1.7.3 Bond Protection Code. For bonds of high reliability, corrosion must be prevented by (1) avoiding the
pairing of dissimilar metals and (2) preventing the entrance of moisture or other electrolytes into the bond area.
Metals to be in direct contact should fall as close together in the galvanic series (see Volume I, Section 18.104.22.168)
as possible. Compatible groupings of the common metals are given in Table 1-24. The corrosive action
between metals of different groups will be greatest when the metallic union is openly exposed to salt spray,
rain, or other liquids. The less exposed the bond, the less the rate of corrosion. The relative degrees of
exposure may be defined as follows (l-8):
Open, unprotected exposure to weather.
Limited protection from direct action of weather. Locations in louvered housings, sheds,
and vehicles offer sheltered exposure.
Located in weatherproof buildings.
When bonds under these different exposure conditions must be made between different groups, they should be
protected as indicated by Table 1-25. Condition A means that the couple must have a protective finish applied
after metal-to-metal contact has been established so that no liquid film can bridge the two elements of the
couple. Condition B means that the two metals may be joined with bare metal exposed at junction surfaces.
The remainder of the bond must be given an appropriate protective finish. Condition C indicates that the
combination cannot be used except under very unusual circumstances where short life expectancy can be
tolerated or when the equipment is normally stored and exposed for only short intervals. Protective coatings
for bonds subjected to weather exposure, corrosive fumes, or excessive dust are mandatory. Consult Table 1-26
for assistance in choosing a method for protecting the bond members against corrosion.
Compatible Groups of Common Metals
Aluminum, aluminum alloys, zinc, cadmium
Carbon steel, iron, lead, tin, lead-tin solder
Nickel, chromium, stainless steel
Copper, silver, gold, platinum, titanium