Formicary Corrosion of Copper Tubes in a Chiller
The tube sections represent a condenser and an evaporator tube from a centrifugal chiller located in an office building. The tubes are approximately 9-inches in length. They are ¾-inch diameter refrigeration grade copper with helical enhanced outside diameter (OD) fins and helical rifling on the inside diameter (ID).
The chiller was fabricated in July. The unit was shipped to the customer in September. The unit was installed and piped up to the recirculation systems, however no service water entered the unit, even though the piping system had been hydro-tested. In April the following year, the unit was leak tested and the condenser bundle was found to have 22 tubes with potential leaks. One of several tubes were pulled from both the condenser and evaporator and the portion containing a suspected leak was sent to Corrosion Testing Laboratories, Inc. (CTL).
The OD surfaces are new-like copper-color, however they have a significant number of circumferential bands of black staining. There is no apparent metal loss or damage to the tube in association with this staining.
The ID surfaces were peppered with intermittent blue to black scale/deposits, similar to other tubes that experienced failures that we have seen in the past. However, in the area identified as the two leak sites, the black scale was very pronounced, covering almost the entire ID surface for a length of approximately 4-inches. Careful examination of the ID surface under optical microscopy (at 40X magnification) revealed microscopic pits reminiscent of those associated with formicary (a.k.a., “ant nest”) corrosion.
A cross-sectional mount was made through the observed pits on both tubes. After polishing, the morphology and extent of pitting confirmed the through-wall failures and the type of attack as formicary corrosion, as seen in the figures below.
|Figure 1. Branched tunnels associated with formicary corrosion in the evaporator tube. (150X magnification)
|Figure 2. Round pit with spurs of branched tunnels, also associated with formicary attack. (63X magnification)
On the evaporator tube, there were indications of localized attack on the flat, non-enhanced tube associated with a tube support. A specimen was cut from this area and a cross-section mount was prepared. The figure below shows the extent and depth to which the formicary damage exists.
|Figure 3. Cross-section through a flat, land area of the evaporator tube. Note the many sites of pitting and the extent of depth. (30X magnification)