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Historic Preservation - Technical Procedures

Spectitle:

Aluminum: Charcteristics, Uses And Problems

Procedure code:

0501008S

Source:

Developed For Hspg (Nps - Sero)

Division:

Metals

Section:

Metal Materials

Last Modified:

01/07/2014

Details:

Aluminum: Charcteristics, Uses And Problems



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
deterioration.


INTRODUCTION

Characteristics of Aluminum:

-    Lightweight

-    Corrosion-resistant

-    Nonmagnetic

-    Has a low melting point

-    Has a moderately high coefficient of expansion

-    Has a good thermal and electrical conductivity

-    Malleable

-    Very soft

-    Ductile

Aluminum found in historic buildings may be finished in one of the
following ways:

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
         corrosion.

    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
         and indoors.

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
         corrosion.

    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
              anodizing.

    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.

4.   Painted/lacquered:  

    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
         finish.  

    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
    aluminum surface.  

    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
         be used.  

    b.   Plated aluminum is most commonly used for features that
         may be subjected to heavy abrasion, such as stair
         railings.  

6.   Porcelain enameled:  A baked-on ceramic coating applied in the
    factory.

    a.   It is hard and impervious to soils, many acids and
         alkalies.  It is available in many colors and surface
         textures.

    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
    the 1970s.


TYPICAL USES

Typical historical uses for aluminum in the late 19th century
included:

-    Stairs

-    Elevators

-    Grilles

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:  1) Natural
or inherent problems based on the characteristics of the material
and the conditions of the exposure, and 2) 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

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
         Manufacturer's Association.

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
         mastics.


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
    problem.

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
    contraction.

    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
    indoor locations:

    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
         roofing.

    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
 


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