Surface Preparation for Electroplating

Preparation for plating is one of the most critical of all cleaning operations, because maximum adhesion of the plated coating to the substrate is the major requirement for quality work. Maximum adhesion depends on both the elimination of surface contaminants in order to induce a metallurgical bond whenever possible and the generation of a completely active surface to initiate plating on all areas. In addition to pickling or other descaling operations, adequate cleaning requires multistage cycles, usually comprised of the following steps: (1) precleaning with a solvent to remove most of the soil; (2) intermediate cleaning with alkaline cleaners; (3) electrocleaning to remove the last traces of solids and other contaminants that are especially adherent; (4) acid treatment and surface conditioning to remove light oxide films formed during previous cleaning processes and to microetch the surface; and (5) electrolytic (anodic) desmutting to remove any smut formed during acid pickling of heat treated high-carbon steel parts. Low-carbon steels do not require this desmutting step. Anodic electrocleaning also offers oxidation or conditioning of scale. The oxidized or softened scale is easily removed in subsequent acid pickling. The types of cleaning usually employed in the above steps are:

• Precleaning: cold solvent, vapor degreasing, emulsifiable solvent, solvent emulsion spray, or alkaline spray with or without solvent emulsion

• Intermediate alkaline cleaning: soak cleaning with 30 to 90 g/L (4 to 12 oz/gal) of cleaner at 82 °C (180 °F) to boiling, spray cleaning with 4 to 15 g/L (0.5 to 2 oz/gal) at 66 to 82 °C (150 to 180 °F), and barrel cleaning with 7.5 to 45 g/L (1 to 6 oz/gal) at temperatures below 82 °C (180 °F)

• Electrocleaning: cathodic, anodic, or periodic-reverse

• Acid treatment: practice is highly specific for the metal being processed

• Anodic desmutting: necessary to remove carbon smut

ASTM recommended practices for cleaning various metals prior to plating are given below:

A 380

Descaling and cleaning of stainless steel surfaces

B 183

Preparation of low-carbon steel for electroplating

B 242

Preparation of high-carbon steel for electroplating

B 252

Preparation of zinc-based die castings for electroplating

B 253

Preparation of and electroplating on aluminum alloys

B 254

Preparation of and electroplating on stainless steel

B 281

Preparation of copper and copper-based alloys for electroplating

B 319

Preparation of lead and lead alloys for electroplating

B 480

Preparation of magnesium and magnesium alloys for electroplating

B 322

Cleaning metals before electroplating

Process sequences and operating details in surface preparation for electroplating are presented in articles in this Volume on cadmium plating, finishing of stainless steel, finishing of aluminum alloys, finishing of copper alloys, finishing of magnesium alloys, and finishing of titanium alloys. The procedures used for preparing the surfaces of high-carbon and low-alloy steels, low-carbon steel, and zinc-base die castings are discussed below.

Steels may be cleaned and otherwise prepared for electroplating according to the procedures outlined by the flow charts in Fig. 4 and operating conditions in Table 9. The preparation of low-carbon steel for electroplating consists essentially of cleaning to remove oil and caked-on grease, pickling to remove scale and oxide films, cleaning to remove smut left on the surface, and reactivating the surface for plating.

Corrosion Types Flow Chart
Fig. 4 Process flow charts for preparation of steels for electroplating. See Table 9 for operating conditions.

Table 9 Solutions and operating conditions for preparation of steels for electroplating

Table 9 Solutions and operating conditions for preparation of steels for electroplating

Solution no.

Type of Solution

Composition

Amount

Operating temperature

Cycle time, s

°C

°F

High-carbon and low-alloy steels, spring tempter

1

Acid pickle

HCl hno3

20-80 vol% 1-5 vol%

Room temperature

(a)

2

Anodic alkaline cleaner(b)

NaCN

20-45 g/L (3-6 oz/gal)

49-54

120-130

30-60

High-carbon and low-alloy steels other than spring temper

3

Acid dip

HCl

1-10 vol%

Room temperature

(a)

4

Anodic alkaline cleaner(b)

NaCN

20-45 g/L (3-6 oz/gal)

Room temperature

30-60

5

Anodic acid etch(c)

H2SO4

250-1005 g/L (33.5-134 oz/gal)

30 max

86 max

60 max

Low-carbon steel bulk-processed parts

6

Alkaline cleaner(d)

Alkali

30-60 g/L (4-8 oz/gal)

82-99

180-210

60-120

7

Acid pickle

HCl

25-85 vol%

Room temperature

5-15

8

Acid dip

H2SO4

4-10 vol%

Room temperature

5-15

Low-carbon steel racked parts(e)

9

Acid pickle

HCl

25-85 vol%

Room temperature

(a)

10

Anodic alkaline cleaner(f)

Alkali

60-120 g/l (8-16 oz/gal)

93-99

200-210

60-120

11

Acid dip

H2SO4

4-10 vol%

Room temperature

5-15

12

Acid dip

h2so4

1 vol%

Room temperature

5-10

(a) Minimum time for removal of scale.

(b) Current density, 1.5 to 2.0 A/dm2 (15 to 20 A/ft2).

(d) Tumble, without current.

(e) Cycles for copper plating included in chart are applicable to all steels here, except that for high-carbon and low-alloy steels, a cyanide copper strike precedes cyanide copper plating.

(f) Current density, 5.0 to 10.0 A/dm2 (50 to 100 A/ft2)

Plating on low-carbon steels represents the bulk of industrial plating. The steps generally used before plating low-carbon

steels are:

1.

Vapor degrease, if necessary

2.

Alkaline soak clean

3.

Water rinse

4.

Descale, if necessary

5.

Water rinse

6.

Alkaline electroclean

7.

Water rinse

8.

Acid activate

9.

Water rinse

10.

Plate, as required

These steps are a general guideline and should not be construed as firm recommendations. The actual required cycle would depend on extent of grease and oil contamination, type of scale, and facilities available for the plating operation. Some of the options available to the plater are:

• Emulsion cleaning may be used in place of vapor degreasing. In this case, additional water rinsing is required.

• Anodic electrocleaning is preferred over cathodic cleaning which can cause smut on parts because of plating of polar soils in the cleaner. Electrocleaners are generally used at 60 to 75 g/L (8 to 10 oz/gal) and at 8.0 to 10.0 A/dm2 (80 to 100 A/ft2). Temperature will depend on the type of cleaner. Low-temperature cleaners operate at 27 to 49 °C (80 to 120 °F); high-temperature cleaners operate at 82 to 93 °C (180 to 200 °F).

• If parts are not excessively dirty, soak cleaning can be used instead of electrocleaning. Specially compounded alkaline cleaners are sometimes used to remove slight amounts of oxides. Elevated temperatures are recommended for all alkaline cleaning.

• Alkaline cleaners are difficult to rinse. Carryover of residues can produce staining, skip plating, or loss of adhesion. Warm water is recommended in the first rinse along with good agitation. Two or more countercurrent (cascade) rinses are highly desirable both from the standpoint of good rinsing and conservation of water. If both alkaline soak cleaning and alkaline electrocleaning are used, the two cleaning steps should be separated with a thorough rinse.

• Plating is initiated on an active surface. A wide variety of activators is available, and most are acidic in nature. Hydrochloric, sulfuric, or fluoboric acids are commonly used.

• Water rinse after activation is critical to avoid contaminating the sensitive plating solution. Countercurrent rinsing with two or more rinse tanks is desirable.

• High-carbon and low-alloy steels are susceptible to hydrogen embrittlement.

• Proprietary inhibited acid pickles are available for the effective removal of scale and rust with reduced danger of hydrogen embrittlement and base metal attack.

Unless the acids used contain inhibiting agents, the acid treatments for surface preparation must be very mild and of short duration. If electrolysis is necessary, it should be used with anodic current. This is especially significant for spring-temper parts and parts that have been case hardened. Mechanical methods of descaling can often eliminate the need for pickling.

During the anodic etch, a high acid content, low solution temperature, and high current density will minimize smut formation. Carryover of water into the anodic etching solution should be held to a minimum, and long transfer times after the anodic etch should be avoided.

Cold rolled steel that has been subjected to deep drawing and certain prepickled hot rolled steels with glazed brownish-colored surfaces may be exceedingly difficult to clean. For these materials, a solution of 25 to 85 vol% nitric acid has proved effective.

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