US2164042A - Art of producing phosphate coatings on surfaces of iron, zinc, or alloys thereof - Google Patents

Description

Patented June 27, 1939 UNITED STATES PATEN 2.104.042 ART or raonocmc rnosrmmicoar- 2,164,042 T OFFICE INGS ,ON SURFACES OF IRON, ZINC, OR

ALLOYS THEREOF Gerald G. Romig,

Company, Ambler,

a corporation of Delaware No Drawing. App jcation May 27, 1938I Serial No. 210,425 18 Claims. o1. 148-6) This invention relates to the art of producing phosphate coatings 1 alloys thereof and is particularly useful in connection with that type of phosphate coating process in which rapid production of a phosphate coating of maximum smoothness, adherence, and corrosion resistance is of primary importance, such, for example, as in the preparation of the metal for the reception of a final finish of paint, varnish, lacquer, japan, or the like.

The principal object of the invention is to pro-v vide a process for producing phosphatecoatings of exceptionally smooth, adherent and .corrosion feet of this mechanical treatment will resistant character with a minimum consumption of time and labor and with maximum uniformity. in quality'regardless of certain variables which ordinarily, adversely affect coatings of the kind in question.

The invention is also of especial significance in connection with the coating of articles of iron,

zinc or their alloys, the surfaces of which, at some stage of their manufacture or fabrication, have been subjected to a cleaning process, especially a chemical cleaning process such as pickling or alkali cleaning. Cleaning processes and particularly the cleaning processes just mentioned introduce certain difliculties and variables into a phosphatic coating process which are .usually com- ,pletely eliminated by my invention. Another object of the invention, therefore, can be stated ,as residing in the provision of a chemical pretreatment for metal surfaces which have been cleaned, which pretreatment insures the subsequent formation during the phosphate coating procedure of a finely crystalline, exceedingly smooth, and highly adherent and corrosion resistant coating which is substantially free of pin holes or of a relatively rough, coarsely crystalline and non-uniform coating.

Before proceeding-with a description of the invention or further elaborating its objects and advantages, it is desired to state briefly some experimental facts with respect tothe phosphate coating art which, it is believed,'will materially aid in an understanding and appreciation of the invention.

It has long been observed that surfaces of iron, steel,'or zinc, or alloys thereof will, under most conditions, acquire a phosphate coating more rapidly'and perfectly in an appropriate phosphate coating solution whenthe surfaces are first wiped, brushed or otherwise mechanically treated before they are subjected to the phosphate, coating solution.- In the discussion to follow the efgenerally be referred to as the wiping effect. However, it has also been observed that even surfaces which have been so wiped may be "spoiled for the reception of a phosphate coaton surfaces of iron, zinc or 'phate coating solution,

other imperfectly coated areas, instead but it is obvious that it ing if they are subjected to aqueous solutions of electrolytes such as acids, bases and salts, and that even a comparativelybrief exposure to pure water will deleteriously affect the rapidity with I which these surfaces will acquire a phosphate coating as well as with its uniformity, smoothness and finely crystalline-character.

In additionto the foregoing, the phosphate coating art generally recognizes that it is usually necessary to clean metal surfaces before subjecting them to the action since the presence of grease, dirt, oxide and other impurities. interferes with the uniformity and quality of the coating. Here again, difiiculties are encountered because the common methods of removing such impurities (including pickling, alkali cleaning, etc.) bring about the deleterious changes in the behavior of the surfaces tions which have already been referred to. Mechanical action, by the wiping effect above described, almost invariably: improves the behavior of metal surfaces cleaned, but it should be .noted that under the influence of prolonged exposure to solutions of strong alkalies, iron or steel surfaces may be so badly impaired that even vigorous mechanical rubbing or wiping will not correct the condition. In such instances, it has been found that a brief exposure of the surface to a dilute solution of mineral acid, such as nitric acid, sulphuric acid or hydrochloric acid, will once more make the surfaces susceptible to the benefits of the wiping effect".- In this connection, however, it should also be noted that treatment with these acids will not in itself produce the desired effect. In

of an appropriate phos-v toward the coating soluwhich have been so other words, an acid-treated surface may be put a into condition to behave perfectlyin the phosphate coating solution if it is first wiped, but an alkali treated or cleaned surface may or may not the wiping effect,

Aonther variable familiar to those skilled in this-art is introduced by the fact that no two samples of metal to be treated will behave in the same manner toward a phosphate coatingsoli tion, but substantial] uniformity in the application of a phosphate coating may be attained if the pieces are subjected to the wiping effect or mechanical action.

The wiping effect" just discussed has fresurface whatsoever to quently been resorted to in phosphate coating art,

cause of the amount .of time and labor involved is a costly procedure bein its use. The present invention is based upon the discovery that the wiping effect can in most instances be substantially duplicated-in fact, almost exactly equaled or paralleled-without any mechanical action whatsoever, simply by exposing the surface briefiy to an acid solution comprising oxalic acid. It is an object of the invention, therefore, to so duplicate the wiping effect by chemical means and without the necessity for time-consuming, expensive and laborious mechanical action. The treatment of the invention, when suitably carried out as described below, seems in almost every instance to affect the surface to be coated in exactly the'same way as does the mechanical action above referred to.

In its broader aspects, the invention consists in pretreating a surface, which is to be phosphate coated, with an acid solution comprising oxalic acid. Aqueous solutions of oxalic acid alone may be used over a wide range of concentrations. However, the pretreating solution may contain a variety of substances of an inorganic or organic nature, without great detriment to the effect of the pretreating solution. Among the inorganic substances which may be present in fairly large amounts along with oxalic acid, without doing. harm, are acids such as sulphuric acid, nitric acid, hydrochloric acid, acetic acid, phosphoric acid; salts such as sodium or potassium phosphate, sulphate, chloride, nitrate, acetate, etc. Organic substances which may be present, without detriment, along with the oxalic acid of my pretreating solution are alcohols, ethers, acetone, etc.; wetting agents such as isopropyl naphthalene sulphonic acid; organic corrosion inhibitors such as bases derived from coal tar, thioureas, sulphite pulp waste liquor, condensation products of aldehydes and amines, etc., may be present in small amounts without detriment to the solution. In general, such a wide variety of materials has been tested in conjunction with the oxalic acid of my pretreating solution that I think myself justified in stating that any pretreating solutions areeifective for my purpose which contain oxalic acid and which are definitely acid in reaction. In my experience, however, none of the above list of comparatively harmless substances markedly improves the effect produced by a pretreating solution consisting of an aqueous solution of oxalic acid alone, with one exception. Surfaces which have become so badly impaired as no longer to respond to the wiping effect may be put into condition to respond to this effect or to the effectof my improved pretreating solution, if they are first immersed for a brief period in a dilute solution of a mineral acid.

For such surfaces I may use a pretreating solution containing, in addition to oxalic acid, some other strong acid like hydrochloric acid or sulphuric acid.

In order to secure the full effect of the pretreatment of a metal surface with my pretreating solution, a certain minimum of time of treatment is necessary, depending on such factors as the nature and surface condition of the metal, the temperature, the relative the surface and the solution, and the concentration of the oxalic acid present. Sumce it to say that, in general, the greater the concentration of oxalic acid, the higher the temperature, the greater the rate of relative motion between the surface and the solution, the more rapid is the completion of the beneficial effect of the solution. Thus, for example, on a certain type of steel surface a 0.25% solution of oxalic acid would rate of motion .of

not produce its full effect on simple immersion at room temperature until after about seconds. A 1% to 2% solution at room temperature would produce the full effect in less than 15 seconds. At F., a 0.25% solution of oxalic acid would produce its full effect inside of 15 seconds.

The presence of other substances usually has litttle effect on the necessary time of treatment, unless these substances are present in large amounts.

However, I have discovered that the presence of a small amount of copper or silver dissolved in my pretreating solution somewhat accelerates its action. Thus, a steel panel which had been alkali cleaned required 15 minutes immersion in a certain zinc phosphate coating solution, at 180 F., to produce a coating of standard corrosion resistance. A similar alkali-cleaned panel dipped for 15 seconds into a 0.25% solution of oxalic acid at room temperature behaved similarly when immersed in the same coating solution. Neither of these panels acquired a smooth phosphate coating; both'panels were rough and coarsely crystalline. A similar panel dipped for 90 seconds into the same oxalic acid solution, under the same conditions, acquired a phosphate coating of standard corrosion resistance in the same zinc phosphate bath in 9 minutes. Another similar panel dipped for 15 seconds into the same 0.25% oxalic acid solution to which had been added 0.02% copper as copper sulphate, likewise acquired a coating of standard corrosion resistance in 9 minutes. Both of these last two panels had smooth, very finely crystalline coatings.

I have found that the copper may be added in any desired form, so long as sufficient copper remains dissolved in the solution. Thus the copper may be added as the sulphate, chloride, carbonate, oxide, etc., to any acid pretreating solution, along with another substance if necessary, to bring a sufficient quantity of copper into solution. The amount of copper permissible in my pretreating solution is not critical. However, large quantities of copper in the solution tend to give heavy visible deposits of copper on the metal to be phosphate coated which are undesirable for reasons well understood in the art of phosphate coating. Therefore, I prefer to use an amount of copper insufficient to give a visible coating on the surface to be phosphate coated, although a very thin, barely visible copper film or deposit seems not to be especially harmful to the subsequently produced phosphate coating.

It should be noted at this point that silver also serves to accelerate the action of my pretreating solution and if employed should be used in amounts substantially equivalent to the amounts of copper which are utilized.

At this point I should like to call attention to the fact that oxalate coatings have been produced on metal by the action of oxalic acid solutions, especially in the presence of such oxidizers as hydrogen peroxide, and also that coatings have been produced on zinc by the action of solutions of oxalic acid and many other acids containing oxidizing agents, but in connection with such oxalate coatings my experiments have developed the following facts which are of importance in the successful employment of my invention. When clean iron or steel surfaces are subjected to the action of a dilute oxalic acid solution, they are at first etched and then, after a comparatively long period of time, there is formed thereon a coating-probably of ferrous oxalate. The surfaces which have been so etched by the action of -acquire a smooth and desirable phosphate layer,

Y being coated and the but if the surfaces have been so long subjected tothe oxalic acid solution as to be coated with an oxalate deposit then a subsequently applied phosphate coating exhibits diminished corrosion and heat-resistance and is otherwise less satisfactory for the reception of a final finish of paint or the like.

It will be seen, therefore, that in utilizing my oxalic acid pretreating solutions, it is preferable to employ them in such a way as to avoid the formation of any oxalate coating on the treated surface. This is not difficult to accomplish since a very brief treatment of a metal surface with a solution containing oxalic acid serves to accomplish the function of this invention, whereas much longer treatment is necessary to produce an oxalate coating thereon. Furthermore, I have found that by adding to the oxalic acid solution some other strong mineral acid, n0 oxalate coating will form, no matter how long the time of immersion.

At this point I should like to call attention to the fact that phosphate coating solutions, especially those for useon iron or steel surfaces, are of two general types. The first type is employed to produce heavy, crystalline coatings which are generally left unpainted, although they are frequently'oiled or dyed. Coatings of this type are produced by boiling the surfaces for comparatively long periods of time generally ranging from 30 to 120 minutes, and the solutions generally consist of iron or manganese phosphate or of combinations of these. The resulting coatings are essentially coarsely crystalline, but they achieve relatively great thickness to which latter fact much of their corrosion resistantv value is due. Although thestructure and appearance of this type of coating commonly produced in boiling solutions for protracted periods may be modifled to some extent by various treatments of the surface. to be coated before immersion in the coating solution, the use of myimproved pretreating solution prior to such a coating process is often without significant effect, since a visibly crystalline structure is characteristic of, and generally considered desirablein this kind of coating.

The second type of phosphate coacting solution and process in common use accounts for the greater bulk of metal surface area at present solutions employed generally contain zinc and/or manganese phosphate, although they may accumulate a certain amount of ferrous phosphate during use. Many expedients have been employed for expediting their coating action in order to meet mass production methods and requirements, and the solutions are usually used at temperatures considerably below the'boiling point. The accelerating means commonly employed may be physical, electrical and/or chemical. For example, the coating speed may be due to the violent impingement of the solution onto the surfaces tobe coated, or the piece to be coated may be used as an electrode in the coating solution as an electrolyte with passage therethroughof an alternating current via the surface being coated. Chemical accelerators are also em-' ployed, and these include'oxidizing agents, metals less basic than iron, such as copper, and other metals, such as cobalt and nickel.

In this second type of operation and solution, the coated surfaces ,are generally intended to be covered with a final finish of paint, varnish,

of the surface a number of coats of paint in order toattain the lacquer, japan, or the like, and any roughness necessitates the use of a greater desired smoothness and luster in the'iinal'ilnish. It is in connection with this second type of processes that the use of my invention is of greatest importance. In these processes, .the

stances it has been necessary to subject the surfaces to mechanical action before proceeding effect ,lsmost evident, and in many inrequired for coating alkali cleaned surfaces,

and particularly the roughness of the resultant coating has in many instances prevented the use of alkali cleaning, and in these instances vapor degreasing has ing, but this procedure is also costly. My in-'- vention overcomes all of these difliculties, and

been employed followed by wipmakes it possible to employ ordinary alkali cleaning which is considerably more economical than vapor degreasing.

As a specific example, I will describe my improved proeess as it may be applied to the coatifng of a steel stamping such as an automobile ender.

extraneous matter, and the manner in which this is accomplished depends, of course. upon the nature of the extraneous matter. Rust and scale may be removed with an acid pickle followed by a wash with water, and oil and grease .may be removed either by vapor degreasing or by dipping into an oil solvent, but preferably by spraying the stamping with an alkaline cleaning agent, such as a solution of caustic soda and soap. After washing off theexcess of alkali cleaning solution by means of 'a spray of clean water, the stampingis immersed for 15 seconds,

more or less, in a solution of the following com-.

position:

. Formula No. 1 Oxalic acid oz 1 Water gal 1 This solution is preferably heated to approximately 180 F. and after treatment therewith the stamping is rinsed by a brief immersion in clean water.

The stamping may then be immersed in a phosphate coating solution prepared in accordance with the following instructions:

Formula No. 2

Pounds Zinc oxide 1.50 Phosphoric acid 75% 6.70 Water 4.17

In making the solution of Formula No. 2, the zinc oxide should be the acid should be gradually added until the solution is clear. Suflicient water cpntaining 3% of the solution of the foregoing formula, by

volume, is placed in a vat to submerge the surfaces to be treated and preferably heated to a temperature of approximately 180 F. The stamping is immersed until the coating is completed which generallyrequires from about 7 to .The stamping is first freed from suspended. in the water and- Instead of the solution of Formula No. 1, the

following solution ,may be employed:

Formula No. 3

Oxalic acid w oz 0.5 Copper sulphate pentahydrate oz 0.1 Water -gal 1.0

This solution may be used at any temperature from room temperature to the boiling point. The immersion of the surface to be pretreated should preferably not exceed 30 seconds.

In place of the phosphate coating solution given in Formula No. 2, the following solution may be substituted:

Formula No. 4

Manganese carbonate (85% commercial) lbs 1.750 Phosphoric acid 75% lbs- 5.000 Sodium nitrate lb 0.500 Cupric nitrate trihydrate -lb 0.016 Water to make gal 1.000

The concentrated solution resulting from the above mixture is dilutedwith water to a concentration of 3% by volume. It is used under the same conditions as the solution of Formula No. 2 above. It is to be noted that the above solution will product phosphate coatings which contain a very small amount of metallic copper. The time of coating will be somewhat shorter than with the solution of Formula No. 2.

Another example of the use of my improved process and penetrating solution is for the coating of die castings of zinc alloy. The castings ordinarily require no pre-cleaningor grease removal, although this may be done by conventional methods if desired. A solution is prepared in accordance with the following formula:

Eormula No. 5 Oxalic acid ozs 3 Water gal 1 The castings to be coated are dipped for 30 seconds, more or less, in the solution of the above formula which may be at room temperature or slightly above. They are then rinsed with clean water. A solution is prepared in accordance with the following formula:

Formula No. 6

Zinc oxide lbs 1.5300 Nickel carbonate lb 0.2480 Phosphoric acid 75% gal 0.5000 Nitric acid 38 B gal 0.0487 Water to make gal 1.0000

particular type of phosphate coating solution is concerned, because I have found that my invention is useful with all of the well known phosphate coating solutions with which I am familiar,

and many of them are well known to those skilled in the art. As to the pretreating solutions formulated above, these too are merely illustrative and are not intended to be limiting.

So far as I have been able to determine there is no upper or lower limit upon which my oxalic acid pretreating solutions do not function, although as stated, at very low concentrations the solutions should be employed at comparatively high temperatures and for comparatively long times.

One further example of asolution which may be used both for the cleaning and pretreatment of surfaces which are to be subjected to the action of rustproofing solutions is given in the following formula:

Formula No. 7

Phosphoric acid 75% lbs-.. 3.110 Butyl alcohol gal 0.045 Denatured alcohol gal 0.042 Oxalic acid -ozs 5.000 Water to make gal 1.000

The above material may be applied to iron or steel surfaces, allowed to act for a sufficient time, and may then be washed or wiped off. The iron or steel surfaces may then be subjected to a rust-' proofing solution such as the solution of Formula No. 2 or Formula No. 4. If the above solution is applied to a galvanized iron or zinc surface, the surface will be cleaned and etched. It may then be washed or wiped off and subjected to a solution such as the solution of Formula. No. 6. In both cases the surfaces will be found to acquire a very uniform, smooth phosphase coating.

Certain further observations on the use of my improved pretreating solutions will prove useful. The pretreating solution need not be washed off the pretreated surface before the surface is subjected to the action of the phosphate coating solution. However, the oxalic acid or other ingredient present in the pretreating solution may be harmful to the continued correct operation of the phosphate coating solution. In this case it is desirable to rinse off the pretreating solution before the surface is subjected to the action of the coating solution. It is not necessary to dry the surface after rinsing it, but this may be done without harm by means of a low temperature oven or other convenient means.

As stated, my experiments have demonstrated that my improved process of producing phosphate coatings has been found operable with every type of phosphate coating solution at present known to me. However, the degree of benefit obtained by the use of my new pretreating solution and process varies somewhat, as previously pointed out, with the nature of the phosphate coating solution, the method by which the coating solution is applied, etc. Present experience indicates that my new pretreatment is of maximum benefit when the phosphate coating solutions are rapid acting and when they are used at moderate temperatures not too closely approaching the boiling point and not, in general, below approximately 100 F. The smoothness of the coating produced by slow-acting solutions which are excessively acid reacting, such as solutions of manganese and ferrous phosphates used at or near the boiling temperature, and solutions whose coating ingredient consists entirely or very largely of ferrous phosphate, is not greatly improved by my novel pretreatment. Phosphate coating solutions applied to the surfaces to be coated by violent impingement from nozzles, etc., produce phate salt produce practically successful coatings solution comprising oxalic acid, and then sub-.

only under conditions involving the more rigorous control of temperature, acidity, content of accelerating agents, etc. by these monocalcium phosphate solutions are at best difficult to produce uniformly, and it is likewise difiicult to improve their characteristics by my novel pretreatment.

I claim: 1

1. The method of coating surfaces of iron, zinc and alloys of either, which comprises treating the surface with an acid solution which comprises oxalic acid, and then subjecting it to the action of a phosphate coating solution comprising metal from the group consisting of zinc and manganese as its principal metallic coating constituent.

2. The method of coating surfaces of iron, zinc and alloys of either, which comprises chemically cleaning the surface, treating it with an acid jecting it to the action of a phosphate coating solution comprising metal from the group consisting of zinc and manganese as its principal metallic coating constituent.

3. The method of coating surfaces of iron, zinc and alloys of either, which comprises cleaning the surface, treating it with an acid solution comprising oxalic acidand dissolved metal from the group consisting of copper and silver, and then subjecting the surface to the action of a phosphate coating solution comprising metal from the group consisting of zinc and manganese asits principal metallic coating constituent.

4. The method of claim 2 wherein the oxalic acid treating solution also includes dissolved metal from the group consisting of copper and silver.

5. The method of coating surfaces of iron, zinc and alloys of either, which comprises cleaning the surface, treating it with an; acid solution comprising oxalic acid, and then subjecting it to the action of a phosphate coating solution comprising metal from the group consisting of zinc and manganese as its principal metallic coating constituent. I

6. In a process of forming an adherent phosphatic coating on surfaces of iron zinc and alloys of either by subjecting the surface to the action of a phosphate coating solution comprising metal from the group consisting of zinc and manganese tion.

7. In a processof forming an adherent phosphatic coating on surfaces of iron, zinc and alloys of either by subjecting the surface to the action of a phosphate coating solution comprising metal from the group consisting of zinc and manganese as the principal metallic coating constituent.

the combination of steps which includes cleaning the said surface and treating it with an acid solution comprising oxalic acid and dissolved metal from the group consisting of copper and silver,

The coatings produced prior to subjecting it to the action of the phosphate coating solution. 1

8. A process of preparing surfaces of iron, zinc and alloys of either for the reception of a phosphate coating, which process comprises chemically cleaning the surface and treating it with an acid solution comprising oxalic acid.

9. The process of claim 8 wherein the oxalic acid treating solution also includes dissolved.- metal from the group consisting of copper and silver.

10. A conditioning treatment for articles having surfaces of iron, zinc and alloys thereof which articles are to be coated with a phosphate coating solution, said conditioning treatment comprising subjecting the article to the action of an acid solution comprising oxalic acid.

11. The method of substantially duplicating the wiping effect in the art of coating surfaces of iron, zinc and alloys thereof by treating them with a phosphate coating solution-which comprises subjecting the surface of the metal prior to the phosphate treatment to the effect of an acid solution comprising oxalic acid.

12. The method of claim 10 wherein the oxalic acid solution also includes dissolved metal from the group consisting of copper and silver.

13. The method of claim 1 wherein the residues of the treatment with oxalic acid are removed by rinsing followed by application of the phosphate coating solution before the article has dried.

14. The method of coating surfaces of iron, zinc and alloys of either which comprises chemically cleaning the surface, treating it with an acid solution comprising oxalic acid, removing the residues of the treatment with said acid solution, and then subjecting it to the action of a phosphate coating solution before the surface has dried, said coating solution comprising metal f:om the group consisting of zinc and manganese as the principal metallic coating constituent.

15. A pretreating solution for conditioning surfaces of iron, zinc'and alloys of either for the reception of a phosphate coating from phosphate coating solutions, said pretreating solution comprising oxalic acid and dissolved metal from the group consisting of copper and silver.

16; A pretrcating solution for conditioning surfaces of iron, zinc and alloys of either for the reception of a phosphate coating from phosphate coating solutions, said pretreating solution comprising substantially the following:

Oxalic acid oz o.5 Copper sulphate pentahydrate oz 0.1 Water gal 1.0

17. A pretreating solution for conditioning surfaces of iron, zinc and alloys of either for the reception of a phosphate coating from phosphate coating solutions, said pretreating solution comprising substantially the following:

Phosphoric acid 75% "lbs.-- 3.110 Butyl alcohol gal.. 0.045 Denatured alcohol gal 0.042 Oxalic acid oz 5.000- Water to make gal 1.000

18. An admixture for use in preparing surfaces of iron, zinc and alloys of either for the reception of a phosphate coating from phosphate coating solutions, said admixture including oxalic acid and metal from the group consisting of copper and silver.

' GERALD C. ROMIG.