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(11) | EP 0 060 716 A1 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Phosphating metal surfaces |
| (57) lron- or zinc-based metal surfaces, especially on a component having both and iron-based
surface and a zinc-based surface, for example car bodies, are phosphated by dipping
them into an acidic aqueous solution containing:
(a) from 0.5 to 1.5 g/l of zinc ion; (b) from 5 to 30 g/l of phosphate ion; (c) from 0.6 to 3 g/l of manganese ion; and (d) a conversion coating accelerator. The phosphated metal surfaces are then suitable especially for electrocoating. |
[0001] This invention relates to a process for phosphating metal surfaces, and to a composition suitable for use in that process.
[0002] Japanese Patent Publication (unexamined) No.107784/ 1980 (Nippon Paint Co.Ltd.) concerns a method of phosphating iron-based metal surfaces which is particularly suitable for treating manufactured products having complicated-surfaces, such as -automobile bodies. The method is. in use commercially in the automotive-industry for pretreating automobile bodies prior to cationic electrocoating, which is a, coating process now used extensively in this industry. The phosphating method is carried-out by first subjecting the metal surface to a dipping treatment with an acidic aqueous solution containing 0.5 to 1.5 g/l of zinc ion, 5 to 30 g/l of phosphate-ion, and 0.01 to 0.2 g/1 of nitrite ion and/or 0.05 to 2 g/1 of m-nitrobenzenesulfonate ion at a bath temperature of 40 to 70°C for 15 seconds or more, followed by spraying with the above solution for 2 seconds or more.
[0003] Recently, in the automotive industry, with the aim of improving corrosion-resistance after the application of a siccative coating, steel components which are plated on one surface only with zinc or a zinc alloy have come to be used as materials for automobile bodies. when the process of the above Japanese Patent Publication is applied to such materials (i.e. to metal components having both iron-based metal surfaces and zinc-based metal surfaces), the iron-based surfaces are provided with a phosphate coating film having a low film thickness with uniform and dense cubic crystals, as well as excellent adhesion and corrosion-resistance. Such phosphate coating on the iron-based surface is suitable as a substrate for cationic electrocoating. However, in the case of the phosphate coating film formed on the zinc-based surfaces, the resistance to salt water spraying after the application of a cationic electrocoat thereto is insufficient, and secondary adhesion (tested by immersion of the surface bearing the film with cross-hatched scratches in warm water) after the sequence cationic electrocoating - intermediate coating - top coating is greatly inferior to that on the iron-based surfaces.
[0004] In addition to the Japanese Patent Publication, the following references disclose phosphating compositions for metal surfaces:
U.S. Patent 3,338,755 (Jenkins et al) discloses a process for phosphating metal surfaces with a phosphating solution containing zinc, manganese, phosphate, nitrate, and nitrite, as essential ingredients, in stated proportions.
German Patent 29 31 693 (Fosfa-Col) discloses a phosphating process using a solution containing zinc, manganese, phosphate, nitrate, and chlorate ions in stated gram-atom relationships.
[0005] The present invention provides an improved phosphating method for metal surfaces, which is particularly suitable for treating metal surfaces, such as-those of car bodies, which have both iron-based surfaces and zinc-based surfaces. The method is especially advantageous for forming phosphate coating films suitable for electrocoating particularly cationic electrocoating.
[0006] Accordingly, the invention provides a process for phosphating an iron- or zinc-based metal surface comprising subjecting the metal surface to a dipping treatment in an acidic aqueous solution characterised in that the solution contains:
(a) from 0.5 to 1.5 g/1, preferably 0.7 to 1.2 g/1, of zinc ion;
(b) from 5 to 30 g/1, preferably 10 to 20 g/l, of phosphate ion;
(c) from 0.6 to 3 g/1, preferably 0.8 to 2 g/1, of manganese ion; and
(d) a conversion coating accelerator.
[0007] The invention also provides an acidic aqueous composition for phosphating an iron- or zinc-based metal surface characterised in that the composition is this solution.
[0008] The conversion coating accelerator is preferably at least one of the following:
(i) from 0.01 to 0.2 g/l, preferably 0.04 to 0.15 g/1, of nitrite ion;
(ii) from 0.05 to 2 g/1, preferably 0.1 to 1.5 g/l,of m-nitrobenzenesulfonate ion; and
(iii) . from 0.5 to 5 g/1, preferably 1 to 4 g/l,of hydrogen peroxide (based on 100%. H202)
[0009] Optionally, the acidic aqueous solution may also contain one or more of the following:
(e) from 0.1 to 4 g/1, preferably 0.3 to 2 g/l, of nickel ion ;
(f) from 1 to 10 g/l, preferably 2 to 8 g/l, of nitrate ion; and.
(g) from 0.05 to 2 g/l, preferably 0.2 to 1.5 g/l, of chlorate ion.
[0010] Following the phosphating treatment, the metal surface can then be coated with a siccative coating, for example by a known electrocoating process, preferably by the cationic electrocoating process.
[0011] The term "iron- or zinc-based metal surface" as used herein means iron-based surfaces, iron alloy-based surfaces, zinc-based surfaces, and zinc alloy-based surfaces. Zinc- and zinc alloy-based surfaces include, for example, zinc plated steel plate formed by hot dipping, alloyed zinc plated steel plate formed by hot dipping, zinc plated steel plate formed by electroplating and alloyed zinc plated steel plate formed by electroplating.
[0012] An important advantage of the present invention is that surfaces of metal components, such as car bodies, that contain both iron-based surfaces and zinc-based surfaces can be treated by the process of the invention with excellent results. In fact, the process of the invention produces better conversion coatings than are obtainable with conventional dip treating processes, and the amount of etching of the metal surfaces during the present process is only 2/3 to 4/5 that of conventional processes, so that both the quantity of chemicals used in the process as well as sludget formation is only from 2/3 to 4/5 that of conventional processes. The present process is equally applicable to the treatment of a single-metal surface of a type described above.
[0013] A particularly useful method of carrying out the present process is first to degrease the metal surface to be phosphated for example by spraying and/or dipping the metal-surface in a known alkaline-degreasing agent at 50°-60°C for two minutes; washing the metal surface with tap water; spraying and/or dip treating the metal surface with a known surface conditioner at room temperature for 10-30 seconds; dipping the surface into the present acidic aqueous solution at 40°-70°C for at least 15 seconds; and washing the metal surface with tap water followed by deionized water.
[0014] The present acidic aqueous solution has a zinc ion concentration within the range of 0.5 to 1.5 g/1. When the amount of zinc ion is less than about 0.5 g/l, an even phosphate film is not formed on an iron-based surface, and a partially blue-coloured film is formed. When the amount of zinc ion exceeds about 1.5 g/1, then though an even phosphate film is formed, the film that forms on an iron-based surface tends to be in the-form of leaf-like crystals, such as those formed in the spray process, which are unsuitable as a substrate for cationic electrocoating.
[0015] When the amount of phosphate ion in the solutions is less than about 5 g/l, an uneven film results. When the amount of phosphate ion exceeds about 30 g/l, no further improvement in the phosphate film is realized and hence, while not harmful, use of phosphate ion above about 30 g/1 is uneconomical.
[0016] When the amount of manganese-ion is less than about 0.6 g/l, the manganese content in the film formed on the zinc-based surface is insufficient, resulting in inadequate adhesivity of the coating film to the phosphate substrate after cationic electrocoating. When the amount of manganese ion exceeds about 3 g/l, no further improvement in the phosphate coating is realized, and hence it is uneconomical to use amounts in excess of about 3 g/l.
[0017] With respect to the preferred conversion coating accelerators specified above, when they are present in amount less than the designated lower amount, the conversion coating on iron-based surfaces is inadequate, forming yellow rust, etc. When the amount of the accelerator exceeds the designated higher amount, a blue-coloured uneven film is formed on the iron-based surface.
[0018] As an example of a source of zinc ions for use in the practice of the invention, one or more of the following can be employed: zinc oxide, zinc carbonate, and zinc nitrate.
[0019] As an example-of a source of phosphate ions, one or more of the following can be used: sodium phosphate, zinc phosphate, and manganese · phosphate.
[0020] As an example of a source of manganese ions, one or more of the following can be employed: manganese carbonate, manganese nitrate, manganese chloride, and manganese phosphate.
[0021] As an example of a source of conversion coating accelerators, one or more of the following can be employed: sodium nitrite, ammonium nitrite, sodium m-nitrobenzenesulfonate, and hydrogen peroxide.
[0022] With respect to the optional additional ingredients specified above, the addition of the nickel ion results in further improvement in the performance of the phosphate conversion coating, so that the adhesion and the corrosion-resistance of the film produced by cationic electrocoating are also further improved.
[0023] As examples of sources of the optional ingredients, nickel carbonate, nickel nitrate, nickel chloride or nickel phosphate can be used for nickel ions; sodium nitrate, ammonium-nitrate, zinc nitrate, manganese nitrate or nickel nitrate for nitrate ions; and chloric acid, sodium chlorate or ammonium chlorate for chlorate ions.
[0024] The present process is usually carried out at a temperature-in the range of 40° to 70°C, preferably 45° to 60°C. When temperatures below about 40°C are employed, the conversion coating deteriorates, and long periods of treatment time are required to obtain a satisfactory coating. When the temperature is higher than about 70°C, the conversion coating accelerators begin to decompose at an unacceptable rate, changing the composition of the bath and resulting in an unacceptable conversion coating. Also, precipitates begin to form in the bath.
[0025] The duration that the metal surface contacts the solution in the dip treatment is usually at least 15 seconds, and preferably is from 30 to 120 seconds. When treatment times shorter than about 15 seconds are employed, an adequate phosphate film is not formed. In treating metal components having complicated surface profiles, such as car bodies have, the components can advantageously be subjected first to dipping treatments for 15 seconds or more, preferably 30 to 90 seconds, and then to spray treatment with the solution for 2 seconds or more, preferably for 5 to 45 seconds. In order to wash out the sludge which adheres during dipping, the spray treatment is preferably carried out for as long a period within the above range as the speed of the production line will permit. Accordingly, the dipping treatment according to the present invention includes the combination of dipping followed by spraying.
EXAMPLES I -XIV
[0027] EXAMPLES I to VIII are Examples of the process and compositions of the invention. EXAMPLESIX toXIV are Examples using known compositions, given for comparison purposes.
[0028] The treating process used, which is common to all the Examples, is given below, with the aqueous coating compositions of each Example being set forth in Table I, while the metal treated and the test results obtained following the phosphate treatment are set forth in Table 2.
[0029] Samples of all four of the metal surfaces specified in Table 2 were treated simultaneously according to the following procedure:
(a) degreasing, using an alkaline degreasing agent (Nippon Paint Co., "RIDOLINE SD200", 2% by weight) which was sprayed on the metal surfaces at 60°C for 1 minute, followed by dipping in the solution for 2 minutes;
(b) the metal surfaces were then washed with tap water at room temperature for 15 seconds;
(c) the metal surfaces were next dipped into a surface conditioner (Nippon Paint Co., "FIXODINE 5N5", 0.1% by weight) at room temperature for 15 seconds;
(d) the metal surfaces were then dipped into the acidic aqueous solution specified in Table 1 at 52°C for 120 seconds;
(e) the metal surfaces were washed with tap water at room temperature for 15 seconds;
(f) the metal surfaces were then dipped into deionized water at room temperature for 15 seconds;
(g) the surfaces were then dried in hot air at 100°C for 10 minutes. At this stage the appearance and film weight of the treated metal surfaces was determined, with the results set forth in Table 2; and
(h) a cationic- electrocoating material (Nippon Paint., Power. Top U-30 Dark Gray") was coated to- 20 µ thickness onto the treated metal surfaces (voltage 180V., treatment time 3 minutes); followed by baking at 180°C for 30 minutes. - One sample of each electrocoated plate so obtained was subjected to the brine spray test.
[0030] A second sample of each electrocoated plate so obtained was coated with an intermediate coating material (Nippon Paint Co., "ORGA. T0778 Gray") to= 30µ thickness, followed by baking at l40°C for 20 minutes, and a top coating material (Nippon Paint Co., "ORGA T0626 Margaret White.") in 40µ thickness was then applied, followed by baking as above. Accordingly, coated plates with a total of 3 coatings and 3 bakings were obtained. All the thus coated plates were subjected to the adhesion test, and the thus coated cold rolled steel plate also to the spot rusting test.
[0031] The testing procedures referred to above are described below:
(A) Brine spraying test (JIS-Z-2871): Cross-cuts were made on an electrocoated plate; 5% brine was sprayed thereon for 500 hours (zinc plated steel plate) or 1000 hours (cold-rolled-steel plate).
(B) Adhesion test: After dipping a coated plate in deionized water at 50°C for 10 days, grids (100 squares) were made at 1 mm intervals or at 2 mm intervals using a sharp cutter; an adhesive-tape was attached to each surface; and the number of squares of coating film that remained on the plate after the removal of the adhesive tape were counted.
(C) Spot rusting test: A coated plate was set at a 15 degree angle to the horizontal
plane, and an arrow with a cone shaped head with a 90 degree vertical angle, made
of alloyed steel (material quality, JIS-G-4404, hardness Hv 700 or higher), weighing
1.00 g and 14.0 mm in total length was dropped repeatedly from a distance of 150 cm,
until 25 scratches were made on the coated surface. Subsequently, the coated plate
was subjected to 4 cycles of testing, each cycle consisting of first the brine spray
test (JIS-Z-2871, 24 hours), second, a moisture test (temperature of 40°C, relative
humidity 85%, 120 hours), and third, standing at room temperature (24 hours). After
testing, the average value (mm) of the largest diameter of rust spots and blisters
was obtained, with the results shown in Table 2.
(a) from 0.5 to 1.5 g/1 of zinc ion;
(b) from 5 to 30 g/1 of phosphate ion;
(c) from 0.6 to 3 g/l of manganese ion; and
(d) a conversion coating accelerator.
(i) from 0.01 to 0.2 g/l of nitrite ion,
(ii) from 0.05 to 2 g/1 of m-nitrobenzenesulfonate ion, and
(iii) from 0.5 to 5 g/1 of hydrogen peroxide.