Selecting the optimum material and finish for threaded fasteners

A good example is one where black stainless steel screws were specified using a treatment involving oil; however, the retained oil was a significant problem in that it contaminated the substrate and substantially marred the appearance of the final assembly. It subsequently turned out that stainless steel was not necessary and that it was perfectly satisfactory to use normal steel with a different blackening process not requiring an oil - with absolutely no detriment to the application.

In truth, screws and nuts may be produced in so many materials and alloys from mild steel, high tensile steels, stainless steels, brass, nylon, copper, titanium, duplex stainless steels, exotics such as Inconel, Incoloy, Monel, Hastelloy, Zirconium, Molybdenum, Tantalum, Waspaloy, Tungsten, Silicon Bronze, Aluminium, Ferralium, Polyoxymethylene and even fibreglass or glass-reinforced plastics. So, it is not surprising that the selection may be very confusing and potentially problematic, especially when overlain with the further multiplicity of additional finishes available.

Typically, these finishes are most applicable to steels because of their ability to change the properties of screws made from this low-cost material. Primarily aimed at improving corrosion resistance or changing appearance, the most common include zinc, in various forms, occasionally cadmium, also black japanned, chemical black, chrome, electro brass, copper, stayblack or powder coating. Chromate conversion coatings are often used as part of the plating process to passivate the coated surface which enhances corrosion protection and can add a decorative effect in the cases of zinc & yellow and zinc & black. Using which substrate and which finish can be simplified with experienced advice available from a specialist fastener supplier.

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