Formulations for surface pretreatment by chemical conversion of oxide layers of titanium or titanium alloys

Abstract

A formulation for surface pre-treatment of titanium or titanium alloys 5 comprising 200 g / l to 400 g / l NaOH and 10 g / l to 150 g / l MGDA in water, wherein the formulation has a pH value of at least 12.

Description

Technical Field

[0001] The present invention relates to a formulation for surface pretreatment by chemically converting an oxide layer of titanium or titanium alloy, and a method for surface pretreatment of an article formed of titanium or titanium alloy, the method comprising contacting the article formed of titanium or titanium alloy with such formulation. Background Technology

[0002] Over the past 30 years, the use of titanium in the aerospace industry has been steadily increasing, with titanium accounting for up to 15% of the structural weight in the latest generation of aircraft (such as the Airbus A350XWB and Boeing 787). This increase can be attributed to problems caused by electrochemical corrosion, necessitating the replacement of aluminum structures at the joints between composite and metal structures. The corrosion resistance and strength-to-weight ratio of titanium and its alloys make them significant for new design concepts. However, titanium materials have exhibited issues with long-term stable adhesion. Other problems include rivet misalignment at titanium rivet heads and cracking impact. Color fading or delamination of fiber-metal laminates are some examples of this type of problem.

[0003] Therefore, surface treatment is the most important step in ensuring a durable bond. To improve durability, surface modification methods for titanium have been developed.

[0004] Mechanical surface treatments such as sandblasting are primarily used to create macroscopically rough surfaces and to remove residues.

[0005] To achieve long-term stable bonded adhesion, physical (e.g., plasma or laser) or wet chemical processes (e.g., etching or anodizing) are commonly used. Anodizing is used in the aerospace industry for pretreating titanium. The most common method for achieving porous oxide layers is chromate anodizing (CAA), which involves a small amount of fluoride in the electrolyte. Nanostructures are generated through localized chemical dissolution (fluoride ions) via controlled field-assisted oxidation and dissolution reactions. CAA produces high resistance to bonding. In addition to anodizing in acidic electrolytes, alkaline electrolytes have also been discussed as pretreatment for structural bonding of titanium. Porous oxide layers can be generated on titanium using sodium hydroxide-based electrolytes. Good durability of bonded adhesion under moisture and stress can be achieved through sodium hydroxide anodizing. DE 3427543 discloses the use of an electrolyte containing sodium hydroxide in conjunction with a complexing agent such as ethylenediaminetetraacetic acid (EDTA) to improve the dissolution rate. High-porosity oxide layers and good long-term durability can be achieved through this NaTESi process.

[0006] DE 102011106764 B4 discloses an anodizing method based on sodium hydroxide, methylglycine diacetic acid, disodium tartrate dihydrate, and pentasodium phosphate.

[0007] In addition, the sol-gel process, the Rocatec method, and the heat-resistant (Pyrosil) treatment are known.

[0008] There is no existing technology that can guarantee long-term stable adhesion to titanium without using an energy source (such as a laser or electric field) or a non-permanently compliant chemical composition.

[0009] Anodizing methods have limitations in application. Due to physics (the Faraday effect), it cannot be used in tubes, cavities, or channels without additional costs. The shading effect can also lead to uneven oxide layer growth. Typically, anodizing parameters need to be adapted to each titanium alloy used.

[0010] Laser processing can produce nanostructured titanium surfaces that exhibit excellent long-term adhesion. However, because laser processing is a line-of-sight method, perpendicular approach to the surface must be ensured. In complex components, this is impractical without significant cost. Laser processing also involves a surface melting process, which, for nanosecond pulse systems, inherently results in a heat-affected zone with properties different from the base material.

[0011] There is no existing technology that can guarantee long-term stable adhesion to titanium without using an energy source (such as a laser or electric field) or a non-permanently compliant chemical composition. Summary of the Invention

[0012] Completely unexpected by those skilled in the art, it has now been proven that a formulation for surface pretreatment of titanium or titanium alloys overcomes the deficiencies of the prior art, the formulation comprising 200 g / L to 400 g / L NaOH and 10 g / L to 150 g / L MGDA in water, wherein the formulation has a pH of at least 12, preferably at least 13. For the purposes of this invention, surface pretreatment should be understood as a chemical transformation of the oxide layer of titanium or titanium alloy. The titanium alloy is primarily composed of titanium by atomic percent. The NaOH content advantageously does not exceed 590 g / L. Using the formulation of this invention, existing oxide layers, whether naturally formed or artificially prepared, can be transformed into nanostructured porous surfaces. Such nanostructured porous surfaces can also be referred to as nanostructured networks. These nanostructured porous surfaces enable long-term stable adhesion of organic coatings to titanium substrates. Furthermore, the formulation according to the invention is free of buffers (such as citrate-citric acid buffers), sulfates, and enzymes (especially amylases or proteases). The surfactant content is very low and does not exceed 5% by weight of MGDA. The formulations according to the invention are also free of lead-containing reagents and silicates. The formulations according to the invention allow for the treatment of titanium or titanium alloy surfaces in a current-free manner and / or at low temperatures. MGDA is methylglycine diacetic acid, and is also used to refer to salts of this acid, such as its trisodium salt. This acid has the following structural formula.

[0013] (HOOC-CH2-)2N-CH(CH3)-COOH.

[0014] It is a biodegradable water softening additive used in dishwashing detergents, developed to avoid the less environmentally friendly use of phosphates or poorly biodegradable water softeners in machine-washing dishwashing detergents.

[0015] The values ​​described in this document are based on the trisodium salt of MGDA.

[0016] The present invention also includes a method for surface pretreatment of articles formed of titanium or titanium alloys, the method comprising contacting the articles formed of titanium or titanium alloys with a formulation at 20°C to 80°C for 5 to 60 minutes, the formulation comprising 200 g / L to 400 g / L of NaOH and 10 g / L to 150 g / L of MGDA in water, wherein the content of other inclusion substances is less than 1 g / L and the pH value of the formulation is at least 12, preferably at least 13.

[0017] This contact can be achieved through immersion, spraying, or coating. The method according to the invention can produce nanostructures with dimensions below 100 nm, thereby achieving mechanical and chemical anchoring through increased surface area, thus improving durability and adhesion properties on such treated titanium or titanium alloys.

[0018] Preferably, in the formulation according to the invention, the fluoride content, based on the fluoride contained in the formulation used, is undeterminable or less than 0.001% by weight. Also preferably, the NaOH content is 300 g / L to 375 g / L, preferably 350 g / L, and the MGDA content is 30 g / L to 100 g / L, preferably 60 g / L. Also preferably, the formulation according to the invention contains a certain amount of a polymeric thickener. Preferably, xanthan gum or agar is used as a thickener and / or the thickener is present at a concentration of 2 g / L to 40 g / L, preferably 10 g / L to 15 g / L. Also preferably, the content of other contained substances in the formulation according to the invention is less than 0.5 g / L, preferably 0.3 g / L. Also preferably, the contact is carried out by impregnation. Also preferably, the contact is carried out at 40°C to 70°C for 10 to 30 minutes. Also preferably, the contact is carried out at 60°C for 20 minutes.

[0019] Particularly preferably, the method according to the invention includes pretreatment, in particular with a surfactant, to ensure the wettability of titanium or titanium alloys. The method according to the invention also preferably includes post-treatment, particularly by means of rinsing with aquademin, to wash away the formulations according to the invention.

[0020] The product according to the invention is an article formed of titanium or a titanium alloy, which can be obtained by the method according to the invention or by using the formulation according to the invention. Preferably, the article formed of titanium or a titanium alloy according to the invention has a porous layer on its surface, wherein the pores are mostly open and have a number-average pore size of less than 100 nm, preferably 30 nm to 70 nm.

[0021] Additionally, the present invention includes an aircraft, particularly an airplane, having articles made of titanium or titanium alloys according to the present invention.

[0022] The above-mentioned and other aspects, features and advantages of the present invention can also be obtained from examples of embodiments, which are described below with reference to the accompanying drawings. Detailed Implementation

[0023] These examples demonstrate different treatments for various titanium alloys. Conditions are given separately. Material samples were cleaned with isopropanol or an alkaline degreasing agent (Metaclean T2001, Experiments 7 and 8 only) prior to treatment. After drying, the samples were treated by immersion in the given aqueous solution without movement or stirring. The solution had a pH of approximately 14. The resulting surfaces were visually evaluated after treatment. Surfaces exhibiting iridescent colors indicate surface modifications (nanostructured surfaces) with characteristic dimensions within the wavelength range of light.

[0024]

[0025]

[0026] Mass loss was observed only in Experiment 5.

[0027] SEM images of the surfaces obtained in Experiment 5 show a sponge-like surface structure with pore sizes ranging from 30 nm to 100 nm. Here, the pore walls are formed as open and irregular meshes.

[0028] In the rolling peel test according to DIN 2243-2, it showed excellent adhesion compared to untreated titanium plates: only adhesive layer failure occurred at room temperature, with no adhesive detachment, and 95% adhesive failure occurred at -55°C. (Surface pretreatment was performed by alkaline cleaning, followed by treatment with an HNO3 / HF mixture, then similar to Example 5, Ti peel plate: 300*210*0.4mm, primer BR 127 (Cytec), adhesive FM 94 (Cytec); room temperature: 174.9N (100% cohesive failure); -55°C, 141.2N (95% cohesive failure).

[0029] It should be noted that the described implementations are merely illustrative and not restrictive.

[0030] While the invention has been shown and described in detail in the embodiments and the foregoing description, it should be noted that such showing and description are merely illustrative or exemplary and not restrictive, so that the invention is not limited to the disclosed embodiments. In this document, the word "having" does not exclude other elements, and the indefinite article "a" does not exclude a plurality.

[0031] The mention of certain features in different embodiments does not in itself limit the subject matter of the invention. Combinations of these features can also be used advantageously.

Claims

1. A method for surface pretreatment of articles formed of titanium or titanium alloys, the method comprising contacting the articles formed of titanium or titanium alloys with a formulation at 20°C to 80°C for 5 to 60 minutes, the formulation comprising 200 g / L to 400 g / L NaOH and 10 g / L to 150 g / L MGDA in water, and the formulation having a polymeric thickener present at a concentration of 2 g / L to 40 g / L, wherein the content of other inclusions is less than 1 g / L and the pH of the formulation is at least 12, wherein the surface of titanium or titanium alloys is treated in a current-free manner using the formulation, and wherein the amount of MGDA is calculated as trisodium MGDA.

2. The method according to claim 1, wherein the preparation has a pH value of at least 13.

3. The method according to claim 1 or 2, characterized in that, The content of fluoride in the formulation used is less than 0.001 by weight.

4. The method according to claim 1 or 2, characterized in that, The content of NaOH is 300 g / l to 375 g / l, and the content of MGDA is 30 g / l to 100 g / l.

5. The method according to claim 4, characterized in that, The content of NaOH is 350 g / l, and the content of MGDA is 60 g / l.

6. The method according to claim 1, characterized in that, Use xanthan gum or agar as a thickener.

7. The method according to claim 1, characterized in that, The thickener is present at a concentration of 10 g / L to 15 g / L.

8. The method according to claim 1 or 2, characterized in that, The content of other internal substances is less than 0.5 g / l.

9. The method according to claim 8, characterized in that, The content of other internal substances is less than 0.3 g / l.

10. The method according to claim 1 or 2, characterized in that, The contact is achieved through immersion.

11. The method according to claim 1 or 2, characterized in that, The contact was performed at 40 to 70°C for 10 to 30 minutes.

12. The method according to claim 1 or 2, characterized in that, The contact was performed at 60°C for 20 minutes.

13. An article formed of titanium or a titanium alloy, said article being obtainable by the method according to any one of claims 1 to 12.

14. The article formed of titanium or titanium alloy according to claim 13, wherein the article has a porous layer at its surface, wherein the pores are mostly open and have a number-average pore size of less than 100 nm.

15. The article formed of titanium or titanium alloy according to claim 14, wherein the pore has a number-average pore size of 30 nm to 70 nm.

16. An aircraft having articles formed of titanium or titanium alloys according to any one of claims 13 to 15.

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