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Under-Coating Corrosion of Tin Coated Copper

EQUIPMENT: Radiator Fins

ENVIRONMENT:

De-Ionized Water Spray

MATERIAL:

Tin Coated Copper

SERVICE TIME:

2 to 3 years

FAILURE:

Under-Coating Corrosion

 

The radiator fins are joined to copper tubing with 100% tin solder by dipping the pre-assembled parts in a molten tin bath. The radiator assemblies are fundamental parts of humidifiers installed in the paint rooms of an automotive assembly plant. The humidifying effect is achieved by spraying de-ionized water over the heated copper tubing. Excess water is collected and recirculated.

Figure 1. Pit initiation under coating (1000X Original Magnification)

Visual examination of the fins revealed areas of varying degrees of corrosive attack. Areas containing very light attack with small pin holes through the tin coating to heavily attacked areas where the copper substrate was exposed and through penetration pitting was observed.

SEM (scanning electron microscope) equipped with EDS (energy dispersive x-ray spectroscopy was used to identify the elemental components of the corrosion products. Green deposits observed on the surface were found to contain mainly tin, copper and chlorine. Green and black corrosion products from inside the pits were found to contain mainly copper, chlorine and sulfur. The chloride content of the corrosion products found in the pits was noticeably higher than that found on the surface.

 

A cross-section revealed substantial under-coating corrosion (Figure 1). In this type of corrosion, pits develop in the substrate material at imperfections in the coating. As the pit enlarges, the substrate no longer provides support for the coating. Eventually, the coating will either spall off or cave in. The progression of "cave-in" under coating corrosion is illustrated in Figures 2 and 3.

This failure was most likely caused as a result of the water supply becoming contaminated with sulfur and chloride compounds.

 
Figure 2. Tin coating "cave-in" (400X Original Magnification) Figure 3. Advanced pitting of copper under tin coating (200X Original Magnification )

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