Copper roofing

Very often the environment is reflected in the composition of corrosion products, eg, the composition of the green patina formed on copper roofs over a period of years. The determination of the chemical composition of this green patina was one of the first systematic corrosion studies ever made (see Copper). The composition varied considerably depending on the location of the stmcture as shown in Table 2 (26,27). 

A note of caution about roof fasteners. A common mistake is to fix a galvanised or aluminium roof in place with nails or screws of a different metal copper or brass, for instance. The copper acts as cathode, and the zinc or aluminium corrodes away rapidly near to the fastening. A similar sort of goof has been known to occur when copper roofing sheet has been secured with steel nails. As Fig. 24.6 shows, this sort of situation leads to catastrophically rapid corrosion not only because the iron is anodic, but because it is so easy for the electrons generated by the anodic corrosion to get away to the large copper cathode. 

Drainage water from copper affects zinc in a similar way. Zinc sheets must never be fixed with copper nails, nor should copper roofs drain into zinc or galvanised gutters. Copper lightning arrestors provide further potential hazards to zinc work when a copper lightning strip has to pass over or near a zinc roof, it should be either well insulated or heavily tinned. 

Many metals are fairly easily oxidized. The atmosphere contains a powerful oxidizing agent oxygen. Because metals are constantly in contact with oxygen, they are vulnerable to corrosion. In fact, the term corrosion is sometimes defined as the oxidation of metals exposed to the environment. In North America, about 20% to 25% of iron and steel production is used to replace objects that have been damaged or destroyed by corrosion. However, not all corrosion is harmful. For example, the green layer formed by the corrosion of a copper roof is considered attractive by many people. 

In more exposed environments, copper roofs and pipes quickly become covered in verdigris. Verdigris is green in colour (Figure 10.26) and is composed of copper salts formed on copper. The composition of verdigris varies depending on the atmospheric conditions, but includes mixed copper(n) carbonate and copper(n) hydroxide (CuC03.Cu(0H)2). 

Figure 10.26 Verdigris soon covers copper roofs in exposed environments.

Copper roofs and bronze statues, such as the Statue of Liberty, turn green in air because Cu3(0H)4S04 and Cu4(0H)6S04 form. 

Copper is a moderately active metal. It dissolves in most acids and in alkalis. An alkali is a chemical with properties opposite those of an acid. Sodium hydroxide, commonly found in bleach and drain cleaners, is an example of an alkali. An important chemical property of copper is the way it reacts with oxygen. In moist air, it combines with water and carbon dioxide. The product of this reaction is called hydrated copper carbonate (Cu2(0H)2C03), which changes copper s reddish-brown color to a beautiful greenish color, called a patina. Copper roofs eventually develop this color. 

As soon as one comes across a green copper roof , one should attempt to remove the green layer until one reaches the pure red-brown copper metal. If one dissolves the green substance in diluted hydrochloric acid, bubbles form. The addition of limewater helps to prove the existence of carbon dioxide. The green substance must be a type of copper carbonate, a completely different substance than the red-brown metal. The formation of green carbonate can be explained by reactions of copper with the solution of carbon dioxide in rainwater, or the formation of blue copper sulfate by the reaction of copper with industrial acidic rain . 

Different properties have to be associated with different substances. Comparing the green substance of the copper roof with various copper carbonates, copper sulfates and copper chlorides, one realizes that there are different green or blue copper compounds, but no green copper exists The discussion should result in the statement that copper is covered with a green copper compound, called copper hydroxo carbonate. 

The evaluation of the data shows that approximately half of the ordinary level students attributed the precipitation of copper by using familiar everyday explanations. These explanations are at times so dominant, that even the explicitly named copper color is not taken into account iron comes into contact with the copper and colors it green the copper-colored coating is green-turquoise-blue . In these cases, copper is associated with the color green of well-known copper roofs which are really coated by a special copper carbonate compound. 

Producing verdigris artificially on a copper object isn t an easy task. Artists usually resort to using acrylic paints to mimic the natural reaction of copper with acidic compounds in the air. By the way, the patina on copper roofs is a mixture of copper carbonate and copper sulfate. 

Copper roofs turn green (so-called patina). 

One of the most familiar redox reactions is corrosion of a metal (T FIGURE 4.11). In some instances corrosion is limited to the surface of the metal, with the green coating that forms on copper roofs and statues being one such case. In other instances the corrosion goes deeper, eventually compromising the structural int ity of the metaL Iron rusting is an important example. 

For centuries copper has been used as an architectural material. Copper roofs are still in existence on many castles and monumental buildings that are hundreds of centuries old. 

Copper experiences a low corrosion rate when exposed to atmospheric conditions. Because of this copper has long been used for building structures such as roofs, facades, and gutters. Many copper roofs have lasted for centuries on castles and other monumental buildings. 

Perhaps you have never heard of an oxidation and reduction reaction. However, this type of reaction has many important applications in your everyday life. When you see tarnish on a silver spoon, rust on iron tools, or the green patina on a copper roof, you are observing oxidation. 

Where copper is used as flashing on roofs, corrosion has been encountered at the edge of the shingles as a continuous groove. This effect is more pronounced when the atmosphere contains both chlorides and sulfides, and with wood shingles as compared with roofs of other composition. Tests indicate that all-copper roofe 0.5 mm or more in thickness would last several centuries in urban atmospheres. 

Copper and copper alloys are well-known for their excellent resistance to corrosion in corroding environments. Copper is one of the earliest metal known to mankind and it was the most exploited material because of its ease of extraction from the ore. Copper alloy artifacts have been found in excellent conditions after having been buried for thousands of years. Copper roofing in rural areas have been found to corrode at a rate less than 0.4 mm after 200 years. Several statues of bronze and brass stood for hundreds of years before being destroyed by environmental pollutants. Copper artifacts and copper roofs, hundreds of years old, in historical buildings and castles are still in good state of preservation. 

An old copper roof, like that on Chartres Cathedral, will have a pale green tint—the result of copper oxidizing over the centuries—called the patina. But you can also apply patina to your metal projects to create an aged look. 

Green patina finishes. The much admired natural protective coating of a blue-green patina characterizes older copper roofs, including ancient cathedrals, as well as bronze statues and other copper metal surfaces exposed to the weather. Because of the time required to achieve this, much research has been done on artificial patina. The major coloring agent in natural patina is a film of basic copper sulfate. Carbonate and chloride salts of copper may also be present in varying concentrations. In seacoast locations, chloride salts may form an essential part of the patina film. The basic chloride salts of copper are not only fairly soluble but photosensitive as well. ... 

However, nniform corrosion is relatively easily measnred and predicted, making disastrons failnres relatively rare. In many cases, it is objectionable only from an appearance standpoint. As corrosion occnrs nniformly over the entire snrface of the metal component, it can be practically controlled by cathodic protection, nse of coatings or paints, or simply by specifying a corrosion allowance. In other cases, nniform corrosion adds color and appeal to a surface. Two classic examples in this respect are the patina created by naturally tarnishing copper roofs and the rust hues produced on weathering steels. 

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