Bimetallic or electrolytic corrosion with resulting rapid consumption of the zinc coating is likely if a galvanized article is installed in contact with brass or copper, particularly in a moist environment. Contact between aluminium, cadmium and galvanized surfaces is normally satisfactory.
Bimetallic corrosion occurs for the same electrochemical reasons as those by which zinc provides cathodic protection for steel but the rate of consumption of zinc coatings by galvanic corrosion may be extremely high.
A guide to compatibility of metals and alloys in contact is given in our manual After Fabrication Hot Dip Galvanizing.
Galvanized surfaces in contact
For maximum corrosion resistance under conditions of extreme humidity, overlapping galvanized surfaces should be isolated from each other by the application of an inhibitive jointing compound such as Dulux Foster C1 Mastic or equivalent. Alternatively a suitable paint may be used. Galvanized surfaces in contact with other materials may also require isolation.
Galvanized members in contact with aluminium conductors may require the use of an electrical conductive compound at joint faces to repel moisture and inhibit corrosion. Galvanizers Association of Australia can provide further information on this.
Copper and copper alloys
Bimetallic corrosion requires electrical contact in the presence of an electrolyte and cannot occur in the absence of these factors. However, run-off water from copper surfaces frequently contains small quantities of dissolved copper, sufficient to cause attack and rapid deterioration of zinc coatings through chemical deposition of copper.
Where use of copper or brass together with galvanized steel in the presence of an electrolyte cannot be avoided, precautions should be taken to prevent electrical contact between the dissimilar metals. Joint faces should be insulated using nonconducting gaskets or mastics and connections should be made with insulating grommet-type fasteners. The design should be arranged so that water flows from the galvanized surface onto the brass or copper surface and not the reverse.
A rubber ring separates the stainless steel wire from the hot dip galvanizing