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Contributor
FAILURE INVESTIGATION OF A CORRODED HEAT EXCHANGER ( PART 2 )
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 .
Images supplied by One Eighty
… continued from part one . 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 .
Microstructural image – sample A location 3 , 50x magnification .
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
Ryan Rennie , from the Spada- Rennie Group .
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
“ Several of the welded joints also showed to have completely dislodged themselves from the copper material .”
Supplied by Ryan Rennie
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RACA Journal I April 2024 www . refrigerationandaircon . co . za