Black Oxide is a versatile finish.
Black Oxide is a conversion coating formed by a chemical reaction
produced when parts are immersed in an alkaline aqueous salt solution at
approximately 285 degrees F. Although this process will produce similar
results on some non-ferrous metals and alloys, it is usually found on
iron, steel, and stainless steels. The reaction between the iron of the
ferrous alloy and the hot oxidizing bath produces a magnetite (Fe3 O4)
on the surface of the parts.
When one speaks of a Black Oxide process, they are usually referring
to the hot bath process. There are in fact both hot and cold processes,
as well as a process using an intermediate operating temperature. The
hot bath Black Oxide process works best between 285 and 290 degrees F.
As this is an aqueous solution, there is considerable evaporation loss,
which must be accommodated by the introduction of replacement water. As
the temperature of the bath is well above the boiling point of water,
the replacement water must be added very carefully, so as to avoid
entrapment of water, resulting in bursts, or explosions, of steam. This
is the element that makes the process dangerous for the inexperienced or
To avoid the danger posed by the formation of super-heated steam,
both a cold process, and a "warm" process have been developed. The cold
process is accomplished at room temperature. The results are uniformly
disappointing, as the resulting color is not uniform, and as the final
finish is almost always "smutty", i.e., it rubs off on your hands and
clothing. The Birchwood-Casey Company has developed a process that
operates at 190 degrees F., below the boiling point of water, and
reportedly gets better results than the cold process.
Hot black oxide can be accomplished from generic mixtures of caustic
soda, sodium nitrate/nitrite, wetting agents and stabizers, or from
proprietary mixtures. Some have inquired regarding gun-bluing and/or
steel-brown patinas. Most gun bluing solutions are identical with the
hot bath black oxide formulations; some employ more sodium cyanide,
which lends the bluish cast to the black. Likwise, the steel-brown is
accomplished with the addition of a different salt.
There are five steps to a successful black oxide coating:
Clean, Rinse, Black Oxide, Rinse, After-finish. It is most obvious that
without proper cleaning, and rinsing, the black oxide coating may be
less than uniform in color or coverage. What is not as obvious is that
without a proper after-finish, flash-rusting almost always results.
After-finishes fall into two major categories, oil, and wax. Lacquers
are also used. If none is specified, the default is an oil. The
preference is determined by application of the item, and the appearance
desired. An oil after-finish will be more toward a glossy appearance,
while a wax after-finish will be more to the matte.
The resulting black oxide finish will have a significant degree of
water- and rust-resistance; however, black oxide coatings cannot be
expected to perfom well in long-term exterior, or harsh marine
environments. Water-resistant, yes; water-proof, no.
Occasionally, it may be necessary to remove the black oxide coating
from a few parts. This can be accomplished with Muriatic Acid (a 10%
solution of Hydrochloric Acid). Be sure to take all necessary
precautions for personal, and environmental, protection if and when such
removal becomes necessary. The black oxide coating will also be removed
by high heat. The after-finish (oil or wax) will fail between 200 and
300 degrees F., then the color will become mottled and finally "burn
off" as the temperature approaches 900 or 1000 degrees F. If the parts
are to be hot-forged to achieve a decorative effect, there is no need
for the acid bath, as the heat of the forge will accomplish the removal
of the black oxide.