US3109757A - Method and material for applying phosphate conversion coatings on zinciferous surfaces - Google Patents

Description

United States Patent 3,109,757 METHOD AND MATERIAL FOR APPLYING PHOS- PHATE CGNVERSION COATINGS ON ZINCIF- EROUS SURFACES Earl R. Reinhold, Levittown, Pm, assignor to Arneheln Produc w, Inc., Ambler, Pa, a corporation of Delaware N0 Drawings. Filed Jan. 26, 1962, Ser. No. 169,997 6 Citations. (Cl. 148--6.15)

This invention relates to the art of coating zinciferous surfaces and involves a method and solutions for producing phosphate conversion coatings on such surfaces which coatings have highly improved characteristics, especially with respect to fineness of grain structure without serious sacrifice of coating weight, ability to promote the adhesion of siccative finishes and resistance to stresses set up by bending, folding or shaping the metal after the coating has been applied.

As used herein the expression zinciferous surfaces or zinc surfaces are to be understood as applying to alloys of zinc with other metals wherein zinc is the principal ingredient, to hot-dipped galvanized surfaces, to electrodeposited zinc surfaces, as well as to essentially pure zinc surfaces.

The art of applying phosphate conversion coatings to zinc surfaces, of course, is well developed and is based upon the familiar practice of subjecting the surfaces to the action of an aqueous acid phosphate coating solution until the desired coating has been deposited. Following this the surface is customarily rinsed either with water or with dilute chromic and/or phosphoric acid rinses. Probably the most familiar of the aqueous acid phosphate coating solutions is a zinc solution and the present invention is directed to the improvement of methods involving such a solution.

In this art of coating zinc surfaces with aqueous acid zinc phosphate coating solutions various solution formulations are well known to the art and the present invention is broadly applicable to solutions of this general type although, as will be pointed out below, maximum benefits from the present invention are obtainable where the aqueous acid zinc phosphate solutions conform to the specifications recited hereinafter.

In the phosphate coating of zinc surfaces the art has long realized that the conversion coated surfaces and any siccative finish that may have been applied thereto do not lend themselves very well to bending or folding without serious attendant cracking and peeling with subsequent loss of or injury to the siccative film. This is particularly exemplified by what happens to an acrylic base paint which is often objectionably impaired as the result of cracking of the phosphate coating in situations requiring bending or shaping operations such, for example, as in the use of galvanized iron sheeting as siding in the construction industry.

With the foregoing in mind, the principal object of the present invention is the provision of a method and solutions for applying an improved, adhesion-promoting, zinc phosphate conversion coating on zinc surfaces which will withstand or resist to a marked extent flexing, shaping and bending operations even to an extent involving a 180 bend without failure of the siccative finish or film.

The invention is broadly based upon the discovery that if the following agents are added to an aqueous acid zinc phosphate coating solution of generally standard characteristics and such solution is then utilized in the treatment of Zinciferous surfaces it will formon the surfaces zinc phosphate conversion coatings which are smoother, finer grained, more tightly adherent, unimpaired as to coating weight, and better suited to serve as a substrate for subsequent sicca-tive finishes than are the familiar zinc ice phosphate conversion coatings presently known to the art. The addition agents employed with my invention are:

(a) From 0.1% to 1.0% by weight of a compound selected from the group consisting of glycerophosphoric acids and water-soluble salts thereof; and

(b) At least 0.025% by weight of complex fluoride radical selected from the group consisting of fluoboric, fluosilicic, fluotitanic and fluozirconic acids and alkali metal and ammonium salts thereof.

The glycerophosphoric acids or water-soluble salts there- 'of include mono and di-glycerophosphate compounds. Soluble salts are preferred over the acids in view of their greater solubility in the acid phosphate coating media. Particularly preferred salts include the alkali metal and ammonium mono and di-glycerophosphates, since these are more readily prepared, and provide an easily handled concentrated liquid solution.

Where the amount of glycerophosphate, or water-soluble salt thereof, which is employed, is less than the 0.1% by weight, minimum concentration, the improved paintadhesion promoting coatings will not be obtained, and subsequent bending or flexing of the phosphate coated and painted zinc surfaces will result in cracking and peeling of the siccative film. As the amount of glycerophosphate compound in the phosphate coating solution is increased, the paint bonding qualities of subsequently produced coatings on zinc surfaces also increases up to an apparent maximum improvement at about 1% concentration, by weigh-t. Use of glycerophosphates in amounts greater than 1% concentration has no noticeable deleterious effect upon either the coating reaction, or upon subsequent paint adhesion on the coated surfaces. However, as no additional benefit is obtained when employing more than about 1% glycerophosphate, it is preferred, in the interests of economy and to prevent waste, to operate between a range of from 0.1% to 1.0% concentration thereof as stated above.

As for the quantity of complex fluorides, I have found that, in order to realize the benefits of this invention, they should be added in an amount equal to at least 0.25% by weight of the coating solution. If less than this minimum quantity is present the resulting conversion coatings will not provide the highly flexible, paint-bonding qualities obtainable by my invention.

Insofar as an upper concentration limit of complex fluoride is concerned, I have found that there is nothing critical about the quantity employed since solubility considerations pretty well limit the amounts thereof which can be dissolved in zinc phosphate coating solutions of this general character. For example, the complex fluoride acids, i.e. fiuoroboric, fluosilicic, etc., and the ammonium salts thereof have been found to be more soluble in the coating baths or solutions than the alkali metal salts. However, the maximum solubility of any of these complex fluoride compounds in the customary aqueous acid zinc phosphate coating solutions is about 2.5% by weight thereof. Use of a relatively low concentration of complex fluoride compound, for instance, about 0.03%, provides the same enhanced adhesion-promoting characteristics as does the use of large amounts, even those amounts which approach the saturation limitation. Therefore, in the interests of preventing waste and securing maximum economy of operation, I prefer to employ a range of complex fluoride concentration of from 0.005% to 0.25% by weight of the coating solution.

While the use of complex fluorides from the class described is essential for successful operation of the process of this invention, it has been found that the inclusion of simple fluoride ions into the coating solution of this invention has no apparent deleterious effect upon coating formation. However, use of simple fluoride ion alone will not provide the improved results obtainable with the complex fluorides as defined hereinabove.

As a further guide in attainment of the maximum benefits of my invention I wish to call attention to the followmetals are employed) of less than 0.3% of nickel and as little as 0.0003% to 0.001% of copper.

A concentrate which is suitable for dilution with water, as hereinafter described, to produce the coating solutions i .f of this invention, and which is also suitable for addition The phosphate ion content which yields the best results to partially exhaust solutions as a treplenisher, can be ranges from about 0.5% to about 2.5% by Weight of the prepared according to the following exampl coating solution with the minimum quantity being at least sufiicient to form the dihydrogen phosphate with the zinc EXAMPLE I ions present and the content of zinc ions should be in Percent by Welght an amount which does not exceed about 0.5% by weight Zno of the coating solution. H?PO4 (75%) Another factor which should be observed in obtaining 3 9 the maximum benefits of my invention arises in connec- 3 (38 tion with the presence of any oxidizing ion such as nitrate Water 42-88 and mixtures of nitrate and nitrite. These oxidizing agents, of course, are very familiar to those skilled in the 100-00 art and for best results with my invention the quantity h abovg f l h dil d i h watmto a of nitrate ion should be about 0.2% to about 1.0% with Strength f 4% by volume d h i dd d h m major benefits being Obtalnable when the l f 'fiy 116$ 0.25% (weight/volume) of caustic soda, has the followbetween about 0.3% to about 0.5%. Where nitrite 1s also ing analysis; included in the bath as an oxidizing agent substantially T 1 smaller amounts thereof are required in order to yield Q i i 3 0 the results which are secured by this familiar ingredient. i-i 0 For example, the range of nitrite ion when used in com- 25 erwm mo bination with nitrate ion is from about 0.0002% to about The acidity referred to was determined by the method 0.008% and with my invention I have found that optimum indicated above. flexible paint-bonding properties in the phosphate con- To a dilute solution prepared as above described there version coatings are obtainable with the use of less than should then be added from 0.1% to 1.0% by weight of about 0.5% nitrate ion and less than about 0.005% of a glycerophosphate compound as hereinabove described nitrite ion. and at least 0.025% by weight of a complex fluoride as Insofar as total acidity of the solution is concerned, previously described. The solution should then be heated I have found that it is preferable to utilize zinc phosphate to between about 120 F. and 160 F. and preferably coating solutions which have a total acidity of less than between 130 F. and 160 F. and can then be utilized with 40 points. Total acidity is the milliliters of tenth normal greatest benefit in applying zinc phosphate conversion sodium hydroxide (points) required to titrate a 10 ml. coatings to zinciferous surfaces and it will be found that sample of the bath to a phenolphthalein endpoint, whereas it will produce excellent, highly flexible, smooth and finethe free acidity is the mls. of tenth normal sodium hydroxgrained, paint-bonding phosphate conversion coatings ide (points) required to neutralize a 10 ml. sample of which are ideally suitable as a substrate for siccative the bath to a brom cresol green endpoint. finishes.

Beyond the foregoing, it an aqueous acid zinc phos- The advantages attainable with my invention are amply phate coating solution is employed which contains nickel demonstrated by the following comparative tests which I and/or copper ion (these ions of course being well have conducted. A 4% dilution of the concentrate of known to the art), it is important to observe the following Example I given above was prepared and aliquot samples facts in order to secure the maximum benefits of my inof the diluted solution were utilized in accordance with vention. In the case of nickel ion there should be emthe examples given in Table I below. The metal being ployed somewhere between about 0.01% and 0.4% and in coated was hot-dipped galvanized steel. A series of simithe case of copper between about 0.0003% and 0.005%. lar panels from such galvanized steel were prepared and Use of higher amounts than these of nickel and copper cleaned in accordance with well established prior art pracwill not yield the full advantages otherwise obtainable tices. The reported paint adhesion results were deterwith the process and solutions of my invention. Actually, mined following application of an epoxy primary paint best results are secured with the utilization (where these system.

Table I Complex fluoride Paint adhesion test (ASTM D52241) Example Percent Coating N o. glycerowt.,

phosphate rug/ft. Percent Percent Type paint Paint appearance loss 2 None None 462 50 Heavy cracking.

3 None 0.05 (NH4)2ZHFB 487 10 Moderate crack- 4 None 0.126 HzSiFs 469 30 H e y cracking.

5 None 0.033 HZTiFB- 442 20 D0.

6 0.1 None 501 5 Fine cracking.

7 0.1 0.05 (NHQzZnF n 504 1 Very fine cracking. s 0.2 0.05 N11 2111 420 1 Do.

9 0.4 0.05 (N114): Z1111. 402 1 Do.

10 0.1 0.126 H2SlFfl 477 2 Fine cracking.

11 0.2 456 1 Very fine cracking. 12 0.4 468 1 Do.

13 0.2 417 1 l4 0. 2 494 1 Very fine cracking.

1 Examples 8 and 14 also contained 0.005% copper ion.

2 The panels of Example 13 were inadvertently lost and were not available for this test but visual observation thereof indicated that their coatings would be equally satisfactory as those in Examples 7 to 16, inclusive.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,109,757 November 5 1963 Earl R. Reinhold It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 2, line 40, for "0.25%" read 0.025% column 4, line 6, for "exhaust" read exhausted Signed and sealed this 28th day of April 1964.

(SEAL) Attest:

ERNEST W. SWIDER Attesting Officer EDWARD J, BRENNER Commissioner of Patents

Claims (1)

1. IN THE ART OF PHOSPHATGE COATING ZINCIFEROUS SURFACES,M THE METHOD WHICH COMPRISES SUBJECTING SAID SURFACE TO THE ACTION OF AN AQUEOUS ACID ZINC PHOSPHATE COATING SOLUTION, THE COATING PRODUCING INGREDIENTS OF WHICH CONSIST ESSENTIALLY OF PHOSPHATE ION FROM 0.5% TO 2.5% BY WEIGHT AND NITRATE ION FROM 0.2% TO 1.0% BYW EIGHT, WHICH SOLUTION ALSO CONTAINS AS ADDITION AGENTS: (A) AT LEAST 0.1% BY WEIGHT OF THE SOLUTION OF A COMPOUND SELECTED FROM THE GROUP WHICH CONSISTS OF GLYCEROPHOISPHORIC ACIDS AND WATER SOLUBLE SALTS THEREOF; AND (B) AT LEAST 0.025% BY WEIGHT OF THE SOLUTION OF COMPLEX FLUORIDE RADICAL SELECTED FROM THE GROUP CONSISTING OF FLUORBORIC, FLUOSILICIC, FLUOTITANIC AND FLUOZIRCONIC ACIDS AND ALKALI METAL AND AMMONIUM SALTS THEREOF; AND REPENLISHING THE SOLUTION AS REQUIRED TO MAINTAIN ITS INGREDIENTS AS DEFINED.