Relatively high cost and toxic to the environment.Ī secondary dipping process after plating increasing corrosion resistance, adding colour or brilliance.Ī chemical discoloration which does not add to part thickness. Superior rust-resisting qualities used in marine and aviation applications. Good rust-resisting qualities, appearance, and low cost. Most would agree HDG provides superior protection largely due to the Zinc Steel alloy layers, but Class 4 protection is getting up there and has the advantage of leaving clean recesses and sharp threads. The advantage is uniformity and smoothness of the coating thickness with no build up in recesses or on thread peaks. Mechanical energy from a rotating barrel and glass impact media are utilised to cold weld a Zinc / Tin metal powder to pre-cleaned fasteners. Mechanical Galvanising (Class 3 and 4) Commonly available on cladding fasteners, Mechanical Galvanising (Galv) looks similar to HDG, but with a smoother surface. Galvanised male threads cannot be assembled with plain or zinc plated nuts, or into standard tapped threads In terms of cost HDG is slightly more expensive than zinc plating, but this is far outweighed by the superior corrosion protection. The Zinc on the male thread protects the bare internal thread. Internal threads are tapped after Galvanising to prevent two zinc surfaces galling. It is standard practice to achieve this by tapping nuts and other internal threads oversize. HDG coating is so thick there must be some allowance for thread fit. Life expectancy of Hot Dip Galvanising can exceed 50 years in a rural environment. A further benefit accrues from the relative hardness of the Zinc Steel alloy layers which are difficult to “scrape” off. Hot Dip Galvanising applies a coating more than ten times as thick as electroplating, (0.055mm for items 8mm and over in diameter) and as corrosion protection is directly related to coating thickness, providing more than ten times the protection of zinc plating. Control of the coating thickness and the removal of surplus zinc is achieved by centrifuging the fasteners immediately on withdrawal from the zinc bath. The zinc adheres to the fastener during the dipping, forming a series of layers of Zinc Steel alloys, and finally a coating pure of zinc on the surface. Hot Galvanised (HDG) One of the more common finishes applied to fastenings above 6mm diameter, the process involves dipping the fastener in a bath of molten zinc. The yellow chromate is heavier than the clear or blue, offering marginally better corrosion protection. The former may display a slight bluish tinge, while the latter may appear iridescent. The two most common chromate finishes are “clear” and “yellow”. This produces a hard film, which provides the first barrier between the fastener and the environment. To supplement the thin layer of zinc a final process known as chromate conversion is applied. Zinc is a soft material, which can be easily scratched exposing the base steel of the fastener. The zinc coating is very thin, in the region of 0.004 mm, and consequently thread fit is not an issue. Bright Zinc Plated (Electrogalvanized, Class 2) Zinc is deposited on the fastener by an electroplating process. Plain finish offers no corrosion protection, and rusting may commence within days of installation. In both cases there will probably be some light oil present from the manufacturing process. Mild steel products will appear grey in appearance, while high tensile product will appear black. Plain Finish (Black, Self-Colour) The term plain finish means no coating at all. The team at Boltmasters hope you find this helpful. THE BOLTS & NUTS OF BOLT FINISHES This month we have a few words on the surface treatment of steel fasteners.
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