Process Capabilities

Plating Thickness. The thickness of mechanical plating deposits ranges from 5 to 75 pm (0.2 to 3 mils). The heavier coatings are referred to as mechanical galvanizing or cold-impact galvanizing coatings. Because the coating thickness is somewhat independent of cycle time and is controlled by the plating metal additions, a heavy "galvanized" coating thickness can be applied in almost as little time as thinner commercial coating thicknesses. Mechanical galvanizing provides uniform, smooth, and adherent coatings, and the coated parts require no thread chasing. Extensive exposure and salt spray testing have confirmed that the corrosion resistance of mechanical galvanized parts is comparable to that of hot-dip galvanized parts and parts coated with other types of zinc deposits at equivalent coating weights (Table 1).

Table 1 Corrosion resistance of threaded bolts with various coatings

Coating

Coating thickness, ^m

Hours to red rust(a)

Zinc-aluminum

50 25

5000 2200

Zinc galvanized

50

490

Cadmium plated

25

1100

Dacromet

1250

Zinc electroplated

12.5

144

Note: All parts except Dacromet were mechanically plated. Dacromet is a zinc flake/chromate dispersion coating applied like dip/spin paint with a double cure.

(a) ASTM B 117 salt spray test

Applicable Parts. Many parts for which coating options were formerly limited to electroplating or hot-dip galvanizing are now successfully being mechanically plated. Part types now universally accepted as candidates for mechanical plating or galvanizing include nails; chain and wire forms of all types; bolts, nuts, and washers; offshore drilling hardware; poleline hardware for telephone and cable television lines; certain marine fasteners; ASTM A 325 structural bolts; aircraft hardware; and automotive hardware of all types.

Whether or not parts can be successfully coated using mechanical plating or galvanizing depends on their size, weight, and geometry. Parts that would tend not to withstand the vigorous tumbling action of the process are not suitable. Parts heavier than 1 to 2 kg (2.2 to 4.4 lb) or longer than 0.5 m (1.64 ft) are not usually coated using mechanical plating.

Specific Characteristics, Advantages, and Limitations. Because of the dust or powder form in which metals are deposited, the surfaces of mechanical coatings will have matte to medium-bright luster. It is possible to achieve very smooth surfaces, but the mirrorlike surfaces typical of electroplated parts cannot be obtained. For this reason, mechanical plating finishes are considered to be functional rather than decorative.

Parts with holes and recesses that are difficult to electroplate to the desired thickness usually can be properly plated via mechanical plating. Parts with hole diameters as small as 0.78 mm (0.03 in.)--even where the depth of the hole is greater than the diameter--frequently can be plated successfully. Mechanical plating is often more economical and more flexible for parts that require special racking or special anode configurations to electroplate recessed areas.

Powder metallurgy parts can be coated by mechanical plating without prior sealing of their surfaces. Because mechanical plating solutions are generally chemically consumed, little excess is available to be entrapped in the pores of the substrate. The initial galvanically deposited copper strike will permeate such pores, and the metal powder will fill and bridge them. Also, because the potential for hydrogen embrittlement is extremely low, any pores in the part should not accumulate significant amounts of hydrogen.

Quality Control. In order to ensure the quality of the mechanically plated parts, the operator must pay constant attention to the coating adhesion, weight and thickness, consolidation, uniformity, and general appearance and brightness. The main causes of nonuniform appearance include:

• Use of glass media in poor condition

• Insufficient cleanliness of parts prior to plating

• Insufficient burnishing time at the end of the plating cycle

• Wet parts kept in surge hopper too long

• Conductivity of water too high

• Zinc added to cycle too quickly or in incorrect amounts

Applicable Specifications. ASTM B 635, B 695, and B 696 are standard specifications for mechanical plating. The military standard is covered by MIL C 81562. Many large original equipment manufacturers have their own specifications, as do many state highway departments. Mechanical galvanizing conforms to the coating weight and general requirements specified in ASTM A 153, which is widely used in the hot-dip galvanizing industry.

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