Weathering, Corrosion, and Staining
Architectural Considerations: One of the most important issues concerning the use of copper is the chemical reaction between copper and other materials. Chemical reactions are responsible for corrosion, staining, and even the green patina that develops on copper surfaces over time.
Weathering and Patination:
Architectural Considerations: The oxidation process that gives copper its characteristic green patina is a result of exposure to an acidic atmosphere. The process is, therefore, faster in some metropolitan, marine, and industrial areas, where higher concentrations of pollutants exist. When acidic moisture comes in contact with exposed copper surfaces, it reacts with the copper to form copper sulfate. The acid is neutralized during the reaction with the copper. This patina eventually covers the surface and adheres tightly to it, thus providing a protective layer against further weathering.
All metals have a property called nobility. It is a measure of a metal’s resistance to corrosion when in contact with another metal. A greater relative difference in nobility between the two metals in contact indicates a greater corrosion potential.
Table 1.1.4 ranks the most common metals used in construction in increasing nobility, called the galvanic number.
The Nobility of Common Metals
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. The electrolyte may be rainwater running from one surface to another, or moisture from the air containing enough acid to cause it to act as an electrolyte.
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. It will, however, cause corrosion of the other metals if in direct contact. The solution is to prevent such direct contact with the use of separating materials, such as specific paints or gaskets.
It is not necessary to isolate copper from lead, tin or stainless steel under most circumstances. The principal metals of concern in terms of direct contact are aluminum and zinc.
Architectural Considerations: If paints or coatings are used for isolation, they must be compatible with both metals. Bituminous or zinc chromate primers can be used between copper and aluminum. Either of these or a red lead primer can be effective in separating copper from iron and other ferrous metals.
Taping or gasketing with nonabsorptive materials or sealants are effective methods of separating copper from all other metals. In areas with severe exposure, lead or similar gasketing materials should be used, except between copper and aluminum.
Regardless of the method used to separate the metals, wash from copper surfaces should be prevented from draining onto exposed aluminum. Traces of copper salts in the wash may accelerate corrosion of the aluminum.
Another type of corrosion, which affects copper, is caused by the flow of acidic water concentrated on a small area of copper. This type, often called “erosion corrosion”, occurs when rain falls on a non-copper roof such as tile, slate, wood, or asphalt. The acidic water is not neutralized as it flows over the inert material. 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. Another type of corrosion occurs at the drip edge of inert roofing material conducting water into a copper gutter or valley. If shingles rest directly on the copper, the corrosive effect is amplified because moisture is held along the edge by capillary action resulting in “line-corrosion”. The solution is to raise the lower edge of the shingles with a cant strip or to provide a replaceable reinforcing strip between the shingles and the copper.
The wash of water over copper surfaces can have additional impact. Moisture in contact with copper surfaces tends to pick up small quantities of copper salts. When this moisture contacts porous material, such as marble or limestone, it is absorbed. As the moisture evaporates, it leaves behind the copper salts as a stain on these materials. The green stain is particularly visible on light colored surfaces.
The condition does not occur with heavy rains or similar rapid run-off since the dwell time of the moisture on the copper is short and little copper salt is picked up. Staining results from the slow bleeding action of copper laden moisture.
There are a number of ways to reduce staining or its visual impact. Two common methods are: collecting run-off in gutters and directing it away from the building via downspouts; and designing drip edges to a minimum of one inch, helping reduce the amount of copper laden moisture that comes into contact with material below. Coating the adjacent surface of the porous material with a clear silicone sealant can reduce staining by minimizing the amount of moisture absorbed into the surface. The use of lead-coated copper results in a black or gray stain, which may blend better with some building materials.
Other materials used as substrates for copper include concrete, brick, masonry units, terra cotta, and stucco. The guidelines discussed above apply to these materials as well. Smooth, dry surfaces, compatibility with copper, and provision for fasteners are all required for an acceptable substrate.