|Soil Type||pH||Years Buried||Average Corrosion Rate (mm)||Maximum Pitting Rate (mm)|
|Wet Acid Peat||4.2||10.5||0.007||0.02|
|Moist Acid Clay||4.6||10.5||0.005||0.04|
|Made Up Ground||9.8||0.0003||0.002|
|Dry Acid Sound||5.1||11.0||0.0002||0.0002|
|Moist Normally Clay||7.2||11.0||0.0002||0.0002|
|Slightly Acid Sand||6.2||11.0||0.0001||0.0002|
It would appear that where serious corrosion occurs, it is not due to any one characteristic of the soil or climate, but to a combination of high sulphate and/or chloride content with poor drainage and considerable moisture capacity and a moderate or heavy rainfall.
Soils containing a large proportion of organic matter and moisture such as tidal marsh, peat and muck possess the combination of properties mentioned above as particularly conducive to corrosion. Cinders when left moist are highly corrosive, either because of the very aggressive sulphide in them or possibly owing to galvanic action between the carbon in them and copper.
In a test conducted by both the National Bureau of Standards in the United States and the BNF Metal Research in the UK, copper proved to be the most satisfactory material.
A simple calculation shows that for all but the most aggressive soils, class 2 copper tube would provide excellent service, however, if any doubt exists as to the corrosivity of the soil, heavy wall class 3 tubing will provide superior service. It is further recommended that tubes be taped with CXP tape or Denso tape. When using Denso make sure that the tape is smoothed to seal properly.
Many existing domestic water systems provide excellent examples of mixed metal systems. Copper hot water cylinders, zinc coated steel pipes and brass taps. There is no record of serious galvanic corrosion problems from such systems largely as a result of the water quality and the favourable anode/cathode area ratio. An additional corrosion mechanism falling within the realms of galvanic corrosion should be considered. In a mixed metal plumbing system it is a well-documented fact that copper ions taken into solution from the copper portions of the system can cause corrosion of the downstream zinc coated system. What happens is that the copper ions plate out on the less noble zinc and once plated out form a localised cathode of an active galvanic cell. However, having made such a statement the practical situation is somewhat different. Reef waters - our electrolyte - at the present pH and TDS ranges quickly forms an oxide layer in the copper portion of the system which effectively prevents copper ions entering the solution and in addition forms a protective scale on the zinc coating preventing the plating out of the copper ions. It should be emphasised, however, that new mixed metal systems are most susceptible to this form of corrosion and it is strongly recommended that copper tubing always be installed downstream to zinc coated steel in new installations.
The high incidence of corrosion failures in zinc coated steel systems, however, raises a number of questions when replacement is considered. Copper tubing can be used to replace any portion of a failed system which has been in service a number of years as exposed zinc metal is unlikely to be present. Possible subsequent failure of the remaining old installation cannot be attributed to the installation of the copper and the advisability of partial replacement of a system should receive careful consideration.
Copper and zinc feature in many modern building applications. Copper and galvanised sheeting for roofing purposes and brass and copper for fasteners. Zinc sheets should never be fixed with copper nails nor should brass screws be used to attach aluminium plates. Copper roofs should not drain into zinc or galvanised gutters for the same reason that zinc coated pipe should not be installed downstream from copper tubing.
An electro-chemical potential almost always exists between two dissimilar metals when they are immersed in a conductive solution, if two dissimilar metals are in electrical contact with each other and immersed in a conductive solution, a potential results which electropositive member (the cathode).
Although we often refer to the metals being immersed in the electrolyte we must not forget that the same conditions exist when the electrolyte is contained in a tube or a pipe.