Reference

1. M. Rubinstein, Electrochemical Metallizing, Vinmar Press, 1987 Advantages and Limitations

The key advantage of selective plating is portability. Many systems can be moved to various locations in a production facility or be transported to the job site. Selective plating is also versatile; it permits most electroplate types to be deposited onto any conductive substrate that can be touched with an electrode. Cast iron, copper, steel, stainless steel, high-temperature nickel-base alloys, aluminum, and zinc are commonly plated by this method and exhibit good adhesion. Limited adhesion can be obtained with other materials, such as titanium, tungsten, and tantalum.

Selective plating allows higher current densities than tank plating, which translate into higher deposition rates, up to 0.010 mm/min (0.0004 in./min). In addition, inherently precise thickness control permits plate buildup or repair without the need for subsequent machining. In regard to deposit hardness, a 70 HRC trivalent chrome is available for selective plating of thick deposits, which places selective plating on a par with hexavalent tank chrome. The exceptions are the harder deposits of cobalt and gold (Table 2). Table 3 provides a point-by-point comparison of selective plating with competitive processes, including tank plating.

Table 2 De posit hardness attainable with selective plating versus bath plating

Table 2 De

Metal type

Microhardness, DPH

Bath plating

Selective plating

Cadmium

30-50

20-27

Chromium

750-1100(a)

850-1100

Cobalt

180-440

510

Copper

53-350

140-210

Gold

40-100

140-150

Lead

4-20

7

Nickel

150-760

280-580

Palladium

85-450

375

Rhodium

550-1000

800

Silver

42-190

70-140

Tin

4-15

7

Zinc

35-125

(a) Usual range, but hardnesses of 280-1200 DPH are possible.

Table 3 Selective plating versus other processes

Characteristic

Selective plating

Welding

Flame spray or plasma metallizing

Electroplating

Precision buildup capability

Excellent

Poor

Poor

Fair to good

Quality of bond

Excellent

Excellent

Fair to good

Good

Heat distortion or stresses

None

Frequently

Sometimes

None

Heat cracking

None

Frequently

Sometimes

None

Speed of deposit

Fast

Very fast

Very fast

Slow

Density of deposit (porosity)

Very dense(a)

Very dense, but with blowholes

70-90% of theoretical density

Moderately dense

Portability

Yes

Yes

Sometimes, but overspray precludes its use

No

Requirement for post-machining

Not required on thicknesses up to 0.254 mm (0.010 in.) on smooth surface

Always required

Almost always required

Usually required

Hydrogen

No(b)

No

No

Yes

embrittlement

Source: Ref 1

(a) Generally 25% less porous than electroplating and 70% less porous than flame spray or plasma metallizing.

(b) Specific cadmium, nickel, and nickel-tungsten deposits have been tested on high-strength steel and were found to be nonembrittling. Other deposits may not cause embrittlement.

Besides electroplating, selective plating systems can perform several other ancillary operations:

• Electrostripping for deplating of many metals and alloys

• Anodizing for protecting aluminum and alloys

• Electromilling for removing base metal, as in chemical milling

• Electroetching for permanently identifying parts

• Electropolishing for refining a surface chemically

Depending on part size, dimensional considerations, and required surface characteristics, all of these operations can be done with the same equipment and similar electrodes. Only the solutions are different.

Selective plating of small parts is more the exception than the rule, and large volumes of small parts are more economically plated by high-production-rate processes, such as barrel plating. Plating of entire components with complex geometries is better left to processes such as tank plating, which is more economical because solutions are less costly and throughput is higher. Another limitation is deposit rate; both flame spraying and welding deposit metal at a considerably faster rate.

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