Copper Alloys Methods of Prevention of Corrosion
In most cases, if copper is exposed to the atmosphere, tarnishing will be the most severe form of corrosion encountered.
Tarnish can be prevented by coating copper with a clear lacquer.
A black oxide chemical conversion coating, provides a good base for adhesive bonding or for organic coatings. It is, however, primarily a decorative coating.
Cadmium plate, and tin plate, are suitable finishes for electrical bonding.
Tin provides solderability and corrosion protection. It may be applied by hot-dipping or it may be electroplated and reflowed.
Copper should not be used under a silver or gold plate. Instead nickel should be used under silver or gold or between copper and silver.
Normally, copper and copper alloys are not painted; however, they may be painted for decorative or other purposes.
General or uniform corrosion is not a problem in most of the environments in which copper and copper alloys are used.
If general corrosion is a problem however, one of the following copper alloys corrosion prevention methods should be applied:
1. Increase the thickness of the cross section to obtain an adequate service life.
2. Choose a more resistant material or copper alloy.
3. Choose from the various types of coatings that may be useful.
All of the copper alloys are susceptible to some degree to crevice corrosion. Prevention of crevice corrosion is primarily a design consideration therefore, during design, crevices and water traps should be eliminated or minimized. If crevices cannot be ruled out, a more resistant alloy should be chosen.
Many of the high-strength copper alloys, especially in the cold-worked condition, are susceptible to stress corrosion cracking. The most susceptible alloys usually contain zinc as an alloying element. Stress-relief annealing is effective for some alloys in some applications.
However, the usual approach to eliminating the problem is to use a less susceptible or even immune alloy.
For a given cold-worked product, the exact minimum annealing temperature varies with such factors as composition, degree of cold work, and microstructure. As an alternative, overanneaIing can be performed if the resultant mechanical properties are adequate for the intended service. Overannealing softens the alloy enough so that hardness or microstmcture can be used as an indicator of stress relief.
Brasses that contain at least 8570 copper and admiralty brasses that contain 1% tin and inhibiting additions of arsenic, antimony, or phosphorus have high resistance to dezincikation. Alloy 688 is a high-strength brass particularly suitable to severe environments.
This alloy has unusual resistance todezincification in seawater. Another of copper alloy maybe specified, such as a cupronickel, that has a high resistance to dealloying.
Copper-aluminum alloys that contain about 4% nickel and a relatively low amount of aluminum have improved
resistance to dealloying attack.
The use of a temper anneal heat treatment in the range of 649° to 704° C (1200°F to 1300°F) reduces the susceptibility of these nickel-aluminum-bronze alloys to dealloying attack.
The copper-nickel alloys are susceptible to attack by stildes in seawater at low concentrations. Sulfldes as low as 0.007 ppm can induce pitting of 90:10 and 70:30 alloys.
Accelerated attack occurs at higher concentrations of sulphide. If the copper-nickel alloy is likely to be exposed to polluted seawater for extensive periods, an alternative material should be specified.
source - MIL-HDBK-735
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