Environmental Engineering Reference
In-Depth Information
suitable Ni-Cr alloy followed by aluminizing produces a coating with a superficial
region rich in chromium, which imparts superior oxidation resistance.
Overlay Coatings
Weld Overlays.
The most important type of overlay coating is weld overlay,
which is the most widely applied of all the methods for bulk coatings production
in largest tonnage. It is often used to protect the inner surfaces of reaction vessels
in chemical engineering processes. In this process [72], the deposit is laid down
by melting the coating alloy on the work surface by gas or arc welding processes.
The coating material is supplied in the form of powder, paste, rod, strip, or wire.
Many of the standard welding processes, such as oxyacetylene, manual metal arc
(MMA), metal inert gas (MIG), tungsten inert gas (TIG), submerged arc, etc.,
in manual, semiautomatic, and fully automatic modes, are often employed to
develop such coatings on components ranging from small intricate shapes to large
areas of flat or cylindrical shape.
Roll Bonding and Coextrusion.
Metallurgical methods of bonding protective
layers to the outside surfaces of components without resulting fusion at the inter-
face are widely used in industry. In such techniques, the coating is bonded to the
substrate by solid-state methods that rely on a combination of surface cleanliness,
temperature, and pressure to generate an intimate atomic contact and interdiffu-
sion, which produce a true metallurgical bond, often usually stronger than the
parent metal itself. High-rate techniques [72] comprise explosive cladding and
electromagnetic impact bonding (EMIB), while a slower rate process utilizes hot
isostatic pressure (HIP), to bond both powder and solid layers onto a component.
Medium rate methods involve processing times of the order of 1 min and com-
prise such processes as roll cladding and coextrusion. The virtue of these methods
is that the coating material can be carefully controlled and must itself have a
reasonable ductility, so that it is unlikely to adversely affect the mechanical prop-
erties of the component. Explosive cladding is a solid-phase welding process in
which bonding is produced by an oblique, high-velocity collision between the
two pieces to be joined, the required force for deformation being supplied by
chemical energy. HIP is a diffusion bonding process in which a metallurgical
bond is formed by diffusion across the interface. In EMIB, the force is applied
to the component by an intense magnetic field developed by a sudden surge of
current through a coil. Coextrusion is most frequently used for products of rod
and tubular form, particularly for creep-resisting steel bodies with internal or
external cladding of stainless steels and nickel-based alloys.
Ion Implantation.
This technique is of very recent origin and is carried out in
a fairly high vacuum chamber. The process involves bombardment of the metal/
alloy surface with a high-energy beam of ions of the chosen element to be im-
planted. The ions are accelerated to an energy of about 100 keV, but their penetra-
