MIL-DTL-14538D
(g) Lot size, if different (see 4.2).
(h) Number of samples to be inspected from the lot for nondestructive tests, and for
destructive tests if the number of items in a lot is four or less (see 4.3.1).
6.3 Dimensional change. Black chromium deposits involve only a slight dimensional change of
approximately 0.0002 inches. In the usual applications, however, the dimensional increase is of little or
no importance. For that reason it has not been considered necessary to specify an exact thickness of
coating which should be no more than necessary to meet the required performance.
6.4 Adaptability. Black chromium plate may be applied to metals the same as bright chromium
plate. It may also be applied over bright chromium plate with an increase in plating time to obtain added
corrosion protection.
6.5 Equipment and processing. Plating tanks of mild steel or polyvinyl chloride have been found
entirely satisfactory. Lead or butyl rubber lined tanks are not suitable due to the high acetic acid content
of the solution. Tanks need not be equipped with steam coils. An adequate exhaust system is required
to remove the acetic acid fumes. Anodes of mild steel are preferred to carbon anodes due to the
attrition of carbon anodes. Parts with deeply recessed areas may require auxillary iron wire or platinum
anodes as the throwing power of black chromium plating solution is not appreciably greater than that of
ordinary chromium solutions. The processing operations require careful control and proper maintenance
of the solution. Plating can be accomplished at normal room temperature or temperature as high as
110oF (44oC). The usual plating time is 30 to 45 minutes. Racks and hooks should be kept clean for
good contact and have adequate current carrying capacity. A stable power supply of 6 to 12 volts at a
current density of 40-90 amp. per sq. ft. is required. A solution that can be used for general application
is as follows:
Chromium Trioxide, Technical - Cr03
33-40 oz./gal.
Federal Specification 0-C-303
Acetic Acid, Glacial, Technical-CH3C00H
28.2 oz./gal. (by wt.)
Federal Specification 0-A-76
Water - H20
Remainder
Barium Carbonate, Powder - BaC03 or
Barium Acetate, Crystal - Ba(CH3C00)2
1 oz./gal.
Whenever an addition of Cr03 is made to the solution, barium carbonate or barium acetate should also
be added.
6.5.1 Solution maintenance. The bath may become sluggish and exhibit poor throwing power in
use. The addition of one gallon of water per one hundred gallons of solution will generally improve the
throwing power.
6.5.2 Solution analysis. The chromic acid and acetic acid content is determined by using an
electrometric titrator with standard 0.1 Normal sodium hydroxide as the titrant. A curve is also made
from a standard solution made up according to the formula in 6.5. The chromic acid is determined by
comparing the volumes of standard sodium hydroxide for the unknown with the standard at pH 3.7.
6.5.3 Acetic acid. Acetic acid content is determined by a comparison of volumes at pH 5.7.
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