T.O. 1-1A-9
groups are considered dissimilar to one another. The metal/material referred to in the groups is the metal on the surface of the part;
E.G., zinc includes all zinc parts such as castings as well as zinc coated parts, whether the zinc is electro deposited, applied by hot
dipping, or by metal spraying over similar or dissimilar metal parts. The four groups are as follows:
a. GROUP I Magnesium and its alloys. Aluminum alloys 5052, 5056, 5356, 6061 and 606.q.
b. GROUP II Cadmium, zinc, and aluminum and their alloys (Including the aluminum alloys in Group I).
c. GROUP II Iron, lead, and tin and their alloys (except stainless steels).
d. GROUP IV Copper, chromium, nickel, silver, gold, platinum, titanium, cobalt, rhodium and rhodium alloys; stainless
steels; and graphite.
NOTE
The above groups do not apply to standard attaching parts such as rivets, bolts, nuts and washers
which are component parts of assemblies, which will be painted prior to being placed in service unless
other wise specified by specifications MIL-F7179, or other approved data.
2-285. TYPES OF PLATING.
2-286. CADMIUM PLATING (QQ-P-416). The primary purpose of cadmium plating is to retard or prevent surface corrosion of
parts. Unless otherwise specified, the plating shall be applied after all machining, brazing, welding, forming and perforating of the
item has been completed. Proper safety precautions should be observed in the event any welding or soldering operations are
required on cadmium plated parts because of danger from toxic vapors during such operations. Cadmium coatings should not be
used on parts subjected to temperatures of 450°F or higher. All steel parts having a hardness of Rockwell C40 (180,000 PSI) and
higher shall be baked at 375° 25°F for 3 hours minimum after plating for hydrogen embrittlement relief. All steel parts having an
ultimate tensile strength of 220,000 PSI or above shall not be plated, unless otherwise specified. When permission is granted, a low
embrittlement cadmium plating bath shall be used. Federal Specifications QQ-P-416 should be used for cadmium plate
requirements. Critical parts should be magnafluxed after plating.
2-287. Zinc Plating (QQ-Z-325). The primary purpose of zinc coatings is to retard or prevent the formation of corrosion products on
exposed surfaces. Unless otherwise specified, the plating shall be applied after all machining, brazing, welding, forming and
perforating have been completed. All parts having a hardness greater than Rockwell C40 and higher shall be baked at 375° 25°F
for 3 hours after plating for hydrogen embrittlement relief. Zinc shall be deposited directly on the basic metal without a preliminary
plating of other metal, except in the case of parts made from corrosion resisting steels on which a preliminary plating of nickel is
permissible. Zinc plating(Type 1) should not be used in the following applications:
a. Parts which in service are subjected to a temperature of 700°F or higher.
b. Parts in contact with structural fabric structure.
c. Parts in functional contact where gouging or binding may be a factor or where corrosion might interfere with normal
functions.
d. Grounding contacts where the increased electrical resistance of zinc plated surfaces would be objectionable.
e. Surfaces where free circulation of air does not exist and condensation of moisture is likely to occur. For additional
information, refer to QQ-Z-325.
CAUTION
Chromium and nickel electro deposits severely reduce the fatigue strength of high strength steels. All
steel parts having a tensile strength of 180,000 PSI or above should be shot-peened prior to electro
plating. In addition high strength steels are susceptible to detrimental hydrogen embrittlement when
electro plated. All steel having an ultimate strength of 220,000 PSI or above shall not be electro plated
without specific approval of the procuring service or responsible engineering activity.
2-288. Nickel Plating (QQ-N-290). This coating is divided into two classes. Class I, plating is intended for decorative plating, and
Class II, plating is intended for wear and abrasion resistance. Unless otherwise specified, the plating shall be applied after all base
metal heat treatments and mechanical operations such as machining, brazing, welding, forming and perforating on the article have
been completed, all steel parts shall be given a stress relief at 375° 25°F(191° 14°C) for 3 hours or more prior to cleaning and
plating, as required, to relieve residual tensile caused by machining, grinding or cold forming. Steel parts having a hardness of
Rockwell C40 and higher shall be baked at 375° 25F° for 3 hours or more and within eight (8) hours after plating to provide
embrittlement relief.
Parts shall not be reworked flexed or subjected to any form of stress loads after placing and prior to the hydrogen embrittlement relief
treatment. The general requirements for nickel plating are specified in QQ-N-290. Nickel shall be used for the following application
only in accordance with MIL-S5002:
Change 3 2-123
