What is Titanium Anodizing? How to Anodize Titanium

Titanium anodizing is an electrolytic finishing process that manipulates the oxide layer on the surface of titanium via electric current. The titanium item forms the anode (positive electrode) of an electrolytic cell; hence the name “anodize.”

Anodizing has a long history in the aerospace industry, where it was first used in 1923 to protect British seaplane parts from salt-water corrosion. Aerospace companies continue to use anodizing processes today to protect metals from effects of aging, wear and corrosion.

The medical device industry also widely uses the titanium anodizing process, since anodized titanium parts are nontoxic and suitable for biomedical applications, such as orthopedic implants.

Types of Titanium Anodizing

There are two commonly used types of titanium anodizing: Type 2 and Type 3. Type 1 is far less common, and is used in specialized high-temperature treatments.

Type 2 Titanium Anodizing

Type 2 is mainly for wear purposes: It protects the metal surface against the effects of wear. When untreated titanium parts rub against each other, they produce titanium dust – a result that is not desirable with orthopedic implants, for example. Type 2 anodizing provides a wear-resistant surface and helps prevent seizing or friction between sliding titanium surfaces.

The reduced friction, or lubricity, from Type 2 anodized titanium also helps patients with orthopedic implants by improving mobility of joints.

For most Type 2 titanium anodizing lines, AMS 2488D is the applicable specification. SAE International, the aerospace standards organization, is the governing authority for the specification for Type 2, which was last reviewed and reaffirmed in February 2018.

For aerospace applications, Type 2 anodized parts are compatible with aircraft fluids and can withstand extreme temperatures within a range of -70 °F to 260 °F. Type 2 anodized titanium resists corrosion from exposure to salt water and humidity.

Type 2 anodized titanium parts are a distinctive gray color. This makes them easily distinguished from stainless steel, or from color anodized titanium.

In this photo of dental implant hardware, the middle screw is made of Type 2 anodized titanium.

Type 3 Titanium Anodizing – Color Titanium

Type 3 titanium anodizing is also called color anodizing. Type 3 anodizing is widely used in the medical world for quick visual identification of parts. For example, an orthopedic surgeon in mid-procedure can simply ask for a blue bone screw, without having to specify the 12 mm length of the screw.

Another example of color coding to assist doctors and physicians is bone fixation plates that an orthopedic surgeon uses to treat trauma fractures. These bone fixation plates have drill guides of different colors to indicate the anatomical difference between the left and right plates.

Type 3 color anodizing is less common in the aerospace industry, but sometimes is used for quick visual identification in complicated assemblies.

Outside the medical device and aerospace industries, Type 3 colored titanium is also used in jewelry manufacturing.

Sample of titanium colors available with Type 3 anodizing

In contrast to Type 2, Type 3 color anodizing lacks an overarching industry specification. AMS 2488D does not address color anodizing, and no industry-wide standards exist to define specific colors in the Type 3 spectrum. This makes color matching of parts from one batch to another a real challenge.

The lack of standards for Type 3 color anodizing means that manufacturers must build their own process validation from the ground up, rather than starting with the framework offered by an industry specification. Best Technology’s process equipment experts offer process validation consulting services for manufacturers installing a new Type 3 titanium anodizing line.

Key differences between Type 2 and Type 3 Titanium Anodizing

Type 2 differs significantly from Type 3 titanium anodizing.

Anodizing Equipment and Tanks for Titanium Color Anodize

How does titanium color anodizing work?

Anodizing machines manipulate the oxide layer on the surface of titanium to produce an “illusion of color.” The titanium oxide layer gives the perception of color due to an interference phenomenon, similar to a prism. Light reflects from both the oxide layer and the underlying titanium at different angles and those reflections interfere with each other. Certain wavelengths of light cancel each other out or combine, so that the remaining light is perceived as color. Unlike aluminum anodizing, no dyes are required to produce the color perception, thus adding to the biomedical safety of the finished part.

An oxide layer forms naturally on the surface of titanium upon exposure to the atmospheric conditions, as oxygen reacts with the surface of titanium. This process of oxidation naturally occurs with many elements and the thin protective oxide layer helps protect it from further reactions to air or water.

Under normal conditions, the oxide layer is about 1-2 nanometers thick (10-20 angstroms) when it first forms, but it will continue to grow in open air. A typical, non-anodized oxide layer would be 20-25 nanometers (200-250 angstroms).

In color titanium anodization, however, the thickness of the oxide layer is augmented and manipulated. For example, the color bronze – the thinnest color layer on the titanium color spectrum – can be achieved by building the the oxide layer thickness to about 300 to 350 angstroms. At the other end of the titanium color spectrum, the color green – the thickest color layer – can be achieved with an oxide layer thickness of 500 to 550 angstroms.

Since the entire range of color for the titanium oxide layer is within 25 billionths of a meter (a nanometer is 1-billionth of a meter), it is little wonder that the process requires careful precision and high-quality anodizing equipment to reach the best results.

Electrochemical processing controls the thickness of the titanium oxide layer

The process of manipulating the oxide layer of titanium is an electrochemical process: It uses electricity and chemistry. The titanium part, serving as the anode (positive electrode), is immersed in an aqueous electrolyte solution such as trisodium phosphate (TSP) or various salts. When electrical current is applied, water molecules undergo hydrolysis and split into hydrogen and oxygen. The electrical potential forces the oxygen to the titanium surface, adding to the thin layer of titanium oxide. The final perceived color depends on the thickness of the oxide layer, which can be adjusted by varying the voltage and the immersion time.

How to do Type 3 color titanium anodizing

Chemistry

In Type 3 color anodizing, a titanium part is immersed in an electrolyte solution. The most common chemistry uses trisodium phosphate (TSP), forming an alkaline solution; however, there are many options. The chemistry serves to provide ions for the electrolytic process, but it does not directly affect the titanium surface.

Electricity

The electrical part of the anodizing process requires a specialized rectifier, an electrical device that converts alternating current (AC) to direct current (DC). The rectifier allows you to regulate the voltage to a level that you specify and to control the amperage.

In titanium anodizing, the voltage dictates the color. The volt range for color titanium anodizing is between 15 and 110 volts. A bronze color, with the thinnest oxide layer, can be achieved with about 16 volts. A green color, with the thickest oxide layer, can be achieved with 106 volts.

In contrast, the amperage dictates the time required. By increasing the amperage, the oxide layer forms more quickly. But the increased amperage carries a risk of inconsistency in color output, because it can become too difficult to stop the oxide layer buildup at the correct color. Product size drives the amperage; typically, the larger the product, the more amperage is used.

Process steps for Titanium Anodizing Equipment

Best Technology and its partners have developed custom chemical blends of solutions and electrolytes that greatly increase the manufacturing yield of this process, which are not offered by anyone else in the industry. The chemistry for a titanium anodizing line includes both cleaning and electrochemical electrolytes.

For manufacturers looking to take their titanium color anodizing in-house and build their own Type 3 color anodize line, here is an example of process steps:

1. High alkaline clean – A general washing in a high-alkaline cleaner ensures that any organics are removed. Organics block the anodize prep step and prevent an even, consistent color.
2. Cold rinse – Ensure any alkaline detergents are removed from the part. It’s recommended to perform a water-break test. (Water should sheet off the part, not bead off. Beading off means there is still oil or grease on the part and will cause failure in anodizing.)
3. Hot rinse
4. Anodize Prep – Preparing the surface of the titanium to receive the oxide layer is the most important step of the process. To achieve consistent color, the oxide layer must be distributed evenly throughout the surface of the part. There are many different options for prep chemistry, for example:
   • Nitric and hydrofluoric acid
   • Nitric acid
   • Hydrochloric acid, etc.
5. Cold rinse
6. Alkaline neutralization
7. Cold rinse
8. Anodize Process – Sodium bicarbonate
9. Cold rinse
10. Hot rinse

How to get the best results with color titanium anodizing

Surface preparation, also called anodize prep, is crucial for achieving consistent color for titanium. The titanium surface must be prepared in a way that supports the even distribution of the oxide layer. Anodize prep requires removing a thin layer of material to create a uniform surface for color anodizing. The titanium grade determines the level of prep; the purer grades of titanium such as grade 2 require more aggressive surface prep than titanium alloys such as grade 5, titanium 6AI-4V.

Timing is critical after surface preparation. The anodize process should occur almost immediately after the anodize prep. Otherwise within 6-8 hours of exposure to oxygen (air, water) the oxide layer will form apart from anodizing. Anodizing parts after the oxide layer has already formed results in a splotchy color pattern that is not desirable.

Surface finish is one of the biggest factors that can affect the appearance of color. If the part is machined poorly, some of the surface areas can become work-hardened or smeared. Upon application of electricity, the work-hardened or smeared area slows down the electrical current, and the resulting color is inconsistent and not harmonious. For example, the titanium part might have magenta coloration with gold spots, or a blue coloration with bronze spots.

The surface finish also dictates the brilliance of the final color. In other words, if the titanium part has a blasted finish before anodizing, the finished part will continue to have a matte appearance after anodizing. Unlike Type 2, the titanium color anodize process does not affect the brilliance of the surface finish.

Recovering from mistakes in color titanium anodizing

Because anodizing involves manipulating the oxide layer on titanium, that layer can easily be stripped with a high-alkaline cleaner. This makes it relatively easy to recover from mistakes in color anodizing. To recover from the following types of issues, strip the oxide layer with a high-alkaline cleaner, and then repeat the anodize prep and anodize process.

• Over-anodizing: For example, if a part reaches a magenta color but was intended to be a gold color.
• Non-harmonious color or splotchy color due to delay between anodize prep and anodize process.

Processing the parts in titanium anodizing equipment specifically made for color titanium anodizing helps control the process to achieve consistent titanium colors.

Equipment for anodizing titanium

Medical device manufacturers depend on Best Technology’s expertise for process design and development with anodizing machines that are designed for safe use with the required chemistries.

Best Technology has the experience and the resources to build Ti anodize tanks and equipment, and to develop processes for Ti anodizing applications both large and small. Let our application engineers design a titanium anodizing line for your products today!