WO1992001085A1

WO1992001085A1 - Composition and method for chromating treatment of metals

Info

Publication number: WO1992001085A1
Application number: PCT/US1991/004549
Authority: WO
Inventors: Arata Suda; Takao Ogino; Takayuki Aoki
Original assignee: Henkel Corporation
Priority date: 1990-07-06
Filing date: 1991-06-26
Publication date: 1992-01-23
Also published as: AU8009391A; JPH0466672A; JP2770864B2

Abstract

Prior art chromating treatments for metal surfaces are improved by including blocked isocyanates therein but excluding other types of organic resins.

Description

COMPOSITION AND METHOD FOR CHROMATING TREATMENT OF METALS

TECHNICAL FIELD

The present invention relates to a chromate treatment composition or bath and a method of using it which provides an excellent corrosion resistance, paint film adherence, and electrodeposition paintability on metal substrates treated with the composition.

BACKGROUND ART

The following have already been proposed in order to improve the performance of chromate treatment baths: (a) In the method disclosed in Japanese Patent Publication Number 52-35620 [35,620/77], an aqueous solution con¬ taining water-soluble resin in low concentrations is applied as a treatment bath based on hydrated chromium oxide, in order to improve the corrosion resistance, paintability, and resistance to the elution of hexa- valent chromium ion. Examples of the water-soluble resin in this case are oleoresins, alkyds, phenol alkyds, araino alkyds, and acrylic resins.

(b) The method disclosed in Japanese Patent Publication Number 58-47230 [47,230/83] proposes the treatment of metal surfaces with an aqueous solution which contains both hexavalent chromium ion and acrylic resin.

(c) In the method disclosed in Japanese Patent Publication Number 58-53069 [53,069/83], a chromium-containing aqueous solution (pH < 3.5) is prepared whose prin¬ cipal components are trivalent chromium, sulfate ion, and polycarboxylic acid resin. Its application is followed by a water wash. The next step is the ap¬ plication of an aqueous organic polymer solution with a specific composition, the main component being water dispersible olefin resin, and this is followed by drying.

(d) Japanese Patent Application Laid Open [Kokai or Un- exa ined] Number 60-228682 [228,682/85] proposes the application of an aqueous solution in which a fluorine complex and acrylic resin are dispersed in CrO .

(e) In the method disclosed in Japanese Patent Application Laid Open Number 61-584 [584/86], an aqueous solution is prepared which contains chromic anhydride, fluorine ion or fluorine containing complex ions, and a silicic acid compound, the chromic anhydride is then reduced with a reducing agent, and the pH is adjusted to 1.5 to 4. To the solution thus obtained are added a spec¬ ified quantity of a particular silane coupling agent and a specified quantity of a water-soluble or water dispersible resin which has a good mixing stability with chromic acid. This is applied after dispersing or dissolving to homogeneity, and the dry film weight

(f) The method disclosed in Japanese Patent Application Laid Open Number 63-143265 [143,265/88] proposes the addition to a chromate treatment bath of reducing agent and also at least 1 selection from acid radi¬ cals, resins, and silica.

Each of these chromate treatment baths seeks to im¬ prove the several properties listed above through the addition of organic resin to the chromate treatment bath. DESCRIPTION OF THE INVENTION Problems to Be Solved by the Invention

Because the quantity of water-soluble component in the resulting chromate film cannot be reduced after the chro- mate treatment, the addition of organic resin to a chromate treatment bath as described hereinbefore is associated with an environmental pollution risk due to chromium elution when the treated workpiece is used in an unpainted condi¬ tion. This places inherent limitations on the service applications of organic resin-containing chromate coatings. In addition, with regard to service as a paint undercoat- ing, the film formed on the treatment substrate by an or¬ ganic resin containing chromate treatment bath has a poorer paint adherence than the same type of chromate without or- ganic resin. This is due to the effects of the activator used to disperse the organic resin.

Moreover, the improvement in the corrosion resistance of the chromate/organic resin film is limited merely to the addition of the barrier activity exercised by the resin film to the effects exercised by the chromate film. Fur¬ thermore, as the resin use quantity is increased, the ef- feet of chromate ions in promoting self-repairability of the film is compromised and the corrosion resistance of the chromate/organic resin film comes to depend simply on the barrier effect of the resin film. As a consequence, the corrosion resistance is ultimately lower than that of the chromate film by itself.

A further problem accruing to chromate/organic resin systems is the increased insulation character exhibited by the chromate film after chromate treatment, which leads to a very large decline in the electrical conductivity. The electrical conductivity of chromate films strongly affects their electrodeposition paintability and spot weldability, for example, in the case of ordinary car body steel sheet and precoated metal for household appliances. Accordingly, chromate/organic resin films sacrifice the electrodeposi- tion paintability and weldability which are so essential to these types of applications. Summary of the Invention

The present invention solves the problems arising in the prior art, particularly with reference to paint adher- ence, electrodeposition paintability, and corrosion resist¬ ance, through the application to the treatment workpiece of a chromate treatment bath which is characterized by the addition of at least one type of blocked isocyanate to a chromic acid-based chromate treatment bath and does not contain any other type of organic resin. In a preferred embodiment, the ratio between the weight of isocyanate groups in the chromate treatment bath and the total chrom¬ ium weight in chromium containing compounds in the bath has values between 0.01 to l. The chromate treatment bath according to the present invention may be used to treat any metal or metal alloy. Details of Preferred Embodiments of the Invention

Chromate treatment baths within the scope of the present invention must not contain organic resin (other than the blocked isocyanate and any reaction product from it that may form) and must contain chromic acid, but their composition is not otherwise restricted, so that they may be otherwise like known chromate treatment baths. Among the components of chromate treatment baths, chromic acid is known to be an essential component relative to the corro- sion resistance, regardless of the treatment method (reac¬ tive or coatable, nonrinse baking chromate) , and it is an indispensable component in the treatment bath according to the invention. Chromate treatment baths also typically contain anionic components such as phosphoric acid and the like.

The isocyanate in the chromate treatment bath is ex¬ emplified by methylphenyl diisocyanate (MDI) , xylylene di- isocyanate (XDI) , hexamethylene diisocyanate (HDI) , toluene diisocyanate (TDI) , prepolymers of the preceding, and com- binations of two or more of the preceding. Other compounds may be used so long as they contain at least two isocyanate groups in each molecule.

The blocking agent in the blocked isocyanate employed by the chromate treatment bath according to the present in- vention functions to inhibit the reaction of the isocyanate group with water. The blocking agents may be, for example, alcohols, oximes, lactams, bisulfites, phenols, cyano types, and the like, but are not limited to these.

A surfactant may also be used in the compositions according to the invention, in order to disperse the blocked isocyanate in the water and improve the stability.

The benefits associated with the present invention, and particularly the improved adherence with an overcoat, are usually insignificant when the ratio of the total weight of isocyanate group in the treatment bath to the total weight of Cr in the chromium containing compounds in the bath falls below 0.01. On the other hand, when the ratio of the isocyanate group weight to the total Cr ion weight exceeds l, a self-polymerization reaction of the blocked isocyanate usually occurs. While this improves the corrosion resistance, the electrical conductivity of the chromate film layer is reduced, and this compromises such properties as the electrodeposition paintability and spot weldability. Of course, a chromate bath with a composition ratio outside the preferred ranges specified above can be employed when the treated material is to be used in appli- cations which do not require spot welding or electrodeposi¬ tion coating, or where paint adherence is not highly criti¬ cal, and thus where the aforementioned features need be present only to a modest degree.

The chromate bath is preferably applied in an amount

₂ to yield, as Cr add-on after drying), from 0.5 to 450 mg/m in the case of aluminum sheet, 0.5 to 450 mg/m 2 in the case of Zn/Ni-plated steel sheet, and 0.5 to 450 mg/m 2 in the case of galvannealed hot-dip galvanized steel sheet.

While the invention is not to be limited by any theo- ry, it is believed that the blocking agent thermally dis¬ sociates from the isocyanate group during the drying pro¬ cess which normally follows chromate treatment, with the result that isocyanate is produced in the dried chromate film. When the chromate film is subsequently coated with a resin which contains active hydrogen, this isocyanate may react with the active hydrogen in the resin, to cause a substantial increase in the interlaminar adhesion and co¬ hesion between the chromate film and the overlying paint film and within the chromate film itself. In addition, the isocyanate may react with the residual water in the chrom¬ ate film to produce amine, which would further react with isocyanate to produce the urea linkage. The net result would be the elimination of the water in the chromate film and a strengthening or solidification of the chromate film layer and thus a reduction in chromium ion elution and a further improvement in the corrosion resistance. In ad¬ dition, although probably less importantly, the isocyanate may undergo a moderate self-polymerization reaction when the film is dried, which again can result in an improvement in the corrosion resistance.

Examples of the present invention as well as co pari- son examples are provided below, without limiting the in¬ vention.

Examples

General Conditions Applicable to the Examples

The chromate treatment bath, in each case with the composition as reported below under Examples 1 through 5 and Comparison Examples 1 through 4, was applied using a roll coater to aluminum sheet, Zn/Ni-plated steel sheet, and galvannealed hot-dip galvanized steel sheet. This was dried at 180° C without an intervening water wash. The specimens thus obtained were electrodeposition coated with an electrodeposition paint (EL-9400 from Kansai Paint Company, Limited) so as to provide a high quality ex¬ ternal appearance, and they were then baked in an oven at 165° C for 20 minutes to afford the respective test speci- mens.

The chromium add-on in the chromate film layer was measured by X-ray fluorescence, and was approximately 70

2 mg/m in all the cases reported below.

The test specimens thus obtained were subjected to each of the following tests in order to evaluate the per¬ formance: ( 1) Primary adhesion testing

(a) Checkerboard adhesion testing: One hundred squares (1 mm sides) were placed using a cutter so as to reach the base metal. This was overlaid with cello¬ phane tape, which was subsequently stripped off. The percentage residual paint film was then scored.

(b) Dupont impact test: A weight (diameter = 12.7 mm (1/2 inch) , weight = 500 g) was dropped onto the coated surface from a height of 50 cm. Irregularities in the painted surface were then inspected visually. (c) Erichsen extrusion testing: The coated surface was extruded 6 mm using an Erichsen extruder, and irreg¬ ularities in the painted surface (e. g. , cracking, peeling) were visually inspected.

The pai•nt fi.lm adherence in the preceding tests was scored on the following four-level scale based on the degree of paint film exfoliation.

+ + + paint film exfoliation 0%

+ + paint film exfoliation less than 10% + paint film exfoliation at least 10%, but less than 30% x paint film exfoliation at least 30%

(2 ) Salt-sprav testing

In accordance with JIS Z 2371, a cross was scribed using a cutter from the paint film to reach the base metal, and this was subjected to a salt spray for 1,500 hours. The corrosion resistance was evaluated based on the area of rust development as a percentage of the entire surface area of the test specimen. + + + area of rust development 0%

+ + area of rust development less than 10%

+ area of rust development at least 10%, but less than 30% x area of rust development at least 30%

(3) Secondary adhesion testing

After the painted surface had been exposed to salt- spray for 1,500 hours, it was subjected to the same check¬ erboard adhesion test as in primary adhesion testing. The method of evaluation was also the same as in primary adhe- sion testing.

(4) Electrodeposition coatability

In order to evaluate the electrodeposition coatability separately from the evaluation of electrodepositior coat¬ ability under (1) above, the electrodeposition voltage was set to 350 V, and the number of craters produced in the

₂ painted surface was measured. The number per 1 dm was classified according to the following four-level scale.

+ + + number of craters less than 20

+ + number of craters >. 20 but less than 40

+ number of craters _> 40 but less than 60 x number of craters > 60

(5) Spot weldability

A continuous spot welding test was executed, under the conditions noted below, on SPCC sheet, Zn/Ni-plated sheet, hot-dip galvanized steel sheet, and galvannealed hot-dip galvanized steel sheet, which in each case had not been electrodeposition coated during the specimen preparation process. The chromate-treated surface was employed as the welding electrode contact surface. The dimensions of the welding test specimen were 30 x 100 mm, and 1000 spot welds were produced on the test specimen. Evaluation was based on the number of spots for which a test specimen tensile shear strength of at least 400 kg could be maintained.

Welding surface: chromate-treated surface (according to examples and comparison examples) or untreated surface

Welding pressure: 200 kg

Current: 8.5 kA

Weld time: 10 cycles

Electrode: R40 (radius type) of chromium-copper material Scoring:

+ + + at least 1,000 spot welds

+ + at least 900, but less than 1,000 spot welds

+ at least 800, but less than 900 spot welds x less than 800 spot welds

Below, the ratio of {total isocyanate groups weight} to {total Cr in chromium containing compounds weight} is denoted as "I/Cr".

Example 1 An aqueous solution was prepared by first dissolving 130 g chromic anhydride in 870 g pure water, followed by the addition of 9 g methanol for reduction of part of the hexavalent chromium. To this was added blocked isocyanate produced according to Example 4 of Japanese Patent Applica¬ tion Laid Open Number 52-96698 [96,698/77] so as to give I/Cr = 0.50, thus affording the chromate treatment bath.

Example 2

An aqueous solution was prepared by first dissolving 130 g chromic anhydride in 870 g pure water, followed by the addition of 9 g methanol for reduction of part of the hexavalent chromium. To this was added blocked isocyanate produced according to Example 4 of Japanese Patent Applica¬ tion Laid Open Number 52-96698 [96,698/77] so as to give I/Cr = 0.01, thus affording the chromate treatment bath.

Example 3 An aqueous solution was prepared by first dissolving 200 g chromic anhydride and 60 g 75% phosphoric acid (in¬ dustrial grade) in 740 g pure water, followed by the addi¬ tion of 18 g of methanol for reduction of part of the hexa¬ valent chromium. To this was added blocked isocyanate pro¬ duced according to Example 4 of Japanese Patent Applica- tion Laid Open Number 52-96698 [96,698/77] so as to give I/Cr = l.oo, thus affording the chromate treatment bath.

Example 4 An aqueous solution was prepared by first dissolving 200 g chromic anhydride and 60 g of 75% phosphoric acid (industrial grade) in 740 g pure water, followed by the addition of 18 g of methanol for reduction of part of the hexavalent chromium. To this was added blocked isocyanate produced according to Example 1 of Japanese Patent Appli¬ cation Laid Open Number 55-82119 [82,119/80] so as to give I/Cr = 0.3, thus affording the chromate treatment bath.

Example 5 An aqueous solution was prepared by first dissolving 200 g chromic anhydride and 120 g 75% phosphoric acid (in¬ dustrial grade) in 680 g pure water, followed by the addi- tion of 26 g methanol for reduction of part of the hexaval¬ ent chromium. To this was added blocked isocyanate pro- duced according to Example 1 of Japanese Patent Application Laid Open Number 58-80320 [80,320/83] so as to give I/Cr = 0.05, thus affording the chromate treatment bath.

Comparison Example l A chromate treatment bath was prepared by dissolving 130 g chromic anhydride in 870 g pure water, followed by the addition of 9 g methanol for reduction of part of the hexavalent chromium, thus affording the chromate treatment bath.

Comparison Example 2 A chromate treatment bath was prepared by dissolving 200 g chromic anhydride and 60 g 75% phosphoric acid (in¬ dustrial grade) in 740 g pure water, followed by the addi¬ tion of 18 g methanol for reduction of part of the hexaval- ent chromium, thus affording the chromate treatment bath.

Comparison Example 3 An aqueous solution was prepared as in Example l by first dissolving 130 g chromic anhydride in 870 g pure wat¬ er, followed by the addition of 9 g methanol for reduction of part of the hexavalent chromium. To this was added blocked isocyanate produced according to Example 4 of Jap¬ anese Patent Application Laid Open Number 52-96698 [96,698/ 77] so as to give I/Cr = 0.005, thus affording the chromate treatment bath.

Comparison Example 4

An aqueous solution was prepared as in Example 1 by first dissolving 130 g chromic anhydride in 870 g pure water, followed by the addition of 9 g methanol for reduc¬ tion of part of the hexavalent chromium. To this was added blocked isocyanate produced according to Example 4 of Japa¬ nese Patent Application Laid Open Number 52-96698 [96,698/ 77] so as to give I/Cr = 1.1, thus affording the chromate treatment bath. The results of the examples and comparative examples are shown in Tables 1, 2, and 3 for aluminum, zinc-nickel plated steel, and galvannealed hot-dip galvanized steel re¬ spectively. Excellent corrosion resistance, adherence to paint overcoat, electrodeposition paintability, and weld¬ ability were achieved on all substrates tested.

Table 1.

sheet receiving treatment: aluminum sheet

Table 2.

sheet receiving treatment: Zn/Ni-plated steel sheet

Table 3.

sheet receiving treatment: galvannealed hot-dip galvanized steel sheet

Claims

1. A chromic acid-based chromate treatment composition for metals, characterized in that it contains at least one type of molecule containing at least two blocked isocyan- ate groups but does not contain any other organic resin.

2. A composition according to claim l, wherein the ratio between the weight of blocked isocyanate groups present in the chromate treatment composition and the total chromium weight present in the form of chromium compounds in the composition is between 0.01 and 1.

3. A composition according to claim 2, wherein the isocy¬ anate molecules are selected from the group consisting of methylphenyl diisocyanate, xylylene diisocyanate, hexa- methylene diisocyanate, toluene diisocyanate, prepolymers of any of these aforementioned isocyanates, and mixtures of any two or more of these.

4. A composition according to claim l, wherein the isocy¬ anate molecules are selected from the group consisting of methylphenyl diisocyanate, xylylene diisocyanate, hexa- methylene diisocyanate, toluene diisocyanate, prepolymers of any of these aforementioned isocyanates, and mixtures of any two or more of these.

5. A composition according to any one of claims 1 - 4, wherein part of the chromium oxide initially present in the composition is reduced by addition of a reducing agent to the composition before the composition is used.

6. A composition according to claim 5, wherein the reduc¬ ing agent is methanol.

7. A method for treating a metal substrate by contacting the metal substrate for a sufficient time to form a chrom¬ ium containing conversion coating thereon with a liquid chromic acid based chromate treatment composition, char- acterized in that the liquid chromating composition con¬ tains at least one type of molecule containing at least two blocked isocyanate groups but does not contain any other organic resin.

8. A method according to claim 7, wherein the ratio be- tween the weight of blocked isocyanate groups present in the chromate treatment composition and the total chromium weight present in the form of chromium compounds in the composition is between 0.01 and 1.

9. A method according to claim 8, wherein the isocyanate molecules are selected from the group consisting of meth¬ ylphenyl diisocyanate, xylylene diisocyanate, hexamethyl- ene diisocyanate, toluene diisocyanate, prepolymers of any of these aforementioned isocyanates, and mixtures of any two or more of these.

10. A method according to claim 7, wherein the isocyanate molecules are selected from the group consisting of meth¬ ylphenyl diisocyanate, xylylene diisocyanate, hexamethyl- ene diisocyanate, toluene diisocyanate, prepolymers of any of these aforementioned isocyanates, and mixtures of any two or more of these.

11. A method according to claim 10, wherein part of the chromium oxide initially present in the composition is reduced by addition of a reducing agent to the composition before the composition is used.

12. A method according to claim 9, wherein part of the chromium oxide initially present in the composition is reduced by addition of a reducing agent to the composition before the composition is used.

13. A method according to claim 8, wherein part of the chromium oxide initially present in the composition is reduced by addition of a reducing agent to the composition before the composition is used.

14. A method according to claim 7, wherein part of the chromium oxide initially present in the composition is reduced by addition of a reducing agent to the composition before the composition is used.

15. A method according to claim 14, wherein the reducing agent is methanol.

16. A method according to claim 13, wherein the reducing agent is methanol.

17. A method according to claim 12, wherein the reducing agent is methanol.

18. A method according to claim 11, wherein the reducing agent is methanol.

19. A method according to any one of claims 7 - 18, where¬ in the metal treated is selected from the group consisting of aluminum sheet, zinc-nickel plated sheet, and galvan- nealed hot-dip galvanized steel sheet, and the chromium add-on after drying after treatment is in the range from 0.5 to 450 milligrams per square meter of metal surface treated.