RELIABILITY OF TEST METHODS FOR METALLIC PRODUCTS - FINAL REPORT TO THE DRINKING WATER INSPECTORATE
DWI0825

December 2002

EXECUTIVE SUMMARY

I Benefits

The development of a short term laboratory procedure to test a metal's acceptability for use with drinking water would minimise the costs involved as well as facilitate the development of new alloys or the determination of limitations on use based on water compositions of established alloys.

II Objectives

  1. To Pilot and Round Robin test a variant of the test procedure used in the first Round Robin i.e. using disposal plastic centrifuge tubes as the reaction vessels and a more stringent control of temperature.
  2. To develop a statistical based method to determine the outcome of a test.

III Reasons

The results of the first Round Robin test (see WRc-NSF report DWI 4079 July 2002) showed the good intra-laboratory and poor inter-laboratory reproducibility observed in previous studies. Significant correlations between the concentrations of metals leached were observed a possible interpretation of which was competition for the dissolved oxygen in the test water. To test this idea, a procedure was devised using closed test tubes as the reaction vessels. By using disposable plastic centrifuge tubes, the new procedure also significantly reduced the cost of testing.

Although the Pilot established that the original ideas as to the cause of the correlations between the concentrations of metal leached was not correct, it was decided that a limited second Round Robin test of the new procedure would be of value.

IV Conclusions

  1. This work has developed a test procedure, based on plastic centrifuge tubes as the reaction vessels, that significantly increases the practical convenience and significantly reduces the cost of carrying out the testing of metals.
  2. Strong correlations have been found between the concentrations of the major elements, i.e. copper, lead and zinc, that leach from the copper alloys. All show an exponential increase in concentration with time in the leachate water.
  3. The amount of metal leached showed little sensitivity to the depth of immersion of the test coupon, suggesting little sensitivity to the level of dissolved oxygen in the test water. This in turn suggests that the contamination level achieved cannot be under the simple control of the alloy corrosion rate.
  4. The exponential increases with time of a particular metal in the leachate water, found by the different test laboratories, in most cases do not appear to be going to the same equilibrium value. This suggests that the contamination level achieved cannot be under the simple control of the solubility of the corrosion product formed.
  5. The levels of copper and zinc leached in the tests were so far below their respective PCV's that the acceptability for these elements of the alloys tested could be made unambiguously. This was not the case for lead.
  6. Although the differences between measured lead concentrations at the three laboratories were considerable, a statistical procedure to decide the outcome of the test, that takes these 'between laboratory' differences into account, can be suggested.

Copies of this report may be available as an Acrobat pdf download under the 'Post 2000 Reports' heading on the DWI website.