Surface treatment chemicals and bath for aluminum or its alloy and surface treatment method

Abstract: A surface treatment chemicals for aluminum or its alloy consisting essentially of 10-1000 parts by weight of niobium ion and/or tantalum ion, and 1-50 parts by weight of effective fluorine ion, and optionally 10-500 parts by weight of zirconium ion and/or titanium ion and 10-500 parts by weight of phosphate ion. A surface treatment bath is produced by diluting the surface treatment chemicals so as to have a pH of 1.5-4.0. Excellent resistance to blackening by boiling water and adhesion to an overlying polymer coating film can be obtained by a surface treatment using this bath.

Description: BACKGROUND OF THE INVENTION The present invention relates to a chemicals or bath for surface-treating aluminum or its alloy, and more particularly to a surface treatment chemicals or bath suitable for the surface treatment of aluminum cans for drinks. Aluminum and its alloy are conventionally subjected to a chemical treatment to provide them with corrosion resistance and to form undercoating layers thereon. A typical example of such chemical treatment is a treatment with a solution containing chromic acid, phosphoric acid and hydrofluoric acid. This method can provide a coating having high resistance to blackening by boiling water and high adhesion to a polymer coating film formed thereon. However, since the solution contains chromium (VI), it is hazardous to health and also causes problems of waste water treatment. Thus, various surface treatment solutions containing no chromium (VI) have already been developed. For instance, Japanese Patent Publication No. 56-33468 discloses a coating solution for the surface treatment of aluminum, which contains zirconium, phosphate and an effective fluoride and has a pH of 1.5-4.0. Japanese Patent Laid-Open No. 56-136978 discloses a chemical treatment solution for aluminum or its alloy containing a vanadium compound, and a zirconium compound or a silicon fluoride compound. Further, Japanese Patent Publication No. 60-13427 discloses an acidic aqueous composition containing hafnium ion and fluorine ion. With respect to the coating solution disclosed in Japanese Patent Publication No. 56-33468, it shows sufficient properties when it is a fresh solution, namely a newly prepared solution. However, after repeated use for chemical treatment, aluminum is accumulated in the solution by etching of the aluminum plates or sheets with fluorine. A conversion coating produced by such a coating solution does not show high resistance to blackening by boiling water which is used for sterilization, and it also has poor adhesion to a polymer coating film produced by paints, inks, lacquers, etc. Further, the treatment solution disclosed in Japanese Patent Laid-Open No. 56-136978 needs a treatment at a relatively high temperature for a long period of time, preferably at 50-80°C for 3-5 minutes, and the formed conversion coating does not have sufficient resistance to blackening by boiling water and sufficient adhesion to a polymer coating film. In addition, since the formed conversion coating is grayish, it cannot be suitably applied to aluminum cans for drinks. The composition disclosed in Japanese Patent Publication No. 60-13427 is also insufficient in resistance to blackening by boiling water and adhesion to a polymer coating film. EP-A-15020 discloses a process for treating a metal surface by applying a phosphate coating, wherein the surface is wet with the phosphatizing liquid containing at least one metal cation of valance two or greater and at least one metal ion selected from the group consisting of soluble molybdate, tungstate, vanadate, niobate and tantalate ions, and wherein the formed liquid film is subsequently dried in situ. The phosphatizing liquid includes phosphate ions in an amount of 10 to 40 gram per liter and fluorozinconates or fluorotitanates. OBJECT AND SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide an aqueous surface treatment bath for aluminum or its alloy free from the above problems inherent in the conventional techniques, which makes it possible to conduct a surface treatment at a low temperature for short time to provide a conversion coating excellent in resistance to blackening by boiling water and in adhesion to a polymer coating film formed thereon, and which suffers from little deterioration with time, so that it can provide a conversion coating having the above properties even when it is not a fresh one. Another object of the present invention is to provide a composition for preparing such an aqueous surface treatment bath for aluminum or its alloy and a method of the surface treatment of aluminum or its alloy. As a result of intense research in view of the above objects, the inventors have found that a combination of particular proportions of niobium ion and/or tantalum ion, and free fluoride ion, and optionally zirconium ion and/or titanium ion, and phosphate ion can provide a composition for the preparation of a surface treatment bath and bath free from any problems of the conventional techniques. The present invention is based on this finding. Thus, the first composition for preparing an aqueous surface treatment bath for aluminum or its alloy according to the present invention contains as main components 10-1000 parts by weight of niobium ion and/or tantalum ion added as hexafluoroniobates and/or hexafluorotantalates and 1-50 parts by weight of free fluoride ion added as HF, NH₄F, NH₄HF₂, NaF or NaHF₂. The second composition for preparing an aqueous surface treatment bath for aluminum or its alloy according to the present invention contains as main components 10-1000 parts by weight of niobium ion and/or tantalum ion added as hexafluoroniobates and/or hexafluorotantalates, 10-500 parts by weight of zirconium ion and/or titanium ion, 10-500 parts by weight of phosphate ion, and 1-50 parts by weight of free fluoride ion added as HF, NH₄F, NH₄HF₂, NaF or NaHF₂. The first aqueous surface treatment bath for aluminum or its alloy according to the present invention contains as main components in addition to water 10-1000 ppm of niobium ion and/or tantalum ion added as hexafluoroniobates and/or hexafluorotantalates, and 1-50 ppm of free fluoride ion added as HF, NH₄F, NH₄HF₂, NaF or NaHF₂ and has a pH of 1.5-4.0. The second aqueous surface treatment bath for aluminum or its alloy according to the present invention contains as main components in addition to water 10-1000 ppm of niobium ion and/or tantalum ion added as hexafluoroniobates and/or hexafluorotantalates, 10-500 ppm of zirconium ion and/or titanium ion, 10-500 ppm of phosphate ion, and 1-50 ppm of free fluoride ion added as HF, NH₄F, NH₄HF₂, NaF or NaHF₂, and has a pH of 1.5-4.0. The first method of surface-treating aluminum or its alloy comprises the steps of (a) applying to said aluminum or its alloy an aqueous surface treatment bath containing as main components in addition to water 10-1000 ppm of niobium ion and/or tantalum ion added as hexafluoroniobates and/or hexafluorotantalates and 1-50 ppm of free fluoride ion added as HF, NH₄F, NH₄HF₂, NaF or NaHF₂, and having a pH of 1.5-4.0, at a temperature between room temperature and 50°C and (b) washing the treated surface with water. The second method of surface-treating aluminum or its alloy comprises the steps of (a) applying to said aluminum or its alloy an aqueous surface treatment bath containing as main components in addition to water 10-1000 ppm of niobium ion and/or tantalum added as hexafluoroniobates and/or hexafluorotantalates, 10-500 ppm of zirconium ion and/or titanium ion, 10-500 ppm of phosphate ion and 1-50 ppm of free fluoride ion added as HF, NH₄F, NH₄HF₂, NaF or NaHF₂, and having a pH of 1.5-4.0, at a temperature between room temperature and 50°C and (b) washing the treated surface with water. DETAILED DESCRIPTION OF THE INVENTION The composition for the preparation of an aqueous surface treatment bath of the present invention contains particular proportions of substances suitable for the surface treatment of aluminum or its alloy, and it is diluted to a proper concentration as a surface treatment bath. Specifically, the first composition (first aqueous surface treatment bath) contains 10-1000 parts by weight of niobium ion and/or tantalum ion (10-1000 ppm as a concentration in a surface treatment bathe same in the following). When the content of niobium ion and/or tantalum ion is less than 10 parts by weight (10 ppm), a conversion coating-forming rate is extremely low, failing to produce a sufficient conversion coating. On the other hand, when it exceeds 1000 parts by weight (1000 ppm), further improvement due to the addition of niobium ion and/or tantalum ion cannot be obtained. Thus, from the economic point of view, 1000 parts by weight (1000 ppm) of niobium ion and/or tantalum ion is sufficient. The preferred content of niobium ion and/or tantalum ion is 15-100 parts by weight (15-100 ppm). Sources of niobium ion and tantalum ion include hexafluoroniobates and hexafluorotantalates such as NaNbF₆, NH₄NbF₆, NaTaF₆, NH₄TaF₆, etc., and particularly the ammonium salts are preferable. The first composition (first aqueous surface treatment bath) of the present invention further contains 1-50 parts by weight (1-50 ppm), preferably 3-20 parts by weight (3-20 ppm) of free fluoride ion. When the content of free fluoride ion is less than 1 part by weight (1 ppm), substantially no etching reaction of aluminum takes place, failing to form a conversion coating. On the other hand, when it exceeds 50 parts by weight (50 ppm), an aluminum etching rate becomes higher than a conversion coating-forming rate, deterring the formation of the conversion coating. In addition, even though a conversion coating is formed, it is poor in resistance to blackening by boiling water and adhesion to a polymer coating film. Incidentally, the term "free fluoride ion" means isolated fluoride ion, and its concentration can be determined by measuring a treatment solution by a meter with a fluoride ion electrode. Thus, fluoride compounds from which fluoride ion is not isolated in the surface treatment solution cannot be regarded as the sources of free fluoride ion. The suitable sources of free fluoride ion include HF, NH₄F, NH₄HF₂, NaF, NaHF₂, and particularly HF is preferable. The first aqueous surface treatment bath is generally produced by diluting the first composition to a proper concentration. The resulting first aqueous surface treatment bath should have a pH of 1.5-4.0. When the pH of the first aqueous surface treatment bath is lower than 1.5, too much etching reaction of aluminum takes place, deterring the formation of the conversion coating. On the other hands when it exceeds 4.0, the etching reaction rather becomes too slow, deterring the formation of the conversion coating. The preferred pH of the first surface treatment bath is 2.5-3.3. The second composition (second aqueous surface treatment bath) of the present invention further contains, in addition to niobium ion and/or tantalum ion and free fluoride ion in amounts as described above, 10-500 parts by weight (10-500 ppm) of zirconium ion and/or titanium ion, and 10-500 parts by weight (10-500 ppm) of phosphate ion. By adding zirconium ion and/or titanium ion together with phosphate ion, the resulting coating has further improved resistance to blackening by boiling water and adhesion to a polymer coating film. When the content of zirconium ion and/or titanium ion is less than 10 parts by weight (10 ppm), no improvement is obtained by the addition thereof. However, even though it exceeds 500 parts by weight (500 ppm), further effects cannot be obtained. Thus, from the economic point of view, it would be sufficient if it is up to 500 parts by weight (500 ppm). The preferred content of zirconium ion and/or titanium ion is 20-100 parts by weight (20-100 ppm). When the content of phosphate ion is less than 10 parts by weight, no improvement is obtained by the addition of phosphate ion, and when it exceeds 500 parts by weight, the resulting coating rather has a poor resistance to blackening by boiling water and adhesion to a polymer coating film. The preferred content of phosphate ion is 25-200 parts by weight (25-200 ppm). The sources of zirconium ion and titanium ion include complex fluorides such as H₂ZrF₆, H₂TiF₆, (NH₄)₂ZrF₆, (NH₄)₂TiF₆, Na₂ZrF₆, etc., nitrates such as Zr(NO₃)₄, Ti(NO₃)₄, etc., sulfates such as Zr(SO₄)₂, Ti(SO₄)₂, etc., and particularly (NH₄)₂ZrF₆ and (NH₄)₂TiF₆ are preferable. The sources of phosphate ion include H₃PO₄, NaH₂PO₄, (NH₄)H₂PO₄, etc., and particularly H₃PO₄ is preferable. The second aqueous surface treatment bath should have a pH of 1.5-4.0, and the preferred pH is 2.5-3.3 for the same reasons as in the first aqueous surface treatment bath. Incidentally, the pH of each aqueous surface treatment bath may be controlled by pH-adjusting agents. The pH-adjusting agents are preferably nitric acid, sulfuric acid, ammonium aqueous solution, etc. Phosphoric acid can serve as a pH-adjusting agent, but it should be noted that it cannot be added in an amount exceeding the above range because it acts to deteriorate the properties of the resulting conversion coating. The compositions (aqueous surface treatment bath) of the present invention may optionally contain organic chelating agents for aluminum, such as gluconic acid (or its salt), heptonic acid (or its salt), etc. The compositions of the present invention may be prepared by adding the above components to water as an aqueous concentrated solution, and it may be diluted by a proper amount of water to a predetermined concentration with its pH adjusted, if necessary, to provide the aqueous surface treatment bath of the present invention. The application of the aqueous surface treatment bath to aluminum or its alloy can be conducted by any methods such as an immersion method, a spraying methods a roll coat method, etc. The application is usually conducted between room temperature and 50°C, preferably at a temperature of 30-40°C. The treatment time may vary depending upon the treatment method and the treatment temperature, but it is usually as short as 5-60 sec. Incidentally, aluminum or its alloy to which the aqueous surface treatment bath of the present invention is applicable includes aluminum, aluminum-copper alloy, aluminum-manganese alloy, aluminum-silicon alloy, aluminum-magnesium alloy, aluminum-magnesium-silicon alloy, aluminum-zinc alloy, alulminum-zinc-magnesium alloy, etc. It may be used in any shape such as a plate, a rod, a wire, a pipe, etc. Particularly, the aqueous surface treatment bath of the present invention is suitable for treating aluminum cane for soft drinks: alcohol beverages, etc. By treating aluminum or its alloy with the aqueous surface treatment bath of the present invention, the aluminum is etched with free fluoride ion, and forms a double salt with the niobium ion, tantalum ion and fluoride ion to produce slightly soluble coating made of aluminum fluoroniobate and/or aluminum fluorotantalate, thereby forming a strong conversion coating. Strong corrosion resistance and adhesion to a polymer coating layer appears to contribute to the improvement in resistance to blackening by boiling water. The present invention will be explained in further detail by the following Examples and Comparative Examples. In Examples and Comparative Examples, (1) resistance to blackening by boiling water and (2) adhesion to a polymer coating film are evaluated as follows: (1) Resistance to blackening by boiling water Each aluminum can treated with a surface treatment bath is dried, and a bottom portion is cut off from the can, and then immersed in boiling water at 100°C for 30 minutes. After that, the degree of blackening is evaluated as follows: Excel.: Not blackened at all. Good: Slightly blackened. Fair: Lightly blackened (No problem for practical purposes). Poor: Considerably blackened. Very poor: Completely blackened. (2) Adhesion to polymer coating film Each aluminum can treated with a surface treatment bath is dried, and its outer surface is further coated with an epoxy-phenol paint (Finishes A, manufactured by Toyo Ink Manufacturing Co., Ltd.) and then baked. A polyamide film of 40 µm in thickness (Diamide Film 7000 manufactured by Daicel Chemical Industries, Ltd.) is interposed between two of the resulting coated plates and subjected to hot pressing. A 5-mm-wide test piece is cut off from the hot pressed plates, and to evaluate the adhesion of each test piece, its peel strength is measured by a T-peel method and a 180° peel method. The unit of the peel strength is kgf/5 mm. Incidentally, the adhesion measured on a test piece before immersion in boiling water is called "primary adhesion," and the adhesion measured on a test piece after immersion in tap water at 90°C for 7.5 hours is called "secondary adhesion." Examples 1-11 An aluminum sheet (JIS A 3004) is formed into a can by a Drawing & Ironing method, and degreased by spraying an acidic cleaner (Surfcleaner NHC 100 manufactured by Nippon Paint Co., Ltd.). After washing with water, it is sprayed with a surface treatment bath having the composition and pH shown in Table 1 at 40°C for 30 sec. Next, it is washed with water and then with deionized water, and then dried in an oven at 200°C. After drying, each can is tested with respect to resistance to blackening by boiling water and adhesion to a polymer coating film. The results are also shown in Table 1. Comparative Examples 1-8 For comparison, surface treatment baths having the compositions and pH shown in Table 2 are prepared. The same surface treatment of an aluminum can as in Example 1 is conducted by using each surface treatment bath, and the same tests as in Example 1 are conducted. The results are also shown in Table 2. As is clear from the above results, in the case of treatment with the surface treatment bath of the present invention (Examples 1-11), the formed conversion coatings are good in resistance to blackening by boiling water and in adhesion to a polymer coating film. On the other hand, when the content of niobium ion and/or tantalum ion is less than 10 ppm (10 parts by weight) (Comparative Examples 1, 2, 6 and 7) or when free fluoride ion is less than 1 ppm (part by weight) (Comparative Example 3), the formed conversion coatings suffer from poor resistance to blackening by boiling water and adhesion to a polymer coating film. When the pH of the surface treatment bath is less than 1.5 (Comparative Example 4), a conversion coating is not easily formed, and the formed conversion coating is slightly blackened and shows poor adhesion to a polymer coating film. On the other hand, when the pH exceeds 4.0 (Comparative Example 5), the treating bath becomes cloudy because of precipitation, and the resulting conversion coating is slightly poor in resistance to blackening by boiling water and also shows poor adhesion to a polymer coating film. As described above in detail, with the compositions (aqueous surface treatment baths) of the present invention, a conversion coating having extremely high corrosion resistance can be formed on a surface of aluminum or its alloy at a low temperature in a very short time. The conversion coating thus formed is highly resistant to blackening even when immersed in boiling water, meaning that it has excellent resistance to blackening by boiling water even in a thin layer. In addition, when a polymer coating film is formed on the conversion coating by painting or printing, extremely strong adhesion between them can be achieved. Further, since the conversion coating shows good slidability, it is extremely advantageous in conveying. Since the compositions (aqueous surface treatment baths) of the present invention shows sufficient characteristics even though its concentration is varied, it is not required to strictly control the concentration of the surface treatment bath. The compositions (aqueous surface treatment baths) having such advantages are highly suitable for the surface treatment of aluminum cans.

Claims: An aqueous surface treatment bath for aluminum or its alloy containing as main components in addition to water 10-1000 ppm of niobium ion and/or tantalum ion added as hexafluoroniobates and/or hexafluorotantalates, and 1-50 ppm of free fluoride ion added as HF, NH₄F, NH₄HF₂, NaF or NaHF₂, and having a pH of 1.5-4.0. The surface treatment bath according to claim 1, wherein the amount of said niobium ion and/or tantalum ion is 15-100 ppm, of said free fluoride ion is 3-20 ppm, and said bath has a pH of 2.5-3.3. An aqueous surface treatment bath for aluminum or its alloy containing as main components in addition to water 10-1000 ppm of niobium ion and/or tantalum ion added as hexafluoroniobates and/or hexafluorotantalates, 10-500 ppm of zirconium ion and/or titanium ion, 10-500 ppm of phosphate ion, and 1-50 ppm of free fluoride ion added as HF, NH₄F, NH₄HF₂, NaF or NaHF₂, and having a pH of 1.5-4.0. The surface treatment bath according to claim 3, wherein the amount of said niobium ion and/or tantalum ion is 15-100 ppm, of said zirconium ion and/or titanium ion is 20-100 ppm, of said phosphate ion is 25-200 ppm, and of said free fluoride ion is 3-20 ppm, and said bath has a pH of 2.5-3.3. A composition for the preparation of an aqueous surface treatment bath for aluminum or its alloy containing as main components 10-1000 parts by weight of niobium ion and/or tantalum ion added as hexafluoroniobates and/or hexafluorotantalates, and 1-50 parts by weight of free fluoride ion added as HF, NH₄F, NH₄HF₂, NaF or NaHF₂. The composition according to claim 5, wherein the amount of said niobium ion and/or tantalum ion is 15-100 parts by weight, and of said free fluoride ion is 3-20 parts by weight. A composition for the preparation of an aqueous surface treatment bath for aluminum or its alloy containing as main components 10-1000 parts by weight of niobium ion and/or tantalum ion added as hexafluoroniobates and/or hexafluorotantalates, 10-500 parts by weight of zirconium ion and/or titanium ion, 10-500 parts of weight phosphate ion, and 1-50 parts by weight of free fluoride ion added as HF, NH₄F, NH₄HF₂, NaF or NaHF₂. The composition according to claim 7, wherein the amount of said niobium ion and/or tantalum ion is 15-100 parts by weight, of said zirconium ion and/or titanium ion is 20-100 parts by weight, of said phosphate ion is 25-200 parts by weight, and of said free fluoride ion is 3-20 parts by weight. A method of surface-treating aluminum or its alloy comprising the steps of (a) applying to said aluminum or its alloy an aqueous surface treatment bath containing as main components in addition to water 10-1000 ppm of niobium ion and/or tantalum ion added as hexafluoroniobates and/or hexafluorotantalates, and 1-50 ppm of free fluoride ion added as HF, NH₄F, NH₄HF₂, NaF or NaHF₂, and having a pH of 1.5-4.0, at a temperature between room temperature and 50°C, and (b) washing a treated surface with water. The method according to claim 9, wherein the amount of said niobium ion and/or tantalum ion is 15-100 ppm, of said free fluoride ion is 3-20 ppm, and said bath has a pH of 2.5-3.3. The method according to claim 9 or 10, wherein the temperature of said surface treatment bath is 30-40°C, and the surface treatment time is 5-60 seconds. A method of surface-treating aluminum or its alloy comprising the steps of (a) applying to said aluminum or its alloy an aqueous surface treatment bath containing as main components in addition to water 10-1000 ppm of niobium ion and/or tantalum ion added as hexafluoroniobates and/or hexafluorotantalates, 10-550 ppm of zirconium ion and/or titanium ion, 10-500 ppm of phosphate ion, and 1-50 ppm of free fluoride ion added as HF, NH₄F, NH₄HF₂, NaF or NaHF₂, and having a pH of 1.5-4.0, at a temperature between room temperature and 50°C, and (b) washing a treated surface with water. The method according to claim 12, wherein the amount of said niobium ion and/or tantalum ion is 15-100 ppm, of said zirconium ion and/or titanium ion is 20-100 ppm, of said phosphate ion is 25-200 ppm, of said free fluoride ion is 3-20 ppm, and said bath has a pH of 2.5-3.3. The method according to claim 12 and 13, wherein the temperature of said surface treatment bath is 30-40°C, and the surface treatment time is 5-60 seconds.

Inventors: IKEDA SATOSHI, MEGURO SHIGEYUKI, IKEDA, SATOSHI, MEGURO, SHIGEYUKI
Applicants: NIPPON PAINT CO LTD, NIPPON PAINT CO., LTD.
Application year: 1990
Main classification: C23C22
IPC codes: C23C22