Written by Jody Muelaner
An example of galvanic corrosion. Materials used in fasteners must be compatible with the parts being joined together and the conditions for use.
There are many considerations when selecting the ideal type and material for fasteners. For example, the material selected typically must be reliable, cost-effective, and maintain durability in the environment it’s in.
This often means the fastener material should be corrosion-resistance.
The corrosion-resistance of a particular fastener will also depend on the materials being joined because dissimilar metals that are in contact with each other may cause galvanic corrosion.
The most common material for mechanical fasteners, such as bolts and screws, is steel. Different grades of steel are available, depending on the strength required. If rust or corrosion is a concern, stainless steel, brass, or titanium may be used. Surface coatings, such as bright zinc plating, are also often used to prevent corrosion.
Polymer fasteners are ideal for some applications, when corrosion-resistance with moderate strength is required. These materials include nylon and polytetrafluoroethylene (PTFE).
Standard steel bolts use different grades of steel and heat treatments depending on the required strength. The grade is indicated by two numbers, usually stamped on the head and separated by a point, which should not be mistaken for a decimal.
The first of the two numbers is the ultimate tensile strength (UTS), given in MPa divided by 100. The second number is the ratio of the yield strength to the ultimate tensile strength. Common classes are 5.8, 8.8, and 10.9. For example, a grade 10.9 bolt would fail at 1,000 MPa and will yield at 90% of this value (900 MPa).
Aluminum is also sometimes used for bolts and screws although it is relatively rare in this application. Aluminum pop rivets are, however, very common for low-cost applications. Stainless steel is typically used where corrosion is an issue.
For fusion welded joints, the filler material is usually closely matched to the material of the parts being joined. However, brazing, soldering and friction stir welding may all use different materials to fill the weld than those being joined.
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