Anodizing Aluminum at Home

Aluminum has a very high chemical affinity for oxygen. Therefore, aluminum oxide can be formed easily. If you merely expose aluminum to air, a very thin oxide film of a few angstroms (1 angstrom is 10^-8 cm) will be formed on the aluminum surface. This oxide film is called "air-formed oxide film" or "natural oxide film." The thickness of the air-formed oxide film is very small; therefore, it cannot be used as a protective film. If aluminum is used as the anode and electrolysis carried out using an electrolyte, an oxide film is formed on the surface of the aluminum. This electrolytic process is called "anodizing of aluminum." The quality of the film is very sensitive to electrolyte temperature, concentration and the alloy used. The thickness is dependent on electrolyzing time and current density. If the temperature of the electrolytic bath is low, the growth of the oxide film is good and a hard oxide film can be formed. An oxide film formed in a sulfuric acid bath at approximately 0°C by anodizing of aluminum is called "hard anodizing film." This type of film is poor for dying and is usually naturally dark drown to black. At high electrolytic bath temperatures of 60 to 75°C the oxide film formed is thin and soft, and the surface will easily scratch and may even rub off.

Table 1

Effects of Anodizing Different Alloys of Aluminum
Material Designation		Nominal Composition	Protective anodizing	Anodizing and dyeing	Suitability for Bright anodizing	Hard Anodizing
	(1)	99.99%Al	E	E	E	E
1080 A	(1A)	99.8%Al	E	E	E	E
1050 A	(1B)	99.5%Al	E	VG	VG	E
1200	(1C)	99.0%Al	VG	VG	VG	E

2011	(FC1)	Al-5.5%Cu, 0.4%Pb, 0.4%Bi	M-G	M-G*	U	G
2014A	(H15)	Al-4.25%Cu, 0.75%Si, 0.75%Mn, 0.5%Mg	M	M*	U	G
2031	(H12)	Al-2%Cu, 1%Ni, 0.9%Mg, 0.8%Si	M	M*	U	G

3103	(N3)	Al-1.25%Mn	G	G	M	G
3105	(N31)	Al-0.6%Mn, 0.5%Mg	G	G	M	G

4043A	(N21)	Al-5%Si	G	M*	U	G

5005	(N41)	Al-1%Mg	E	VG	G	E
5056A	(N6)	Al-5%Mg	G	G	M	E
5083	(N8)	Al-4.5%Mg, 0.7%Mn, 0.15%Cr	G	G	M	G
5154A	(N5)	Al-3.5%Mg, 0.3%Mn	VG	VG	G	E
5251	(N4)	Al-2.25%Mg	VG	VG	G	E
5454	(N51)	Al-2.7%Mg, 0.75%Mn	VG	VG	G	E

6061	(H20)	Al-1%Mg, 0.6%Si, 0.2%Cr	VG	G	M	VG
6063	(H9)	Al-0.75%, 0.4%Si	E	VG	G	E
6082	(H30)	Al-1%Mg, 1%Si, 0.7%Mn	G	G	M	G
6463		99.8%Al-0.75%Mg, 0.4%Si	VG	VG	VG	VG

7020	(H17)	Al-4.5%Zn, 1%Mg	G	G	M	G


E Excellent; VG Very Good; G Good; M Moderate; U Unsuitable
* Only suitable for dark colours

Stripping Anodizing

To strip off an anodic film an aqueous sodium hydroxide solution is used. The concentration should be 20-30%(wt) and temperature should be 70-80°C. The stripping process takes from 5 to 10 minutes. Sodium hydroxide will attack the base metal so care must be taken not to over strip the part. When the part comes out it will be covered in a layer of black sludge. This sludge is then removed by de-smutting. The part will also be etched or pitted if left in too long. This is a poor method of stripping and is not used commercially. A widely used method of stripping anodic film which will have very insignificant effects on the base metal is with a phosphoric/chromic acid solution.

Phosphoric acid (d. 1.75(90%wt)) 35ml/l or 7%(vol)

Chromic acid 20g/l or 4%(wt.)

Use at boiling point

De-smutting Aluminum

When aluminum is etched with alkali, the surface of aluminum turns a colour from gray to black. This black substance sticking on the surface is called 'smut.' Smut is formed when insoluble impurities or alloy contents of Si, Mg, Fe, or Cu included in the aluminum, deposit on the surface. The aluminum surface is de-smutted by immersing it in an aqueous solution of 25-50%(vol) nitric acid at 20°C. Care must be taken not to contaminate the anodizing solution with nitric acid. Even when the acid is neutralized the nitrate ion (NO₃^-) is still present and if this is transported to the sulfuric acid bath for anodizing it will hinder the formation of the anodic oxide film. Similar to the chlorine ion, the NO₃^-ion causes pitting corrosion of the oxide film during electrolysis.

Cleaning Aluminum

Parts must have a high degree of cleanness to ensure the successful application of the finishing operation. If oil or grease is allowed to remain on the surface, patchy coatings are obtained and the electrolyte solution itself will not operate satisfactorily if contaminated. Surfaces must not only be physically free from oils and dirt, they must be chemically clean as any oxide skin on the metal will interfere with film formation. Chemical cleanness is often confused with the ‘water break test’ i.e. the ability of the metal to be ‘wetted’ by water. While freedom from water-break on the rinsing often constitutes a rough guide as to the presence or absence of oils it depends on the surface tension of the contaminates and the thickness of the water film, and may be obscured by the presence of wetting agents on the metal surface i.e. detergent.

Solvent cleaning

This is the first stage in cleaning and is used as a pretreatment to etch cleaning. Solvent cleaning removes heavy oils, dirt, and polishing compounds. Solvent cleaning however will not produce a chemically clean surface and so it is not meant to substitute etch cleaning. For very dirty work with heavy grease or polishing compound deposits an initial soak in kerosene is used. Due to kerosene’s slow evaporation, which can leave a residue, it must be followed by a soak with a lighter solvent such as mineral spirits, lacquer thinner or toluene. If the part is only lightly oiled the kerosene soak can be omitted.

Rinsing

It is important that the part is rinsed in clean hot water. Rinsing in contaminated water can undo the previous steps. Rinse the parts in hot water in a range of 60-70°C.

Alkaline Cleaning

Aluminum is amphoteric and is readily attacked by alkaline solutions. Alkaline cleaners for use with aluminum and its alloys must work, therefore, at comparatively low alkalinity, and are inhibited in order to form a protective film over the surface, which will prevent attack of the metal during cleaning.

Cleaning solution: sodium carbonate 5-15g/l

trisodium phosphate 5g/l

temperature 80-95°C

Acid Cleaning

These non-etching solutions usually contain nitric acid or chromic acid, the former attacking aluminum only very slowly while chromic acid both passivates aluminum and tends to confer a passivating action on any other solution to which it is added. An effective cleaner generally used for removing oxides including anodically formed coatings is a mixture of chromic and phosphoric acid. A typical composition is 7% by volume phosphoric acid and 4% by weight chromic acid used at 98-100°C which will rapidly dissolve any oxide with no effect at all on aluminum and most of its alloys. Three other mixtures for cleaning aluminum are as follows:

1) chromic acid 4%

sulphuric acid 15%(vol)

2) chromic acid 175g/l

sulphuric acid 35g/l

3) 50% chromic acid solution 1pt(vol)

conc. Sulphuric acid 10pt(vol)

Such mixtures attack aluminum only very slowly. It is important that the parts are dry before cleaning. The article is immersed for up to 20min. at a temperature of 40-65°C a clean, slightly etched surface being obtained.

Surface Treatments

Producing a Matt Finish

To produce a matt finish or a ‘dust’ finish a 5%-20% sodium hydroxide solution is used. The temperature should be from 40 to 70°C and emersion time 1-10 min. This will etch a fine matt texture in the base metal and is done before anodizing. The roughness of the etch increases with increased concentration, temperature or immersion time.

Chemically Polishing

Chemical polishing may be used as a finishing treatment for mechanically polished surfaces. The solutions involved in chemical polishing include a mixture of an attacking acid and a passifiing acid to suppress etching. A phosphoric/ nitric acid mixture is commonly used where the etching is almost entirely suppressed. The action of these solutions are carefully balanced and require good control over solution concentrations. Evaporated water must be taken into consideration as these solutions are used at elevated temperatures.

Optimum range Typical solute

1) phosphoric acid 73-83%(vol) 80.5%(vol)

nitric acid 2-5% 3.5%

water 14-23% 16%

temperature approx 90°C same

2) nitric acid 2.8-3.2%(wt) 4.9-5.6%(vol)

water 17-23%(vol)

aluminum phosphate 10-12%

phosphoric acid 64-70%

copper 0.01-0.02%(wt)

The following graph can be used to troubleshoot a polishing solution.

Fig. 1. Effect of composition of a phosphoric-nitric acid polishing bath on the finish. Bath operated at 88-99°C and containing 2.8-3.2% wt. nitric acid and 0.01-0.02% copper.

Anodizing Jigging and Equipment

Jigging

The objective of jigging is, first to form a positive contact with the work, and second, to make handling as easy as possible without damaging the work. Points of contact should be in a position hidden from the eye as no film is formed at such points. Jigging material is often made of aluminum ideally of the same or similar alloy as the material being anodized. In practice the material is normally Al-Mg-Si alloys 6063(HE9) or 6082(HE30) which have good spring characteristics. The 2000 series Al-Cu alloys should not be used due to their high current consumption and rapid deterioration.

Tanks

Acid resist materials such as polypropylene, PVC, or rubber as well as most plastics can be used. For small home operations double walled plastic food coolers or Tupperware containers are ideal.

Temperature control

The control of temperature in the anodizing electrolyte is of fundamental importance in almost all processes, and temperature control to within ±1°C or even ±0.5°C, of the desired temperature is often necessary. For home anodizing the temperature can manually be maintained with plastic milk bags filled with a brine solution and ice. The bags are dipped right into the anodizing solution as needed to maintain the temperature. For a much better cooling solution, aluminum or lead cooling coils can be fabricated which go directly into the electrolyte. Chilled water is then pumped through the coils. The pump can then be controlled with a thermostat or manually.

Agitation

Agitation of the electrolyte is also an essential requirement for successful anodizing, mainly to ensure that heat is taken away from the surface of the film and that the electrolyte temperature is uniform. The most common method of agitation is by means of air which is clean and free from oil. This is fed from a blower into perforated PVC or polypropylene pipes attached to the bottom of the tank. In commercial practices the amount of agitating air in the tank at any given time is at a level of 0.22-0.45m³ per m² of surface area. Aquarium air pumps will work well for a small tank as they produce an oil free low pressure high volume flow.

Power supply

An ideal power supply would be a 24 volt rectifier with variable stepless voltage control. An ammeter connected in series with one of the power leads will be necessary for current monitoring. The voltage can then be adjusted to obtain the desired current. For a low budget home operation a battery charger will work, a manual one is preferred as automatic ones could cut out during anodizing which can result in soft film formation.

Cathodes

Cathodes are made from aluminum, usually in the form of 1050 or 1200 alloy plates or 6063 extrusions. It is important to maintain a minimum separation distance of 25cm between the cathode and anode. To minimize excess current effects the cathode should not be extended below the lowest workpiece on the load and the cathode should be shielded at the solution surface. A general guideline for cathode size is in the ratio of anode to cathode surface area of 3:1.

Anodizing process

The particular operating conditions of the anodizing process will dramatically change the quality of the film and its ability to absorb dyestuff so care must be taken. Sulphuric type anodizing is particularly sensitive to temperature. Dilute solutions and lower temperatures favor harder less clear finishes with poor dying abilities while more concentrated solutions favor softer, brighter films with better dying abilities.

Table 2

Anodizing Electrolyte Makeup

product	electrolyte concentration(% by wt of H₂SO₄)	electrolyte temperature (°C)	current density(A/dm2)	typical voltage(V)	approximate limiting film thickness(microns)
bright dyed anodizing	18-24	22-24	1.0-1.2	14-15	30-35
Architectural	15-18	18-22	1.4-1.8	17-20	40-50
Hard anodizing	15-16	0-5	2.0-3.0	25-50	80+

Typical process sequence would be:

Jig.
Solvent degrease.
Hot water rinse.
Alkali clean (10min. at 70°C).
Cold water rinse.
De-smut (30% (vol) nitric acid at room temperature).
Cold water rinse.
Anodizing
Cold water rinse.
Organic Dying
Cold water rinse.
Cold water rinse distilled.
Steam or Boil Seal (40-60 min distilled water)
De-jig.

Table 3

Anodic oxide film thickness specifications
Minimum average thickness		Service conditions
(microns)	(mil)
25	1.0	Aggressive environments; permanent external
20	0.8	architectural applications.
15	0.6	Outdoor architectural use when cleaned
10	0.4	frequently; arduous indoor conditions
5	0.2	Special outdoor use with very frequent cleaning
3	0.12	e.g., decorative car trim; indoor use
1	0.04	Reflectors and paint base.

Table 4

Anodizing Line Baths
Use	Solution Makeup	Temperature	Duration	Air Agitation
Stripper	Phosphoric acid (d. 1.75(90%wt)) 35ml/l or 7%(vol)	98-100°C	Untill Stripped	no
	Chromic acid 20g/l or 4%(wt.)
De-Smutting/Bloom Removal	nitric acid 30%(vol)	room	5min(De-Smut) 10min(Bloom)	No
Solvent Soak	kerosene	room	10min	No
Alkaline Cleaning	sodium carbonate 5-15g/l	80-95°C	20min	Yes
	trisodium phosphate 5g/l
Acid Cleaning	chromic acid 4%	40-65°C	20min	Optional
	sulphuric acid 15%(vol)
Electrolyte	sulphuric acid 18-24%(vol)	22-24°C ±1°C	50min	Yes
Organic Dyeing	dye [varies], distilled water	55-70°C	20min	Yes
Bleaching	nitric acid 27%(wt)	room
Sealing	distilled water	99-110°C	50min	No

Effects of Varying Operating Conditions

This following diagram illustrates the effect of anodizing time on film thickness. This graph can be used to give a rough guide of the anodizing times needed to get to a specific film thickness.

Fig. 2. Anodic coating thickness of aluminum alloys (15% H₂SO₄, 1.3 A/dm², 0-120 minutes)

Electrolyte Concentration

Sulphuric acid anodizing electrolyte is usually maintained between 15-24%(wt). Different concentrations are used for different types of anodizing as shown in table 2. At low concentrations of about 1%, the resistance of the electrolyte is excessively high thus, bath voltage becomes excessively high, no anodic oxide film is formed, and black spots appear on the surface of the aluminum. Anodic oxide films are also formed in sulfuric acid baths with high concentrations in excess of 24%. Since the electrolytic bath has a strong dissolving power, the growth rate of film is poor, and a thick film cannot be formed easily. The film formed is also soft. Concentrations that favor dissolving of the film have higher dye affinity but at the expense of lower abrasion resistant. Therefore the absolute maximum concentration of sulphuric acid is 24%. See table 2 for typical concentrations.

Current Density

Current density will affect the rate at which the film is formed. If the current density is excessively high, variation is likely to occur in film thickness. Also in order to obtain a high current density bath voltage must be increased. The number of pores formed decrease as bath voltage increases, generally dye affinity decreases if voltages is in excess of 18-20 volts. If electrolysis is carried out at a low current density of about 0.5 A/dm² for a prolonged period, the abrasion resistance and the corrosion resistance tend to deteriorate; in general, a current density of 1 to 2 A/dm² is recommended. The high variation in film thickness at high current densities can also be observed in the thickness of electroplated films. High current density means "a very high reaction speed." If the reaction speed is excessively high, there is a difficulty in obtaining reactions at a uniform rate. The performance of anodic oxide film deteriorates at a current density of 0.5 A/dm² because of chemical dissolution of the film. At 0.5 A/dm², the growth rate of the film is low. For instance, if aluminum is anodized for a prolonged period with a low current density in order to form a film of thickness 10 microns, the anodic oxide film becomes extremely thin because of chemical dissolution of the pore walls in the electrolytic bath. The abrasion resistance and the corrosion resistance of anodic oxide film with very thin pore walls are poor.

Electrolyte Temperature

Anodic oxide film has a large electrical resistance, therefore, when current flows through the film, Joule heat (Q) is generated.

Q = Vi = i²R

Here, (V) is the voltage, (i) is the current and (R) is the resistance.

During anodizing of aluminum, the temperature of the electrolytic bath increases due to "Joule heat." That is why the bath is cooled during the anodizing process to maintain it at a constant temperature.

At a temperature of 20°C, the abrasion resistance of the film is excellent. If the temperature rises beyond this point, the durability of the film deteriorates and brightness and dye affinity increases. Moreover, the abrasion resistance of the film tends to improve at lower temperatures. In general, at lower temperatures, the brightness of the film deteriorates and the colour turns gray to grayish black. For instance, if aluminum is anodized at 0 to 5°C using the hard anodizing method, the abrasion resistance of the film improves remarkably but the colour of the film turns gray to grayish black.

If the temperature of the electrolytic solution decreases, the ability to absorb dyestuffs decreases, resulting in non-uniform and pale colours.

The reason for the unsatisfactory brightness and the grayish or blackish colour of anodic oxide film in low temperature baths is the poor dissolving power of the electrolytic bath. Moreover, light is reflected irregularly because of this unevenness, further reducing the brightness and colour of the film. Metallic impurities in aluminum do not dissolve easily in low-temperature baths and are frequently included in the anodic oxide film.

Fig. 3. Shows the relationship beteen current density and electrolyte temperature.

Fig. 3. Influence of current density and temperature of 15% (wt) H₂SO₄ electrolyte

on the film characteristics of bright anodized 6063 extrusions

Electrolyte Contaminates

A very damaging contaminate in a sulphuric acid electrolyte is chloride, and at levels above 200mg/l it can lead to pitting during the anodizing process. Heavy metal ions also give problems, particularly when trying to bright anodize. Iron above 50mg/l, Cu above 125mg/l and lead above 59mg/l are detrimental. 10g of aluminum is dissolved per m² surface anodized (20 micron film) so it will always be present in an anodizing electrolyte. 5g/l of aluminum is said to actually be beneficial rather that detrimental. The upper limit of dissolved aluminum is usually 15-20g/l any higher and abrasion resistance if affected. Oil in the anodizing bath will readily be taken up by the anodized surface and prevent dying and will cause staining. Grease or oil present on the water prier to anodizing may inhibit film formation completely.

Colouring Anodic Oxide Coating

The main requirements of anodic films that are to be dyed are: (1) the coating is of adequate thickness, the thickness varying the shade of the dye, i.e. dark colour tones will require thicker coatings; (2) the coating itself has a suitable colour; and (3) that it is free from blemishes such as scratches, pits etc. The precise anodizing conditions used have a strong influence with the coating’s ability to take up dyestuff. Conditions favoring film attack will always give the best dye absorption. Thus good films(above 10 microns) formed with higher acid concentration and anodizing temperature than normal will give the deepest dyings, but this must be balanced against other coating requirements such as abrasion resistance or weathering properties.

Dying is carried out in, enamel, stainless steel, plastic, glass or rubber-lined tanks. Tanks of iron or lead should not be used and aluminum or copper is not generally recommended.

Optimum dying conditions

The temperature of the dying bath is maintained between 55 and 70°C. Bad dying results if the temperature is too low. The light-fastness tends to decrease with short dyeing times or low temperature. On the other hand if the dying temperature is too high, there is a danger of sealing occurring before sufficient dye is absorbed. Lower bath temperatures are used when dyes are to be stripped for fade effects and splash anodizing or pale colours are needed. Very high temperatures (above 80°C) are sometimes used if the dyes tend to bleed out in the sealing baths.

Fig. 4. Influence of temperature of dyebath and duration

of dyeing on maximum depth of shade (E)

Contaminates in the dye such as oil can lead to local staining. Other contaminates such as phosphates, chlorides, silicates and fluorides even at very low levels can block out dying completely. Thus distilled water is used in making up dye solutions. Faults in dying are usually a result of inadequate control of anodizing and rinsing practices. Entrapment of air can also cause a lack of dyeing and in some cases, may also have prevented adequate anodizing. Irregular, cloudy effects on dying are nearly always caused by incorrect rinsing or anodizing practice and local pale or undyed areas may be caused by inadequate degreasing or later oil contamination. If bleeding during sealing is causing problems it can be minimized by: dyeing at the highest permissible temperature, dyeing for relatively long times, using the best quality water for dye bath make-up, rinsing briefly between dying and sealing, immersing the dyed work in boiling work for 20 second and plunging it quickly in cold water followed by normal sealing method.

Removing Dye

This is used in multi colour splash dying and fade effects. This is done previous to sealing. The dyestuff can be removed without damage to the anodic oxide coating by using 27%(wt) nitric acid or 5ml/l sulphuric acid at 25°C. Some dyes can not be removed completely by acid bleaching and sometimes can be removed with sodium hypochlorite 10g/l at 20°C.

Multi Colour Dying

With multi colour splash dying the part is dyed in the first colour masked with rubber cement or lacquer, bleached and dyed with the next colour. To remove rubber cement use kerosene or mineral spirits to remove lacquer use lacquer thinner. The resists must be completely dry, if they are not dry and immersed in the dying bath they can cause the dye to coagulate and precipitate out of solution ruining the dye bath and causing spotted dying.

Typical Splash anodizing process:

Anodize, rinse thoroughly
dye with first colour
apply dye resist and allow to dry is a low humidity area
bleach unprotected areas with 27%(wt) nitric acid
cold water rinse
dye again in second colour; rinse, and dry with warm air not exceeding 110°C
repeat 2 to 6 for as many colours as required
remove the resist with suitable solvent soak; rinse
seal in boiling water or steam

Sealing Anodic Oxide Coatings

Sealing in distilled water by immersion at or near to its boiling point is the simplest technique and most commercially used. The sealing should normally be carried out at between 98 and 100°C a temperature of 95°C being perceptibly slower while at temperatures below 90°C the rate of sealing is substantially reduced. The water must be distilled because contaminates of phosphates are potent inhibitors of sealing, silicates also hinder sealing but not to the extent of phosphates.

Fig. 5. Effect of temperature and pH in various sealing processes (sealing time

30 minutes. The sealing efficiency was determined by overdyeing with 3.5g/l

Aluminum Blue LLW for 15 minutes at 60°C