Because the oxide film is somewhat porous. it
can be impregnated with dyes of various colors. Anodized gears possess not only improved
appearance, but also other significant protection against many corrosive
atmospheres and salt sprays.
15.2 Chromate Coatings
Applicable to aluminum, bronze, zinc and magnesium, these are
low-temperature dip-bath processes that produce a chemical film of chromate
which is extremely thin and does not alter dimensions. However, the thin film
has little wear resistance and offers corrosion protection only against
non-abrasive environments. Coating color varies with the particular metal and
alloy. Most often there is an iridescent color, which generated the common trade
name Iridite, Dyes can be added to produce a wide assortment of colors. Because
there is no dimensional change, chromating can be applied to all gears,
including precision, after tooth cutting.
15.3 Passlvatlon
This is not a coating, in the strict sense, but a
conditioning of the surface. It is particularly applicable to stainless steels.
The process is essentially a low strength nitric acid dip. It results in an
invisible oxide film that develops the "stainless" property, removes "tramp
iron" and reduces the metal’s anodic potential in the galvanic series.
Passivation causes no dimensional changes and does not discolor or otherwise
alter the natural surface. If anything, it prevents random staining due to
"free iron" particles left from machining. All quality stainless steel
gears can be passivated after complete machining since dimensions and stability
are unaffected.
15.4 Plating.
The common electroplating materials, such as cadmium,
chromium, nickel and copper, are not suitable for gear surfaces since they alter
dimensions. Also, susceptibility to localized buildup precludes their use on any
precision part. Use of these platings should be limited to the application of
coatingt prior to cutting of the teeth and of any other gear dimensions
requiring close tolerances.
15.5 Special Coatings
In recent years, special extra thin precision coatings have
been developed and are available under different commercial names, Some claim
surface hardness, wear resistance, low coefficient of friction, anti-corrosive
qualities, etc. There are many successful applications on record. Each case
however should be investigated and tested.
15.6 Application of Coating.
It is advisable to finish coat all gears which operate in a
corrosive environment or must meet the requirements of military equipment
applications. In addition, appearance considerations may
compel a protective finish.
Aluminum gears are best protected when anodized in a natural color but not on
the tooth surfaces.
A chromate coating is adequate for many applications
and is acceptable in many military
equipment specifications.
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