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Deposition of Aluminum Using Ionic Liquids


Megan O’Meara, Aurelie Alemany, Matthias Maase, Uwe Vagt, and Itamar Malkowsky, BASF SE

Surface coating is of major importance technically as well as economically. Aluminum surface finishing processes are used not only in traditional fields such as mechanical and plant engineering, aircraft and spacecraft, automotive, and medical technology, but they are a key technology in many other industries as well. The application of functional coatings can modify the properties of material surfaces—for example, their hardness, roughness, chemical resistance, or wettability—to adapt them to the specific technical requirements they must meet. Of special importance in this respect is a coating’s protection against corrosion and wear and tear.

To date, the electroplating processes used to produce metallic surface coatings were based mostly on electrochemical deposition of metals from aqueous solutions of electrolytes. However, reactive metals such as aluminum and aluminum alloys cannot be deposited by these traditional techniques because the electrochemical window of aqueous electrolytes is too narrow; water starts to decompose, forming hydrogen and oxygen before the metals begin to deposit.

An alternative option to be considered is physical vapor deposition (PVD). Although this vacuum-based metal-coating process allows a broad range of materials to be deposited by means of vapor, including aluminum, its deposition rate is low, which implies poor space–time yield and requires investment in expensive plants and equipment.

In the late 1950s, Ziegler and Lehmkuhl developed an alternative electroplating process using alumino-organic compounds.1 The major drawback to this method is that handling these compounds is extremely difficult because they are sensitive to humidity and air on the one hand, and pyrophoric and self-igniting on the other. Therefore, it is necessary to invest in additional safety devices to guarantee safe industrial-scale processes.

BASF SE of Ludwigshafen, Germany, has developed a new laboratory-scale process that is based on the use of ionic liquids and offers a range of advantages. Among these: it allows aluminum to be deposited faster; the system can be easily and safely managed; and it is cost-efficient because the ionic liquid is captured and recycled. The coating applied using this method adheres exceptionally well to the substrate and is both solid and homogeneous. Moreover, its finish quality is attractive, too. A crucial benefit is that the process does not require any major changes in the flow of established electroplating techniques. It is, in fact, a new process made up of mostly well-known steps.

Ionic liquids are salts that are in the liquid state at low temperatures due to their chemical structure, comprising mostly voluminous, organic cations and a wide range of anions. These liquids consist solely of ions; they do not contain other non-ionic components, such as organic solvents or water. In a way, liquid salts combine the properties of solids and liquids in a single material, resulting in unique characteristics and physical properties that no other material achieves. These characteristics pave the path to many innovative solutions in terms of processes and products, some of which have already been successfully established.

The overwhelmingly broad range of applications that use ionic liquids is remarkable, indeed. As early as 2003, BASF had successfully applied these liquid salts for the first time in a chemical process, now known as the BASIL process. Building upon this, BASF collaborated with customers to develop additional applications that use ionic liquids as electrolytes. Another focus currently in development is a method that dissolves and reshapes cellulose. The process for the electrodeposition of aluminum described herein adds another new field of application to BASF’s portfolio.

The new aluminum deposition process comprises three individual steps: First, the substrate is cleaned, degreased, and pickled in a traditional pretreatment process. However, as opposed to traditional electroplating methods, the substrate is then dried. Second, the electrodeposition step that uses an ionic liquid as its electrolyte is initiated. To achieve high ion mobility and a high deposition rate, the process is run at temperatures ranging from 60°C to 100°C. The aluminum that is deposited in this process comes from an aluminum anode, whereas the substrate serves as the cathode. In all cases, the deposition—which occurs rapidly—results from chloroaluminate complexes that are formed as intermediaries.

In contrast to processes using alumino-organic compounds, which require a strict separation from atmospheric oxygen and moisture, this type of aluminum coating process requires simple precautions to ensure that moisture is largely excluded, as the system remains stable up to a water content of 0.1% by weight. At higher water content, however, the electrolytic bath does form interfering aluminum oxychloride compounds and undesirable hydrochloric acid vapors. Therefore, nitrogen gas is used as a protective “blanket” to prevent water from being introduced into the system, thereby suppressing the formation of these unwanted by-products.

The electrolyte is based on 1-ethyl-3-methyl imidazolium chloride (EMIM chloride), a product from BASF’s Basionic portfolio, and is currently available on an industrial scale. Special—and expensive—purification of the basic electrolyte is unnecessary. By contrast, the choice of additives used with the basic electrolyte is decisive where specifically 1.5-molar equivalents of aluminum chloride are used along with other additives.
The coating grows at a speed of up to one micron per minute and forms a solid film with a varying thickness and quality that is dependent upon the additives in the system. For example, deposition from an additive-free electrolyte produces dendrite-like structures that have poor adhesion and lack solidity while not producing a good-quality finish. Inclusion of these additives, developed by BASF, ensures that the film deposited is free of dendrites, highly solid, and adheres superbly to the substrate. In addition, the coatings resulting from this process are distinguished by excellent finish quality. The process achieves these results without incorporating the additives into the coating. Substrates coated in this manner may include a range of metals, such as copper, iron, steel, or nickel.

Finally, the third step of the process consists of finishing the work piece that has been coated. First, any ionic liquid still clinging to the work piece is removed and recycled back into the electrolytic process. The aluminum surface is then passivated (if necessary) and further modified as required. Mechanical polishing, for example, will turn the aluminum coat into a top-quality, high-gloss surface with a mirror finish.

In summary, BASF has developed an aluminum deposition process that combines a number of advantages (see Table). The appeal of the new process lies in the high purity and density of the aluminum coating it produces, in the excellent adhesion between the coating and the substrate, and in the options it offers for modifying the surface finish. BASF is interested in supplying the proprietary ionic liquid-based electrolyte(s) used in this process, licensing of base technology, and finding partners to aide in further development.

Advantages of the Ionic Liquid Aluminum Deposition Process
  • Utilizes standard pretreatment procedures
  • Current density range from 400 to 800 amperes per square meter
  • Electrolysis at less than 100°C
  • Use of a robust electrolyte system that tolerates up to 0.1% by weight of water
  • Minimal loss of ionic liquid

 

Note

1. Ziegler, K., Lehmkuhl, H. Chemie 1956;283:414.

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