Electrocleaning and Electropolishing

Electrocleaning, an electropolishing technique, is a useful alternative to pickling treatments (Ref 5). Although electrocleaning is not covered under ASTM A 380, it is widely used to remove imperfections from the surface of stainless steel after fabrication. It removes embedded iron particles and similar film defects as does pickling. Unlike pickling, electrocleaning does not roughen the surface, but makes it smoother. A 12 V dc power source with variable current capability is connected to the stainless steel, making it the anode. A copper cathode and an electrolyte--usually phosphoric acid (H3PO4)--are then used to corrode away the protective film and several layers of the surface in a controlled manner by varying the current and dwell time.

Electrocleaning can be performed in most plating shops by immersion. Localized electrocleaning with field kits is widely practiced to remove heat tint and weld-related defects from the heat-affected zone (see Fig. 3).

Fig. 3 Equipment for localized electrocleaning of heat tint from the surface of stainless steel. Source: Ref 5

Electropolishing is the same process as electrocleaning, but is generally performed for a longer time (Ref 5). Electropolishing is used primarily to produce a very smooth, bright, easily cleaned surface with maximum corrosion resistance. It removes the surface layer of a metal by anodic treatment in an acid bath. Conditions for electropolishing of stainless steels in acid electrolytes are given in Table 10.

Electrolyte

Electrocleaning

Fig. 3 Equipment for localized electrocleaning of heat tint from the surface of stainless steel. Source: Ref 5

power source

Electrolyte power source

Table 10 Conditions for electropolishing in acid electrolytes

Type of metal (and product)

Purpose of treatment

Bath volume

Installed power

Current density

Polishing cycle, min

Daily production

Operators

No. of parts

Area

L

gal

A

V

A/dm2

A/ft2

m

ft2

Sulfuric-phosphoric acid electrolytes

302 and 430 stainless (job-shop work)

Bright finish

1150

300

1500

15

30

300

3-8

3500

25

250

2

302 and 202 stainless (plumbingware)

Bright finish

1150

300

1500

12

30

300

3-4

10002000

1

303 stainless (food-processing equipment)

Bright finish

2650

700

2500

18

20

200

4-10

400500

190370

20004000

2

Series 300 and 400 stainless (job-shop work)

Various

2250

600

3000

18

10(avg)

25-400

5-45

50-500

2

304 stainless

Brighten; deburr

1300

350

2000

12

30

300

4

3000

5575

600800

1

Stainless steel (aircraft components)

2650

700

3000

18

25-30

250300

5

200

30

300

1

430 stainless

Bright finish

1500

400

1500

14

1-2

7000

75

800

1

430 stainless (trim items)

Brighten; deburr

1500

400

750

18

3

12,000

230

2500

1

430 stainless (automotive trim)

Bright finish

3800

1000

3000

18

30

300

4

250/h

1

430 stainless (automotive rain shields)

Bright finish

3800

1000

3000

18

25

250

5

450/h

1

Stainless and carbon steels (job-shop work)

Brighten; deburr

1500

400

1500

12

25-40

250400

Varies

Varies

1

Sulfuric-phosphoric-chromic acid electrolytes

302 stainless (surgical

Smooth;

3600

950

3000

18

30

300

5

5000

1

instruments)

polish

Electropolishing is applicable to all stainless steel grades, hot or cold finished, cast or wrought. The amount of metal removed is subject to close control, depending on the desired result. The resulting surfaces have a bright, passive finish. The process is most frequently applied to cold-finished surfaces, because they yield a smoother finish than can conventionally be obtained on hot-finished surfaces. As in electroplating, the results depend on the contour and shape of the part. The end-grain surfaces of the free-machining stainless grades, such as types 303 and 416, will appear frosty after electropolishing due to removal of the sulfide inclusions.

Electropolishing can be used as a preliminary brightening operation before final buffing, particularly on drawn parts with burrs, sharp radii, or recessed areas, and it serves to reduce the amount of buffing required. Electropolishing is applied to decorative automotive parts and accessories, conveyor systems for food-handling equipment, animal cages, and pharmacy equipment. It provides an economical finish on many parts that are difficult or impossible to finish by conventional polishing, such as items made from wire.

In contrast to mechanical finishing methods, electropolishing may make inclusions in the material more visible. Some types of inclusions are dissolved out, whereas others remain in relief. Electropolishing has been used as a surface inspection technique to reveal residual foreign material, such as embedded scale and particles of iron, carbide precipitation, and weld defects. The surface obtained by electropolishing is directly related to the original surface quality--the process cannot be used to remove digs, gouges, scratches, and the like.

Chemical polishing is another method for providing a smooth and bright surface on stainless steel. Unlike electropolishing, chemical polishing can be done without the use of electricity and without racking of individual parts. Thus, chemical polishing offers significant savings in capital investment and labor. In addition, chemical polishing offers a greater degree of freedom in polishing items with blind holes and other recessed areas. However, it does not produce the high specular reflectivity (brightness) obtained with electropolishing.

Proprietary products for chemical polishing are available on the market. Generally, they are based on combinations of H3PO4, HNO3, H2SO4, hydrochloric acid (HCl), organic acids, and special surfactants and stabilizers to promote a high degree of brightness and long bath life. Unlike the HNO3-HF mixtures that are used in chemical cleaning, the proprietary chemical bright dips do not cause severe attack on the grain boundaries or intergranular corrosion.

+1 0

Post a comment