metals in building - oten · metals in building introduction metals have the following properties:...

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 © New South Wales Technical and Further Education Commission, 2015 (TAFE NSW – WSI)

• 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

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


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).

Rolling in a rolling mill

Cold rolled sections are used for:

Alloy steels Alloy steels contain certain added elements that provide special properties such as ultra-high

strength or resistance to corrosion or heat.


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).


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 non-

metallic 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.


Painting should be considered as a complete system that includes surface preparation, pre-

treatment 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.


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 silver-

white 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.


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.


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 non-

metallic 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 non-

metallic 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.


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.


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


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.


metals in building - oten · metals in building introduction metals have the following properties:...

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