Aluminum: Charcteristics, Uses And Problems
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This standard includes general information on the characteristics and common uses of aluminum and identifies typical problems associated with this material along with common causes of its deterioration.
Characteristics of Aluminum:
- Has a low melting point
- Has a moderately high coefficient of expansion
- Has a good thermal and electrical conductivity
- Very soft
Aluminum found in historic buildings may be finished in one of the following ways:
- Nonfinished: A bare aluminum surface.
- Upon exposure to the air, bare aluminum develops a thin layer of natural oxide. This patina layer is thin, transparent, tough, and protects the aluminum from corrosion.
- The texture of bare aluminum may be smooth, highly polished or brushed, or it may obtain its texture from casting, extruding, or machining.
- Nonfinished aluminum is the most common type of finish found on historic buildings (1920 -1950), both outdoors and indoors.
- Anodized: An oxide coating applied by passing an electrical current through the aluminum.
- This tough coating is approximately 0.05 to 1.5 mils thick and provides greater resistance to atmospheric corrosion.
- Anodized aluminum surfaces appear off-white in color and have more of a smooth finish than ordinary aluminum.
- The anodic coating may be transparent or integrally colored by adding pigments or dyes before it is sealed.
- In the 1950s, colored aluminum was achieved by adding dyes. Colors of red, blue and green often faded nonuniformly and appeared blotchy. Colors of gold, brown, grey and black, however, usually retained their original color.
- Today colored coatings are produced by varying the alloy content, which results surface only during the anodizing process. Any working of the metal and any texturizing of the surface is applied to the aluminum before anodizing.
- Anodizing aluminum was invented in 1923 and began to be used for architectural elements in the 1950s.
- Chemical conversion: A coating formed by chemical processes.
- This type of coating is thinner and less abrasion resistant than anodic coatings. It is often used as a base coating before painting.
- The final finish of a chemical conversion coating may appear clear or colored. Some colors include gold, gray, golden brown, green, or blue-green.
- Pigmented (paint) or clear (lacquered) types of organic coatings were used in the 1930s on aluminum doors, frames, and radiator cabinets to create a wood grain finish.
- Today paint is usually applied over chemical conversion finishes. During construction anodized surfaces are often given a clear coating for protection against alkaline building products.
- Aluminum siding with a baked-on paint finish came on the market in the 1950s.
- Plated: The process of electrodepositing a metal onto the aluminum surface.
- The most common metals used for plating are chromium and nickel. To achieve a smoother finish, copper may be used as an intermediate layer. Tin, silver or gold may also be used.
- Plated aluminum is most commonly used for features that may be subjected to heavy abrasion, such as stair railings.
- Porcelain enameled: A baked-on ceramic coating applied in the factory.
- It is hard and impervious to soils, many acids and alkalies. It is available in many colors and surface textures.
- Seldom found in today's historic buildings, its use as an exterior wall cladding beginning in the 1970s will make it an historic material in the not so distant future.
- Laminated: Fabricated by bonding wood, cloth, plastic, etc. onto the aluminum. These types of finishes were introduced in the 1970s.
Typical historical uses for aluminum in the late 19th century included:
Typical uses for aluminum in the early 20th century included:
- Decorative detailing
- Roofing, wall panels, and spandrels (since it could be rolled into sheets)
- Window mullions and frames, storefront surrounds, doors, and door trims (as it could be extruded into lengths of specialized profiles or cross sections)
Problems and deterioration
Problems may be classified into two broad categories:
- Natural or inherent problems based on the characteristics of the material and the conditions of the exposure, and
- Vandalism and human- induced problems.
Although there is some overlap between the two categories, the inherent material deterioration problems generally occur gradually over long periods of time, at predictable rates and require appropriate routine or preventive maintenance to control. Conversely, many human induced problems, (especially vandalism), are random in occurrence; can produce catastrophic results; are difficult to prevent, and require emergency action to mitigate. Some human induced problems, however, are predictable and occur routinely.
Natural or Inherent Problems
- Upon exposure to the air, aluminum protects itself by developing a layer of white aluminum oxide which covers the exposed surface. This layer is thin, transparent, tough, and to a great extent protective.
- Heavy deposits of soiling occur when the aluminum surface has been neglected and not cleaned regularly.
- Aluminum has good corrosion resistance to: Atmospheric gases, moisture and soil.
Aluminum has poor corrosion resistance to:
- Alkalis, hydrochloric acid, lead-based paints, some wood preservatives, and chlorides.
- Aluminum may also corrode when in contact with wet lime mortar, Portland cement, plaster, or concrete before they are fully cured, damp, porous brickwork and stonework. To protect aluminum against contact with masonry, apply a coating of bituminous paint, followed by 2 coats of aluminum metal and masonry paint.
- Acids from unseasoned wood, damp oak, cedar, and redwood may also attack aluminum.
- Corrosion will result from direct contact between wet wood and aluminum.
- Water draining off a roof of unweathered wood shingles will also corrode aluminum.
- Corrosion may be accelerated on an aluminum roof where condensation develops on the underside of the roof, much like a terne- or tin plated roof. If standing water is acidic, corrosion cells will develop on the aluminum.
- Aluminum may be protected from corrosion by applying a paint or other coating as recommended by the Paint Manufacturer's Association.
Galvanic (Electrochemical) Corrosion:
- Galvanic action will occur, causing the aluminum to corrode, if the aluminum comes in contact with other metals such as tin, iron and steel (if they are not painted), and especially copper.
- Aluminum is compatible with zinc, cadmium, lead, galvanized steel, monel, magnesium, and usually nonmagnetic stainless steel. Nonmagnetic stainless steel is sometimes corrosive to aluminum when the two metals come into contact in industrial environments.
- Protect aluminum against galvanic corrosion by electrically insulating it with a coating of paint or mastics.
Vandalism or Human-Induced Problems
Mechanical or physical deterioration:
- Erosion: Aluminum features are extremely vulnerable to erosion because this metal is so soft. When exposed to abrasive agents, erosion of aluminum can be a critical
- Fatigue: Aluminum has a high coefficient of thermal expansion. Fatigue is one of the most common failures resulting from the stresses associated with expansion and contraction.
- The lack of an adequate number of transverse joints or welts in a length of sheet aluminum between bays will result in cracking of the sheets.
- Improperly sized bays (space between vertical seams) and an inadequate number of fasteners can also cause aluminum roofing to bow, buckle and eventually crack. Using aluminum sheets which are not rigid enough to resist this movement will exacerbate this problem.
- Inadequate support from the underside, such as spaced rather than tight sheathing boards, will also result in buckling and sagging of the sheet metal, ultimately causing the metal to crack and tear.
- Human Error: The alloy specified for a job may not be the best choice for the function and location, or the material used in the fabrication is not the alloy specified. These mistakes can cause exfoliation of the aluminum, where the aluminum alloy begins to flake off in layers, similar to rusting wrought iron. The corrosion material lifts out from the surface as if the metal had exploded.
- A number of aluminum alloys have been developed to improve certain properties. These include different alloys for exposed outdoor locations, unexposed outdoor locations, and indoor locations:
- Nonheat-treatable alloys, which include 1-1/4% manganese and 2 to 7% magnesium, are of relatively high strength and are used for cladding and also for corrugated roofing.
- Heat-treatable alloys contain varying proportions of aluminum, magnesium, silicon, and sometimes copper. These have high strength and are, therefore, used for fasteners and for light structural members.
- Aluminum alloys used for casting usually contain silicon, silicon and copper, or silicon and magnesium.