MIL-STD-1250A
e. Appropriate measures shall be taken to avoid metals widely separated in the galvanic series. This can be accomplished by applying to the cathodic member a sacrificial metal coating having a potential similar to or near that of the anodic member; by sealing to insure that faying surfaces are watertight; by painting or coating all surfaces to increase the resistance of the electrical circuit.
f. A small anodic area relative to the cathodic area should be avoided. The same metal or more noble (cathodic) metals should be used for small fasteners or bolts. The larger the relative anode area, the lower the galvanic current density on the anode, and the lesser the attack. The galvanic corrosion effect may be considered as inverse to the node-cathode area ratio.
5.7 Joining. The joining method, whether primarily for mechanical strength or for electrical applications, shall be scrutinized with regard to possible corrosion problems. Joints shall be continuous and impervious or shall be sealed to prevent moisture intrusion and/or entrapment. Where it becomes necessary that relatively incompatible metals must be assembled, the following precautions and joining methods are provided for alleviation of galvanic corrosion:
a. Select materials which are indicated to be more compatible in accordance with
the galvanic series; design metal couples so that the area of the cathode is smaller (appreciably) than the area of the anodic metal. For example, bolts or screws of stainless steel for fastening aluminum sheet, but not the reverse. Interpose a compatible metallic gasket or washer between the dissimilar metals prior to fastening or plate the cathodic member with a metal compatible to the anode. These precautions are applicable to couples which are to serve as an electrical connection.
b. Interpose a non-absorbing, inert gasketing material or washer between the dissimilar materials prior to connecting them. This is applicable to couples which do not serve as electrical conductors.
c. Seal all faying edges to preclude the entrance of liquids.
d. Where practical or where it will not interfere with the proposed use of the assembly, the external joint should be coated externally with an effective paint system.
e. Welded or brazed dissimilar metal assemblies should be coated with a paint system or other suitable protective coatings to at least 0.33 inch beyond the heat affected zone.
f. In protective environments (usually referred to as humidity-controlled), caution should be used in dissimilar metal combination treatments. If the assumption is made that no corrosion will occur because humidity control will be maintained, the stringent requirements would be unnecessary. It must be recognized that Humidity and moisture controlled environments can be assured only by hermetically sealed compartments or containers in which the moisture vapor content has been adequately reduced so as to preclude condensation of water at the lowest temperature expected to be encountered on the actual surface of the item. If humidity and condensate control cannot be maintained or is uncertain,
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