Alpha brasses

Sheet, strip, plate and wire

Low-zinc brasses

The low-zinc brasses, or gilding metals, are used for architectural metalwork, costume jewellery etc. because of their golden colour but require a clear lacquer or other form of protection to preserve their appearance without tarnishing. The benzotriazole-inhibited lacquer Incralac is recommended for most applications (see Section 4). For service involving heavy wear or rough handling this air-drying, acrylic ester lacquer is not really sufficiently hard and a shop-applied, benzotriazole-inhibited, polyurethane lacquer (such as BNF-CB) is preferable ().

Brasses for cold working

As previously mentioned, CuZn30, CuZn33, CuZn36 and CuZn37 are used for a great variety of purposes involving cold working by forming, drawing, spinning etc., CuZn30 having the greatest ductility, is used when deep drawing operations are involved. Deep drawn or other heavily worked articles should be stress relief heat treated to avoid possible stress corrosion in service (see Section 3) but for most purposes no other corrosion protection is called for. CuZn37, with 63% Cu and 37% Zn, has a zinc content very close to the maximum for all-alpha brass. Modern methods of strip production entail rapid cooling after annealing and can result in small amounts of beta phase being retained - with consequent reduction of corrosion resistance. To avoid this, the 64/36 composition is now more commonly employed. Except in particularly aggressive environments CuZn30, CuZn33, CuZn36 and CuZn37 tarnish slowly to a uniform dull bronze colour with no pitting or localised attack. Their original appearance can be preserved, if required, by lacquering or regular polishing.

Since these brasses do not contain arsenic to inhibit dezincification, they are not recommended for service in contact with sea water. They are however the standard materials for core tubes and header tanks of motor car radiators and similar coolers operating on recirculating fresh water or inhibited anti freeze solutions, and are completely satisfactory for that purpose.

Aluminium brass, CuZn20Al2As (CZ110), strip is used when resistance to sea water is essential - for example header tanks and tube plates of aluminium brass tubed air-coolers operated on sea water. It is sometimes employed in plate-type heat exchangers but the extremely high local flow rates involved make it unreliable for this purpose with sea water as the coolant; titanium plates are usually preferred.

Rolled Plate

An arsenic-inhibited version of 70/30 brass, CuZn30As, CZ105, supplied in plate form may be used for tube-and-shell heat exchanger tubeplates. Aluminium brass plate is also used for that purpose and for the production of welded, large diameter sea water pipe systems.

Alpha-beta Brasses

Alpha-beta brasses have a wide range of use in a variety of forms including hot-rolled plate, extruded sections, machining stock, forgings, sand castings and diecastings - as well as for brazing or "bronze welding" fillers. Some examples of their use, in which corrosion resistance is an important factor, are given.

60/40 brass (Muntz Metal)

CuZn40 (CZ123) is used for tube plates of condensers and heat exchangers with brass or copper-nickel tubes - especially in USA; naval brass is generally preferred in UK. CuZn40 is subject to dezincification in sea water service, but the tube plates may be cathodically protected to prevent this. The uncoated cast iron water boxes often used for small heat exchangers will themselves provide sacrificial cathodic protection to brass tube plates but the water box will, as a result, suffer accelerated corrosion - especially near its interface with the tube plate. If the water box is coated for corrosion protection the tube plate can be protected with iron or zinc sacrificial anodes or by an impressed current cathodic protection system. Since, however, tube plates are very thick a considerable amount of dezincification can usually be tolerated before replacement or repair need be considered.

Naval Brass

The presence of 1% tin in naval brass, CZ112, gives it higher resistance to dezincification than CuZn40. This is particularly true if the copper content is near the top end of the range since this reduces the amount of beta phase present. For large tube plates the practical limit to the copper content is governed by the increasing difficulty of hot rolling but for small heat exchangers some manufacturers use cast tube plates of high-copper naval brass. In this material the beta phase is discontinuous and is largely enveloped by a delta phase of high tin content which protects it from dezincification in normal sea water service.

In the UK rolled naval brass is the most usual choice for tube plates for condensers and other large heat exchangers - usually with aluminium brass or copper-nickel tubes. 70/30 copper nickel tubes produce some galvanic acceleration of attack on the tube plate but, as with 60/40 brass, the thickness of the tube plate is such that the dezincification is rarely sufficient to require any remedial action. As with 60/40 brass, cathodic protection with iron or zinc anodes or by impressed current is often provided.

In condensers with titanium tubes galvanic action causes seriously accelerated attack on naval brass tube plates. Some success has been experienced with epoxy coatings to protect naval brass tube plates in condensers originally tubed with brass or copper-nickel and subsequently retubed, wholly or in part, with titanium. This procedure does, however, rely heavily on the integrity of the coating and, for new construction, naval brass tube plates are not considered suitable if titanium tubes are to be used. Nickel aluminium bronze, alloy CA105, is than recommended.

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