|
Return to
Failure Analysis Case Histories
|
Corrosion of Heat Exchanger Copper Tube Assembly
|
|
ENVIRONMENT:
|
Warehouse |
|
EQUIPMENT:
|
Heat Exchanger |
|
MATERIAL: |
Copper Tubes |
|
SERVICE TIME |
never used; 6 months in storage |
|
FAILURE:
|
Corrosion due to Contamination |
|
|
|
The heat exchanger tube assembly consisted of
copper tubes covered with copper fins joined together via U-bend copper tubes.
The finned portions of each tube were used in a Type 304 stainless steel sheath.
All of the copper parts were coated with 70Pb / 30Sn solder.
|
|
During assembly, the copper fins were soldered
onto the individual copper tubes. The soldered and finned tubes were then fitted
with a 304 stainless steel sheath. These assemblies were brazed to U-bends and
the brazed U-bend were coated with solder.
|
|
During storage, the assemblies discolored in an
area just above the brazed joints on the sheathed portion of the finned tubes.
The discoloration was a white and green powder on the solder coated copper and a
brown colored powder on the stainless steel sheath. This discoloration was
deemed unacceptable by the customer.
|
|
SEM (scanning electron microscope) equipped with
an EDS (energy dispersive x-ray spectroscopy) was used to identify the elemental
composition of the powders causing the discoloration. High levels of zinc and
chlorine were found in each of the powders. Copper, lead, tin, iron, chromium
and nickel were also found in the powdery deposits.
|
|
During the brazing process zinc chloride was used
as a fluxing compound. Insufficient removal of zinc chloride from the fins was
evidenced by the water spotting found on the discolored fins (Figure 1). The
spots were found to contain high levels of zinc and chlorine. The presence of
chlorides was causing the discoloration, which was an indication of corrosive attack.
The occurring corrosive attack was most likely due to contamination of the
surfaces by zinc chloride.
|
