MIL-DTL-14072F
3.11 Finishes or processes other than painting or plating (E). Finishes or
processes other than painting or plating shall be in accordance with Table IV. Specific
finishes shall be selected with respect to surface classification (Type I or Type II) and the
basis metal to be protected. Table IV calls out finishes by the letter E followed by a
number. Thus E513 is a chemical-film finish for use on aluminum or aluminum base
parts to be subjected to Type II exposure.
3.12 Electrical conductivity of passivated finishes. Table V contains information
on the electrical conductivity of passivated finishes. Designers specifying a passivated
finish(es) for coating electronic equipment enclosures, mounting racks or associated
structural framework should consider the following characteristics prior to making a
selection from Table V:
1. The equipment conducting frequency mechanism i.e. Power or Radio
Frequency (RF).
2. The requirement for the chassis mounting points to access, or be isolated from,
any or all potential grounding points.
In addition to a statement, either "yes" or "no", regarding electrical conductivity at power
frequencies and radio frequencies, Table V contains a remark column which gives data
concerning the use of each listed finish.
3.13 Intermetallic-couples. The finishing of metallic areas to be placed in contact
presents a special problem. Intermetallic contact of dissimilar metals will result in
electrolytic couples. If precautions are not taken, these sites may promote corrosion
through galvanic action. Table VI shall be used in determining the need for or degree of
protection to be applied to couple members. This depends on the relative position of the
coupled members in the galvanic series. Table VI shows metals and alloys (or platings)
by groups which have common electro-motive forces (EMF) within 0.05 volt when
coupled with a saturated calomel electrode in sea-water at room temperature. All
members of a group regardless of metallurgical similarity or dissimilarity are considered
compatible. Compatible couples between groups have been specified in Table VI based
on a potential difference of 0.25 volt maximum. Permissible couple series are shown in
Table VI by the graphs at the right. Member of groups connected by lines will form
permissible couples. An "o" indicates the most cathodic member of each series, "o" an
anodic member, the arrow the anodic direction. Table VI shows, in addition the EMF
against a calomel electrode, a derived "anodic index" with Group 1 (gold, etc.) as and
Group 18 (magnesium, etc.) as 175. Subtraction of one group anodic index from another
gives the EMF difference in hundredths of a volt. To provide the corrosion protection
required in ground electronic equipment, intermetallic couples should be restricted, where
possible, to those permitted in Table VI having a minimal tendency to galvanically
interact.
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