Report No DWI0017

Dec 1986

The objectives of the reported work were to measure the corrosion potentials of lead solder coupled to copper (using corrosion cells) for selected water supplies, in collaboration with water undertakings, and assess the factors controlling the level of corrosion potential. Additional objectives were to test the effect of treatment to reduce corrosion potential and to identify methods for the local slow-release of zinc corrosion inhibitor into plumbing systems.

The work was carried out in three phases. In Phase I the corrosion potentials of thirty-eight different waters were measured using corrosion cells. The waters were selected to represent typical extremes of water quality parameters previously established to have significant effects on the galvanic corrosion.

In Phase II various treatments were tested on four different waters which had been selected for their high corrosion potentials. The treatments tested were the dosing of silicate, sulphate, zinc and orthophosphates, and the last two in combination. Corrosion potentials again were measured using corrosion cells.

In Phase III a review was made of methods for small-scale dosing of zinc, a cost comparison was made of alternatives for dealing with lead solder and tests were carried out on the dissolution of zinc-rich polyphosphate glasses in water.

The survey of corrosion potentials in water supplies showed that galvanic corrosion potential is increased by increase in chloride and nitrate concentrations. The effect of chloride depends on its ratio with sulphate. Corrosion potential is decreased by increase in sulphate and silicate.

The tests on treatments to reduce corrosion showed that zinc dosing, especially in combination with orthophosphate, was the most effective treatment for all waters.

The review of methods for dosing zinc identified the use of sacrificial anodes, dosing solutions of zinc salts with pumps and the use of zinc-rich glasses as the most feasible. The subsequent cost comparison of alternative strategies showed that the use of sacrificial anodes would probable be more expensive than substituting lead-free solder for lead solder in new plumbing. The comparison also showed flushing to waste water most likely to be contaminated is a low-cost practical solution for small buildings. Also for small systems dosing of zinc using glasses is cheaper than dosing solutions with pumps.

Contamination of tap water by lead solder may be more widespread than generally appreciated. The risk of contamination is greatest in institutional buildings. High corrosion potentials are associated with waters having high chloride-sulphate concentration ratios and high nitrate concentrations.

Dosing of zinc, especially in combination with orthophosphate, is an effective treatment for reducing corrosion potentials. Dosing zinc solutions by pump is more reliable than use of zinc-rich polyphosphate glasses. Flushing to waste is a low-cost practical solution for small buildings. The cost of substituting non-lead for lead solders in new plumbing systems is relatively small.

The merits of flushing to waste tap water most likely to be contaminated by lead solder should be publicised.

In view of its relatively low cost, simplicity and permanence early consideration should be given to ensuring the use of non-lead solders in new buildings.

Copies of this report may be available as an Acrobat pdf download under the 'Find Completed Research' heading on the DWI website.