Fasteners and fixing coating coatings finish finishes fastener market segment segments
Materials from which commercially available threaded fasteners are produced range through aluminium and titanium to high-strength steels and super alloys and on to various polymers . Beyond the choice of materials, there are a number of fastener coatings, these can range from those that are simply to prevent or delay the onset of corrosion, through to those that seek to change the behaviour of the material or to combat difficult environments.
We are familiar with seeing black steel fasteners, Socket head cap screws being a popularly fastener in the uncoated black condition. The 'as forged' black surface treatment does not alter the part dimensionally, and retains oil on the surface of the fastener, preventing corrosion. Also used for corrosion prevention are a number of metallic plating processes, zinc, chromium and cadmium. There are real dangers in the electroplating of high tensile fasteners, Hydrogen embrittlement can lead to sudden failure of fasteners, and care needs to be taken to avoid this.
Titanium is famous for its tendency to gall at low levels of applied surface stress, even under very low sliding velocities and most especially against other titanium components. It is common therefore to find titanium fasteners that have their threads coated. Coatings involving lubricious compounds such as molybdenum or tungsten disulphide are often used and may be found to be combined with other surface treatments such as anodising.
The coating of fasteners with silver is common for motorsport use, especially where they are used in high-temperature applications such as exhausts or turbochargers. Silver has a low shear strength, making it a good choice as a solid lubricant, and it also is very resistant to oxidation. High-strength silver-plated steel nuts are commercially available in both metric and imperial thread forms.
Barrel electro plating is a surface covering in which a metal is deposited on a conductive surface. plating is an old process and has been around
for hundreds of years; it is critical for modern technology.
Barrel electroplating involves a chemical solution with the ionic form of the metal, an anode (positively charged) which may consist of the metal being plated (a soluble anode)
or an insoluble anode (usually carbon, platinum, titanium, lead, or steel), and finally,
a cathode (negatively charged) where electrons are supplied to produce a film of non-ionic metal.
Plating of fasteners is primarily for corrosion inhibition, also to improve the ability for the
fastener to be soldered, to surface harden the Fastener, to improve wear life, to
reduce friction, to improve paint adhesion, to alter conductivity, and to
improve and to give a decorative finish.
In addition to the Zinc protection against corrosion a post-plating dip is applied
to the zinc coating which is called a passivate or conversion coating.
This additional process leaves a thin chromate film which screens the zinc from oxidising.
The effectiveness of this two layer system determines the overall corrosion
performance of the coating. This can be measured by salt spray testing, which
will show how many hours it will take for the salt spray to remove the
passivate film (time to go white) and for the base material to show red rust (time to red).
Mechanical plating is a term usually given to the method of coating parts with Zinc by “cold welding” particles of Zinc powder (sometimes mixed with other metals) onto mainly steel components. It does not induce lasting Hydrogen Embrittlement so it is often used for coating high tensile fasteners class 10.9 and above.
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. Care needs to be taken to stop beads being trapped in some parts.
Tin or Aluminium can be mixed with the Zinc powder to give a mixed coating used for specific purposes.
This process can only be performed in a barrel, so parts larger than about 120mm long cannot be processed.
The deposits can be treated and passivated similar to an electroplated deposit.
Normal thicknesses of deposit will give about 240 hrs. salt spray.
As Mechanical Plating does not rely on 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
ZINC FLAKE FASTENER COATINGS
Zinc Flake 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.
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 onto 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.
Coating Large Fasteners
Fasteners that cannot be bulk coated due to damage problems (usually 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.
Sherardizing - Sherardising - Sherardized Fasteners
The englishman Sherard Cowper-Coles developed a new galvanizing method over 100 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 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.
Pre-treatment is required to remove any contamination or oxidation on the fastener such as mill scale, and are generally removed by shot blasting.
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 surface.
Coating composition of the gamma layer with 21-27% iron forms in and on the base material while, above that, the delta compact layer forms on the surface with 8-13% iron.
Uniform layers of thickness of between 10 µm to 75 µm are possible.
Sherardizing 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.
ZRC Galvilite® Galvanizing Repair
TYPE: Single pack, premixed, ready-to-apply, liquid organic zinc compound with a shiny finish to closely approximate the color of hot-dip galvanizing
- Designed specifically for field and shop repair of hot dip galvanizing
- Silver finish closely blends in with the colour of new hot dip galvanizing
- Meets and exceeds galvanizing repair specifications ASTM Des. A 780, SSPC-Paint 20 and DOD-P-21035A
- Industry-leading 95% zinc in the dried film
- Apply by brush or roller
- Meets VOC standards
GENERAL PROPERTIES: Galvilite® is a high-quality cold galvanizing compound based on a trade secret organic binder which affords true galvanic anti-corrosion protection to metals.
The dried film of GALVILITE is 95% metallic zinc.
GALVILITE is the only coating of its type to be recognized under the Component Program of Underwriter's Laboratories
RECOMMENDED USES: Galvilite is widely used without topcoat in place of hot-dip galvanizing. Specially formulated for repair of damaged hot-dip galvanizing Galvalume® and zinc metallizing, regalvanizing worn hot-dip galvanizing, repair of weldments, and repair of damaged inorganic zinc coatings. Excellent for long-term protection of structural steel, water and waste facilities, power plants, transmission towers and antennae, steel decking and bridges, and numerous OEM uses.
SUBSTRATES: Apply to properly prepared carbon steel, cast iron, hot-dip galvanized, and aluminum surfaces.
NOT RECOMMENDED FOR: Contact with potable water, strong acids or strong alkalis
TEMPERATURE RESISTANCE (non-immersion):
Continuous: 350°F max/177°C max
Non-Continuous: 750°F max/399°C max
SALT FOG EXPOSURE: 2,852 Hours (ASTM B117)
PROHESION/QUV EXPOSURE: 2,000 Hours (ASTM D5894)
H.D.S. Galvanized Fasteners
Pre Treatment consists of 4 separate stages; the Steel Fastener is immersed in a degreasing solution which removes light oil contamination. Pickling acid strips the fasteners steel surface back to clean base steel. A flux solution is applied to the steel fastener to assist with the galvanizing reaction.
Fasteners are placed in baskets to be galvanized then immersed in molten zinc at a temperature of around 450 degrees until the temperature of the work is the same as the zinc. During this process the molten zinc reacts with the surface of the steel to form a series of zinc/iron alloys. As work is removed from the zinc, the zinc on its surface will begin to solidify and is rapidly removed from the bath before the zinc on the work solidifies. The basket is then placed in a centrifuge and spun for several seconds, this removes excess zinc from the surface and prevents the items sticking together to ensure a smooth finish. Work is then transferred to a quench tank where it is cooled to allow handling.
Nuts, bolts and washers 8mm and above can be Hot Dip Spun Galvanized, in practice, it is normal for bolts and screws to be galvanized. Nuts should be galvanized as blanks and thentapped 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 high temperature (around 550°C) tend to take on a uniform, matt grey. The fastener specification should clearly state "that the fastener coating should conform to BS 7371: Part 6: 1998".
The initial cost of hot dip galvanizing threaded fasteners is generally a little higher than zinc plating the cost per year of rust-free life prove that hot dip galvanizing is by far the most economical coating.
High strength bolts up to grade 8.8 can be galvanized without difficulty.
Grade 10.9 bolts may require 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.
The 'lock-up' galling in the threads of galvanized fasteners can be overcome with
lubrication ensuring the correct clamping force is developed. Beeswax has been
found to be a most effective lubricant and molybdenum disulphides, or tallow,
have also been specified for this purpose.
Decorative Barrel Chrome plated Fasteners
Decorative chrome is designed to be aesthetically pleasing and durable.
Thicknesses range from 0.05 to 0.5 µm, however they are usually between
0.13 and 0.25 µm.
The chromium plating is usually applied over bright nickel plating and polishing
can take place after coating.
Hard chrome plated Fasteners
Hard chrome, also known as industrial chrome or engineered chrome, is used to
reduce friction, improve durability through abrasion tolerance and wear resistance,
minimize galling or seizing of parts, expand chemical inertness to include a broader
set of conditions, primarily oxidation resistance, and bulking material for worn parts
to restore their original dimensions. It is very hard, measuring between
65 to 69 HRC.
Hard chrome tends to be thicker than decorative chrome, with standard
thicknesses 200 to 600 µm, but it can be thicker for extreme wear resistance
Blackodize, Chemical Black and Gun Barrel Blue
This finish has a number of names Blackodize, Chemical Black and Gun Barrel Blue.
Black oxide is a coating for fasteners, It is used to add mild corrosion resistance,
for appearance and to minimise light reflection. To achieve maximum corrosion
resistance the black oxide must be impregnated with oil or wax.
One of its advantages over other coatings is its minimal deposit.
A hot bath of Sodium Hydroxide, nitrates and nitrites at 141°C are used to
convert the surface of the material into magnetite (Fe3O4).
Water must be periodically added to the bath, with proper controls
to prevent a steam explosion.
Hot blackening involves dipping the fastener into various tanks. These tanks contain, in order,
alkaline cleaner, water, caustic soda at 140.5°C (the blackening compound), and
finally the sealant, which is usually oil.
The caustic soda bonds chemically to the surface of the metal, creating a
porous base layer on the Fastener. Oil is then applied to the heated part,
which seals it by "sinking" into the applied porous layer. It is the oil that
prevents the corrosion of the Fastener.
There are many advantages of blackening, mainly:
blackening can be done in large batches, ideal for small fasteners
No significant dimensional impact , the blacking process creates
a layer about a micrometre thick.
It is cheaper than similar corrosion protection systems,
such as electroplating.
Generally known by its trade name Xylan® produced by Whitford is one of the most well-known industrial 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.
Depending upon grade the properties that this coating offers:
- Abrasion resistance
- Acid rain resistant
- Chemical resistance
- Corrosion resistance
- Road chemical resistant
- Salt water resistant
- Wear resistance
Wide operating temperature range: from -250°C to +285°C.
Flexible curing, ambient to 440°C
Low coefficient of Friction
Excellent adhesion to most metals.
Can be supplied in a variety of colours.
Rilsan® PA 11 coatings may be applied to all types of steel fasteners. Polyamide 11 (PA 11) or Nylon 11 is a polyamide bioplastic, its properties are similar to PA12, although it has a lower environmental impact, consumes less non-renewable resources to be produced and has superior thermal resistance. It is weaker but more resilient than the most common types of Nylon 6 and 66.
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 with Rilsan® products. Following this pre-treatment, a primer may be applied to improve the adhesion of Rilsan® onto the surface of the fastener, and maximize the anticorrosion properties.
Rilsan® has high corrosion resistance in water, waste water and salt water.
Rilsan® has high chemical resistance with hydrocarbons, solvents, acids, salts and alkalis.
Rilsan® outstanding features are abrasion resistance, impact resistance, flexibility, thermal resistance, weathering and chalking.
Rilsan® 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).
Anodising (UK) anodizing (USA) is an electrolytic passivation process used to increase the thickness of the natural oxide layer on the surface of metal parts.
The process is called anodising because the part to be treated forms the anode electrode of an electrical circuit. Anodising increases corrosion resistance and 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.
Anodising is also used to prevent galling of threaded components. Anodic films are most commonly applied to protect Aluminium alloys.
Ferrous metals are commonly anodized electrolytically in nitric acid or by treatment with red fuming nitric acid, to form hard black ferric oxide.
Anodising changes the microscopic texture of the surface and changes the crystal structure of the metal near the surface. Thick coatings are normally porous, a sealing process is often needed to achieve corrosion resistance. Anodised aluminium surfaces are harder than aluminium but have low to moderate wear resistance that can be improved with increasing thickness or by applying suitable sealing substances. Anodic films are generally much stronger and more adherent than most types of paint and metal plating, but also more brittle. This makes them less likely to crack and peel from aging and wear, but more susceptible to cracking from thermal stress.
Phosphate coatings are a crystalline conversion coating that is formed on a steel fasteners.
Phosphate coating is employed for the purpose of pre-treatment prior to coating or painting,
increasing corrosion protection and improving friction properties of sliding components.
In other instances, phosphate coatings are applied to threaded parts and top coated with oil
to add anti-galling and rust inhibiting characteristics. The phosphate process relies on the
basic pickling reaction that occurs on the metal fastener when the process solution comes
in contact with the metal.
The main benefits that phosphate provides are strong adhesion and corrosion protection.
Zinc phosphate coatings are also available and are mainly used for rust proofing steel fasteners. They can be applied by immersion or spraying. Zinc phosphate is a lighter alternative to manganese phosphate, while providing resistance to harsh elements that tend to wear products quickly.
Manganese phosphate coatings are for corrosion protection, anti-galling and lubrication.
There are a large number of phosphate coating available, manganese phosphate coatings
are the hardest, while providing unbeatable corrosion and abrasion protection. In comparison
to zinc phosphate coatings, manganese phosphate coatings offer continued wear protection
following the breaking in of components that are subject to wearing. These coatings are
applied only by immersion.
A chemical conversion which modifies the passive oxide layer of the stainless steel
fastener leaving the coated steel matt black. Stayblack is not electrolytic, it is a
barrel or vat process and so there is no possibility of hydrogen embrittlement.
The surface coating will not chip, peel or flake. Stayblack fasteners can be
delivered dry or lubricated.
Stayblack is used more specifically in military applications where a black, matt,
non-reflective surface is essential for stainless steel particular for fasteners which
need to retain a consistent and durable black appearance.
Stayblack is specified for fastener components in medical, optical and scientific
equipment as well as for many varied aerospace applications.Stayblack withstands
hot water, atmospheric weathering, 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 or change the
dimensions of the fastener to any significant amount.
Stayblack does not affect the inherent corrosion resistance of the base stainless steel
and will remain colour consistent longer than an untreated stainless steel fastener.
Coloured stainless steel fasteners
Chemical colouring of stainless steel uses a mixture of chromic and sulphuric acids that
develop the thickness of the naturally occurring passive film on the steel surface,
depending on the immersion time. It is a two stage immersion process that requires close
control on the operation's parameters and a considerable operator skill to get the
required colour consistency.
The colours produced are the result of light wave interference effects, as light is reflected
from the surface of the thickened oxide passive layer and interface of the passive layer and steel surface.
The sequence of colours formed as the film grows in thickness ranges through bronze, blue,
black, charcoal, gold, red-violet and green. With care, intermediate colours are possible.
The finished thickness of the passive film ranges from 0.02 microns to produce a bronze colour effect to
0.36 microns to produce a green colour effect between the passive layer and the steel surface of the sheet.
Nut Tapping before or after coating
Nuts and other tapped fasteners that are Hot Dip Spun Galvanised, Sherardised or have another heavy deposits will require re-tapping to clear coating material from the threads in the nuts. re-tapped tapping
Nuts can also be tapped over size before coating to allow for heavy coating deposits.
Because hot-dip galvanizing is a coating of corrosion-inhibiting, highly abrasion-resistant zinc on bare steel, the original steel becomes slightly thicker. When talking about tapped holes and fasteners, the increased thickness is important.
In consideration of the added thickness, holes need to be overtapped (enlarged) in order to accommodate the increased size of the fastener to be inserted. While chasing or retapping the nuts after galvanizing results in an uncoated female thread, the zinc coating on the engaged male thread will protect both components from corrosion. For economy, nuts are usually galvanized as blanks and the threads tapped oversize after galvanizing.
Tapping of all holes after galvanizing is recommended to eliminate double-tapping costs and the possibility of cross-threading. If, after galvanizing, the hole is still not large enough, it can be chased or retapped. Chasing or retapping will not necessitate regalvanizing. Because of the zinc coating’s cathodic properties, the coating on the mating fastener will protect chased or retapped nuts.
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