Environmental Engineering Reference
In-Depth Information
is a combination of evaporation and glow discharge (plasma formation). In CVD
the coating metal is produced and alloyed at the surface of the workpiece through
gas phase reaction with a volatilized compound.
The films produced by vapor deposition are dense and pore-free; therefore,
they are resistant to penetration by moisture and gases. However, the cost and
complexity of the processes have restricted their industrial application for corro-
sion protection purposes. Nevertheless, the vapor-deposited aluminum coating
is finding wide applications, particularly in the aerospace industry. The brittle
intermetallic layer of iron-aluminum alloy produced in hot-dipped steel, which
hampers the formability of steel, is absent in steels with vapor-deposited alumi-
num coating. Cadmium electroplating produces hydrogen embrittlement in some
varieties of steel; replacement by vapor-deposited aluminum coating eliminates
this problem. Sputtered chromium and stainless steel are also making inroads in
corrosion applications.
Surface Modification
Surface composition or structure of a metallic part can be modified drastically
with respect to the bulk material by use of directed energy or particle beams.
The modification is principally metallurgical in that it involves alloy formation
or the formation of an amorphous layer, in contrast to chemical conversion coat-
ings (see section ''Chemical Conversion Coatings'') which are inorganic in na-
ture. However, the structural changes achieved differ significantly from those
obtained by conventional processes and this becomes a contributing factor to the
improved performance of surface-modified metals in corrosion applications. The
principal surface modification methods are ion implantation and laser process-
ing.
Ion implantation involves the deposition of ions of metals or nonmetals on
the metal substrate and several techniques are employed. Both conventional and
unconventional alloying have been carried out, the latter being more attractive.
For example, tantalum, which cannot be alloyed easily with iron, can be easily
implanted on steel. Amorphous layers produced by phosphorus or boron implan-
tation in iron-chromium alloys increase corrosion resistance and improve passiv-
ation characteristics of the alloys. Even the implantation of chromium or molyb-
denum on iron to achieve the properties of stainless steel means a lot of saving
on these expensive alloying elements. Similarly, the noble metal bulk alloying
can be avoided by simple ion implantation, e.g., palladium on titanium.
Laser processing is carried out by high-energy laser beams ranging from 0.5
to 10 kW. The surface of the metal may simply be modified to an amorphous layer
by melting and rapid quenching. Alloying elements added to the beam produces
surface-alloying plain-carbon steels. Laser surface alloyed with chromium and
molybdenum has been reported to exhibit the same passivation behavior of the
bulk alloys of corresponding composition. Laser-modified type 304 stainless steel
