Bolts are typically marked on the head to show what grade bolt they are. For a list of the most common grade markings see our Material Grade Identification and Properties Chart. Note that, in addition to the grade marking, many bolts also have a manufacturer's mark.

  • Grade 2 is a standard hardware grade steel. This is the most common grade of steel fastener and is the least expensive. Except a possible manufacturer's mark, Grade 2 bolts have no head marking.
  • Grade 5 bolts are hardened to increase strength and are the most common bolts found in automotive applications. Grade 5 bolts have 3 evenly spaced radial lines on the head. Grade F is roughly equivalent to Grade 5. Grade F nuts are used with Grade 5 bolts.
  • Grade 8 bolts have been hardened more than grade 5 bolts. Thus they are stronger and are used in demanding applications such as automotive suspensions. Grade 8 bolts have 6 evenly spaced radial lines on the head.
  • Alloy steel bolts are made from a high strength steel alloy and are further heat treated. Alloy steel bolts are typically not plated, resulting in a dull black finish. Alloy steel bolts are extremely strong but very brittle. 

 

​Silicon bronze, often referred to simply as bronze, is an alloy made mostly of copper and tin with a small amount of silicon. Bronze is used primarily in marine environments. It is preferred over stainless in wooden boat construction and re-fastening due to its superior corrosion resistance, and over brass due to its higher strength. Bronze is similar to copper in color and is also sometimes seen in fine woodworking where it is used for its appearance. The main drawback of bronze is its high cost.

Brass is an alloy of primarily copper and zinc. Brass is highly corrosion resistant and electrically conductive. However, its use as a fastener is somewhat limited due to its relative softness. It is used primarily for its appearance.

Aluminium is a light, soft, corrosion resistant metal. Like stainless steel, aluminum's corrosion resistance is inherent to the material. Therefore, scratches and nicks will not effect the corrosion resistance. Fasteners are made from a variety of aluminum alloys, with elements such as manganese, silicon, iron, magnesium, zinc, copper, and silicon being added to increase strength and melting point. Rivets are often made from aluminum alloys in the 5000-series, which uses magnesium as the primary alloying element.

Zinc Plating. Many steel fasteners are electroplated with zinc for better corrosion resistance. Fasteners that have been zinc plated have a shiny, silvery or golden appearance, referred to as clear or yellow zinc respectively. They are fairly corrosion resistant but will rust if the coating is destroyed or if exposed to a marine environment.

Hot Dip Galvanizing. Galvanizing is another coating involving the application of a layer of zinc. Hot dip galvanizing puts the thickest possible coating on the metal, resulting in superior corrosion resistance. Due to the thickness of the coating hot dipped galvanized bolts are not compatible with other nuts. Galvanized nuts are tapped slightly larger than other nuts to accommodate this coating. Hot dipped galvanized fasteners are frequently used outdoors, especially in coastal environments.

Chrome Fasteners are chrome plated and polished for appearance. Chrome plating provides similar corrosion resistance to zinc plating. The main drawback of polished chrome is its high cost. If more corrosion resistance is required, stainless steel may be chrome plated, preventing any corrosion should the chrome be penetrated.

Galvanic corrosion occurs when dissimilar metals are in electrical contact in water especially salt water. As a small electrical current flows from one metal to the other, one metal will begin corroding faster than normal (the anode) and the other will corrode more slowly than normal (the cathode). The result is that the anode material will be eaten away much more quickly than the cathode material.

Through sound design and material choice galvanic corrosion can be reduced. This involves avoiding dissimilar material combinations. In addition, a dissimilar metal may be purposely used as a ‘sacrificial anode’. This is done by purposely creating a corrosive situation where the metal being corroded faster is simply a meaningless block of metal (often zinc) thus resulting in the important materials corroding more slowly.

This information was sourced from theboltdepot.com