Aluminium
Aluminium
General Characteristics
A wide variety of mechanical characteristics, or tempers, are available through various combinations of cold working and heat treatment.
A good resistance to corrosion on surfaces exposed to the atmosphere is achieved due to the formation of a thin transparent oxide film that protects the substrate from further oxidisation.
Attractive appearance, low maintenance, and light weight together with reflectivity are additional features of aluminium roofing materials.
Finishes available are plain mill finish, and stucco embossed finish which reduces glare.
Alloy and Temper
The four-digit system used for designating the alloys is an international one which has been adopted by all major aluminium producers. The first digit indicates the alloy group. The second digit indicates modifications of the original alloy or impurity limits. The last two digits identify the specific alloy and aluminium purity.
Alloys recommended for roofing and wall cladding products are 5052, 5005 and 5251. In particular, the magnesium and chromium content of 5052 makes this alloy appropriate for recommended use in marine environments, and this alloy is in common use in New Zealand for roofing and wall cladding products.
The temper designation describes the combination of cold working and heat treatment performed to give the material appropriate forming and end use mechanical properties. The tempers used for roofing and wall cladding products are H34 for crimp curving and folding, and H36 for rollforming.
These tempers involve strain-hardening and then low temperature heating to slightly lower the material strength and increase
ductility.
Material Thickness
Common thicknesses for use as roofing and wall cladding are 0.90mm and 1.2mm.
Working and Finishing
All aluminium alloys work harden and gain strength during forming operations. Thus the appropriate alloy and temper must be selected to allow the appropriate degree of formability, without excessive hardening and consequent splitting.
Mechanical fasteners, whether rivets or screws, are available in aluminium. Where the use of aluminium fasteners is plainly unsuitable, e.g. self-drilling screws, stainless steel is normally regarded as the best alternative.
With correct pretreatment procedures, aluminium is an excellent substrate for applied coatings. The pretreatment uses phosphate or chromate conversion coatings to chemically remove the aluminium oxide layer.
Durability
Aluminium alloys have excellent durability and corrosion resistance, but their behaviour can be influenced by the way in which
they are used.
Aluminium’s natural affinity with oxygen results in the formation of a transparent oxide film when aluminium is exposed to air. This film is extremely hard, chemically stable, corrosion resistant, and adheres strongly to the parent metal surface. Once formed it prevents further oxidation, and, if damaged in any way, will reform if sufficient oxygen is available.
If the surface is pitted by air-borne pollutants found in industrial or marine atmospheres (e.g. sulphuric acid, sodium chloride) the resulting chemical reaction produces a larger volume of powdered corrosion product than the volume of original pit. This seals the surface and inhibits further corrosion.
Ongoing protection against corrosion in this way requires oxygen to be available at the surface. Therefore aluminium alloys can suffer corrosion when used in ways that prevent this, e.g. continuously wet environments like sheet end laps or under accumulated debris.
Contact with Other Materials
In general, direct chemical attack of aluminium only occurs when the pH is below 5 or above 8.
Contact with other metals can result in corrosion due to galvanic reactions. Copper, steel, stainless steel and lead will all cause attack of the aluminium alloy. Contact with zinc will cause the zinc to be attacked.
Entrapment of moisture between adjacent aluminium surfaces will cause water stains. The stain is a superficial condition, and the mechanical properties of the aluminium alloy are not affected. The degree of staining may be judged by the relative roughness of the stained area. If the surface is reasonably smooth the stain appearance may be able to be improved by abrading with steel wool and oil.
Maintenance
Regular cleaning is required to maintain an unaffected surface for good aesthetic quality in industrial and marine environments.
This can be achieved naturally by rain washing but surfaces not adequately washed by rain must be cleaned by water washing and scrubbing with a soft brush at sufficient frequency to prevent build up of salt or dirt deposits. In severe marine locations the washing frequency may be every 4-6 months for surfaces not washed by rain.
