Sterling silver, an alloy of silver and copper, contains two metallic elements; steel contains one metallic element iron and one non-metallic element carbon. Brasses are alloys of mainly copper and zinc, with small percentages of other elements such as tin, lead or arsenic. In many alloys, corrosion may result in the loss of the more reactive component of the alloy, with the less reactive component left behind. General terms for this process are "dealloying," "selective corrosion" or "selective leaching.
The actual mechanism of dezincification is still not completely agreed upon. For many years, there were two competing proposals Weisser In one, the zinc preferentially corrodes and is removed from the alloy, leaving the copper behind. In the other, both the copper and zinc corrode and are removed from the alloy, but the copper ions in solution plate back onto the surface.
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In the first proposal, the surface of the metal should become porous after dezincification but, otherwise, should not change. The second proposal, although more complicated, is needed to explain cases where copper crystals appear on the surface after dezincification Walker In this mechanism, the zinc dissolves out of the brass, leaving behind the copper, and then the copper rearranges on the surface of the metal, leading to the formation of copper crystals. This rearrangement is possible because the copper on the surface is attracted by negative ions in the solution.
The attraction is not enough to dissolve the copper, but it does weaken the binding of the copper to the surface, allowing the copper to move around more rapidly Erlebacher et al. Dezincification usually occurs in relatively mild conditions, such as in slightly acidic or alkaline solutions Moss For example, Weisser observed the dezincification of a brass object after treatment in an alkaline solution.
In strong acids, however, both the copper and zinc dissolve and the surface is not enriched in copper. Dezincification can also occur when brass is exposed to solutions that contain chloride ions, such as seawater Moss One example is dezincification of brass doors and brass kickplates that had been exposed to de-icing salt. Morissette reported dezincification of a set of brass doors that had changed colour from yellow to pink after being cleaned with hydrochloric acid.
There are several possible atomic arrangements of copper and zinc in brass, but only the alpha and beta phases are important in commercial brasses. Commercial brasses are either alpha brass or duplex brass. Duplex brass is even more prone to dezincification than alpha brass Scott The first sign of dezincification of brass is a change in colour, from the yellow typically found in brass to the salmon pink of pure copper metal. The pink colour may then turn reddish and then brown if the surface copper corrodes to form cuprite. More severe dezincification produces a porous, weak metal, which is mainly copper Dinnappa and Mayanna Severe dezincification of brass plumbing fixtures can perforate the brass and cause leaks.
Figure 1 compares the colour of brass to the colours of pure copper and zinc. CCI Figure 1. From top to bottom: cartridge brass, pure copper and pure zinc. When a metal or an alloy corrodes, metal atoms lose electrons through an electrochemical reaction and either dissolve in the solution as ions or are incorporated into a corrosion product such as an oxide.
In an alloy, as opposed to a pure metal, the more reactive component has a greater tendency to react. In brass, zinc is more reactive than copper, and so the zinc is lost preferentially. The relative reactivity of zinc and copper can be estimated through their position on electrochemical scales.
There are two common scales that can be used for this estimate. Alternatively, zinc and copper can be compared using the galvanic series, which gives the potentials of metals measured in some solution, usually seawater. Here, zinc is about 0. The galvanic series is discussed further in the CCI learning resource Understanding galvanic corrosion. Figure 2 shows a French horn affected by dezincification; for comparison, Figure 3 shows a similar horn in pristine condition.
The horns are made of brass, and the moving slides, supports and stays are made of nickel-silver an alloy of copper, zinc and nickel. The dezincified horn was used in a school band for about thirty years, and was rarely, if ever, polished or cleaned. The dezincification was caused by the handling of the horn with bare hands. In the musical community, dezincification of brass in musical instruments is called "red rot," but this term is used more often in conservation to describe the deterioration of leather. CCI Figure 2.
French horn made circa , showing pink areas typical of dezincification. CCI Figure 3. Modern French horn made in showing the typical yellow colour of polished brass. Some commercial polishes marketed for copper alloys are acidic and can cause dezincification. This is normally not noticed, because the polish also contains an abrasive. When the polish is rubbed on the surface, the abrasive removes the copper-rich surface as fast as the dezincification occurs.
If the polish is left on the surface, however, dezincification can be observed. Figure 4 shows a brass tray with dezincification produced by a commercial polish that contained citric acid. The pink areas in the photograph were originally covered with masking tape, which was applied around a small rectangular strip that was left uncovered. The central strip and the masking tape around it were covered with acidic commercial polish and left overnight. After the polish was rubbed off, the central strip was clean and shiny, probably because of the abrasive in the polish.
When the masking tape was pulled off, the pink areas were revealed.
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Those areas had undergone dezincification because liquid from the polish had seeped under or through the tape. Dezincification also occurs when brass is cleaned with a mixture of salt and vinegar. CCI Figure 4. Part of a brass tray after exposure to an acidic commercial polish overnight. The pink regions, which had been covered with masking tape, have been affected by dezincification, whereas any signs of dezincification on the central strip, where there was no tape, were removed when the polish was rubbed off.
Other elements can be added in brass with higher zinc concentrations to make the brass more resistant to dezincification. Ancient brass alloys, which generally contain tin or impurity elements, resist dezincification better than modern brass alloys that contain only copper and zinc Scott Modern tin brass has about 0. When this amount of tin is added to cartridge brass, the resulting alloy is called admiralty brass.
Further protection against dezincification is obtained if smaller amounts of arsenic, antimony or phosphorus, in the range of 0. Today's admiralty brass generally contains one of these three elements in addition to tin. Brass should be kept clean and dust free. It should be handled with gloves to avoid contact with salts and acids in perspiration.
Brass in public spaces should be cleaned regularly. If possible, commercial polishes should be avoided. Some of these contain acids to speed up cleaning; others contain ammonia to remove dirt and grease. Both acids and alkaline solutions can cause dezincification. An abrasive slurry could be prepared instead, based on precipitated calcium carbonate or other harder abrasives.
Whenever brass is cleaned, it should be rinsed well so that any residues from cleaning are removed. Residues of commercial polish left on copper alloys may react with the copper to produce green-blue compounds.
- Ask Jason.
- Le Rosaire (Fiction et Littérature) (French Edition);
- Difference Between Copper, Brass and Bronze | Metal Supermarkets.
- Bricks & Brass;
- World & European Brass Association - Home.
Citric acid, for example, is found in some polishes and will produce green copper citrate. Even non-reactive residues will be noticeable if they accumulate in crevices. Polished brass is often coated for example, with a clear lacquer or wax to protect the shiny surface from tarnishing. I can provide your team with individuated coaching to help them own their anxiety.
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