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Corrosion of Heat Exchanger Copper Tube Assembly




Heat Exchanger


Copper Tubes


never used; 6 months in storage


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.

Figure 1. Fin surface showing water spots (2X Original Magnification)

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