Atmospheric Corrosion
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roll bonding (e.g., stainless steel to non-heat-treatable aluminum alloys),
by chrome plating, or by vapor deposition or ion deposition of a variety of
metals. The latter methods are expensive and have size limitations. Unlike
Alclad coatings, metal barrier coatings do not provide any electrochemical
protection at a break in the coating.
Conversion coatings and paint coatings are the more conventional barrier coatings.
Anodized coatings. Anodized coatings are produced by an electrolyte process. The surface of the aluminum is immersed in an acidic bath and made
the anode, and a direct current is applied. Oxidation of the surface produces
a hard, porous film of aluminum oxide that is bound as tenaciously to the
alloy as the natural oxide film but is much thicker. Most anodized coatings are produced using either a sulfuric acid, chromic acid, or phosphoric
acid bath. Other solutions can also be used (e.g., boric acid, oxalic acid). The
porous oxide film is sealed by immersion in boiling water, which imparts
impermeability to the film. The porous film may be dyed to impart color
before sealing.
The anodized surfaces are shinier and more wear resistant than the natural oxide surface and are easier to keep clean. Eventually they will wear or
roughen through use. The primary attributes of anodizing are in decorative
appearance and ease of cleaning.
Anodized coatings are not resistant in most environments having a pH
outside the range of 4 to 9. Within this range, the corrosion resistance is
improved by an order of magnitude or more. Atmospheric weather tests show
that the number of pits that develop in the base metal decrease exponentially
with the thickness of the coating. As a general rule, anodized coatings do
not provide sufficient protection to make aluminum alloys suitable for environments where without the coatings, the alloys themselves are unsuitable
because of their poor resistance in them.
Anodized surfaces are excellent for subsequent painting but usually less
expensive treatments are used for this purpose.
Paints. Aluminum is painted for both decorative and protective purposes.
As with any metal, surface preparation is extremely important. Conversion
coatings, either of the chromate or phosphate type, are recommended for
the preparation of aluminum alloys. In environments containing chlorides,
a chromated primer should be applied first. Water-based paints are particularly suitable for use with aluminum alloys.
4.7.7.2 Applications
Aluminum and aluminum alloys find applications in such outdoor atmospheres as roofing, building facades, and window frames. They are particularly useful in SO 2 -polluted atmospheres because of their relatively good
resistance. After decades of exposure, aluminum structures in urban atmospheres have been reported to be in excellent condition.
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roll bonding (e.g., stainless steel to non-heat-treatable aluminum alloys),
by chrome plating, or by vapor deposition or ion deposition of a variety of
metals. The latter methods are expensive and have size limitations. Unlike
Alclad coatings, metal barrier coatings do not provide any electrochemical
protection at a break in the coating.
Conversion coatings and paint coatings are the more conventional barrier coatings.
Anodized coatings. Anodized coatings are produced by an electrolyte process. The surface of the aluminum is immersed in an acidic bath and made
the anode, and a direct current is applied. Oxidation of the surface produces
a hard, porous film of aluminum oxide that is bound as tenaciously to the
alloy as the natural oxide film but is much thicker. Most anodized coatings are produced using either a sulfuric acid, chromic acid, or phosphoric
acid bath. Other solutions can also be used (e.g., boric acid, oxalic acid). The
porous oxide film is sealed by immersion in boiling water, which imparts
impermeability to the film. The porous film may be dyed to impart color
before sealing.
The anodized surfaces are shinier and more wear resistant than the natural oxide surface and are easier to keep clean. Eventually they will wear or
roughen through use. The primary attributes of anodizing are in decorative
appearance and ease of cleaning.
Anodized coatings are not resistant in most environments having a pH
outside the range of 4 to 9. Within this range, the corrosion resistance is
improved by an order of magnitude or more. Atmospheric weather tests show
that the number of pits that develop in the base metal decrease exponentially
with the thickness of the coating. As a general rule, anodized coatings do
not provide sufficient protection to make aluminum alloys suitable for environments where without the coatings, the alloys themselves are unsuitable
because of their poor resistance in them.
Anodized surfaces are excellent for subsequent painting but usually less
expensive treatments are used for this purpose.
Paints. Aluminum is painted for both decorative and protective purposes.
As with any metal, surface preparation is extremely important. Conversion
coatings, either of the chromate or phosphate type, are recommended for
the preparation of aluminum alloys. In environments containing chlorides,
a chromated primer should be applied first. Water-based paints are particularly suitable for use with aluminum alloys.
4.7.7.2 Applications
Aluminum and aluminum alloys find applications in such outdoor atmospheres as roofing, building facades, and window frames. They are particularly useful in SO 2 -polluted atmospheres because of their relatively good
resistance. After decades of exposure, aluminum structures in urban atmospheres have been reported to be in excellent condition.