MIL-R-193090
3.3.4 Materials. Unless otherwise specified in the contract, requisition, or order, all materials and purchased parts entering into the final assembly of the relay shall conform strictly to the applicable Federal or Military specifications. Materials which are not covered by specification or which are not specifically designated herein shall be of the best quality, or the lightest practicable weight consistent with strength and durability, and entirely suited to the purpose for which intended under all conditions of test and service operation. Recommendations concerning the use of a superior substitute for any part or material specified herein shall be accompanied by a description of the proposed substitute, the reasons therefore, together with evidence to substantiate any claim as to its suitability. At the discretion of the Naval Surface Warfare Center, test samples may be required to prove the suitability of the proposed substitute.
3.3.4.1 Critical materials. Materials shall be selected on the basis of suitability and availability, taking into consideration the additional restrictions created during a time of national emergency. Critical materials shall be used only when essential for satisfactory operation of the particular item or equipment. Substitution of less critical materials for materials required by this specification is subject to approval by the Naval Surface Warfare Center.
3.3.4.2 Magnesium. The use of magnesium requires the specific approval of the Naval Surface Warfare Center or its appointed agency.
3.3.4.3 Flammable materials. Materials that support combustion or which are capable of causing an explosion shall not be used.
3.3.4.4 Toxic materials. Materials that produce harmful toxic effects under conditions encountered in the Naval
Service shall not be used.
3.3.5 Moisture, fungus, and corrosion. The relay shall be rendered moisture, fungus, and corrosion resistant as required by NAVOR0 OS 6341.
3.3.6 Finishes and coatings.
3.3.6.1 General. All machining operations shall be completed before the application of the corrosion-resistant treatment.
3.3.6.2 Plating. Electrodeposited metallic coatings shall conform to the following. The plating thickness in all cases shall be adequate to ensure conformance with the parts or equipment specification for conductivity and corrosion resistance:
Zinc ASTM-8633, Type II, Class 2 or 3
Nickel SAE-AMS-QQ-N-290
Chromium SAE-AMS-QQ-C-320, Type I Silver ASTM 8700
NOTE: Cadmium plating shall not be used.
3.3.6.3 Tin and lead-tin alloys. Plating of tin and lead-tin alloys, when used, shall be of adequate thickness and so continuous as to pass the marine environment test.
3.3.6.4 Corrosion resistant steel.
3.3.6.4.1 Cleaning. Remove all oil, grease, dirt, and other foreign matter from the metal by vapor degreasing, washing with petroleum spirits, MIL-PRF-680, Grade 1, or by immersion in a boiling solution of trisodium phosphate, O-S-642, followed by rinsing in cold water and a final rinse in hot water. If necessary because of scale, immerse in the following solution held at 60°C (140°F) until the scale is removed:
Nitric acid 20 percent by volume Hydrofluoric acid 1 to 3 percent by volume Water 77 to 79 percent by volume
Then rinse in cold water and blow dry with compressed air. If structures are fabricated with overlapping joints, the preferred method of cleaning is by vapor degreasing to avoid entrapment of caustic or acid solutions.
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