plating processes, procedures & solutions
ANODIZING OF ALUMINUM
BY CHARLES A. GRUBBS
CHARLIE GRUBBS CONSULTING, LAKELAND, FLA.
An aluminum part, when made the anode in an electrolytic cell, forms ananodic oxide on the surface of the aluminum part. By utilizing this process,known as anodizing, the aluminum metal can be used in many applicationsfor which it might not otherwise be suitable. The anodizing process forms anoxide film, which grows from the base metal as an integral part of the metaland when properly applied imparts to the aluminum a hard, corrosion- andabrasion-resistant coating with excellent wear properties. This porous coatingmay also be colored using a number of methods.Many acidic solutions can be used for anodizing, but sulfuric acid solutionsare by far the most common. Chromic, oxalic, and phosphoric acids are alsoused in certain applications.The morphology of the oxide formed is controlled by the electrolyte andanodizing conditions used. If the oxide is not soluble in the electrolyte, it willgrow only as long as the resistance of the oxide allows current to flow. The resultantoxide is very thin, nonporous, and nonconductive. This particular propertyof the anodic oxide is useful in the production of electrolytic capacitors usingboric and/or tartaric acids.If the anodic oxide is slightly soluble in the electrolyte, then porous oxidesare formed. As the oxide grows under the influence of the applied DC current,it also dissolves, and pores develop. It is this property that allows us to color theoxide using organic dyes, pigment impregnation, or electrolytic deposition ofvarious metals into the pores of thecoating.By balancing the conditions used in the anodizing process, one can produceoxides with almost any desired properties, from the thin oxides used in decorativeapplications to the extremely hard, wear-resistant oxides used in engineeringapplications (hardcoating).Colored anodized aluminum is used in a wide variety of applications rangingfrom giftware and novelties through automotive trim and bumper systems. Suchdemanding situations as exterior architectural applications or wear-resistant,abrasive conditions, such aslanding gears on airplanes, are not beyond the scopeof anodized aluminum. Semiprecious and precious metals can be duplicatedusing anodized aluminum. Gold, silver, copper, and brass imitations are regularlyfabricated. New and interesting finishes are constantly being developed,which gain wide appeal across the spectrum of purchasers.The utilization of electropolishing or chemical bright dipping in conjunctionwith a thin anodic oxide produces a finish whose appeal cannot be duplicated byother means. Matte finishes produced by etching the aluminum surface, affordsthe “pewter” look, which is oftentimes desired. Matte finishes are also the finishof choice of most architects.
EQUIPMENTTanks
A wide variety of materials can and have been used to build anodizing tanks.Lead-lined steel, stainless steel, lead lined wood, fiberglass-lined concrete, andplastic tanks have all been used in the past. A metallic tank can be used as thecathode, but adequate distance between the work and the tank must be maintainedto prevent shorting. Some problems are experienced using metal tanks.For instance, the anode-to-cathode ratio is generally out of balance; also, sincethe entire tank is an electrical conductor, uneven current flow is possible leadingto uneven oxide thickness formation. This uneven oxide formation causes widecolor variations in organically dyed materials and is not generally recommended.Generally, the use of inert materials in the construction (or lining) of theanodize tank is recommended. PVC, polypropylene, or fiberglass are good inertmaterials for this application.
Cathodes
Cathodes can be aluminum, lead, carbon, or stainless steel. Almost all newinstallations are using aluminum cathodes because of their ability to reducethe energy requirements of the process. Because of the better conductivity ofaluminum, the anode-to-cathode ratio becomes extremely important. It hasbeen found that an anode-to-cathode ratio of approximately 3:1 is best formost applications. Cathode placement is also of vital importance. It is recommendedthat the cathodes be no longer (deeper) than the work being anodized.Placement of the cathodes along the tank sides should be such that they extendno further than the normal work length. For example most 30-ft long tankscan only handle 28-ft lengths; therefore, the cathodes should be positioned atleast 1 ft from either end of the tank to keep the work material from “seeing”too much cathode and anodizing to a thicker oxide on the ends. The depth ofthe cathodes in the tank should not exceed the normal depth of the work beingprocessed. If the cathodes extend deeper into the tank than the parts beinganodized, there will be excessive oxide growth on the parts in the lower portionof the anodizing tank. This will result in color differences in the oxide andsubsequently colored parts.The correct alloy and temper for aluminum cathodes is vital, 6063 or 6101alloys in the T-6 or T-5 condition are best. The overaged T-52 tempershouldnever be used!Cathode material should be welded to an aluminum header barusing 5356 alloy welding wire. Bolted joints are not recommended due to thepossibility of “hot joints.”Employment of aluminum cathodes has done much to improve the overallquality of anodized finishes in all areas of application.
Temperature Control
This is one of the most important factors influencing the properties of theanodic oxide and must be closely controlled to produce consistent quality. Thetemperature should be held to plus or minus 2OF. Most installations have somemeans of temperature control, since large amounts of heat are generated in theanodizing process.Lead cooling coils have been used in the past, but newer plants use externalheat exchangers. The external heat exchanger has been found to be moreefficient in cooling the solution while offering additional agitation. Again, asmentioned above, the presence of other metals in the tank, in conjunction withthe aluminum cathodes, can cause undo electrical problems.One of the added benefits of using a heat exchanger is agitation. Proper placement of the intake and outlet piping can insure goodagitation as well asminimization of temperature variations within the tank. This type of acid movementassures one of better anodizing.Recently, the use of acid “spargers” in the bottom of the anodize tank hasbecome popular. These spargers replace the more common air spargers nowbeing used and give much better acid circulation and temperature control.
Agitation
To prevent localized high temperatures, some form of agitation is requiredin the bath. Low-pressure air, provided it is clean and oil-free, is often used.Mechanical agitation and pumping of the electrolyte through external heatexchangers are also used. Generally, compressed air is not recommended dueto the presence of oils in the lines. Multiple filters in the air lines when usingcompressed air have not proven to be completely effective in keeping oil out ofthe anodize tank.
Racks
The two most common rack materials are aluminum and titanium. If aluminumis used, it should be of the same alloy as the work, or at least not be analloy that contains copper (2xxx series). Alloys 6063 and 6061 are excellent rackmaterials. It must be remembered that aluminum racks will anodize along withthe work and must be stripped before being used again. Titanium racks are moreexpensive, initially, but do not require stripping and are generally not attackedby the baths used in the anodizing process. Only commercially pure titaniumcan be used as rack material.Titanium racks are not suitable for low temperature anodizing (hardcoating)where high voltages are required. The lower conductivity of the metal causesheating of the racks and eventual burning of the aluminum parts being anodized.
Power Equipment
For normal (Type II) sulfuric acid anodizing (68-72OF), a DC-power sourcecapable of producing up to 35 V and 10 to 24 A/ft2should be suitable.Some processes such as phosphoric acid, oxalic acid, hard coating, or integralcolor may require voltages as high as 150 V.Power supplies come with a variety of options. Such things as constant currentcontrol, constant voltage control, adjustable ramping, end-of-cycle timers/signals/shut-offs, and a variety of other options make the anodizing processeasier and more controllable.Power supplies for hardcoat anodizing require more stringent capabilities.Those used for Type III low temperature anodizing (28-32OF) will require voltagesapproaching 90 V and amperages equivalent to 48 A/ft2. Power suppliesused for “room temperature” hardcoating (50-65OF) will require only 36 V andsufficient current to reach 36 to 46 A/ft2.
SURFACE PREPARATION
The type of surface preparation prior to anodizing gives the metal finisher achoice of effects. By combining mechanical techniques, such as scratch brushingor sandblasting with buffing and bright dipping, interesting effects can beachieved. Sandblasting and shot peening have also been used to give interestingsurface treatments.The beauty of dyed anodized aluminum can be further enhanced by colorbuffing the work after it is sealed and dried, using a lime-type composition,preferably containing some wax. In addition to actually polishing the coating,this step removes any traces of the sealing smut.Irregular shaped parts, castings, etc. are best finished by brushing with aTampico brush or by tumbling with sawdust or other suitable media.
PRETREATMENT
Cleaning
Proper and thorough cleaning of the aluminum surface prior to anodizingis one of the most important steps in the finishing process. Improperly cleanedmaterial accounts for more reruns and rejected parts than any other single factor.It is essential that all machining oils, greases, body oils, and other surfacecontaminants be removed prior to the continuation of the anodizing sequence.Both alkaline- and acid-based proprietary cleaners are available that will do anadequate job. If the oils or greases are specific in nature, some cleaners may needto be “customized” for adequate results.What is clean? Generally, we speak of a part being clean if it exhibits a “waterbreak-free” surface. Thismeans that if the water rinses off of the metal surfacein a continuous sheet,the work is considered to be clean. If, on the other hand, the water “beads”up or forms water breaks, the part still has foreign matter on the surface andcontinued cleaning is necessary. Once the part has been determined to be clean,subsequent finishing steps can proceed.
Etching
Etching is the removal of some of the aluminum surface from a part usingchemical solutions. There are a number of reasons for etching aluminum:
1. To impart a matte finish to the material (lower the specularity or gloss).
2. To remove surface contaminants.
3. To hide surface imperfections (scratches, die lines, etc.)
4. To produce an overall uniform finish.
Chemical etching is accomplished using both alkaline and acid solutions.The most frequently used etch media is sodium hydroxide. Time, temperature,concentration, and contaminant level will affect the type of finish possible inan etch bath. Many proprietary solutions are available from the chemical suppliers.Close attention to the technical information included with the chemicalsis important.
Rinsing
Probably one of the most abused steps in the finishing of aluminum is rinsing.Most anodizers practice some form of “water management,” usually to thedetriment of the other process tanks. Improper rinsing causes poor surfacefinish due to cross reactions of chemicals left on the surface from previousprocessing tanks reacting with the chemicals in further processing tanks. Crosscontamination of expensive solutions is another fallacy of “water management.”Cascading rinses, spray rings, or just cleaner rinse tanks with adequate overflowwill go a long way in reducing poor finish and cross contamination.
Deoxidizing/Desmutting
After etching, a “smut” of residual metallic alloying materials is left on thealuminum surface. This must be removed before further processing. The use ofdeoxidizer/desmutters will accomplish this, leaving the treated surface clean forsubsequent finishing steps.Many alloys, during their heat treatment steps, will form heat treat oxides. Ifthese oxides are not removed prior to etching or bright dipping, a differentialetch pattern can develop, which will cause rejection of the parts. In this instancea deoxidizer must be used. The deoxidizer is designed to remove oxides, but isalso extremely good at removing smut. A desmutter, on the other hand, will notremove oxides. It is apparent that a deoxidizer would be the preferred solutionto have in an aluminum finishing line.Remember, a deoxidizer will desmut but adesmutter will not deoxidize.
Bright Dipping and Electrobrightening
A chemical or electrobrightening treatment is required where an extremelyhigh luster is to be obtained on the aluminum surface. The electrobrighteningor electropolishing treatment is particularly applicable to the super-purity aluminumnow used extensively in the jewelry and optical field. Proprietary chemicalsfor these treatments are available from a number of suppliers. Chemicalbrightening is most commonly used for most applications because of it’s easeof operation. A number of companies offer proprietary solutions, which willgive you the bright finish you desire. Specifics on the makeup and use of thesesolutions is available from the chemical suppliers.
ANODIZING
Properties of the Oxide Film
The anodizing process conditions have a great influence on the properties ofthe oxide formed. The use of low temperatures and acid concentration will yieldless porous, harder films (hardcoating). Higher temperatures, acid contents, andlonger times will produce softer, more porous, and even powdery coatings. Itmust be remembered that changing one parameter will change the others, sincethey are all interrelated.It should also be pointed out that the alloy being processed may significantlyalter the relationship between the voltage and current density, often leading topoor quality coatings. This is particularly true when finishing assembled components,which may contain more than one alloy.
Factors Influencing Shade
In order to obtain reproducible results from batch to batch, a large numberof variables must be kept under close control. First to be considered are thosethat affect the nature of the oxide.
Alloy
The particular aluminum alloy being used has a pronounced effect on shade,especially with certain dyes. The brightest and clearest anodic oxides are producedon the purest form of aluminum, the oxides becoming duller as theamount of alloying constituents are increased. Super-purity aluminum (99.99%Al) and its alloys with small amounts of magnesium produce an extremely brightoxide, which does not become cloudy upon being anodized for extended periods.Alloys containing copper, such as 2011, 2017, 2024, and 2219, although forminga thinner and less durable oxide than the purer forms, produce a heavier andduller shade. Magnesium in excess of 2% has a similar effect although not aspronounced. The presence of silicon imparts a gray color to the coating; alloyscontaining more than 5% silicon are not recommended for use with bright colors.Iron in the alloy can lead to very cloudy or “foggy” oxides.The majority of casting alloys contain appreciable amounts of silicon, rangingas high as 13%, and present difficulty in anodizing. Use of a mixed acid dip(normally containing hydrofluoric and nitric acids) prior to anodizing is of valuewhen high-silicon alloys are encountered.Since the various alloys produce different shades when anodized identically,the designer of an assembled part must use the same alloy throughout if theshades of the individual components are to match.
Anodizing Conditions
Other variables affecting the nature of the oxide i.e., its thickness, hardness,and porosity) are the acid concentration and temperature of the anodizing bath,the current density (or the applied voltage, which actually controls the currentdensity), and the time of anodizing. These factors must be rigidly controlled inorder to achieve consistent results.The “standard” sulfuric acid anodizing bath (Type II) produces the best oxidesfor coloring. The standard anodizing solution consists of:Sulfuric acid, 180-200 g/LAluminum, 4-12 g/LTemperature, 68-72OFAs the anodizing temperature is increased, the oxide becomes more porousand improves in its ability to absorb color; however, it also loses its hardness andits luster, due to the dissolution action of the acid on the oxide surface. As thepore size increases, sealing becomes more difficult and a greater amount of coloris bled (leached) out into the sealing bath. The ideal anodizing temperature,except where a special effect is desired, is 70OF.Oxides produced by anodizing in chromic acid solutions may also be dyed.The opaque nature of the oxide film produced in this manner has a dullingeffect upon the appearance of the dyed work. Consequently, some dyes, notablythe reds, which produce pleasing shades on sulfuric acid anodized metal, areunsuitable for use with a chromic acid coating. Fade resistance of this type ofdyed oxide is extremely poor, possibly because the oxide is not thick enough tocontain the amount of dye needed for good lightfastness. The best chromic acidcoatings for dyeing are produced with a 6 to 10% by weight solution operatedat 120OF. A potential of 40 to 60 V is used, depending upon alloy, copper- andsilicon-bearing materials requiring the lower voltage. The usual time is from 40to 60 minutes.
DECORATIVE ANODIZING
Decorative anodic oxides are used in a great many applications, from lightingreflectors to automotive trim. The thickness of the oxide might range from 0.1to 0.5 mil (2.5 to 12 microns). As mentioned above the most common electrolyteis sulfuric acid and typical conditions are listed below. Parts that are to be givenbright specular finishes are usually produced from special alloys formulated fortheir bright finishing capabilities.Typical decorative anodizing conditions are:Sulfuric acid, 165-180 g/LTemperature, 60-80OFCurrent density, 10-15 A/ft2Voltage, depends on current density, temperature, and electrolyteTime, 12-30 minutes depending on film thickness desired. Longer timesproduce thicker coatings.
ARCHITECTURAL ANODIZING
The conditions used in architectural anodizing are not much different thanthose used for decorative applications, except the anodizing time is usuallylonger and the current density may be slightly higher. In general the thicknessof the oxide will be greater than for decorative coatings, and this relates to thetreatment time.
Interior
For interior applications the coating will be probably 0.4 mil thick (10microns). This means an anodizing time of about 20 minutes at 15 A/ft2.
Exterior
For exterior uses the coating will be a minimum of 0.7 mil thick (18 microns)and this means an anodizing time of about 39 minutes at 15 A/ft2.
INTEGRAL COLOR ANODIZING
This process, used mainly for architectural applications, requires the use ofspecially formulated electrolytes, usually containing organic sulfo acids with lowcontents of sulfuric acid and aluminum content, to produce a series of bronzeto black shades. The color produced is dependent upon the time of treatmentand the final voltage used. Specially formulated alloys are also required. Largeamounts of heat are generated in the process due to the high current densitiesemployed (up to 45 A/ft2), so efficient heat exchange equipment is needed tokeep the bath cool.
HARDCOATING
Hardcoating (Type III) is a name used to describe a special form of anodizing.The process, which usually employs higher acid concentrations, lower temperatures,and higher voltages and current densities is sometimes referred to as an“engineering hardcoat.” This is due to the fact that hardcoating imparts a veryhard, dense, abrasion-resistant oxide on the surface of the aluminum. A denseoxide is formed due to the cooling effect of the cold electrolyte (usually 30-40OF).At these temperatures, the sulfuric acid does not attack the oxide as fast as atelevated temperatures. Because of the lower temperature, the voltages needed tomaintain the higher current densities also help form smaller, more dense pores,thus accounting for the hardness and excellent abrasion resistance.Normal low temperature hardcoating is carried out under the followingconditions:Acid concentration, 180-225 g/LAluminum content, 4-15 g/LTemperature, 28-32OFThere have been a number of organic additives developed in the past few yearsthat allow the anodizer to hardcoat at elevated temperatures (50-70OF). Theseadditives, by virtue of their chemical reaction in the oxide pores, help cool thematerial being anodized and retard acid dissolution of the coating.
COLORING OF ANODIC COATINGS
The coloring of anodic oxides is accomplished by using organic and inorganicdyes, electrolytic coloring, precipitation pigmentation, or combinationsof organic dyeing and electrolytic coloring. After the anodizing step, the partsare simply immersed in the subject bath for coloring.The thickness of the anodic oxide can range from 0.1 mil for pastel shadesup to 1.0 mil for very dark shades and blacks. Application of electrolytic coloringwill be discussed below. Suffice it to say, the combination of organic dyeingand electrolytic coloring gives a more complete palette of colors from which tochoose.
Organic Dyes
The actual process of dyeing the aluminum oxide is very simple. A watersolution of 0.025 to 1.0% of dyestuff at a temperature of 140OF composes thedyebath. The aluminum, previously anodized, is simply immersed in this bathfor a short period of time, usually 10 to 30 minutes, The work is then sealed andis resistant to further dyeing or staining.The equipment required, in addition to that needed for the actual anodizingoperation, consists of rinse tanks with clean, flowing water; a dye tank for eachcolor desired; and a sealing bath preferably equipped with continuous filtration.The dye tanks must be of stainless steel, plastic, fiberglass, or some otherinert substance; never of copper or steel. They must be supplied with means ofmaintaining a constant 140OF temperature and should be equipped with someform of agitation. Usual plant practice is to use air agitation; however, withproper filtration, the filter itself can be used as the source of agitation. With airagitation the use of water and oil traps, plus a filter on the air supply, isnecessary to prevent contamination of the dye solution. A few drops of oilspread on the surface of the dyebath is very often the cause of streaked andspotted work. Typically, the use of blower air agitation is preferred over compressedair.Rinsing after anodizing, followed by immediate dyeing, is of prime importance.Since some dyes will not dye aluminum in the presence of sulfate ion, poorrinsing can cause streaks and discolorations. Even in the case of dyes not affectedby sulfates, any carry-over of acid causes a lowering of the pH of the dyebath,which means shade variations in succeeding batches of work.In the design of parts to be color anodized, care must be taken to avoid theuse of closed heads or seams, which are impossible to rinse. In the case of partscontaining recesses, which are difficult to rinse, a neutralizing bath of sodiumbicarbonate is of value. In workingwith coated racks, care must be taken thatthe rack coating does not separate, thereby forming pockets that can entrapsulfuric acid, later allowingit to seep out into the dyebath. Work must not beallowed to stand in the rinse tanks between anodizing and dyeing, but shouldbe dyed immediately, following a thorough rinsing. For most effective rinsing,three tanks should be used. In this way the final tank, usually deionized water,will remain relatively free of acid.The variables in the dyebath are time, temperature, concentration, and pH.Time and temperature are readily controlled in plant practice; however, regulationof concentration presents some difficulties. Fortunately, in the case of mostsingle component dyes, concentration control is not very critical, a variation of100% causing little change in depth of shade.The usual dyebath concentration for full shades is 2 g/L except for black,which requires from 6 to 10 g/L. In the case of pastel shades concentrations ofconsiderably less than 2 g/L may be required in order that the shade does notbecome too deep. This reduction in concentration will have a negative effect onthe dye lightfastness.Control of pH is important and a daily check (more often in smaller tanksor where high volume is a factor) should be made. The pH range between 6.0and 7.0 gives the best results with the majority of dyes; however, a few are moreeffective at values close to 5.0. Initial adjustments should always be made since itis not practical for the manufacturer to standardize the dyes with respect to thepH of their solutions. These adjustments are made by addition of small amountsof acetic acid to lower the pH value and dilute sodium hydroxide or acetate toraise it. Solutions may be buffered against possible carry-in of sulfuric acid byadding 1 g/L of sodium acetate and adding sufficient acetic acid to reduce thepH to the desired value.
COLORFASTNESS OF THE DYED COATING
Of the many dyes that color anodized aluminum, possibly several hundred,it should be understood that only a few possess sufficient inherent resistanceto fading to be considered for applications where exposure to direct sunlight isintended. Where items of long life expectancy are involved, for example, architecturalcomponents, even greater selectivity must be imposed, since all organiccolorants now known will exhibit some fading when subjected to sunlight ofsufficient intensity and duration. Also, the parameters of application as well asthe colorant are involved in the resistance to premature loss or change of color.The following additional factors are considered by most authorities as affectingthe lightfastness of the dyed coating.
Coating Thickness and Penetration of the Dyestuff
Accelerated and long-term exposure tests and practical experience both hereand abroad verify that an anodic oxide thickness in the order of 0.8 mil (20microns) and its complete penetration by the colorant is required for optimumresistance to fading and weathering. This means that, in some applications, thedye time may be extended to 30 minutes for complete dye saturation.
Intensity of Shade
Usually, the greater the amount of dye absorbed, the better its resistance tofading. Also, whatever fading may occur will be less apparent to the observer.Pastel shades may, therefore, be expected to exhibit inferior light and weatherfastness as compared to full strength dyeing.
Type and Degree of Sealing
Those dyes that are reactive with the nickel or cobalt salts present in thesealing bath usually require this treatment for optimum performance. It isreported that certain selected dyestuffs benefit from after-treatment with otherheavy metals; for example, lead, copper, zinc, or chromium. Generally, suchtreatments are not utilized because of the requirement of an individual sealingtank for each dye.In the case of extremely porous anodic oxides, for example, those formed onalloys of high copper content, effective sealing is particularly important withcertain dyes to prevent color loss from sublimation of the dye or by chemicalreaction in oxidizing or reducing environments.
ELECTROLYTIC COLORING (2-STEP)
This electrolytic coloring process consists of conventional sulfuric acid anodizingfollowed by an AC treatment in a bath containing tin, nickel, cobalt, orother metal salts to produce a series of bronze to black colors as well as blues,greens, burgundies, and golds. The most common bath is one containing tin.The colors produced are not alloy or thickness dependent and are easier tocontrol. The process is not as energy intensive as the integral color process. It isfor this reason that this process has almost entirely replaced the integral colorprocess in recent years. Unlike sulfuric acid anodizing, the coloring process iscontrolled by voltage and time, rather than by current density. Depending uponthe bath used, the coloring time can range from 20 sec for champagne to 10 minfor black. The use of specially built AC power supplies, using electronic timingand voltage control, helps produce a finish that is reproducible time after time.Proprietary baths containing bath stabilizers, color enhancers, and other additivesare being marketed and used throughout the finishing industry.
PIGMENTATION BY PRECIPITATION OF INSOLUBLE COMPOUNDS
Before the development of special organic dyes for coloring anodized aluminum,the precipitation of various insoluble metal compounds within theanodic oxide was used commercially. The treatment consisted of alternativelyimmersing the anodized surface in concentrated solutions of suitable metalsalts until a sufficient amount of the pigment was precipitated to produce thedesired color. Although seldom usedin today’s state of the art, a number of thesereactions are listed below:Lead nitrate (or acetate) with potassium dichromate—yellowLead nitrate (or acetate) with potassium permanganate—redCopper sulfate with ammonium sulfide—greenFerric sulfate with potassium ferrocyanide—blueCobalt acetate with ammonium sulfide—blackFerric oxalates(ferric ammonium oxalate or ferric sodium oxalate) appliedto conventional anodic oxides in the same manner as organic dyes are, underproper conditions, hydrolyzed to deposit ferric hydroxide within the coatingpores, imparting a gold to orange color of outstanding resistance to fading.Special proprietary chemicals are available for this treatment.The deposit of ferric oxide produced in the above manner may, in addition, beconverted to ferric sulfide, the resultant shade of which is black. Alternatively, abronze shade may be formed by reduction of the ferric oxide with pyrogallic acid.Cobalt acetatereduction, although commercially used in Europe, is not wellknown in the U.S. It consists of saturating a conventional anodic oxide with thecobalt solution and then reacting this with potassium permanganate to producea cobalt-manganese dioxide complex. The resultant bronze shade has excellentlightfastness and offers some potential for architectural applications.
MULTICOLOR ANODIZING
The application of two or more colors for the production of nameplates,instrument panels, automotive and appliance trim, etc. has now achieved sufficientcommercial importance that a number of large firms deal exclusivelywith such items.The following methods of multicolor anodizing are possible:Themultiple anodizing process,which entails a complete cycle of anodizing, dyeing,and sealing; application of a resist to selected areas; stripping of the entireanodic oxide from the remaining unprotected surfaces; and repetition of thisentire procedure for each color.Thesingle anodizing method, wherein an anodic oxide of sufficient thicknessand porosity to absorb the dye required for the darkest shade is first applied.This oxide is then dyed and left unsealed, a resist applied, and the dye alonedischarged or bleached out with a solution that leaves the anodic oxide intact.The operation is then repeated for each successive shade. Finally, the resist isremoved with a suitable solvent, and the entire surface sealed. In certain cases,where a dark shade is to be applied after a pastel shade, a modification of thistechnique omits the bleaching step with the supplementary dye being applieddirectly over the preceding color.The use of aspecialized combination ink-and-resistenables information or designsto be printed directly on the previously formed anodic oxide in several colors.The background color may then be applied by conventional dyeing methods,while the ink serves as a stop-off for the printed areas.Preanodized, photo-sensitizedaluminum alloy material is available, whereinthe image, in black, may be produced by photographic methods, and the backgroundcolored by the conventional dye immersion method.
SEALING OF ANODIC COATINGS
Hydrothermal Sealing (200-212°F)
To achieve the maximum protective qualities and corrosion resistancerequired for finished articles, the anodic oxide must be sealed after it is formedand/or colored. The sealing process consists of immersing the anodized partsin a solution of boiling water or other solution such as nickel acetate, whereinthe aluminum oxide is hydrated. The hydrated form of the oxide has greatervolume than the unhydrated form and thus the pores of the coating are filledor plugged and the coating becomes resistant to further staining and corrosion.The use of nickel containing seals will, in most cases, prevent leaching of dyesduring the sealing operation.When sealing with the nickel acetate bath, a smutty deposit may form on thework. This can be minimized by the addition of 0.5% boric acid to the bath orby the use of acetic acid to lower the pH of the solution to 5.3 to 5.5. Too low apH, however, causes leaching out of the dye. Use of 0.1% wetting agent in thisbath also aids in preventing formation of the smut. Proprietary sealing materialsdesigned to completely eliminate this smut are now available from chemicalsuppliers.The sealing tank should be of stainless steel or other inert material and mustbe maintained at 200OF. Use of a filter enables a number of colors to be sealedin the same bath without danger of contamination.
Mid-Temperature Sealing (160-190°F)
Due to the higher energy costs inherent in hydrothermal sealing, chemicalmanufacturers have developed “mid-temperature” seals (160-190OF). Theseseals, which contain metal salts such as nickel, magnesium, lithium, and others,have become very popular due to the lower energy costs and their ease ofoperation.One disadvantage of the lower temperature is the tendency of organicallydyed parts to leach during sealing. This can be compensated for by a slightincrease in the bath concentration and by operating the solution at the uppertemperature limits (190OF).“Nickel-free” seals (or more “environmentally friendly” seals, as they arecalled) are fast becoming the seal of choice whereclear or electrolyticallycolored parts are concerned. Because there is nothing to leach, these midtemperatureseals accomplish hydration of the oxide without the use of theheavy metal ions. When the seals become contaminated or are no longereffective, they can be discharged to the sewer without subsequent treatment(except possible pH adjustment). This offers the finisher a safer alternative tothe effluent treating necessary with heavy metal containing seals.
Room Temperature (Cold) Seals (70-90°F)
A significant modification in the sealing of anodized aluminum was thedevelopment of “room temperature sealing” (70-90OF). Unlike the high temperatureand mid-temperature seals, which depend on hydration for sealing,the cold seals rely on a chemical reaction between the aluminum oxide andthe nickel fluoride contained in the seal solution. Unfortunately, this reactionis slow at ambient temperatures and the sealing process can proceed up to 24hours; however, it has been found that a warm water rinse (160OF) after the coldseal immersion will accelerate the sealing process, allowing for handling andpacking of the sealed parts. The sealing of organically dyed parts in cold sealshas been found to be advantageous. Light stability testing (fade resistance) hasshown that parts sealed in cold seals gain additional lightfastness.
OTHER ELECTROLYTES
A number of other electrolytes are used for specialized applications.Chromic acidis used in marine environments, on aircraft as a prepaint treatment,and in some cases when finishing assemblies where acid may be entrapped.Although the film produced is extremely thin, it has excellent corrosion resistanceand can be colored if desired.A typical bath might contain from 50 to 100 g/L of chromic acid, and be runat about 95 to 105OF. There are two main processes, one using 40 V and a newerprocess using 20 V. The equipment needed is similar to that used in sulfuricacid processes.Oxalic acidis sometimes used as an anodizing electrolyte using similar equipment.This bath will produce films as thick as 2 mils without the use of very lowtemperatures and usually gives a gold or golden bronze color on most alloys. Thetypical concentration is from 3 to 10% oxalic acid at about 80 to 90OF, using aDC voltage of about 50 V.Phosphoric acidbaths are used in the aircraft industry as a pretreatment foradhesive bonding. They are also very good treatments before plating onto aluminum.A typical bath might contain from 3 to 20% of phosphoric acid at about90OF, with voltages as high as 60 V.
SUMMARY
Aluminum is a most versatile metal. It can be finished in a variety of ways. Itcan be made to resemble other metals, or can be finished to have a colorful aswell as a hard, durable finish unique unto itself. Only the imagination limits thefinish and colors possible with anodized aluminum.
