Another type of corrosion occurs at the drip edge of inert roofing material conducting water into a copper gutter or valley. When water, collected over a large surface, is diverted or collected by a relatively small copper flashing or gutter, the copper may deteriorate before it develops a protective patina. The acidic water is not neutralized as it flows over the inert material. This type, often called "erosion corrosion", occurs when rain falls on a non-copper roof such as tile, slate, wood, or asphalt. Traces of copper salts in the wash may accelerate corrosion of the aluminum.Īnother type of corrosion, which affects copper, is caused by the flow of acidic water concentrated on a small area of copper. Regardless of the method used to separate the metals, wash from copper surfaces should be prevented from draining onto exposed aluminum. In areas with severe exposure, lead or similar gasketing materials should be used, except between copper and aluminum. Taping or gasketing with nonabsorptive materials or sealants are effective methods of separating copper from all other metals. Either of these or a red lead primer can be effective in separating copper from iron and other ferrous metals. Bituminous or zinc chromate primers can be used between copper and aluminum. If paints or coatings are used for isolation, they must be compatible with both metals. Iron and steel are generally not a problem unless their mass is similar or smaller than that of the copper. The principal metals of concern in terms of direct contact are aluminum and zinc. It is not necessary to isolate copper from lead, tin or stainless steel under most circumstances. The solution is to prevent such direct contact with the use of separating materials, such as specific paints or gaskets. It will, however, cause corrosion of the other metals if in direct contact. Since copper has one of the highest galvanic numbers or nobility of the active metals, it will not be harmed by contact with any of them. The electrolyte may be rain water running from one surface to another, or moisture from the air containing enough acid to cause it to act as an electrolyte. When dissimilar metals are in contact with one another in the presence of an electrolyte, galvanic action occurs, resulting in the deterioration of the metal with the lower galvanic number. Table 1.3A. The Nobility of Common Metals Table 1.3A ranks the most common metals used in construction in increasing nobility, called the galvanic number. A greater relative difference in nobility between the two metals in contact indicates a greater corrosion potential. It is a measure of a metal's resistance to corrosion when in contact with another metal. Natural Weathering Color Chart - Typical for Moist Industrial Climates, Timespan Varies UnexposedĪll metals have a property called nobility. This patina eventually covers the surface and adheres tightly to it, thus providing a protective layer against further weathering. The acid is neutralized during the reaction with the copper. When acidic moisture comes in contact with exposed copper surfaces, it reacts with the copper to form copper sulfate. The process is, therefore, faster in some metropolitan, marine, and industrial areas, where higher concentrations of pollutants exist. The oxidation process that gives copper its characteristic green patina is a result of exposure to an acidic atmosphere. Chemical reactions are responsible for corrosion, staining, and even the green patina that develops on copper surfaces over time. One of the most important issues concerning the use of copper is the chemical reaction between copper and other materials. Architectural Considerations Weathering, Corrosion, Staining, Substrate, Solder, Sealants
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