Improving the appearance of a finished product
for purely aesthetic reasons can be important
because it often increases the salability
of the product. The increased product performance
and safety provided by proper edge and surface
finishing is also important. The removal
of burrs and sharp edges improves safety
for both the worker and product user by
eliminating the possibility of cuts and
making parts easier to handle. For critical
components, the surface condition and edge
geometry can be a major influence on component
performance and durability.
From strictly an engineering
point-of-view, surface finishing is primarily
good for one thing: preventing corrosion.
Historically, man has employed three different
mechanisms to stop corrosion: (1) barrier
coatings, (2) inhibitive primers and (3)
the use of zinc anodes.
Barrier coatings are simply
paints applied to the surface of metal to
create a "Barrier" or "Wall" between the
metal and the corrosion-initiating exterior
atmosphere. To protect yourself on a rainy
day, you put on a raincoat to keep you dry.
The paint of a barrier coating acts as a
raincoat for the steel. This method is only
partially effective since barrier coats
are not completely impermeable to moisture
and will eventually breakdown, allowing
the corrosion process to begin, and will
only protect as long as the coating is intact.
If a barrier coating is scratched or damaged
in some way exposing the underlying metal,
employ special pigments which provide corrosion
protection through their ability to release
inhibitive ions which are carried to the
metal surface as water penetrates the coating.
At the metal surface, these ions modify
anode and/or cathode reactions, and force
the steel's potential to corrode into a
passive mode. These coatings are somewhat
effective but give limited service life
and will allow corrosion to occur at damaged
areas, like barrier coatings.
Back in the 1700's a man
named Luigi Galvani discovered that if you
place two dissimilar metals in direct, electrical
contact with each other and subject them
to an electrolytic solution, ions from the
least noble metal go into solution, liberating
electrons and causing a current flow into
the more noble metal preventing it's ions
from going into solution. The process described,
which became known as "Galvanizing" (aptly
named for Mr. Galvani), employs the use
of zinc as the anode, or least noble metal.
The zinc slowly releases it's ions causing
the current to flow into the metal it's
applied to. The "hot-dip" galvanizing process,
where iron or steel is dipped into molten
zinc at 850 degrees F, was born by a French
chemist named Melovin in 1742. Since then,
hot-dip galvanizing has been considered
by many to be the epitome of corrosion protection.