AYA FASTENERS Guide | Bolt Surface Treatment

news-800-800 When it comes to fasteners the majority of people will often think about the underlying material while not paying as much attention to the surface coating. However, the surface coating can often have the same importance as the underlying material as it can help give specific properties to the material, including adding extra protection to the threads and shanks. Usually, the type of coating you will choose for your bolt will depend on the environment in which you are going to be using your fastener. To help you better understand how the bolt surface treatment can fundamentally affect your construction we have created this guide that will help you get a better idea about the different types of joints and surfaces.

How are surface finishes for bolts applied?
The surface application is an additional step in the manufacturing process for bolts. This surface engineering process results in a surface of material being overlayed on top of the existing bolt. This material could include, powder, bulk, or film and will change based on the intended usage of the bolt.

What is the most common reason for adding surface coating to a bolt?
The most common reason why surface coating is added to a bolt is that it can help increase the fastener's corrosion resistance. Normally, depending on the environment you are in you might find that different surface materials would give you better corrosion resistance results. This helps increase the longevity of your project and ensures that you will not have to worry about your bolts getting corroded over time.

What are the best factors to determine the type of coating needed for your bolts?

There are many different factors that should be considered when it comes to deciding the coating for your fasteners, but the main three are:

Mechanical Application and the Environment
The service environment will usually be what dictates the type of material you need your fastener to be, including the type of surface overlay that would work best. When it comes to the environmental conditions that are more frequently used for determining the surface coating, the first one to consider is loading conditions. If the fastener you are using will be in a cyclic loading environment in a corrosion fluid the surface coating you will require will be different from what it would have been if it was in open air and had a constant and static load. Therefore the mechanical application as well as the environmental conditions in which the fastener is placed is going to have a tremendous effect on the fastener.

Fastener material
Naturally, the type of material you have used for the bolt is also going to have an effect on which surface coatings are best for your bolt. Most fasteners are created using steel alloys which means that in most cases they are able to have most of the common surface coatings applied to them.
Another thing to consider is the qualities of the fastener material as those can also dictate which areas need to be strengthened through the use of a coating. Steel fasteners for example are far more likely to suffer from corrosion and oxidation, which would make surface coating necessary in the majority of environments. Non-steel fasteners on the other hand, such as aluminum, brass, and stainless steel are more limited in the available coating options. Stainless steel bolts as an example are unlikely to have any coating as the material that the bolt is created from is already quite resistant to corrosion. Aluminum fasteners however will usually receive a surface treatment known as Anodizing rather than have a different material coated on its surface. Anodizing will instead increase the natural aluminum oxide layer on the surface of the material through the use of an electrolytic process.

Fastener appearance
Finally, the appearance of the fastener might also be important, especially in cases where the bolt might be located in a visible place. Different coatings will give a different appearance to your bolt both in terms of coloring and thickness.
If you want your bolt to have a darker appearance then you most likely will require Black Oxide coatings, while for shinier appearances you can use either nickel or chrome coatings. Finally, Zinc coating is known for having a more muted appearance which could be the look you are going for with your bolts.
It is more common for the fastener appearance to be considered in cases where the bolt is going to be used indoors in an item that has a decorative aspect. However, the appearance should not be the only thing you consider when choosing a coating, as each material and technique could give you a different result in terms of coating thickness.

What are the most common materials for fasteners?

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Stainless Steel
Stainless steel is known for giving very good resistance to heat and corrosion while also having high weldability. There are various different performance grades for stainless steel (45,50,60,70, and 80) with most of them consisting of austenite. Different types of stainless steel also have slightly different qualities.
A1, A2, A4 martensite, and 13% C are known for their high strength and wear resistance but their corrosion resistance is low. C1, C2, and C4 are considered to be ferritic stainless steel, while 18 percent CR has a far higher level of corrosion resistance.

Carbon Steel
Carbon steel is divided into low-carbon steel, medium-carbon steel, and high-carbon steel. There is also alloy steel by carbon content but that type of carbon steel is not normally available in the market.

What are the most common types of fastener surface coatings and treatments?

news-800-800 Electro galvanizing
Electro galvanizing is a low-cost surface treatment that can leave you with fasteners that have a beautiful appearance. However, while it is a relatively cost-effective method, its anti-corrosion performance is not as good as one might hope, and it is generally considered one of the methods with the lowest anti-corrosion performance.
Galvanizing is also likely to lead to hydrogen embrittlement which is why it is not a method used with bolts that have a grade higher than 10.9. While there is a process for removing hydrogen that can be used after galvanizing, the passivation film is going to be damaged by this process which requires a temperature of more than 60 degrees. As a result, unless it is required by the customer most manufacturers are not going to proceed with hydrogen removal unless necessary.

Surface phosphating
This process is even cheaper than galvanizing but the corrosion resistance it provides the bolt is even lower. Fasteners need to be coated with oil once this process is completed, as corrosion resistance is actually connected to the oil’s performance.
Industrial screw fasteners are often taken through the phosphating and oiling process as the performance of this method is fairly consistent and the toque and pre-tightening forces can have a good performance overall.

Oxidation
news-800-800 Oxidation, also known as blackening and bluing, is a process that is used for the rapid oxidization of steel and iron products. This creates a dense oxide film protective layer. Blackening has many benefits including that it can be safe to perform without electricity. It is also a low-cost and fast method with very simple materials.
This is the cheapest surface treatment available for industrial fasteners which is why it is often used by manufacturers.
However, blackening does not have any antirust abilities, which means that even after the material is oiled it could end up being rusted. Blackening does not improve the pre-tightening force or torque of a fastener either.

Silver and nickel plating
Silver coating is extremely effective in preventing corrosion and it can also act as a solid lubricant for fasteners. However, the high cost of the silver plating means that it is usually nuts and smaller bolts that are coated while larger bolts are not normally coated. Silver plating also adds to the high-temperature resistance of the bolt and can help prevent oxidization. Nickel plating is also effective when high corrosion resistance and conductivity are necessary, including in the vehicle battery terminal.

Conclusion
When it comes to the fasteners you use, the surface plating can have a particular importance as it will help ensure the longevity of your fastener. Depending on the environment and mechanical application you need the bolt for, the type of surface plating you might need will change. More importantly, the different types of fastener surface coatings can have a big effect on the price of the bolts you are using, as such certain bolts will cost more just because of the coating they have and the surface treatment they received.

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