Anodizing Processes

The three principal types of anodizing processes are chromic processes, in which the electrolyte is chromic acid; sulfuric processes, in which the electrolyte is sulfuric acid; and hard anodic processes that use sulfuric acid alone or with additives. Other processes, used less frequently or for special purposes, use sulfuric acid with oxalic acid, phosphoric acid, oxalic acid, boric acid, sulfosalicylic acid, sulfophthalic acid, or tartaric acid. Except for thicker coatings produced by hard anodizing processes, most anodic coatings range in thickness from 5 to 18 /'m (0.2 to 0.7 mil). Table 2 describes a few applications in which anodizing is used as a step in final finishing. The sequence of operations typically employed in anodizing from surface preparation through sealing is illustrated in Fig. 1.

Table 2 Typical products for which anodizing is used in final finishing

Product

Size

Alloy

Finishing before anodizing

Anodizing process

Post-treatment

Service requirements or environments

mm

in.

Auto head lamp

diam, 11 4

55571125

Buff, chemical brighten

Sulfuric acid(a)

Seal

Atmospheric exposure

Canopy track

760 mm T-extrusion

30-in. T-extrusion

7075

Machine

Hard

None

Resist wear, sea air

Gelatin molds

150-205 mm overall

6-8 overall

drawn

Sulfuric acid

Dye, seal

Food

Landing gear

205 mm diam by 1.4 m

8 in. diam by 41 ft 2

7079-T6

(b)

Chromic acid

Paint

Corrosion resistance

mm by 100

mm

12 ft by 7 by 4

6063-T6

(d)

Sulfuric acid(e)

Seal, lacquer(f)

Urban atmosphere

Name plates

Various sizes

Various sizes

30031114

(g)

Sulfuric acid

Dye, seal

Atmospheric exposure

Percolator shell

125 mm diam by 150

5 in. diam by 6

Buff, chemical brighten

Sulfuric acid

Seal

Coffee

Seaplane-hull skin

2850 by 1020

T6

(g)

Chromic acid

■ (h) corrosion

Seat-

stanchion tube

50 mm diam by 610

2 in. diam by 24

7075-T6

Machine

Hard

None

Wear resistance

Signal-

cartridge container

by 6l 2

3003-0

As drawn

Chromic acid

Prime, paint

Marine atmosphere

Tray, household

430 mm diam

17 in. diam

Butler

Sulfuric acid

Seal, buff

Food

Utensil covers

Up to 0.20 m2 total area

Up to 2 ft2 total area

1100

Buff, chemical brighten

Sulfuric acid®

Dye, seal

Steam, cooked foods®

Voice transmitter

50 mm diam

2 in. diam

5052-0

Burnish, alkaline etch

Sulfuric acid

Dye, seal®

Gas mask

mm area

Up to 8 in.2 area

6151

Machine

Sulfuric acid®

corrosion

Computer chip hat

160 by 160

6.2 by 6.2

6063-T6

Non-etch clean

Sulfuric acid

Deionized water seal

High dielectric, thermally conductive

Ice cream scoop

400 by 50

8 by 2

6061-T6

Light etch

Hard

Polytetrafluoroethylene seal

Food; good release

(b) Partially machine, clean with nonetching cleaner, and remove surface oxide.

(d) Lined finish (180-mesh grit) on 100-mm (4-in.) face; other surfaces alkaline etched.

(e) Anodized for 80 min; minimum coating thickness, 30 ^m (1.2 mils).

(f) Sealed for 20 to 30 min. Methacrylate lacquer, 8 ^m (0.3 mil) minimum.

(g) Clean with nonetching cleaner; remove surface oxide.

(h) Maximum resistance required.

(i) Anodic coating 5 ^m (0.2 mil) thick. (j) Must not discolor during service. (k) Sealed in dichromate solution.

Anodized in sulfuric acid solution (30% H2SO4) at 21 °C (70 °F) for 70 min at 2.5 A/dm2 (25 A/ft2).

(m) In presence of hydraulic brake fluids

Fig. 1 Typical process sequence for anodizing operations

Surface Preparation. A chemically clean surface (free of all grease and oil, corrosion products, and the naturally occurring aluminum oxide found on even the cleanest-appearing aluminum) is a basic requirement for successful anodizing. The cleaning method is selected on the basis of the type of soils or contaminants that must be removed and the dimensional tolerance. Traditionally the first step employed was vapor degreasing; however, due to restrictions on ozone-depleting compounds, many of these degreasing solvents, such as trichloroethylene, are no longer in wide use. Alternatives to vapor degreasing, such as solvent wiping or alkaline soak cleaning, are now predominantly used for removing the major organic contaminants. The main function of this cleaning stage is to provide a chemically clean aluminum surface so that subsequent acid pickles or caustic etches can react uniformly over the entire surface.

After cleaning, the work is etched, pickled, or otherwise deoxidized to remove surface oxides. When specular surfaces are required, the work is treated in a brightening solution. After etching or brightening, desmutting usually is required for the removal of heavy metal deposits resulting from the preceding operations.

In order to treat precision-machined aluminum components, anodize pretreatment procedures that require neither etching nor pickling have been developed and are now widely employed.

Chromic Acid Process. The sequence of operations used in this process depends on the type of part, the alloy to be anodized, and the principal objective for anodizing. Due to the corrosive nature of sulfuric acid, chromic acid anodizing is the preferred process on components such as riveted or welded assemblies where it is difficult or impossible to remove all of the anodizing solution. This process yields a yellow to dark-olive finish, depending on the anodic film thickness. Color is gray on high-copper alloys. Table 3 gives a typical sequence of operations that meets the requirements of military specification MIL-A-8625.

Table 3 Sequence of operations for chromic acid anodizing

Operation

Solution

Solution temperature

Treatment time,

min

°C

°F

Vapor degrease

Suitable solvent

Alkaline clean

Alkaline cleaner

(a)

(a)

(a)

Rinse®

Water

Ambient

Ambient

1

Desmut(c)

HNO3, 10-25 vol%

Ambient

Ambient

As required

Rinse®

Water

Ambient

Ambient

1

Anodize

CrO3, 46 g/L (5 4 oz/gal)(d)

32-35

90-95

30(e)

Rinse®

Water

Ambient

Ambient

1

Seal(f)

Water®

90-100

190-210

10-15

Air dry

105 max®

225 max®

As required

(a) According to individual specifications.

(b) Running water or spray.

(c) Generally used in conjunction with alkaline-etch type of cleaning.

(e) Approximate; time may be increased to produce maximum coating weight desired.

(f) Dependent on application.

(g) Water may be slightly acidulated with chromic acid, to a pH of 4 to 6.

(h) Drying at elevated temperature is optional.

Chromic acid anodizing solutions contain from 3 to 10 wt% CrO3. A solution is made up by filling the tank about half full of water, dissolving the acid in water, and then adding water to adjust to the desired operating level.

A chromic acid anodizing solution should not be used unless: • pH is between 0.5 and 1.0.

• The concentration of chlorides (as sodium chloride) is less than 0.02%.

• The concentration of sulfates (as sulfuric acid) is less than 0.05%.

• The total chromic acid content, as determined by pH and Baume readings, is less than 10%. When this percentage is exceeded, part of the bath is withdrawn and is replaced with fresh solution.

Figure 2 shows the amount of chromic acid that is required for reducing the pH from the observed value to an operating value of 0.5.

Fig. 2 Control of pH of chromic acid anodizing solutions. The graph shows the amount of chromic acid required to reduce pH to 0.5 from observed pH.

When anodizing is started, the voltage is controlled so that it will increase from 0 to 40 V within 5 to 8 min. The voltage is regulated to produce a current density of not less than 0.1 A/dm2 (1.0 A/ft2), and anodizing is continued for the required time, generally 30 to 40 min. Certain alloys, typically those in the 7xxx series, such as 7075, fail to develop a coating at 40 V, but running the process at 22 V produces acceptable results. Casting alloys should also be processed at 22 ± 2 V, as specified in military specification MIL-A-8625, type 18. Because of the porous structure of the casting alloys, processing them at higher voltages can cause excessive current densities that can be extremely damaging to the components. When the 22 V process is employed, times should be lengthened to 40 to 60 min. At the end of the cycle the current is gradually reduced to zero, and the parts are removed from the bath within 15 s, rinsed, and sealed.

According to MIL-A-8625, revision F, the coating weight should be checked prior to sealing, and depending on the type of alloy, the minimum coating weight should be 200 mg/ft2. Measuring coating weight prior to sealing will allow the parts to be put back in the chromic anodizing tank so that anodizing can continue, if needed, and subsequent stripping can be avoided.

Sulfuric Acid Process. The basic operations for the sulfuric acid process are the same as for the chromic acid process. Parts or assemblies that contain joints or recesses that could entrap the electrolyte should not be anodized in the sulfuric acid bath. The concentration of sulfuric acid (1.84 sp gr) in the anodizing solution is 12 to 20 wt%. A solution containing 36 L (9.5 gal) of H2SO4 per 380 L (100 gal) of solution is capable of producing an anodic coating that when sealed meets the requirements of MIL-A-8625.

A sulfuric acid anodizing solution should not be used unless:

• The concentration of chlorides (as sodium chloride) is less than 0.02%.

• The aluminum concentration is less than 20 g/L (2.7 oz/gal), or less than 15 g/L (2 oz/gal) for dyed work.

• The sulfuric acid content is between 165 and 200 g/L (22 to 27 oz/gal).

At the start of the anodizing operation, the voltage is adjusted to produce a current density of 0.9 to 1.5 A/dm2 (9 to 15 A/ft2). Figure 3 shows the voltage required to anodize at two different temperatures with current density of 1.2 A/dm2 (12 A/ft2). The voltage will increase slightly as the aluminum content of the bath increases. The approximate voltages required for anodizing various wrought and cast aluminum alloys in a sulfuric acid bath at 1.2 A/dm2 (12 A/ft2) are:

Alloy

Volts

Wrought alloys

1100

15.0

2011

20.0

2014

21.0

2017

21.0

2024

21.0

2117

16.5

3003

16.0

3004

15.0

5005

15.0

5050

15.0

5052

14.5

5056

16.0

5357

15.0

6053

15.5

6061

15.0

6063

15.0

6151

15.0

7075

16.0

Casting alloys

413.0

26.0

443.0

18.0

242.0

13.0

295.0

21.0

514.0(a)

10.0

518.0(a)

10.0

319.0

23.0

355.0

17.0

356.0

19.0

380.0

23.0

1

k

Te.Ti pH r ü T urc o r20 =C °F

bath

I

^ 2!

1

Anodiiing t-ime, min

Anodiiing t-ime, min

Fig. 3 Voltages required during sulfuric acid anodizing. To maintain a current density of 1.2 A/dm2 (12 A/ft2), a bath temperature of between 20 and 25 °C (68 and 77 °F) must be maintained.

When a current density of 1.2 A/dm2 (12 A/ft2) is attained, the anodizing process is continued until the specified weight of coating is produced, after which the flow of current is stopped and the parts are withdrawn immediately from the solution and rinsed. Figure 4 shows the effect of time on the weight of the coating developed on automotive trim anodized in 15% sulfuric acid solutions at 20 and 25 °C (68 and 77 °F), operated at a current density of 1.2 A/dm2 (12 A/ft2).

Te

mperatur-Ë

ot bath, 2<

°C (60 F

/ i 'C<77dF)

-

Anodizing time, min

ra Q

10 15 20

Anodizing time, min

Fig. 4 Effect of anodizing time on weight of anodic coating. Data were derived from aluminum-alloy automotive trim anodized in 15% sulfuric acid solutions at 20 and 25 °C (68 and 77 °F) and at 1.2 A/dm2 (12 A/ft2).

A flow chart and a table of operating conditions for operations typically used in anodizing architectural parts by the sulfuric acid process are presented in Fig. 5; similar information, for the anodizing of automotive bright trim, is given in Fig. 6.

Solution

Type of solution

Composition

Operating

Cycle

No.

temperature

time,

min

°C

°F

min

1

Alkaline cleaning

Alkali, inhibited

60-71

140-160

2-4

2

Alkaline etching

NaOH, 5 wt%

50-71

120-160

2-20

3

Desmutting

HNO3, 25-35 vol%

Room

Room

2

4

Anodizing

H2SO4, 15 wt%

21-25

70-75

5-60

5

Sealing

Water (pH 5.5-6.5)

100

212

5-20

Fig. 5 Operations sequence in sulfuric acid anodizing of architectural parts

Fig. 5 Operations sequence in sulfuric acid anodizing of architectural parts

Solution

No.

Type of solution

Composition

Operating temperature

Cycle time, min

°C

°F

1

Alkaline cleaning

Alkali, inhibited

60-71

140-160

2-4

2

Chemical brightening

H3PO4 and HNO3

88-110

190-230

1-5 2

3

Desmutting

HNO3, 25-35 vol%

Room

Room

2

4

Anodizing

H2SO4, 15 wt%

21-25

70-75

5-60

0 0

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