Bolt Finishes

Tuesday, 1 August 2023
This month we have a few words on the surface treatment of steel fasteners. The team at Boltmasters hope you find this helpful.
This month we have a few words on the surface treatment of steel fasteners. The team at Boltmasters hope you find this helpful.

Plain Finish (Black, Self-Colour)
The term plain finish means no coating at all. Mild steel products will appear grey in appearance, while high tensile product will appear black. In both cases there will probably be some light oil present from the manufacturing process. Plain finish offers no corrosion protection, and rusting may commence within days of installation.
Bright Zinc Plated (Electrogalvanized, Class 2)
Zinc is deposited on the fastener by an electroplating process. The zinc coating is very thin, in the region of 0.004 mm, and consequently thread fit is not an issue. It is an excellent “indoor” finish.
Zinc is a soft material, which can be easily scratched exposing the base steel of the fastener. To supplement the thin layer of zinc a final process known as chromate conversion is applied. This produces a hard film, which provides the first barrier between the fastener and the environment.
The two most common chromate finishes are “clear” and “yellow”. The former may display a slight bluish tinge, while the latter may appear iridescent. The yellow chromate is heavier than the clear or blue, offering marginally better corrosion protection.
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 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. Control of the coating thickness and the removal of surplus zinc is achieved by centrifuging the fasteners immediately on withdrawal from the zinc bath.
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. A further benefit accrues from the relative hardness of the Zinc Steel alloy layers which are difficult to “scrape” off. Life expectancy of Hot Dip Galvanising can exceed 50 years in a rural environment.
HDG coating is so thick there must be some allowance for thread fit. It is standard practice to achieve this by tapping nuts and other internal threads oversize. Internal threads are tapped after Galvanising to prevent two zinc surfaces galling. The Zinc on the male thread protects the bare internal thread. 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.
Mechanical Galvanising (Class 3 and 4)
Commonly available on cladding fasteners, Mechanical Galvanising (Galv) looks similar to HDG, but with a smoother surface.   Mechanical energy from a rotating barrel and glass impact media are utilised to cold weld a Zinc / Tin metal powder to pre-cleaned fasteners. 
The advantage is uniformity and smoothness of the coating thickness with no build up in recesses or on thread peaks. 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.
Boltmasters Fastener Finish Chart


MaterialCorrosion Resistance
White to blue greyAll metalsGoodMost common used plating. Good rust-resisting qualities, appearance, and low cost.
Bright or dull silver greyAll metalsExcellentSuperior rust-resisting qualities used in marine and aviation applications. Relatively high cost and toxic to the environment.
ChromateYellow, olive drab, black, blue/whiteZinc & cadmium plated partsVery goodA secondary dipping process after plating increasing corrosion resistance, adding colour or brilliance.
Black OxideBlackFerrous metals & stainless steelFairA chemical discoloration which does not add to part thickness. Usually combined with an oil dip. Rust resistance comes from the oil only.
Phosphate & OilCharcoal grey or blackSteelGoodZinc or manganese phosphate used with a rust-inhibiting oil dip. Low cost.
Colour PhosphateBlue, green, red, purple, etc.SteelVery GoodChemically produced coating superior to regular phosphate and oil.
IriditeOlive drab, green, black, red, blue, bronzeAll metalsGoodApplied on top of zinc or cadmium plating as a die for colour and additional corrosion protection.
NickelSilverAll metalsVery goodHard stable finish, relatively expensive and sometimes hard to apply.
ChromiumBright blue/whiteAll metalsVery goodHard lustrous finish adds wear resistance and is very expensive.
Hot Dip (galvanising)Dull greyAll metalsVery GoodParts are dipped in pure zinc. Gives maximum corrosion protection. Adds a thick irregular coating. Size must be adjusted to allow for thickness of coat.
PassivatingBright - etchedStainless steelExcellentParts are dipped in nitric acid which removes iron particles and brightens the finish. Produces a passive corrosion-resistant finish.
AnodisingFrosty - etchedAluminiumExcellentAcid dip produces a hard oxide surface. Can be colour dipped after anodizing for preferred finish.