Metals in building. Introduction. Joining metals. Metals have the following properties:

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1 Metals in building Introduction Metals have the following properties: malleability able to be hammered or ductility drawn out as wire able to be melted and formed into shapes in moulds. most metals can be polished. conductors of energy such as heat and electricity. Other characteristics possessed by metals may vary considerably from metal to metal. Some metals (e.g. stainless steel) have good strength qualities, whereas others (e.g. tin) have very little strength. All metals, however, will lose strength when repeated force is applied to them a process known as metal fatigue. The degree of hardness of a metal will vary according to its natural characteristics (lead and tin, for example, are soft metals; chromium and nickel are hard) and according to the degree to which the metal is worked. When a metal is worked at normal temperatures (by being rolled or forged, for instance) the result will be an increase in its hardness and strength this is called work hardening. Most metals are subject to corrosion, which occurs when the surface of the metal combines with oxygen in the air to form a coat or crust that is no longer metallic (eg rust on iron or steel). Corrosive liquids and gases can actually eat away metals. (We can see the effect of salt air or spray on aluminium.) The process of corrosion is usually greatly speeded up by the action of heat and moisture. Some metals have very low corrosion-resistance, while others have a good degree of corrosion-resistance. Metals with a high degree of corrosion resistance (e.g. chromium) are often used as coatings or as alloys with other metals to increase their resistance to corrosive agents. Joining metals Metals can be joined by a variety of methods, including the following: Mechanical joints: Bolts, screws or rivets are used to join metal components together. Topic 2.5 Metals CPCCBS6001 Ed 1 1

2 Soldering and brazing: Most metals can be joined using an alloy which is a mixture of two or more metals that melt at a lower temperature than the melting point of the metals being joined. Soldering usually refers to tin-lead and lead-silver alloys which melt below 300 C. Brazing: Gives stronger joints than soldering; however, as it is done at higher temperatures (over 600 C), brazing cannot be used on metals such as lead which have low melting points. Welding: Most welding involves a metal being heated to a temperature below its melting point, and the soft metal being hammered together. This traditional blacksmithing method has been replaced by gas welding (using oxyacetylene or propane) and arc welding (using an electric arc struck between the work and a welding rod or a carbon electrode). Both brazing and welding involve heating the adjacent metal to extremely high temperatures which allows the metals to flow together and form one continuous unit. 2 Topic 2.5 Metals CPCCBS6001 Ed 1 New South Wales Technical and Further Education Commission, 2015 (TAFE NSW WSI

3 Structural steel Structural steel products are available in hot rolled sections and cold formed sections. Hot rolled sections These are formed while the steel is at elevated temperatures and include the following profiles: Hot rolled sections Topic 2.5 Metals CPCCBS6001 Ed 1 3

4 Cold formed sections These are formed while the material is cold. Unlike hot rolled sections, cold formed sections have constant thickness. Cold formed sections may be formed by rolling in a rolling mill (for material up to 20mm in thickness). Cold rolled sections are used for: Rolling in a rolling mill Alloy steels Alloy steels contain certain added elements that provide special properties such as ultra-high strength or resistance to corrosion or heat. 4 Topic 2.5 Metals CPCCBS6001 Ed 1 New South Wales Technical and Further Education Commission, 2015 (TAFE NSW WSI

5 Stainless steel (containing chromium and nickel) is one such steel alloy which, although much more expensive than mild steel, is being increasingly used in building in a wide variety of applications because of its durability and low maintenance needs (even under extreme conditions of atmospheric pollution, as it has excellent resistance to corrosion). Stainless steel has outstanding structural advantages because its hardness and toughness allows it to be used in very light sections, thus reducing greatly the weight of finished articles. Even more importantly, it is less affected by extreme heat, such as in a fire. Except for very simple cutting or drilling on site, all shaping and fitting of stainless steel must be done in suitably equipped factories and workshops. Stainless steel is also used for sanitary ware (e.g. sinks and benchtops). Topic 2.5 Metals CPCCBS6001 Ed 1 5

6 Prevention of corrosion in steel Upon exposure to the atmosphere ferrous metals combine with oxygen to form a red oxide (ie rust). Rust corrodes the metal and eventually wears it away, leaving behind a red powdery residue. This not only affects the appearance of the metal but substantially reduces its strength. One way of making steel rust resistant is by applying one of many protective coatings available for steel products. These are roughly in two groups: metallic coatings and nonmetallic coatings. As most require scrupulously clean conditions and special surface preparation of the steel for successful application, factory application of surface coatings is preferable. Metallic protective coatings These function by taking advantage of electro-chemical differences between different metals. In adverse atmospheric conditions it is the surface coating that is sacrificed rather than the base metal. A number of methods are used to apply metallic coatings, such as electroplating, spraying and hot dipping. Metals used to coat the steel include cadmium, zinc, tin, aluminium and copper. Zinc aluminium alloy applied by the hot dip process has effectively replaced galvanised steel in applications such as roofing because of its greatly increased durability. Non-metallic coatings These are available in a wide variety of colours and include: paints baked epoxy finishes vinyl coatings bituminous coatings vitreous enamel coatings. Baked epoxy finishes are applied to zinc-aluminium coated steel which is chemically treated to assist bonding. An epoxy primer and then the final colour coat are baked on separately. This type of finish is popular for domestic and commercial roofing and wall cladding for normal conditions. In marine and polluted industrial conditions steel can be coated with a tough vinyl which is laminated to the steel substrate. The vinyl coating locks out moisture, making an extremely corrosion-resistant finish. Vitreous enamel coatings comprise a layer of glass fused to a properly prepared steel base. 6 Topic 2.5 Metals CPCCBS6001 Ed 1 New South Wales Technical and Further Education Commission, 2015 (TAFE NSW WSI

7 Painting should be considered as a complete system that includes surface preparation, pretreatment to facilitate adhesion, primer, intermediate coat or coats and finish coat. Different types of steel require different pre-treatments and coatings. Bituminous coatings are based on bituminous resins such as coal tar or asphalt. The bituminous resins perform well underground and in contact with water but do not have good weather durability when exposed to sunlight. Topic 2.5 Metals CPCCBS6001 Ed 1 7

8 Nonferrous metals Most nonferrous metals are metals that don t contain iron and are more costly to produce than ferrous metals. However, they often have much better working properties and resistance to corrosion. The more common nonferrous metals are copper, aluminium, zinc, lead, nickel, tin and cadmium. Copper Copper has been used for sheets, pipes, and other building products for thousands of years. Copper is a pinkish coloured metal and is easily hammered into sheets. It is much more expensive than some alternatives but its extreme resistance to corrosion outweighs this disadvantage in certain applications. Upon exposure to the atmosphere, copper forms a protective copper oxide coating which is light green in colour. Its resistance to corrosion has made it popular for use as water pipes and tanks. It also conducts electricity very well, hence its use for electrical wiring. Other uses include roofing, roof plumbing, flashing and damp courses. Brass Brass is an alloy of copper and zinc, and is an attractive golden colour. Brass is used for plumber s hardware (eg pipe connectors and fittings; taps and outlet spouts, often chrome finished). Screws, nails, grilles, hinges, door locks and latches and chains are often made from brass. Aluminium Aluminium is a lightweight metal (approximately one-third the weight of iron) and is silverwhite in colour. Aluminium was introduced as a building material after World War II in competition with traditional building metals, such as steel and copper. Probably the major characteristic that has helped aluminium gain widespread acceptance in the building industry is its suitability for extrusion production methods. This means that very complicated shapes can be produced economically. Uses Aluminium products are extensively used in the building industry for domestic windows, doors and insect screens; for commercial windows and curtain walls for residential and industrial roofing and rainwater goods; for balustrades and railings and for reflective insulation. 8 Topic 2.5 Metals CPCCBS6001 Ed 1 New South Wales Technical and Further Education Commission, 2015 (TAFE NSW WSI

9 Corrosion resistance One of the most significant properties of aluminium is its excellent resistance to atmospheric corrosion. On exposure to the atmosphere, a whitish coating of aluminium oxide forms which then protects the surface from further corrosion. The structural integrity is not impaired as a result of this process. Thus, untreated aluminium can be used for roofing, cladding and so on, but where long-term appearance is important the aluminium should be finished. Compatibility with other building materials Corrosion of a metal may be accelerated through contact with another metal of very different electro-chemical properties especially in the presence of an electrically conductive solution, such as sea spray or industrially polluted moisture. Copper, brass and nickel alloys, all have a large potential difference to aluminium and in a salt solution cause it to rapidly corrode. Some other building materials are also incompatible with aluminium and direct physical contact with those materials should be avoided or barriers should be used. The table below broadly indicates the types of barriers suitable for most building construction applications. Compatibility of aluminium with various building materials Contact material Compatibility Recommended barrier Stainless steel 18/18 type or 300 series should be specified Satisfactory. Recommended for all fastenings. No protective barrier required. Zinc Under severe environments such as coastal or industrial, zinc will suffer from preferential attack. In severe environments metal contact surfaces should be coated with a bitumastic paint. Galvanised steel As for zinc. As for zinc. Mild steel Aluminium will corrode in contact with mild steel in presence of an electrolyte. Coat contact surfaces with bitumastic paint or yellow zinc chromate paint. Topic 2.5 Metals CPCCBS6001 Ed 1 9

10 Lead Corrosion of the aluminium will only occur in marine or severe industrial environments. In severe environments separate contact surfaces with non metallic spacers or bitumastic paint. Copper and brass (including nickel alloys) Attack of the aluminium surface in contact with these materials will occur in most atmospheric conditions. Copper and brass must be plated with nickel and/or chromium; otherwise use nonmetallic separators. Concrete, cement, lime etc, stone and brick Wet or green products can cause severe attack on aluminium. Surfaces in contact with these products must be protected by painting or separating with nonmetallic material. Wash thoroughly with clean water if contact occurs. Damp or unseasoned timber Because of their acidic nature can cause aluminium to corrode. Timber must be primed with yellow zinc chromate undercoat and sealed with suitable protective paint. Treated timber Wood preservatives use salts of heavy metals such as mercury or copper, or certain chlorides. Timber should be coated with caulking compound or mastic. Hardboard, plasterboard The absorption of moisture into hardboard or plaster board may give rise to poultice corrosion. Seal using suitable primer. Plastics, rubber No corrosive effect. No special treatment required. Adhesives, sealants, etc These should not contain chlorides in excess of 0.1% and those containing water soluble sulphates should be fully tested before use. Carefully select adhesives and sealants compatible with aluminium. 10 Topic 2.5 Metals CPCCBS6001 Ed 1 New South Wales Technical and Further Education Commission, 2015 (TAFE NSW WSI

11 Zinc Zinc is a soft, greyish metal which can be hammered or rolled into sheets, often used for roofing rainwater goods. Today, zinc s most important function in the building industry is as a protective coating on steel. The zinc coating acts first as a barrier to corrosion. However, should the coating be scratched or damaged, exposing the steel, the zinc surrounding the damaged part will itself corrode instead of the steel. Thus by sacrificing the zinc the steel is protected and will not rust until all available zinc is used. Galvanising Galvanising is the process of coating steel and iron with zinc to form a protective coating. The steel is lowered into a molten bath of zinc heated to approximately 500 C and emerges with a shiny coating of zinc. The zinc coating acts as a sacrificial anode and corrodes to protect the steel. Since its rate of corrosion is slow, the steel can remain protected for hundreds of years, depending on the environment. Zincalume Zincalume is a newer protective coating and is a combination of zinc and aluminium (45% and 55% respectively), which is applied in a factory process to sheet steel used for roofing and cladding in the building industry. Lead Lead is soft and easily worked, but its great density makes it heavy to handle, and thin sheets and pipes will not even support their own weight. Lead has been used for thousands of years: lead water pipes were used by the Romans, and our word plumber comes from the Latin word plumbum meaning lead. Due to its toxic properties, however, lead is no longer used for water pipes. In the past, it was used for roofing and roof plumbing, but today its use is limited although in certain roof plumbing situations, its weight and malleability still make it a useful and preferred material. Lead is used: for flashing and damp coursing for solder (as an alloy with other metals) as sheet lead lining for sound proofing. Topic 2.5 Metals CPCCBS6001 Ed 1 11

12 Metal frame construction Domestic and commercial buildings can both be of metal frame construction. This type of construction is versatile, light, strong, time and labour saving, economical, and stable. Walls, roofs and floors can all be constructed this way. The metal frames made from steel are pre-fabricated in the workshop or before being erected. They can be joined together using rivets, welds, screws or bolts. Metal framing for a brick veneer house 12 Topic 2.5 Metals CPCCBS6001 Ed 1 New South Wales Technical and Further Education Commission, 2015 (TAFE NSW WSI

13 Steel framing for a residential building. This building will have a Hebel Powerpanel used as the outer cladding. Additional reading: This topic has been an introduction to steel and other metals used in residential construction. To gain a more detailed knowledge of metals in construction, you should now read the Metals in building reading material provided. Topic 2.5 Metals CPCCBS6001 Ed 1 13