Coatings are developed to offer protection from corrosion
and erosion to secure the material from chemical and physical interaction with
its environment. Corrosion and wear issues are of great relevance in diverse
industrial applications as they result in the degradation and eventual failure
of components and systems in the processing and manufacturing industries and in
the service life of various components. Different technologies are used to
deposit the suitable surface protection to provide protection under specific
conditions. They are often distinguished by coating thickness.
Most thermal coating processes are used at atmospheric
pressure in air, except plasma spraying, usually served in soft vacuum. Plasma
spraying can be performed in an inert condition or vacuum and cold spray is
usually performed at atmospheric pressure however in a controlled condition
chamber to collect reuse the spray gas due to the large gas flow rates
required.
Extreme uses of arc spray wire are now in culture
against wear and corrosion as well as heat and also for functional purpose. The
choice of coating process is firmly based on the required coating properties
for the application and coating cost. Coating characteristics are determined by
the coating material and the form in which it is used, as well as the setoff
parameters used to conduct the coating process. Thermal spray coatings are
usually featured by a lamellar structure and the real contact between the
splats and substrate or the earlier deposited layers determine to a large level
of the coating properties like heat conductivity, Young modulus etc.
The real contact area ranges between 20-60% of the coating
surface parallel to the substrate. It increases with impact speed of particles.
Therefore the coating density increases with increase from flame, wire arc,
plasma and gun spraying and thereafter re-fused.
The variety of corrosions particularly for coatings can be
categorized as general corrosion, related to 30% failure, where the average
rate of corrosion on the surface is uniform and localized corrosion, about 70%
of failures. The galvanic corrosion occurs when two different metals are in
contact with each other in a conductive solution, the more anodic metal is
attacked where each other in a conductive solution, the more anodic metal is
attacked, while the more cathodic one is uninfluenced. The electrolyte plays a
significant role, and the relative surface contact area, small anodic to
cathodic area ratio refers to severity of anodic metal corrosion.
The corrosion can also be intergranular as well as transgranular
when cracking occurs. The coating material and is microstructure plays a
crucial role in this type of corrosion. The coatings provide protection from
corrosion, sacrificial coatings, thicker coatings offer longer protection.
Corrosive wear occurs when the influence of corrosion and
wear are combined, resulting into faster failure of material surface. A surface
that is oxidized can be mechanically weakened and can wear at a higher rate.
Stress corrosion failure results from the combined influence of stress and corrosion.
With thermal spray coatings, such types of metallic failures can be
significantly reduced.
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