Metal Coatings

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Metal Coatings

• Many new materials have been developed, but steel
  remains the principal construction material for
  automobiles, appliances, and industrial
  machinery. Because of steel's vulnerability to
  attack by aggressive chemical environments or
  even from simple atmospheric oxidation, coatings
  are necessary to provide various degrees of
  protection. They range from hot-dipped and
  electroplated metals to tough polymers and
  flame-sprayed ceramics.
• In general, corrosive environments contain more
  than one active material, and the coating must
  resist penetration by a combination of oxidizers,
  solvents, or both. Thus, the best barrier is one
  that resists "broadband" corrosion.

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• Physical integrity of the coating is as important
  as its chemical barrier properties in many
  applications. For instance, coatings on impellers
  that mix abrasive slurries can be abraded
  quickly coatings on pipe joints will cold-flow
  away from a loaded area if the creep rate is not
  low and coatings on flanges and support brackets
  can be chipped or penetrated during assembly if
  impact strength is inadequate. Selecting the best
  coating for an application requires evaluating
  all effects of the specific environment,
  including thermal and mechanical conditions.

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• Zinc One of the most common and inexpensive
  protection methods for steel is provided by zinc.
  Zinc-coated, or galvanized, steel is produced by
  various hot-dipping techniques, but more steel
  companies today are moving into
  electrogalvanizing so they can provide both.
• Oxidation protection of steel by zinc operates in
  two ways -- first as a barrier coating, then as a
  sacrificial coating. If the zinc coating is
  scratched or penetrated, it continues to provide
  protection by galvanic action until the zinc
  layer is depleted. This sacrificial action also
  prevents corrosion around punched holes and at
  cut edges.

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• The grades of zinc-coated steel commercialized in
  recent years have been designed to overcome the
  drawbacks of traditional galvanized steel, which
  has been difficult to weld and to paint to a
  smooth finish. The newer materials are intended
  specifically for stamped automotive components,
  which are usually joined by spot welding and
  which require a smooth, Class A painted finish.
• Among today's improved galvanized steels is
  Inland Steel's Paint-Tite B family of three
  products. These galvannealed (heat-treated)
  steels have a hot-dipped zinc coating on one side
  and a light, gas-jet-wiped coating on the smooth
  side. They are thermally treated to provide a
  uniform, spangle-free, paintable, weldable,
  zinc-iron alloy coating. Because the zinc-iron
  crystals on the coated side are very fine, the
  sheet can be formed or rolled with no
  "print-through" in the top surface.

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• Another improved zinc-coated steel is Armco's
  Ultrasmooth, which has a hot-dipped zinc coating
  on both sides and surfaces comparable to a Class
  1 cold-rolled steel finish. Jets of nitrogen,
  which surround the steel as it emerges from the
  molten zinc, control the coating thickness. The
  resulting surfaces are free from ripples and
  oxide patterns commonly found on conventional
  hot-dipped galvanized steels.
• Armco's newest zinc-coated product,
  Electrasmooth, has an electrogalvanized coating
  on one or both sides. Made by the vertical-cell
  process, it has a smooth, uniform surface
  suitable for painted automotive panels, appliance
  wrappers, and architectural products.
  Electrasmooth coatings are available on the full
  range of commercial and drawing-quality grades.

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• Another new process for zinc coating of steels is
  Galfan, developed by the International Lead Zinc
  Research Organization (ILZRO). In addition to
  zinc, the coating contains mischmetal (a mixture
  of cerium, lanthium, and other rare-earth metals)
  and 5 aluminum. Galfan is claimed to provide
  improved formability and paintability, and to
  have two to three times the corrosion resistance
  of conventional, hot-dipped galvanized steel in
  salt-spray, sulfur-dioxide, and humidity tests.
• Licenses to produce the material in the U.S. have
  been granted to Gregory Galvanizing Metal
  Processing Inc., Canton, Ohio, and Weirton Steel
  Corp., Weirton, W.Va. Galfan has been used in
  Europe and Japan in applications such as building
  panels, automobile and appliance parts, and
  marine wire rope. U.S. applications include
  panels for washing machines, fence posts and
  related components, and garage-door hardware.
  Automotive applications include tubing for
  transmission cooling lines, various under-hood
  brackets and housings and, for the 1988 Ford
  T-Bird, a fuel-tank shield.

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• Another corrosion-resistant coated-steel product
  involving zinc is Zincrometal (Metal Coatings
  International), which is processed with a
  proprietary, two-coat system. The base coat is a
  chromium-base inorganic material called Dacromet.
  After the coating is baked, the coil is coated
  with a second material -- Zincromet, an
  epoxy-based, zinc-rich topcoat -- followed by
  another baking cycle.
• Like the other zinc-coated materials, Zincrometal
  is used principally for truck and automobile
  exterior panels. Minimum dry-film thickness for
  such applications is 0.5 mil. With proper
  lubricants and drawing compounds, the formability
  of the material is comparable to that of
  cold-rolled steel. Welding characteristics are
  claimed to be similar to those of the uncoated
  steel.

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• Unlike hot-dip galvanized or electrogalvanized
  materials, Zincrometal provides barrier, rather
  than sacrificial, protection. Thus, it is
  recommended for use where corrosion protection is
  the primary need, where components are not
  subjected to abrasion or mechanical abuse.
  Zincrometal, which is manufactured on a toll
  basis by coil-coating companies, is essentially a
  one-sided product, however, and is being replaced
  in some automotive applications by two-side
  coated steels.
• Aluminum Two types of aluminum-coated steel are
  produced, each for a different kind of corrosion
  protection. Type 1 has a hot-dipped
  aluminum-silicon coating to provide resistance to
  both heat and corrosion. Type 2 has a hot-dipped
  coating of commercially pure aluminum, which
  provides excellent durability and protection from
  atmospheric corrosion. Both grades, developed by
  Armco, the producer of these aluminized steels,
  are usually used unpainted.

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• Type 1 aluminum-coated steel resists heat scaling
  to 1,250F and has excellent heat reflectivity to
  900F. Nominal aluminum-alloy coating is about 1
  mil on each side. The sheet is supplied with a
  soft, satiny finish. Typical applications include
  reflectors and housings for industrial heater
  panels, interior panels and heat exchangers for
  residential furnaces, microwave ovens, automobile
  and truck muffler systems, heat shields for
  catalytic converters, and pollution-control
  equipment.
• Type 2 aluminized steel, with an aluminum coating
  of about 1.5 mil on each side, resists
  atmospheric corrosion and is claimed to outlast
  zinc-coated sheet in industrial environments by
  as much as five to one. Typical applications are
  industrial and commercial roofing and siding,
  drying ovens, silo roofs, and housings for
  outdoor lighting fixtures and air conditioners.

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• For higher temperature applications, Type 1
  coatings are supplied on two new products.
  Aluma-Ti is a vacuum-degassed, interstitial-free
  steel containing columbium and titanium that can
  be used in cyclic service to 1,400F. The second
  product, Aluma-Fuse, operates to a maximum metal
  temperature of 1,600F. Its high-temperature
  properties come from a combination of the
  diffused coating and a low-alloy steel base metal
  containing chromium, aluminum, silicon,
  columbium, and titanium, plus a heat treatment.
• Armco has also developed a process for aluminum
  coating of Type 409 stainless steel. This
  product, first tested in some 1987 automobiles,
  is expected to extend exhaust-system life to five
  years or more because of its superior resistance
  to corrosion by hot exhaust condensates and road
  salts. Another benefit of the aluminum coating is
  cosmetic. It prevents red rust staining
  indefinitely.

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• Electroplating Use of protective electroplated
  metals has changed in recent years, mainly
  because of rulings by the Environmental
  Protection Agency. Cyanide plating solutions and
  cadmium and lead-bearing finishes are severely
  restricted or banned entirely. Chromium and
  nickel platings are much in use, however, applied
  both by conventional electroplating techniques
  and by new, more efficient methods such as Fast
  Rate Electrodeposition (FRED). This latter method
  has also been used successfully by Battelle
  Columbus Labs to deposit stainless steel on
  ferrous substrates.
• Functional chromium, or "hard chrome," plating is
  used for antigalling and low-friction
  characteristics as well as for corrosion
  protection. These platings are usually applied
  without copper or nickel underplates in
  thicknesses from about 0.3 to 2 mil. Hard-chrome
  plating is recommended for use in saline
  environments to protect ferrous components