Description METHOD OF PRE-TREATING METAL SUBSTRATES PRIOR TO PAINTING

Technical Field

This invention concerns a method of pre-treating various metal substrates, for example steel, steel sheet, zinc based plated steel sheet and aluminum and its alloys. In more practical terms, the invention concerns a method of pre-treat- ing metal substrates prior to painting which provides excellent paint film adhesion in those cases where painting is carried out with a cationic electrodeposition type paint, for example. Background Art

At the present time, phosphate based conversion coating forming liquid compositions and chromate treatments are generally used as methods of pre- treating various metal substrates prior to painting. From among these, the zinc phosphate based conversion coating forming liquid compositions are advantage¬ ous when compared with the chromate treatments, which involve environmentally harmful hexavalent chromium, in that they have excellent effluent treatment properties, and in that various metal substrates, such as cold-rolled steel sheet and zinc based plated steel sheet for example, can be treated at the same time, and these methods are widely used as pre-treatments prior to the painting of automobile bodies and domestic electrical appliances.

The adhesion between the paint film and the base metal (paint film adhe- sion) and the corrosion resistance after painting, in cases where the paint film is damaged in such a way as to reach the base metal, can be cited as typical indi¬ cators of performance required when a metal substrate is painted. When a metal substrate is painted, it is possible to improve the aforementioned aspects of painting performance by subjecting the metal to a zinc phosphate based conver- sion coating forming liquid composition as a pre-treatment prior to painting. Fur¬ thermore, even in systems where painting is carried out after a zinc phosphate based conversion coating is in place, the painting performance can be further im¬ proved by improving the surface characteristics of the zinc phosphate based con¬ version coating itself. A zinc phosphate based conversion coating is comprised in the main of

zinc phosphate crystals (hopeite) but, on iron and steel materials, the iron com¬ ponent which is dissolved out from the base is taken into the film and co-precipi¬ tated in part as zinc iron phosphate crystals (phosphophilite). Zinc iron phos¬ phate crystals are chemically stable when compared with zinc phosphate crys- tals, i.e., they have better resistance to dissolution by acids and alkalies, so that the painting performance can be improved by increasing the co-precipitation rate of zinc iron phosphate crystals. The diffusion of iron ions dissolved out by etch¬ ing is slow, and since dipping methods where iron ions are readily taken up into the film are better than spraying methods for improving the zinc iron phosphate co-precipitation rate, at the present time the zinc phosphate based conversion coating forming pre-treatment prior to painting is generally carried out by dipping.

Furthermore, various investigations have been carried out in connection with means of improving the zinc iron phosphate crystals themselves, and in practical terms, conversion coating processes in which heavy metal ions other than zinc, such as nickel ions and manganese ions for example, are added to the treatment liquid have been widely used, and the painting performance is im¬ proved by incorporating these metals into the zinc phosphate crystals. However, for nickel ions, which have an especially pronounced favorable effect on painting performance, investigations into reducing the amount which is to be used or dis- continuing its use altogether in the future have started for environmental reasons, and it is thought that there is a limit to the improvement of the zinc phosphate based coatings themselves.

On the other hand, the ability of chromate treatments, with hexavalent chromium containing aqueous solutions, as an after-treatment following a zinc phosphate based conversion coating to improve further the performance of the zinc phosphate based conversion coatings has been confirmed, but there are problems with waste water treatment after a chromate treatment.

Furthermore, a method whereby the corrosion resistance and paint adhe¬ sion properties of a metal surface are improved by bringing the metal surface into contact with an aqueous solution or aqueous dispersion of a poly-4-vinylphenol derivative has been disclosed in Japanese Patent Kokai 58-185661 as a resin based after-treatment, but the cost is high since a new after-treatment process

must be established with this treatment technology, and the waste water treat¬ ment of the aqueous resin solution is also difficult. Disclosure of the Invention

Problems to Be Solved bv the Invention This present invention resolves these problems of the prior art and pro¬ vides a method of pre-treating metal substrate prior to painting with which it is possible to reduce the costs, with low polluting characteristics, and to obtain ade¬ quate painting performance (adhesion of the paint film, corrosion resistance after painting) in cases where the improving effect of the film is inadequate and in cases where it is not possible to use a chromate treatment or resin based treat¬ ment as an after-treatment, without adding components for improving the film properties, such as nickel ions, to the zinc phosphate based conversion coating forming liquid compositions.

Summary of the Invention The inventors have thoroughly investigated a double method consisting of a zinc phosphate conversion coating and an after-treatment as a means of re¬ solving the abovementioned problems and improving painting performance. As a result, it has been newly found that (i) by bringing the surface of a metal sub¬ strate, following a zinc phosphate based conversion coating, into contact with an aqueous solution which comprises, preferably consists essentially of, or more preferably consists of, water, copper(ll) cations, counterions for the copper cat¬ ions, and, if needed to adjust the pH to a value from 4 to 6, other acidic or alka¬ line materials, the painting performance, and especially the paint film adhesion, can be greatly improved, and that this after-treatment is more effective than add- ing copper ions directly to the zinc phosphate based conversion coating forming liquid composition. The invention is based upon these findings.

That is to say, this present invention provides a method of pre-treating metal substrates prior to painting, characterized in that the surface of the metal substrate, following treatment with a zinc phosphate based conversion coating forming liquid composition, is brought into contact with an aqueous solution con¬ taining 2 - 200 parts per million by weight (hereinafter usually abbreviated "ppm") of copper(ll) cations at pH 4 - 6. Moreover, the preferred temperature conditions

of the said aqueous solution are 20 - 45 ° C.

More Detailed Description of Preferred Embodiments

The metal substrate which is treated in this present invention is, in the main, steel, steel sheet, zinc based plated steel sheet or aluminum alloy, but it is not limited to these materials.

The aqueous solution which is used in the treatment of this present inven¬ tion (referred to below as the treatment liquid) is an aqueous solution containing copper(ll) cations at a concentration of 2 - 200 ppm, along with some counterions for the copper ions. If the copper ions concentration is less than 2 ppm, then an adequate painting performance usually is not obtained, and if the copper ions concentration exceeds 200 ppm, then not only does the corrosion resistance after painting generally deteriorate but undesirable corrosion of the apparatus is also accelerated. The preferable concentration of copper ions is 5 - 50 ppm. No particular limitation is imposed upon the source of supply of the copper ions, and they can be added in the form of a salt, such as copper sulfate, copper nitrate, copper phosphate or copper chloride for example.

The pH of the treatment liquid should be from 4 -6. In those cases where it is less than pH 4, the zinc phosphate based chemically formed film which has been produced earlier is often dissolved away completely, and, conversely, in those cases where the pH exceeds 6, the copper ions are usually precipitated as hydroxide. Moreover, acid components, such as nitric acid, sulfuric acid, phos¬ phoric acid, and hydrochloric acid for example, or alkaline components, such as sodium hydroxide, sodium carbonate, and aqueous ammonia for example, can be used to adjust the pH of the treatment liquid. Furthermore, the temperature of the treatment liquid is preferably 20 - 45

° C. If it is less than 20° C, then not only is an adequate after-treatment effect not usually obtained, but the treatment liquid may also require cooling, depending on the season of the year, and if cooling apparatus must be provided there is an economic disadvantage. If, on the other hand, the temperature exceeds 45° C, no better after-treatment effect is usually obtained and there is an economic dis¬ advantage because of the cost of the energy required to maintain the tempera¬ ture above normal ambient conditions.

The fundamental zinc phosphate based conversion coating process for a metal substrate generally involves an alkaline degreasing, a first water rinse, treating with a zinc phosphate based conversion coating forming liquid composi¬ tion, a second water rinse, and a rinse with deionized water. The phrase "following the zinc phosphate based conversion coating" in this description of the present invention signifies a process step between treatment with the zinc phos¬ phate based conversion coating forming liquid composition and the rinse with de¬ ionized water. In practical terms, the after treatment with a solution containing copper ions according to this invention preferably is carried out between treat- ment with a zinc phosphate based conversion coating forming liquid composition and the second water rinse or between the second water rinse and the rinse with deionized water, and at least one treatment with a solution containing copper ions is necessary. (Treatment with a solution containing copper ions could also be carried out after the deionized water rinse, but this would be disadvantageous in most cases because another deionized water rinse probably would then be required for maximum benefit.) Moreover, each process step as described above may be a multi-stage treatment, and a surface activation process with an activating agent of the titanium colloid type, as is well known in the zinc phos- phating art, usually is preferably included as a process step prior to the treatment with a zinc phosphate based conversion coating forming liquid composition, with a view to providing finer crystals in the phosphate coating formed.

No particular limitation is imposed upon the method of establishing contact between the liquid treatment compositions and the metal substrates treated in this present invention; both spraying methods and dipping methods can be used, along with any other convenient method. A treatment time, i.e., time of contact between the liquid treatment compositions and the metal substrates, of 10 - 120 seconds is preferred.

Known liquid zinc phosphate based conversion coating forming liquid compositions can be used for the treatment liquid in the zinc phosphate based conversion coating forming process. Thus, a treatment liquid which contains zinc ions and phosphate ions as the main components, at least one species selected from among nitrate ion, nitrite ion and chlorate ion as an oxidizing agent, and

sodium ions for example as neutral ions, can be used. Furthermore, additional metal ions, such as nickel ion and manganese ion for example, and fluoride, for example, as an etching agent, may advantageously be added to the zinc phos¬ phate based conversion coating forming liquid compositions, with a view to im- proving painting performance, as is generally known in the art.

In general, when metal substrates are treated with a zinc phosphate based conversion coating forming liquid composition and painted over the top, the adhesion of the paint film is determined by the strength of adhesion at the in¬ terface between the zinc phosphate based conversion coating thereby formed and the paint film. Hence, the strength of adhesion of the paint film can be in¬ creased by improving the zinc phosphate conversion coating surface to a surface condition with which the strength of adhesion with an organic paint film is strong¬ er. If the adhesion of the paint film is improved, then the progress of corrosion is also suppressed, so that an improvement of corrosion resistance can also be expected.

The method of treatment according to this present invention is aimed at improving the zinc phosphate based conversion coating surface, and it is thought that a surface state which provides good adhesion with an organic paint film is achieved by covering the film surface with a copper compound such as copper phosphate.

However, base metals such as iron, zinc and aluminum are less noble metals than copper, and if these metal substrates are brought into contact with an aqueous solution containing copper ions, some metallic copper is almost al¬ ways deposited on the base metal, because baser metals normally spontaneous- ly dissolve in the presence of ions of nobler metals that can be deposited, in amounts corresponding to the amount of nobler metal deposited. Metallic cop¬ per deposited in this way can result in a deterioration of corrosion resistance, by way of a local electrical cell action with the base metal. Hence, it is advantage¬ ous to suppress the amount of metallic copper deposition to the lowest limit that can be achieved, consistent with the primary objective of improving adhesion. The deposition of metallic copper in excess can readily occur in those cases where copper ions are added to a zinc phosphate based conversion coating

forming liquid composition, and also in those cases where the copper ions con¬ centration becomes excessive in the treatment of this present invention, and this is liable to result in an undesirable deterioration in corrosion resistance.

Some illustrative examples of actual treatments are described below, along with comparative examples, and the effect of the invention is described in more practical terms.

Examples Rectangular cold-rolled steel ("CRS") sheets 70 millimeters (hereinafter usually abbreviated "mm") wide x 150 mm long and 0.8 mm thick (type SPCC- SD), electrogalvanized ("EG") steel sheets (zinc coating: 20 grams/meter ² ) and aluminum alloy ("AA") sheets (type JIS-A5052) were used, and these were sub¬ jected to treatment using the treatment processes described below. Also, the paint film adhesion and corrosion resistance were evaluated with test sheets which had been processed through the final painting, Treatment Process Steps and Conditions

(1 ) Degreasing: Alkaline cleaner "FineCleaner™ L4460" (manufactured by Ni¬ non Parkerizing Co. Ltd.) 43° C, 120 seconds, spray.

(2) Water Rinse: Tap water, ambient temperature, 30 seconds, spray.

(3) Activation: Titanium colloid based surface preparing agent "Preparen™ ZN" (manufactured by Nihon Parkerizing Co. Ltd., ambient temperature,

30 seconds, spray.

(4) Zinc phosphate based conversion coating: A nickel-free treatment liquid consisting of water and: zinc ions, 1.3 grams per liter (hereinafter usually abbreviated "g/l"); manganese ions: 0.5 g/l; phosphate ions, 15 g/l; nitrate ions, 7 g/l; fluosilicate ions, 1.0 g/l; nitrite ions in an amount to yield 3

"points" of accelerant as measured by the saccharometer method; free acid points, i.e., the number of milliliters (hereinafter usually abbreviated "ml") of 0.1 N sodium hydroxide needed to titrate 10 ml of treatment liquid with bromophenol blue as indicator, 0.9; temperature, 43° C; contact time, 120 seconds; contact method, dipping.

(5) Water rinse: tap water, ambient temperature, 30 seconds, spray.

(6) After-treatment: 20 seconds dipping into one of the treatment composi¬ tions shown in Table 1.

Table 1 : COMPOSITIONS OF THE COPPER IONS CONTAINING TREATMENT LIQUIDS

Compo¬ ppm of Cu Chemical Source pH of the Treatment sition Ions in of the Cu Ions Composition Temperature, Number Composition ° C

1 20 Copper sulfate 4.5 35

2 80 Copper sulfate 5.0 30

3 180 Copper nitrate 5.7 22

4 3 Copper sulfate 4.2 42

5 50 Copper chloride 5.0 55

6 220 Copper sulfate 4.2 43

7 1 Copper nitrate 5.8 23

8 100 Copper chloride 3.7 42

(7) Water rinse: Deionized water (electrical conductivity less than 0.2 μS/cm), ambient temperature, 20 seconds, spray.

(8) Draining and drying: 110° C, 180 seconds. (9) Electrodeposition painting with cationic electrodeposition paint "Elecron™ 9210", manufactured by the Kansai Paint Co.; paint bath temperature, 28° C; voltage, 250 Volts; time of electrodeposition, 180 seconds; film thick¬ ness, 20 micrometers; baking, 170° C, maintained for 20 minutes. Evaluation Test Methods

Measurement of Film Weight

The weight in grams of the treated sheet after conversion coating ("W1") was measured, then the film was stripped away from the treated sheet with the stripping liquid and conditions indicated below for the particular substrate metal that had been conversion coated, and the weight in grams ("W2") was measured again. The coating weight was calculated using the following equation: Coating weight in grams/meter ² = (W1-W2)/0.021.

(a) For Cold-Rolled Steel Sheet: stripping liquid 5 % aqueous chromic acid anhydride solution; dip at 75° C for 15 minutes.

(b) For Electrogalvanized Steel Sheet: stripping liquid, ammonium bichromate-25 % aqueous ammonia-pure water in a weight ratio s of 2:49:49 respectively; dip stripping at 75° C for 15 minutes.

(c) For Aluminum Alloy Sheet: stripping liquid, 5 % aqueous chromic acid anhydride solution; dip stripping at ambient temperature for 5 minutes.

Paint Film Adhesion Tests (Water Resistance Secondary Adhesion Tesf) o The test sheets prepared in accordance with the process steps (1 )

- (9) above were immersed in deionized water at 40° C for 240 hours, and then cross cuts which reached the base material were made with a sharp cutter on the test sheets which had been removed from the water. A 3 mm extrusion, centering on the cross cuts, was carried out with an Erich- 5 sen tester, and the degree of peeling of the paint film, after application of cellophane tape to the film and subsequent removal of the cellophane tape together with whatever paint had adhered to it, was evaluated. The state of peeling of the paint film was reported on the following scale, after measuring the proportion of the peeled area with respect to the total ex- o truded area of the test specimen:

+ +: Paint film peeled area fraction less than 15 %.

+: Paint film peeled area fraction more than 15 % but less than

30 %. x: Paint film peeled area fraction more than 30%. 5 Corrosion Resistance Tests (Outdoor Exposure Tests)

Cross cuts reaching the base material were introduced with a sharp cutter on the painted sheets prepared using the process steps (1) - (9) de¬ scribed above, and outdoor exposure tests were carried out. Twice a week, salt water spraying (5 % salt water) was carried out during the out- 0 door exposure. The tests were continued for 9 months, after which the width of the greatest extension of corrosion in the width direction from the cross cuts was measured, and the evaluation was reported on the follow-

ing scale:

+ +: Maximum sideways extending width less than 3 mm. +: Maximum sideways extending width more than 3 mm but less than 6 mm. x: Maximum sideways extending width more than 6 mm.

The results of the evaluations are shown in Table 2. The following facts can be deduced from Examples 1 - 10 and Compar¬ ative Examples 1 - 8.

(1 ) In examples 1 - 10 in which the copper ions concentration in the treatment liquid and the pH of the treatment liquid were within the scope of this invention, the performances with respect to adhesion of the paint film after painting and cor¬ rosion resistance were good, irrespective of the type of base material.

(2) On the other hand, Comparative Examples 1 - 6, in which either the cop¬ per ion concentration of the treatment liquid or the pH of the treatment liquid was outside the range of this invention, did not have adequate painting performance. With Comparative Examples 1 and 2 in particular, where the copper ion concen¬ tration exceeded the upper limit value, the paint film adhesion was good but there was a clear and pronounced deterioration in corrosion resistance, believed to be due to the excessive deposition of metallic copper. (3) Moreover, Comparative Examples 7 and 8 are controls in which the after- treatment following the zinc phosphate based conversion coating was omitted. In Comparative Example 7, there was no change in the composition used to form the zinc phosphate coating, and in Comparative Example 8, copper ions were added in an amount of 100 ppm to the conversion coating forming liquid compo- sition, but in both cases the paint film adhesion and corrosion resistance were poor. Moreover, a blue-colored precipitate of copper hydroxide was inevitably produced on adjusting the pH of an aqueous solution with a copper ions concen¬ tration of 100 ppm to 6.2. Hence, evaluation was impossible with controls where the pH exceeded 6.0.

Table 2: TEST RESULTS FOR THE EXAMPLES AND COMPARATIVE EXAMPLES

Example Treatment Substrate Coating Paint Film Corrosion or Com¬ Composi¬ Treated Weight, Adhesion Resistance parative tion Num¬ grams per Rating Rating Example ber square Number meter

Ex l 1 CRS 2.2 + + + +

Ex 2 1 EG 3.4 + + + +

Ex 3 1 AA 1.7 + + + +

Ex 4 2 CRS 2.2 + + + +

Ex 5 2 EG 3.4 + + + +

Ex 6 3 CRS 2.2 + + + +

Ex 7 3 AA 1.7 + + + +

Ex 8 4 CRS 2.1 + + + +

Ex 9 4 EG 3.3 + + + +

Ex 10 5 CRS 2.2 + + + +

CE 1 6 CRS 2.1 + + X

CE 2 6 EG 3.2 + + X

CE 3 7 CRS 2.2 + X

CE 4 7 AA 1.7 x +

CE 5 8 CR 1.9 + X

CE 6 8 EG 2.7 X X

CE 7 Control 1 CRS 2.2 + X

CE 8 Control 2 EG 3.7 + X

Abbreviations for Table 2

Ex = Example; CE = Comparative Example; CRS = Cold Rolled Steel; EG ^: Electrogal- vanized Steel; AA = Aluminum Alloy.

Benefits of the Invention

The method of pre-treatment prior to painting of this present invention is characterized by treatment with a dilute solution of copper ions as an after-treat¬ ment following a zinc phosphate based conversion coating, and, in comparison with conventional after-treatments which have been used in practice, there are improvements in terms of waste water treatment and chemical costs, and the in¬ vention can be carried out even with the conventional water rinsing process and with little limitation in terms of processing.