MIL-DTL-14072F
3.10 Plated finishes. Plated finishes shall be in accordance with Table III. The
plating process shall provide a continuous flaw free deposit. Specific finishes shall be
selected with due regard to surface classification (Type I or II) and the basis metal to be
protected. Plated finishes are designated by the letter M followed by a number. For
example, M212 is a plated nickel finish for use on iron or iron-base alloy parts to be
subjected to a Type I exposure. As specified in Table III, Finish M212 consists of
0.0152mm minimum nickel over 0.0127mm minimum copper under coat over the basis
metal.
3.10.1 Plating thickness substitution. If dimensionally acceptable, any plating
thickness specified for Type I surfaces may be used in lieu of the thickness required for
Type II surfaces. For example, a part extending in assembly from an unsheltered zone to
a sheltered zone would be finished (as applicable) with a Type I plate. Similarly, Type II
platings may be used on basis metals in unsheltered zones provided the surfaces
extending to and exposed to Type I conditions are painted with an applicable paint
system.
3.10.2 Hydrogen-embrittlement relief. Hydrogen embrittlement of hardened,
plated steel parts intended for resilient or high stress service, such as a spring or fastener,
can be a serious problem. No visual or chemical inspection method to reveal this defect
is known. Designers are, therefore, cautioned to avoid the use of hardened ferrous alloys
requiring plating whenever possible. When this is not feasible, steels employed should
be of the lowest carbon content and lowest hardness which will provide the necessary
mechanical performance of the part. The microstructure of such steels should be
essentially tempered martensite. The use of sharp bends in the part should be avoided,
particularly if the part is to be hardened after forming. Plating baths and pre-plate
cleaning processes shall be adjusted and operated in a mode designed to prevent
hydrogen embrittlement. Oxide and scale removal shall be conducted by tumbling or
other mechanical means wherever possible. A brief dip in an acid containing an inhibitor
may be used if necessary. Cleaning should be accomplished by means of soaking or
anodic treatment in alkaline cleaners. All temper-hardened parts shall be stress relieved
by baking at 25°C below draw temperature for one hour per square inch of cross section
area. All hardened steel parts subject to hydrogen embrittlement shall be treated in
accordance with Finish E200 (See Table IV). Finish requirements, when specified on
drawings or other pertinent documentation, shall include that finish designation. Thus the
finish for a Type I cadmium plated steel spring would be called out on drawings at
"M225 and E200."
3.10.3 High strength steel (i.e. 16.87 kg/sq cm and up). Special finishing
methods, such as those specified in MIL-STD-1500, MIL-STD-1501, and MIL-C-87115,
are required for materials in this class to reduce the risk of hydrogen embrittlement.
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