Coatings and Treatments for Aluminum
Randall Marks
OPTI 521
12/01//2008
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 alumina1. 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 O2- 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.
Fig. 1. Sketch illustrating ion transport through the oxide film. From Reference #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.
Fig. 2. Idealized structure of anodic porous aluminum oxide. From Reference #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
topcoat3. 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 injection4. 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 performance5.
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 performance6.
Fig. 3. 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.
Fig. 4. Protected Aluminum Reflectance. From Reference #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.
Fig. 5. Enhanced Aluminum Reflectance. From Reference #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.
1
Robert S. Alwitt, Anodizing, Electrochemistry Encyclopedia, http://electrochem.cwru.edu/ed/encycl/arta02-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
J. 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