MIL-DTL-14072F
3.13.1 Use of compatible couples. The following should be considered in the
selection and application of compatible couples:
a. Passivated coatings. For a compatible couple selection, passivated coatings
specified herein shall be ignored and only the plating or basis metal considered. For
example, all chromate or phosphate treatments of zinc or cadmium specified in tables III
and IV shall be ignored in making couple selections and only zinc or cadmium
considered as acting in galvanic corrosion. Hard anodic films on aluminum-base alloys
are impervious nonconductors and, therefore, contact may be made with any dissimilar
metal.
b. Surface area of contacting metals. In intermetallic couples, the member
with the higher anodic index is anodic to the member with the lower anodic index. It will
therefore be susceptible to corrosion in the presence of an electrolytic cell or media. If
the surface area of the cathodic part is significantly greater than that of the anodic part,
the corrosive attack on the contact area of the anodic part may be greatly accelerated.
Material selection for intermetallic contact parts should therefore establish the smaller
part as the cathodic member of the couple.
c. Platings. For couple selection only the contacting metallic surfaces shall
be considered for compatibility. For example, when a plated part is intended for
assembly with aluminum, the compatibility of the plating, not the basis metal, with
aluminum shall be considered. Likewise, when two plated parts are intended to be
coupled, the compatibility of the platings not the basis metals is to be considered.
3.13.2 Exceptions to Table VI. The following exceptions apply to the selection
of compatible couples in accordance with Table VI:
a. Special service conditions. Table VI shall be employed for ground based
parts and electronic equipment(s) to be subjected to the variables of exposure(s) expected
under military service conditions. For special conditions or considerations, Table VI may
not be applicable. These may include factors such as operational temperature differences
between parts, plating thickness(es), longevity, polarity reversals, multiple contacts, etc.
Zinc, for example, may become cathodic to iron in hot tap water, and tin anodic to iron in
tap water at ordinary temperatures. Both of these, as isolated cases, vary from the general
experience reflected by the compatibility graphs of Table VI. In isolated cases, where
couples are not exposed to weather or salt-laden air but are subjected to wide temperature
and humidity variation with possible moisture condensation, restriction of couple
members to those shown in Table VI might serve to provide a part life abnormally longer
than the functional requirements of the equipment. In such cases, finish needs may be
met by extending the anodic index differences to 50. Generally, couple selection should
be governed by previous experience with such special conditions.
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