Machine Design
The Basics of Coatings and Finishes for Metal Hardware

The Basics of Coatings and Finishes for Metal Hardware

Engineers have a range of coatings and finishes they can specify for metal hardware such as retaining rings, wave springs, latches, hinges, and fasteners. The finishes can be functional or aesthetic. Here is a list of the most popular coating and finishes.

Vapor degrease/Ultrasonic cleaning

Vapor degreasing is the standard cleaning and finish for all stainless steels. It removes oil and other organic compounds from the steel’s surface by using a chlorinated solvent. The solvent effectively removes oil and grease from the exposed surfaces. Ultrasonics are used to force the solvent into cracks and crevices.

Vibratory deburring/Hand deburring

Though all surfaces and edges of fasteners and other hardware are usually smooth, there can always be sharp corners present on gap and part ends due to cut-off operations. To eliminate the sharp corners and get a blended, smooth surface finish, parts may be deburred by hand or by automated vibratory deburring machines to meet specifications.

Oil dip

This is the standard finish for many pieces of hardware made from carbon steel. The oil provides corrosion resistance during transport and normal storage. The oil-dip finish should not be considered a permanent finish.

Black oxide

MIL-DTL-13924, Class 1

This finish provides a flat-black cosmetic finish that also improves corrosion resistance.

Zinc phosphate

MIL-DTL-16232, Type Z, Class 2

This finish, sometimes called “Parkerizing,” appears gray-black in color. The phosphate boosts corrosion resistance compared to black oxide, but cannot outperform cadmium plating or stainless steel for total corrosion resistance. Phosphate cannot be applied to stainless steel.

Passivation

AMS 2700, Method 1, Type 2, Class 3

Passivation gives stainless steel a bright finish while increasing its corrosion resistance. The process dissolves iron particles and other substances that have become imbedded in the surface of stainless steel during production. If not dissolved, these foreign particles can promote rusting, discoloration, and even pitting.

In theory, stainless steel’s corrosion resistance is due to a thin, invisible oxide film that completely covers the surface and prevents further oxidation. Removing contaminants prevents breaks in the oxide film and optimizes corrosion resistance.

Cadmium plating

Cadmium Plate, AMS-QQ-P-416, Class 2, Type I
Cadmium Plate w/Chromate Dip, AMS-QQ-P-416, Class 2, Type II

Cadmium plating on carbon steel increases the steel’s corrosion resistance. The process of cadmium plating spiral retaining rings is costly and subjects the ring to the possibility of hydrogen embrittlement. Smalley (www.smalley.com) offers stainless steel as the preferable option to cadmium.

Ken Masset is the Vice President of Engineering Sales at Smalley Steel Ring Co. in Lake Zurich, Ill. 

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