Plumbing Tubes: Mixing Materials

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.

Behaviour of mixed metals

Copper metals are almost always cathodic to other common structural metals such as steel, aluminium and zinc. When steel, aluminium or zinc is put in contact with a copper metal, the corrosion rate of the steel, aluminium or zinc increases where as that of the copper metal decreases. When copper metals are coupled to the normal grades of stainless steel variable behaviour is experienced; copper metals may be either anodic or cathodic to the stainless steel depending on conditions of exposure.

Copper metals usually corrode preferentially when coupled with high nickel alloys, titanium or graphite.

When Copper is alloyed with zinc to form the brasses or aluminium to form the aluminium bronzes, the potential is moved towards the anodic (more electro negative) end of the range; additions of tin to form the bronze alloys or nickel to form the cupro-nickels move the potential towards the cathodic (less electro negative) end. Galvanic corrosion between two metals is seldom a significant problem because the difference in potential is small.

The Galvanic series

The galvanic series of metals and alloys provides a more practical means of predicting the corrosion behaviour of dissimilar metals but should not be used without the consideration of the conditions present in the system. Galvanic series generally refer to metals and alloys under specific conditions.

Table 5 on the next page, lists a galvanic series of metals and alloys valid for dilute solutions such as seawater and weak acids. The metals that are grouped together may be coupled to each other without significant galvanic damage.

However, connecting metals from different groups leads to damage of the more anodic metals; the larger the difference in galvanic potential between groups the greater the corrosion is likely to be.

Table 5:

Galvanic Series in seawater

Mixing Material

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