ASTM B766 Cadmium Plated Fasteners
ASTM B766 specification covers the requirements for Electrodeposited Cadmium coatings on products of iron, steel, and other metals.
The coating is provided in various thicknesses up to and including 25 µm either as electrodeposited or with supplementary finishes.
Cadmium coatings are used for corrosion resistance and for corrosion prevention of the basis metal part. The asdeposited coating (Type I) is useful for the lowest cost protection in a mild or noncorrosive environment where early formation
of white corrosion products is not detrimental or harmful to the function of a component. The prime purpose of the supplementary chromate finishes (Types II and III) on the electroplated cadmium is to increase corrosion resistance. Chromating
will retard or prevent the formation of white corrosion products on surfaces exposed to various environmental conditions as well as delay the appearance of corrosion from the basis metal.
Cadmium plating is used to minimize bi-metallic corrosion between high-strength steel fasteners and aluminum in the aerospace industry. Undercutting of threads on fastener parts is not necessary as the cadmium coating has a low coefficient
of friction that reduces the tightening torque required and allows repetitive dismantling.
|Cad Class Number
||Thickness µm (Inch)
||As-plated without treatment
||With colored chromate treatment
||With colorless chromate treatment
|ULTIMATE TENSILE STRENGTH
|STRESS RELIEF BEFORE PLATING
|HYDROGEN EMBRITTLEMENT RELIEF
|CERTIFICATION OF COMPLIANCE
|ADDITIONAL REQUIREMENTS OR TESTING
||List all that apply
The thickness of electrodeposited cadmium coatings shall be determined by Test Methods B487, B499, B504, B567, or B568, as applicable. Other methods may be used, if it can be demonstrated that the uncertainty of measurements with these methods is less
The thickness measurements of Types II and III deposits shall be made after application of the supplementary treatments. Whenever Test Method B504 or B567 is used, remove the supplementary treatment from the test area before measuring the thickness.
Remove by using a mild abrasive (a paste of levigated alumina or magnesium oxide) and rubbing gently with the finger.
- The electrodeposited cadmium coating shall be tested for adhesion to the basis metal or substrate layer in a manner that is consistent with the service requirements of the coated article. When examined at a magnification of approximately 4× diameters,
the coating shall not show separation at the interface. The formation of cracks in the coating caused by rupture of the basis metal that does not result in flaking, peeling, or blistering of the coating shall not be considered as nonconformance.
Use one of the following methods for determining adhesion
- Abrasion Resistance
- The abrasion resistance of the supplementary chromate films (Types II and III) shall be determined in accordance with Practice B20
- Clear (Colorless) Type III Coatings
- The presence of a clear Type III coating shall be determined in accordance with Practice B201.
- Salt Spray Corrosion Resistance
- The 5% neutral salt spray (fog) test as defined in Test Method B117 shall be used when there is a steel or iron substrate. Parts with supplementary chromate film shall be aged at room temperature for at least 24 h before subjection to the test.
- Hydrogen Embrittlement
- The test to indicate freedom from hydrogen embrittlement failure of coated parts or specimens need not be conducted unless the method is specified in the purchase order. The method description shall include specimen configuration, means of applying
a load to the part, stress or load level, and duration of test. Parts shall not crack or fail by fracture when tested.
- Visual Examination
- The coating shall be examined for compliance with the requirements of luster, workmanship, and finish.