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Corrosion Fatigue of Copper Tubes in a Subcooler

ENVIRONMENT:

Refrigerant and water

EQUIPMENT:

Condenser

MATERIAL:

Copper
SERVICE TIME: 5 to 6 months

FAILURE MODE:

Corrosion Fatigue

 

Summary

Longitudinal corrosion fatigue cracks developed in preexisting defects (notches) on the outer surface of the tube near the lands.  The corrosion fatigue cracks served as precursors for the circumferential fatigue cracks that lead to the ultimate failure. Corrosive chemical species in the refrigerant on the outside of the tube contributed to the failure.

 

Background

The submitted tube was removed from the condenser/subcooler in the chiller unit that had been in service for five to six months.   The refrigerant was circulated on the shell side of the tube and cooling tower water was on the tube side.

 

Description of Material

CTL received seven sections of pipe, Figure 1.  In service, four lands about 2-inch long supported the tube and occurred about every 45-inch.  A flat section on the bottom characterized the lands and top of the tube and two indentation marks 2-inch apart on each side of the land.  The overall length of the tube was about 16-ft.

 

Figure 1.   The relative location of the lands and tube sheet with respect to the copper tubing is revealed. 

A crack occurred toward the end of the second land; the final separation of the crack tube occurred during the removal process and was characterized by wall thinning and a ductile tear.  It was assumed that this tearing occurred as the tube was being extracted. The side of the tube had a circumferential thick crack.   
Findings

The outside of the tube was of a uniform new copper color.  The subject fracture occurred on the second land, Figure 1.  An examination of the fracture under a stereoscopic microscope revealed that through-wall longitudinal cracks were present, Figure 2.  In this area it was not possible to determine which cracks occurred first or their origin.

Fine longitudinal cracks were found on the third land on the bottom side of the tube. Circumferential cracks could be found originating from some of the longitudinal cracks.

At higher magnifications, shallow pits a few mils in depth could be found on the outside surface of the tube in the same area as the longitudinal cracks, shown in Figure 3. 

A cross section of the copper tubing was taken near Land #3 in the same area that the longitudinal cracks were found.  Small defects in the outer surface of the copper tube were detected that have the general form of v-shaped notches.  Cracks initiated at the bottom of many of these notches. 

One of the cross sections with longitudinal cracks discussed above was etched.  The longitudinal cracks contained a large amount of oxides, Figure 4.

 

Figure 2. Fracture surface with the stereoscopic microscope.  Note the longitudinal cracks.  (11X original magnification)

Figure 3. Shallow pits near the longitudinal cracks on Land #3  (125X original magnification)
 

Figure 4. Etched crack section.  Crack full of oxide corrosion products.  (1250 X Original Magnification)

 

DISCUSSION

Several modes of failure were in progress simultaneously to reduce the life of this tube.  The mode that resulted in the failure of the tube was a fatigue crack that ran, for the most part, circumferentially around the tube.  It was assumed that the ductile part of this fracture occurred as the tube was being removed. 

The nature of the longitudinal cracks supports corrosion accelerated by fatigue, or corrosion fatigue.  The small mechanical defects, assumed to be present from the drawing of the tube, served as the nucleation site of the corrosion/fatigue.  The oxide corrosion product in these cracks and the branching, transgranular structure of the cracks provided further support that corrosion was active.  The pits, although not directly implicated in the failure, provided additional evidence that corrosive conditions existed on the outside of the tube.  Ultimately, the longitudinal cracks became the stress concentrators for the circumferential fatigue crack.  The fatigue crack started in one of the branches of the longitudinal cracks in the very high stress area next to the land.  Once the fatigue crack started, failure was rapid in the tube already weakened by the longitudinal cracks. 

Chemical analysis both inside and outside of the tube provided additional confirmation that the cracks originated on the outside of the tube.  The particles found in the tube contained some chlorides and chemicals normally associated with treated cooling water (Si, P, Ca, Al).  In contrast, the corrosion products in the crack only contained aluminum, oxygen, copper, and sulfur. 

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