Method of plating with titanium and a substrate plated with titanium

Description

[0001] This invention relates to a surface treatment method using a corrosion-resistant metal film, and more particularly to methods of plating an iron substrate material with titanium or a titanium alloy by electro deposition and plating aluminium, magnesium or the like with titanium or a titanium alloy such as Ti-Cr-Al by non-electrolytic treatment.

[0002] Titanium and its oxides are excellent in corrosion resistance and in heat resistance and they would be of great industrial utility when employed, as protective films, in surface treatment of other metals. Heretofore, many attempts have been made to utilize them for such purpose, but no satisfactory method has been realized yet on an industrial scale. The main reasons therefor are that the merits of titanium cannot fully be utilized in plating techniques due to the difficulty of growing the titanium film and its porosity, and because titanium is difficult to dissolve for plating. Recently, use has been made chiefly of physical treatment methods such as evaporation or ion plating methods. However in view of the problems of the design of plating equipment and auxiliary instruments in dependence on the composition, configuration or area of the material to be plated, a chemical or electrochemical method using a plating bath is more practical.

[0003] Acids which are used for dissolving titanium to obtain a plating bath are chiefly hydrofluoric or hydrochloric acid. Hydrofluoric acid violently reacts with titanium to dissolve it with ease, but the use of this acid is impractical in that when employed as a plating bath, the resulting solution encounters difficulties in the growth of a plated film, incurs degradation of the material plated and allows mixing of foreign matter into the plated film. It is rather more difficult to dissolve titanium in hydrochloric acid as compared with the hydrofluoric acid. The method of the present invention overcomesthe abovesaid defects and is capable of dissolving titanium to obtain a plating liquid in a short time. Since the time for dissolving titanium is reduced in proportion to an increase in the surface area of the titanium to be dissolved, embodiments of the present invention described later employ sponge titanium and adopt a 35% content of hydrochloric acid, and the pH value of the resulting titanium solution ranges from 1 to 5 in dependence on the content of the hydrochloric acid used. Further, the larger the content of a rare (concentrated) acid becomes, the more the plating when bath is oxidized, so that when a pH value above 5 is desired, glacial acetic acid, glycolic acid (hydroxyacetic acid) or the like is added as a reducing agent to the plating bath to prevent its oxidation. It is possible, of course, to use, as the material to be dissolved, a metal reduced from titanium dioxide or (ordinary) metallic titanium as well as the aforementioned sponge titanium. Titanium which is used as starting material is the metallic titanium, sponge titanium or titanium oxide (Ti0₂) mentioned above. The titanium is obtained by collecting titanium dioxide which is reduced by heating when immersed in acetic acid added with a sugar, in accordance with a standard metal oxide reducing method.

[0004] Titanium is difficult to ionize chemically, but when added with an agent for facilitating its elution, such as hydrofluoric acid, the titanium undergoes vigorous fuming and reaction, which is poisonous and hence is dangerous.

[0005] An object of the present invention is to provide a surface treatment method which overcomes the above said defects of the prior art to obtain, in a short time, a titanium solution without directly heating titanium and chemicals used therewith, and which is capable of forming a highly corrosion-resistant protective film by using the titanium solution or a mixed solution composed principally of titanium.

[0006] The present inventor has previously proposed a titanium plating solution and its manufacturing method and, further, has now discovered a titanium and a titanium alloy plating method which enables the industrial application of such solutions. These methods are very important from the viewpoint of non- environmental pollution. Furthermore, the titanium alloy plating method can be carried into practice by using existing equipment, in particular chromium plating arrangements, taking advantage of the property of titanium that when plated, it is alloyed with another metal to form an alloy film.

[0007] In order to achieve the abovesaid object, according to the present invention, titanium is ionized into titanium trichloride, titanium tetrachloride, titanous sulfate or the like to fit for a particular surface treatment, and then for example a solution of chromic acid anhydride or sodium bichromate and aluminium chloride or nickel sulfate, or nickel chloride is added to the ionized titanium to obtain a mixed solution for the formation of a corrosion-resistant protective film.

[0008] The surface treatment which forms a protecting film of titanium chemically or electrochemically by ionizing it is excellent in terms of industrial applications, but in view of the porosity of the protective film thus formed, it is desirable to employ techniques for correcting the configuration of the film during its growth. Further, aluminium, magnesium or the like which is an active material to be plated, can be coated with a protective film even by a plating treatment in which chromic acid anhydride (Cr0₃), sodium bichromate (Na₂Cr₂0₇.2H₂0) and glycolic acid (HOCH₂COOH) or glacial acetic acid (CH₃COOH) are added to and mixed with the ionic titanium solution applying the active factor of aluminium and magnesium; however the resulting film is not always completely satisfactory as a stable, corrosion-resistant protective film.

[0009] In accordance with the present invention, a titanium alloy (e.g. a titanium-chromium-aluminium alloy) film can be formed by electrolytic or non-electrolytic treatment on an iron, aluminium and magnesium substrates; industrially speaking, a very stable, corrosion-resistant protective film can be formed, fully utilizing the existing facilities.

[0010] According to the invention there is provided a method of plating a metal substrate with titanium or a titanium alloy which comprises treating the substrate in a bath comprising titanium in ionic form in a manner such as to deposit the titanium or titanium alloy in film form on the substrate.

[0011] In the case where the substrate is iron or contains iron the deposit may be formed electrolytically using the substrate as cathode and carbon as anode.

[0012] Alternatively the substrate may be aluminium or magnesium, or have a coating of aluminium or magnesium, in which case the deposit is formed non-electrolytically.

[0013] When an alloy is deposited this may contain for example titanium and one or more metals selected from aluminium, chromium and nickel, or even cobalt. The bath which is used in accordance with the method is preferably held at a temperature from ambient to 45⁰C, and may comprise for example ionic titanium in the form of titanium trichloride, titanium tetrachloride or titanous sulphate, and optionally one or more additives selected from pH adjusting agents, reducing agents, deoxidants and agents for promoting film growth.

[0014] The method may include the additional step of treating the plated substrate with an agent such as sodium silicate for sealing the pores of the film of titanium or titanium alloy.

[0015] Some of the following Examples illustrate the invention.

I. Production of titanium plating 'solutions.

[0016] To produce the titanium plating solutions two vessels were employed,these being vessel A made of ordinary glass for containing the titanium metal in a mixture of hydrochloric acid and water, and vessel B for containing vessel A in hot water at 80 to 100°C. Vessel B was of 2000 cc capacity, cylindrical, and formed from 0.2 mm thick ordinary soft iron plate. To produce the titanium solutions vessel A was placed in vessel B and the mixture in vessel A was boiled at 80 to 100°C for about two hours in order to dissolve the titanium in the hydrochloric acid.

Example A

[0017] 50 g of sponge titanium was immersed in a solution of 150 cc of hydrochloric acid and 500 cc of water in vessel A, which was then placed in'vessel B containing 2000 cc of water at 75°C. The water in vessel B was heated by means of a 300 watt electric heater, and after 25 minutes the temperature in vessel A reached 85°C, thus causing its contents to boil. When the temperature in vessel A reached 92°C (after a further 15 minutes) the heater was turned off, by which time the temperature of the hot water in vessel B was 95°C. By using this technique it is ensured that the temperature of vessel B, and hence vessel A, does not exceed 100°C, thus making it possible to prevent degeneration of the titanium solution and gas generation by excess heating. After switching off the heat source, water was added to the solution in vessel A in an amount to compensate for that evaporated by heating, so that the volume of titanium solution produced in vessel A was equal to the total amount of hydrochloric acid and hot water added initially, that is, 650 cc. By this time, the solubility limit of the solution had already been reached, this being an hour after switching off the heat source and when the water in vessel B was at 78°C and the solution in vessel A was at 79°C. The time from the start of heating to the completion of the production of the titanium solution was an hour and forty minutes.

[0018] In the method described above, the time required for dissolution was found to vary slightly with the amount of acid used: the quantity of acid and the conditions for the titanium dissolution could be adjusted according to the kind and composition of the material to be plated. It is preferred to use hydrochloric acid to produce the plating solution on an industrial scale in a short time, since the resulting solution is substantially non-poisonous and leads to a rapid growth of plated film.

Example B

[0019] 5 to 20% of sulfuric acid was added to the titanium solution produced in Example A to obtain a titanous sulfate atmosphere liquid (Ti₂(SO₄)₃). In this case, since the pH value of the liquid was less than 1, the liquid was not suitable for use as a plating solution; therefore, its pH value was increased to 3 to 7 by using an alkaline additive such as caustic soda, ammonium hydroxide or sodium citrate. Further, a reducing agent such as acetic acid, glacial acetic acid, glycolic acid (hydroxyacetic acid) was added in an amount of about 5%,in order to prevent decomposition and oxidation of the liquid.

Example C

[0020] 3 to 10% of nitric acid was added to the titanium solution produced in Example A to obtain a titanium tetrachloride atmosphere liquid. In this case also, since the pH value of the liquid was less than 1, the liquid was not suitable for use as a plating solution; therefore, the pH value of the liquid was increased to 3 to 11, by using an additive with a pH value above 10, such as caustic soda, ammonium, sodium citrate or sodium silicate. A reducing agent such as acetic acid, glacial acetic acid, glycolic acid (hydroxyacetic acid) was added in amount of about 5% to prevent decomposition and oxidation.

[0021] The titanium solution or titanium-based mixtures according to Examples A, B and C were found to permit stable and minute titanium plating without the necessity of pre-treatment in a potassium cyanide bath as is required by conventional plating technology.

II Formation of corrosion-restistan protective films

Example 1 ,

[0022] Several tests were carried out in which, to 150 cc of a titanium chloride solution with a pH of from 3 to 6.5 was added, as reducing agent, glacial acetic acid or glycolic acid (hydroxyacetic acid) in an amount of 2 to 10% based on the titanium solution. The solution temperature was set in the range 15 to 45⁰C. In each test a member to be plated was immersed in the solution and, with the member as cathode and graphite as anode, a current of up to 600 mA was passed for 30 to 50 minutes. A film was thereby electrodeposited to a thickness of 2 to 7 ^p. The durability and thickness of the electrodeposited films was found to vary with the kind of material to be plated, the current density, the liquid temperature and the pH value. It was also found to be effective to add 1 to 3 cc of phosphoric acid or sodium phosphate or 5 to 40 cc of caustic soda to the plating bath.

Example la

[0023] In one specific test of this series a soft iron plate 0.5 mm thick was used as the substrate and cathode. To the 150 cc of titanium chloride solution was added, as reducing agent, 10 cc of glacial acetic acid.In this case, less than 1 g of tannin (C₁₄H₁₀O₉·xH₂O) or less than 2 g of tartaric acid (C₂H₂(OH)₂(COOH)₂) could also have been used as the reducing agent. A current of 200 mA was applied for 20 minutes at a voltage of 2 V, with the solution being maintained at 45°C and a graphite plate being used as the anode. A film was electrodeposited to a thickness of 3 p when the pH value of the plating solution was 3. In this particular test the treatment was carried out at 45°C; however in other tests temperatures from 45°C down to room temperature were used, when it was found that the treatment required a little more time, In each test, prior to plating, the substrate was polished and then degreased. After plating, the plated substrate was immersed in sodium silicate and heat-treated at 200^uC for 5 minutes in order to seal the pores of the electrodeposited film.

Example 2

[0024] In another series of tests, 10% of sodium acetate (CH₃COONa.3H₂O) and oxalic acid (H₂C₂O₄.2H₂O) was added to a titanium chloride solution having a pH value adjusted to less than 6.5. The solution temperature was held in the range of room temperature to 45⁰C. As in Example la, a soft iron plate served as substrate and cathode, and graphite as the anode. By applying a current of 200 to 600 mA for 20 to 40 minutes, electrodeposited films 2 to 5 µ thick were obtained, The treatments before and after plating were the same as in Example la.

Example 3

[0025] In yet another series of tests, electrodeposited films of a titanium-nickel-aluminium alloy were obtained, by using a mixed plating bath based on a titanium chloride solution having its pH value adjusted to 3.5 to 6.5, a nickel chloride solution and an aluminium chloride solution. The plating bath comprised 40 to 80 cc of the titanium chloride solution, 10 to 80 cc of the nickel chloride solution and 10 to 50 cc of the aluminium chloride solution. The bath, which was held at 45°C, additionally contained 10 to 30 cc of a saturated boracic acid solution and 1 to 3 cc of phosphoric acid. By applying a current of 200 to 600 mA for 30 to 50 minutes, using a substrate metal plate as the cathode, films of 2 to 5 p were electrodeposited. The thickness and durability of the electrodeposited films were found to vary with the pH value of the plating bath, the current density and the kind of material to be plated.

Example 3a

[0026] In one specific test of this series, a soft iron plate 0.5 mm thick was used as substrate and 70 cc of titanium chloride solution, 50 cc of nickel chloride solution and 30 cc of aluminium chloride solution were mixed together as the plating solution. As additives, 20 cc of boracic acid and 2 cc of phosphoric acid were employed. The liquid temperature was held at 45⁰C, and a current of 220 mA was applied at a voltage of 2 V for 40 minutes, using carbon as the anode. The electrodeposited film was 5 µ thick when the pH value of the bath was adjusted to 3. Prior to plating, the - substrate was polished and greased; after plating, the plated substrate was immersed in sodium silicate and heat-treated for 5 minutes to seal the pores of the electrodeposited film. In an endurance and a corrosion resistance test, no change was found in the electrodeposited film after it had been sprayed with salt water; neither was the film oxidized when placed in an atmosphere of sulfurous acid gas in a concentration of 1000 ppm. The purpose of using the mixed plating solution is that in such a mixed bath, the advantages and disadvantages of the components, in terms of the electrodeposition of a film, are complementary to one another. The actual components of the mixture are decided on in consideration of the required growth and density of the electrodeposited film and its required corrosion resistance. For example a solution of cobalt chloride may sometimes be added to the above mixed bath, depending on the intended use of the material in which the film is electrodeposited.

Exampl-e 4

[0027] A titanium tetrachloride plating solution was treated with 5 to 50% of a saturated solution of sodium citrate in glacial acetic acid or glycolic acid to prevent decomposition. Then 5 to 50% of ammonia, ammonium chloride or sodium hydroxide was added to the mixed solution to adjust the pH to 7 to 11.

[0028] Since this method usually employs an alkaline plating bath, the material to be plated is covered with a protective film of an active metal which is effective for non-electrolytic treatment, such as aluminium or magnesium. A similar protective film is also obtainable from plating solutions formed by adding 5 to 20% of sulfuric acid or nitric acid to a solution of titanium tetrachloride to give a titanium sulfate bath atmosphere, or by adding 20 to 50% of glacial acetic acid or sodium citrate to the titanium chloride solution to form aplating liquid which then has its pH adjusted to 7 to 11 by the addition of ammonia, ammonium chloride or caustic soda.

Example 4(a)

[0029] As a specimen to be plated, use was made of a coat member of magnesium AZ91.AZ80 together with a spreader (a spreading and casting member). 20 to 600 cc of glacial acetic acid was added to 500 cc of a titanium chloride solution, and then 50 to 100 cc of a saturated solution of citric acid and 100 to 200 cc of 20 mol% ammonia were added to the mixed solution to produce a plating bath, in which the specimen was subjected to plating for 20 minutes. Before plating, the specimen was greased, and after plating, the plated specimen was rinsed with water for 5 minutes and immersed in sodium silicate, thereafter being subjected to a pore-sealing treatment at 150°C.

Example 4(b)

[0030] The specimen used was the same as that in Example 4(a) 20 cc of a saturated solution of sodium citrate and 30 cc of caustic soda were added to 50 cc of a titanium chloride solution to obtain a plating bath, in which the specimen was subjected to plating for 20 minutes, The treatments before and after plating were the same as those in Example 4(a),

Example 4(c)

[0031] The specimen used was the same as that in Example 4(a). A plating bath was produced by adding 10 cc of glacial acetic acid to 50 cc of the titanium tetrachloride solution of Example 3, and adding 25 cc of a saturated solution of citric acid and 25 cc of aqueous ammonia thereto. The specimen was subjected to plating in this bath for 15 minutes, with the treatments before and after plating being the same as those in Example 4(a).

Example 4(d)

[0032] The solution of titanium sulfate prepared in Example 4 was used on the specimen according to Example 4(a) under plating conditions which were the same as those of Example 4(c).

Example 4(e)

[0033] The specimen used was the same as that in Example 4(a). 40 to 50 cc of sodium silicate (150 g/1) was added to 50 cc of a solution of titanium tetrachloride to produce a plating bath, in which the specimen was subjected to plating for 2 to 4 minutes. The treatments before and after plating were the same as those in Example 4(a).

Example 5

[0034] As plating bath, use was made of a bath produced by adding 5 to 20% of titanium sulfate to a solution of titanium chloride, or a bath of a titanium tetrachloride a produced by adding 3 to 10% of nitric acid to_/solution of titanium chloride.

Example 5(a)

[0035] A cast aluminium member was used as a specimen to be plated, together with a spreader, (a spreading and casting member). The specimen was subjected to plating for 20 to 30 minutes in a plating bath produced by adding 20 to 50 cc of a solution of sodium citrate or sodium silicate and 20 to 50 cc of caustic soda to 50 cc of a titanium tetrachloride solution. Before plating, the specimen was degreased and, if necessary, it was finished by buffing and then degreased and washed. After plating, the specimen was rinsed with water and immersed in sodium silicate, thereafter being subjected to a pore-sealing treatment at 200°C for 5 minutes.

Example 5(b)

[0036] The specimen used was the same as that in Example 5(a). The specimen was subjected to plating, for 30 minutes, in a plating bath formed by adding 10 cc of a solution of sodium citrate (100 g/1) and 30 to 50 cc of aminoacetic acid to 50 cc of a titanium chloride solution having a pH adjusted to 4. The treatments before and after plating were the same as those in Example 5(a).

Example 5(c)

[0037] Various specimens were subjected to plating for 25 to 30 minutes in a plating bath produced by adding 20 cc of sodium citrate (100 g/1), 5 cc of glacial acetic acid and 20 to 40 cc of ammonia to 50 cc of a titanium sulfate solution. Depending on the kind of specimen used, 20 to 30 cc of a saturated solution of alum was also added to the plating bath. The treatments before and after plating were the same as those in Example 5(a).

[0038] The electrodeposited films obtained by the methods described above were subjected to spraying with salt water for 24 hours and to an exposure test for 1000 hours, but no change was found in the films,

Example 6

[0039] A mixed plating bath was produced by adding aluminium chloride, chromic acid anhydride, an antioxidant, a reducing agent and a pH adjusting agent to a titanium chloride solution having a pH of 3 to 6. More specifically, 100 cc of aluminium chloride solution, 200 cc of sodium citrate and 50 cc of glycolic acid were added to 500 cc of the titanium chloride solution, and then 300 cc of chromic acid anhydride and 200 cc of caustic soda were added to adjust the pH of the mixture to 2 to 4.

[0040] A cast member of magnesium AZ91.AZ80, selected as a member to be plated, was placed, together with a spreader, in this plating bath and subjected to plating for 1 to 5 minutes. Before plating, the specimen was polished and degreased or washed for degreasing; after plating it was rinsed with water for 5 minutes and then immersed in sodium silicate, thereafter being subjected to a pore-sealing heat treatment for 2 minutes.

[0041] In accordance with the above method, no buffers need be employed, and an electroplating or non- electroplating technique may be used, depending on the kind of substrate to be treated. Thus an electroplated film obtained by the above method was subjected to a corrosion resistance test during which it withstood sea water spraying for 48 hours. This indicates that a film electrodeposited by the method described above is satisfactory for industrial applications,

[0042] As may be deduced from the foregoing Examples, titanium plating according to the present invention using the defined solutions generally results in the electrodeposition of a highly corrosion- and heat-resistant film; therefore, the plating method of the present invention is applicable to a very wide variety of materials. Depending on the material to be plated, the plating solution may be diluted with water, and may contain an inorganic chemical for promoting the growth of the plated film and for reinforcing it. In this case, in order to prevent the plating solution from being hydrolysed, an appropriate amount of reducing agent, for example an acetic acid system such as (CH2COOH.HOCH2COOH.H2NCH2COOH) or (COONaCH₂C(OH) (COONa)CH₂COONa.2H₂0) is preferably added. Furthermore, it is possible to perform, after plating a pore-sealing treatment, for example using sodium silicate.

[0043] As will be appreciated from the foregoing, the present invjention is characterized by the non-use of potassium cyanide and sodium cyanide in the plating of the substrate materials; this makes it possible to perform titanium plating on an industrial scale which has been impossible hitherto.

[0044] Certain preferred embodiments of the invention are listed below.

[0045] A first preferred embodiment of the titanium plating method of the invention is characterized in that a titanium chloride solution having a pH value of 3 to 6.5 is used as a starting solution for titanium plating, and is added with a reducing agent, preferably in the ratio of 5 to 10% with respect to the former to obtain a plating bath; and a film is electrodeposited on a substrate material, holding the solution at a temperature which is preferably in the range of room temperature to 45⁰C and using the substrate material as the cathode and graphite as the anode.

[0046] A second preferred embodiment of the titanium plating method of the invention is characterized in that a mixed solution of a titanium chloride solution, nickel chloride and an aluminium chloride solution is used as a starting solution for titanium plating and is added with a saturated solution of boric acid and phosphoric acid; and a film is electrodeposited on a substrate material, holding the solution at a temperature which is preferably in the range of room temperature to 45°C and using the substrate material as the cathode and graphite as the anode.

[0047] A third preferred embodiment of the titanium plating method of the invention is characterized in that a titanium chloride solution is used as a starting solution for titanium plating and is added with an additive, preferably in the ratio of 5 to 50% and a pH adjusting agent, preferably in the ratio of 10 to 50% with respect to the former to obtain a plating bath having a pH value of 7 to 11; and a corrosion-resistant protective film is plated by non-electrolytic treatment on an aluminium or magnesium substrate in the plating bath.

[0048] A fourth preferred embodiment of the titanium plating method of the invention is characterized in that a titanium chloride solution is used as a starting solution for titanium plating and is added with sulfuric acid, preferably in the ratio of 5 to 20% with respect to the former and an alkaline additive and a reducing agent to obtain a plating bath having a pH value of 3 to 7; and a corrosion-resistant protective film is plated by non-electrolytic treatment on an aluminium or magnesium substrate in the plating bath.

[0049] A fifth preferred embodiment of the titanium plating method of the invention is characterized in that a titanium chloride solution having a pH value of 3 to 6 is used as a starting solution for titanium plating and is added with nitric acid, preferably in the ratio of 3 to 10% with respect to the former and an alkaline additive and a reducing agent to obtain a plating bath having a pH value of 3 to 11; and a corrosion-resistant protective film is plated by non-electrolytic treatment on an aluminium or magnesium substrate in the plating bath.

[0050] A sixth preferred embodiment of the titanium plating method of the invention is characterized in that a titanium chloride solution having a pH value of 3 to 6 is used as a starting solution for titanium plating and is added with a deoxidant, a reducing agent, aluminium chloride and chromic acid anhydride to obtain a plating bath having a pH value of 2 to 4; and a corrosion-resistant protective film of a titanium alloy is plated by non-electrolytic treatment on a magnesium substrate.

Claims

1, A method of plating a metal substrate with titanium or a titanium alloy characterised in that the substrate is treated in a bath comprising titanium in ionic form in a manner such as to deposit the titanium or titanium alloy in film form on the substrate.

2. A method as claimed in claim 1 characterised in that the substrate comprises iron and the deposit is formed electrolytically using the substrate as cathode and carbon as anode.

3. A method as claimed in claim 1 characterised in that the substrate comprises magnesium or aluminium, or has a coating of magnesium or aluminium, and the deposit is formed non-electrolytically.

4. A method as claimed in any one of the preceding claims characterised in that the alloy contains titanium and one or more metals selected from aluminium, chromium and nickel.

5. A method as claimed in any one of the preceding claims characterised in that the bath is used at a temperature of from ambient to 45°C.

6. A method as claimed in any one of the preceding claims characterised in that the bath comprises titanium trichloride, titanium tetrachloride or titanous sulphate, and optionally a pH adjusting agent and/or a reducing agent and/or a deoxidant and/or an agent for promoting film growth.

7. A method according to any one of the preceding claims characterised in that an additional step is carried out, which step comprises treating the plated substrate with an agent such as sodium silicate for sealing the pores of the film.

8. A substrate whenever plated with titanium or a titanium alloy in accordance with the method as claimed in any one of the preceding claims.