Fastener Coatings and Surface Treatment
Surface treatment determines how long a fastener survives in service. S.R. Forgings supplies bolts, nuts, washers and studs with a range of protective coatings, selected according to the working environment, the required corrosion life and the specification the project is built to.
The Dacromet and Geomet processes can be used to protect fasteners, springs, brake discs and similar components. Both comprise a uniform deposition of overlapping zinc and aluminium flakes on an inorganic binder, applied as two coats with two bakes. The coefficient of friction can be modified with an additional top coat, which also improves protection against corrosion. The difference between the two is chemistry: the Dacromet process contains hexavalent chromium, while the Geomet process is completely chrome free.
Black Oxide
Black oxide is a conversion coating on iron and steel that converts the surface of the material to black iron oxide, giving a dark finish with mild corrosion resistance and minimal dimensional change.
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Black Phosphate Coating
Black phosphate is a crystalline conversion coating formed on a ferrous substrate. It provides a base for oils and paints, improves lubricity and reduces galling during assembly.
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Zinc Flake Coating (Dacromet / Geomet)
Zinc and aluminium flakes in an inorganic binder, applied in two coats with two bakes. Because it is not electroplated there is no hydrogen charging of the steel, which is why zinc flake is specified for high tensile fasteners at property class 10.9 and 12.9. The coating is thin and dimensionally predictable, so threads assemble without the fit problems a heavy galvanised coat can cause. Available on all sizes in the S.R. Forgings range.
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Cadmium Plating
Cadmium coatings are used in electrical, electronic, mining, offshore and defence applications, where they are applied to bolts, fasteners and other components. Note that cadmium is restricted in several markets and is supplied only where the specification calls for it.
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Hot Dip Galvanizing
HDG is relatively maintenance-free and provides corrosion protection to steel structures for 50 to 70 years in most atmospheric environments. It works in two ways: a barrier coating that seals the steel from the atmosphere, and cathodic protection, where the zinc corrodes sacrificially so the steel beneath does not. Galvanising adds thickness to the thread, so nuts are tapped oversize after coating and supplied matched to their bolts, lot for lot.
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Electroplating
Electroplating begins with cleaning the fastener in an alkaline detergent solution, followed by acid treatment to remove rust and surface scale before the zinc layer is deposited. Suited to general-purpose fasteners in mild environments.
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