Anodizing Aluminum. By J.C. HECKER, JR. Aluminum Consultants Madison, Wisconsin

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1 Anodizing Aluminum By J.C. HECKER, JR. Aluminum Consultants Madison, Wisconsin Anodizing, unlike electroplating and organic coatings, is commercially unique to aluminum. Developed in the early 1930 s, it has greatly extended the applications of aluminum in products and uses where the metal might otherwise not be utilized. The finish is readily available from finishing job shops throughout the world and is relatively inexpensive. Anodizing converts the surface of aluminum to an oxide. While aluminum naturally forms aluminum oxide on its surface, this is a very thin film. Anodizing provides a much thicker oxide coatingseveral mils thick if required. Coating Properties The hardness of this aluminum oxide coating rivals that of diamond. Thus anodizing improves abrasion resistance. Anodizing (with suitable prefinishing) also can appreciably alter and improve the appearance of aluminum. By using dyes and special anodizing procedures, the finisher can make aluminum look like pewter, stainless steel, copper, or brushed bronze. Anodizing improves corrosion resistance, especially when the metal surface is exposed to industrial, humid and marine atmospheres. The electrical insulating properties of the anodic finish find application when dielectric properties are important for electrical components. 284 Anodized aluminum is easy to clean and resists heat to the temperature at which the aluminum itself melts. Coating Formation Anodizing is an electrochemical conversion process, not an applied coating. The surface of the aluminum metal is converted to aluminum oxide as a result of reactions occurring at the anode in an acidic solution. The thickness and properties of the anodic coating will vary with alloy, anodizing process employed and cycle time (ampere-hours). Oxide formation proceeds inward, toward the source of fresh metal. The first formed oxide remains in contact with the anodizing solution throughout the process cycle; the last formed oxide is at the metal interface. The coating is pct thicker than the metal it replaces, since the volume of oxide produced is greater than that of the metal replaced. Structure of most common anodic coatings is predominantly cellular/porous. There is a very thin non-porous barrier layer at the interface. Sealing is normally required to set dyed colors, prevent staining and improve corrosion resistance of the anodized surface. Existing Processes Anodic coatings can be formed in a variety of chemica1 solutions, although only

2 a handful have been commercialized and are in industrial use. Some of the existing processes and their characteristics are as follows: Chromic Acid. The fairly thin ( mil), normally grayish-colored coating formed in a chromic acid electrolyte is used primarily to improve corrosion resistance and as a base for paint in aircraft and marine applications. Typically produced in a 3-10 pct chromic acid solution at 40 volts for min, chromic acid anodizing produces coatings that are thinner and less abrasion resistant than those produced in sulfuric acid electrolytes. Oxalic Acid. Primarily used in Japan. Characteristic yellowish color. Typical process conditions: three pct solution, asf, 75-95F, for min. Electrical power may be AC only or DC superimposed on AC. The coating is somewhat harder and more abrasion resistant than that of a conventional sulfuric acid anodic coating of comparable thickness. Phosphoric Acid. Very limited use as a base for electroplating (large pore size) and as surface preparation for adhesive bonding. Coatings produced in 3-20 pct by volume electrolyte at 85-95F, volts for min. Boric Acid. A thin, non-porous, barrierlayer-type anodic coating is produced in hot boric acid solution. Used as a dielectric for capacitors. Coatings are applied to continuous aluminum strip at voltages up to 600. Sulfuric Acid. The predominant anodizing process. Coatings 0.l-1.0 mil thick formed (typically) in a 15 pct solution, 12 asf, volts, 70F for min. The basic anodic coatings may exhibit yellowish, tan or gray colors, depending upon aluminum alloy and coating thickness. This coating is commonly dyed or otherwise colored with mineral pigments or precipitated metals (electrolytic two-step process). 286 Sulfuric/Oxalic. A mixed-acid variation used at high current density (24-36 asf), low temperature (30-50F) and necessitating high voltage ( volts) to produce coatings one to three mils thick. Very dense hard coatings produced. Used where extreme wear and/or corrosion resistance required. Organic Acids. The most widely used integral processes employ g/liter solutions of organic acids, containing a small amount of sulfuric acid (for increased conductivity). Operating conditions: 70-80F, 24 asf, voltages up to 75. Produce amber, bronze and black coatings, primarily for architectural applications. Coating thickness varies from about 0.4 mil to slightly over 1.0 mil. Color is achieved by aluminum alloy/ process variations, without need of secondary coloring agents. The oxide coating is hard and dense, similar to hard coatings. It is light fast and weather resistant. This finish is losing popularity to the electrolytic two-step coloring process because of energy saving, lower equipment costs, color flexibility and claims for better color uniformity and greater tolerance for variations in material. Producing Good Anodic Coatings Process control on the anodizing line starts with metal quality, precleaning and racking, and ends with unracking, possible clean-up and final inspection. The job shop finisher or captive anodizing department usually has the least control over a very important necessity for good anodizing: metal quality. Type of mill product, quality of that product, alloy, temper, gauge and so on have significant effects on the appearance, oxide coating properties and functional properties (abrasion and corrosion resistance) of the end product. Specific information on the effect of metal vs. anodizing (and other finishes)

3 may be found in the Conference Proceedings for Aluminum Finishing 87, a conference held October 28-30, 1986 by PRODUCTS FINISHING. Suffice it to say, it behooves the finisher to know the metal that is to be treated. Certain process changes may have to be made, and specified quality standards may be difficult to achieve if other than the proper alloy is being anodized. Finally, the point often arises - whose fault is it-the metal supplier or the finisher-if work is rejected by the buyer or architect? In investigating such conflicts it pays to know the characteristics of the aluminum being anodized. Pretreatment. Pretreating may be necessary prior to racking of aluminum parts for the anodizing line. Mechanical finishing may be desirable to remove extrusion die lines or casting parting lines. Protective paper and adhesive on sheet panels will have to be removed manually and by solvent wiping. Excessive machining oil may have to be removed in a vapor degreaser to avoid gross contamination of a soak cleaner solution in the anodizing line. And the list goes on. Racking. Racking is the first step necessary to produce good anodic coatings. The parts must make good electrical contact with the rack, which carries current to the part during anodizing. Poor contact because of insufficient rackcontact area or because of loose contacts can cause iridescent appearance, powdery coatings, poor dyed-color match, burning and other problems. The continuing build-up of insulating oxide tends to break flow of current to the part. The thicker the coating the more pressure is exerted. Inadequate racking may also cause parts to loosen due to mechanical, rather than electrical, action. Considerable force is exerted by entry and exit from solutions and by air agitation. Rack design and part placement on the rack are important. A good rack design will hold parts securely, conduct current adequately and carry a full load without shielding ( robbing current = nonuniform coating thickness). Part position must allow for good drainage and avoidance of air pockets. Small, lightweight pieces are racked on finger clip racks or on coiled springs. Large, heavy sections, on the other hand, will require strong work bars of sufficient current-carrying capacity, and possibly bolted contacts. Aluminum, titanium and combination racks are used. Aluminum racks must be stripped after each cycle. Titanium racks last longer but are more expensive and require greater contact area because of their lower electrical conductivity. Racks may be plastisol coated, but solution entrapment (and subsequent contamination) can be a problem, as the plastisol is undercut with use. Adequate cleaning is the first required tank process operation. Many organic compounds will act as a resist to later etching and anodizing steps. They must be removed. The most common type cleaner in large lines is probably the non-silicated, inhibited alkaline soak cleaner, which is available from a large number of chemical companies. It will remove most surface soils such as plant dust, oils and light buffing compound. A typical operating temperature might be F. Too high a temperature may produce a dried-on foam pattern. Overly vigorous agitation can also produce excessive foaming and carry-over on the rack and parts. Process control will include control of cleaner concentration and temperature. Avoid any oil accumulation on the tank surface. Cleaned and rinsed parts should present a water-break-free surface. Thorough rinsing must follow each 287

4 chemical step in the sequence. These may be single- or multiple-tank rinses and/or spray or immersion. Water should be clean, flowing and equipped with an overflow lip at the end or along the side. Bottom inlet and top outlet are usually recommended. Conductivity meters also have been utilized in order to control water purity and conserve water. Tanks should be free of galvanic currents (insulating pads and dielectric pipe connections) and be, equipped with insulated work-bar pads on the tank ends. Deoxidizing would normally follow cleaning and rinsing. Using an acid solution at somewhat elevated temperature ( F), its purpose is to remove nonuniform oxide films present on the surface of many aluminum products and other contaminants not dissolved by the soak cleaner. Work not properly prepared in this manner may not etch uniformly in the following step. Deoxidizers are typically mixtures of chromic, sulfuric, nitric or phosphoric acids. They require chemically resistant tank linings. Analytical procedures and test kits are often provided by suppliers of proprietary solutions. Use of these plus temperature control and visual observation of cleaned metal surfaces would constitute usual process control. The work is now ready for etching, a treatment step designed to remove the natural shiny aluminum look and provide a soft, matte, textured appearance. (Bright dip, on the other hand, is used to enhance the pre-polished specular surface). Etching is carried out for periods of 3-5 min at F in nominally five pct sodium hydroxide solutions. Excessive solution temperature can produce caustic burning, a non-uniform etch pattern that will usually make it necessary to reject the work. Both generic and proprietary etch so- 288 lutions are used. Small amounts of sodium gluconate are often added at low concentration to sequester sodium aluminate built up in the etch solution. This will prevent precipitation of hard, alumina hydrate scale on tank walls and burner tubes. It is normal practice to dump a portion of the etch tank when dissolved aluminum content gets too high (about 40 g/liter). At this point the solution is quite viscous and difficult to rinse. A fairly recent development is the never dump etch. In these proprietary solutions, aluminum is allowed to build to a point where the rate of dissolution is balanced by the rate of aluminum removed by dragout. The tank may be operated for a long period of time under these conditions and etching is said to be more uniform. In either case, analysis by titration is used to monitor and maintain alkali content and aluminum concentration. Etched parts are usually desmutted in the acid deoxidizer, although they may be carried directly to the sulfuric acid anodizing tank. Acid desmutting removes most of the aluminum alloy metallic constituents not dissolved by the caustic etch and clinging to the surface as smut. These copper, iron, manganese, silicon and other elements tend to contaminate the anodizing solution. And they may be carried through the entire process and show on the finished parts as a darkish film if not previously removed by a desmutting operation. Anodizing is usually carried out in a nominal 15 pct sulfuric acid solution at 70-80F, depending upon whether the coating is to be clear or dyed. Temperature should be held within a few degrees to produce consistent coating properties. Control of current flow (12-16 asf) is normally recommended, but many plants operate at fixed voltages. The amount of oxide produced will be

5 Anodizing Aluminum... ANODIZING line at left, with colored parts emerging from electrolytic coloring station. a function of amperes x time, as with all electrochemical reactions. Agitation is necessary to prevent localized overheating on parts and to provide uniform solution temperature throughout the tank. Cathode location is important, particularly with large sections. Even though the oxide buildup is self-limiting and throwing power is good, those surfaces closest to the cathode will receive thicker anodic coatings. With a metal tank connected as cathode, it may be necessary to selectively mask certain areas with a nonconductor. Acid concentration and aluminum content are determined by titration. You should avoid solution contamination by chlorides, fluorides, iron, copper, mercury and so on. Aluminum content is normally allowed to rise until it reaches approximately 20 g/liter. Then a portion of 290 the solution is discarded and replaced with fresh acid. Some companies offer equipment for removal of aluminum to eliminate the need for this procedure. Production sheets should record all anodizing parameters as well as concentrations, temperatures and chemical additions made in cleaning, etching, dyeing and sealing solutions. Inspection at the anodizing tank would usually include general appearance and coating thickness on selected parts. Where desired, the dyeing operation is next carried out in tanks of various watersoluble, organic-dye solutions. Dye concentration, ph and temperature are recommended by the dyestuff manufacturer. Air (if recommended) or mechanical agitation should be employed to maintain uniform concentration and temperature. Very short (less than about three min) dye times should be avoided. Longer immer-

6 Anodizing Aluminum... sion times will allow for deeper dye penetration into pores of the oxide coating and this is desirable. It is necessary to avoid galvanic currents and chemical contamination in dye solutions. Impurities such as aluminum, sulfates, phosphates, silicates and iron can affect absorption characteristics and dyestuff service life. Tanks also should be covered to prevent introduction of dirt, rust and oil and unnecessary exposure to light. Dye concentration and ph should be controlled. Concentration may be checked by spectrophotometer and/or standardized dyeings using test samples. Dye performance will vary with the specific dye, amount of dye absorbed, sealing treatment and exposure conditions. A very important final process step is sealing. Unsealed, the oxide pores are subject to staining and lowered corrosion resistance. For clear coatings, sealing in boiling deionized water converts the amorphous form of aluminum oxide to a more stable crystalline hydrate form. This reaction tends to plug and cap the oxide pores. Dyed anodized aluminum requires specialized sealing in nickel acetate, to prevent bleeding and to improve light fastness. Sealing times may be 3-5 min for nickel acetate and up to min for water. Double seals are also employed. A dichromate seal may be used for improved corrosion resistance. It imparts a light greenish color to the anodic coating. The ph must be controlled closely to insure efficient sealing. Again, it is also necessary to prevent chemical contamination by phosphates, silicates and some metallic elements. Deionized water is recommended for tank make-up and additions. Slight agitation is normally used to insure uniform temperature distribution. Cover the tank to protect the seal solution from overhead contaminants. Solution filtration helps to remove suspended particles. Surface smut after the sealing treatment is fairly common. Degree of smut may vary with alloy, ph, water purity, sealing time and other factors. It can be removed by wiping. Chemical companies also market additives that tend to decrease smut formation. Final inspection includes checking for appearance and coating thickness and performance of one or more of the seal tests. Coating weight and salt-spray resistance also may be required for some specifications. Newer Developments Electrolytic coloring processes were introduced into the United States during the late 1970 s, largely because of the need to reduce energy consumption. They were in use in Europe and Asia for some time before that. The two-step name indicates a dual process: anodizing in a conventional sulfuric acid solution and coloring in a subsequent operation. It differs from organic dyeing or mineral pigmentation in that coloring is produced by electrochemical action. In a proprietary solution, applied AC power deposits metallic particles of tin, nickel or cobalt in the pores of the previously formed anodic coating. This causes colors to be developed as a result of optical effects produced by light scattering that occurs when these other metals are deposited in the pores. Color range is similar to that described for the integral colors. Light fastness is said to be very good. The processes do not require the amount of energy needed for integral anodizing because lower current, voltage and time are used. Each of the five to ten electrolytic processes being actively promoted in this country has its 291

7 Anodizing Aluminum... own characteristic process parameters and stated advantages (or disadvantages). Typical coloring conditions are 60-80F, one to five asf and five-25 volts AC. Coloring time, largely independent of oxide-coating thickness, may be as little as 10 sec and up to min. Detailed information on these processes should be obtained from the chemical supplier who markets and/or licenses a specific treatment. Two variations of the electrolytic coloring process are over-dyeing and the possible application of a wide range of colors through interference effects. In the first method, bronze background colors produced electrolytically are dyed with conventional organic dyestuffs before sealing. This three-step process is said to be in use at several plants in Europe. Light-fast dyes are employed for architectural applications. Integral bronze colors also may be over-dyed. The second coloring procedure, not yet in production, is considerably more complicated. It involves modification of the pore structure produced in sulfuric acid by a subsequent anodic treatment in phosphoric acid. Pore enlargement occurs at the base of the pore. Metal deposition at this location produces colors ranging from blue, green and yellow to red. The colors are apparently caused by opticalinterference effects, rather than by light scattering as with the basic electrolytic coloring process. The feasibility of this approach will have to await further development. Spray dyeing is a relatively new technique said to offer several advantages over conventional immersion in dyes. As the name implies, anodized and rinsed parts are introduced into a closed chamber and sprayed with dye emerging from a series of spray nozzles. Time, temperature and concentration of the dye solutions are similar to those for immersion 292 coloring. Rinsing also may be carried out in the spray chamber. This process reduces space requirements considerably. Instead of individual dye tanks in the line for each color, one spray chamber is used and various colors are fed as needed from auxiliary storage tanks off-line. Capital investment in dye solutions is reduced, since the quantities used in spray dyeing are much less (recirculation to storage). Solution life reportedly is extended, since the dye baths, in closed containers, are protected from light, oil and dirt. Coil anodizing should be mentioned, not because it is new-which it is notbut because the reader should be aware that anodized material is available in wide (60 inches) sheet and foil form. Finishes available from at least five production facilities include mechanical, etched and bright dip pretreatments followed by clear, dyed and, within the last seven years, electrolytic coloring. Advantages of this system include prefinished stock for fabrication; lower cost; product uniformity; and less handling (vs. many individual parts). Possible disadvantages, on the other hand, include gauge limitation (.080-inch max depending on vendor), raw edges (where slit to width) and crazing. Some anodizing treatments such as hard coatings and integral colors are not available in coil form. As in many other industries, automation, particularly of process-cycle control, power supplies and cranes is becoming more common with new lines. Once manually racked, the parts may be cleaned, etched, / anodized, dyed and sealed automatically. Cycle times in each tank are pre-programmed. Hoists operate at prescribed speeds (horizontal and vertical) and tilt for drainage. Rectifiers may ramp and then maintain constant

8 Anodizing Aluminum... current, constant voltage and/or amp-hr control for set cycles. Conveyorized automatic processing offers better control of process parameters (temperature, concentration, agitation and so on). This type of equipment should find greater application in the future. In the hard-coating field, improved power supplies have decreased the tendency for burning and made it easier to process difficult-to-run high-copperbearing alloys. Two modifications are pulsed power and AC superimposed on DC. The first system applies 5-30 millisecond pulses of direct current superimposed on a base current. Pulsing apparently prevents overheating of the metal surface and resultant burning. One rectifier manufacturer states that (pulsed) current densities as high as 200 asf may be applied without causing any burning. The second method employs alternating current of variable potential superimposed on DC to accomplish similar goals. Being able to run thin gauge, critical machined parts and/or high-copper alloys without excessive process time or burning is a great benefit to this industry. Energy conservation, and reduced heating cost, has prompted the development and promotion of low-temperuture sealing solutions during the past few years. Several proprietary chemicals are being offered, allowing sealing of anodic coatings at temperatures of F down to ambient temperature (80-9OF). The cold seal is said to involve chemical impregnation of the pores, followed by reaction to form a resistant barrier. Vendor claims, in addition to energy savings, include reduced sealing times, less smut and greater tolerance for seal bath contaminants. Although these sealing treatments will pass all of the standard seal tests, there is a reluctance to use them for exterior architectural applications because of the lack of long-term exposure data. A final, fairly new development that currently is of interest is the use of organic additives to sulfuric acid for higher speed anodizing. Organic additives have been promoted through the years for hard coating. More recently, however, several companies have expanded the use of organic additives to conventional sulfuric acid anodizing. Claims are made that these additives will improve productivity and reduce operating costs by allowing the use of higher current densities (at higher temperatures). The anodic coatings are said to be comparable to the normal 70-72F coating in hardness, and burning has not been encountered at the higher current densities. Chemical cost is higher than that of straight sulfuric acid and the additive must be monitored. PFD 293