Introduction

Aluminum is a very common material used for structural components due to it being fairly inexpensive to procure and machine. If these structural components must maintain their integrity in some adverse environments, the aluminum must be treated to avoid degradation of performance and/or appearance. Since it is also relatively inexpensive to deposit thin layers of aluminum on a polished substrate, it is also quite commonly used for inexpensive mirrors. In this case, it may be coated to improve resistance to corrosion and/or improve optical performance.

Structural Aluminum

Aluminum naturally oxidizes fairly quickly, and it can be assumed that all untreated aluminum that has been exposed to oxygen has approximately a 2-3 nm thick layer of Al₂O₃, or alumina [1]. This protects the metal from further oxidation under most conditions, giving it a very good resistance to weather. Since alumina is clear when fairly uniform and thin, it does not greatly alter the appearance of the aluminum. For many applications, this oxide layer is adequate to protect the aluminum. However, many applications require that the aluminum be exposed to corrosive environments, and therefore additional precautions must be taken.

One of the most common methods of protecting aluminum is the process of anodizing. A much more complete discussion of this process can be reviewed at reference [1], however, it will be summarized here. The aluminum part is inserted into an electrolytic liquid. The anode of a battery or other electrical source is then attached to the aluminum part while the cathode is inserted into the solution. The electric potential across the already existent oxide layer causes the Al³⁺ ions to cross from the metal, and the O^2- to cross from the electrolyte. Once these reach the other side of the oxide, they form Al₂O₃. See Figure 1 for a representation of this process. There are two essential categories of this formation: barrier oxide and porous oxide.

Fig1: Sketch illustrating ion transport through the oxide film [1].

Barrier oxide is formed by immersing aluminum with the natural oxide layer into a near neutral solution such as ammonium borate. Since the oxide has very low solubility in the solution, this process forms a fairly uniform oxide layer. Since the oxide supports a field of around 1 V/nm, 1000 V are required to create a 1 um thick layer. This layer is amorphous, and a fairly good electrical insulator with a volume resistivity of greater than 10¹⁴ ohm*cm². Barrier oxide is generally too thin for mechanical applications, but it can be used in electrical components as a dielectric, such as in a capacitor.

Porous anodize is a much more common and useful protective coating. It is formed by utilizing an acidic electrolytic solution during the process. The properties can vary based on the acid, but the most common is 1 molar sulfuric acid. Because the oxide is somewhat soluble in the acid, it produces the porous structure during oxidation. Approximately 60% of the aluminum oxide formed becomes suspended in the solution. Because of the nature of the process, cells are produced with a pore in the center, running down almost to the surface of the aluminum. See Figure 2 for an example structure. The thickness of the oxide layer near the aluminum remains relatively constant during the process, and thus relatively low voltages can be used to produce fairly thick oxide layers, on the order of 100’s of um. As the anodize layer is formed, the pores and surrounding oxide grow towards the Aluminum, causing the Aluminum to be eaten away, in effect. These pores are on the order of around 50 to 300 nm, with the cell having typically 2 to 3 times that diameter. With the addition of oxygen atoms, the anodized layer ends up increasing the thickness of the part by approximately half of its width. For example, if one face of an aluminum part is anodized with a 50 um thick layer, the part thickness is increased by about 25 um.

Fig2: Idealized structure of anodic porous aluminum oxide [1].

After the anodize is the correct thickness, the pores are then sealed using hot water. The hot water reacts with the anodized coating, forming a less dense hydrous oxide, which fills the pores, leaving a nice, thick passivation layer. If different colors such as black or blue are desired, a dye can coat the part prior to sealing, which will seal in the dye within the pores, leaving the part a specific color. In optics, it is quite common to black anodize parts in order to reduce the amount of stray light spreading throughout the system. If one strongly wishes to reduce this light even further, the aluminum part can be bead blasted, a process by which the part is hammered with small glass beads. This forms a matte finish, which can be black anodized, and will further reduce stray light by all but eliminating speculer reflections. The indentations in the surface will tend to capture much more of the light than the flat surface would have.

For tougher color applications where the aluminum object will be exposed to sunlight and weather, and will be expected not to fade or wear, a different, more durable type of coloring must be used. All dyes will eventually fade, even when encased in anodize. Therefore, an interference color can be formed by depositing metal at the bottom of the pores using AC after the DC sulfuric acid step.

The oxide layer formed when aluminum is anodized is one of the hardest materials known. However, it is fairly thin and brittle, and will be fairly likely to break off under very high stresses. As long as the coating does not catastrophically fail, the coating has very good wear characteristics, and can protect the aluminum from a great deal of corrosive environments.

In order to improve the wear characteristics, the part can be hard anodized. In this process, the aluminum is anodized in sulfuric acid at low temperatures. The oxide produced by this method has larger cells with smaller diameter pores. This coating is much more hard and durable, it improves corrosion resistance, and it is often used in bearings because it is not damaged easily by the sliding motions and pressures involved.

It is often used without the hot water sealing to expose the pores to either a lubricant or to prep for adhesive application.

Structural aluminum can also be coated with any number of paints, epoxies, polishing compounds, or plastic laminate to improve its wear resistance. In corrosive environments, the aluminum can be coated with chromate primers, with a polyurethane topcoat [3]. It can also be treated similarly with silane primers. These coatings can protect against corrosion fatigue cracking, erosion corrosion, and microbiologically induced corrosion. There are also other, more elaborate treatment techniques such as laser surface treatment with Silicon Carbide powder injection [4]. These methods are much less used in optics applications due to the fact that most optical systems will have to be isolated from these kinds of harmful environments to protect the glasses, filters, and coatings. Furthermore, depending on the specific need, these types of treatments can vary a great deal, and providing an exhaustive list would be nearly impossible.

For most applications, some type of anodize will be acceptable for structural aluminum. Generally, the aluminum will be treated to have some type of porous anodize, and then sealed. This produces a decent electrical insulation, maintains a relatively high thermal conductivity, and improves wear and corrosion resistance of the aluminum. However, if the aluminum is to be used in strong acids or bases, or requires more stringent or stronger coating, other types of coatings will be used, and vary widely based on the specific application.

Optical Aluminum

Aluminum can also be used as a mirror due to its high reflectivity. Due to difficulties in polishing aluminum to optical grades, aluminum is generally deposited onto a glass substrate, which is much easier to polish into whatever shape is necessary. These deposition techniques leave a layer around 100 nm thick on top of the glass substrate. This thickness is much too thin to anodize, and three basic aluminum mirror types are readily available: bare aluminum, protected aluminum, and enhanced aluminum.

Bare aluminum is just how it sounds: uncoated vapor deposited aluminum. These mirrors form an oxide layer over time, which reduces the reflectivity of the aluminum considerably by adding roughness to its surface and creating a non-optimal interference layer above the aluminum. These mirrors are highly susceptible to environmental conditions and cannot be contact cleaned without damaging the aluminum. In fact, these coatings must be stripped and reapplied fairly often to maintain high performance [5]. These mirrors have the widest wavelength range of applications because they do not have interference coatings on top of them. See Figure 3 bare aluminum performance [6].

Fig3:  Bare Aluminum Reflectance  From Reference [6].

Protected aluminum is the same thickness of aluminum, but with a dielectric coating on top to prevent oxidation of the aluminum and to isolate it from environmental conditions. This coating is generally fairly hard in order to prevent scratches while cleaning. Since the dielectric layer reflects some light, it can be tailored to be a 1 layer interference filter for a specific wavelength, which improves the performance of the mirror over a fairly wide range of wavelengths. See Figure 4 for a protected aluminum reflectance curve. This dielectric coating can be one of many materials depending on the specific application, but two of the most common are Si₂O₃ and MgF₂. Si₂O₃ is generally used in visible light mirrors, while MgF₂ is used in UV testing because of its low absorption curve out into the UV.

Fig4: Protected Aluminum Reflectance [6].

Enhanced aluminum mirrors simply consist of more dielectric layers than the protected aluminum. The increased number of layers creates an interference filter to improve the performance of the mirror over a much more limited range of wavelengths. See Figure 5 for a typical reflectance curve. Care must be taken to finish the coating with an environmentally stable dielectric, especially if one wishes to be able to contact clean the surface.

Fig5: Enhanced Aluminum Reflectance [6].

Summary

Aluminum has very different coatings depending on the application. For aluminum used in structural applications, the most common treatment is to anodize the aluminum. This process produces a rather thick oxide layer to protect the aluminum and provide a much harder and stronger surface for mounting or sliding. When aluminum is used as a mirror, one or several dielectric layers deposited on top of it to provide both chemical protection and improve the performance of the mirror.

Reference

1. Robert S. Alwitt, Anodizing, Electrochemistry Encyclopedia, http://electrochem.cwru.edu/ed/encycl/art-a02-anodizing.htm

2. Accuratus Website, http://www.accuratus.com/alumox.html

3. Wim J. van Ooij, Protection of Aluminum Alloys Against Corrosion Fatigue Cracking, Erosion Corrosion, and Microbiologically-Induced Corrosion, Department of Civiland Environmental Engineering, University of Cincinnati.

4. A. Vreeling, Laser Melt Injection in Aluminum Alloys: on the Role of the Oxide Skin, Elsevier Science, Acta mater. 48 (2000) 4225-4233

5. Optical Mechanics, Long Term Study of Aluminum Coatings for Astronomical Mirrors, http://www.opticalmechanics.com/reflective_coatings/coating_study/index.html

6. Melles Griot, Optical Coatings Catalogue, http://www.mellesgriot.com/products/optics/oc_5_1.htm