There are many finishes or coating applied to fasteners; some corrosion protective, some decorative, or there may be no added coating at all.
An 'as produced' finish on carbon steel products having an oil residue which provides some shelf life but no real corrosion protection when in use.
The most economic and common fastener finish, comprising a thin coating of zinc applied either by electroplating or mechanically. A shiny silver grey appearance, it will normally be enhanced by a chemical chromate passivation conversion which applies a harder surface film. This can be clear (bluish tinge), or iridescent yellow which is thicker and gives marginally better protection. Clear is referred to as zinc, zinc clear, blue zinc. Yellow is referred to as zinc plate gold (ZPG), zinc yellow chromate (ZYC), zinc di-chromate, zinc yellow pass.
Formerly a popular electroplated or mechanically applied finish, looking like but giving slightly better protection than zinc and providing increased lubricity; also chromate converted. Very seldom used today due to its toxicity and environmental non-acceptability. If specified, it is usually through habit, error or ignorance and possible confusion with zinc.
Hot dip galvanising will allow much heavier coatings, the normal commercial coating is approximately 50 microns (µm), which necessitates the over-tapping of the mating thread and is the maximum practical to avoid serious compromise of the fastener's strength. Unlike electroplating, the concentration of deposits is in the thread roots and internal corners. For this reason, thread diameters of less than M10 are not normally galvanised unless a subsequent light re-roll of the thread is performed. Nuts supplied with galvanised bolts will have over-tapped threads to allow for the galvanised build-up on bolt threads and to reduce assembly galling.
In addition to corrosion being caused by exposure of uncoated materials, it is also caused or enhanced by the combination of dissimilar or incompatible materials.The following chart gives guidelines for the selection of materials or finishes based on this galvanic action:
A- The corrosion of the base metal is not increased by the fastener.
B- The corrosion of the base metal is marginally increased by the fastener.
C- The corrosion of the base metal may be markedly increased by the fastener.
D-The plating on the fastener is rapidly consumed, leaving the bare fastener material.
E-The corrosion of the fastener is increased by the base metal.
NR Not recommended.
Xylan® Coatings are a family of fluoropolymer coatings designed for use on various types of OEM components and fasteners, including xylan coated bolts to prevent corrosion. Most Xylan® coatings contain PTFE or other types of lubricants and are applied in thin films. Xylan coating is a dry film lubricant.
Electroless nickel plating is an auto-catalytic reaction used to deposit a coating of nickel on a substrate. Unlike electroplating, it is not necessary to pass an electric current through the solution to form a deposit. This plating technique is to prevent corrosion and wear. EN techniques can also be used to manufacture composite coatings by suspending powder in the bath. Electroless nickel plating has several advantages versus electroplating. Free from flux-density and power supply issues, it provides an even deposit regardless of workpiece geometry, and with the proper pre-plate catalyst, can deposit on non-conductive surfaces.
Geomet is a water-based high performance thin layer zinc flake anti-corrosion system, which is none embrittling and is available in a number of grades depending upon the application need. Corrosion performance of the Geomet product range will depend upon specification, coating thickness, surface preparation and any top coat applied. The normal range of expectation would be 500- 1500hrs to ASTM B117. Geomet is approved by most of the world’s major automotive manufacturers.
DACROMET® is the leading inorganic coating specified by automotive companies worldwide and is a proven coating system in many industries. A water-based, VOC compliant coating, DACROMET® is comprised mainly of overlapping zinc and aluminum flake in an inorganic binder.