The job of coated fasteners
Screws, bolts and fasteners serve a function, but how well they do their job usually depends on the coating. It’s the coating that allows all fastener types to perform in harsh conditions by providing higher strength, preventing corrosion and resisting extreme temperatures or chemicals.
A fastener’s application determines which coating you need, of course. Coatings should also maintain uniform thickness and critical tolerances for parts with complicated geometries. If you’re part of a global supply chain, consistency in worldwide environmental compliance will be especially critical.
For instance, in the European automotive industry, regulations on end-of-life-vehicles (ELV) must be considered. Certain materials are off limits. Manufacturers must also adhere to the EU Regulation for the Registration, Evaluation, Authorisation and Restriction of Chemicals (REACH), and the Restriction of the Use of Hazardous Substances (RoHS).
Which materials are popular with industries that require heavy-duty fastener protection and why?
Automotive
Different coated fasteners are needed throughout the vehicle. For example, powertrain fasteners are located under the bonnet, around the engine, and within transmission systems. The coating will need to resist corrosion, heat and oil and offer consistent coefficient of friction properties for maximum joint security, without exceeding the proof load of the fastener. For this reason, zinc-based coatings are often used.
Exterior fasteners on a vehicle need to provide aesthetics as well as corrosion resistance, which makes zinc-nickel coatings popular, thanks to its high-wear resistance. You’ll also find Polytetrafluoroethylene (PTFE) coated fasteners within a vehicle due to its electrical insulation capabilities. This coating is also used for decorative touches on car parts.
Aerospace
According to Machine Design, Virgin Atlantic Airlines estimates that a one-pound reduction on every aircraft across their fleet would result in fuel savings of 14,000 gallons per year.
Lightweight is the key word, which makes PTFE popular with the aerospace industry. Another way to reduce weight is to use fastener covers instead of coated fasteners. The material has to be resistant to environmental factors, so zinc, nickle, silver, phosphate, cadium and black oxide are often used.
A few manufacturers use organic-based coatings, which include material from vegetable or animal matter or from compounds rich in carbon. The coats act as a barrier between the substrate and atmosphere. However, when the coating is broken, protection is lost.
Construction and mining
Coated fasteners have to withstand whatever the elements throw at them. For this reason, hot dip galvanising (HDG) is often used. It’s relatively maintenance free and provides excellent resistance to corrosion for steel fasteners. HDG provides a thicker layer of zinc on the surface. Because zinc reacts 10 times slower to corrosion than steel, it protects the substrate by corroding first –but because of its resistance, this will take significantly longer than most other coatings.
PTFE is also used to facilitate driving through metal without disturbing work carried out by other contractors.
Heat treating steel fasteners are an excellent choice for masonry or concrete work. The heat treatment process isn’t corrosion resistant, so are recommended for interior applications. However, heat treated fasteners can also have a galvanized coating, which works for exterior applications.
Oil and gas
Perhaps no other industry suffers direct and indirect costs caused by corrosion than Oil and Gas. For this reason, PTFE coatings are used quite often, due to its ability to provide high-corrosion resistance, along with accurate torque loadings. Its low-friction properties minimise ‘break-out’ torque, which allows you to remove the bolting with a wrench instead of a blowtorch.
Hot dip spun galvanised coating is sometimes used as well, for its resistance to corrosion in coastal and severe marine conditions. Zinc plating is another option, as is stainless steel. The steel will oxidise over time, but it will never lose its strength due to corrosion.
A look at coatings
Not all coatings are right for every application. Here are some additional coatings used across a range of industries:
Brass or Bronze
Often used for electronics or indoor decorative applications where the fastener is left visible.
Yellow or Electro-Plated Zinc
Yellow zinc provides corrosion resistance for interior uses. It’s similar to an electro galvanized fastener but may offer slightly better corrosion protection. Electro Galvanised fasteners are generally used bathrooms, kitchens and other areas that are susceptible to some water or humidity.
Black oxide
For dry, non-corrosive applications, used to minimise light reflections and provide reduced conductivity.
Clear Zinc Plated
Offers corrosion resistance in humid environments.
Chrome- or Nickel-Plated
Provides some level of corrosion resistance, but mostly used for their aesthetic qualities.
Phosphate
Provides lubrication and therefore, reducing friction. It also offers some corrosion resistance, but should only be used indoors. Avoid using on treated lumber.
Vinyl Coated
Reduces friction while driving. Use indoors when you don’t need corrosion protection.
The best results come with the right coating - The more you know your materials and what will work best for your application, the better your results will be.
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Questions?
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