x magnification
The microstructure of the copper tubing was consistent with copper; however, inconsistent grain sizes indicate that improper heat input may have been present during the fabrication/joining process.
Microstructural image – sample A location 1, 50x magnification
Microstructural image – sample A location 1, 100x magnification.
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