Introduction

MECHANICAL PLATING is a method for coating ferrous metals, copper alloys, lead, stainless steel, and certain types of castings. The process applies a malleable, metallic, corrosion-resistant coating of zinc, cadmium, tin, copper, or aluminum; combinations of metals can be applied as codeposits or as "sandwich" layered deposits. Mechanical plating has been used internationally for over 40 years and is referred to by a variety of names, including peen plating, impact plating, and mechanical galvanizing. Mechanical plating often can solve engineering, economic, and pollution-related plating problems. It offers a straightforward alternative method for achieving desired mechanical and galvanic properties with an extremely low risk of hydrogen embrittlement. In some circumstances, it offers a potential cost advantage over electroplating (Fig. 1).

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Fig. 1 Cost comparison of mechanical plating and electroplating. Approximate costs shown are for chemicals only and do not include labor or overhead.

Mechanical plating is accomplished at room temperature, without the electrical charge passing through the plating medium that is necessary with electroplating or electrocoating. The metallic coating is produced by tumbling the parts in a mixture of glass beads, metallic dust or powder, "promoter" or "accelerator" chemicals, and water. The glass beads provide impacting and hammering energy, which serves to pound the metallic particles against the surfaces of the parts. The result is a tight, adherent metallic coating produced by "cold welding" fine, powdered metallic particles to the surfaces of parts.

Recent improvements in deposit quality, cost-effectiveness, and ease of application have induced many finishing engineers to investigate and adopt mechanical plating for certain applications. Special advantages of the mechanical plating and galvanizing process are that it:

• Greatly reduces part susceptibility to hydrogen embrittlement

• Can be used to deposit a wide variety of metals in a broad range of coating thicknesses

• Consumes comparatively low amounts of energy

• Does not require the use of toxic chemicals

• Simplifies waste treatment

• Does not require baking in most cases

• Provides greater uniformity of coatings (when used for galvanizing)

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