Hot-Dip Galvanizing Performance in Contact with Other Metals

Where Zinc contact with another metal under atmospheric or aqueous condition, the potential for corrosion through a bimetallic couple exists. The extent of the corrosion will depend upon the position of the other metal relative to the zinc in the galvanic series, the relative surface areas of the two metals in contact, and the conductivity of the electrolyte on the surface of the two metals.

If an installation requires contact between galvanized materials and copper or brass in a moist or humid environment, rapid corrosion may occur. Even runoff water from copper or brass surfaces can contain enough dissolved copper to cause rapid corrosion of zinc coatings. If the use of copper or brass in contact with galvanized items is unavoidable, precautions should be taken to prevent electrical contact between the two metals. Joint faces should be insulated with non-conducting gaskets and connections should be made with insulating grommet-type fasteners. The design should ensure that water is not recirculated and flows from the galvanized surface towards the copper or brass surface, and not the reverse.

Any time a bimetallic assembly contains metal systems that are subject to galvanic corrosion, the ratio of the cathodic area to that of the anode must be carefully considered. The corrosion current that flows between the cathode and anode is independent of area, but the rate of penetration at the anode is dependent on the current per unit area, that is, current density. Therefore, it is undesirable to have a large cathode surface in contact with a small anode surface. The rate of penetration from corrosion increases as the ratio of the cathode to anode surface area increases. When using a stainless steel plate with a zinc rivet, the ratio of the cathode surface area to the anode surface area is large, and the rivet will fail rapidly because of accelerated corrosion. When combining zinc plate with stainless steel rivet, the area ratio between cathode and anode is reversed, although more surface area affected, the depth of penetration small; the fastener shouldn't fail because of corrosion.

Under atmospheric conditions of moderate to mild humidity, contact between a galvanized surface and aluminum or stainless steel is unlikely to cause substantial galvanic corrosion. However, under very humid conditions, the galvanized surface may require electrical isolation through the use of paint or joining compounds. The galvanic behavior of galvanized coatings in contact with various metals in atmospheric and immersion environments is summarized below.

General Notes: Ratings in brackets are based on very limited evidence and hence are less certain than other values shown. The table is in terms of additional corrosion and the symbol “0” should not be taken to imply that the metals in contact need no protection under all conditions of exposure.

Source: British Standard Institute, pp 6484: 1979, Table 23

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