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Pitting of Copper Condenser Tubes

ENVIRONMENT:

Office Building

EQUIPMENT:

Chiller

SERVICE TIME:

 4 years

FAILURE:

Pitting caused by formicary corrosion, underdeposit corrosion and aggressive ion dissolution

 

Background

An eddy current test report indicated 83% of the chiller tubes “were detected to have indications of internal diameter (ID) corrosion damage exceeding 20% wall loss.  Of these, 17% have damage exceeding 60%.” 

The chiller has reportedly been in service for four years.  The unit has over 26,000 operating hours, operating 85% of the approximately 3.5 years it has been in service.

 

Description of Material

CTL received two (2) full-length condenser tubes each cut into four (4) pieces for convenient shipping purposes.  The overall length of each tube was approximately 14-feet.  The pieces were individually identified so as to be able to reconstruct the tube.   

The tubes are measured to be ¾-inch in outer diameter.  They are soft refrigerant grade copper (UNS C1200) with helical, rifled inside (ID) bore and helical rolled fins on the outside (OD).

Also received was a 1-Liter bottle of water, represented to have come from the recirculating cooling tower water system, which flows through the condenser tubes of the chiller.

 

Findings

Water Analysis

Analysis of the received water sample has been performed according to standard analytical chemistry procedures that have been adopted by the United States Environmental Protection Agency (USEPA), including digestion, photometric and titration procedures.  The results of the analysis performed are presented in Table 1. 

 

Table 1.

Chemical Analysis of Water Samples Received

Compared to Typical Analysis Reported for This Unit 

 

Identification

pH

Cl-(mg/L)

Fe (mg/L)

Cu (mg/L)

Hardness (mg/l as CaCO3)

Alkalinity (mg/L as CaCO3)

Conductivity (mmhos/cm3)

As-received

5.8

112

1.13

0.19

335

7.2

950

Typical

7.2 (av.)

175 to 200

1.0 to 1.8

0.48 (av.)

350 to 400

35 (av.)

1035 (av.)

 

Location of Leaks

The ends of the tube sections were sealed and pressurized with 120-psig air while holding them underwater to pinpoint possible leak locations.  Several through-wall penetrations were discovered as evidenced by air bubbles on the OD surfaces.  The locations were marked with waterproof paint. 

Visual and Macro-Examination

In the as-received condition, the two 14-foot tubes had been cut to a convenient length (approximately 42-inches, 4-sections per tube) for shipping.  The tubes had been cut at the flat, un-enhanced OD and ID portions that served as areas for tube supports within the condenser shell.  The tubes had a helical rifled surface on the ID and helical low finning on the OD surface.

 

 

Figure 1.  ID surface of the as-received tubes.  The entire length was covered with a greenish-tan scale resembling dried mud.  There were intermittent deposits of blue–green color, as seen at the arrows. (10X Original Magnification)

 

The OD surfaces of the tube sections have short helical fins.  They were examined along their entire length ocularly and under a stereo microscope up to 40X magnification. There is no significant corrosion on the OD surface to indicate any activity that would lead to through-wall penetration and failure. 

The tube sections were saw cut lengthwise to examine the interior surfaces.  The tubes are internally enhanced by rifling.  The appearance of all tube section ID’s are essentially identical, displaying greenish-tan deposits along the entire length with intermittent areas of blue-green mounds in the valley of the rifled grooves, Figure 1. 

The overall appearance of the tube ID surfaces before and after cleaning in inhibited hydrochloric acid can be seen.  Comparing the two, one can see that where there was a blue-green deposit before cleaning, there is a black stain after cleaning.  Within each black stain there is a pit of varying size.  The pit density is about 1 per centimeter of tube length (2 per inch).
A pit is clearly within a lap (a surface discontinuity), as can be seen a higher magnification in Figures 2 & 3. 

 

 

Figure 2.   Large pit (approximately 1-mm in diameter) in the root of the rifling.  Note the extent of black staining that represents surface corrosion. (20X Original Magnification))

Figure 3.   Close-up of a pit.  The pit is clearly associated with a lap.  (40X Original Magnification)

 

Metallographic Analysis

A cross-section was taken transversely to the spiral fins through the pit.  The specimen was mounted in clear acrylic plastic and prepared according to ASTM E 3 (Metallographic Specimens) procedures, using an appropriate range of grinding and polishing papers.

Figure 4 shows the cross-section of the through-wall penetration.  There was a copious amount of corrosion product within the pit as well as product just outside the pit on the OD surface.  Figure 5 reveals "tunnel-like" features at the bottom of a pit.

 

  

Figure 4.   Through-wall penetration.  Note the copious corrosion product within the pit, as well as deposits on the OD. (62.5X Original Magnification)

Figure 5.  Cross-section of a pit.  Note the branching,  “tunnel-like” features at the bottom of the pit. (200X Original Magnification)

 Observed during the cross-sectional examination was the extensive amount of lapping in the root of the rifling.  Figure 6 show the extent of this artifact. 

Figure 7.  A close-up view of a lap seen  Note the build-up of a scale within the lap, at arrows.(500X Original Magnification)

 

Microanalytical Examination

A specimen from one tube was carbon-coated for conductance and placed on the goniometer stage of a scanning electron microscope (SEM) augmented with an energy dispersant x-ray spectrograph (EDS) to analyze for elemental composition. 

Several locations along the length of the tube specimen were analyzed close to the edge of a visible pit and within the pit itself.  Significant concentrations of chloride was present in the pit bottom.  The remaining elements were typical for water.

 

Discussion

Characterization of Tube Damage

Our examination of the tube sections received (cross-sectional metallurgical microscopy and microstructural analysis) indicates that the tubes are representative of refrigeration grade copper tubing (i.e., DHP copper, UNS C12200).   

The water we received did not appear to be overly aggressive, except for the pH level being on the acid side, and this may be explained by biological activity during shipment.  It has not been reported if a water treatment program was in place during the years of operation.  Nor has it been reported if the chemical balance of the cooling tower ever experienced upsets.

Based on the examination of the samples received as being representative of leaking tubes, it is apparent that the tubes have suffered from internal corrosion in the form of localized pitting.  There is no evidence of OD corrosion attack or OD mechanical deformation.  The internal pitting is extensive and along the entire length of each tube.  Pits range in size from incipient to through-wall penetration. 

The morphology of damage suggests that this has occurred over the years and is not a recent event. 

Analysis of Deposits

The extent of corrosion was distributed circumferentially aorund the ID with blue-green deposits.  Had there just been stagnation alone, the deposits would have formed along the bottom quadrant of the tube owing to gravitational settling.  But in this current situation, the deposits are 360 degrees around the tube, and therefore not related specifically to stagnant conditions.  For the deposits to have formed thusly, aggressive water conditions must have been present to establish cells of copper oxide and copper complexes, leading to mounds (or tubercles) of corrosion product on all quadrants of the tubes. 

Metallurgical Analysis

Examination of the cleaned tubes revealed the presence of laps in the root of the rifling.  These artifacts do present crevices where an occluded corrosion cell could be established under conditions of aggressive cooling water.   

The evidence that some of the pits are acutely associated with laps does not rule out their role in the pitting mechanism in the chiller.  In fact, Figure 6 shows evidence of an oxide layer within the lap that could just as easily have grown into a pit. However, since there are numerous pits not directly associated with laps in the rifling and present on the rifled sidewall and ridges, this manufacturing defect cannot be blamed for 100% of the damage.  

Copper tubing is usually quite forgiving on the variance in water chemistry associated with cooling tower systems and it requires an unusual set of circumstances for it to be attacked in the way observed during this investigation.  Based on the morphology of the cross-sectional pits examined, the initiation of the pits originated as “formicary corrosion” (also known as “ant-nest corrosion”) after tube manufacturer and before operation in December 1995.  It is known that lubricating fluids in the tube manufacturer and chiller fabrication can remain on the metal surface, and given the proper set of conditions of moisture, air and a warm environment, can decompose to primary organic acids.(1),(2)  These acids have been demonstrated to aggressively attack copper tubing in the heating, ventilation and air conditioning (HVAC) industry.

A formicary is defined as an ant’s nest, with its diversified and branching tunnels.  The root definition is based on the chemical species (formic acid, a carboxylic acid) used by ants to protect themselves as venom and to aid in the digestion of food. 

The morphology of the pits, observed in this investigation, resembles a formicary pit that has stopped propagating and has been replaced by another pit propagator, under deposit corrosion. 

 

(1) Corbett, R.A. and P. Elliott, “Ant-Nest Corrosion – Digging the Tunnels,” Corrosion 2000, Paper 00646.

(2) Tetley, G., M. Heidenreich and K. Smith, “The basics of Formicary Corrosion,” Air Conditioning, Heating & Refrigeration News, March 30, 1998, Vol. 203, No. 13, Serial No. 3598.

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