Materials
Screws and bolts are usually made of steel. Where great resistance to weather or corrosion is required, like in very small screws or medical implants, materials such as stainless steel, brass, titanium, bronze, silicon bronze or monel may be used.
Galvanic corrosion of dissimilar metals can be prevented (using aluminum screws for double-glazing tracks for example) by a careful choice of material. Some types of plastic, such as nylon or polytetrafluoroethylene (PTFE), can be threaded and used for fastenings requiring moderate strength and great resistance to corrosion or for the purpose of electrical insulation.
Often a surface coating is used to protect the fastening from corrosion (e.g. Bright Zinc Plating for steel screws), to impart a decorative finish (e.g. jappaning) or otherwise alter the properties of the base material.
Selection criteria of the screw materials include: size, required strength, resistance to corrosion, joint material, cost and temperature.
Read more about this topic: Screw
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