Extruded Brasses

Sections for engineering components requiring extra corrosion resistance

The innumerable applications of extruded sections for the manufacture of engineering components follow from the combination of close dimensional tolerances maintained in the wide variety of sections available, free-machining properties, good electrical conductivity and, of course, corrosion resistance that they provide. CuZn43Pb2Al (CZ130) is the brass most widely employed in this form. The aluminium-free version is selected if the application involves soft soldering or plating; otherwise the version containing 0.5% aluminium is chosen for its brighter appearance and particularly good tarnish resistance. Typical uses for which no corrosion protection is normally required include machine parts, instruments, electrical appliances, switchgear, fusegear and hinges.

Sections for architectural use

CuZn43Pb2Al (CZ130) with aluminium provides sections for shopfitting etc. which have a bright yellow appearance and good tarnish resistance. CuZn40Mn1Pb1 (CZ136) or a manganese brass (no BS designation) containing 38% zinc with 2% manganese, 1% lead and 0.5% each of tin and iron is sometimes preferred since it is stronger, and superficial oxidation occurring during extrusion gives it a chocolate brown colour which obviously cannot tarnish. It does become dull and eventually start to form a green patina on outdoor exposure but can be preserved by rubbing over with very light oil at one or two-monthly intervals or by lacquering. Wax polish is sometimes used - especially for indoor service involving handling. The same treatments can be applied to preserve the original appearance of CZ130 sections.

Architectural brass sections are often used in conjunction with gilding metal panels, both being toned to the same "brown bronze" colour with proprietary colouring compounds based for example on antimony sulphide. If used on outdoor items these patinated finishes should be preserved by oiling or lacquering. Wax polish is often used indoors.

Rod and bar for machining

The choice of zinc and lead contents of free cutting brass is made according to the requirements for machinability and cold working ability. All have similar corrosion resistance, being subject to dezincification in dezincifying environments but requiring no special protection for most purposes. Naval brass CZ112 and the free machining leaded versions of this alloy have slightly better resistance to dezincification but for use in sea water, acidic conditions or supply waters that cause meringue dezincification the dezincification resistant brass, CuZn35Pb2As (CZ132), is required.

Higher strength, with susceptibility to dezincification similar to that of CZ112, is provided by the high tensile brasses CZ114 and 116. These are used for example as bolting materials and for valve spindles in situations where significant dezincification is unlikely to occur. They should not be used in sea water unless cathodically protected. Because of the possibility of stress corrosion cracking occurring after prolonged service, high tensile brasses are not employed in situations where stress corrosion failure could have serious consequences, for example as load-bearing masonry fixings.

Forgings (Hot Stampings)

All the alpha-beta brasses are similar from the corrosion point of view, having excellent corrosion resistance without need of protection under most conditions of service. They are subject to dezincification in sea water, in supply waters that cause meringue dezincification or if buried in corrosive soils. For these purposes the dezincification-resistant brass is employed.

One of the biggest uses for hot stampings in CuZn40Pb2 (CZ122) is for water fittings, gas fittings and other pipe connectors or valves. These are sometimes chromium plated or gold plated for decorative effect but will provide almost indefinite service without any need for corrosion protection. Watch cases are plated both for decorative purposes and to prevent tarnish and staining from contact with perspiration. Stampings used for machine parts or instrument parts normally neither receive nor require any corrosion protection.

Naval Brass, CZ112, and the high-tensile brasses, CZ114, 115 and 116, are also used for hot stampings. Like CZ122 these alpha-beta alloys are not suitable for service in environments conducive to rapid dezincification though their corrosion resistance is, in general, slightly superior. One particular purpose for which the high tensile brass CZ115 has been found inferior to the "ordinary" leaded forging brass CuZn39Pb2 (CZ128) is for valves fitted to high pressure carbon dioxide cylinders. Following some failures of CZ115 valves by stress corrosion cracking from the inside, laboratory stress corrosion tests in carbon dioxide plus water at high pressure produced similar cracking in valves of CZ115 but not in CZ128. The mode of cracking, both in service and in the laboratory tests, was transgranular and through the beta phase. Since it is evident that high tensile brass valves can sometimes fail by stress corrosion cracking in service with carbon dioxide, the standard material for carbon dioxide cylinder valves is now CZ128.

Dezincification-resistant brasses

Dezincification resistant brasses for hot working or diecasting have been given a sub-section of their own because they are alpha-beta brasses above about 550oC but alpha brasses in the heat-treated condition in which they are used. The most important dezincification resistant brass is CuZn36Pb2As (CZ132). It is most used, in the form of hot stampings and items machined from rod or bar, for the production of water fittings for use in areas where the supply causes meringue dezincification of alpha-beta brass. In the illustration of a stop tap, note the ‘CR’ mark indicating that fittings of the same type, supplied by the same manufacturer, have been tested for dezincification resistance, as laid down in the BS2872 and BS2874 specifications for CZ132, and are approved by the water industry’s Fittings Approvals Board.

A stop tap typically employs hot stampings in CZ132 for the body, bonnet and washer plate, the spindle being machined from CZ132 rod. The gland nut does not come into contact with water and may therefore be of alpha-beta brass unless the tap is for underground use, in which case it must also be in CZ132. The capstan head does not need to be dezincification resistant and may be a hot stamping in CuZn40Pb2 (CZ122).

Such fittings frequently have ends machined for capillary soldered connection to 15mm copper tube. CZ132 is suitable for all conventional soft soldering procedures but if it is heated above 550oC beta phase is formed and its dezincification resistant lost. Capillary brazing is, therefore, not satisfactory. Silver soldering can be employed for the manufacture of mixer valve components, etc. from CZ132 parts provided that the silver solder used is itself resistant to dezincification and the component is heat treated, according to the requirements of BS2874 for CZ132, after fabrication.

The Tee shown in Figure 10 is a hot stamping in CZ132 with Type A compression coupling ends (BS 864: Part 2). The nuts on this type of fitting do not come into contact with the water and are usually of alpha beta brass. For fittings to be used underground, however, the nuts must be of CZ132.

Proprietary dezincification-resistant brasses, formulated on the same principle as CZ132 but usually containing silicon and/or manganese for greater fluidity, are used as diecastings for valve and water meter bodies, etc. The need for heat-treatment after casting, to ensure an all-alpha structure, can sometimes be avoided by controlled slow cooling through the temperature range 550oC to 450oC. It is not easy, by this method, to achieve the degree of dezincification resistance required to qualify for the ‘CR’ mark in UK though the casting may meet the standard required for classification as dezincification resistant brass in Scandinavia (see Section 3). Users requiring diecast fittings that are fully dezincification-resistant are advised to use only those bearing the ‘CR’ mark.

Further information is given later in this section where corrosion mechanisms are discussed.

Castings

The gravity diecasting brasses CuZn38Al-C (DCB1) and CuZn39Pb1Al-C (DCB3) are much used for tap bodies and similar objects required in large numbers but of too complicated a shape to be hot stamped. They are often plated for decorative effect but otherwise are used without special corrosion protection. Being alpha-beta brasses they are subject to dezincification in unsuitable environments (DCB3 somewhat less than DCB1) but it is worth repeating here, since one of the largest uses of brass gravity diecastings is for terminal taps, that terminal taps for service in meringue dezincification areas do not need to be dezincification-resistant, though stop taps do.

Pressure diecastings in PCB 1 are used when large numbers are required and it is desired to take advantage of the thinner wall sections achievable by this process. The copper content of PCB 1 is slightly lower than that of DCB1, and the thinner-walled pressure diecastings often cool more rapidly from the casting temperature. Both of these factors tend to give a higher beta content in the product but any consequent difference in resistance to dezincification is marginal. Pressure diecastings in brass are, in any case, mostly used for purposes, such as instrument parts, where the environment will not cause dezincification. They are generally used unprotected but may be plated or painted for appearances sake.

High tensile brasses are used when the strength of the conventional cast 60/40 brasses may be inadequate. Of the conventional British alloys, the higher tin content and duplex structure of CuZn35Mn2Al1Fe1-C (HTB1) gives it much better corrosion resistance than the stronger CuZn25Al5Mn4Fe3-C (HTB3) ‘beta’ brass.

Bronze welding fillers

The filler alloys specified in BS1724: Bronze welding by gas are designated C2, C4, C5 and C6 in BS1453: Filler metals for gas welding. Each is an alpha-beta brass alloy containing approximately 40% zinc with between 0.2 and 0.5% silicon and an optional addition of up to 0.5% tin; C4 contains, in addition, small amounts of iron and manganese while C5 and C6 contain 10% nickel and 15% nickel to improve their mechanical properties.

As with other alpha-beta brasses, the possibility of dezincification has to be considered. C5 and C6 are used for joining ferrous materials from which they will usually receive galvanic protection in a corrosive environment. C2 and C4 are used both for ferrous materials, which will provide galvanic protection, and for copper which will cause galvanic stimulation of corrosion. Consequently, while bronze welding is generally satisfactory for copper drainage lines and for copper plumbing systems handling water with little tendency to cause dezincification, it is not suitable for copper or copper alloy sea water or brackish water lines. For such service, and for plumbing systems generally, capillary brazed joints made with silver-brazing or copper-phosphorus brazing alloys are safer. The note of caution concerning plumbing systems generally is because the galvanic effect of a large area of copper acting upon a small area of joint filler can cause serious damage in waters that normally cause only an acceptable degree of dezincification in brass fittings. The larger the diameter of the copper pipe the greater this effect will be.

Nickel Silver

Nickel silver is available as sheet, strip, wire, stamping, extrusions, and hot stampings. It is the base metal on which silver is plated to give "EPNS" for good quality tableware. It is also used for architectural purposes to give a warm, silvery-coloured facade, doorway or balustrade when required. Nickel silver sheet, extrusions in nickel brass (known as "silver bronze") and nickel silver castings are used for these purposes - especially in prestige buildings. The RIBA building in Portland Place, London is perhaps the prime example but many, much more recent buildings also display nickel silver used to good effect. The combination of cold working, hot working and cast forms still offers exceptional scope for architectural craftsmanship.

Nickel silvers and nickel brasses show superior tarnish resistance and require no protection or special attention when used indoors, though uniform slight yellowing of the original silver-white colour will occur on the lower-nickel alloys. Outdoors, treatment with very light oil, wax polish or lacquer is required to prevent eventual development of a light powdery green patina. Incralac is recommended. One of the early uses made of this benzotriazole-inhibited lacquer was on the large wrought nickel silver gates of the Air Forces Memorial at Egham. Although situated facing south, on an exposed hilltop, in the flight path westward from Heathrow only 8km away, they were effectively protected by the Incralac for 10-12 years. Unfortunately, it was several more years before they were stripped and relacquered - using an uninhibited lacquer that has proved much less satisfactory.

An important specialised use of the 12% and 18% nickel silvers NS 104 and 107 is for relay contact springs in telecommunications and other equipment. For this purpose their spring properties, solderability and resistance to corrosion by the atmosphere, and by the acidic coronets liable to be generated from organic insulating materials in an enclosed space, are all important.

The leaded nickel silvers, NS111, 112 and 113, are used where machinability, good appearance, corrosion resistance and wear resistance are required. Common examples are cylinder lock keys, screws, gears, pinions and other parts for clocks, cameras and musical instruments.