In Contact with Other Metals
Where zinc comes into contact with another metal, the potential for corrosion through a bimetallic couple exists. The extent of the corrosion depends upon the position of the other metal relative to zinc in the galvanic series, and to a lesser degree on the relative size of the surface area of the two metals in contact.
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.
For example, when using a bare 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 a zinc plate with a stainless steel rivet, the area ratio between the cathode and anode is reversed, and although more surface area is affected, the depth of penetration is small; the fastener should not fail because of corrosion.
The behavior of galvanized coatings in contact with various metals is summarized in the table below. The information given is provided as a guide to avoid situations where corrosion may occur when galvanized surfaces are in contact with another metal.
The following information provides more detail on other common metals used in construction that may come in contact with hot-dip galvanized steel.
Copper and Brass
If an installation requires contact between galvanized materials and copper or brass in a moist or humid environment, rapid corrosion of the zinc may occur. Even runoff water from copper or brass surfaces can contain enough dissolved copper to cause rapid corrosion. 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; connections should be made with insulating, grommet-type fasteners. The design should ensure water is not recirculated and water flows from the galvanized surface towards the copper or brass surface and not the reverse.
Aluminum and Stainless Steel
Under atmospheric conditions of moderate to mild humidity, contact between a galvanized surface and aluminum or stainless steel is unlikely to cause substantial incremental corrosion. However, under very humid conditions, the galvanized surface may require electrical isolation from the aluminum or stainless steel.
When galvanized bolts are used on weathering steel, the zinc will initially sacrifice itself until a protective layer of rust develops on the weathering steel. Once this rust layer develops, it forms an insulating layer that prevents further sacrificial action from the zinc. The zinc coating has to be thick enough to last until the rust layer forms, usually several years. Most hot-dip galvanized bolts have enough zinc coating to last until the protective rust layer develops on the weathering steel, with only a minimal loss in coating life.
|Metal in Contact||Rural||Industrial |
|Aluminum and aluminum alloys||0||0 to 1||0 to 1||1||2 to 3|
|Aluminum bronzes and silicon bronzes||0 to 1||1||1 to 2||1 to 2||2 to 3|
|Brasses including high tensile (HT) brass ( manganese bronze)||0 to 1||1||0 to 2||1 to 2||2 to 3|
|Cast Irons||0 to 1||1||1 to 2||1 to 2||1 to 3|
|Cast Iron (austenitic)||0 to 1||1||1 to 2||1 to 2||2 to 3|
|Chromium||0 to 1||1 to 2||1 to 2||1 to 2||2 to 3|
|Copper||0 to 1||1 to 2||1 to 2||1 to 2||2 to 3|
|Cupro-nickels||0 to 1||0 to 1||1 to 2||1 to 2||2 to 3|
|Gold||(0 to 1)||(1 to 2)||(1 to 2)||(1 to 2)||(2 to 3)|
|Gunmetals, phosphor bronzes and tine bronzes||0 to 1||1||1 to 2||1 to 2||2 to 3|
|Lead||0 to 1||0 to 1||0 to 1||0 to 2||(0 to 2)|
|Magnesium and Magnesium alloys||0||0||0||0||0|
|Nickel||0 to 1||1||1 to 2||1 to 2||2 to 3|
|Nickel copper alloys||0 to 1||1||1 to 2||1 to 2||2 to 3|
|Nickel-chromium-iron alloys||(0 to 1)||(1)||(1 to 2)||(1 to 2)||(1 to 3)|
|Nickel-chromium-molybdenum alloys||(0 to 1)||(1)||(1 to 2)||(1 to 2)||(1 to 3)|
|Nickel silvers||0 to 1||1||1 to 2||1 to 2||1 to 3|
|Platinum||(0 to 1)||(1 to 2)||(1 to 2)||(1 to 2)||(2 to 3)|
|Rhodium||(0 to 1)||(1 to 2)||(1 to 2)||(1 to 2)||(2 to 3)|
|Silver||(0 to 1)||(1 to 2)||(1 to 2)||(1 to 2)||(2 to 3)|
|Solders hard||0 to 1||1||1 to 2||1 to 2||2 to 3|
|Stainless Steel (austenitic and other grades containing approximately 13% chromium)||0 to 1||0 to 1||0 to 1||0 to 2||1 to 2|
|Stainless Steel (martensitic grades containing approximately 13% chromium)||0 to 1||0 to 1||0 to 1||0 to 2||1 to 2|
|Steels (carbon and low alloy)||0 to 1||1||1 to 2||1 to 2||1 to 2|
|Tin||0||0 to 1||1||1||1 to 2|
|Titanium and titanium alloys||(0 to 1)||(1)||(1 to 2)||(0 to 2)||(1 to 3)|
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