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What Is AccuBlack? The Benefits Of Choosing Chemi Black Screws & Bolts

What Are AccuBlack Screws & Bolts?An AccuBlack Socket Countersunk Screw

AccuBlack is a chemical blackening process that converts the finish of stainless steel components from natural to matte black.

Chemical blackening, also known as black oxide coating, is a chemical reaction that alters the surface finish of stainless steel.

This process can be used on our wide range of stainless steel precision engineering components, for instance, to produce blackened bolts from standard Stainless Steel Hexagon Bolts.

AccuBlack utilises a specialist hot PX-3 chemical blackening process to create a durable matte black finish.

This finish still maintains the high precision, tensile strength and corrosion resistance you would find on all of our natural finish stainless steel components.

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What Happens To Stainless Steel Components During The Chemical Blackening Process?

The AccuBlack chemical blackening process partially converts the top layer of stainless steel into matte black magnetite, ferrous-ferric oxide Fe3O4 (FeO-Fe2O3).

Magnetite is a compound of iron and oxygen - an iron oxide. Iron is not pure when it comes out of the ground, this ore is essentially the mineral form of iron, before it has been purified into a “usable” metal. Iron is the main element which makes up all steels.AccuBlack Screws & Natural Stainless Steel Screws

This conversion is achieved by submerging the components in a heated bath of Blackening Solution. This results in the stainless steel components maintaining their original precise dimensions, surface texture, tensile strength and integrity. 

This process does not affect the chromium and molybdenum content of the stainless steel, therefore the blackened stainless steel components maintain their high corrosion resistance against uniform and aqueous corrosion.

However, it is worth noting that this high corrosion resistance doesn’t completely prevent components from being susceptible to corrosion, this can also occur when the component is in contact with carbon steel, or other less noble materials. 

Why Choose A Chemical Matte Black Finish For Stainless Steel Fasteners?

In comparison to conventional oxidising methods, AccuBlack coatings have an increased abrasion resistance, ensuring the blackened appearance is durable during application.

Following extensive testing under strict laboratory conditions, AccuBlack coatings have been proven to be superior to standard cold blackening finishes, such as passing an independent 240 hour salt spray test.

Whilst typical cold black finishes are carried out at room temperature using a copper selenium compound that leaves a sooty residue on the components, our coating offers a smooth matte blackening finish. 

As mentioned earlier, the AccuBlack finish maintains the tight tolerances needed in precision engineering as the chemical process converts the outermost layer of the stainless steel into a black oxide layer.

This is unlike similar finishes such as wet paint or platings, which add a solid layer to the fixing, negatively affecting the surface texture and precise dimensions of the blackened components.

As the overall depth of the AccuBlack coating is less than a micron, this makes it an ideal choice for precision engineering.

J Foster Architects Chose AccuBlack Screws In Their Bespoke Staircase DesignWhat Applications Are Blackened Bolts Commonly Used In?

Accu’s PX-3 coatings are used in a wide range of applications, including automotive, aerospace and military stealth projects.

AccuBlack is also fully compliant with military specification MIL-DTL-13924D Class 4 for controlling reflective glare, making it suitable for optics applications.

Chemi black screws and bolts are also ideal for applications where appearance is considered essential. They may be chosen to match a black assembly when you do not want the component to visually interrupt a dark or black design.

Such can be seen in this bespoke architecture project, where Accu’s black screws were chosen to seamlessly blend in with the overall design of the powder coated black staircase, complimenting the intended aesthetic. 

What components are available with an AccuBlack Chemical Matte Blackening Finish?

AccuBlack chemical blackening is available for application on almost all of Accu’s Stainless Steel Precision Screws and most other Stainless Steel components upon request.

If chemical blacking is not listed as a standard finish on any of our products, customers can request this through our Custom Manufacturing Service.

AccuBlack is even available with our AccuLock Thread Locking Patch. Our anti-vibration patches increase the friction between threaded surfaces, to offer improved resistance to vibration, loosening, and leakage.

AccuLock Patches can be very useful in increasing the effectiveness and longevity of crucial fastenings within a sub-assembly. If you would like to enquire about this process on our blackened range of precision engineering, please contact our engineering team for pricing and lead time information.

Care & Maintenance of Chemical Black Screws & Bolts 

Our black oxide coating offers mild corrosion resistance, but when combined with wax or oil the corrosion resistance is greatly enhanced against uniform and aqueous corrosion, especially in environments with high moisture levels.

It is worth noting that whilst AccuBlack components have passed an independent 240 hours salt spray test, some surface corrosion may take place.

For more information on our PX-3 chemical blacking process, please see our Chemical Black Coating Guide.

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