Popular Fastener Coatings You Should Know About

After fasteners are manufactured, coatings are often applied to provide enhanced properties.

Fastener coatings are used to provide rustproof screws and corrosion-resistant nuts and bolts and decrease the torque required during installation, among other advantages. Examples are fluoropolymer coatings/lubricant blends that decrease friction during installation. Molybdenum disulfide serves a similar purpose but is suitable for high-pressure loads. Epoxy coating is used to provide superior impact resistance in addition to corrosion and abrasion protection. Inorganic zinc coating provides excellent corrosion resistance on steel fasteners ideal for outdoor applications. Phenolic coatings provide the best protection in low pH, high-temperature environments. Phosphate ferrous metal coatings provide anti-galling and minor corrosion resistance. There is a coating available for nearly every fastener application.

Since steel and aluminum fasteners are typically heat treated, the coating is applied afterward. The coatings can be water- or solvent-based, depending on the formulation. In order to provide a consistent finish and maintain desired production rates, the coated fasteners are typically cured in a batch or conveyor oven at temperatures of 95-205°C (200-400°F) for a period of 15-45 minutes. The time and temperature is dependent on the coating itself and is specified by the coating manufacturer.

Low VOC (volatile organic compound) coatings are now available that provide excellent results while reducing the release of volatile vapors during the application and curing processes. Low-VOC coatings simplify cleanup, reduce disposal costs and eliminate the need for pollution control.

For example, this electrically heated belt conveyor oven  (Fig 1)is used to quickly dry a coating on titanium and stainless steel fasteners. The design features a stainless steel wire mesh belt to carry the parts on trays. Fasteners are processed at a rate of 1,300 parts per hour, running two shifts per day for a total production of over 5 million fasteners per year. The oven uses a small footprint to optimize floor space.

The conveyor oven has a maximum temperature rating of 205°C (400°F) and a work chamber of 1,372 mm wide x 2,210 mm long x 152 mm high (4.5 feet wide x 7.25 feet long x 6 inches high). The recirculation system utilizes a 10,200 m3/hour (6,000 cfm) direct-driven blower and top-down airflow, with a 72-kW heating system to maximize heating rates and temperature uniformity of the product. The conveyor belt includes a variable-speed drive to allow changing of the heating times. A digital paperless recorder is included to record and archive the oven operating temperature using Ethernet communication.

When you are looking for some ideal fasteners for your project, you must consider all their characteristics, such as their material of construction, performance in various environments, and durability. However, the last two factors depend on the material and the type of coating used. The coating on the fasteners helps improve their performance and lifespan. There are different types of fastener coatings available in the market. This post discusses 5 important fastener coatings that you can consider while buying fasteners for your industrial, commercial, or personal uses.

21 Popular Types of Fastener Coatings for Your Consideration

The following are the 20 most popular types of coatings for fasteners that may meet your requirements:

1. Clear Zinc: This is one of the most popular fastener coatings available in the market. Inexpensiveness is one of the main reasons for their immense popularity. The clear zinc coated fasteners can withstand oxidative damage. Thus, the clear zinc coatings are recommended for conditions, where they are exposed to high humidity, and stand a high risk of rusting.
2. Yellow Zinc: The yellow zinc coating is applied electrically to the fasteners. These coatings are ideal for conditions where the fasteners may come in contact with water or other liquids. The yellow zinc coating is also ideal for humid conditions. These coatings need to be applied precisely to inhibit corrosion. Fasteners in particular steel fasteners are electroplated with zinc for better corrosion resistance, zinc being the prime material for electroplated fasteners; zinc alloys are also electroplated; zinc-nickel and zinc-iron. Zinc plating applies a thin coating of zinc to fasteners, giving them a layer of protection from the surrounding environment, the process of electroplating fasteners is called barrel plating. Electroplated zinc is a storage finish, fasteners are subject to the heat and moisture variations of a typical fastener warehouse and zinc gives a level of protection. Zinc plated fasteners will rust if placed in a wet environment and are not suitable for outside use.  Fasteners that have been zinc plated and clear passivated have a shiny, silvery appearance and are also available in zinc and yellow passivation that has a shiny yellow gold appearance; passivation is available in a range of colours. Zinc electroplating can provide thicknesses from a negligible flash of colour, for appearance, through commercial coatings of 3-5 microns (µm), on to specified heavy coatings up to 12 microns (0.0005 in). Electroplating does not give an even cover; thicker concentration of deposit occurs on corners, points, thread crests and thinner concentrations on thread flanks and roots. This may cause thread galling on coatings above 8 microns average and adjustment by over - tapping of the nut may be required. Caution: should be exercised in relation to the plating of high strength fasteners 10.9 and above, USA grade 8 and above, because of the risk of hydrogen embrittlement.
3. Black Zinc: Bolts electroplated with an electro zinc flash coating and black chrome passivation are described as Black zinc. This finish is used where a colour coating is important for identification or aesthetics.
4. Zinc-Cobalt: Zinc-cobalt plating of fasteners is a process that achieves enhanced corrosion resistance compared to traditional zinc plating of the same thickness. Zinc-Cobalt gives Bolts and screws have a bright lustre. By electroplating zinc and cobalt the end result is a uniform ductility that will withstand up to six times the corrosion resistance of conventional zinc plating. Zinc-cobalt alloy plating is also becoming more popular because of its affordable operation costs compared to other zinc alloy coatings. 
5. Nickel Palting: Screws Nickel plated are initially copper plated and the Nickel coat is electrolytically applied to the copper finish. Used in fasteners for electrical appliances and areas of condensation. Decorative applications account for about 80% of the nickel consumed in plating. 
6. Electro Brass: Electro brass is used for decorative fasteners; it has been Commercially electroplated since it's development in the 1850s. A brass finish applied by electroplating. Appears similar to brass and is used in furniture or architectural fittings. 
7. Tin: Nuts and bolts of steel and copper can be electro tinned. Tin plating is an industrial process which involves the coating of metal objects with a thin layer of tin. Tin plating increases the performance and durability of objects is it added to and improves corrosion resistance. Tin creates suitable properties for soldering thanks to the oxidation protection that tin-plating provides; which ensures that exposure to air is far less likely to tarnish the surfaces of tin plated fasteners. The conductive properties of tin ensure that it is commonly used in the electrical and electro-mechanical industries. Tin plating is also widely implemented in the food packaging trade thanks to its non-toxic and corrosion resistant properties. 
8. Cadmium plated: For many years fasteners were coated in Cadmium it was the most popular electroplated finish giving a superior finish protection than zinc plate and lubricity, it could also take a chromate passivation. Cadmium is rarely used today due to its toxicity and environmental non-acceptability. The aircraft industry is one of the last users of cadmium, it is a sacrificial coating for steel, and readily takes a chromate conversion coating which improves the corrosion resistance, importantly it's galvanic characteristics are compatible with aluminium. 
9. Chromium Plating: Fasteners are chrome plated and often polished for appearance. Chrome plating provides similar corrosion resistance to zinc plating. Fasteners decoratively chrome plated are designed to be aesthetically pleasing and durable. Thicknesses range from 0.05 to 0.5 μm and are usually between 0.13 and 0.25 μm. Chromium plating is applied on top of bright nickel plating. Fastener base materials range from steel; copper or zinc alloys and aluminium. Because of the double coating, decorative chrome plating is corrosion resistant but can be problematic if applied to very small threaded parts. Hard chrome, is also known as industrial chrome or engineered chrome and is rarely used on fasteners because of the deposit thickness of between 20 to 40 μm.
10. Chromate Passivation: The corrosion resistance of fastener coatings is improved by a ‘passivation’ barrel process, which builds up a passive shield layer of metal oxide on top of the zinc layer on the bolt or screw and can add colour identification. Chromates have been used in passivation for many years. There have been changes in legislation placing restrictions on the use of Chrome 6 in passivates applied to fastener products, Chromium can exist in two states: 

                                           Hexavalent Chrome six, Cr VI,  Cr ⁶.

In the hexavalent state chromium compounds are carcinogenic, and accordingly, there has been legislation in the automotive and electrical industries to ban its use.

                                            Trivalent Chrome three, Cr III, Cr ³

The change in legislation has created a move to other passivates or coloured zinc coatings the most popular is trivalent chromium 3 that has become the substitute for chrome 6 in clear passivation. 

Hexavalent Chromium Passivation

Chromium(VI), Cr(VI), chromium 6, Cr6+

Hexavalent chromium ban imposed by European directives. Hexavalent Cr6+ is replaced with the non-toxic Trivalent chromium Cr3; Cr3 does not have the same healing properties of Cr6.

Chromium plating traditionally was Hexavalent chromium being used for decorative and functional finishes. Hexavalent chromium plating is produced by submerging substrates into a bath of chromium trioxide (CrO3) and sulfuric acid (SO4). This type of chromium plating provides corrosion and wear resistance, as well as aesthetic appeal.

Hexavalent chromium plating disadvantage is that this type of plating produces several by-products which are hazardous waste, including lead chromates and barium sulfate. Hexavalent chromium is a dangerous substance and carcinogen and is heavily regulated. In recent years, automotive OEMs have replaced hexavalent chromium finishes with more eco-friendly finishes. 

Fasteners passivated with Hexavalent chromium are the major issue in that the passivation was designed to improve resistance to corrosion, it replacement Trivalent chromium passivation has been generally problem free but is not as effective.

Trivalent Chromium Passivation

Chromium (iii). Cr3

Trivalent chromium is the alternative method of decorative chrome plating, and is the environmentally acceptable alternative to hexavalent chromium, with many of the same characteristics; just like hexavalent chrome finishes, trivalent chrome provides scratch and corrosion resistance and are available in a variety of colour options. Trivalent chromium plating uses chromium sulfate or chromium chloride as its main ingredient, instead of chromium trioxide; making trivalent chromium less toxic than hexavalent chromium. 

While the trivalent chromium plating process is more difficult to control, and the necessary chemicals more expensive than that used for hexavalent chromium, the advantages of this method make it cost-competitive with other methods of finishing. The trivalent process requires less energy than the hexavalent process and can withstand current interruptions, making it more robust. Trivalent chromium’s lower toxicity means that it is regulated less stringently, reducing hazardous waste and other compliance costs.

11. Gray Phosphate: This is another important fastener coating which is designed to improve the lifespan of fasteners. However, this type of coating is not recommended for conditions where there is a high risk of corrosion. For example: gray phosphate coated fasteners are not ideal to use in marine applications. This means they should only be used in dry settings such as vehicles or interiors of buildings, etc. Blackodized, Black oxide, Gun barrel blue and blackening are all names for this conversion coating. Produced by placing the fastener in a bath of sodium hydroxide, nitrates and nitrites at 140°C this converts the surface of the steel bolt into magnetite (Fe3O4). It is used to add mild corrosion resistance, for appearance and to minimise light reflection. To achieve improved corrosion resistance the black oxide must be impregnated with oil or wax. The advantage over other coatings is its minimal build-up and is used where close tolerances need to be maintained. Phosphate coated fasteners have a thin dull grey coating obtained by immersing the steel fasteners in a phosphoric acid bath. Dipping into the bath allows chemical conversion of the fastener surface and a layer of phosphate to form. Gives a lower level of protection than zinc in mild environments, but gives an excellent base for painting and lubrication. Often used in automotive industry plain or with an oil coat. 

12. Hot Dip Galvanized: Also, referred to as HD, this coating is applied on steel to protect it from corrosion. The hot dip galvanized fasteners are coated with zinc. These fasteners are generally considered for outdoor applications where they will be exposed to extreme weather conditions such as snow, heat, and rain. The HD fasteners are also ideal for coastal areas where the salt content is higher. One of the most effective coatings for steel fasteners both performance and cost is hot dipped spun galvanized zinc. Hot dip zinc finish is dull and rougher then electroplated zinc coating. Fastener Galvanizing involves the application of a layer of zinc by dipping the fastener in a bath of molten zinc then centrifuge spinning the fasteners for an even distribution of zinc and the removal of the excess. Typical coating thicknesses are 50 microns (µm), this is the thickest zinc coating to go onto the fastener resulting in superior corrosion resistance.Due to the thickness of the coating and the concentration of zinc deposits is in the thread roots and internal corners, thread diameters of less than M10 are not normally galvanised. Hot dipped galvanized bolts are only compatible with Galvanized nuts that are tapped slightly larger than other nuts to accommodate this heavy deposit of zinc; to reduce assembly galling, a wax coat will provide assembly lubrication.Hot dipped galvanized fasteners are frequently used outdoors, especially in coastal and severe marine environments.The coating process also avoids the possibility of hydrogen embrittlement but can create tempering back due to the high temperature of molten zinc, due to the reduction in hardness 12.9 and 10.9 fasteners should not be hot dip galvanised. 

Galvanised Nuts: Nuts should be galvanized as blanks and then tapped oversize, the threads then lightly oiled.  When assembled, the nut thread is protected by contact with the coating on the bolt. Even after many years of service, galvanized nuts can readily be unfastened even though the threads have never been galvanized. There is some tendency for hot dip galvanizing to be thicker in thread roots. Fasteners hot dip galvanized at a high temperature of around 550°C tend to take on a uniform, dull grey.

Galvanizing Fastener Strength Considerations: Galvanising bolts up to class 8.8 USA grade 5 is normal practice and fasteners can be galvanized without difficulty. Grade 10.9 bolts may require shot blast-cleaning as an alternative treatment prior to hot-dip galvanizing and there may be some tempering back. Grade 12.9 bolts and higher strength fasteners should not be galvanized. 

Galvanised Thread Galling: Galvanised fasteners can suffer with 'Thread lock-up' normally called 'Galling' it occurs in the threads of galvanized fasteners where there is a coating build up in the thread roots; this can be overcome with lubrication ensuring the correct clamping force is developed. Beeswax has been found to be a most effective lubricant, Molybdenum disulphides, or tallow, have also been specified for this purpose.

Galvanising Fasteners  Europe: There have been incidents of European undersized galvanised bolts being supplied as a standard thread for use with UK overtapped nuts. It is crucial to recognise that the continental European practice is to use an undersized thread bolt that when galvanised is the same thread dimensions as an uncoated bolt it will then easily mated with a standard thread nut; the reverse is the practice in the UK in that the nut is retapped to fit the Galvanised bolt.

 Galvanized Bolts: Hot dip spun galvanizing H.D.S.Galvanising as it is also called is a molten zinc coating on threaded fasteners and is superior to electro zinc plating; the cost per year of rust-free life prove that hot-dip galvanizing is by far the most economical coating compared to zinc electroplating.

Galvanizing: Galvanizing has for over 100 years been seen as the most environmentally friendly fastener finishing process that prevents corrosion. Galvanizing is sustainable and produces little waste, zinc that does not form a coat on the bolt or nut remains in the galvanizing bath for reuse.

Galvanized Bolt: Zinc is not a ferrous metal its properties make it recyclable without loss of chemical or physical properties.

Galvanized fastenings: Galvanized bolts and nuts can be assembled or installed, and can also be removed, re-galvanized and re-used. As part of an end of life recycling process zinc can be easily removed in the scrap steel production process.

Galvanized fasteners offer:A permanently bonded tough coating

Easy to clean surface

Long maintenance-free life

Lowest overall cost compared to other coatings

13. Electro Galvanized (EG): These fasteners have a thin layer of zinc coating which offers frugal corrosion protection. The EG fasteners are used in areas where less corrosion protection is needed such as kitchens, bathrooms, or other areas that are susceptible to humidity or moisture. Most fasteners used in roofing are electro galvanized and are replaced before the fastener starts wearing.

Although all the above-mentioned coatings are designed to protect fasteners they are not all the same. The coatings have diverse properties and are not suitable for all conditions. Thus, you need to be extra careful while selecting them for your application. The wrong selection is going to affect the performance of your fasteners.  

14. Mechanical zinc: After cleaning, the parts are put into a rotating barrel with an inert medium that is usually Glass beads. After an activation process that uses small amounts of copper and tin, Zinc powder is added and the motion of the barrel and the “impingement” of the beads “cold welds” the zinc onto the part surface. The process is carried out at around ambient temperature. The thickness of the deposit is controlled by the amount of zinc added. This means that thicknesses from about 3-5µm up to 40µm can be applied as required.  

The glass beads used are small (from about 1mm. to 5mm diameter) and are mixed to ensure good coverage. They are removed from the load after coating usually with a magnetic separator. Tin or Aluminium can be mixed with the Zinc powder to give a mixed coating used for specific purposes. 

Zinc Mechanically coated fasteners: Zinc Mechanically cold welded fasteners can receive a chromate passivation similar to an electroplated deposit. Normal thicknesses of deposit will give about 240 hrs. salt spray. Mechanical Zinc Plating does not rely on the conductivity of parts, and it is a room temperature process, it can be used effectively on composite assemblies such as parts with rubber or plastic mouldings attached. The national specification for this coating is BS 7371 Part 7. Specification for mechanically applied zinc coatings. 

Mechanical Zinc Plating Process: Fasteners to be mechanically coated are placed into a tumbling barrel containing glass beads, reagents and catalysts, which activate and prepare the surface. The coating to be applied is added, in metallic powder form and glass beads of varying sizes ‘cold weld’ the coating on to the activated surface of the Bolt or Screw. Similar passivates to electroplating are then applied, prior to drying the parts, lubricants may also be added as part of this process. Mechanical zinc can be used as an undercoat to enhance the performance of organic paint systems. 

It is important that all alloy coatings are supplied with the correct amount of alloying metal, which can now be tested non-destructively using X-ray fluorescence techniques. The equipment also determines thickness to an accuracy of ±5% which is better than any other means of nondestructive testing. 

Advantages of Mechanical Coating:

• Uniform coating-galling reduced for threaded fasteners.
• Porous substrates can be coated satisfactorily.
• Cost-effective replacement for galvanising, removing the heat and depth of coating issues. 

Dis-Advantages of Mechanical Coating:

• This process can only be performed in a barrel, so parts larger than about 120mm long cannot be processed.
• Care needs to be taken to stop beads being trapped in some parts
• Large diameter bolts can ensure thread damage during the barreling process

Mixed Metal Applications: 

Mechanical plating can be used to apply zinc, tin or aluminium coatings, either singly or in combination. It is, essentially, a ‘cold welding’ concept that applies the coating using mechanical energy, at room temperature. Mixed Metal Coatings can provide specific advantages such as ductility or high corrosion resistance, especially in certain environments or in contact with other metals. These include Zinc, Tin, Almac and Inverplex. 

Almac: 

Almac® coatings are combinations of aluminium and zinc which give substantially increased corrosion resistance compared with zinc. They are more ductile than zinc and are very advantageous when used in contact with aluminium.

Inverplex: 

Inverplex® is a mixed coating of zinc and tin that has better conductivity and corrosion resistance than zinc. It may be supplied passivated and is used for earthing screws.

15. Al/ZN coat or DACROMET (R) or Zinc Flake Coating: 

Zinc Flake thread Coating is a generic term for the coatings marketed by different suppliers under their respective brand names. The coatings consist predominantly of Zinc Flakes usually with a small quantity of Aluminium. The flakes are about 1µm thick and are bonded together with inorganic or organic compounds. 

Development of these coatings began around the 1970’s to give better corrosion resistance than Zinc Electroplated coatings. The coatings come under brand names such as Geomet® and Dacromet® to name two and are used extensively by the Automobile Industry to give salt spray resistances exceeding 600 hours with a thin coating, making them ideal for fasteners. 

This coating does not induce Hydrogen Embrittlement, which means that high tensile fasteners can be safely coated. 

Some of these coatings have integral lubricants to facilitate assembly of fasteners, or they can have lubricated topcoats. 

All Zinc Flake materials are similar to paints, so they can be applied by dip coating or spraying.

Zinc Flake Dip spin coating: 

This process is ideal for small parts including small to medium size fasteners.

As with all coating processes the parts must first be cleaned to remove any oils, greases or other contaminants, usually by submersion in an alkaline cleaner. The scale or rust is then removed, either by submersion in an inhibited acid solution, but more likely to be blast cleaned in a belt type shot blaster, usually using a Wheelabrator type of shot accelerator with fine steel shot. (Some coatings are applied to a phosphate layer) 

The parts are then dipped into the Zinc Flake mixture in a basket; the raised basket is then centrifuged to remove excess material. The parts are then dried and cured, (at temperatures between 200 and 300ºC).  

When processed in bulk there will be touch marks, the process is repeated to ensure overall coverage. In some cases, further coats are applied to build up the thickness to improve corrosion resistance, or a topcoat used to alter lubricity or colour.

Zinc Flake Coating Large Fasteners: 

Fasteners that cannot be bulk coated due to damage problems; usually an issue with bolts above M16 and longer than 150mm. can be spray coated, or occasionally they are installed onto racks and dipped and spun similar to bulk processing. 

Extra thickness can be applied to increase corrosion resistance, (up to 3000 hrs. salt spray) in some cases thread allowance may be required. 

Specifications have been written by many OEM’s; the appropriate International specification is ISO 10683 –Fasteners: non-electrolytically applied zinc flake coatings.

16. Spray Coatings PTFE XYLAN TEFLON: 

Xylan Screw Coating: 

Fastener coating known by its trade name Xylan® produced by Whitford is one of the most well-known industrial and fastener coatings. 

It is a fluoropolymer and has a high resistance to solvents, acids, and bases. 

Xylan can be obtained in over 300 grades many of which contain other additives such as PTFE. PTF materials with these additives are also known as Dry Film, and the materials are dry to touch. 

Xylan Fastener Properties

Depending upon the grade chosen the properties that this coating offers:

• Abrasion resistance 

• Acid rain resistant

• Chemical resistance

• Corrosion resistance

• Road chemical resistant

• Saltwater resistant

• Wear resistance

 Xylan Fasteners have a wide operating temperature range: from -250°C to +285°C. 

 Flexible curing, ambient to 440°C 

 Low Coefficient of Friction, UV stable, Excellent adhesion to most metals. Can be supplied in a variety of colours.

17. Rilsan: 

Rilsan® PA 11 coatings may be applied to all types of steel fasteners.  Processing is quite simple and consists of depositing a Rilsan® film onto the metal surface of the fastener. Careful surface preparation is absolutely necessary beforehand if optimum performance is to be achieved. Following this pre-treatment, a primer may be applied to improve the adhesion of Rilsan® onto the surface of the fastener and maximise the anticorrosion properties. Rilsan has high corrosion resistance in water, wastewater and salt water and has high chemical resistance to hydrocarbons, solvents, acids, salts and alkalis. 

Rilsan’ s outstanding features are abrasion resistance, impact resistance, flexibility, thermal resistance, weathering and chalking, it has low water absorption high dimensional stability, good resistance to stress cracking and a low coefficient of friction. 

Rilsan is an environmentally green product it is made from a renewable raw material, namely castor seeds 'castor oil'.

18. Stayblack: 

Stayblack is a chemical conversion which modifies the passive oxide layer of the stainless steel leaving the stainless steel matt black. The primary applications of this are fasteners and small pressings which need to retain a consistent and durable black appearance. It is used more specifically in military applications where a black, matt, non-reflective surface is essential. Stayblack is specified for components in medical, optical and scientific equipment as well as for many varied aerospace applications.

Automotive applications include external and under bonnet fasteners.   IMDS reference number for this coating is 3130589.

Stayblack withstands atmospheric weathering, hot water and temperatures up to 500°C without any change in appearance. The coating is less than 1µm thick and so will not alter any tolerance fits nor change the dimensions of the component by any significant amount. Stayblack does not affect the inherent corrosion resistance of the base stainless steel and will remain colour consistent.

It is a non-electrolytic process and so there is no possibility of hydrogen embrittlement,  the coating will not flake, chip or peel. Stayblack is dry to touch but lubricants can be added.

19. Anodising: 

Anodising (UK spelling)  Anodizing (USA spelling) is an electrolytic passivation process that increases the thickness of the natural oxide layer on the surface of the fastener. 

Anodised fasteners form the anode electrode of an electrical circuit. Anodising increases corrosion resistance, wear resistance and provides better adhesion for paint primers. Anodic films can also be used for a number of cosmetic effects, either with thick porous coatings that can absorb dyes or with thin transparent coatings that add interference effects to reflected light. 

The anodised coating on aluminium increases corrosion resistance and wear resistance, is electrically insulative, provides a good key for paint or adhesive and is often used as a decorative finish.

The oxide layer that forms on iron or carbon steel is commonly known as rust, which readily flakes off and actually promotes the corrosion of the underlying material. So an anodising process for steel may not be terribly useful, saying that; Anodising is also used to prevent galling of threaded fasteners; ferrous metals are commonly anodized electrolytically in nitric acid or by treatment with red fuming nitric acid, to form a hard black ferric oxide.

20. Sherardized coatings: 

Sherard Cowper-Coles the English metallurgist developed a new zinc galvanizing method and patented it over 120 years ago. The method which has been named after him made it possible to create zinc layers with unique properties. The difference primarily was in the uniformity of the zinc layer thickness, the anti-corrosive properties and the wear resistance. In the twentieth century, the method was further refined into a modern and high-quality surface treatment process for fasteners, nuts and bolts. 

Sherardised Process

Pre-treatment is required to remove any contamination or oxidation of the fastener such as mill scale or rust by barrelling or shot blasting. 

Sherardise

Diffusion galvanizing occurs when the fasteners are heated together with zinc powder in closed rotating drums. The dry diffusion process occurs at temperatures between 320ºC and 420ºC, during the vapour phase, and the zinc-iron alloy layers then form in and on the fastener surface. 

Sherardize

The coating composition of the gamma layer with 21-27% iron forms in and on the Bolts base material, above that, the Delta compact layer forms on the surface with 8-13% iron. Uniform layers of a thickness of between 10 µm to 75 µm are possible.

Sherardised Fasteners

Sherardising is a lasting, anti-corrosive, temperature and wear resistant layer which is applied uniformly over the entire surface of the fastener. Sherardizing produces an ideal surface for other processes such as rubber to metal bonding and the application of organic oils, stains, lubricants and sealants and provides a cost-effective, high-performance alternative to hot-dip galvanizing.

Sheradising Dis-advantages

Sheradising is a barreling process, thread damage may occur with larger bolts knocking together in the barrel.

Sherardising requires heat to complete the process this will produce tempering back on fasteners of class 10.9 and above.

Sheraplex

Sheraplex™ is a sherardizing process followed by a duplex coating, the coating is an organic barrier layer applied by either a dip and spin or spray technique, the layer is subsequently cured at 200ºC resulting in a uniform, complete coverage coating. The process outperforms HDS Galvanising and Sherardizing due to the additional coating but is more expensive. Sheraplex bolts require undercut nuts because of the depth of deposit and are not suitable for high-grade fasteners because of tempering back. 

21. RUSPERT: 

High-grade anti-corrosion metal surface treatment. Ruspert treatment is a high-grade anti-corrosion metal surface treatment technology,it consists of three layers:Metallic zinc layer,special chemical film and rustproof coating.Forming combined film by chemical reaction among three layers. It does not attribute its good anti-corrosion performance to merely a single film as ordinary metal surface treatment. It provide superior corrosion resistance by combined film.

A tough alloy coating combined with a zinc nickel alloy plating substrate and a top coat having further lubricating ability, which enhance the anti-corrosion performance of the products even after being drilled. It is totally secure even being used under high temperature environment thanks to the zinc nickel plating and inorganic coating combination that has a superior thermal resistance.

Environment-friendly high-grade anti-corrosion metal surface treatment. Non-Chrome Ruspert treatment is completely chrome-free and environmental friendly Ruspert treatment. It 's successful to achieve superior anti-corrosion performance without hexavalent chromiumand trivalent chromium.Various color are available as same as Ruspert treatment.