Galvanic corrosion (bimetallic corrosion) of a hot water pipe

A soldered joint in a hot water pipe fell apart after the tin solder corroded. All that remained was the nobler, two-layer Cu-Sn compound layer (Cu6Sn5) on the copper tube (Fig. 2), whose grain structure appears like a freely solidified surface in the SEM (Fig. 3). Figures 4-6 show the already half-corroded adjacent soldered joint in parallel cut.