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Formicary Corrosion of Unused Copper Tubes






Formicary Corrosion


Five chiller units were installed two years ago after fabrication. The units were never put into operation.  The mechanics installing the units found 3 of the 5 units to have leaking condenser and evaporator tubes.  Forty (40) evaporator tubes were found to be leaking.  Suspect tubes were pulled and pressurized to 150 psi with nitrogen to identify specific through-wall penetrations.  

The five samples received by Corrosion Testing Laboratories were sealed and air underwater tested to 120 pounds of pressure to confirm the marked areas of leaks.  All tubes experienced leaks as verified by bubbles present on the OD surface during air testing.  The sites of air leakage corresponded with markings painted on the OD. 

The OD surfaces of all five tubes were observed to have a dull, brownish copper patina, typical of air oxidized copper. There was no apparent metal loss or damage to the tubes in association with this appearance. 

All five samples were then split longitudinally to expose the ID surface.  The ID surfaces were covered with black scale/deposits, with some tubes having substantial black product build-up.  

The ID rifled surfaces were examined under an optical microscope (at 40X magnification).  The areas of heaviest deposits correlated with the areas marked as leak sites on the OD.  In these areas, deposits of red-to-purple were seen, and within this area at least one pit was observed, as typified by the figure below. 

The scale was chemically removed with inhibited hydrochloric acid, revealing microscopic pits in the black deposited areas; reminiscent of those associated with formicary (a.k.a., “ant nest”) corrosion.  A cross-sectional mount was made through one of the observed pits.  After polishing, the morphology and extent of pitting confirmed the through-wall failure and the type of attack as formicary corrosion, as seen below. 



Figure 1. An ID pit observed near red-to-purple deposits (40X Original Magnification)

Figure 2. Formicary corrosion is observed in a cross-section of a copper tube (62.5X Original Magnification)

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