7.6.3 Platings and Inorganic Finishes.
Many metals are plated or coated with other metals or are treated to produce surface films to achieve
improved wearability or provide corrosion resistance. Metal platings such as gold, silver, nickel, cadmium, tin,
and rhodium should have all foreign solid materials removed by brushing or scraping and all organic materials
removed with an appropriate solvent. Since such platings are usually very thin, acids and other strong etchants
should not be used. Once the foreign substances are removed, the bond surfaces should be burnished to a bright
shiny condition with fine steel wool or fine grit sandpaper. Care must be exercised to see that excessive metal
is not removed. Finally, the surfaces should be wiped with a cloth dampened in a denatured alcohol or dry
cleaning solvent and allowed to dry before completing the bond.
Chromate coatings such as iridite 14, iridite 18P, oadkite 36, and alodine 1000 offer low resistance as well as
provide corrosion resistance. These coatings should not be removed. In general, any chromate coatings meeting
the requirements of MIL-C-5541 (7-11) should be left in place.
Many aluminum products are anodized for appearance and corrosion resistance. Since these anodic films are
excellent insulators, they must be removed prior to bonding. Those aluminum parts to be electrically bonded
either should not be anodized or the anodic coating must be removed from the bond area.
7.6.4 Corrosion By-Products. Oxides, sulfides, sulfates, and other corrosion by-products must be removed
because they restrict or prevent metallic contact. Soft products such as iron oxide and copper sulfate can be
removed with a stiff wire brush, steel wool, or other abrasives. Removal down to a bright metal finish is
generally adequate. When pitting has occurred, refinishing of the surface by grinding or milling may be
necessary to achieve a smooth, even contact surface. Some sulfides are difficult to remove mechanically and
chemical cleaning and polishing may be necessary. Oxides of aluminum are clear and thus the appearance of
the surface cannot be relied upon as an indication of the need for cleaning. Although the oxides are hard, they
are brittle and roughening of the surface with a file or coarse abrasive is an effective way to prepare aluminum
surfaces for bonding.
7.7 COMPLETION OF THE BOND.
After cleaning of the mating surfaces, the bond members should be assembled or attached as soon as possible.
Assembly should be completed within 30 minutes if at all possible. If more than 2 hours is required between
cleaning and assembly, a temporary protective coating must be applied. Of course, this coating must also be
removed before completing the bond.
The bond surfaces must be kept free of moisture before assembly and the completed bond must be sealed
against the entrance of moisture into the mating region. Acceptable sealants are paint, silicone rubber, grease,
and polysulfates. Where paint has been removed prior to bonding, the completed bond should be repainted to
match the original finish. Excessively thinned paint should be avoided; otherwise, the paint may seep under the
edges of the bonded components and impair the quality of the connection. Compression bonds between copper
conductors or between compatible aluminum alloys located in readily accessible areas not subject to weather
exposure, corrosive fumes, or excessive dust do not require sealing. This is subject to the approval of the
responsible civil engineer or the local authorized approval representative.