Fastening Finishes

Monday, 1 January 2024
Zinc Finishes for Steel Fasteners
This month we have a few words on the surface treatment of steel fasteners.  Hope it’s helpful.
Plain Finish (Black, Self Colour)
The term Plain Finish means no coating at all.   Mild steel product 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 (Electrogalvanised, Class 2)
Zinc is deposited on the fastener by an electroplating process.  The zinc coating is very thin, in the region of 0.004mm, 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 bluise tinge, while the latter may appear iridescent.  The yellow chromate is heavier than the clear or blue, offering marginally better corrosion protection.
Hot Dip Galvanised (HDG)
One of the more common finishes applied normally 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 of 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, provides 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.
Because the coating is so thick there must be some allowance for the thread fit.  It is standard practice to achieve this by tapping nuts and other internal threads oversized.  Internal threads are tapped after galvanizing 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 offered.
Mechanical Galvanising (Class 3 and 4)
Commonly available on cladding fasteners, Mechanical Galv looks similar to Hot Dip Galv, but with a smooth surface.  Mechanical energy from a rotation barrel and glass impact media are utilized to cold weld a Zinc / Tin metal powder to pre-cleaned fasteners.  Its advantage is the uniformity and smoothness of coating thickness with no build up in recesses or on thread peaks.  Most would agree Hot Dip Galv 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.