MIL-STD-171F
5.1 IN0RGANIC FINISHES
5.1.1 Metallic Coatings. Metals treated with electrodeposited coatings are subject to hydrogen embrittlement. Hydrogen embrittlement during plating may be avoided by the use of vacuum deposited coatings such as AMS-C-8837 or mechanically deposited coatings such as AMS-C-81562 when specified for use. The basis metal shall be substantially free from flaws or defects that will be detrimental to the appearance or performance of the deposited metal. The surface shall be cleaned and coated as required by the detail specification, drawing or contract. The procuring activity shall select the desired finish from those shown in table II, and shall reference it on drawings, in contracts or in item specifications by the number shown in the table. Unless otherwise specified, steel parts Rockwell C-35 or above hardness, shall not be electrodeposited without a proper stress relief prior to and a hydrogen embrittlement relief treatment after plating or the specific approval of the procuring activity. These parts may be
either vacuum coated with cadmium in accordance with AMS-C-8837 or mechanically coated with cadmium or zinc in accordance with AMS-C-81562. There are specific applications which allow plating of steel parts exceeding HRC 35, e.g., chromium plating for high strength steel landing
gear parts; reference should be made to the applicable drawing on item specification for
exceptions. The plated coatings listed in table II are applied by the usual or conventional plating techniques. Where in-place plating touch-up, build-up, or repair of metallic parts or surfaces are necessary, brush plating techniques may be used. Reference should be made to MIL-STD-865.
5.1.1.1 Stress relief of ferrous alloys. Unless otherwise specified for a particular end item specification or drawing, after forming and hardening, and prior to cleaning and plating, objectionable residual stress in ferrous alloy parts having a hardness greater than Rockwell C-35 shall be relieved by suitable heat treatment. The temperature shall be such that maximum relief is given without hardness being reduced to less than the specified minimum. Stress relief is not necessary where it has been demonstrated that plating has no harmful effects on the plated part. When prestressed wire springs are to be plated, they shall be stress relieved immediately after winding.
5.1.1.2 Embrittlement relief. Unless otherwise specified by a specific end item plating requirement, all steel parts having a hardness of Rockwell C-35 and higher shall be baked at
3750 ± 25 0 F (191 0 ± 14 0 C) for three hours or more as soon after plating as practicable. Plated springs or other parts subjected to flexure shall not be flexed prior to the baking operations. If the
plated part (such as cadmium or zinc plate) is to be given a supplementary surface conversion treatment, such as chromate or phosphate, it should be treated to relieve hydrogen embrittlement
before applying the conversion treatment, which could be rendered ineffective by baking.
5.1.2 Conversion coatings. Unless otherwise specified, stress relief and hydrogen embrittlement relief for steel parts HRC 39 and above shall be performed to MIL-DTL-16232 or TT-C-490. Hydrogen embrittlement relief treatments above 180 0 F (820 C) for zinc phosphate and 2400 F (115.50 C) for manganese phosphate may adversely affect the corrosion resistance of the coating.
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