US2298312A - Method of phosphate coating ferrous metal surfaces - Google Patents

Description

Patented Oct. 13, 1942 METHOD OF PHOSPHATE COATING FER- ROUS METAL SURFACES Gerald G. Bomig, 'Melrose Park, Pa., assignor to American Chemical Paint Company, Ambler, Pa., a corporation of Delaware No Drawing. Application August 3, 1940, Serial No. 351,131"

3 Claims. (01. 148-65) This invention relates to the art of treating ferrous metal surfaces with an acid zinc phos'- phate solution in order to produce thereon adherent, rust-resistant and paint-retentive, water-. insoluble phosphate coatings and is particularlyv concerned with the interrelationship which exists between the coating step per se and the cleaning step which precedes it. The invention is I especially useful in processes which employ relatively cold coating solutions. I

Ferrous surfaces which are to be phosphate coated by treatment with an acid zinc phosphate solution almost invariably require cleaning because of the occurrence thereon of various contaminations including oilor grease, drawing compound, dirt, filings, soldering flux, and the like. Customarily these may be removed by any one of a number of well known cleaning procedures, such as vapor degreasing, wiping with grease solvents, or treatment with alkali'clean- .ing solutions. Of these, the alkali cleaning solutions are for most purposes the most effective and are frequently preferred to any of the others. I have found, however, that the degree of alkalinity ofthe cleaning solution has a very marked efiect upon the subsequent coating step, especially where the coating solution is relatively cold and also where it contains any one ofa number of the common so-called oxidizing agents and particularly where it contains hydrogen peroxide. For example, a high degree of alkalinity, such as is characteristic of many of the industrial cleaners which employ caustic soda, silicates, phosphates, etc., either with or without soap, has a tendency to cause the subsequently produced phosphate coatings to be coarsely crystalline or otherwise unsatisfactory and even non-adherent as well as to be formed more slowly. I have also found that the control of the phosphate coating step is much more critical where the alkalinity of the washing solution is relatively high andthat considerably more chemicals are necessary in th coating step than would otherwise be required.

With the foregoing in mind, the principal objects of my invention will be better understood and maybe specifically recited as including the provision of a combination of cleaning and coating steps which will result in greatly improved and more rapidly applied coatings; in greater ease and simplicity of control of the coating step; in the consumption of a minimum amount I of chemicals; and in less sensitivity to the degree of alkalinity in the cleaner. It is also an object to provid a method of cleaning and coat- 5 ing in which the temperature of the coating bath can be maintained relatively low, 1. e., below approximately F.

Before proceeding with a. detailed description of my invention, I should like to point out that my invention is not directed to the technique of the phosphate coating step per se because this is now well developed and its principles very well understood. In myprior Patent 2,132,883', issued on October ll, 1938, I disclose employing sodium nitrite or its equivalent in a relatively supersaturated zinc phosphate bath for the purpose of removing the iron which enters the bath from the work being processed. Another example of the use of a supersaturated bath is found in my copending application Serial No. 375,448 which is a continuation in part of my earlier application Serial No. 322,176. In the process therein disclosed it is pointed out how hydrogen peroxide may be employed in supersaturated acid zinc prosphate baths for use in the high speed coating of ferrous 'metal surfaces.

In connection with this phosphate coating technique, two examples of which ar referred to just above, I have found that the objects of the present invention, as previously recited, can be attained by combining with the phosphate coating stepa preliminary cleaning step employing an alkaline cleaner in which is emulsified a solvent as described below.

The type'of emulsion cleaner which I have found to be satisfactory for th purpose includes a solvent oil consisting primarily of terpenic,

cyclic 0r poly-cyclic hydrocarbons, which oil is chosen from the class consisting of dead oil, anthracene oil, creosote oil, pine oil and high flash coal tar naphtha. A sufllcient quantity of one or more of these solvent oilsis emulsified in an aqueous solution containing an ordinary soap which may be a soap or an alkali metal, ammonia, or an amine, such as triethanol amine, the fatty acid of which soap may be any of those commonly present in soaps, such as oleic, stearic, ricinoleic, abietic, and natural or artificial'mixtures of them, etc. In addition, the solution should contain one or more of th industrial alkalis, such as are commonly employed for the cleaning of metals. Emulsions of the solvents of the class listed above are very effective in removing grease and are particularly effective, when used in reasonably high concentrations, in removing dirt, carbon smut, filings, etc., which cannot be removed by emulsions of solvents, such as aliphatic hydrocarbons, petroleum fractions, etc.

Although the process of my invention is operable with solvents chosen from the class given just above without respect to the boiling range of the solvent, yet I-have found that there is a considerable advantage in the use of solvents having an initial boiling point beginning at about 150 C. or above. Typicalfofthese are neutral hydrocarbon oil from coal tar, sometimes referred to as "dead oil, creosote oil, etc.

As examples of concentrated emulsions which may be used in making up emulsion type cleaning baths suitable for use with my invention are the Any of the foregoing formulae may be formed into stable emulsions by methods known to those skilled in the art and they are to be added to the cleaning bath proper in the proportion of from about /2 to about three gallons per 100 gallons of cleaning solution.

In the cleaning bath which may be prepared as above mentioned by adding a small amount of one of the concentrated emulsions of Formulas 1 to 3 for example, to water or to a dilute alkaline cleaning solution, I prefer to have present a free alkalinity calculated as free caustic soda not in excess of approximately 0.005 normal. In other words, I prefer to maintain the free alkali not over a titration of 0.5 m1. when determined by titrating a 10 ml. sample with 0.10 normal sulphuric acid using phenolphthalein as the indicator. .If the alkalinity of the diluted cleaning emulsion is not within the limits specified, adjustment of the alkalinity may be made by appropriate additions of alkali or acid. For the present purpose, I prefer to use phosphoric acid as a neutralizer in case this is necessary.

The use of an alkaline cleaner in which is emulsifled a solvent as described just above almost completely overcomes the disadvantages of alkali cleaning hereinbefore noted. For example, if the metal to be coatedis cleaned with a cleaner containing alkali in' any appreciable strength, when such alkali is used alone. or in customary alkali soap cleaning solutions, the subsequently applied phosphate coating is markedly inferior in character unless compensating changes are made inthe composition of thecoating bath, which changes involve corresponding increases in the amount of chemicals consumed.- -But with when using any of the foregoing cleaners or their equivalent within the disclosure of this speciflcation, I have found that it is possible to employ in the coating bath a zinc concentration which lies between approximately 144% and 288%. This is unusually low, especially for supersaturated baths which ordinarily tend to produoe considerable sludge. Furthermore, in a coating process employing hydrogen peroxideand myinvention is'particularly applicable in such a combination-Abe hydrogen peroxide concentration can be maintained as low as between approximately '.0044'% and .011'1% which, also,

-is extremely low. In addition, my invention makes it possible to maintain these low concentrations even in relatively cold coating solutions, i. e., those employed below approximately 130 1".

As is well known in the art, the zinc content and the hydrogen peroxide content of the coat- ,ing bath will become depleted as processing contimes, and it is customary to renew them either periodically or substantially continuously under careful regulation. The zinc may be and is preferably added as a concentrated solution of zinc phosphate and the hydrogen peroxide may be added either as such or in the form of somesuitable chemical which will yield hydrogen peroxide in the coating bath, such, for example, as sodium peroxide. Although the details of these replenishing procedures in and of themselves form no part of the present invention, yet reference can be made to my prior patent and to my copending application above referred to for fuller discussions thereof if so desired.

In order that my invention may be more fully appreciated, especially in connection with itsadvantage in reducing the quantity of coating chemicals necessary, I wish to refer to the fact that in order to maintain the requisite pH and supersaturation of the coating bath it is frequently necessary in practice, particularly in a relatively cold process employing hydrogen peroxide, to add to the coating bath periodically, a quantity of caustic soda or other pH raising chemical. This is especially true where' the alkalinity of the cleaning solution is relatively high. By my invention, therefore, I am enabled to save not only much of the cost of the caustic soda or other pH raising chemical, but also much of the cost'that is involved in unnecessary precipitation of zinc sludge which always occurs where it is necessary to raise the pH or maintain the supersaturation at a point which is so high as to cause excessive precipitation on the bottom and sides of the tank in which the solution is kept.

Experience with the present invention has shown that in order to maintain the supersaturation of the coatingbath at a satisfactory value, it is only necessary to add a quantity of caustic soda (or its equivalent) ranging from about .7 to about 1.1 lbs. for every lb. of zinc added as a concentrated zinc phosphate solution. This is considerably less than has heretofore been cusmy invention it is possible to obtain the advantages associated with the excellent grease and dirt removing properties ofan alkali cleaner without the resulting deleterious effects heretofore incident to other alkali cleaners. In fact a much tomary in this art and it is my belief that the saving can be attribuated to'the herein described cleaning step which I have combined with the coating step. 3

. a The advantages of my invention are particularly noticeable in connection with a spray process of cleaning and coating, the details of which are well known in the art but certain of which will 3 be referred to. so that the invention may be more clearly understood and appreciated. In an ordinary spray process of coating, it is customary -76 to suspend the pieces to be treated upon an-end-* collected for reuse.

nel having suitable spray nozzles, draft connections, drainage basins, collecting chambers, conduits, pumps, etc. In a system of this kind the first stage is the'cleaning stage, the second stage is a preliminary rinsing stage generally employing plain hot water; the third stage is a second rinsing stage frequently employing a rinse which is maintained at a pH between approximately /2 and by the addition of caustic soda or phosphoric acid as may be required for this purpose; and the fourth stage is the coating stage proper. In all of the stages the bath is sprayed against the work by suitable nozzles and the run-off is Between stages there are socalled draining spaces so that the articles being treated, as they pass through the equipment, will have an opportunity to drain away any excess material before they enter the succeeding stage.

In this way unnecessary losses of solutions are avoided and excess contamination of one solution with the preceding solution can be prevented. These features, of course, are well known to those skilled in this art, but my invention fits with peculiar advantage into this situation because the atmosphere in the treating tunnels where spraying operations are carried out is necessarily exceedingly moist and warm-so much so, in fact, that during passage of the pieces being treated through the drainage sections, chemical action is set up on the surface of the metal which, in ways not yet fully understood, seems to have an adverse efiect upon the coating processas a whole. I have found that my invention reduces the difficulties to a very marked degree because, for reasons which are not yet fully understood but which have been proven repeatedly in practice, an emulsion type cleaner of the character herein specified improves the final coating, minimizes consumption of chemicals and ensures the formation of excellent and extremely adherent coatings in aminimum space of time.

I claim:

1. In the art of coating ferrous metal surfaces with a water-insoluble phosphate by treating them with an acid zinc phosphate solution. the

method which includes cleaning the surface preparatory to the coating step by washing it with an alkaline cleaner in which is emulsified a'solvent oil chosen from the class consisting of dead oil, anthracene oil, creosote oil, pine oil and high flash coal tar naphtha; rinsing the surface. and then treating it with the coating solution.

2. In the art of coating ferrous metal surfaces with a water-insoluble phosphate by treating them with an acid zinc phosphate solution containing hydrogen peroxide, the method which includes cleaning the surface preparatory to the coating step by washing it with an alkaline cleaner in which is emulsified a solvent oil chosen from the class consisting of dead oil, anthracene oil, creosote oil, pine oil and high flash coal .tar naphtha; rinsing the surface, and then treating it with the coating solution.

3. The 'method of claim 1 wherein the coating solution is maintained at a temperature below approximately F.

4. The method of claim 2 wherein the coating solution is maintained at a temperature below approximately 130 F.

'5. The method of claim 1' wherein the solvent oil has an initial boiling point beginning at about C. or above.

6. The method of claim 2 wherein the solvent oil has an initial boiling point beginning at about 150 C. or above.

'7. In the art of coating ferrous metal surfaces with a water-insoluble phosphate by treating them with an acid zinc phosphate solution containing hydrogen peroxide, the method which includes cleaning the surface preparatoryto the coating step by washing it, with an alkaline cleaner in which is emulsified a solvent oil chosen from the class consisting of dead ofl, anthrac'ene oil, creosote oil, pine oil and high flash coal tar naphtha; rinsing the surface, and then treating it with the coating solution, the zinc concentration in the coating solution lying between approximately 144% and 288% and the hydrogen peroxide concentration lying between approximately .0044% and .0117%.

8. The method of claim '7 wherein the supersaturation of the coating solution is maintained by adding thereto as required from about .7 to

about 1.1 lbs. of caustic soda for every lb. of

zinc added as a concentrated zinc phosphate solution.

GERALD C. 301416. I