Copper sheet is available in pre-patinated form. As well as green, other attractive colours such as brown, gold and silver are available. See www.kme.com and www.aurubis.co.uk.
It reduces by about 3%.
Suggest DEF STAN 02-834, which is a naval grade of aluminium bronze with a low magnetic permeability. This meets MOD requirements.
Various copper alloys have particular strengths under specific sea water conditions but on balance 90-10 copper-nickel has excellent corrosion resistance in many applications and is perhaps the most versatile. There is a detailed description of the properties and applications of this and other popular copper nickel alloys on the www.coppernickel.org website. Of course, aluminium bronze alloys, particularly nickel aluminium bronze, are also recognised for their corrosion resistance in severe seawater conditions and for that reason also have established applications as propellers and pumps and valves.
CW505L is the most ductile brass, but for simple forming the more economical 63/37 Common Brass (CW508L) should be used.
70/30 brasses CZ106 (CW505L).
(a) Remove surface oxide with dilute (10%) hydrochloric acid
(b) Neutralise with bicarbonate solution (lye)
(c) Seal by chromate conversion
(f) Clean with detergent annually.
After exposure to seawater, copper alloys develop a surface oxide film which provides protection from corrosion.
The first job is to clean the surface. Use a detergent with a kitchen scouring sponge (usually green). For a badly stained surface it may be necessary to incorporate pumice powder with the detergent .For large areas it may even be necessary to shot blast the surface. After cleaning dry the surface with hot air and as soon as possible wax the surface using natural wax. Natural wax is essential since it will enhance the true colour and form a protective coating, repelling water. Artificial wax should not be used since it will crack in time and draw water in.
A clear acrylic resin finish containing tarnish inhibitor. It is an air drying ester lacquer containing Benzotriazole, a corrosion inhibitor. It can be relied upon to provide protection for 3-8 years outdoors and for much longer indoors .
Copper strip and wire may be produced with a combination of high elastic limit and good corrosion resistance, making them an ideal choice for springs. The main groups of alloys are:
- Phosphor bronze, typically CW451K (PB102) with a yield strength of about 500N/mm2.
- Nickel silver, typically CW409J (NS106).This is silver coloured and has excellent tarnish resistance.
- Copper beryllium CW100C (CB101). This alloy is used where very high yield strength is required (greater than 1000N/mm2). These alloys are expensive and phosphor bronze will be adequate for most engineering applications.
A useful Specification is BS EN 1654:1998 (Copper and copper alloys. Strip for springs and connectors).