Environmental Engineering Reference
In-Depth Information
Hot dipping is the application of coating by immersion of the metal piece in
the molten bath of a low-melting-point metal like zinc, tin, aluminum, and lead.
Galvanizing of steel by hot dipping in molten zinc is a widely used practice for
wires, roofing sheets, and a host of other components.
Galval
coating, accom-
plished with the incorporation of aluminum in molten zinc, provides improved
performance in marine conditions.
Electroplating is the deposition of a metal from its aqueous salt solution by
passing direct current in a cell in which the metal to be plated constitutes the
cathode. Electroplating is normally suited for small components. Zinc, nickel,
tin, and cadmium are plated on steel in large tonnage. Nickel plating is particu-
larly popular for anticorrosive and decorative applications. A base of copper coat-
ing on steel is usually employed for better adhesion of nickel coating. It is often
overlaid with a thin chromium coating for appearance and also for resisting the
attack of sulfur-bearing industrial atmospheres. Such coating
is known as
''chrome plate.''
Flame spraying is a suitable method for providing metallic coatings on large
structures like bridges, ship hulls, tank cars, and vessels of all kinds. Metal wire
or powder is fed to the steel surface through a moving flame and the coating is
achieved. Oxygen or oxyacetylene is commonly used for the melting flame.
Plasma jet is used for coatings of high-melting metals. The coatings achieved by
flame spraying are usually porous, but they provide a good base for a subsequent
paint coating. Flame spraying is industrially known as ''metallizing.''
Diffusion coating involves alloy formation on the metal surface by high-tem-
perature diffusion of the coating metal. The latter may be supplied in the powder
form, dissolved in molten calcium, or obtained through a gaseous environment
containing the metal. Coatings of zinc, chromium, and aluminum are delivered
by this method; the corresponding processes are called sherardizing, chromizing,
and calorizing.
Cladding refers to a surface layer of a corrosion-resistant sheet metal, usually
accomplished by rolling, on a cheaper and stronger base. The cladding can also
be applied by explosive bonding or spot welding. Nickel, aluminum, copper,
titanium, and stainless steels are often used as cladding for steel. A lot of saving
on the costlier and more corrosion-resistant materials is achieved. Cladding of
high—strength aluminum alloys with commercially pure aluminum saves them
from SCC.
The vapor deposition processes fall into two major categories: physical vapor
deposition (PVD) and chemical vapor deposition (CVD). In PVD process the
coating metal is vaporized in a high-vacuum chamber and the vapor deposits on
the parts to be coated. The coating can also be achieved by sputtering, which
involves the transport of the coating material from a source (target) to the sub-
strate by means of the bombardment of the target by gas ions that have been
accelerated by a high voltage. There is also a third method, ion plating, which
