Anodizing Aluminum (Frederick a. Lowenheim)

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Anodizing Aluminum( Frederick A. Lowenheim )

Anodizing is an electroly tic process in which a metal is made the anode in a suitable electroly te so that

when an electric current is passed through the electroly te, the metal surface is converted to a form of its

oxide that has useful decorative, protective, or other desirable properties. The electroly te prov ides

oxy gen ions that react with metal ions to form the oxide, and hy drogen is released at the metal or carbon

cathode. Depending upon the solvent action of the selected electroly te on the anodize oxide, the operating

conditions employ ed, and voltage/current relationships, the metal anode continues to be consumed and

converted to an oxide coating which progresses inward. The last-formed oxide is adjacent to the metal-

coating interface.

Anodizing differs from electroplating in two significant respects. In electroplating, the work is made the

cathode, and metallic coatings are deposited on the work. In anodizing, the work is made the anode, and

its surface is converted to a form of its oxide that is integral with the metal substrate.

Anodizing processes have been developed for many metals. However, those used with aluminum are of the

greatest commercial significance. Magnesium is anodized for improved resistance to corrosion and

abrasion by procedures similar to those used with aluminum . Zinc can be electrochemically treated as an

anode in a complex proprietary aqueous electroly te developed under the auspices of the International

Lead Zinc Research Organization. Although this process is commercially referred to as 'anodizing', it does

not produce an oxide coating. Rather, v ia high voltage spark discharge, a fritted semi-fused surface that

enhances resistance to abrasion and corrosion is formed. Other metals including copper, cadmium, silver

and steel usually are anodized to achieve decorative effects which often are fugitive unless they are

protected by an organic over-coating.

Theory of anodic oxide formation

The mechanism of anodic oxidation is complex, and some aspects are not completely understood. In

accordance with Faraday 's law, 1 gram equivalent of pure aluminum( 8.9938g ) reacts electrochemically

when 96,500 C of electricity is passed through the aluminum anode. However, not all of this aluminum

appears as aluminum oxide in the coating. A significant ratio can be calculated between the weight of

coating and the metallic aluminum or aluminum alloy removed. If all the aluminum were converted to

oxide, this 'coating ratio', would be 1 .89( Al2O3/2Al.With the porous and absorbent ty pe of coating, this

ratio is significantly lower, seldom exceeding about 1 .60. The observed coating ratios for several

electroly tes and coating times are given in Table 22-1 . The coating ratio is lowered significantly by an

increase in the sulfuric acid content or temperature of the electroly te.The ratio is also lower with

aluminum alloy s than with the pure metal. A lowering of the coating ratio indicates, in general, an increase

in porosity and decrease in abrasion resistance. It is known,moreover, that the coating is not all aluminum

oxide, but contains chemically bonded and absorbed substances such as sulfate and water from the

electroly te.

Based on studies conducted with a sulfuric acid electroly te, it has been suggested that even under the most

favorable conditions, anodic coating weight is only 61 percent of the theoretical weight calculated from

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Faraday 's law. Considering the fact that as much as 13 percent of the coating is a form of sulfate, the

divergence from the theoretical amount of aluminum oxide is even greater.

Anodic coatings formed on aluminum in an electroly te that has little or no capacity to dissolve the oxide

are called barrier coatings. They are essentially non porous and have a limited thickness proportional to

the applied voltage ( 1 .3-1 .4 nm/V ). The barrier thickness represents the distance through which ions

penetrate the lay er of oxide under the influence of the applied potential. Once the limiting barrier

thickness has been reached, it becomes an effective barrier to further ionic and electron movement;

current flow drops to a very low leakage value, and oxide formation ceases.

When electroly tes that have appreciable solvent action on the oxide are employ ed, the barrier lay er does

not reach its limiting thickness, and current continues to flow. This results in the development of a 'porous'

oxide structure. Porous coatings may be several tens of micrometers thick. However, a thin lay er of

barrier oxide alway s remains at the metal-oxide interface.

Figure 22-1 represents a model of a porous ty pe of anodic coating as env isioned by keller, Hunter, and

Robinson and illustrates ty pical dimensional relationships of barrier to porous oxide as formed in a

phosphoric acid electroly te. This concept has been confirmed essentially by others with only minor

modifications in pore configuration and dimensions.

Morphology of anodic coatings

Advances in the science of microscopy and especially the availability of the electron microscope have

enabled researchers to describe more clearly the structural features of anodic coatings.

Electron micrographs of the underside of an anodized oxide coating reveal the presence of close packed

cells of essentially amorphous oxide. There can be billions of cells per cm2, their size being a function of

anodizing voltage. As can be seen in fig.22-4, which depicts a surface and cross-sectional v iew, each cell

has a single pore. Pore size is influenced by a number of factors, including ty pe of electroly te,

temperature, and voltage/current relationships. Pores extend downward to the barrier oxide. The

structures of anodize coatings formed in phosphoric, sulfuric, chromic, and oxalic acids differ only in pore

and cell dimensions.

Anodizing in sulfuric acid

Many electroly tes have been suggested for anodizing aluminum, some of which might be classified as

general purpose, whereas others are intended to achieve some specific objective. The processes most

widely used employ sulfuric acid in water at concentrations of 12 to 25%( wt.). This ty pe of electroly te is

relatively inexpensive and easy to control, and it results in coatings with a wide range of aesthetic or

functional properties.

General purpose or 'conventional' sulfuric acid anodic coatings applied for decorative and protective

purposes range in thickness from 2.5 to 30 microns. Ty pically , they are produced at a temperature 21

deg.C, a current density of 130 to 260 A/m2. and a voltage range of 12 to 22 V.

The effects of time of oxidation and temperature of the electroly te on the thickness of the oxide coating are

illustrated in Fig 22-6. To obtain these data, 1100 alloy sheet was anodized in 15% sulfuric acid electroly te

at a temperature of 21 deg C, the increase in thickness of the coating is linearly proportional to the time of

oxidation over a period of about an hour or longer. If the temperature of the electroly te is increased, the

coating thickness obtained for a given oxidation period decreases, and the linear portion of the curve is

shortened . This has led to the selection of a temperature of about 21 deg C with 15% sulfuric acid

electroly te for the production of general purpose coatings.

The oxide coating occupies a greater volume than the aluminum metal from which it is formed, therefore,

when there is no appreciable solvent action upon the oxide coating, the thickness of the section increases.



This increase in thickness is about one third the thickness of the coating. For example, a cy linder with an

oxide coating 24 micron in thickness would be increased 8 micron per surface, or 16 micron in diameter.

However, when the solvent action of the electroly te is greater, the increase will be smaller of there may

even be a decrease in the diameter of the piece. These changes are ordinarily small, but where close

tolerances are specified for parts that must fit together, they must be considered.

The weight of the oxide coating is a more sensitive measure of the solvent action of the electroly te than is

the thickness of the coating. Oxide, for example, may be dissolved from within the pores, reducing the

weight without appreciably decreasing the thickness of the coating. The effect of time of oxidation on

weight of coating is illustrated by the data of fig.22-7 , which were obtained at three different electroly te

temperatures. In comparing figs 22-6 and 22-7 , it can be seen that temperature has little effect on coating

thickness, whereas coating weight is lowered substantially as temperature is increased. This accounts for

the increase in porosity and loss of abrasion resistance as the temperature of the electroly te increases.

With long oxidation periods, for instance, 1 to 2 hours, and with other conditions such as high temperature

and low current density , there may even be a net loss in weight of an article after oxidation.

Other anodizing electrolytes

Dozens of electroly tes have been developed for producing special effects or characteristics on aluminum.

Those that have attained some commercial significance follow.

Chromic acid anodic coatings are opaque, are limited to a maximum thickness of about 10 micron, and are

rarely used for decorative purposes. Ty pically , they are formed in a 3-10% solution of chromic acid at 40

deg.C .Standard practice is to raise the voltage slowly to 40 V and continue anodizing for about 30 mins at

this constant voltage. Chromic acid anodizing processes are among the earliest developed for aluminum.

Before the advent of chromate chemical conversion coating processes, this ty pe of coating was used as a

base for paints, especially for military applications. They continue to be used for this purpose and for

anodizing complex parts where complete rinsing of the anodizing electroly te is very difficult.

Phosphoric acid anodizing processes have been suggested as a pretreatment for electroplating, since this

ty pe of anodic coating is rather porous and prov ides a substrate for mechanical locking of the electroplate.

Oxalic acid electroly tes produce y ellow coatings that are somewhat harder than conventional sulfuric

anodic coatings. Because this process generally is more expensive than sulfuric acid processes, it is not

used extensively in the United States. Oxalic acid-sulfuric acid mixtures are employ ed for producing hard

anodic coatings, although low-temperatures hard anodizing processes based on sulfuric acid alone are

competitive.

Sulfonated organic acids in combination with sulfuric acid are employ ed to develop so called integrally

colored anodic coatings on controlled alloy s. Shades of bronze, gold, gray , and black have found wide

acceptance for architectural applications. These coatings are harder and denser than conventional sulfuric

acid coatings, and the color develops as a result of the alloy and temper employ ed together with highly

controlled anodizing procedures. Details on these processes are proprietary .

Boric acid electroly tes, often with additions of borax, are popular for producing thin barrier oxide

coatings for electrical capacitors. Citrates and tartrates also are employ ed for this purpose.

Effect of alloy composition

Since anodic oxidation or anodizing involves conversion of the aluminum surface into an oxide coating,

the alloy and its metallurgical structure have important effects on the characteristics of this coating. Not

only does the alloy composition have a pronounced effect on the density of anodic coating, but it also

affects its appearance. Alloy constituents or impurities may impart coloration to the coating. Sometimes

they do not cause coloration; instead, their presence makes coatings look opaque rather than transparent.



The appearance of such coatings is dull and lacks a metallic luster. Wrought alloy s represented by sheet,

plate, forgings, wire, rod, bar and extrusions generally are more suitable for anodizing than cast alloy s.

Nonuniform appearance after anodizing is often experienced with castings owing to large grains, surface

porosity , segregation of alloy constituents, dross inclusions, or flow lines. Also, anodic coatings on cast

products can be less dense and less protective, especially if the alloy contains appreciable amounts of

copper. Pure aluminum develops the most transparent anodic coatings. With the exception of magnesium,

most major alloy constituents reduce the transparency of the anodic coating. Additionally , certain

constituents can impart a discrete color to the coating:e.g., silicon turns it gray : chromium or copper,

gold; and manganese, tan to brown. Primary producers of aluminum offer specialty alloy s, or they

specially control the production of conventional alloy s for applications where uniform appearance and a

specific aesthetic effect are important. For competitive reasons, composition, temper and fabrication

control often are not published. However, guidance on alloy selection can be obtained readily . The

characteristics of some of the more popular aluminum alloy s when anodized in a sulfuric acid electroly te

are shown in table 22-2.

T able 22-2 Anodizing Characteristics of som e alum inum alloy s anodized in conventional

sulfuric acid electroly te

Alloy Ty pe and form Characteristics

5252 High purity Al alloy ed Luster is below that of super purity base Al alloy s,

5457 with Mg in sheet form but they respond well to bright anodizing have good

5657 mechanical properties, and are economical for

applications such as automotive and appliance

trim. Alloy ing constituents do not tint coating.

6063 Al-Mg-Si extrusion alloy s These heat treated alloy s combine strength and

6463 good response to anodizing. Heat treatment dissolves

Mg2Si, improving strength and luster

after anodizing. Applications included automotive,

appliance and architectural trim.

7 005 Al-Zn-Mg alloy s in sheet, Give bright to diffuse luster, depending upon base

7 046 plate or extrusions purity . Age naturally at room temperature to give

high strengths or may be heat treated

1100 Sheet and plate Alloy 1100 is commercially pure Al hav ing some

5005 Si and Fe which diminish luster of anodized finish.

Presence of Si can give coating a grey tint. Alloy

5005 has some Mg for modest strengthening.



3003 Al-Mn alloy in sheet or Has excellent formability , and though widely used

plate for anodic finishing, has less luster than 1100 or 5005

because of Mn. Since MnO2 is brown, the thicker

anodic coatings have a tan or brownish tin

5052 Al-Mg-Cr alloy in sheet or Structural alloy widely used for welded parts .

plates Presence of Cr gives thicker anodic coatings a

y ellowish tint.

6061 Al-Mg-Si-Cu-Cr and Al- Heat treated alloy s have lower luster than 5052

7 07 5 Zn-Mg-Cu-Cr in extru because Al purity is lower. High strength alloy s

sions, sheet, and plate containing Cr and Cu give the thicker anodic

coatings a y ellowish tint.

2011 High Cu alloy s of al in Heat treated, high strength alloy s. Give lower

2014 extrusions, sheet, and density anodic coatings because most of the Cu

2024 plate dissolves out during anodizing. Thicker coatings

y ellowish tint.

Coloring anodic Coatings

In addition to so called integral color anodic coatings, where the color results from the metallurgical

characteristics of the aluminum alloy and specially developed anodizing procedures, most aluminum

alloy s can be anodized in general purpose electroly tes and subsequently colored. Porous ty pe anodic

coatings can be colored with organic dy es, certain inorganic pigments, and electroly tically deposited

metals.

Organic Dyes

After anodizing and through cold water rinsing, parts are immersed in a heated 65 deg.C aqueous or

organic solvent solution containing several grams per liter of dy e. Dy e concentration and pH control vary

with the particular dy e, and recommendations are available from dy estuff manufacturers. Immersion

times of 5 to 15 min are ty pical. After dy eing, parts are rinsed in cold water and sealed.



Mineral Pigments

Impregnation of anodic coatings with mineral pigments involves precipitation of insoluble materials such

as metal oxides, sulfides, and ferrocy anides in the oxide pores. Sometimes this is a two step process.

Examples of several processes are shown in table 22-3. Generally , colored anodic coatings achieved v ia

organic dy estuffs are not as resistant to fading as those resulting from mineral pigmentation processes. On

the other hand, the former are much easier to control and of much greater commercial value as a finishing

sy stem. They are especially suitable to multicolor applications since they can be resist preferentially

and bleached to allow repeated dy eings in alternate colors.

Electrolytic Deposition

After anodizing in sulfuric acid, metals can be electroly tically deposited in the oxide pores to achieve

shades of gray , bronze, gold, black, and red. Proprietary processes have been developed based on coloring

electroly tes containing nickel, cobalt, tin, selenium, tellurium vanadium, cadmium, copper, iron,

magnesium, lead, and calcium although those employ ing nickel, tin or cobalt are of greatest commercial

significance.Essentially , the technology of electroly tically depositing metals into the oxide coating can be

likened to electroplating. Many colored anodic coatings of this ty pe have good resistance to heat and

fading and have been exploited primarily for architectural applications.

Sealing Anodic coatings

The utility and performance of anodic coatings on aluminum often depend upon the ty pe and quality of

post anodizing treatment employ ed. The term sealing generally denotes a treatment which renders the

coating non absorptive or introduces into the coating a material that enhances or modifies the

characteristics of the anodic coating. Sealing usually involves subjecting the anodic coating to a hot

aqueous environment which causes hy dration to the coating.With certain aqueous solutions, components

of the sealant are absorbed by the coating. Other solutions permit precipitation of materials into the

coating by hy droly sis of specific metal compounds. When anodic coatings are subjected to pure water at

elevated temperatures the water reacts with the surface of the aluminum oxide to form boehmite :

Al2O3 + H2O ---------> 2AlOOH

The process of sealing involves a dissolution of oxide and re-precipitation of voluminous hy droxide inside

the pore. Prolonged sealing causes aging and densification of the precipitate. Some aqueous sealants

contain metal salts, the oxides or hy droxides of which may be co-precipitated with the aluminum

hy droxide. With such sealants, benefits accrue from this reaction as well as from the hy dration.

Dichromate sealants are useful because they combine the attributes of hy dration with the corrosion

inhibiting characteristics of chromate ions.The resistance of anodic coatings to staining and corrosion

depends upon the absorptiv ity of the coating. Therefore, sealing treatments usually are employ ed

immediately after anodizing. Improperly sealed coatings can account for pitting attack, color change, and

mottling. Highest dielectric values also require sealing of the coating.Choice of sealant depends upon the

kind of env ironment to which the anodic coating will be subjected and any special performance

requirements. Also to be considered are any post sealing surface treatments, eg. painting or adhesive

bonding. It is possible to add materials to a sealant to accomplish specific objectives, and it is practical to

employ a dual sealing treatment in order to capitalize in the peculiar advantages of each

Types of sealants and applications.

Water

The most widely used sealant employ s water, although only high quality water is effective. Distilled or

deionized boiling water- low in solids and free of phosphates, silicates fluorides and chlorides- is required.

Mixed-bed ion exchange sy stems or a two column ty pe charged with a strong base anion and a strong acid



cation exchange resin are satisfactory for furnishing sealing water . In some finishing lines, continuous ion

exchange of the sealing water is used to remove deleterious ions dragged in by racks or anodized items.

The presence of trace amounts of phosphates and silicates ( 5 and 10 ppm respectively ) significantly

retards hy dration of the coating. Chlorides and fluorides also will prevent adequate sealing and even cause

pitting attack of the anodic coating.

Sealing temperature is important and should be 98 to 100 deg C, lower temperatures require much longer

sealing times. The time required for formation of boehmite approaches infinity at temperatures below

about 65 deg. C. To preclude undue attack of the coating and to permit proper hy dration, the sealing water

should be maintained at a pH of 5.5 to 6.5 Buffers such as sodium acetate, are sometimes used to facilitate

pH control. Sealing time should not be shorter than 10 min for thin ( 2.5 micron ) decorative coatings and

can be as long as 60 min for thicker ( 25 micron ) coatings. Extension of sealing time ensures more

complete hy dration, but longer times do not entirely compensate for poor control of other sealing

variables, such as temperature, pH, and water purity .

Surfactants and dispersing agents may be added to water to minimize formation of a powdery smudge on

the work. Care must be exercised that they are of the ty pe that does not adversely affect the adhesion of

paints or other applied coatings used subsequently . Phosphates also are effective for minimizing smudge,

but they retard hy dration significantly .A variation of water sealing is steam sealing. The prime advantage

of this technique is that contaminant free moisture is ensured. The major disadvantage is that equipment

costs are much higher than for tank sealing using boiling water. Compared with conventional water sealing

methods, equivalent sealing is claimed to be faster with steam, the sealing rate increasing with increasing

steam temperature.

Nickel Acetate

It is believed that sealing in solutions of nickel acetate is effected by hy droly sis, resulting in precipitation

of colloidal nickel hy droxide in the pores of the coating . The coating is not colored by this reaction

because the finely div ided nickel hy droxide is almost colorless. Concurrent with the precipitation process,

a reaction of aluminum oxide with water to form boehmite occurs, as in plain water sealing. In the case of

certain dy ed coatings, the nickel ion may combine with the dy e molecule to form a metal complex or a less

soluble nickel salt of the dy e. This reaction may cause a color change depending upon the particular

dy estuff involved. In general, however, nickel acetate sealing minimizes leaching of the dy e during the

sealing treatment and improves light-fastness. Nickel acetate sealed chromic acid anodize coatings are

often used to mask or resist areas on parts to be hard anodized.

The addition of 1 to 5 g/l of nickel acetate in pure water y ields a very popular sealant, especially for dy ed

anodic coatings. The solution is maintained at a pH of 5.2 to 5.5 and at boiling temperature. Immersion

time of 3 to 10 min are ty pical. Pure water , free from phosphates, silicates, halides, and heavy metal ions

other than nickel , is required, just as with water sealing. Nickel acetate solutions are somewhat less

sensitive to the deleterious effects of these contaminants. Nonetheless, it is important to prevent

accumulation of these ions and to regularly discard solutions which have become contaminated.

Other Metal Salts

In addition to nickel acetate, salts of aluminum, cobalt, zinc, copper, lead and chromium have been used

to seal anodic coatings. The choice among these is based upon the utility and cost of the chemicals

employ ed and the compound formed as a result of the reaction between the particular dy estuff and the

metal ion. It is also possible to combine two or more salts in a single sealant. Certain organic dy estuff

metal ion combinations are more stable than others. Usually , the preferred sealant is noted by the

manufacturer of the particular dy estuff or mineral pigment.

Nickel sulfate, nickel nitrate, and alkali metal salts such as sodium, potassium or calcium sulfate for

chloride have been used for sealing but are not of commercial significance in the United States. Sodium



moly bdate has also been used by itself or in combination with other metal salts or with dichromates.

Dichromates

For improved resistance to saline environments, anodic coatings are sealed in a 5% solution of sodium or

potassium dichromate. Such solutions are operated at boiling temperature, immersion time is usually 15

min. At a pH of 5 to 6, maximum chromate-ion absorption and hy dration of the coating occur, assuming

the absence of interfering ions such as phosphates and sulfates. Resulting coatings are y ellow. However, if

the concentrations lowered to 0.5 g/l, only a trace of color is apparent. Dichromate sealed coatings are

not so resistant to staining by aqueous materials as those sealed in water or metal salt seals. For example,

some trace of stain is usually apparent after testing in accordance with the ASTM B 136 dy e stain test.

Neither do dichromate sealed coatings prov ide the maximum dielectric values.For this reason they are

not preferred as treatments when the anodic coating must withstand breakdown by impressed voltages,

such as might be encountered in electrical applications. Also, they are not as effective as nickel acetate

seals for rendering the coating resistant to breakdown during subsequent anodizing treatments.

Potassium or sodium dichromate is sometimes added to sealing water, when the chloride content of the

sealer exceeds 30 ppm and causes surface smudge. A concentration of 0.01 g/l is effective and will not

color the anodic coating.

Silicate

Alkaline metal silicate solutions have been used to seal anodic coatings. However, they are not especially

popular commercially . The mechanism os such treatments is not clear, but in addition to hy dration, it is

probable that aluminum silicates are formed. Sodium or potassium silicate solutions are usually

employ ed, sodium silicate being the more popular. In the case of sodium silicate, the preferred ratio of

Na2O to SiO2 is 1 :3 with potassium silicate, a ratio of 1 :4 is recommended. A boiling 5% solution and an

immersion time of 30 mins are ty pical, although sealing times of less than 1 min may be adequate. Whereas

trace amounts of silicate ( 10 ppm ) in a water sealant appear to retard hy dration of t6he coating, larger

concentrations prov ide a film of' water glass' with its characteristic resistance to alkalis and certain

environments.

Organic Materials

Waxes are often used to seal anodic coatings, especially when good 'nonsticking' or release characteristics

are required. Examples are ice cube tray s and threaded screw machine products. Treatments such as these

do not prov ide maximum resistance to corrosion and weathering .They should be considered as special

treatments to be used for release properties or lubricity in applications where maximum resistance to

corrosion is not of prime importance. Often they are used after other sealing treatments to impart

slipperiness.

Soaps, moly bdenum disulfide, and dispersions of poly tetrafluorethy lene(e,g Teflon ) are also employ ed to

decrease the coefficient of friction of the anodized finish. The phy sical dimensions of some organic

poly mers are so much larger than the ty pical pore diameter of an anodic coating that it is doubtful that

impregnation of the coating can occur. Thus, treatments of this ty pe are not seals per se, and the resultant

surface characteristics may be somewhat transitory .. Lacquers are often applied to anodic coatings to

improve resistance to staining and weathering and to increase dielectric properties. Usually , they are used

after an aqueous sealing treatment, but sometimes are applied directly after anodizing.Therefore, they are

called seals, although this is perhaps a loose generalization. Acry lics or modified acry lics are popular and

are available for dip or spray application. Often such treatments are used to prevent staining and attack of

the anodic coating by alkaline building products used during construction.

Vapor Techniques



Anodic coatings may be impregnated with certain organic materials in the vapor phase. The sealing

material is placed in a closed container with the anodic coating and heated to temperatures ranging from

93 to 260 deg C. In the case of certain resin forming substances, the monomer is vaporized, absorbed by

the coating, and poly merized, thus forming a solid resin in the pores. Wax-like materials can also be

applied by vapor phase treatments. Stearic acid,Carbowax, and paraffin are examples of materials which

can be vaporized. When such materials condense, the pores of anodic coating are plugged with the solid

material. Vapor sealing is not important commercially , primarily because of the rather cumbersome

procedures involved. Its prime utility would be for applications where aqueous sealing sy stems or

hy drated anodic coatings might be undesirable.

Dual Seals

Two step sealing sy stems are sometimes used to capitalize on the special attributes of each sy stem. For

example, a brief ( 1 to 2 mins) nickel acetate seal can be followed by a longer ( 10 to 15 min ) treatment in

boiling pure water. This sequence is employ ed with dy ed coatings to minimize leaching of the color, limit

the formation of surface smudge associated with nickel acetate, and maximize the degree of sealing.

Separate nickel acetate and dichromate treatments also are common, especially when resistance to saline

exposure is paramount. In so far as lacquering treatments are sometimes called seals, another dual

treatment would involve water or metal salt sealing before lacquering. This combination is used to prov ide

maximum resistance to alkaline building products.

General

Whereas sealing treatments are required for many good reasons, certain coating characteristics are

adversely affected by certain sealants. Resistance to abrasion, for example , is lowered by as much as 10

percent when a pure boiling water seal is employ ed ( measured by Taber Abraser instrument ). Also, the

temperature at which an anodic coating will craze is seal dependent. Seemingly , the more complete the

seal, the lower the crazing temperature. Fatigue failure also may be hastened by sealing. For these and

other reasons, the choice of a sealant should be made carefully . Table 22-4 prov ides general guidelines.

Table 22-4 Sealing Systems for Anodic Coatings

Ty pe Composition Time Temperature pH Remarks

Water Pure 10 ~60 mins boiling 5.5~6.5 Most widely

used sy stem

Steam Pure 5~30 neutral

Nickel 1~5 g/l 3~30 mins boiling 5.2~5.5 Recommended

Acetate for most dy ed

coating.

Dichromate 50 g/l 15 mins boiling 5.0~6.0 Imparts y ellow

color, excellent

resistance to

saline environment.

Silicate 50 g/l 30 mins boiling neutral Ratio of Na2O



to SiO2 is 1 :3

Wax proprietary 10~15 mins boiling 6.5~7 .0 Not suitable for

exterior use :

good nonstick

characteristics.

Lacquer Acry lics or - - - Spray or dip

modified Acry lics

Teflon TFE or FEP - - - Application

techniques are

proprietary . Intended

to prov ide low

coefficient of friction.

Equipment

Anodic coatings are produced by batch, bulk, and coil techniques on manual or automatic equipment

similar to that employ ed for electroplating. Irrespective of the technique employ ed, certain basic

considerations must be observed.

Electrical Contact

Anodic coatings are dielectric: therefore, the original positive contact with the aluminum surface must be

maintained throughout the entire anodizing cy cle. Otherwise, the electrical insulating character of the

initially formed anodic oxide effectively impedes flow of current to the work.

Racks

Aluminum or commercially pure titanium are the only practical materials for racking indiv idual parts or

for bulk anodizing. When aluminum is used for racking, the anodic oxide coating must be removed from

contact areas after each anodizing cy cle, usually by etching in caustic soda. For coil anodizing, electrical

contact can be made v ia copper or brass contact rolls that are located outside of the anodizing tank. This

arrangement requires good maintenance of the contact rolls to avoid arcing and localized overheating of

the aluminum web. Liquid contact techniques eliminate arcing problems associated with contact rolls and

permit use of higher anodizing current densities. Essentially , this approach relies on dual anodizing cells

where the aluminum web is made the cathode in the first cell and the anode in the second cell.

Cooling and Agitation

During anodizing, electrical energy is converted to heat which must be removed to maintain the selected

electroly te operating temperature. This is accomplished by air or mechanical agitation of the electroly te

and the use of cooling coils or an external heat exchanger. The refrigeration requirement is determined by

calculating the wattage input of the largest load to be anodized.

Tank Linings

Ty pe 316 stainless steel, antimonial lead or tellurium lead linings are satisfactory and can be used as the

cathode. Inert linings of rubber, plastic or glass can also be used prov ided suitable metallic cathodes in the



form of coils or auxillary electrodes are prov ided.

Power Supply

Motor generators or rectifiers can be used to prov ide the required direct current . Solid state silicon

rectifiers featuring constant current and voltage control prov ide a reliable and versatile power source. For

most anodizing, a power supply of 24V capacity is adequate, although up to 100V may be required for

hard anodizing and certain integral color processes. Copper anode bars are preferred for conducting the

current form the power supply to the rack.

Fume Removal

An exhaust sy stem for removal of fume or spray , constructed of corrosion resistant material is required.

Capacity calculation should be based on an air flow of about 5.0m3 per 0.1 m2 of solution surface.

Nomenclature

The Aluminum Association Designation Sy stem for Aluminum Finishes prov ides a convenient way to

classify anodic finishes as well as chemical and mechanical treatments for aluminum ( Table 22-5 ).

Anodized finishes are designated by the letter A followed by a two digit numeral, the first denoting class of

coating and the second denoting ty pe of coating. Since the introduction of this sy stem in 1964, it has

gained in popularity . However, the Alumilite designation sy stem, introduced in 1931, continues to be used

commercially . Another important designation sy stem relates to military applications ( MIL - A - 8625 )

which classifies chromic and sulfuric acid anodic coatings as Ty pes I and II , respectively , and hard

coatings as Class III.

Applications and properties

Reasons for anodizing are to (1) increase corrosion resistance (2) increase paint adhesion (3) permit

subsequent plating (4) improve decorative appearance (5) prov ide electrical insulation (6) permit

application of photographic and lithographic emulsions (7 ) increase emissiv ity (8) increase abrasion

resistance and (9) detect surface flaws. Even a casual rev iew of the many attributes of anodic coatings and

the markets they serve illustrates clearly the tremendous contribution anodizing has made to the growth

of the aluminum industry .

Corrosion Resistance

Aluminum, in its natural form, has a high inherent resistance to corrosion owing to an ever present thin

oxide film. Thicker, controlled anodic coatings enhance this characteristic so that aluminum is widely

accepted for outdoor applications including marine hardware, automotive trim, architectural curtain

walls, windows and storefronts.

Paint Adhesion

In addition to prov iding an excellent substrate from the standpoint of adhesion, anodic coatings prov ide

an inert and abrasion resistant barrier to attack of the metal should the organic over coating be damaged

or partially removed. Sealed anodic coatings must be free from surface films associated with some sealing

procedures, especially those containing surfactants. Also, final rinsing of sealed coatings requires high

quality water to avoid deposition of contaminants harmful to adhesion of organic coatings. Painted anodic

coatings are particularly valuable for critical aerospace and military hardware. Examples are torpedoes

and helicopter blades.

Plating Substrate

The natural oxide on aluminum prevents it from being electroplated directly . However, porous,



discontinuous anodic coatings, such as can be formed on aluminum in a phosphoric acid electroly te, are

sufficiently electrical conductive to prov ide a suitable substrate for electroplating. This ty pe of anodic

coating is characterized by a relatively irregular profile that enhances mechanical locking or key ing of the

electrodeposit to the anodic coating.

Decorative Appearance

Proper choice of alloy , pre-anodic finishing, sy stem and anodizing procedure enables aluminum to exhibit

an almost endless variety of surface appearances. Anodized products can be bright and specular, dull and

diffuse, directional or non directional in texture, any color or combination of colors; and anodized finishes

can be combined with organic coatings to prov ide a multitude of aesthetic effects. Jewelry , sporting

goods, appliance trim, building components, cooking utensils, hardware, decorative plaques, and

nameplates illustrate successful use of this attribute.

Electrical Insulation

Anodic coatings can withstand as much as 40-V/micron breakdown voltage, and they do not char at

elevated temperatures. They are used for transformer windings, electronic cabinetry , and many high

temperature applications. Special barrier ty pe anodic coatings are the foundation of the aluminum

electroly tic capacitor industry .

Photographic and Lithographic Substrates

When porous anodic coatings are impregnated with light sensitive materials ( silver halides ), the anodized

sheet or plate can function like photographic film. When anodic coatings are used for lithographic plates,

the anodic coating exhibits excellent hy drophilic qualities and further protects image areas from wear.

Emissivity and Reflectivity

These terms usually describe surface characteristics in the infrared and v isible regions of the spectrum of

energy , respectively . Emissiv ity describes a material's capability of giv ing up or re-radiating absorbed

heat to its surroundings. Reflectiv ity is that portion of incident energy that is not transmitted or absorbed

by the material.Since emissiv ity and reflectiv ity are surface phenomena, the surface finish, including the

ty pe and thickness of anodic coating employ ed, can be selected to achieve the desired effect. For

maximum emissiv ity values ( 0.8~0.9 ) in the far infrared region ( 9.3 micron ), sulfuric acid anodic

coatings of about 25 micron are optimum. In the far infrared region, thickness of the anodic coating, not

its color, is the controlling factor. However, at shorter wavelengths, color of the coating influences

emissiv ity . Thinner coatings, and coatings formed in other electroly tes such as oxalic, chromic, and

phosphoric acids, have lower values.

Highest reflectiv ity for v isible radiation ( ca.84%) is achieved with thin sulfuric acid anodic coatings (

2.5~5.0micron) formed on highly polished, brightened, high purity aluminum. Anodic coatings with high

emissiv ity are used in aerospace applications where, under vacuum conditions, heat dissipation is

controlled by radiation. Many applications are also found in the electronics and machinery and equipment

fields where aluminum products function as heat sinks.

Abrasion Resistance

Anodic coatings on aluminum characteristically are hard and resistant to abrasion. Still, the term hard

anodic coating usually is reserved for a special class of extra thick and extra hard anodic coatings used

chiefly for their wear resistant characteristics. Normally , appearance and to a lesser extent, resistance to

corrosion are secondary attributes. There are a number of commercial hard anodizing sy stems identified

by trade names such as Alumilite, Hardas, Martin, and Sanford.

Hard anodic coatings are usually applied in a sulfuric acid electroly te operated at relatively low



temperatures ( -4 to + 10 deg C ) and high current densities ( 250 to 400 A/m2 ). Coatings are usually 25

to 7 5 micron thick.

Hard anodizing most aluminum alloy s causes each surface to grow by an amount equivalent to about one

half of the anodic coating thickness produced.

Excellent resistance to abrasion relates to the hardness of the aluminum oxide coating. However, a

hardness value, as measured by a conventional indentation test ( e.g. Vickers, Rockwell, Brinell, Knoop ),

contributes little to any assessment of the wearing quality of an anodized article. These tests point-load the

hard anodic coating, forcing it into the much softer aluminum substrate. Thus, the value obtained is

influenced by the mechanical properties of the particular alloy and temper from which the coating was

formed.

The Taber Abraser test is used often for determining relative resistance to rubbing wear; it requires flat,

10.2 cm square panels. Weighted abrasive wheels ride on the rotating test panel. Various wheels and

different weights can be used, and test results are reported in different way s. The results of one study using

ty pe CS-17 wheels and 1000 g loads appear in Figure 22-8 . Here the weight loss was div ided by the

density of the material being tested to develop a 'wear index '. These data show a 25 micron thick hard

anodic coating on alloy 6061 T6 to perform almost the same as a case hardened steel and much better than

carbon steel or Ty pe 304 stainless steel. Compared with uncoated alloy 6061 T6 material, the anodized

specimen was about 13 times more resistant to abrasion. Although most anoeic coatings are sealed after

anodizing to enhance resistance to staining and corrosion, hard anodic coatings are normally not sealed

because sealing can reduce abrasion resistance 10 to 20%.

Despite the fact that they are not sealed,these coatings perform well in saline environments. Compared to

conventional anodic coatings which usually must withstand a few hundred hours of salt spray exposure

without failure, hard anodic coatings withstand months of salt spray exposure per ASTM B117 with no

signs of failure. Large quantities of aluminum are hard anodized for hundreds of applications such as gears,

pistons, fan blades, gun scopes, guide tracks, missile components and fuel nozzles.

Surface Analysis

Since anodic coatings reproduce the surface from which they are formed, anodizing especially in chromic

acid is a useful tool to make minute surface flaws more v isible. This technique is used also to study

metallurgical characteristics of aluminum substrates.

Testing and evaluation

The American Society for Testing and Materials, and particularly its Committee B.08 on Electrodeposited

Metallic Coatings and Related Finishes, has developed a number of standards, specifications, test methods,

and recommended practices that are useful to those interested in anodic coatings on aluminum ( Table 22-

6). International Organization for Standardization, TC7 9, Sub. 2 on Anodized Aluminum, has adopted

additional standards that are particularly important for international trade ( Table 22-7 ).

Table 22-6 ASTM Documents Pertinent to Anodic Coatings on Aluminum

B 110 Test for Dielectric Strength of Anodically Coated Aluminum

B 117 Salt Spray ( Fog ) Testing

B 136 Measurement of Stain Resistance of Anodic Coatings on Aluminum

B 137 Measurement of Weight of Coating on Anodically Coated Aluminum



B 244 Measurement of Thickness of Anodic Coatings on Aluminum with Eddy Current

instruments.

B 287 Acetic Acid - Salt Spray ( Fog ) Testing

B 368 Copper - Accelerated Acetic Acid - Salt Spray ( Fog ) Testing ( CASS Test )

B 457 Measurement of Impedance of Anodic Coatings on Aluminum

B 487 Measurement of Metal and Oxide Coating Thicknesses by Microscopical

Examination of a Cross Section.

B 529 Measurement of Coating Thicknesses by the Eddy -Current Test Method; Non

conductive Coatings on Nonmagnetic Basis Metals.

B 538 FACT ( Food Anodized Aluminum Corrosion Test ) Testing.

B 580 Specification for Anodic Oxide Coatings on Aluminum

B 588 Measurement of Thickness of Transparent or Opaque Coatings by Double Beam

Interference Microscope Technique.

B 602 Sampling Procedures for Inspection of Electro-deposited Metallic Coatings and

related Finishes.

D 2244 Standard Method for Instrumental Evaluation of Color Differences of Opaque

materials.

E 167 Standard Recommended Practice for Goniophotometry of Reflecting objects and

materials

E 429 Measurement and Calculation of Reflecting Characteristics of Metallic Surfaces Using

Integrating Sphere Instruments.

E 430 Measurement of Gloss of High Gloss Metal Surfaces Using Abridged

Goniophotometer or Goniophotometer.

G 23 Recommended Practice for Operating Light and Water Exposure apparatus ( Carbon

Arc ty pe ) for Exposure of Nonmetallic Materials.

Table 22-7 ISO Documents Pertinent to Anodic Coating on Aluminum

ISO 237 6 Anodization ( Anodic Oxidation ) of Aluminum and Its Alloy s- Insulation check

by Measurement of Breakdown Potential.

ISO 27 67 Surface Treatments of Metals - Anodic Oxidation of Aluminum and its alloy s -



Specular Reflectance at 45 deg. - Total Reflectance - Image Clarity

ISO 2931 Anodizing of Aluminum and Its Alloy s - Assessment of Quality of Sealed Anodic

Oxide Coatings by Measurement of Admittance or Impedance.

ISO 2932 Anodizing of Aluminum and Its Alloy s - Assessment of Sealing Quality by

Measurement of the Loss of Mass after Immersion in Acid Solution.

ISO 3210 Anodizing of Aluminum and Its Alloy s - Assessment of Sealing Quality by

Measurement of the Loss of Mass after Immersion in Phosphoric-Chromic Acid

Solution.

ISO 3211 Anodizing of Aluminum and Its Alloy s - Assessment of Resistance of Anodic

Coatings to Cracking by Deformation.

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