We are familiar with seeing black steel fasteners, Socket head cap screws being a popular fastener in the uncoated black condition. The 'as forged' black surface treatment does not alter the part dimensionally, and retains oil on the surface of the fastener, preventing corrosion. Also used for corrosion prevention are a number of metallic plating processes, there are real dangers in the electroplating of high tensile fasteners, Hydrogen embrittlement can lead to sudden failure of fasteners, and care needs to be taken to avoid this.
Fasteners of High tensile class 12.9 are typically supplied unplated with an oil quenched finish, where the vast majority of lower tensile fasteners are coated; this includes Metric class 8.8, USA grade 5 and below. Considerable qualities of fasteners will be coated by the fastener manufacturer in their own factory or subcontracted plating plants. Smaller quantities and unique finishes are outsourced to a local coater via the fastener distributor.
The reason for coating is primarily for corrosion resistance but can also be for thread lubrication and in many instances for aesthetics for furniture and similar industries. Non-electroplating finishes are typically applied by barreling, dipping, or spraying
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