Anodizing of titanium alloy

Anodic oxidation process is a traditional technology of forming an oxidation film by electrolytic oxidation on the surface of metal and its alloy. The oxide film made by this technology has uniform color, corrosion resistance, strong adhesion and good biocompatibility, and has been widely used in the field of modern biomedicine and aerospace.

The anodic oxidation process of titanium is to use titanium as the anode and other metals such as stainless steel as the cathode. With the help of certain electrolyte, the surface of titanium alloy can be oxidized through electrochemical reaction to form an oxidation film.This layer of oxide film has very obvious reflection and refraction effect on light, and different thickness of oxide film will show different colors, at the same time has protection, so it is an ideal decorative layer and wear resistant layer, widely used in architecture, aviation, medicine and other fields.

Titanium anodization

 Anodizing of titanium alloy

Anodic oxidation coloring of titanium and its alloys

The anodic oxidation coloring process of titanium and its alloy is as follows:

Degreasing (strong alkali series degreasing agent)→ washing → initial pickling (hydrofluoric acid aqueous solution)→ washing → secondary pickling (hydrofluoric acid + hydrogen peroxide aqueous solution)→ washing → anodic oxidation (constant voltage electrolysis in phosphoric acid aqueous solution)→ washing → sealing → drying.

  1. Remove oil

The purpose of degreasing is to remove any oil that may remain on the surface of the titanium during rolling.Because of the poor water permeability of the attached part of the oil, it is easy to appear uneven coloring when pickling the titanium surface. 

  1. Initial pickling

The purpose of initial pickling is to form a pear skin pattern on the surface of titanium and its alloys.The use of 5% weight hydrofluoric acid on the titanium pickling, can promote the formation of pear skin pattern. 

  1. Secondary pickling

The second pickling is to remove the powdery dirt that formed on the surface after the first pickling.In addition, in order to avoid uneven pickling, it is necessary to use hydrofluoric acid and hydrogen peroxide water solution to make the titanium ions into a complex containing titanium to achieve stability after the hydrofluoric acid is dissolved the surface contamination once. 

  1. Anodic oxidation

The aluminum plate was used as the cathode and the pretreated titanium plate was used as the anode for constant voltage treatment.As the voltage rises, the oxide film on the titanium surface thickens, resulting in a variety of color changes. 

Voltage(V) 5 15 30 50 80 90
color Grayish yellow Brown Blue Yellow Purple Green
Film thickness 100 300 500 700 950 1300

Table: relationship between anodic oxidation voltage, color and film thickness

5, Sealed

In order to improve the corrosion resistance, pollution resistance and wear resistance of the anodic oxidation film, titanium and its alloys need to be sealed with hot water, steam, solution containing inorganic salt and organic matter after anodic oxidation coloring.

6, dry

After sealing, wipe the moisture from the workpiece with a clean cotton cloth and let it dry naturally.

In summary, it can be seen that the anodic oxidation process of titanium and its alloy is relatively simple, with abundant surface color and low cost, so it is easy to be popularized in industry, and it is a promising oxidation coloring technology.

Anodized titanium spheres

A new environmental protection titanium anodic  oxidation process

The invention relates to a method for anodizing the surface of titanium and titanium alloy, the purpose of which is to provide a new fluorine-free environmental protection process method, improve the current titanium and titanium alloy anodic oxidation process generally adopts the technology of hydrofluoric acid high toxic substance, in order to meet the requirements of green production safety and sustainable development.The process steps are as follows: pre-anodic oxidation acceptance → installation → deoilification → flowing hot water washing → flowing cold water washing → electrolytic polishing → flowing cold water washing → deionizing water washing → anodic oxidation → flowing cold water washing → flowing hot deionizing water washing → disassembly → drying → inspection.The anodic oxidation coloring process of the invention has low cost, simple operation, can obtain a high quality oxidation film layer with rich color and bright color, and the surface is anti-fingerprint pollution, strong anti-fouling, and has a good decorative effect.

Titanium and titanium alloys have attracted more and more attention due to their unique properties, so they are widely used in various fields such as advanced national defense technology, civil industry and people’s livelihood articles.For example, in the field of aerospace, titanium and its alloys are used in the production of new-generation aircraft due to their small specific gravity, light weight and high specific strength.In medicine, titanium and titanium alloy has good corrosion resistance, good biocompatibility and low exclusion from human body. It is widely used in dental and bone implants, as well as vascular stents and other medical fields, and has become an attractive biological metal material.In terms of architecture, due to the unique characteristics of titanium and titanium alloy, its consumption has been increasing year by year. Some developed countries have also built titanium buildings.In addition, the daily consumer goods such as watches, mobile phone case, laptop case, eyeglass frames, frames and other applications are also expanding.

However, titanium and titanium alloys also have some shortcomings, such as poor wear resistance, more sensitive to hydrogen and molten salt stress corrosion, contact with other metals are prone to contact corrosion, which limits its application scope.Therefore, in order to further expand the application field of titanium, its surface must be modified.Anodic oxidation of titanium and titanium alloy is a surface treatment technology with simple process, low cost and remarkable effect.In the 1960s and 1970s, foreign countries have attached great importance to the study of titanium anodic oxidation process, which has made great progress and been applied in various industries.However, the research on the anodic oxidation treatment of titanium and titanium alloy surface is relatively late in China, and no corresponding national standard has been established so far.

At present, the solution containing hydrofluoric acid (or fluoride) is the main substance used in the acid corrosion activation step in the anodizing process of titanium and titanium alloy.Hydrofluoric acid is present in both corrosion solutions used in the current industry standard for anodizing processes (HB/ z347-2002).Hydrofluoric acid is a highly toxic substance, which is highly volatile into the air. In the process of using it, it is easy to cause some harm to the health of equipment and workers.In addition, it can only be diluted and discharged in the process of discharge, and there is no other treatment method, which is easy to cause a certain degree of environmental pollution.Therefore, it is urgent to develop a new technology of fluorine-free environmental protection anodic oxidation.In addition, the surface of the oxide film obtained by using the traditional titanium alloy anodic oxidation process is easy to be contaminated, and the color is easy to change after fingerprint contamination, and it is not easy to recover after being contaminated, which seriously affects its decorative effect and limits its application range.In a word, there are several problems and difficulties in anodizing and coloring of titanium and titanium alloy.

Current problems in anodizing and coloring process of titanium and titanium alloy:

  • Anodic oxidation process engineering generally contains acid corrosion activation steps, and the general use of the main substances containing hydrofluoric acid (or fluoride) solution, serious pollution to the environment, not conducive to safe production.
  • the surface of the oxide film obtained by the anodic oxidation process is rough, the color is not brilliant enough, and the gloss is not high enough, which limits its application in the decoration field (such as titanium jewelry).
  • the electrolyte used in the anodic oxidation process is generally dominated by phosphoric acid and phosphate, resulting in the difficult treatment of waste liquid.

Difficulties in anodizing and coloring of titanium and titanium alloy:

  • due to the active chemical properties of titanium, there is not only a naturally formed oxide film on the surface of the processed parts, but also a pollution layer.The presence of these layers causes uneven staining, but there are few ways to remove them.
  • The change of process conditions has a great influence on the coloring effect. In order to obtain uniform coloring effect, the conditions must be strictly controlled, including: temperature, current density, voltage, electrolyte composition, etc.
  • The surface energy of anodized film is high, the surface of film is easy to be polluted due to adsorption of smudgy substances, and the color is easy to change due to fingerprint contamination.Additional, be polluted hind is difficult to restore, affect its beautiful thereby, affect adornment effect.

The purpose of the invention is to solve the above difficulties and problems and make up for the deficiency of the current anodic oxidation process;The aim is to provide a cheap, simple, fluorine-free environmental protection process to achieve green production safety and sustainable development.

The process steps of the invention are as follows:

step 1 — oil removal, cleaning
step 2 — electrolytic polishing
step 3 — second cleaning
step 4 — anodizing
step 5 — third cleaning
step 6 — disassembly
step 7 — drying
step 8 — inspection

step 3 is at 20 ~ 30°C, current density 10 ~ 20A · dm”2, voltage 15 ~ 40V, titanium or titanium alloy workpiece as the anode, stainless steel mesh or titanium mesh as the cathode, the working distance is 2 ~ 15mm, the area ratio of Yin and Yang is 1 ~ 10:1, in the non-electrolytic solution electrolytic polishing;

Step 5: at 15 ~ 35°C, current density 5 ~ ioa-dm “2, and voltage 13 ~ 92V, the pre-oxidized titanium or titanium alloy is placed into the phosphorus-free electrolyte for anodizing 2 ~ lOmin.

The deoiling and cleaning steps include deoiling steps, flowing hot water washing steps and flowing cold water washing steps.

The second cleaning step includes a flowing cold water washing step and a deionizing washing step.

The third cleaning step includes a flowing cold water washing step and a flowing hot deionizing washing step.

The titanium is industrial pure titanium and the titanium alloy is TC4 titanium alloy.

Described the water electrolysis liquid is formamide solvent, add in 100 ~ 300 g · Γ amino sulfonic acid, 1 0. 1 to 0. 6 g · L 1 hexadecyl trimethyl ammonium bromide, and 0. Ol ~ 0. 05 – g · L 1 of surfactant.

The phosphorus-free electrolyte is 30 ~ 60g · L — 1 sodium silicate, 10 ~ 15g · L — 1 tartaric acid, 2 ~ 8g · L — 1 propyl

The process of the invention does not use hydrofluoric acid and other toxic and harmful substances, it is green, safe and environmental friendly, and the process steps are few, each step is simple to operate and easy to maintain.Different color anodic oxidation films can be obtained by setting different voltage values.The surface of the oxide film obtained after treatment is smooth, with brilliant color, bright color and uniform without mottery, and the surface of the oxide film is fingerprint-proof and has good decorative effect.In addition, the oxide film is firmly combined with the matrix, with high surface hardness and good corrosion resistance.The electrolyte can be used continuously by adding the main components and has a good prospect of practical application.

Experiment example 1

Specific implementation method

  • alkaline degreasing, at room temperature, in to ion in aqueous solution (IL), in turn, to join the 100g) sodium carbonate and sodium phosphate (20 g), sodium silicate (I0g), after being dissolved completely heated to 80 ° C, and will be processed into IOmmX IOmmX 2 mm size two pieces of the same samples of TC4 titanium alloy in alkaline solution ultrasonic stirring continuously, processing after 15 min, and the flow of deionized water to clean the water film to the surface of the sample homogeneously.
  • Electrolytic polishing liquid was prepared by adding 200g sulfamic acid, IL formamide and 0.5 g hexadecyl trimethylammonium bromide into the electrolytic cell (1.5L) at room temperature.Put the deoiled titanium alloy A into the polishing electrolyte, take the sample as the anode and the stainless steel network as the cathode, apply 15V voltage and IOA · dm_2 current density, and take it out after treatment for 50s.Rinse with flowing deionizing water.
  • For chemical corrosion, titanium alloy B after oil removal was put into a pickling solution consisting of 40mL · l-1 hydrofluoric acid (40%,) and 150mL · l-1 nitric acid (65% ~ 68%, ^%) for treatment for 60s. After removal, it was washed with flowing deionizing water.
  • anodic oxidation coloring. At room temperature, add 50g sodium silicate, 12g tartaric acid and 6g glycerol in a deionized aqueous solution (SOOmL) after stirring, add deionized water to IL to prepare anodic oxidation coloring solution.The titanium alloys A and B treated above were then put into the solution, with the stainless steel plate as the cathode and the sample as the anode. The constant pressure was increased to 53V by A constant current of 5A · dm”2, and the aiiin was removed after treatment and cleaned with flowing deionized water.
  • After drying, the samples treated above were blow-dried with compressed air, and then placed in an oven at 130°C for 30 minutes before being taken out.

Experiment example 2

  • the oil was removed by alkaline washing. At room temperature, sodium carbonate 100g, sodium phosphate (20g) and sodium silicate (15g) were successively added to the deionized water (IL), and heated to 70°C after the solution was completely dissolved. The titanium workpiece was put into the solution and stirred by ultrasonic wave continuously.
  • electrolytic polishing. At 25°C, the titanium workpiece after alkaline washing was put into the electrolytic polishing solution of example 1, with the sample as the anode and the titanium mesh as the cathode. A voltage of 35V and a current density of 15A*dm_2 were applied.After treatment for 6min, remove and clean with flowing deionized water.
  • Anodic oxidation coloring. At 25°C, 45g sodium silicate, 15g tartaric acid and 5g glycerol were added in turn after stirring in deionized water (SOOmL). Then, deionized water was added to IL to prepare anodic oxidation coloring solution.The titanium alloy after the above treatment was put into the solution. The stainless steel plate was used as the cathode and the sample as the anode. The constant pressure was increased to 18V by 20A · dm”2 constant current.Remove it after 3min and clean it with running deionized water.
  • After drying, the samples treated above were blow-dried with compressed air, and then placed in a 100°C oven for 30 minutes before being taken out.

Performance of anodic oxidation film:

It shows that after anodic oxidation coloring, the oxide film obtained from the workpiece in example 2 has a high gloss, bright color, uniform and beautiful.In addition, oxidation film surface anti – fouling, hand touch without leaving marks.

Experiment example 3

  • alkaline washing for oil removal, same as embodiment 2.
  • At 20°C, 200g of sulfamic acid, IL formamide, 0.5 g of hexadecyl trimethylammonium bromide and 0. 5g of surfactant were added into the electrolytic cell (1.5L) to prepare the electrolytic polishing solution.Put the titanium alloy sample after oil removal into the polishing electrolyte, take the sample as the anode and titanium mesh as the cathode, apply 20V voltage and IOA · dm_2 current density, and take it out after treatment for 50s;Clean with running deionized water.
  • Anodic oxidation coloring. At 25°C, add 40g sodium silicate, I0g tartaric acid, and 2g glycerol in a deionized aqueous solution (500mL), then add deionized water to IL to prepare anodic oxidation coloring solution.The titanium alloy after the above treatment was put into the solution. The stainless steel plate was used as the cathode and the titanium alloy as the anode. The constant pressure was increased to 30V by a constant current of 7A · dm-2After treatment for 5min.Finally, the anodized titanium alloy was put into the flowing deionized water to clean and take out.
  • After drying, the samples treated above were blow-dried with compressed air, and then placed in an oven at 110°C for 30 minutes before being taken out.The performance of the anodic oxide film obtained in embodiment 3 is given below.

Example 3: after anodic oxidation coloring, the oxide film obtained has high gloss, bright color, uniform and beautiful.In addition, oxidation film surface anti – fouling, hand touch without leaving marks.

Experiment example 4

  • alkaline washing. At room temperature, sodium carbonate 05g, sodium phosphate Q5g and sodium silicate (15g) were successively added to deionized aqueous solution (IL), and heated to 65°C after being completely dissolved. TC4 titanium alloy samples were put into alkaline washing solution and continuously stirred by ultrasonic wave.At room temperature, the alkaline washed titanium alloy was washed in flowing deionized water for 5 minutes and then removed
  • Electrolytic polishing: at 30°C, the titanium alloy after alkaline washing was put into the electrolytic polishing solution of embodiment 3, with the sample as the anode and the stainless steel network as the cathode. A voltage of 20V and a current density of 15A · dm_2 were applied. After 30s of treatment, the titanium alloy was removed and cleaned with flowing deionized water.
  • Anodic oxidation coloring. At 30°C, 60g sodium silicate, 16g tartaric acid, and 8g glycerol were added successively after stirring in a deionized aqueous solution (500mL). Then, deionized water was added to IL to prepare anodic oxidation coloring solution.The titanium alloy after the above treatment was put into the solution. The stainless steel plate was used as the cathode and the sample as the anode. The constant pressure was increased to 69V by the constant current of 8A MnT2.At room temperature, the anodized titanium alloy was put into the flowing deionized water and cleaned.
  • Dryness, the sameas experiment example

Example 4: after anodic oxidation coloring, the oxide film obtained has high gloss, bright color, uniform and beautiful.In addition, oxidation film surface anti – fouling, hand touch without leaving marks.

Experiment example 5

  • alkaline washing, same as example4
  • Electropolishing, same as example
  • Anodic oxidation coloring. At 25°C, 45g sodium silicate, 15g tartaric acid, and 5g glycerol were added successively after stirring in a deionized aqueous solution (SOOmL). Then, deionized water was added to IL to prepare anodic oxidation coloring solution.The titanium alloy after the above treatment was put into the solution. The stainless steel plate was used as the cathode and the titanium alloy as the anode. The constant pressure was increased to 92V by a constant current of 9A · dm-2.At room temperature, the anodized titanium alloy was put into the flowing deionized water and cleaned.
  • dryness, same as example4

Example 5: after anodic oxidation coloring, the oxide film obtained has high gloss, bright color, uniform and beautiful.In addition, oxidation film surface anti – fouling, hand touch without leaving marks.

 

 

 

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