US3130086A - Materials and method for use in applying chromate conversion coatings on zinciferous surfaces - Google Patents

Description

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United States Patent f MATERIALS AND METHOD FOR USE IN APPLY- ING CHROMATE CONVERSION COATINGS 0N ZINCIFEROUS SURFACES George F. Otto, Oreland, Pa., assignor to Amchem Products, Inc., Ambler, Pa., a corporation of Delaware No Drawing. Filed July 22, 1963, Ser. No. 296,491

15 Claims. (Cl. 148-621) sistance and paint-bonding qualities has become increasingly diflicult where certain advanced techniques are employed. For example, commercial strip lines are now in operation whereby it is possible to process upwards of 1000 ft. of metal per minute. Such increased production speeds have resulted in serious bottlenecks at the coating stage in strip installations by virtue of the fact that presently available chromate conversion coating solutions are not capable of producing sufficiently high quality coatings under such high-speed operations. While conventional chromate conversion coating solutions and techniques appear capable of coating zinc surfaces in very short periods of time, for example in 5 to 15 seconds, the coatings produced on such rapidly moving zinc surfaces are very often too powdery to form an adequate substrate for the application of siccative finishes. Moreover, it has been found that such coatings frequently contain soluble salts which are leached from the coating during subsequent exposure to humid environment and, as a result thereof, the coated metal surfaces give rise to serious blistering and loss of subsequently applied paint finishes.

Several attempts have been made to overcome this problem. One such attempt involved the use of a chromate conversion coating solution having dissolved therein trivalent as well as hexavalent chromium ions together with one or more of the following, namely sulfites, sulfates, nitrates and acetates. In high-speed production practice, however, solutions of this type failed to provide the required corrosion resistance and paintbonding qualities on the rapidly moving zinc strip.

Another suggestion involved the use of a solution of zinc and hexavalent chromium ions, which solution was impinged upon the zinc surfaces to be coated after such zinc surfaces had been preheated to a temperature well above the boiling point of the solvent contained in the coating solution. This technique necessitates a coating temperature Well in excess of 94 C., and frequently as high as 177 C., and has not proven to be economically adaptable to high-speed industrial production techniques.

With the foregoing in mind the principal object of the present invention will be more fully understood and appreciated and may be said to reside in the provision of a method and material for rapidly applying to zinc surfaces a highly corrosion resistant and paint-bonding chromate conversion coating even at ordinary or living room temperatures.

A concomitant object is the provision of an aqueous acid chromate conversion coating solution which, over long periods of use, is capable of forming consistently high quality coatings on zinc surfaces.

3,130,086 Patented Apr. 21, 1964 Yet another object of this invention is the provision of an aqueous acid chromate conversion coating solution which will essentially completely react with zinc surfaces thereby leaving no water soluble salts thereon after drying even without the benefit of a final rinse.

How these and other objects and advantages are attained with the teachings of this invention become apparent from a study of the following description and examples.

The present invention is based upon the discovery that an aqueous acid solution consisting essentially of:

(a) At least 3.0 grams/liter of zinc ion (calculated as (b) From 5.0 to 35 grams/liter of hexavalent chromium ion (calculated as CrO (c) From 0.05 to 27 grams/liter of sulfate ion (calculated as S0 and (d) At least 0.15 gram/liter of a complex fluoride radical selected from the class consisting essentially of TlF and SIFG,

'avoided if consistently satisfactory coatings are to be obtained, particularly where the coatings are dried upon the treated zinc surfaces without intermediate rinsing.

'By the term substantially no alkali or alkaline earth cations is meant such cations which might be added deliberately as in prior art coating processes: This term is not intended to, and does not, imply that the normal levels of cations found in the usual commercial water supplies must be eliminated prior to use in the coating solutions of this invention.

The zinc ion (calculated as Zn), as noted hereinabove,

must be present in the chromate conversion coating solutions of this invention in an amount of at least 3.0 grams/liter. If the minimum amount of zinc is permitted to fall below 3.0 grams/litenthe zinc surfaces being treated will be subjected to an etching action with very little, if any coating being formed thereon. So far as an upper limit of zinc ion is concerned, there is no apparent limitation with respect to the coating ability of the solution. For example, where the amount of zinc present in the solution exceeds the stoichiometr-ic quantity required for forming hexavalent chromium salts thereof, such excess amounts of zinc will then become associated with other available anions in the coating solution, i.e., sulfate and complex fluoride ions. When the amount of Zinc exceeds the quantity required to become stoichiometrically associated with all of the anions present in the coating solution, then there occurs a precipitation believed to be zinc chromate (ZnCr0 upon the zinc surfaces being treated. However, this salt precipitation has not been found to be detrimental to coating quality, and is undesirable only from the viewpoint of excess chemical consumption. In general, it is preferred to maintain the zinc concentration at levels where this cation will be present in a quantity which does not exceed the stoichiometric amount needed to become associated with the anions dissolved in the acid coating solution.

So far as concerns introduction of zinc into the coating solutions of this invention, this cation may be added as the oxide, or as a salt of one of the essential anions, such as for example zinc sulfate, zinc dichromate or zinc fluosilicate. An important jconsideration tobe observed isthat whatever salt of zinc is employed, the anionportion thereof must not be in any way detrimental to the coating reaction. Anions which have been found to be detrimental when addedto the coating solutions of this inven- .tioninclude chloride and simple fluoride. Theformer stituent be present in amounts ranging from 5.0 to 35 grams/liter. Where less than the minimum amount is employed, the corrosion resistant qualities of the subsequently produced coatings will be found to'be adversely affected. Conversely, the upper limit of hexavalent chromium is equally critical, and should not exceed 35 grams/liter in order to avoid the occurrence of blistering under subsequently applied painted films after exposure of the metal to humid atmospheres.

Hexavalent chromium ion is preferably added to the coating solutions of this invention in the form of chromic trioxide (CrO However, this coating anion may be introduced as a salt of zinc if desired.

'The'amount of sulfate ion (calculated as 80,) which must be employed in the freshly prepared coating solutions of this invention, must, as noted hereinabove, be within the range of' 0.05 to 27 grams/liter. If the sulfate concentration falls below 0.05 gram/liter, any coatings which will be produced, if indeed any are produced at all, will be found to be very thin and of very low weight so that nouseful coatings can be obtained. Conversely, if the amount'of sulfate ion in the coating solution as initiallyfprepared is allowed to exceed 27 grams/liter,

severe etching of the metal surfaces being treated will result.

In connection with the sulfate ion content the following 'shouldfbe noted. 'When using between 0.05 and 27 grams/ liter of sulfate ion the coatings produced will tend tobe of alight golden to dark golden-brown color and, somewhat surprisingly,I have discovered that the addition of nitric acid to the coating solutions of this invention tends not onlyto produce a lighter colored coating of somewhat reduced weight,'but also to increase the salt 'which, as above indicated, must be maintained between 0.62 and'2.5. In view'of this thequantity of nitric acid which can be utilized must not be more than will lower the pH below0.62.

The impact test, referred to above, comprises dropping a inchball upon a test panel under a given inch-pound force. 'The deformed surface is then examined for loose or cracked paint and ratingsin inches of paint failure are reported. The discovery of the effect of the useof nitric 'acid is especially surprising in'view of applicants further discovery that nitric acid, .when used alone in place of the sulfuric acid, fails completely to produce the desired I coatings upon zinc surfaces.

A complex fluoride radical selected from the class consisting essentially of fluotitanate (TiF fluoborate (BF and fluosilicate (SiF must be included within the coating' solutions of this invention in an amount of at least 0.15 gram/liter, in order to insure obtaining the desired results." Depending upon the particular complex fluoride 4 chosen from the class described above, more or less of this constituent can be utilized. For example, where either fluosilicate or fluoborate is employed the upper limit of complex fluoride radical appears to be controlled solely by the limit of solubility" of the respective radical in the acid coating solution. However, it should benoted that where fluotitanate is employed ithas been found that use of more than approximately 10 grams/liter of this material tends somewhat to impair the paint adhesion and the impact and bending qualities of the treated zinc surfaces. However, more than the amount specified of this ingredient has no apparent adverse effect upon the salt spray corrosion resistance. In other words, pact resistance and bending qualities are the major properties to be attained in the coatingsproduce'd by this invention, the quantity of fluotitanate should be limited to 10 grams/liter--otherwise, it is unnecessary to place an upper limit upon this ingredient because ,evenlarger quantities produce unusually fine results with respect to salt spray corrosion resistance.

So far as the minimum limit of thecomplex fluoride radical is concerned, it has been found that if the 0.15 gram/liter amount is not attained, the resultantcoatings will be non-adherent and will lackthe essential paint- .bonding qualities.

In selecting a salt of the complex fluoride radicals for use in preparing or replenishing the coating solutions of this invention, care must be taken to insure that no alkali or alkaline earth cations be included therewith for the reasons stated above. It is thus essential thatthis particular component be utilized in the form of its respective acid, or as a zinc salt of the complex fluoride radical.

, Although a complex fluoride radical from the class described must be included within the coating solutions of this invention, it has been found that simple fluoride ions, asnoted hereinabove, must be carefully-avoided in the coating solutions of this invention in order to prevent etching of the zinc surfaces being treated. Thus, in selecting a, complex fluoride compound suitable for 'use herein, it is important to insure that such compound contain substantially no simple fluoride salts therein.

In addition to all ofthe foregoingrequirements, successful coatings can only be obtained if the solution pH, as measured by standard glass electrodes, is maintained between the values of 0.62 and 2.5. If the coating'solution pH is permitted to fall below 0.62, a marked etching action, at the expense of coating formation, will occur. Moreover, if the solution pH is allowed to increase above 2.5, the resultant coatings will possess poor corrosion resistant qualities. A pH range of 1.0 to 2.0 has been found to'yield optimum results commensurate with consistently high quality coatings, and is the preferred .operating range. 7

While the solutions of the present invention have been designed to coat zinc surfaces at ordinary room temperatures, they are known to be fully operative under colder conditions, i.e., at 12 C.; or at elevated temperatures up to 65 C. However, no noticeable improvement can be attributed to higher operating temperatures, so that such practice is not recommended in the interests of economy.

It has been found, although the reasons therefor are unknown to the inventor, that the quality of the coatings produced in accordance withthe teachings of this invention may be improved by the addition,.to the coating solutions, of some trivalent chromium ions. The amount to be added has been found to be at least 0.25 gram/liter, calculated as Cr. Use of larger quantities of trivalent chromium ions does not appear to have any detrimental effects upon either coating formation or quality, and, where the level of trivalent chromium ion concentration exceeds its solubility limit in the coating bath, such excess amount is precipitated as insoluble salt from the solution."

The coating solutions of the present invention may be applied to zinc surfaces by any conventional means such 5 as spraying, dipping or roller coating. Roller coating is preferred since it has been found to be particularly adaptable to commercial strip line operations and provides consistently high quality results without the need of auxiliary equipment required in dip or spray operations.

After application of the chromate solution to zinc surfaces the resulting coating may be water rinsed and air dried preparatory to the application of a siccative film. If desired, the chromate conversion coating may be baked after application, whether or not a water rinse was utilized. A preferred operation consists in employing a water rinse followed by air drying, and the subsequent application of a siccative film. The painted metal surface is then baked to cure the decorative organic finish.

Zinc surfaces which are to be coated by the process of this invention should be given a suitable cleaning pretreatment for removal of oils, greases, etc. which are frequently present on such surfaces. However, since this particular operation is well known to the art, and since such cleaning forms no part of the present invention, it need not be described in detail.

By way of example, there are presented below a number of illustrations of solutions and operating conditions which can be used to produce the coatings of this invention, but it is to be understood that these are intended as being merely representative and should not be construed as limitations except as defined in the appended claims.

Water to make 1 liter.

This solution, as prepared, had a pH of 2.03 and was applied to clean, galvanized steel panels using a 10 second spray application at 48 C. The coatings produced were iridescent, golden-brown in color, and the panels were subsequently painted with an acrylic paint and cured at 230 C. for three minutes. After 336 hours of salt spray testing (ASTM-B-117-61) the coated and painted panels were examined and found to contain only a trace of corrosion.

Nine additional examples were prepared wherein one or more of the coating constituents were varied in order to demonstrate the high level of corrosion protection obtained in accordance with the teachings of this invention, as follows:

[Grams/liter] Example N0. Coating constituent Zn 7.2 7.4 4.1 7.2 16.2 7.2 4.5 4.5 4.5 OrO; 10. 6 10. 6 10. 6 10. 6 10. 6 10. 6 25 10. 6 10. 6 S 1. 8 1. 8 1. 8 6. 3 20. 8 24. 3 1. 8 1. 8 1. 8 Complex Fluoride" BF. TiF SiF SiFi TiFQ SiFo BF; SiFs SiFo 7.3 7.1 20.6 0.6 0.7 0.6 0.7 0.6 0.7 Cr 0. 25 1. 25 0.44 N01 7 2. 2. 32 2. 2 1. 65 l. 88 2. 32 1. 62 0.9 l. 31 1. 77 Treating,

C 26 24 24 24 24 40 48 32 24 Treating time, seeonds 15 15 8O 25 1O 15 Galvanized steel panels were coated in the solution of Examples 2 to 10 at the temperatures and for the time cycles noted. All of these coated panels were then painted with an acrylic paint and cured at 230 C. for three minutes. After 240 hours of salt spray corrosion testing (ASTMB1 17-61) the coated and painted panels 6 were inspected and found to contain only trace amounts of corrosion.

From the foregoing examples it is readily apparent that the coating solutions and method of this invention are capable of providing excellent, high quality coatings on zinc surfaces in a minimum of time.

It is within the purview of this invention to provide solid, concentrated formulations of the essential coating constituents for purposes of making up and replenishing the coating solutions of this invention. These solid concentrates must contain from 3.5 to 10% of a zinc salt of a complex fluoride radical selected from the class above described, from 40 to 65 CrO or the equivalent amount of CrO in the form of a zinc salt thereof, and the balance to be supplied by a zinc salt selected from the class consisting essentially of the oxide and the sulfate.

The solid, concentrated formulations may in some cases require the addition of sulfuric acid upon dissolution with water so as to provide the desired initial level of the essential sulfate ions.

Typical examples of solid admixtures are as follows:

Example 11 This admixture is dissolved in suflicient dilute sulfuric acid to make a solution containing 5% by weight of the above composition having a pH of 1.25.

I claim:

1. An aqueous acid solution consisting essentially of:

(a) at least 3.0 grams/liter of zinc ion (calculated as Zn),

(b) from 5.0 to 35 grams/liter of hexavalent chromium ion (calculated as CrO (c) from 0.05 to 27 grams/liter of sulfate ion (calculated as S0 and (d) at least 0.15 gram/liter of a complex fluoride radical selected from the class consisting essentially of TiF BF and SiF said solution having a pH between 0:62. and 2.5 and being substantially free of alkali and alkaline earth metal ions.

2. The solution of claim 1 which also contains nitrate 3. The solution of claim 1 which has a pH between 1.0 and 2.0.

4. The solution of claim 1 which also contains at least 0.25 gram/liter of trivalent chromium ion.

5. The solution of claim 2 which also contains at least 0.25 gram/liter of trivalent chromium ion.

6. The method of forming a chromate conversion coating on a zinciferous surface which comprises subjecting the surface to the action of an aqueous acid solution consisting essentially of:

(a) at least 3.0 grams/liter of Zinc ion (calculated as Zn),

(1)) from 5.0 to 35 grams/liter of hexavalent chromium ion (calculated as CrO (c) from 0.05 to 27 grams/liter of sulfate ion (calculated as S0 and (d) at least 0.15 gram/liter of a complex fluoride radi- 7 cal selected from the class consisting essentially of TiF BF and SiFs, 7' i A said solution being substantially free of alkali andalkaline earth cations and being maintained at a pH of from 0.62 to 2.5 andthe treatment being continued until a chromate conversion coating is formedon the surface.

7. The process of claim 6 wherein the zinc ion is supplied from the group consisting ofzinc oxide and the sulfate, dichromate and fluosilicate salts of zinc.

8. The process of claim 6 wherein the zinc ion is supplied from the group consisting of zinc oxide and the sulfate, dichromate and fluosilicate salts of zinc and wherein the chromium ion is supplied in the form of chromium trioxide. i

9. The process of claim 6 wherein the complex fluoride radical is supplied by use of not more than grams/liter of fluotitanate (TiF 10. 'Th e process of claim 9 wherein the complex fluoride radical is supplied from the group consisting of the respective acid and the zinc salt of the complex fluoride acid.

11. The process of claim 6 wherein the zinc ion is supplied from the group consisting of zinc oxide and the sulfate, dichromate and fluosilicate salts of zinc and where in the zinc content is no greater than the quantity required to satisfy the stoichiometric amount needed fully to satisfy all of the anions present in the coating solution.

12. The process of. claim 6.wherein the solution also containsat least,0.05%. by weight ofnitric acid but not sufficient tol'ow er the pHhelow. the minimum of 0.6.2 specified.

13. The process of claim '6 wherein the solution also contains at least 0.25 gram/liter. of trivalentchromium 14. T-heprocess of claim 12 wherein the solution also contains at least 0.25 gram/liter of trivalent, chromium 1011.

15. A solid concentrate for use in preparingan aqueous acidcoating solution upon dissolution in water, said concentrate consisting essentially of (a) 3.5 to 10% of azinc salt of a complex fluoride radical selectedfrom the class consisting of Til- BF and SiF (b) from. 40 to CrO and (c) the balance zinc sulfate.

References Cited in the fileof this patent UNITED STATES PATENTS 2,276,353 Thompson Nov. 17, 1942 2,762,731 Hiller Sept. 11, 1956 4 2,786,062 Vam et a1 Nov. 19, 1957 2,948.643 Pimbly Aug. 9, 1960

Claims (1)

1. 6. THE METHOD OF FORMING A CHROMATE CONVERSION COATING ON A ZINCIFEROUS SURFACE WHICH COMPRISES SUBJECTING THE SURFACE TO THE ACTION OF AN AQUEOUS ACID SOLUTION CONSISTING ESSENTIALLY OF: (A) AT LEAST 3.0 GRAMS/LITER OF ZINC ION (CALCULATED AS ZN), (B) FROM 5.0 TO 35 GRAMS/LITER OF HEXAVALENT CHROMIUM ION (CALCULATED AS CRO3), (C) FROM 0.05 TO 27 GRAMS/LITER OF SULFATE ION (CALCULATED AS SO4), AND (D) AT LEAST 0.15 GRAM/LITER OF A COMPLEX FLUORIDE RADICAL SELECTED FROM THE CLASS CONSISTING ESSENTIALLY OF TIF6, BF4, AND SIF6, SAID SOLUTION BEING SUBSTANTIALLY FREE OF ALKALI AND ALKALINE EARTH CATIONS AND BEING MAINTAINED AT A PH OF FROM 0.62 TO 2.5 AND THE TREATMENT BEING CONTINUED UNTIL A CHROMATE CONVERSION COATING IS FORMED ON THE SURFACE.