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Stress Corrosion Cracking of Copper Tube in Evaporator 

ENVIRONMENT:

Refrigerant system

EQUIPMENT:

Evaporator

MATERIAL:

Two years

FAILURE MODE:

Stress Corrosion Cracking

 

Background

An ethylene glycol solution was circulated on the shell side of the evaporator. A refrigerant was circulated within the tubes.   The tubes were in place for 2 years and saw 1,354 hours of service. The failure allowed the glycol solution to enter the refrigerant system.  

Acetic anhydride is manufactured in the building where the evaporator is housed. The plant occasionally uses ammonia to treat plant water. It is not known whether the water in this system was ever treated with ammonia.  

As reported, the fracture occurred at a steel support within the evaporator. The mating end to the fracture was not retrieved from the evaporator.

 
Description of Material 
One smooth walled diameter copper tube approximately 13 long was received, Fig. At one end was a circumferential fracture.  The opposite end had been crimped shut.
 
Findings

The outside of the tube was of a uniform copper color. The fracture end of the tube was removed and the remaining tube split longitudinally. The internal surfaces of the tube were bright with several spots of discoloration running linearly along one section of the tube.

Visual analysis of the fracture end revealed substantial mechanical damage. The damage observed was consistent with damage that would be expected to have occurred during tube removal caused by rotation of the tube through a tight fitting orifice.  Substantial mechanical damage was also present on the circumferential fracture surface.   

The damage to the circumferential fracture surface was too severe to make any definitive comments.  

Small green deposits on the circumferential fracture surface
The fractured end of the tube was removed and placed into a scanning electron microscope (SEM) equipped with Energy Dispersive x-ray Spectroscopy (EDS). The green spots were analyzed using EDS for elemental composition, Table 1. These spots were comprised primarily of silicon and magnesium with minor quantities of iron, copper, sulfur and calcium. A major carbon peak was also present. The copper present in the spectrum was likely due to the copper tube background.  

 

Table 1.

EDS Analysis of Green Deposits

Elements

Wt%

Silicon

32.

Iron

22.

Copper

17

Magnesium

14

Calcium

6

Sulfur

6

 

Longitudinal through wall fractures extending from the circumferential fracture.

Visual examination revealed three primary longitudinal cracks on the outside diameter (OD) surface. The fracture end section was split to reveal the inside diameter (ID) surface. The cracks were present on the ID as well indicating through wall penetration.  

A SEM image reveals debris trapped in the narrow areas of the cracks (Figure 2). The debris was analyzed by EDS and found to contain primarily copper, silicon, and aluminum with minor quantities of chlorine (chloride) and potassium.  

 

 

Figure 1. Debris in narrow crack on OD surface (300X Original Magnification).

Figure 2. Microstructure of crack, dichromate etch, (500X Original Magnification).

 

A metallographic cross-section of a longitudinal crack was prepared. The polished surface was etched with a dichromate etchant to reveal the grain structure. Several fractures were observed to be initiating from the ID surface and propagating transgranularly, Figures 2.

The observed microstructure was consistent with refrigeration grade copper tubing. No unusual microstructural abnormalities were observed. 

 

Discussion

The longitudinal cracks observed are unusual and are not the result of the circumferential fracture. They were present before the final fracture of the tube occurred.   

The features of the longitudinal cracks are consistent with stress corrosion cracking of copper. While there are many compounds that may cause stress corrosion cracking in copper there are only two primary types of compounds, ammonia and nitrite. Liquid metals (i.e., mercury) are also known to cause stress cracking in copper but there is no evidence to support this type of occurrence.  

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