Anodic oxidation technology of titanium and its alloy

Anodic oxidation is a commonly used surface protection treatment method for titanium and its alloys. Anodic oxidation of titanium refers to the method of generating oxygen on the anode in an electrochemical way and reacting with the surface of the anode titanium to form an oxidation film, also known as titanium electrode with metal oxide coating, it was first used in chlor-alkali production and has been widely used in chemical industry, environmental protection, water electrolysis, water treatment, electrometallurgy, electroplating, foil production, organic synthesis, electrodialysis, cathodic protection and other fields.

There are several ways to anodize titanium workpiece. At present, the anodic oxidation of titanium and titanium alloys is mainly carried out in acidic solution. The color, thickness and properties of the anodized film are different from the anodizing solution and process conditions. Place the titanium workpiece into the anodizing bath which can be regarded as an electrolyte solution. Opposite to a plating process, the current flows from the workpiece in the anodizing process. The following is the composition, content and process conditions of various anodic oxidation solutions:

 

• Oxalic acid oxidation

Oxalate: 55 to 60 g/L

PH: 0.5 to 1.0

Temperature: 18 to 25 ℃

The area ratio of the anode and the cathode: 1:10

Anodic oxidation initial current density of 1.0-1.5 min, maintain the current density gradually will continue to oxidation voltage up to 100-120 – v then 30-50 min, current density automatically dropped to 1.0 0.3 A/d ㎡. Oxalic acid extremely oxygen change got oxygen of the film thickness is only 0.2-0.3 (including m, the pale gray, abrasion resistance and corrosion resistance. If the anodic oxidation under 8 ℃, and can improve the wear resistance of oxide film.

 

• Pulsar oxidation

Sulfuric acid： 360-370 g/L

Phosphate： 16 and 32 g/L

Temperature： 0-13 ℃

The anode and the cathode area ratio： 2:1

Pulse time：0.15-0.30s

Pulse frequency：40-120 / min

Current density：30-7 A/d ㎡

The oxidation time ：10-20min

The solution at the beginning of the oxidation temperature was controlled in the lower, at 1.0 to 1.5 min flat steady current flow degree up to 7 a/d ㎡ keep 1.0 to 2.0 min. Allows for 1.0 2.0 A/d electrical flow dense degree of oxygen 40 ㎡ plus or minus 10 min. The thickness of oxide film is 2 um, brown has good wear resistance, corrosion resistance and prevent stickiness.

 

• Thick film oxidation

Sulfuric acid：350-400 g/L

Hydrochloric acid：60-65 g/L

Temperature：40-50 ℃

Current density：2-4 A/d ㎡

At the rate of 0.3A/3dm, the current density was gradually increased to 2-4A /dm, and this current density was maintained to oxidize to the required oxide film thickness. The thickness of the oxide film obtained by this process can reach 20-40 m. With the increase of the thickness of the oxide film, the color of the oxide film changes from gray to gray-black. The oxide film has high hardness and good wear resistance. The oxidation film is porous and has good adsorption. If the colloidal graphite and dry film lubricant are immersed, the wear resistance can be further improved.

 

• Colour anodic oxidation

There are many formulas for anodizing titanium and titanium alloys. The color of oxidant film was obviously affected by the change of process conditions.

Formula①:

chromic anhydride: 120-150g/L

Boric acid: 3 to 5 g/L

Temperature: 18 to 25 ℃

Formula②：

Phosphate: 50-200 g/L

Organic acids: 20-100 g/L

Temperature: 18 to 25 ℃

When formula① was used, the voltage was gradually increased from 5V to 50V within 15MIN, and the oxide film was first light brown, then blue-purple, and finally gold. If the voltage rises steadily to 50V within 1-2min and is maintained for 15MIN, the oxide film changes from light blue to golden yellow. When using formula② for oxidation, gradually increase the voltage from 5V to 9V within 20MIN, and oxidation films of different colors will appear in different voltage segments.

 

When titanium and titanium alloy anodic oxide film does not meet the requirements, you can also use mechanical processing or blowing sand or chemical method to remove the oxide film according to the size and accuracy of the parts, and then re-anodic oxidation.