Electroplating

Stainless steels may be plated with copper, brass, chromium, nickel, cadmium, and the precious metals for such purposes as:

• Color matching

• Lubrication during cold heading, spring coiling, or wire drawing

• Reduction of scaling at high temperature

• Improvement of wettability or of conductance of heat or electricity

• Prevention of galling

• Decorative uses in such applications as jewelry

• Prevention of superficial rusting

Detailed information on plating bath compositions, control of bath composition, bath temperature, plating equipment, process limitations or advantages, and applications for electrodeposits can be found in the articles contained in the Section on "Plating and Electroplating" in this Volume.

Although a stainless steel surface may be clean and scale-free, an adherent electrodeposit cannot be obtained until the surface is activated for removal of its normally ever-present oxide film. Activation, which is performed immediately before plating, may be accomplished by cathodic treatments, immersion treatments, or simultaneous activation-plating treatments. These treatments, together with other procedures necessary for preparing stainless steel for electroplating, are fully described in ASTM B 254 (Ref 7).

Figure 4 summarizes some of the data from ASTM B 254. The same cleaning procedures would be appropriate before electrodeposition of other metals. When preparing stainless steel for electroplating, the following should be considered:

• Electrocleaning: When bright plating is desired, the alkalinity, current density, and temperature of the cleaning bath should be kept as low as possible, especially for the high-chromium alloys. To minimize the severity of electrocleaning when a bright finish is desired, a short electropolishing treatment may be used in lieu of electrocleaning, provided that the preliminary cleaning is adequate.

• Acid pickling: Freshly prepared pickling baths should be activated by dissolving some iron in them before using the bath; otherwise, the rate of pickling is slow until the bath has dissolved some iron from the steel being treated.

• Activating: The activation step is the most important in surface preparation of stainless steel for electroplating. If the simultaneous activation-plating treatment is used, where the anodes are nickel, the nickel content of the solution gradually increases because of the low cathode efficiency. This can be compensated for by removing a portion of the solution and replenishing with HCl. The activation-plating treatment is usually preferred for stainless steel parts that are to be copper plated.

• Plating: After activating and rinsing, the work should be entered directly into the electrolyte. When possible, the rinse water should be slightly acid (2.5 to 3.5 pH); the required acidity is usually provided by the dragout of acid from the activation treatment. Stainless steel has much lower electrical conductivity than most other metals, and thus adequate attention should be paid to racking when introducing work into copper electrolyte, to ensure an adequate number of contacts to obtain complete coverage in minimum time.

Solution no.

Type of solution

Composition of solution

Operating temperature

Cycle time, min

°C

°F

1

Anodic alkaline cleaner(a)

Alkali, as low as possible

(b)

(b)

(b)

2

Acid pickle

H2SO4, 8-11%

65-70

150-70

10-45

3

Acid pickle

HNO3, 6-10 vol%; HF, 1.5 vol%

Room temperature

Room temperature

10-30

4

Cathodic acid etch(c)

H2SO4, 5-15 vol%

Room temperature

Room temperature

1-5(d)

(a) Current density, as low as possible.

(d) After subsequent rinse, place parts into copper electrolyte while parts are still wet

Fig. 4 Electroplating copper on stainless steels

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