Corrosion of Soldered and Brazed Joints in Tap Water

Abstract

Three test rigs of copper pipes joined with nine different solders or brazing alloys have been exposed to hot tap water in Copenhagen, Gothenburg and Oslo for 18 months under normal service conditions. The alloys investigated were the silver brazing alloys (hard solders) L–Ag55Sn, L–Ag34Sn and L–Ag18Sn, the copper–phosphorus brazing alloys (hard solders) L–Ag15P, L–Ag5P and L–CuP6, and the soft solders L–Sn5Ag and L–SnCu3.

In spite of the considerable differences in water analysis in the three cities, there were no major differences in the degree of corrosion of the joints. Nearly all the joints were noticeably corroded but the form varied with the solder or brazing alloy type.

In all three silver brazing alloys, selective corrosion resulted in deep corrosion attack of up to 3 mm/year. There was a wide variation in corrosion depth and some alloy/water combinations were completely free from corrosion. In all cases the copper–phosphorus alloys were not corroded but the copper adjacent to the joint in samples exposed in Copenhagen and Oslo was corroded up to 0·2mm/year. The soft solders all behaved similarly, irrespective of alloy or water quality: they suffered general corrosion of up to 0·3mm/year. The corrosion products, containing small, unattacked grains of one of the solder phases, remained attached to the solder surface.The rapid corrosion rate indicates that attention should be paid to the contribution of corrosion to the failure of soldered and brazedjoints in tap water. It should be noted that, although the joints were prepared under good workshop conditions, they did contain some cavities.