Screw materials and finishes are an important factor in successful applications, therefore specialist guidance is of great value. Challenge Europe is used to dealing with questions of specification regarding materials and finishes of threaded fasteners. This ideally occurs early in the process of defining new assemblies, but may equally be of value at the service or repair stages.
A good example is one where black stainless-steel screws were specified using a treatment involving oil, however the retained oil was a problem in that it contaminated the substrate and substantially marred the appearance of the assembly. It turned out that stainless steel was not necessary and that it was perfectly fine to use normal steel with a different blackening process – with absolutely no detriment to the application.
What are the options? 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, silicon bronze, aluminium, ferralium 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.
Chromate conversion coatings are often used as part of the plating process to passivate the coated surface. Using which substrate and which finish can be simplified with experienced advice available from a specialist fastener supplier.