Historic Preservation - Technical Procedures
- Aluminum: Charcteristics, Uses And Problems
- Procedure code:
- Developed For Hspg (Nps - Sero)
- Metal Materials
- Last Modified:
- Aluminum: Charcteristics, Uses And Problems
- Last Modified:
ALUMINUM: CHARACTERISTICS, USES AND PROBLEMS
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
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
1. Nonfinished: A bare aluminum surface.
a. 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
b. The texture of bare aluminum may be smooth, highly
polished or brushed, or it may obtain its texture from
casting, extruding, or machining.
c. Nonfinished aluminum is the most common type of finish
found on historic buildings (1920 -1950), both outdoors
2. Anodized: An oxide coating applied by passing an electrical
current through the aluminum.
a. This tough coating is approximately 0.05 to 1.5 mils
thick and provides greater resistance to atmospheric
b. Anodized aluminum surfaces appear off-white in color and
have more of a smooth finish than ordinary aluminum.
c. The anodic coating may be transparent or integrally
colored by adding pigments or dyes before it is sealed.
1) 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.
2) Today colored coatings are produced by varying the
alloy content, which results in color on the
surface only during the anodizing process. Any
working of the metal and any texturizing of the
surface is applied to the aluminum before
d. Anodizing aluminum was invented in 1923 and began to be
used for architectural elements in the 1950s.
3. Chemical conversion: A coating formed by chemical processes.
a. This type of coating is thinner and less abrasion
resistant than anodic coatings. It is often used as a
base coating before painting.
b. The final finish of a chemical conversion coating may
appear clear or colored. Some colors include gold, gray,
golden brown, green, or blue-green.
a. 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
b. 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.
c. Aluminum siding with a baked-on paint finish came on the
market in the 1950s.
5. Plated: The process of electrodepositing a metal onto the
a. 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
b. Plated aluminum is most commonly used for features that
may be subjected to heavy abrasion, such as stair
6. Porcelain enameled: A baked-on ceramic coating applied in the
a. It is hard and impervious to soils, many acids and
alkalies. It is available in many colors and surface
b. 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.
7. Laminated: Fabricated by bonding wood, cloth, plastic, etc.
onto the aluminum. These types of finishes were introduced in
Typical historical uses for aluminum in the late 19th century
Typical uses for aluminum in the early 20th century included:
- Decorative detailing
- Roofing, wall panels, and spandrels (since it could be rolled
- 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: 1) Natural
or inherent problems based on the characteristics of the material
and the conditions of the exposure, and 2) Vandalism and human-
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
NATURAL OR INHERENT PROBLEMS
1. Natural Corrosion:
a. 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.
b. Heavy deposits of soiling occur when the aluminum surface
has been neglected and not cleaned regularly.
2. Chemical Corrosion:
a. Aluminum has good corrosion resistance to: Atmospheric
gases, moisture and soil.
b. Aluminum has poor corrosion resistance to: Alkalis,
hydrochloric acid, lead-based paints, some wood
preservatives, and chlorides.
c. 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.
d. Acids from unseasoned wood, damp oak, cedar, and redwood
may also attack aluminum.
1) Corrosion will result from direct contact between
wet wood and aluminum.
2) Water draining off a roof of unweathered wood
shingles will also corrode aluminum.
e. 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.
f. Aluminum may be protected from corrosion by applying a
paint or other coating as recommended by the Paint
3. Galvanic (Electrochemical) Corrosion:
a. 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.
b. 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.
c. Protect aluminum against galvanic corrosion by
electrically insulating it with a coating of paint or
VANDALISM OR HUMAN-INDUCED PROBLEMS
Mechanical or physical deterioration:
1. 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
2. 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
a. 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.
b. 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.
c. 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.
3. 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.
4. A number of aluminum alloys have been developed to improve
certain properties. These include different alloys for
exposed outdoor locations, unexposed outdoor locations, and
a. 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
b. 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.
c. Aluminum alloys used for casting usually contain silicon,
silicon and copper, or silicon and magnesium.
END OF SECTION