A method for preparing a surface blueing film on a titanium alloy substrate by a thermal oxidation method

Abstract

This invention discloses a method for preparing a bluing film on a titanium alloy substrate by thermal oxidation. First, a bluing solution is coated onto the surface of the titanium alloy substrate. Then, a passivation bluing film containing TiN, TiO2, and Ti4O7 is formed on the titanium alloy surface through high-temperature thermal oxidation. Subsequent saponification treatment yields a bluing film with uniform thickness and strong adhesion. This method effectively reduces the surface energy of the titanium alloy substrate, increases the corrosion potential of the titanium alloy, and reduces the corrosion current on its surface, thereby achieving the overall improvement of the corrosion resistance of the titanium alloy. Simultaneously, it endows the titanium alloy with good wear resistance.

Description

Technical Field

[0001] This invention belongs to the field of titanium alloy surface treatment technology, specifically relating to a method for preparing a bluing film on a titanium alloy substrate by thermal oxidation. Background Technology

[0002] Titanium is a relatively new metal that emerged on the world's technological stage after World War II. It comprises 0.6% of the Earth's crust, ranking fourth among structural metals after aluminum, iron, and magnesium. TC4 titanium alloy was the first practical titanium alloy, accounting for 75%–85% of all titanium alloys used. It is characterized by low density, high specific strength, high specific stiffness, and good high-temperature mechanical properties, making it widely used in aerospace, marine, and other fields. However, titanium alloys suffer from low hardness, low thermal conductivity, and poor wear resistance. Furthermore, as a two-phase metal, the potential difference between the α and β phases of TC4 titanium alloy can lead to the formation of galvanic cells, accelerating corrosion and thus limiting the development of titanium alloys.

[0003] Blue coating has excellent properties such as inhibiting metal surface corrosion and having a beautiful appearance, making it an indispensable protective technology in the production of mechanical parts. It has been widely used in hardware tools, molds and cutting tools, standard parts, spare parts, and military equipment.

[0004] High-temperature chemical oxidation bluing is a traditional bluing method, and its process is very old and mature, especially for high-temperature hot alkaline bluing. Current research focuses on two aspects: firstly, reducing the concentration of alkali or nitrite to save energy, protect equipment, and reduce pollution; and secondly, using a secondary bluing process to overcome certain defects in the bluing film. Furthermore, traditional degreasing processes generally use alkaline degreasing, which is inefficient and time-consuming; research on acidic degreasing is now underway. Acidic degreasing agents are low-cost, less polluting, and highly efficient, making them compatible with rust removal processes and saving energy.

[0005] Yu.I. Kuznetsov et al. investigated the formation characteristics of magnetic iron oxide coatings on low-carbon steel in hot alkaline salt solutions. Unlike traditional high-temperature hot alkaline bluing, they used NH4NO3 solution and a GAMC-5 additive instead of NaOH and NaNO2 solutions, significantly reducing the bluing temperature from around 140℃ to 70–98℃. The corrosion resistance of the coating was determined using electrochemical testing and analysis methods, and digital images were processed using atomic force microscopy and scintillation noise spectroscopy (FNS) to obtain the surface structure of the coating at different nanoscale ranges. The results reveal a fundamental possibility for addressing the standardization of coating surface corrosion resistance based on analysis of surface profiles at the nanoscale.

[0006] He Chengli et al. studied the composition and operating conditions of alkaline chemical oxidation solutions for steel parts. By adding a certain concentration of GCHF-1 additive to the oxidation solution, the improved oxidation process shortened the oxidation time and improved product quality and work efficiency. Lowering the working solution concentration effectively prevented red ash buildup. The consensus is that red ash is caused by ferric hydroxide adhering to the oxide film surface. However, when the iron content is too high, it affects the film formation rate and makes red ash more likely to appear on the oxide film. Therefore, the alkaline chemical oxidation solution for steel parts needs to contain a certain amount of iron, generally controlled at 1–2 g / L, to ensure a dense film and strong adhesion.

[0007] To reduce the amount of NaNO2 used, Li Xiancheng et al. employed a dual-oxidizing agent method to oxidize the oxide film into a bluish color. Sodium nitrite in the oxidation solution could be partially replaced by sodium nitrate, with a NaNO3 concentration of 50–100 g / L and a NaNO2 concentration of 150–200 g / L. Sodium nitrate produced a matte, deep black oxide film, while sodium nitrite gave it a glossy bluish-black color. The combined effect of the dual oxidizing agents improved the appearance and corrosion resistance of the oxide film. Furthermore, adding a small amount of auxiliary oxidizing agent, such as sodium molybdate, could further enhance the blackness and gloss of the oxide film.

[0008] Room temperature bluing processes, due to their advantages of low cost, high efficiency, energy saving, and low pollution, will gradually replace hot alkaline bluing. Current research focuses primarily on the following aspects:

[0009] Firstly, the bluing mechanism needs further study. The room-temperature bluing process is quite complex, and its mechanism remains unclear, which has negatively impacted process development. Therefore, it is crucial to study the film-forming mechanism using multiple methods.

[0010] Secondly, the development of novel low-toxicity or non-toxic bluing solutions is crucial. Traditional bluing film-forming agents mainly focus on the selenium-copper system. However, since selenium is a toxic substance and easily pollutes the environment, selenium-free room-temperature bluing will undoubtedly be the focus of future research. Furthermore, there has been considerable research on auxiliary film-forming agents, pH buffers, complexing agents, and surfactants in additives.

[0011] Third, it is organically combined with other processes to leverage their strengths and compensate for their weaknesses, thereby improving the performance of the bluing film.

[0012] Zhang Fayu et al. used ammonium molybdate to form a molybdate-copper salt system to replace selenite, a toxic substance, in the reaction with copper sulfate to form a film. They then added sodium citrate as a complexing agent and OP-10 emulsifier as an additive. They investigated the mechanism of room-temperature selenium-free bluing and the influence of various parameters on the quality of the bluing film, and developed a new room-temperature selenium-free blackening process formula. This improved working conditions and enhanced the color, adhesion, and corrosion resistance of the blackened film.

[0013] In summary, the currently used bluing technology has several drawbacks, including complex process operation, long bluing time, reduced work efficiency, brittle bluing film, uneven thickness, and poor adhesion and corrosion resistance. Summary of the Invention

[0014] The purpose of this invention is to provide a method for preparing a bluing film on a titanium alloy substrate via thermal oxidation. This method forms a uniformly thick and strongly adherent bluing film on the surface of the titanium alloy substrate, aiming to improve the surface properties of the titanium alloy. By enhancing wear resistance, it expands the application of titanium alloy materials in more fields; by improving corrosion resistance, it adapts to more complex and harsh service environments. The resulting titanium alloy with the bluing film has a self-corrosion potential of -0.17 to -0.03 V and a corrosion current density of 0.18 to 0.92 μA / cm². 2 Friction loss ≤66mg.

[0015] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0016] A method for preparing a bluing film on a titanium alloy substrate by thermal oxidation includes the following steps:

[0017] 1) Preprocessing

[0018] After grinding, degreasing, and cleaning the surface of the titanium alloy substrate, it is ready for use.

[0019] 2) Coating and baking

[0020] The bluing solution is applied to the surface of the titanium alloy substrate, and then the bluing solution is spread evenly on the surface of the titanium alloy substrate using an extrusion roller. The bluing solution is applied repeatedly 3 to 8 times, and then baked at 120 to 150°C for 5 to 10 minutes.

[0021] The bluing solution comprises: NaOH: 270-320 g / L, NaNO2: 120-160 g / L, and silane coupling agent: 30-60 g / L;

[0022] 3) Heat treatment

[0023] The baked titanium alloy sample was placed in a tube furnace, and after evacuation, a nitrogen-oxygen mixture was introduced. The pressure inside the tube furnace was controlled at 0.8 to 2 standard atmospheres. Then, the temperature was raised to 500 to 800°C at a heating rate of 5 to 13°C / min and held for 1 to 4 hours. After that, the sample was cooled to room temperature with the furnace. The volume fraction of nitrogen in the mixture was 70 to 90%.

[0024] 4) Saponification treatment

[0025] Clean the surface of the heat-treated titanium alloy, and then put it into soapy water with a concentration of 40-50 g / L for saponification treatment. The saponification treatment temperature is 353-363 K and the saponification treatment time is 10-20 min.

[0026] 5) Post-processing

[0027] Wash and dry.

[0028] Preferably, in step 1), the grinding is performed using a grinding wheel machine with a grinding wheel speed of 400-800 r / min.

[0029] Preferably, in step 1), the degreasing temperature is 70-90°C and the degreasing time is 10-15 minutes.

[0030] Preferably, in step 1), the degreasing is performed using a mixed solution of sodium hydroxide and sodium carbonate.

[0031] Preferably, the molar ratio of sodium hydroxide to sodium carbonate in the mixed solution is 1:4 to 6.

[0032] The titanium alloy with a bluing film described in this invention has a self-corrosion potential of -0.17 to -0.03 V and a corrosion current density of 0.18 to 0.92 μA / cm². 2 Friction loss ≤66mg.

[0033] The method of preparing a bluing film on the surface of titanium alloys in this invention is fundamentally different from other methods of adding protective coatings to the surface of titanium alloys. This invention utilizes the spontaneous growth of the titanium alloy substrate, combined with additives and heat treatment, to induce the spontaneous growth of a dense and highly corrosion-resistant passivation film on the surface of the titanium alloy substrate; it is not a newly prepared coating. Because this passivation film grows spontaneously, its adhesion to the metal substrate is excellent.

[0034] The bluing solution used in this invention contains an additional silane coupling agent. The silane coupling agent can combine with the titanium alloy substrate, induce passivation of the titanium alloy surface, so that the bluing film can grow better on the titanium alloy surface, thereby giving the bluing film better adhesion to the titanium alloy substrate and making the obtained bluing film more dense inside.

[0035] This invention involves coating a titanium alloy substrate with a bluing solution, followed by high-temperature baking and nitriding / oxygenation at high temperatures. This process creates a composite layer on the titanium alloy surface composed of titanium nitride and titanium oxides. During the heat treatment, the mixed gas contains a high nitrogen content, which leads to the formation of a TiN layer on the titanium alloy surface. TiN is characterized by high hardness, high melting point, good corrosion resistance, and good wear resistance. Furthermore, by using a low oxygen content during the heat treatment, oxygen combines with the titanium substrate in the form of TiO2 and Ti4O7. The titanium suboxide Ti4O7 has advantages such as good acid and alkali resistance and heat resistance, further improving the hardness and wear resistance of the titanium alloy substrate. Moreover, the TiO2 formed at high temperatures is generally gray, while the titanium suboxide Ti4O7 appears blue on the titanium alloy surface, thus creating a brightly colored thin film on the titanium alloy substrate surface, providing a good decorative effect.

[0036] The present invention adds a saponification step after bluing heat treatment on the surface of titanium alloy, which can make the bluing film formed on the surface of titanium alloy smoother.

[0037] The beneficial effects of this invention are:

[0038] This invention employs a bluing solution with added silane coupling agent, combined with subsequent high-temperature nitriding and oxygenation treatments, to form a passivating bluing film containing TiN, TiO2, and Ti4O7 on the surface of titanium alloys. Further saponification treatment yields a bluing film with uniform thickness and strong adhesion, effectively reducing the surface energy of the titanium alloy substrate, increasing its corrosion potential, and decreasing its surface corrosion current, thereby improving the overall corrosion resistance of the titanium alloy. Simultaneously, it imparts excellent wear resistance to the titanium alloy.

[0039] This invention first coats the surface of a titanium alloy substrate with a bluing solution, and then modifies the surface of the titanium alloy substrate by high-temperature thermal oxidation, resulting in the spontaneous growth of a passivation film on the surface of the titanium alloy substrate. This overcomes the technical bottleneck that titanium alloy substrates are prone to combine with oxygen in the air to form titanium oxide films, making them unsuitable for preparing coatings on their surface. The overall method is simple to operate. Detailed Implementation

[0040] The present invention will be further described below with reference to the embodiments.

[0041] The specific process parameters of this invention are shown in Table 1.

[0042] Electrochemical testing: A three-electrode system was used, with the titanium alloy sample obtained by the method of this invention as the working electrode, a platinum electrode as the counter electrode, and a saturated calomel electrode as the reference electrode. A 3.5 wt.% NaCl solution was used as the electrolyte. Potentiodynamic polarization curves were employed for testing, with a scan rate of 1 mV / s and the system controlled in automatic sensitive mode. Changes in potential and current were observed, calculated, and recorded. Specific test results are shown in Table 2.

[0043] The friction loss test of the samples was conducted in accordance with the standard GB-T 12444-2006.

[0044] Table 2 shows that the self-corrosion potential of the titanium alloy with the bluing film is -0.17 to -0.03 V, and the corrosion current density is 0.18 to 0.92 μA / cm². 2 The friction loss is ≤66mg. The self-corrosion potential of the titanium alloy with a bluish film obtained by this invention is -0.17 to -0.03V, which is significantly higher than the self-corrosion potential of ordinary titanium alloys in seawater (-0.5 to -0.7V).

[0045] The corrosion current density of the titanium alloy surface with a bluing film obtained by this invention is reduced to 0.18–0.92 μA / cm². 2 However, when a bluing film was prepared on the surface of titanium alloy using conventional methods, the corrosion current density of the titanium alloy samples obtained was 55–82 μA / cm. 2 The corrosion current density of existing titanium alloys coated with Ni-Cr coatings, which have good corrosion resistance, is 8–48 μA / cm². 2 This demonstrates that the method described in this invention can significantly slow down the corrosion rate of titanium alloys.

[0046]

[0047]

Claims

1. A method for preparing a bluing film on a titanium alloy substrate by thermal oxidation, characterized in that, Includes the following steps: 1) Preprocessing After grinding, degreasing, and cleaning the surface of the titanium alloy substrate, it is ready for use. 2) Coating and baking The bluing solution is applied to the surface of the titanium alloy substrate, and then the bluing solution is spread evenly on the surface of the titanium alloy substrate using an extrusion roller. The bluing solution is applied repeatedly 3 to 8 times, and then baked at 120 to 150°C for 5 to 10 minutes. The bluing solution comprises: NaOH: 270-320 g / L, NaNO2: 120-160 g / L, and silane coupling agent: 30-60 g / L; 3) Heat treatment The baked titanium alloy sample was placed in a tube furnace, and after evacuation, a nitrogen-oxygen mixture was introduced. The pressure inside the tube furnace was controlled at 0.8 to 2 standard atmospheres. Then, the temperature was raised to 500 to 800°C at a heating rate of 5 to 13°C / min and held for 1 to 4 hours. After that, the sample was cooled to room temperature with the furnace. The volume fraction of nitrogen in the mixture was 70 to 90%. 4) Saponification treatment Clean the surface of the heat-treated titanium alloy, and then put it into soapy water with a concentration of 40-50 g / L for saponification treatment. The saponification treatment temperature is 353-363 K and the saponification treatment time is 10-20 min. 5) Post-processing Wash and dry.

2. The method for preparing a bluing film on a titanium alloy substrate by thermal oxidation according to claim 1, characterized in that, In step 1), the grinding is performed using a grinding wheel machine with a speed of 400-800 r / min.

3. The method for preparing a bluing film on a titanium alloy substrate by thermal oxidation according to claim 1, characterized in that, In step 1), the degreasing temperature is 70-90℃ and the degreasing time is 10-15 minutes.

4. The method for preparing a bluing film on a titanium alloy substrate by thermal oxidation according to claim 1 or 3, characterized in that, In step 1), the degreasing is performed using a mixed solution of sodium hydroxide and sodium carbonate.

5. The method for preparing a bluing film on a titanium alloy substrate by thermal oxidation according to claim 4, characterized in that, The molar ratio of sodium hydroxide to sodium carbonate in the mixed solution is 1:4 to 6.

6. A titanium alloy with a bluing film prepared by the method according to any one of claims 1 to 5, characterized in that, The titanium alloy with a bluing film has a self-corrosion potential of -0.17 to -0.03 V and a corrosion current density of 0.18 to 0.92 μA / cm². 2 Friction loss ≤66mg.

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