Metals in building
Introduction
Metals have the following properties:
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malleability – able to be hammered or
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ductility—drawn out as wire
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able to be melted and formed into shapes in moulds.
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most metals can be polished.
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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:
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Mechanical joints: Bolts, screws or rivets are used to join metal components
together.
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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.
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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.
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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.
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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
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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.
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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).
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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:
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paints
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baked epoxy finishes
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vinyl coatings
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bituminous coatings
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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.
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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.
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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.
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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
Satisfactory. Recommended for all
No protective barrier required.
18/18 type or 300
fastenings.
series should be
specified
Zinc
Under severe environments such as
In severe environments metal
coastal or industrial, zinc will suffer
contact surfaces should be
from preferential attack.
coated with a bitumastic paint.
Galvanised steel
As for zinc.
As for zinc.
Mild steel
Aluminium will corrode in contact with
Coat contact surfaces with
mild steel in presence of an electrolyte. bitumastic paint or yellow zinc
chromate paint.
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Lead
Corrosion of the aluminium will only
In severe environments separate
occur in marine or severe industrial
contact surfaces with non
environments.
metallic spacers or bitumastic
paint.
Copper and brass
Attack of the aluminium surface in
Copper and brass must be
(including nickel
contact with these materials will occur
plated with nickel and/or
alloys)
in most atmospheric conditions.
chromium; otherwise use nonmetallic separators.
Concrete, cement,
Wet or ‘green’ products can cause
Surfaces in contact with these
lime etc, stone and
severe attack on aluminium.
products must be protected by
brick
painting or separating with nonmetallic material. Wash
thoroughly with clean water if
contact occurs.
Damp or
Because of their acidic nature can
Timber must be primed with
unseasoned timber
cause aluminium to corrode.
yellow zinc chromate undercoat
and sealed with suitable
protective paint.
Treated timber
Wood preservatives use salts of heavy Timber should be coated with
metals such as mercury or copper, or
caulking compound or mastic.
certain chlorides.
Hardboard,
The absorption of moisture into
plasterboard
hardboard or plaster board may give
Seal using suitable primer.
rise to poultice corrosion.
Plastics, rubber
No corrosive effect.
No special treatment required.
Adhesives,
These should not contain chlorides in
Carefully select adhesives and
sealants, etc
excess of 0.1% and those containing
sealants compatible with
water soluble sulphates should be fully aluminium.
tested before use.
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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:
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for flashing and damp coursing
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for solder (as an alloy with other metals)
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as sheet lead lining for sound proofing.
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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
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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
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