The oxide film formed by anodizing titanium and titanium alloys is colorless and transparent. The visible colors result from interference between the two light beams I1 and I2. Different oxide-film thicknesses produce specific colors.
The resistance of anodized titanium colors to contamination, such as color changes after touching, is mainly affected by skin oils adhering to the workpiece surface. The narrower the color range, the more noticeable the influence of the oil-film thickness on the color. Dark blue, blue, magenta and purple generally have relatively narrow color ranges. A rougher surface also retains a thicker oil film, making the color change more apparent.
The colors produced by titanium anodizing are created through optical interference. Therefore, only interference colors can be achieved; colors such as bright red and orange-yellow cannot be produced.
Under normal environmental conditions, titanium naturally forms an oxide layer approximately 5 nm thick. Anodizing can produce an oxide film up to approximately 300 nm thick. A higher anodizing voltage produces a thicker colored oxide film, while a lower voltage produces a thinner film.
Titanium anodizing causes virtually no dimensional change to polished parts. Sandblasted parts require a longer etching time to remove elements introduced by the blasting process. Under normal conditions, approximately 0.01 mm may be removed from screw threads. For sandblasted parts with tight dimensional tolerances, allowance should be reserved for the color-anodizing process.
Before color anodizing titanium and titanium alloys, the surface must be free of scratches, pits, water stains from cleaning, vibration marks caused by ultrasonic cleaning and oxide scale. The surface and internal holes must also be free of oil contamination. Special attention should be paid to oxide transition layers from rough polishing, wire-cut surfaces, welded areas and the original surface of the raw material.
If the product surface contains scratches, pits, water stains from cleaning, vibration marks caused by ultrasonic cleaning, oxide scale, or oil contamination on the surface or inside holes, these defects may remain visible after anodizing. Scratches and pits cannot be concealed by anodizing. Severe water stains may cause blackening or whitening; ultrasonic-cleaning vibration marks may appear white after anodizing; oxide scale may cause uneven colors or prevent proper coloring; and oil contamination may result in localized mottling.
Before EDS analysis of anodized titanium and titanium alloys, all oil contamination must be thoroughly removed. The product should then be rinsed with purified water, wiped and oven-dried. No water stains may remain after drying. Because EDS is a precise surface-element analysis, any substance adhering to the workpiece may be reflected in the results and cause significant measurement errors.
Titanium alloy and commercially pure titanium do not fundamentally change the anodizing result. In general, titanium-alloy products are easier to anodize, while pure-titanium products are more difficult and require greater surface consistency.
For black-gray anodizing of titanium and titanium alloys, the product surface must be free of deep scratches, pits, oil contamination and wire-cut marks. Black-gray anodizing uses a relatively high voltage and produces a more intense surface reaction, so water stains, slight oil contamination and minor oxide scale generally have less influence on the final result.
Black-gray anodized titanium products can be reworked, but the oxide film must not be removed by chemical etching before rework. Because black-gray anodizing uses a high voltage and produces an intense reaction, a relatively rough transition layer forms between the base material and the outer oxide film. If the outer film is removed by etching, this rough transition layer becomes exposed, significantly worsening the surface finish and potentially preventing the part from meeting an implant surface-roughness requirement below 0.8 μm.
