Mechanical Plating,Mechanical Plating,Mechanical Plating,Mechanical Plating
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 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)
Hydrogen Embrittlement
The need to prevent hydrogen embrittlement was one of the major reasons for the creation and use of mechanical plating.
A critical concern in electroplating and other coating processes used on ferrous parts is the embrittling effects of hydrogen
absorbed by the part. The current used in electroplating acts to enhance the possibility of hydrogen embrittlement--both
because most electroplating generates hydrogen at the cathode and because the negative charge acts to pull hydrogen into
the part. Hydrogen embrittlement can cause sudden development of breaks or cracks in highly stressed areas, with
subsequent total rupture of the part or the assembly. The risk increases for parts that have high hardness from cold
working or heat treating, especially those made of high-carbon steels.
In the normal electroplating process, an important source of hydrogen gas is the reaction between acids and metals present
in the plating solution. The hydrogen migrates through the metallic substrate and concentrates at high stress points and
grain boundaries. The trapped hydrogen builds internal pressures that can lower the tolerance to stresses applied in actual
use. Dangerous failures in critical applications can result.
Other atmospheres that can potentially cause or contribute to hydrogen embrittlement include heat-treating furnaces,
cleaning solutions, and pickling baths.
Advantage of Mechanical Plating in Avoiding Embrittlement. Mechanical plating deposits metals while
eliminating or at least minimizing the risk of embrittlement caused by the coating process itself. A hydrogen-producing
reaction does occur in mechanical plating:
Zn metal + 2H+ ®H2(gas) + Zn++(ion) (Eq 1)
However, this reaction occurs essentially on the surface of the powdered zinc particles, which are approximately 5 to 10
μm in diameter. The reaction proceeds at a relatively slow rate and within a more porous, less oriented metallic grain
structure than that produced by electroplating. Therefore, the hydrogen gas is not likely to be trapped or occluded within
or under the metal particles in the coating. The effusion or escape of hydrogen gas through the deposit and away from the
substrate is more likely than diffusion into the base metal. A substantial portion of the mechanical plating performed
today is done on parts and metals that are predisposed to hydrogen embrittlement. These include spring steels, parts heat
treated to 42 HRC or higher, cold-headed parts, or any part for which service application or structural integrity is highly
critical.

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