Have you ever wondered how an aluminum bicycle frame, carabiner, or iPod case got that brilliant exterior color? Unlike paint which is organic, that colored coating is a thick inorganic oxide layer created by “anodization.” Anodized products can be found across all industries from consumer products to medical devices. Even though several metals can be anodized including titanium, zinc, and magnesium, this article will focus on anodization of aluminum, as that is the most common.
What is Anodized Aluminum?
Aluminum is used in industry because it has a good strength-to-weight ratio. Aluminum also has decent corrosion resistance because it has a very strong affinity for oxygen, which forms a thin and tenacious oxide layer on its surface and protects the underlying metal from further oxidation. However, aluminum isn’t a very hard metal and can be scratched fairly easily. Ceramics (oxides) are much harder than most metals, so to improve the scratch resistance of aluminum, “anodization” is used to thicken the oxide layer. The thicker oxide layer also improves the metals corrosion resistance.
What is the Anodization Process?
Metals are anodized by placing them in an acidic bath, often sulfuric acid, and the acid corrodes the metal resulting in a very thick oxide (ceramic) layer on the metal surface. This oxide layer is porous, and can be up to 50 µm thick (2 mils) and appear as different colors depending on its thickness. Aluminum oxide is typically gray or black in color. The porous surface oxide layer has to be “sealed” to complete the process, and this is accomplished by submerging the sample in warm water which hydrates the oxide and shrinks the pores. Putting dyes in the water during the sealing process allows the as the dye to permeate the oxide layer, this is how colorful products are created.
Common Problems Which Interfere with Anodization
Creating a uniform oxide coating of the correct thickness is the goal, but there are
sample requirements that must be met to obtain an ideal coating. Initially, the part must be very clean before putting it in an acid bath. This is necessary so that the acid can reach the aluminum to grow the oxide layer. Not having a clean surface when exposing the metal to the acid bath can prevent anodization from occurring at the contaminated locations.
Additionally, using an alloy that is not well suited for the anodizing process is problematic. For example, aluminum alloys that contains too much copper or silicon can have discolored anodized coatings like a dark smut. Almost all aluminum alloys can have anodized coatings, but the alloy type will determine how thick of an oxide coating can be tolerated.
In-Service failures of Anodized Coatings
While anodized coating failures are not common, there are two mechanisms which we have observed which can remove the anodized coating. One way an anodized coating can be removed is from exposure to chlorine, as chlorine is very reactive and causes pitting corrosion by removing the oxide layer. The second way to remove an anodized coating is to expose it to a solution that is very acidic (pH<4) or basic (pH>9), which also has the ability to break down the oxide layer and make the underlying aluminum susceptible to corrosion.
Investigating Anodized Coating Failures
If an anodized product is failing to perform as expected, there are testing options available to determine the cause of the problem and pinpoint what changes can be made—either in processing procedures, alloy used, or the environment to which a part is exposed. Multiple analytical techniques can be used to assess anodized coating failures but the workhorse instruments are X-ray Fluorescence Spectroscopy (XRF) and Scanning Electron Microscopy with Elemental Dispersion X-ray Spectroscopy (SEM/EDS).
The XRF is a quick test which determines the samples elemental composition, which indicates if the sample lends itself to successful anodization. This non-destructive testing technique can be used with samples as small as 3 millimeters in diameter, and there is no maximum sample size as the XRF is handheld, which is another advantage of this test.
The SEM/EDS is a microscopy technique used in conjunction with spectroscopy. If you use SEM/EDS to look at a cross-section of an anodized sample, it’s possible to measure the thickness, porosity, and uniformity of the anodized coating. The sample must fit into the sample chamber of the instrument, which often means the test is destructive in order to get the sample to fit within the 4”x4”x0.5” chamber.
Understanding the anodization process and difficulties are great first steps to ensure products meet performance expectations. Should an anodized coating fail to perform as expected, contacting an independent testing lab with the expertise and instrumentation needed to deliver an actionable solution is key—we welcome the opportunity to help in this way.
Alex joined the Polymer Solutions team with a strong educational background and industry experience with metallurgy. He completed his M.S. in Materials Science through Virginia Tech and earned his P.E. licensure in North Carolina. He has years of experience with forensic and metallurgical engineering, which includes failure analysis and quality control work for industrial clients as well as customized testing and failure analysis of materials for litigation.