TECHNICAL BULLETIN

TECHNICAL BULLETIN #1-03 “Electrifying Coloring of Anodized Aluminum” TECHNICAL BULLETIN TECHNICAL

This Technical Bulletin is published by the Aluminum Anodizers Council as an educational and informational service. The information contained in this Technical Bulletin has been obtained from sources believed to be reliable. It should be noted that relevant practices, regulations and technologies are subject to change and this Technical Bulletin is current only as of the publication date. Further, it should not be assumed that all processes, standards or requirements affecting metal finishing are included. AAC makes no warranty, guarantee or representation whatsoever as to the absolute validity or sufficiency of the information or any reference set forth in this Technical Bulletin. AAC does not “approve” or “endorse” any products, services or methods. AAC assumes no responsibility for use or misuse of this Technical Bulletin.

Aluminum Anodizers Council • 1000 N. Rand Road, #214 • Wauconda, Illinois 60084 USA Tel: 847.526.2010 • Fax: 847.526.3993 • [email protected] • www.anodizing.org © 2013 Aluminum Anodizers Council No portion of this Technical Bulletin may be reproduced without the express written permission of the Aluminum Anodizers Council.

Issued June 2013 ELECTROLYTIC COLORING OF ANODIZED ALUMINUM

Electrolytic coloring has been used on aluminum throughout may carry only 113 as many parts as a conventional load, perhaps North America since the late 1970’s. The technology had been 2/3 if a well-designed coloring tank is used. A tank that is 30 developed in Japan and, due in large part to concerns over feet long and six feet deep will effectively color a load that is up energy consumption, the process soon was adopted in various to 500 square feet in surface area. If the tank is wide enough regions of the world. to accommodate central electrodes, this maximum load size doubles to 1,000 square feet. It is usually referred to as electrolytic two-step coloring. The first step is Type II, sulfuric acid anodizing. The second step is the The cost of the power and chemicals associated with the coloring deposition of metal particles deep into the pores of the anodized step is usually insignificant at pennies per square foot of work aluminum. processed. The electrolytic two-step process is seen by many as a favorable alternative to its predecessor, integral coloring. Energy-efficient Equipment electrolytic two-step coloring offers unsurpassed light fastness, Equipment and its associated cost may be an important factor in heat fastness, weather fastness, and chemical resistance. This determining justification for installing the process. In addition to inorganic coloring method enjoys wide acceptance in the a nonconductive coloring tank, two additional tanks for rinsing architectural and other markets. are recommended. The typical process sequence would be as follows: In electrolytic two-step coloring, metal particles are deposited at the anodize ➜ rinse ➜ rinse ➜ color ➜ rinse ➜ rinse ➜ seal base of the pores of the anodic oxide To retrofit an existing line, the question of space may come into layer (see illustration). After being play. It is important to note that the coloring tank contains anodized, the parts are immersed electrodes along the sides, and possibly down the middle; in a bath containing metal ions and therefore the width of this tank should be roughly the width of electrical current is applied. The the anodize tank. The electrodes are typically made of type 316 metal particles build up at the base stainless steel or tin. of the pores and, depending on how long current is applied, the particles A 60 Hz single phase AC power supply capable of delivering a fill the pores to varying depths. peak current of 10 amps/ft2 and an average of 5 amps/ft2 at approximately 20 volts, must be located close to the coloring Unlike conventional plating operations, two-step coloring uses tank. A good quality power source is essential to a smooth alternating current (AC) rather than direct current (DC). It has running operation. Two types of power supply equipment been demonstrated that the use of alternating current results in are available: one combines a step-down transformer with a fewer defects that may otherwise result from contaminants that motorized autotransformer; the other is a solid-state thyristor- may be present in the bath. controlled unit. Colors can vary with choice of metal, but are mainly dependent Over the years, process improvements have been promised by on the amount of metal that is deposited into the anodic oxide the advent of specialty power supplies. For instance, modulated layer. Coloring times for light colors can be as little as 30 seconds. systems, where the anodic and cathodic half waves can be Medium depth colors may take a few minutes, and black can individually controlled, have been shown to speed the process be achieved in about 10 minutes. Coloring baths may use tin, and improve control. Another system uses a three-minute (or nickel, or occasionally cobalt salts. Most of the electrolytic two- longer) DC treatment preceding the AC coloring step to improve step processing done today is used to achieve colors ranging color uniformity by modifying the structure of the coating. Some from bronze to black. enjoy the use of these specialty units; still others who own them Companies considering the installation of electrolytic two-step opt not to use them. coloring capabilities should consider the following: Surprisingly, getting the electrical current from the power supply • Product Marketability to the tank can lead to production challenges. The naturally • Equipment Investment occurring capacitance of the oxide, coupled with unwanted line • Choice of Chemistry inductance, often results in a distorted electrical waveform and, • Production Details. consequently, unwanted color variance. Line inductance can be minimized by braiding the cables that carry the electrical current Marketing Two-Step to the tank. An assessment of marketability begins with an understanding of Other miscellaneous capital expenditures include the purchase of process limitations and costs. Certain groups of alloys are more filtration equipment and may include a heat exchanger capable conducive than others to the application of twostep coloring. of maintaining the temperature of the bath between 65-75°F The preferred alloys are taken from AA 1xxx, AA3xxx, AA5xxx, and (approximately 18-24°C). By their very nature, stannous ions are AA6xxx series. One cannot expect to achieve a reasonable finish unstable. They readily oxidize to form stannic hydroxides, which on castings or AA2xxx series alloys. subsequently drop out of solution as a fine yellow precipitate. It is recommended that continual filtration through a 1O-micron Compared to conventional clear-anodized or dyed parts, filter be carried out on the production bath. Doing so will reduce productivity of two-step is considerably lower, thereby leading the development of so-called fall-out smut on the finished part. to increased overhead costs. As a comparison, a two-step load Chemistry The third important consideration when racking is part spacing. The process requires that electrical current flow to the parts Once the equipment is in place, effective production of in a uniform way. Parts cannot shield one another from the electrolytic two-step begins with the choice of chemistry. In electrodes. As a rule of thumb, depending on the shape of the North America, the most commonly used chemistry is based on part, this means spacing one part from another at a distance no tin deposited from a stannous sulfate bath. Relative to nickel, closer than P/5, where P is the perimeter of the part. (In other tin baths provide the promise of increased productivity at lower words, the minimum distance between parts should be one-fifth expense with minimal environmental impact. All tin baths are the distance measured around the part.) It also means that only comprised of stannous sulfate and organic additives that are one plane (row) of parts should be placed between any pair of essential to the practicality of the process. Without the additives, electrodes. If these rules are not followed, color uniformity will color uniformity is sacrificed and the stability of the bath is be impaired. severely compromised. Once racked, the parts are anodized in the normal way. If the The concentration of dissolved tin is somewhat dependent on target color is black, then it is recommended that a minimum of the choice of electrodes. If stainless steel is used, then all of the 0.7 mil be built up during anodizing. This anodic oxide thickness tin is supplied by the added stannous sulfate and normally the is required to accommodate the quantity of tin deposited within level in the bath is controlled within 15-20 grams per liter (g/l). the pore. Without it, one runs the risk of over-filling the pores, If tin electrodes are used then the concentration of stannous leaving a grayish smut on the surface of the part. sulfate is often kept near 12 g/l and roughly half of the tin used for coloring comes directly from the electrodes. In either case, After anodizing, the parts are immersed in the coloring tank. The the correct level of organic additives must be present to ensure load must be allowed to soak for about one minute before the color uniformity and tin stability. power is initiated. Without this soak, blotchy colors will result. The voltage should be ramped up to operating voltage in 30 Tin electrodes offer the benefit of reduced chemical costs at the to 60 seconds. Doing so too quickly or too slowly may result in expense of higher capital costs. The process consumes the tin deeper or lighter colors around the periphery of the load. The electrodes. Because of this, they routinely must be repositioned operating voltage is typically 18 to 20 volts, normally about 1 volt in the tank to compensate for zones of higher and lower current lower than that used for anodizing. For very light shades, the flow. Typically, tin electrodes are removed and recycled when value may be as little as 10 volts. their mass is reduced by about 60 percent. Stainless electrodes initially are less expensive and they last longer, but they must be At the end of the coloring cycle, the power is discontinued and removed periodically for cleaning. the load removed for a color check. This is commonly done on a section of the unrinsed load that is dried and compared with Chemicals are supplied as premixes or as individual components. light and dark color standards. Some lightening will occur during To prepare a bath, the premixes would be diluted with water rinsing and sealing and this should be taken into account when containing sulfuric acid. Subsequent additions of the premix are the color check is being made. If additional color is required, based on a simple titration for available stannous sulfate. For the load is re-immersed in the coloring tank and the process is those who prefer to add their stannous sulfate separately, the continued until the desired color is achieved. individual components can be added to the bath according to the manufacturer’s recommendations. Following the coloring step, the load is rinsed and sealed. Unlike dyed coatings, a nickel seal offers no benefit over a non-nickel Chemistry based on depositing cobalt from a cobaltous sulfate seal. The most common seals are achieved using either boiling bath is frequently used to impart color to anodic oxide layers water with a dispersant to control seal smut or a mid-temperature that are thinner than those colored using tin. Thin color films are non-nickel seal. typically preferred by the automotive industry, while architectural standards may require thicker colored oxide layers. Summary Production Two-step electrolytic coloring of anodized aluminum is a mature technology that has gained wide acceptance in the architectural Proper racking of the parts is far more important when performing marketplace. Almost all North American companies use tin-based electrolytic two-step coloring than it is when clear anodizing. electrolytic coloring chemistries. The decision to implement the The jigs must be made of aluminum, not titanium; the preferred two-step coloration process must first be based on marketing alloys are either AA6061 or AA6063 in the T6 temper. Either alloy input and weighed against the required capital. Compared to should be used exclusively, not in combination within the same clear anodizing or organic coloring, relatively low productivity system. will increase overhead costs per square foot of colored product. Contacts must be secure. This may be achieved by clamping, spot The process is also somewhat different as a result of the use of AC welding, or by using a twist fixture technique. Spring clips can power to color parts rather than immersion impregnation. The be used, but are normally effective for only about half a ; dozen final product, however, has unsurpassed light fastness, weather cycles before they must be discarded. This is because aluminum fastness, chemical resilience, and heat resistance. racks must be stripped prior to reuse and the clips soon lose their spring. A common problem is loss of electrical contact when the rack is being jostled during transfer from the anodizing tank through the rinse steps to the coloring tank.