GB2155962A

GB2155962A - Composition for treating metal surfaces

Info

Publication number: GB2155962A
Application number: GB08507516A
Authority: GB
Inventors: Thomas J Prescott
Original assignee: Parker Chemical Co
Current assignee: Henkel Corp
Priority date: 1984-03-23
Filing date: 1985-03-22
Publication date: 1985-10-02
Also published as: JPH0422986B2; AU4028085A; AR242247A1; ZA852128B; MX166776B; JPS60218483A; EP0155742B1; NZ211485A; EP0155742A2; GB8507516D0; DE3509556A1; DE3583512D1; BR8501308A; GB2155962B; EP0155742A3; AU580041B2; CA1256003A

Description

SPECIFICATION Compositions and processes for coating metals It is conventional to apply base coatings on metal surfaces to improve corrosion resistance of the surfaces and to provide a base for subsequent painting. The base is required to improve adhesion of the paint and to improve corrosion resistance of the painted surface, for instance as manifested by resistance to humidity and salt spray. When the painted surface is to be deformed after painting adhesion and corrosion resistance after deformation are also important.

In the past, difficulties have often been encountered in developing a composition and method for treating metal surfaces which produces an end product which, when painted, exhibits satisfactory formability adhesion and corrosion characteristics. High coating weights were required for corrosion protection while low coating weights were required for formability.

Generally speaking, conventional compositions and methods for treating metal surfaces have involved multi-stage chemical treatments of the metal with water rinsing after each treatment.

However, multiple-stage treating involves additional personnel, inter-stage contamination problems, extended line length, and problems with regard to disposal of rinse water.

An aqueous composition according to the invention comprises (a) a hexavalent chromium compound; (b) a compound selected from the group consisting of silica and silicates and mixtures thereof and (c) phosphate.

Preferably, the composition also comprises a trivalent chromium compound.

The preferred weight ratios, per part by weight total chromium, of hexavalent chromium, trivalent chromium, compound selected from the group consisting of silica and silicates and mixtures thereof, and phosphate are from about 0.6 to 1.0; from 0.0 to about 0.4: from about 1.0 to about 3.0; and from about 0.1 to about 1.0, respectively. However in less desirable compositions other amounts of silica or silicate and phosphate can be used. For instance the ratio, per part by weight total chromium, of silica or silicate, and phosphate respectively may be about 0.5 to about 5 and about 0.1 to about 5, respectively.

A process according to the invention for forming a coating on a metal surface comprises contacting the surface with an aqueous composition as defined above and curing the resultant aqueous coating.

The phosphate and chromium components of the present invention may be supplied in any form which does not interfere with the quality of the final paint base coating. Addition in the form of alkali metal salts should be minimised and preferably avoided since such salts may interfere with the quality of the coating. The phosphate and hexavalent chromium are preferably added as relatively soluble di- or trivalent metals salts, thermally stable ammonium or amine salts (including double salts with the foregoing metals), or in acid form. Suitable di- or trivalent metals include salts of, for example, zinc, manganese, chromium, nickel, cobalt and iron. The hexavalent chromium component is preferably added as zinc dichromate or chromic anhydride (chromic acid) and the phosphate component is preferably added as zinc dihydrogen phosphate or as phosphoric acid.The preferred ratio by weight hexavalent chromium to total chromium is between 0.7:1 and 1:1.

The compositions often contain trivalent chromium, in a weight ratio based on total chromium of 0.4:1 to 0:1, preferably 0.25:1 to 0:1. The trivalent chromium compound conveniently is prepared by partial reduction of hexavalent chromium, for instance within the composition. For example the trivalent chromium compound can be prepared by the partial reduction of an aqueous solution of chromic acid with starch and heat in a conventional manner such as is disclosed, for example, in U.S. Patent 3,706,603, December 10, 1972 to Vessey, et al. The partially reduced solution will, of course, contain both hexavalent and trivalent chromium compounds.

The amount of hexavalent chromium is preferably 0.9 to 0.95 parts per part total chromium.

The amounts of silica or silicate and of phosphate are preferably, per part by weight total chromium, 1.5 to 2.5 and 0.25 to 0.5 parts respectively.

The silica or silicate must be one capable of dissolving in the aqueous composition or becoming dispersed therein to form a homogeneous, that is colloidal, dispersion. It is therefore preferably used in a finely-divided form. The use of fume or precipitated silica, is preferred, but naturally occurring ground quartz and diatomaceous earth may also be used when the necessary dispersion can be obtained. There may also be used silicates such as montmorillonite or synthetic fluorosilicates, such as complex magnesium fluorosilicates sold under the trade name Laponite. The use of soluble sodium or potassium silicates and fluorosilicates is much less preferred since they tend to form glassy coatings with poor adhesion and therefore the use of a water-insoluble silica or silicate that is nevertheless colloidally dispersible in the solution is preferred.Mixtures of different forms of silica and/or different silicates may be used if desired.

Additional components may be optionally included in the composition of this invention, such as metal di- and trivalent cations such as zinc, manganese, cobalt, nickel and iron; inert colouring agents designed to provide a specific colour to the protective coating; silicon compounds; a conductive material to improve weldability such a pulverulent metal as disclosed in U.S. 3,671,331, or a conductive carbon and emulsifying agents necessary to maintain the resin component in a dispersed state (normally present in commercially available resin aqueous dispersions).

The aqueous composition of this invention may be used without any need for pH adjustment.

Where the components are added in the form of di- and trivalent metal salts or as the acids, pH's of below 2.5 will normally be expected. On the other hand, if the components are added as the ammonium or amine salts, less acid pH's will be expected.

The working composition may be prepared by mixing the essential ingredients in any order to provide the desired weight ratios. However, the silica or silicate is preferably added after partial reduction of hexavalent chromium if heat is employed during the reduction since heat may reduce the dispersable nature of the silica. Since no substantial reaction with the metal surface takes place prior to curing, the components of a film deposited from the bath are initially present in the same concentration as in the bath. Therefore, the concentration of the essential components in a replenishing composition will be substantially the same as in the working composition. This fact simplifies bath control and improves product uniformity.

Depending on the method of application, a wide range of concentrations could be employed herein. A working solution can, for example, comprise from about 0.8% to about 12.0%, preferably 0.8 to 8%, total chromium.

The aqueous composition of the present invention is especially useful for dry-in-place stripline application to cold rolled steel to provide improved corrosion protection and formability for painted stock. However, the composition may also be used over aluminium, zinc, galvanised steel, other ferrous metal surfaces, and alloys thereof.

Application of the aqueous composition to the metal surface may be accomplished in any of the conventional manners so long as sufficient care is taken to obtain a reasonably uniform thickness of the aqueous film. For flat surfaces such as sheet or strip, this control may be accomplished most readily through the use of rollers, or squeegees, however, the composition may be applied by any suitable conventional method such as electrostatic coating or mist-on techniques.

Coating weights may vary from as little as one milligram per square foot to as much as 400 milligrams per square foot or higher. Normally, the coating weight will be between 5 and 100 milligrams per square foot. Coating weights for aluminium surfaces will typically be between about 5 mg/ft2 and about 1 5 mg/ft2, while for ferrous surfaces the coating weight will typically be between about 1 5 mg/ft2 and about 25 mg/ft2 and for zinc between about 10 mg/ft2 to about 20 mg/ft2.

In operation, processing variables will normally be determined based upon the desired coating weight to be obtained. Depending upon the overall concentration of the components in the working composition, a film of predetermined thickness of the aqueous treating composition will be applied to the metal surface from a working bath and then cured by heating. As the surface is cured, the composition becomes concentrated and a reaction will begin to take place between the components of the composition and the metal surface to form the coating of this invention.

Normal ambient temperatures are suitable for the working bath. However, the working bath and/or the metal surface may be preheated in order to hasten the curing process.

Metal temperatures of up to 93"C or higher may be employed for immersion or roll-on applications without degrading the bath. Higher temperatures may be employed in connection with mist-on application. The manner of curing is not critical so long as the liquid film is not unduly disrupted, e.g. by hot air currents or physical contact during the curing process.

However, the manner of curing may affect the temperature required to effect a cure. For example, curing is obtained at lower peak metal temperatures in an infra red oven than in a conventional oven. Under normal operations, it is desirable to use elevated oven temperatures and warm air streams of velocity insufficient to disturb the wet film. From a practical standpoint, the oven temperature should result in a metal temperature of between about 50 and 1 80to.

The paint can be applied to the cured coated surface by any conventional means. While the particular paint employed will affect the overall corrosion resistance and adhesion, with most commercial paints tested, the process of this invention will give results comparable to those obtained by conventional two or three-stage base coat processes. The invention is of particular value when the painted surface is subsequently cold formed.

The following are examples of the invention: EXAMPLE 1 A treatment solution is made up from a solution of chromium anhydride (cur03); partially reduced chromic anhydride (Cr+6:Cr+3 = 60:40 from reduction of CrO3 with starch and heat); phosphoric acid, and a 10% w/v solution of Aerosil 200 silica (milled in H20 to form a suspension) to give Cr+6:PO4:SiO2:Cr' 'a' weight ratios of 0.87:0.97:2.0:1.0, respectively. The solution was applied to cold rolled steel panels by means of a grooved squeegee roll at coating weights of both 37 and 1 7 mg/ft2 with the coating being cured by drying the treated panels in a conventional oven to a peak metal temperature of approximately 82-93"C. The panels sat for about 2 weeks.Then some panels were painted with a single coat of polyester paint, some panels with a single coat of vinyl and some panels with a 2 coat epoxy-polyester system. All panels had good paint adhesion corrosion protection and formability.

EXAMPLES 2-4 Four treatment solutions were prepared as in Example 1 to the following Cr+e:PO4:SiO2:Crtotal ratios: Example 2 Example 3 Example 4 0.6:0.32:2.0:1.0 0.6:0.65:2.0:1.0 0.6:0.97:2.0:1.0 Coatings were again applied as in Example 1 to cold rolled steel panels which coated panels were then dried to 82-93"C peak metal temperature in a conventional oven before painting as in Example 1. Comparison of the panels of Examples 2-4 found that corrosion protection (salt spray) improved but formability diminished as the P04 level was increased.

EXAMPLE 5 Cold rolled steel panels were treated with 20 mg/ft2 coatigs from a solution made as in Example 1 to give a Cr+6:PO4: SiO2:Crtota' ratio of 0.93:0.32:2.0:1.0 using the previously mentioned grooved roll squeegee coater. The panels were cured in a conventional convection oven at 150"C peak metal temperature and after 30 second cooling-off period in a room temperature environment were painted as in Example 1. All panels had good corrosion resistance, paint adhesion characteristics and formability.

EXAMPLE 6 The solution of Example 5 was applied to a surface of cold rolled steel panels which had been preheated to about 93"C. After about 1 minute the panels were painted with a single coat of polyester paint or a two coat epoxy primer polyester top coat with excellent corrosion resistance, paint adhesion and formability being obtained.

Claims

1. An aqueous composition for treating a metallic surface and comprising a hexavalent chromium compound, a compound selected from the group consisting of silica and silicates and mixtures thereof, and phosphate.

2. A composition according to claim 1 in which the weight ratio of phosphate to total chromium is between 0.1:1 and 1:1 and the weight ratio of silica or silicate to total chromium is from 1:1 to 3:1.

3. A composition according to claim 1 or claim 2 in which the weight ratio hexavalent chromium to total chromium is from 0.6:1 to 1:1.

4. A composition according to claim 3 in which the weight ratio hexavalent chromium to total chromium is from 0.75:1 to 1:1.

5. A composition according to any preceding claim comprising a trivalent chromium compound.

6. A composition according to claim 5 in which the weight ratios, per part by weight total chromium, of the hexavalent chromium compound, the silica or silicate and the phosphate are between 0.9 and 0.95, between 1.5 and 2.5 and between 0.25 and 0.5 respectively.

7. A composition according to any preceding claim in which the total chromium content is from 0.8 to 12% by weight.

8. A composition according to claim 1 substantially as herein described with reference to any of the Examples.

9. A process for forming a coating on a metal surface comprising contacting the metal surface with a composition according to any preceding claim and curing the resultant aqueous coating.

10. A process according to claim 9 wherein the surface is of cold rolled steel.

11. A process according to claim 9 or claim 10 in which the surface is subsequently painted.

1 2. A process according to claim 11 in which the surface is subsequently subjected to a forming operation.