![]() ![]() In theory HDG steel may be successfully combined with painted steel, but this assumes a durable paint coating system which is diligently maintained. As zinc coating thickness is directly related to coating longevity, the component with the thinnest zinc coating will be the first to experience corrosion. Common examples include the use of mechanically galvanized bolts to connect high-strength HDG structural steel, and connections between metallized bridge girders and galvanized cross-braces. There is no fear of galvanic corrosion when galvanized steel is in contact with other zinc coatings. 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. Joint faces should be insulated with non-conducting gaskets connections should be made with insulating, grommet-type fasteners. 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. ![]() Even runoff water from copper or brass surfaces can contain enough dissolved copper to cause rapid corrosion. 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. For more detailed information about galvanic corrosion and hot-dip galvanizing in contact with dissimilar metals, see our primary page on Dissimilar Metals in Contact. The following information provides more detail on other common metals used in construction that may come in contact with hot-dip galvanized steel. 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 behavior of galvanized coatings in contact with various metals is summarized in the table below. 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. If corrosion is present on both hot- and cold-water pipes, have your house water evaluated by a water-testing laboratory.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.Be sure to change the water heater’s anode rods every three to five years. Iron deposits from a rusty water heater can also corrode copper pipes.The excess flux gets carried through the pipe and deposited along the way, pitting the interior. On cold-water piping, pitting and corrosion is often caused by an excess amount of flux residue. ![]()
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