Titanium Anodizing Colors: What You Need To Know
Titanium is a strong and hard-wearing metal with wide applications. Aside from this, titanium can be anodized to take on vibrant colors that won’t fade. This process, called Type III titanium anodizing, is capable of imparting a variety of monochromatic finishes to the titanium, ranging from bronze to blue, and even magenta and green.
Further, this anodization improves certain properties of the titanium, such as its corrosion resistance and biocompatibility. This article will discuss titanium anodizing colors, their importance, the types of anodization processes and components, as well as provide tips to get the best results.
Anodized titanium is titanium (including some titanium alloys) that has had its surface converted to titanium oxide by an electrochemical reaction called anodization. This titanium oxide surface has a specific depth that has been controlled by the parameters of the electrochemical reaction, such as the applied voltage. Titanium anodization can be carried out in different ways with different effects. For example, Type II anodization increases wear resistance, while Type III anodization gives vibrant colors.
The available colors for anodized titanium are: bronze, blue, yellow, magenta (pink/purple), cyan (teal), and green. The color of the anodized titanium depends on the thickness of the oxidized layer created on the surface of the titanium. Therefore, a range of colors is possible depending on the exact thickness of the oxidized layer created.
There are multiple purposes for anodizing titanium. Anodizing titanium thickens the oxide layer on the surface of the titanium. One purpose of this is to provide corrosion resistance. A second purpose is to ensure biocompatibility for implants, screws, and other orthopedic fittings—the oxidized layer is robust and does not degrade and release dust or particles of titanium. Thirdly, anodizing titanium can decrease friction along the surface, giving increased lubricity. Finally, another purpose for anodizing titanium is for aesthetic reasons, as it can provide colors that add interest to titanium components.
The anodizing of titanium is important as it enhances the titanium’s surface properties. Anodizing generally improves the wear resistance, corrosion resistance, and biocompatibility of titanium. Anodizing titanium, therefore, broadens the application of the metal.
Type II anodizing, in particular, improves the wear resistance and lubricity (reduced friction during movement) of titanium. Type III anodizing also enhances the aesthetics of titanium by imparting different colors according to the thickness of the oxide film on the surface. These colors are also stable and unfading, which is a significant benefit.
To learn more, see our guide on Everything You Need to Know About Titanium Anodizing.
Anodizing of titanium can be carried out according to three broad types. Each type of anodizing is similar, but with different chemistry, and produces quite different results:
Type I titanium anodizing is a specialized process with limited application. It is a high-temperature electrolytic oxidation process applied to titanium.
Type II anodization gives a gray color to the surface of the titanium. It enhances its mechanical properties like lubricity (low friction) and wear resistance. Type II anodization of titanium is carried out electrochemically. The titanium is placed in an electrolyte solution and a voltage is applied. This voltage is set to maintain a certain current density, and the oxide film that is formed penetrates the titanium base metal. Therefore, the overall dimensions of the titanium component are unchanged after anodizing.
Type III anodization of titanium gives a colored appearance. It is the most visually distinctive anodizing process as a wide range of colors can be created, from bronze to blue to green. The process of anodizing uses an electrochemical cell to create an oxide layer on the surface of the titanium article, which interferes with the reflection and refraction of light off the surface.
The process of performing a Type III anodization of titanium takes place in an electrochemical cell. It can be explained as two components—the chemistry of what happens during anodization, and then the electrical process that drives that chemistry.
The chemistry involved in anodization creates an oxide layer on the surface of the titanium. The titanium at the surface of the part is converted to titanium oxide. This occurs in an electrolytic solution, which allows water to be split into hydrogen and hydroxide ions. The oxygen from the hydroxide ion is paired with the titanium to make titanium dioxide.
The electrolytic solution does not need to have a specific chemical makeup, it only needs to allow the transmission of electricity. However, for titanium anodization, common electrolyte solutions include sulfuric acid and trisodium phosphate. These solutions provide low electrical resistance, allowing the process to be more efficient.
Electricity is necessary to enable the chemical processes of the anodizing to take place. In other words, an electrochemical cell is required for anodizing. An electrochemical cell comprises two electrodes (an anode and a cathode) and an electrolyte solution. The titanium part to be anodized is connected as the anode; another conductive material is selected as the cathode. The electrolyte solution could be sulfuric acid, trisodium phosphate, or another solution, as described above.
The circuit needs to be connected to a voltage source, such as a battery, to complete the circuit. The voltage is a key parameter in the anodizing process, as it dictates the final color of the anodized titanium. This is because the voltage is the driving force for the anodizing reaction, and therefore determines the final thickness of the oxide film on the surface of the titanium. The thickness of the film in turn determines the final color of the anodized titanium. The applied voltage is in the range of 12 V for a bronze color (thin film) all the way up to 100 V for a green color (thick film).
There are a number of steps to be followed in the coloring process of type III titanium anodizing. These steps are:
- Clean the titanium part to be anodized. The alkaline clean specifically removes organic residue such as oils from fingerprints. These organic residues would otherwise interfere with the anodizing process.
- Rinse the alkaline cleaner off the titanium article. It is important to see the water running off the titanium in sheets, rather than in beads. If the water beads, it shows that there is still some residual oil on the surface (oil is hydrophobic, causing the water to bead).
- Hot rinse the part. A hot rinse allows the water to evaporate after the rinse so that minimal water is introduced at the next step.
- Prepare the surface with an acid clean for the best results in color anodizing. The purpose of this preparation step is to remove the residual oxide film that would have formed over time with exposure to air. If this oxide film is stripped off, exposing the base metal, it provides the best starting point for the controlled anodization to give a uniform color.
- Remove the excess acid with a cold water rinse.
- An alkaline solution can be used to neutralize any residual acid.
- Rinse off the alkaline solution.
- The anodizing process can now be carried out. Submerge the titanium component in an electrolytic solution (such as trisodium phosphate) and apply an appropriate voltage. The electrochemical reaction takes place to form the oxide film of a specific thickness on the surface of the titanium.
- After the titanium component is removed from the electrolyte solution, rinse off any remaining solution from the component.
- Execute a final hot rinse. A final hot rinse ensures that no residual solution is left, and the rinse water will evaporate to leave a dry, colored piece of anodized titanium.
There are two key aspects to getting the best results with color titanium anodizing. The first is the surface finish of the titanium article to be anodized. This will depend on the manufacturing process of the component, particularly if it was machined. It may be necessary to polish the item beforehand to achieve a smooth and consistent finish over the whole surface. If the machining process leaves a rough surface, or areas with a work-hardened or smeared finish, this will affect the electrical current flow through the titanium part. This results in an uneven film thickness and uneven color.
The second important aspect that should be considered carefully is the surface preparation of the titanium part before anodizing. The preparation essentially removes a thin layer of material to expose the base metal before anodizing. This preparation needs to be completed to give consistent results. Further, the anodization should happen immediately after the preparation is complete. If the titanium part is left exposed to air for some time (a number of hours), the base metal will naturally begin to react with oxygen in the air. This will interfere with the anodizing process and result in a splotchy color. Also, if any contaminants are deposited on the surface after preparation (even fingerprints), these will also result in uneven coloration.
To recover from mistakes while anodizing titanium for color (Type III), simply remove the oxide layer and start the process again with a fresh metal surface. Mistakes that may occur include arriving at the wrong color (by making the oxide film too thick) or getting a splotchy, uneven color (by not cleaning and preparing the surface correctly before anodizing).
A strong acid, usually hydrochloric acid, will remove the oxide layer. This is commonly an ingredient in household rust removers or oven cleaners. Another method for removing the oxide layer is by abrasion, using sandpaper to remove it mechanically.
The equipment used for anodizing titanium is essentially the equipment needed for an electrochemical reaction. It includes a bath (or tank) to hold the electrolytic solution, an electrode as the cathode, a conductive rack for holding the titanium parts that will function as the anodes, and a power supply. Other tanks or baths are necessary for the surface cleaning and preparation before anodizing, and for the rinse afterwards. All these items of equipment can be combined efficiently in purpose-built titanium anodizing machines.
Yes, Type II anodized titanium has a distinctive dull gray color. It is easily distinguished from non-anodized titanium or other metals. However, Type II anodized titanium cannot take on any other colors. It cannot have colors such as: blue, bronze, or magenta like Type III anodized titanium. Type II is always a dull gray.
Yes, the color on anodized titanium does last for a long time. It is a permanent chemical change of the surface film of the titanium. The color is present due to the reflection of light on the anodized surface. It is not a paint or a dye, so it does not fade. However, the anodized film on the surface of the titanium can be damaged mechanically (scuffed or scraped) and chemically (highly alkaline or acidic solutions). If the surface film is damaged, then the color will be affected or lost.
Yes, anodized titanium is more expensive than anodized aluminum. This is primarily due to stock titanium being more expensive than aluminum as titanium is less abundant and more valuable than aluminum. Further, the anodizing process for titanium is also more expensive than for aluminum as it is more complex.
Yes, anodized titanium is shiny if the process used was Type III anodization of titanium (colored). The Type III anodizing creates a thin oxide film that enhances color in the spectrum of white light and results in a shiny surface. However, Type II anodized titanium has a dull, gray surface.
Anodized titanium changes color due to the way the oxide layer on the titanium surface interferes with the reflection and refraction of light off the surface. If white light (sunlight—made up of its well-known spectrum of colors) shines onto anodized titanium, the oxide layer reflects a portion of that light, and also allows some light to pass through (refraction). This light is then reflected by the base metal underneath the oxide layer. This portion of light is then displaced from that which was reflected off the oxide layer by a distance that depends on the thickness of the layer. The two beams of light interact with each other, and the wavelengths of the various colors of light interact with each other. Depending on the distance of the displacement of the refracted light, some color wavelengths “cancel” each other, while other color wavelengths constructively amplify each other. This effect filters out some colors while enhancing others. The colors that are filtered or enhanced depend on the thickness of the oxide layer, as this determines the distance that the refracted light is shifted as it passes through the layer.
No, the 720 rule does not apply to anodizing titanium. The 720 rule is applied for anodizing aluminum. It is used to calculate the length of time (in minutes) that it will take to create a specific thickness of oxide film. However, with titanium anodizing, and Type III in particular, the thickness of the oxide film is set by the voltage that is applied, rather than the length of time that the titanium is exposed to the electrochemical reaction. Therefore, the 720 rule cannot apply to anodizing titanium.
To learn more, see our guide on What is 720 Rule for Anodizing?
No, the color in anodized titanium does not fade. The color of Type III anodized titanium is due to a permanent chemical change in the surface film of the titanium, which then affects the reflection and refraction of the light that falls on the surface, amplifying the wavelength of a particular color. In other words, the color of anodized titanium is not due to a dye or paint that may fade but is due to a chemically stable film on the surface, which therefore does not fade.
No, etching of titanium is not required before anodizing, but etching can be used to enhance the resulting color of Type III anodizing. Etching is an optional step for anodizing titanium—good results can be achieved with surface preparation limited to a good clean. However, if the titanium is etched with an acid solution after cleaning, it can give an even brighter color to the titanium after anodizing.
To learn more, see our guide on Importance of Etching Before Anodizing.
This article presented titanium anodizing colors, explained them, and discussed colors available and types of coloring processes. To learn more about titanium anodizing colors, contact a Xometry representative.
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