Finishes on a bolt is described as the coating used on a bolt material. Depending on the kind of application required, finishes can be of very high to minimal quality.
An anti-corrosive finish is a common example of a very high quality finish whereas decorative finishes which serve the purpose of giving the bolt some color, are classified as low quality finishes. At the very least, some bolts may be available with no coating.
There are various finishes for bolts in use today and they are classified as follows:
|Plain finish||No coating. Also called natural, self-colour, black. When a finish isn’t specified it is automatically understood that the bolt has no coating. Plain finish products provide no resistance against corrosion.|
|Zinc plated||Zinc coated bolts. Thicker coatings provide higher resistance to corrosion and vice versa. Widely used as it is inexpensive. Coating is smooth and the finish looks good on bolts. These finishes are also clean to handle and provides a surface for painting, if required.|
|Zinc and clear chromate||Also known as zinc, clear zinc, bright zinc or clear. Very silvery in appearance. Offers resistance to corrosion.|
|Zinc and yellow chromate||Also known as zinc and yellow, z/chr, or zinc and chromate. This mixture is heavier than that of clear chromate and therefore offers better resistance to corrosion.|
|Hot dipped galvanized (HDG)||A process in which bolts are submerged in molten zinc. Zinc sticks to fasteners when dipped. Coating thickness is much more than that of zinc plating due to submerging of bolts. Therefore, provides superior resistance to corrosion.|
|Class 3||Bolts that have a zinc/tin coating. Corrosion resistance properties similar to that of hot dipped galvanized finish. Provides better socket fit than HDG.|
|Cadmium plated||Not used in present day due to environmental issues surrounding the poisonous cadmium. Replaced by zinc plating.|
|Decorative finish||Fundamentally used to produce a decorative or colorful outlook. Does not provide resistance to corrosion. Ideal for indoor applications. Important examples of such finishes include bronze metal antique (BMA), florentine bronze, brassed and nickel plated.|
Selection of a suitable fastener material for a particular application is vital. This is because bolts are used for various heavy duty as well as light weight applications and therefore the materials of these bolts have to be chosen accordingly.
A bolt made of steel as opposed to one made of aluminium can hugely affect the quality and durability of the joint it forms. Other factors such as environmental situations, presence of corrosive components as well as structural stability can alter a material’s effectiveness.
Some common bolt materials include:
Most commonly used material due to high formability, tensile strength and durability. Steel is inexpensively fabricated and widely available.
Available in various alloys to provide varying degrees of strength to suit different applications.
An alloy that mixes the attributes of low carbon steel and specific amounts of chromium and nickel.
The chromium in stainless steel makes it corrosion resistant. Low carbon content provides protection from hardening. Stainless steel can be martensitic or austenitic.
Martensitic stainless steel is strong and durable and can be strengthened via heat applications but provide low resistance to corrosion.
Austenitic stainless steel is the most common kind of stainless steel. It is made up of high levels of nickel and chromium and therefore provides superior resistance to corrosion. This kind of stainless steel can also bear considerable loads without fracture.
Bronze is an alloy made up of tin and copper. Provides high resistance to corrosion. Therefore, ideal for aquatic applications such as submarines, ships, boats and other underwater constructions.
Brass is an alloy made up of copper and zinc. It also provides high resistance to corrosion and forms a soft material.
A lightweight, synthetic material made of plastic.
Along with being corrosion resistant, it also has superior thermal and electrical insulating properties and can be easily dyed. However, nylon can melt if exposed to extremely high temperatures and can weaken in extremely low temperatures.
Everyday millions of bolts are keeping tall structures as well as small components in place. Their vast variety has emerged from the type of applications that demands their use.
There are various ways to classify the types of bolts and their uses, and all the considerations mentioned above make up the final choice of a bolt to be used in a certain application.