Sit-and-soak tests

Until now simplified methods have been available to monitor metal release by metallic pipe materials. In the United Kingdom a sit-and-soak test is available to determine the metal release from drinking water materials. In The Netherlands, copper and lead pipes in combination with water quality are tested in pipe rigs, and in Germany a protocol for testing pipe materials is available (see below). 

Sit-and-soak tests have been earned out with BS 7766 standard test water and with laboratory tap waters (composition of waters given in Table 4.3). Pipe rig experiments have been carried out with laboratory tap waters, and four materials have been tested copper, brass (eopper with brass fittings for pipe rigs), galvanized steel, and stainless steel. 

Table 4.2 Summary of the laboratory sit-and-soak test procedure (based on BS 7766, 1994)...

Achieving reproducibility with a laboratory sit-and-soak test... 

Having demonstrated the prineiple of a laboratory test, the Conormative Group considered it the task of others to develop a specific test protocol. The question remaining for the current projeet was to consider to what extent a sit-and-soak test represents the situation in authentic plumbing systems and to what extent any deviation is significant. 

The short term nature of the laboratory sit-and-soak test probably means any effect of this nature will be missed by the cmrent test procedure. 

Surface finish of the test coupon Various manufacturing processes can change the physical form of the metal s initial surface or produce surface films, either of which may influence the eharacteristies of the corrosion deposit that is developed when in contact with water. These effeets may influence or change the mechanism controlling the level of eontamination. The standard sit-and-soak test defines a machined smface that will not be representative of a cast surface, for example. In the first set of eonormative tests, aetual galvanized steel pipe was used to overcome precisely this sort of problem. 

Ejfect of flow regime experienced The speed with which a metal s surface comes to equilibrium with a water, and in some cases the characteristics of the corrosion product which is developed, can depend on the flow regime that it has experienced. At present, the standard sit-and-soak test defines static conditions. However, if this mechanism was thought potentially to be significant, different flow regimes could be incorporated into an ageing procedure to cover this. 

In the current project, sit-and-soak tests were carried out using a range of waters. This could be made the standard procedure when testing a metal for general acceptability. The control mechanisms covered by the sit-and-soak test used in this project, and its potential for adaptation, are summarized in Table 4.12. 

Galvanic interaction Not covered by the current sit-and-soak test but galvanic test couples have been used in the past... 

Surface finish of metal Not covered by current sit-and-soak test but the difficulty has been met before by using manufactured pipe sections as test pieces may be practically more difficult with fittings... 

Information from reference rigs is more complete and closer to reality than is information given by a simple sit-and-soak test where only 24-hour stagnation samples are analysed over a short period of time (2 weeks). However, adaptations of sit-and-soak tests could be considered as getting more information on diiferent possible mechanisms (Table 4.13) or to produce stagnation curves. However, static tests cannot cover the influence of flow conditions, which appear also to be an important factor in authentic situations. 

Table 4.13 Comparison of the advantages and disadvantages of a sit-and-soak test versus rig tests...

Experiments carried out with sit-and-soak tests show that it is possible to get reproducible results within laboratories. However, the test has proved to be very sensitive to slight variations in procedure. Further development and improvements in the definition of the procedure will be necessary to achieve reliable inter-laboratory reproducibility. 

The sit-and-soak test procedure used in this study, adapted finm that in BS 7766, tacitly assiunes that the migration of metals in water is governed by the solubility of corrosion products. However, adaptations of the procedure could be possible to cover many of the other potentially controlling mechanisms. 

A simple sit-and-soak test procedure (or a set of such tests) can be useful in identifying which controlling mechanisms are operating in a particular situation. However, there appears to be no ready way of correlating the results of such static tests with those contamination levels expected in authentic situations. Consequently sit-and-soak test procedures cannot be reliably used to produce an absolute evaluation of the potential migration fix)m any particular metal. 

---