Zinc Flake Coated Fasteners

Stud Bolts, Nuts, Screws, Threaded Rod & Washers

Zinc flake coating systems can be applied in bulk or rack process using dip-spin or spray process.
Zinc flake coating is generally a mass process. When lots of small quantity have to be coated, a suitable coating line and/or process may be necessary in order to achieve the required properties and performances for the coated fasteners. For Zinc Flake Coated fasteners of large size or mass or when the risk of thread damage has to be reduced, rack instead of bulk process may be considered.
Zinc Flake Coating can be provided according to variou specifications viz:

• ASTM F1136/F1136M
• ASTM F2833
• ASTM F3019/3019M
• ASTM F3393
• ISO 10683

Curing process (especially with higher temperature and/or longer duration) may have an effect on the properties/performances of fasteners. When the curing temperature is above the tempering temperature, reduction of hardness may affect the performances of case hardened or nitro carburised fasteners (e.g. for thread forming or self- drilling screws), or elastic and plastic deformation (e.g. for clips); for cold worked fasteners or fasteners with thread rolled after heat treatment, residual stresses may be reduced.

Typical Zinc Flake Coating Systems

The choice of a coating system with or without Cr(VI) should take into account national regulations.
An integral lubricant may be chosen to achieve torque/tension properties.
An additional top coat may be chosen to increase corrosion resistance and to achieve other specific properties (e.g. torque/tension properties, resistance to chemicals, mechanical resistance, aspect, colour, thermal resistance, electrical insulation/conductivity, UV resistance).
The selection of the nature of the top coat should be based on desired additional properties:
- Organic top coat: electrical insulation, high resistance to chemicals or colouring possibilities, etc.
- Inorganic top coat: impact/abrasion resistance or thermal resistance, etc. An additional lubricant may be chosen to adjust torque/tension relationship.

Zinc Flake Coated Fastener Properties

Chemical resistance

Organic top coats applied on zinc flake base coats are more resistant to acidic and alkaline chemicals than inorganic top coats

Electrical conductivity

The electrical conductivity of zinc flake base coats with inorganic top coat is generally suitable for application of electrophoretic coatings and antistatic purposes. Zinc flake coating systems are not suitable for electrical grounding.

Galvanic corrosion

In order to reduce the risk of galvanic corrosion, all of the parts of the assembly should be considered (coated fasteners and clamped parts). A direct metal contact with non-coated clamped parts should be avoided, especially e.g. for stainless steel, magnesium, copper or copper alloys. Due to their insulating
effect organic top coats improve the resistance against galvanic corrosion. The items listed in A.2.2 are not exhaustive. All the specific service conditions should be considered when selecting a coating system.

Cleanliness

For cleanliness requirements, the suitability of the zinc flake coating system should be checked (e.g. dust, particle size, particle type, number of particles).