US2316811A - Method of coating ferrous metal surfaces with water insoluble metallic phosphates - Google Patents

Description

Patented Apr. 20, 1943 I UNITED STATES PATENT I OFFICE I Gerald G. Roinlg, Melrose Park, Pa., assignor to American Chemical Paint Company, Ambler, Pa., a corporation of Delaware No Drawing. Application April 13, 1940, Serial No. 329,491

Claims.

This invention relates to the art of applying phosphate coatings to the surface of iron or steel and especially to the production of coatings of this type which are to be used as a base for paint orthe like. Typical instances are the coating and finishing of automobile bodies and fenders; refrigerator, radio and other cabinets; washing machine parts, etc.

As is well known, in industrial fields of the kind mentioned, modern mass production methods generally demand extreme rapidity of operation. Furthermore, the results must be uniform over a long succession of pieces and the process must be absolutely dependable so as to avoid all interruptions to production schedules. My invention, therefore, is particularly concerned with meeting these requirements.

Other objects of the invention are to provide a process of the character described which can be operated practically continuously on a twentyfour hour a day basis without interruptions due either to chemical or mechanical difficulties or limitations; to provide a phosphate coating process in which the production of sludge in the coating solution during the course of its use is reduced to a minimum as well as to modify the character of whatever sludge is produced, so that shut-downs for the purpose of removing sludge either from the tanks used to hold the solution or from pipes, spray nozzles or pumps, etc., used in spraying it, are practically eliminated; the provision of a process which can be employed without the necessity of heating the coating solution so as to avoid the cost and other disadvantages incident to the employment of heating elements; the provision of a method which is less costly to operate not only from the servicing standpoint but also from the standpoint of chemical consumption; and the provision of a method which will yield extremely fine-grained phosphate coatings.

Before proceeding to a description of the details of my invention, I think that its nature and advantages can be better understood if certain principles of phosphate coating are kept in mind. For instance, it is the generally accepted view that, at the time the coating is actually deposited upon the surface of the piece undergoing treatment, it is necessary that the solution immediately adjacent the surface become sufficiently supersaturated under the existing conditions to cause a precipitation of the phosphate on the surface of the metal. This is effected by virtue of the fact that the hydrogen ion content of the solution is locally reduced as the metal of the work dissolves and when sufficient of the metal is so dissolved, the solution can no longer retain or hold its content of coating metal phosphate, with the result that some of it is precipitated upon the piece being treated. Incidentally,

this action also results in the formation of some phosphate from the metal of the piece so that the coating actually includes phosphates of the metal of the piece as well as phosphates of thecoating metal in the solution. Therefore, in the coating of iron or steel surfaces with, for example, a zinc phosphate coating solution, the final coating on the piece will include a mixture of iron and zinc phosphates.

From what has just been said, it can be seen that for any given set of conditions there must exist a certain optimum condition of local supersaturation of the solution immediately adjacent the surface of the work in order that the desired rated solutions, and for this reason they are inherently slower in their action than are the supersaturated solutions. In order to bring such solutions to the proper degree of supersaturation for effecting the coating operation, it is necessary that they be depleted in acid (1. e., hydrogen ion concentration) adjacent the surface of the work, and this is accomplished by virtue of the action of the acid upon the metal of the piece. When sufficient of the metal is dissolved to bring about the proper degree of supersaturation, then the deposit of the coating takes place as already described.

As distinguished from solutions of the type just mentioned, it has also been proposed that a much more saturated solution be employed, i. e., one in which the degree of saturation much more closely approaches that which is necessary to bring about the precipitation of the desired coat. Such solutions, however, are open to the objection that they produce a relatively great amount of sludge, especially when used at elevated temperatures as is generally customary. As is well known, elevation in temperature decreases the solubility of the coating metal phosphate in baths of this kind so that when heating takes place there is a tendency toward local precipitation of coating phosphate on the heating coils or on the bottom of the tank which contains the solution. On the other hand, if lower temperatures be employed, solutions of this kind will not function rapidly because the degree of solubility of the coating material is considerably higher at such temperatures, so that a greater lengthoftimeisrequiredto obtaintheproper degree of supersaturation in the solution immediately adjacent the surface of the work, Indeed. the chemical activity of the solution at the lower temperature is so much reduced that even when greater extent than the advantages incident to Y the employment of a cold solution outweigh its disadvantages.

There is another factor to which I wish to call attention before proceeding to a detailed description of my invention. From what has been said it would appear that very highly supersaturated solutions, i. e., solutions saturated considerably beyond their normal equilibrium point for any given temperature, would, apparently, be the most rapid in their coating action. Experience, however, indicates that, beyond a certain point, this is not so and one explanation which occurs to me is that in these highly supersaturated solutions the quantity of free acid available (1. e.,- the hydrogen ion content) for attacking the metal to the extent and/or in the manner necessary to effect the deposit of the desired coating is insuillcient. If any coating at all is deposited, it seems to be loose and non-adherent so that, as previously carried out, phosphate coating processes have never attempted to employ solutions which are too highly supersaturated.

The essence of the present invention is to employ a cold supersaturated solution which, at the temperature of use, has a pH which is so high that it is normally incapable of producing a satisfactory coating in any reasonably practical or acceptable length of time and then to treat the solution in such a way as to make it effeck tive for the purpose intended and at the same time to maintain it in a satisfactory coating condition by substantially continuous additions of certain materials to be described below and also to process work in the solution at a rate which is sufliciently rapid to aid in controlling the free acidity of the bath, all as will more fully appear hereinafter.

The phosphate coating solutions employed with the present invention should include only such as are non-oxidizable by air, such, for example, as zinc phosphate, manganese phosphate, cadmium phosphate, and calcium phosphate. For practical'and commercial reasons I prefer to use zinc phosphate because of its readier availability and relatively lower cost. Therefore, in the following description I will refer to zinc phosphate, al-

though it is to be understood that the same principles may very well be applied to solutions of other metal phosphates of a similar non-oxidizable-nature.

I will now describe my improved process as it may be applied to the coating of a succession of steel stampings such as automobile fenders, hood covers, splash guards, gasoline tank covers, refrigerator or radio cabinets, etc.

The stampings, as they come from the press,

are generally contaminated with oil, grease,

and/or drawing and sometimes with rust. Unless the rusting is-severe, the parts may be satisfactorily coated without removal of the rust. If severely rusted, the parts may be subjected to pickling or other de-rusting treatment before entering the cleaning and coating system to be described.

Grease, should be removed from the articlestobecoatedbymyprocess. Thismaybe accomplished by any of the well-known conventional means such as wiping or washing with a solvent, vapor degreasing, or-alkaline cleaning.

Although my improved process of producing a uniform coating on a long series of surfaces is not dependent for its success on a particular type of cleaning procedure, yet the production of extremely uniform coatings on surfaces originally contaminated non-uniformly with grease, dirt. etc is facilitated by a cleaning procedure that leaves the surfaces uniform with respect to residual contamination. This can be accomplished by solvent cleaning methods, but the simplicity and economy of alkali cleaning methods makes them preferable. And when the latter are employed it is desirable to use alkali cleaning solutions which are only very mildly alkaline, and to supplement their action, if necessary, by adding thereto common cleaning assistants such as soap, sulphonated oils, emulsified solvents, etc.

After cleaning, the pieces may be treated with a coating solution prepared in accordance with the following example:

Phosphoric acid, gallons 0.500 Water do 0.500

The foregoing is a concentrated solution, and for use should be diluted with water until the concentration of-the formula is about 2 percent by volume.

The diluted solution is placed inasuitable tank which need not be equipped with means for con-' trolling the temperature of the solution.

Measured at room temperature, the above solution'has an initial pH of about 2.5.

I then raise the pH of this solution to a point where the degree of supersaturation is so high as to render the solution incapable of producing the desired coating at the normal room temperature under which it was prepared. This is preferably done by adding sufllcient caustic soda to accomplish the desired result, although other chemicals effective to raise the pH of the solution may be employed just so long as they do not introduce anything deleterious. The degree of supersaturation of zinc phosphate will probably be in the neighborhood of-20% or even greater, but itis difficult to specify just what the percentage is because the exact pH at which such a supersaturation would exist naturally varies with the temperature. However, the lower the temperature, the higher the pH. For example, at 60 F. the pH will be about 3.8, but at F. the pH will be only about 3. The pHs referred to are those measured by a suitable glass electrode and heat is applied to the solution, any increase above ordinary room temperatures or operating conditions being incident to the fact that the pieces to be treated or processed are frequently warm' at the time that they are contacted-with the solution, due to the previous cleaning steps to which they may have been subjected. Heat introduced in this way is merely an incident to the functioning of a complete series of coating steps and does not involve the use of any supplemental heat.

Although my improved process may be carried out'by immersing the pieces to be coated in the solution, yet I prefer to impinge the solution against the surface of the parts by any suitable spraying equipment, the details of which are not illustrated because they may take various forms, dependent upon individual desires or requirements. In spraying equipment, however, it'will be understood that circulation of the solution through the tanks to the spraying nozzlesor jets is preferably provided for by means of a centrifugal pump, the excess material running down from the work being collected in suitable troughs and returned to the tank or reservoir from which the solution is pumped.

The stampings to be coated are subjected to the impinging action of the jets of solution at a rate such as will provide for a treatment of from thirty seconds to one minute, although this period may vary considerably, depending upon individual tastes, requirements or conditions.

While the stampings are being coated, the pumping system is kept in continuous operation in order to provide for continuous circulation of the solution from the tank to the impinging nozzles or jets, and from the jets to the work, and then back again to the tank.

Equipment of the kind described is familiar to those skilled in this art, but I should like to point out that a substantially continuously operating conveyor system of any suitable sort is particularly well adapted for use with my improved process, the stampings being carried past the impinging nozzles at a steady and uniform rate, and the coating action taking place in a remarkably short period of time.

As soon as the process is placed in operation, I immediately add to the coating solution regulated quantities of the solution of Formula No. l and also of a water solution 01 a salt of nitrous acid, preferably sodium nitrite, and these additions are advantageously made at a point where the solution is in more or less violent agitation so as to ensure thorough mixing with the body of the solution. In this connection, however, I should like to call attention to the fact that it is better not to make the additions, especially oi. the nitrite solution, at a point which is too close to the intake of the pump, for the reason that as the process continues in operation ferrous phosphate will tend to accumulate in the solution which phosphate is quickly oxidized by the nitrite to the practically insoluble ferric state, the insoluble material, of course, precipitating out of the solution. If this precipitation takes place too close to or in the pump inlet it is liable to accumulate in the line, in the pump, or in the spray nozzles, and gradually interfere with the physical working of the process by virtue of the choking action which results.

As stated, the zinc solution which is added may be the concentrated solution of Formula No. 1, and the sodium nitrite solution may contain about 60 grams of sodium nitrite per literof water.

5 ings.

The rate at which these solutions are introduced is regulated so as to maintain the zinc content and the nitrite content of the coating solution substantially constant, and to this end suitable determinations of the 'zinc content and of the nitrite content may be made in any desired manner known to the chemist. In addition, the solution must be maintained in a proper state of supersaturation with respect to the zinc phosphate, as will be further discussed hereinafter. Suillce it to say here that the pH of the solution may serve as an index of proper supersaturation, although in many instances, especially as the operators become familiar with the practical working of the process, the appearance of the finished work itself will serve as a sensitive index of the degree of supersaturation.

The zinc content ofthe solution may be determined electrolytically or chemically. A convenient method consists in titrating a small sample of the solution with standard potassium ferrocyanide in accordance with the directions given in standard chemical text books.

The pH may be determined colorimetrically or potentiometrically by any standard method. The potential of the glass electrode versus a saturated calomel electrode furnishes a convenient measure of the pH of the solution.

The zinc content of the solution may be varied over a considerable range without appreciably affecting the operation of the process, but in the interests of economy it is desirable that the zinc content be kept below about 10 grams per liter. Some of the coating solution is inevitably lost by drag-out on the surface of the coated work, and this loss is naturally smaller in the case of more dilute solutions.

In the above reference to the determination or control of the zinc content, the quantity of zinc referred to is the quantity of zinc with which there is associated at least enough phosphate radical to correspond to the formation of zinc dihydrogen phosphate, as this is the portion of the zinc which is primarily useful in forming the coatcompounds, such as zinc nitrate, zinc not associated with an equivalent amount of phosphoric acid has only a minor effect upon the equilibria involved in the solution, probably due to its "common ion effect.

The quantity of sodium nitrite which should be added should be sufllcient to introduce enough of the nitrite radical to assure a suiiiciently rapid rate of reaction between the solution and the metal of the piece to be coated. As I visualize the nature of the chemical reactions which take place, the nitrite radical introduced by the sodium nitrite oxidizes the surface of the piece to be coated, thereby causing it to be brought into solution with the liberation of hydroxyl ions which practically instantaneously unitewith the hydrogen ions of the acid of the solution to form water. In this way the hydrogen ion concentration is reduced so that zinc phosphate will precipitate uponthe work to produce the coating. Incidentally, as mentioned in the beginning of this specification, there is also formed some iron phosphate at the same time, so that the final coating consists of a mixture of iron phosphate and zinc phosphate.

The amount of nitrite radical necessary to accomplish the desired result must be rather carefully regulated because very large quantities of nitrite may lead to the formation of a visible oxide film rather than a phosphate coating upon If the bath contains other soluable zincthe surface of the work, while too small a quantity will not produce enough reaction to accomplish the desired result. To this end I have found that the concentration of nitrite, computed as sodium nitrite, should not be less than about .12 gram per liter. The upper limit of concentration I have found by experience should be not greater than about 5 grams per liter. The presence of greatly excessive quantities of nitrite will be immediately noted because .the work then takes on a bluish or reddish cast and the coating is no longer gray and very finely crystalline as it should be when the proper phosphate coating is being deposited. However, even less nitrite than this may still be an excessive quantity, because the coating produced will be less adherent and have definitely inferior corrosion resisting properties. The nature of the coating itself, therefore, to the eyes of a skilled operator, will be a very accurate guide to the quantity ofnitrite to be added provided all other factors are properly adjusted.

The sodium nitrite solution mentioned above is only typical, and I wish it to be understood that solutions of any convenient strength may be employed, solong as they are added in proper quantities to accomplish the purpose in view.

Similarly, the zinc-containing solution used for maintaining the zinc concentration of the coating solution may likewise be varied considerably in composition and in concentration. It is convenient, however, to use the same solution as was used for making up the original coating bath. Furthermore, there is an advantage in using the concentrated zinc-containing solution of Formula No. 1 not only when originally making up the bath, as described, but also for replenishing purposes for the reason that the dilution with water causes a secondary ionization which converts more of the dihydrogen zinc phosphate into the water-insoluble monohydrogen zinc phosphate or into the normalzinc phosphate, the net efiect of this being to increase the more insoluble coating constituent of the bath, and therefore to increase its supersaturation with respect to such constituent. This is of material benefit in practicing my invention because its successful operation involves the maintenance of a high degree ofsupersaturation, as described, and anything which tends to this end will minimize the amount of pH controlling ingredient, such as the caustic soda mentioned, which it may be necessary to add.

I should now like to consider briefly certain phases of the "mechanism of the operation of the process at least insofar as my understanding of the phenomena is concerned.

As pointed out near the beginning of this specification, it is necessary for the coating solution to become depleted in hydrogen ion concentration before the phosphate coating can be deposited upon the work. With my process this is accomplished by the nitrite radical which causes the bath to bring into solution some of the iron of the piece to be coated so that the solution immediately adjacent the surface of the work becomes so highly supersaturated as to cause a precipitation of the desired phosphate on the surface of the piece. As successive pieces are processed, it is necessary, as already described, to replenish the coating constituent of the bath, and when this is done by adding thereto a solution of zinc phosphate, it will be obvious that there is continuously added to the bath additional quantities of fresh acid. At the same time, there is a loss from the solution oi zinc phosphate, a part of which is deposited upon the work and another part of which simply precipitates generally within the solution due to the tendency of the solution to relieve its super saturation. Under normal conditions, therefore, the bath tends to gain in acidity from two sources, namely, (1) hydrolysis (loss of supersaturation), and (2) addition of acid due' to constant replenishing with a normally less saturated solution. However, the tendency for the bath to become more acid from the two sources just mentioned may be substantially counteracted by processing work sufliciently rapidly to consume such increase. In other words, the attack of the solution on the iron itself may usually be made sufllcient to compensatefor the' gain in acid from the two sources mentioned, by processing work at a sufliciently rapid rate.

However, if the rate at which new uncoated surfaces are being treated is very slow a definite is so high as to develop a pH which may be too high for the production of the most satisfactory coatings. If this situation arises, the pH can be lowered to the desired point by additions of phosphoric acid.

7 During the processing operation there is another source of increase in acidity which must be taken into account if economical and consistently rapid operation is to be obtained. This arises in the following way. In acting upon the iron to be coated the bath accumulates a certain amount of ferrous phosphate. The action 'of air upon such ferrous phosphate, especially in a spraying proc- V ess, leads to the oxidation of the ferrous phosphate to ferric phosphate, which latter is only very sparingly soluble in the solution, any excess over this very small amount being precipitated out in the form of normal ferric phosphate with the liberation'in the solution of free phosphoric acid. The appearance of this free phosphoric acid would likewise tend to cause a fall in the pH of the solution and, therefore, a fail in its supersaturation. This, however, is prevented in my process by the addition of the sodium nitrite which promptly oxidize any ferrous phosphate in the bath to ferric phosphate without the formation of any free phosphoric acid. Instead, there is formed a small amount of entirely harmless monosodium phosphate (or other dlhydrogen phosphate). Thus, the presence of sodium nitrite, or other equivalent material as will be hereinafter mentioned, completely eliminates this third source of increase in free phosphoric acid, thereby preventing the disturbing effects which it would otherwise introduce.

It will be seen, therefore, that the sodium nitrite acts in two very important ways in the operation of my process(1) it causes the bath to bring into solution iron from the surface of the work and (2) it prevents an increase in the acidity of the bath as described Just above.

In the foregoing description I have specifically mentioned sodium nitrite, but other water soluble salts of nitrous acid can be employed for the purpose, such, for example, as barium nitrite and calcium nitrite.

There are, however, certain other agents which can be quite satisfactorily employed, and I wish to'mention those which I have found by experiment to function in the general manner described. They are bromates, iodates, picric acid and quinone. All of these agents will operate to cause the bath to bring the iron of the work quickly into solution and at the same time keep the solution free of ferrous phosphate by oxidizing it to ferric phosphate before such action can be effected by the oxygen of the air.

I wish to point out that it is not necessary to use the same solution in the maintenance of the zinc concentration of the coating bath as was used for making up the original coating solution. Other solutions may be used for this purpose and, in fact, solid crystallized zinc dihydrogen phosphate may be used for the purpose. However, I prefer to use a concentrated solution of .zinc phosphate rather than a solid compound or admixture for the purpose, since the first named is by far the more convenient. It might be thought that replenishment of the solution by means of solid zinc dihydrogen phosphate would be useful, in that its employment would lead directly to a supersaturated solution of less acid phosphates of zinc and would thus avoid the necessity for the employment of a pH-maintaining agent. However, since the operating bath is continuously maintained in a supersaturated condition, the addition to such a solution of solid zinc phosphate actually results in the formation of a heavy sludge in the solution. This sludge tends to be crystalline in nature and to stimulate the rate of loss of supersaturation of the bath itself. The net result is that far from being an agent which in itself tends to regulate the pH of the solution and to maintain it supersaturated, crystallized zinc phosphate has the opposite effect, tending to cause the bath more rapidly to lose its supersaturation, and by its use one does not avoid, but may actually increase, the amount of pH-regulating agent it is necessary to add. Similar considerations apply to the use of this material in making up a fresh coating solution.

The presence of small amounts of foreign anions in the coating solution is generally quite harmless, but large amounts of sulphates, chlorides, etc., should be avoided. Foreign cations. especially of metals more basic than zinc. are almost entirely without influence on the process. For instance, a large quantity of the cations of alkalis and ammonium do no harm.

Solutions of the phosphate of other coating metals except, of course, ferrous phosphate, may be substituted for the zinc solution given in Formula No. 1, and by way of illustration the following may be employed:

Formula N0. 2

Manganous carbonate pounds 2.35 Phosphoric acid, 75% ..gallons 0.500 Water do 0.500

In using this solution in my improved process. the supersaturation and the manganese content of the coating solution are maintained constant at appropriate values in a manner entirely analogous to the maintenance of the supersaturation and the zinc content of the coating solution of the example of Formula No. 1.

In any case, the content of coating metal in the working solution is determined by standard chemical methods forming no part of the present invention. Rapid methods of chemical analysis, such as titration, are naturally to be preferred.

As will be fully appreciated by those skilled in this art, my improved process results in a number of very marked advantages. The fact that artificial heat need not be supplied to the coating solution leads to a great reduction in the amount of water evaporated, especially when the coating solution is applied by spraying. There is thus saved not only the physical, but also the latent heat of the evaporated water. Furthermore, the constant restoral of water with its inevitable content of dissolved impurities is greatly reduced.

Insofar as the sludge is concerned, the present invention is also of outstanding advantage. All of the phosphate coating processes with which I am familiar inevitably precipitate some insoluble sludge, especially where the solutions are maintained in use for long periods of time and where the volume of work coated is relatively large. With my invention this insoluble sludge is deposited in a relatively crystalline, dense and quick-settling form rather than in the voluminous, flocculent or gelatinous form heretofore common in the prior art. This greatly mitigates the troubles incident to sludge because the usual ill effects due to the poor settling qualities of the previous voluminous or gelatinous sludges are completely overcome by my invention, since the sludge which I produce settles rapidly as a sandy precipitate to the bottom of the reservoir, whence it may be removed with case without getting into the pumps, pipes, nozzles, and so forth, as so fre quently happens with the gelatinous types of precipitates heretofore encountered. Furthermore, because of the much lower rate at which zinc phosphate precipitates from this supersaturated solution in the cold, the absolute quantity of sludge formed is greatly reduced with my invention.

The elimination of heating coils likewise eliminates another serious disadvantage which is otherwise present. Owing to the high temperature of the surface of the heating coils themselves, they rapidly become coated with an adherent, heat-insulating scale which so interferes with the passage of heat into the solution that they must be cleaned at comparatively frequent intervals if the proper temperature of the solution is to be maintained. Moreover, this local heating tends to cause the precipitation of zinc phosphate, with a consequent loss of zinc, and furthermore, a loss of supersaturation.

The degree of supersaturation for all conditions is diflficult to specify. It varies somewhat with such variables as the type of steel, the type of cleaning, and the type of pretreatment of the surface, if any, after cleaning. For some solvent-cleaned ordinary steel, values of supersaturation as low as 20 percent may be adequate.

For alkali-cleaned steel, a somewhat higher super-saturation should be maintained which may reach, in the case of solutions moderately dilute with respect to zinc phosphate, a value as high as 50 percent or more supersaturation. As a practical matter, provided the relative proportions of zinc phosphate and other ingredients are not greatly altered during the course of the coating operation, determinations of zinc (phosphate) and of pH will suflice for the maintenance of the correct supersaturation.

required speed is obtained uniformly on all workprocessed, since too low a supersaturation prevents successful operation of the process and too high a supersaturation causes excessive loss of coating metal by useless precipitation of insoluble phosphates.

vIn any case, for a particular solution the correct pH corresponding to working supersatura-' tion is easily found by trial and the quality of the work then serves as a sensitive index of correct conditions.

I claim:

1. In the art of rapidly coating ferrous metal with an adherent,water-insoluble, metallic phosphate, the method which includes treating the surface with a cold solution of acid phosphate of metal from the class which consists of zinc, manganese, cadmium and calcium, the temperature of which solution does not exceed about 120 F.; said solution being upwards of about 20% supersaturated with respect to coating metal phosphate at the temperature of use but not so highly supersaturated as to result in the production of a loose, non-adherent coating; said solution containing an oxidizing agent from the class which consists of bromates, iodat-es, picric acid, quinone and nitrites, the quantity of said agent being at least sufllcient to cause the solution to attack and dissolve iron from the metal being treated at a rate which is not less than the rate at which it would be dissolved by the presence in the solution of from about .12 gram to about grams per liter of sodium nitrite but not sufiicient to produce an oxide film at the existing temperature; processing a succession of pieces with said solution; and adding to the solution as processing continues regulated quantities of the phosphate,

of said agent and of a compound capable of raising the pH of the solution, such additions being adjusted so as to maintain the supersaturation and the content of oxidizing agent substantially as specified.

2. In the art of rapidly coating ferrous metal with an adherent, water-insoluble, metallic phosphate, the method which includes preparing a concentrated cold solution of acid phosphate of metal from the class which consists of zinc, manganese, cadmium and calcium; diluting said solution with water; adding a sufllcient quantity of a compound capable of raising the pH so as to cause the solution to be upwards of about 20% supersaturated with respect to coating metal phosphate and at least to a degree where its hydrogen ion content is insufficient to attack the metal effectively enough to produce the desired adherent phosphate coating at the existing temperature; adding to said solution an oxidizing agent from the class which consists of bromates,

.iodates, picric acid, quinone and nitrites, the

quantity of said agent being at least sumcient to cause the solution to attack and dissolve iron from the metal being treated at a rate which is not less than the rate at which it would be dissolved by the presence in the solution of from about .12 gram to about 5 grams per liter of sodium nitrite but not suflicient to produce an oxide film at the existing temperature, the temperature of the solution not exceeding about F.; processing a succession of pieces with said solution; and adding to the solution as processing continues regulated quantities of the phosphate,

of said agent and of a compound capable of raising the pH of the solution, such additions being adjusted so as to maintain the supersaturation and the content of oxidizing agent substantially as specified.

3. In the art of rapidly coating ferrous metal with an adherent, water-insoluble. metallic phosphate, the method which includes treating the surface with a cold solution of acid zinc phosphate, the temperature of which solution does not exceed about 120 F., said solution being upwards of about 20% supersaturated with respect to zinc phosphate at the temperature of use but not so highly supersaturated as to result in the production of a loose, non-adherent coating; said solution containing sodium nitrite in quantity sumcient to yield a concentration of nitrite ion of from about 0.12 gram to about 5 grams per liter; processing a succession of pieces with said solution; and adding to the solution as processing continues regulated quantities oi the phosphate. of sodium nitrite, and of a compound capable oi raising the pH of the solution, such additions being adjusted so as to maintain the supersaturation and the content 01' sodium nitrite substantially as specified.

4. In the art of rapidly coating ferrous metal with an adherent, water-insoluble, metallic phosphate, the method which includes preparing a dilute cold solution of acid zinc phosphate; adding a suflicient quantity of caustic soda so as to raise the pH of the solution to a point where it is upwards of about 20% supersaturated with respect to zinc phosphate and at least to a degree where its hydrogen ion content is insufllcient to attack the metal effectively enough to produce the desired adherent phosphate coating at the existing temperature; adding sodium nitrite to said solution in quantity suflicient to yield a concentration of nitrite ion of from about 0.12 gram to about 5 grams per liter; the temperature of the solution not exceeding about 120 E: processing a succession of pieces with said solution: and adding to the solution as processing continues regulated quantities of zinc phosphate, of sodium nitrite and of caustic soda, such additions being adjusted so as to maintain the supersaturation and the content of sodium nitrite substantially as specified.

5. For use in the art of rapidly coating ferrous metal surfaces with an adherent, waterinsoluble, metallic phosphate: a dilute solution of acid zinc phosphate having a temperature not exceeding about 120 E, which solution is upwards of about 20% supersaturated with respect to coating metal phosphate and contains an oxidizing agent from the class which consists of bromates, iodates, picric acid, quinone-and ni- -trites in quantity equivalent in iron dissolving eflect at least to 0.12 gram per liter of sodium nitrite, but not more than about 5 grams per liter.

GERALD C. ROMIG.