Failure analysis of copper tube used in a refrigerating plant

摘要

This paper reports the failure analysis of a leaked copper tube used in a refrigerating plant for carrying polyol ester oil for lubrication and cooling of compressor bearings. Failure had occurred at the bent region of the tube in the form of a wide diametric crack. The crack was intergranular and filled with corrosion products. The failed region on the inner diameter side of the tube had red-brown and dull-black surface films which were identified as cuprous oxide. No degradation or decomposition of the lubricant oil was detected. The microscopic examination of the cross-section of the tube showed corrosion degradation on both the outer and inner diameter surfaces with similar morphology. The corrosion morphology consisted of pit tunnels with fine longitudinal cracks bridging individual pitted regions typical of ant-nest corrosion. A few cracks, at the bent region of the tube, had originated through the pits and propagated along the wall thickness. The failure was concluded to be due to ant-nest corrosion of copper in the presence of carboxylic acid. Organic compounds used during the fabrication and joining of the copper tubes, if not properly cleaned, may decompose to carboxylic acid in presence of air and moisture.