US2789070A - Composition and process for phosphatizing metal - Google Patents

Description

United States ?atent Oliice COMPOSITION AND PROCESS FOR PHOS- PHATIZING METAL Harry B. Copelin, Niagara Falls, N. Y., assignor to E. I. du Pont de Nemonrs and Company, Wilmington, Del., a corporation of Delaware No Drawing. Application July 1, 1955, Serial No. 519,670

20 Claims. (Cl. 148-615) This invention relates to a process for producing phosphate coatings on metals. More particularly, it relates to the production of phosphate coatings on steel or other ferrous surfaces in a nonaqueous medium. In one aspect, the invention is concerned with the process for producing phosphate coatings. In another aspect, it is concerned with a solution for producing the coatings.

Phosphate coatings are widely employed today to protect metals against mild oxidation and to provide surfaces conditioned to receive paint finishes. Generally such coatings are formed in somewhat complicated processes involving aqueous solutions at several points. Thus the base metal may sequentially be treated with phosphate applied in hot aqueous solution and then passivated by chromic acid dissolved in water. Degreasing operations and hot or cold water rinses are utilized as necessary. Such procedures yield phosphate layers which themselves afford some protection against corrosion but which are probably of more value as bases for additional coatings of paint.

Various disadvantages appear, however, in presently used. phosphatizing methods. The cumbersome and lengthy sequence of steps necessary is evident from the outline given. The use of aqueous solutions, however, gives rise to other but perhaps less obvious disadvantages. Water, of course, tends to corrode iron if contact is prolonged for any length of time. Workpieces must, therefore, be carefully dried after the rinses or aqueou solutions are applied. In addition, the presence of water greatly increases problems involved in removing grease from or degreasing the metal preliminary to the phosphatizing.

Degreasing can easily be carried out by contacting the metal surface with a degreasing solvent, particularly of the chlorinated hydrocarbon type. Trichlorethylene and perchlorethylene are typical degreasing solvents but others, s'uch asmethylene and ethylene chlorides, may be used as well. These materials, however, are incompatible with water and remain isolated in a separate phase when contacted with the latter. Presence of the two phases may hasten the corrosion of the base metal. Separate phases may be formed either by carrying water into the chlorinated hydrocarbon of the degreaser or by carrying the grease solvent into the aqueous phosphatizing bath. Thus it is sometimes necessary to isolate the degreasing stage from stages involving aqueous solutions by intermediate hot alkaline cleaning solutions and by rinsing and drying steps- As a result of the additional steps necessitated by the incompatability of water and the chlorinated solvents, degreasing by means of these solvents is usually dispensed with in phosphatizing. The simplicity of the degreasing itself and the cleanliness obtainable therewith are thus lost to the phosphatizing process.

A general object of the present invention is, consequently, provision of a new and improved process for developing phosphate coatings on metal articles. .A qfler object is provision of a phosphatizing method simpler than those in customary use.

A specific object of the invention is provision of a phosphatizing process which utilizes no water or aqueous solutions. A further object is provision of a process for phosphatizing metals directly compatible with degreasing operations employing chlorinated hydrocarbons.

An additional specific object is provision of a nonaqueous solution for producing phosphate coatings on metals. A corollary object is the provision of means for solubilizing phosphoric acid in specific nonaqueous media.

The above-mentioned and yet further objects are achieved in accordance with this invention by a process consisting essentially of three steps. These steps, in sequence, are: 1) the ferrous, or other, article to be phosphatized is degreased with a chlorinated hydrocarbon in the customary manner; (2) the degreased article is dipped in a special, nonaqueous solution containing orthophosphoric acid; and (3) the dipped article is degreased to remove excess phosphate. Of these steps, the first is substantially conventional and the third can be accomplished very simply. The heart of the invention thus resides in the second step.

The phosphate coating produced in the nonaqueous bath is somewhat difierent from that generally obtained from aqueous solutions. Thus it is lighter in color than the coatings heretofore produced, i. e., it is gray rather than black. X-ray studies also indicate that there may be some difierences in physical structure. The protection afiorded and the base metal and the value of the phosphate layer as a substrate for paints are, however, comparable to those obtained from aqueous phosphatizing baths.

In carrying out the process of the invention, preliminary degreasing is accomplished in a conventional manner. The workpiece, dry and physically cleaned of loose scale and dirt, is suspended for a short time in the hot vapors above boiling trichlorethylene or tetrachlorethylene. Alternatively, it may be dipped into the boiling solvent. In either case sulficient contact time should be allowed to permit the temperature of the workpiece to reach that of the solvent.

The degreased workpiece is transferred, while still hot, to the phosphatizing solution of this invention. The

phosphatizing solution consists primarily of a chlorinated,

and

wliere ,Rijrepres'ents anialkyl radical containing between" Patented Apr. 16, 1957 land 18 or more carbon atoms. The aliphatic chain may be branched or unsaturated although straight-chain, saturated compounds are preferred. Little solubilization can be achieved with compounds containing less than 6 carbon atoms because of the insolubility of the agents themselves. Best results are found where R contains between 8 and 18 carbon atoms. Specific hydrogen phosphates that may be used include those of the hexyl, octyl, decyl, dodecyl, stearyl, oleyl and like radicals. When diaIkyl phosphates are utilized the two Rs within the same molecule may be different.

The concentration of the solubilizing agent may vary to some extent. A weight of agent equal to not less than about O.50-1% of the weight of the solvent should be'employed for best results. Up to about by weight can be used to advantage when the agent is itself sufiiciently, soluble. Higher percentages of the alkyl phosphateshould not in general be added since they mayunfavorably affect the boiling point and other properties of the solvent. Some of the alkyl acid phosphates are themselves not very soluble in chlorinated hydrocarbons. The quantity of these added will be controlled by their solubility since it is desired to avoid the appearance of a second phase in the solution.

' Chlorinated hydrocarbons containing 0.5l% or more.

by weight of an alkyl hydrogen phosphate will dissolve up to about 0.5% by weight of orthophosphoric acid. Between about 0.1 and 0.5% of the acid, when dissolved, will provide a good phosphate coating on a metallic Workpiece. Concentrations near the upper limit of solubility are, however, preferred since they speed the formation of the coatings and need replacement or replenishment less frequently.

It will be understood. that phosphoric acid must be maintained within the bath employed. Maintenance can be accomplished by regular periodic additions of acid or by determination of the quantity present followed by addition of the requisite amount. The acid may also be replenished by occasional slow additions to the. bath carried out in such a manner that the last small quantity added just fails to dissolve.

Thorough contact of workpiece with phosphatizing solution is readily insured by immersing it in the latter. The only important variables involved in the immersion are the contact time and the temperature, atmospheric or ambient. pressure being assumed throughout.

Phosphate coatings will be formed at any temperature between about 0 and: 120 C. when iron or steel articles are contacted with the solution. Formation is, however, very slow at the lower temperatures. At the higher temperature, e. g., 100-1206 C.,. the alkyl phosphates begin to decompose. This decomposition is very appreciable at 120 C. For these reasons, between. 80 and 100 C. is preferred. 1n some cases it is desirable to maintain heated. vapor in the space above the solution. The

temperature chosen should then be near the boiling point.

of the solution. Trichlorethylen'e boils at around 86 C. and thus is an ideal solvent for the purposes of the invention. Perchlorethylene, boiling at around 120 C., can also be used as the solvent but preferably not at its boiling temperature. Mixtures of perchlorethylene and trichlorethylene can be used to obtain boiling points intermediate those of the constituents.

At temperatures between 80-120 C. suitable coatings form in about 1-10 minutes. At 60-80 C. up to about one-half hour may be required. Below 60 C. the required contact time is too long for practical Under the conditons of pressure, temperature and concentration prevailing when the coating step of the process is operated, small amounts of Water are carried from the bath in the vapors produced. The orthophosphoric acid of commerce can thus be used with good results. Solutions even more dilute may also be employed but are less preferred.

In addition to the ingredients mentioned, i. e., the solvent, the solubilizer and phosphoric acid, a fourth nonaqueouscomponent is preferably included in the bath. This component is a polar compound such as glacial acetic acid which serves, not to improve the phosphatization directly, but to prolong the life of the bath. A concentration of about 1% by weight is desirable. The reflux life of a bath containing no acetic acid is only 10-l2 hours. This can be prolonged to 200 hours by the inclusion of the second acid. 7

Apparatus for carrying out the process of the inventio may be substantially conventional and so simple that it need not be illustrated. If desired a degreaser containing two compartments for liquids and vapors, separated by a partition, and a sump for drainage can be used. The two compartments and the zone above the sump should be in communication in the vapor phase. The entire apparatus can be open to the atmosphere if it is provided with cooling coils at the top to maintain a steady air-vapor interface.

The first compartment is maintained at a temperature above the boiling point of the solvent utilized and the second is maintained at the preselected phosphatization temperature. A workpiece can thus be continuously passed through the vapors of the first compartment, where standard vapor degreasing occurs, and then dipped below the surface of the liquid in the second compartment. After phosphatization the article is withdrawn through the vapors over the bath. During this passage a second degreasing takes place removing excess phosphate from the surface. The workpiece is then drained for a short time over the sump and removed from the apparatus substantially dry. The surface of the metal is immediately ready for further treatments, painting in particular, as desired.

While the invention has been described primarily with reference to iron or alloys thereof, its application is not restricted thereto. Metallic surfaces generally subject to phosphatizing can, in fact, be treated in the chlorinated. hydrocarbon bath. Zinc and aluminum-zinc provide examples of such metals in addition to cast iron and steel.

Advantages of this invention, generally paralleling the objects, are obvious from the foregoing. description. Some, however, may be mentioned:

(1) A phosphatizing bath is provided which. elimihates the need for water and is compatible with chlorinated degreasing solvents;

(2) The large number of steps required in most phosphatizing processes is' reduced to three; and

('3) Requirements for equipment, particularly in rinsing and drying steps, are minimized.

There follow some examples which illustrate the bath and process of this invention in operation. In these examples all. percentages are reported in terms of weight. Pressures in all; vapor-phase. operations were ambient.

Example I To 270- g; of boiling trichlnrethylene; was. added 30 g; of a commercialoctyl hydrogen phosphate carrying dissolved 3 g. of. 85%. orthophosphoric acid. The commercial phosphate consisted. of about 40% of monooctyl dihydrogen phosphate, about 40% ofv dioctyl hydrogen phosphate and a remainder of trioctyl phosphate. The composite solution was boiled until water stopped. distilling over, additional t'ri'chlorethylene being added to maintain the weight. 3 g; of glacial'acetic acid was then added thereto.

e.,raaovo A'number of panels of 22 gauge steel were vapor degrea'sed and wiped to remove smut(carbonac'eous dc posits). They were immediately dipped for two'minutes into the boiling solution described above and then removed, cooled and again vapor degreased in trichlorethylene. The degreased panels were sprayed with two coats of a urea-formaldehyde alkyd interior enamel, commonly used in refrigerators, and baked for 30 minutesat 280 F. These panels are' referred to below as test panels. 7

Control panels of the same 22 gauge steel utilized above were prepared by degreasing the base metal, coating it directly with enamel and baking the enamel. The following tests were made to compare the two sets of panels:

(1) Blister test. After 1 month immersion in water at 100 F. test panels showed negligible blistering. The controls blistered badly, at least 50% of their total surface being blisters; I

(2) Bending test. A test panel was bent sharply. The paint film broke into a multiplicity of tiny cracks along the bend but showed no tendency to peel. The enamel coat on a control panel subjected to the same treatment broke with a single crack peeling away from the bend;

(3) Swelling test. A test panel was immersed for five minutes in boiling tric hlorethylene with essentially no damage to the enamel. Control panels were com pletely stripped of their coating in 10-30 seconds of this treatment;

(4) Filiform corrosion test. Test and control panels were scribed through their respective coatings and exposed for 1 month to the atmosphere over a saturated aqueous solution of potassium chloride containing a small amount of hydrochloric acid. 6080% less corrosion was noted in the test panels; and

(5) Salt spray test. Test and control panels were scribed and exposed at 100 F. for 125-150 hours to a 20% salt-spray fog. i. e., a spray of an atomized 20% salt (NaCl) solution. At the end of the exposure the under-rusting at the inscribed cuts was severe on the control but negligible on the test panels.

It may be noted that the commercial alkyl phosphate mixture of neither this nor of succeeding examples contained very much free orthophosphoric acid. Tests showed that the addition of phosphoric acid was essential to obtain the coatings desired. Some other commercial samples labeled Alkyl Phosphates may contain enough orthophosphoric acid to render the addition unnecessary.

Example 2 Test panels formed as in Example 1 were compared with phosphatized panels prepared as follows: 22 gauge steel was degreased, washed, dipped in a conventional aqueous phosphatizing bath, washed, dipped in chromic acid, washed and finally painted with the alkyd enamel used on the test panels.

The coating on the test panels was found to be slightly harder than that on the controls and somewhat more difiicult to cut with a knife. Corrosion resistance as shown by the filiform and salt spray tests was, however, slightly better on the control panels.

Example 3 Example 1 was substantially duplicated except that the phosphatizing solution, prepared by dissolving in trichlorethyle'ne 1% of the crude commercial octyl hydrogen phosphate containing about 5% of commercial phosphoric acid, was free from acetic acid. Comparison tests with the treated panels gave about the same results as in Example 1.

-- Example 5 Clean steel panels were immersed for one hour at 25 C. in a trichlorethylene solution containing 3% of oleyl dihydrogen phosphate and about 0.15% of orthophosphoric acid. The panels were then vapor degreased, painted and tested as in Example 1 with similar results.

Example 7 The tests of Example 1 were substantially repeated except that a commercial octyl hydrogen phosphate solution was used containing 60% of monooctyl dihydrogen phosphate, around 2030% dioctyl hydrogen phosphate and a remainder of trioctyl phosphate. The test panels produced were not quite as good as those of Example 1.

It will be obvious to those skilled in the art that various modifications can be made in the details described without departing from the spirit of the invention. Thus the phosphate solution can be applied to the workpiece by spraying or by any other method equivalent to the dipping exemplified. Consequently I propose to be bound solely by the appended claims.

Having described my invention, I claim:

1. A uniform, substantially water-free composition of matter suitable for phosphatizing metals consisting essentially of a chlorohydrocarbon degreasing solvent containing between about 1 and 10% by weight of an acid alkyl phosphate in which the alkyl radical contains between about 6 and 18 carbon atoms and an effective phosphatizing quantity of orthophosphoric acid.

2. The composition of claim 1 containing additionally between about 0.5 and 1.5% by weight of glacial acetic acid.

3. A uniform, substantially water-free composition of matter suitable for phosphatizing metals which comprises around 98% by weight of a chlorohydrocarbon degreasing solvent, up to around 10% by weight of an acid alkyl phosphate in which the alkyl radical contains 6-18 carbon atoms and about 0.1-0.5% by weight of orthophosphoric acid.

4. The composition of claim 3 containing additionally between about 0.5 and 1.5 by weight of glacial acetic acid.

5. The composition of claim 3 in which the chlorohydrocarbon degreasing solvent is a member of the group consisting of trichlorethylene and perchlorethylene.

6. The composition of claim 3 in which the alkyl radical is a member of the group consisting of octyl and oleyl radicals.

7. A composition consisting essentially of trichlorethylene containing, dissolved, up to about 10% by weight of octyl acid phosphate and about 0.l-0.5% by weight of orthophosphoric acid.

8. The composition of claim 7 containing additionally about 0.5-1.5% by weight of glacial acetic acid.

9. A composition consisting essentially of trichlorethylene containing, dissolved, up to about 10% by weight of oleyl acid phosphate and about 0.1-0.5 by weight of orthophosphoric acid.

10. The composition of claim 9 containing additionally about 0.5-1.5% by weight of glacial acetic acid.

11. The process of phosphate-coating an article made at a m ta of the. c s cap ble of reacting with phosphoric acid to form a metal'phosphate which comprises imm rsin he article for etwee about 2 and 60. minme; a a t mpera ure b twee about 25 and 120 n a comp ion cons sting essentially of a chlor ydrocarbon degreasing solvent containing about 1 and by weight of an acid alkyl phosphate in which the alkyl radical contains between about 6 and 18 carbon atoms and an efiective phosphatizing quantity of orthophosphoric acid.

12. The process of claim 11 in which the chlorohydrocarbon degreasing solvent contains additionally between about 0.5 and 1.5% by weight of glacial acetic acid.

13. The process of claim 11 in which the chlorohydrocarbondegreasing solvent is a member of the group consisting of trichlorethylene and perchlorethylene.

14.. The process of claim 11 in which the alkyl radical is a member of the group consisting of octyl and oleyl radicals. 1 i

15 The process of phosphatizing a metal capable of being phosphatized which comprises immersing the same for between about 2 and 60 minutes at -100"'-C. in a composition consisting essentially of trichlorethylene containing about 1 10% by weight of octyl acid phosphate and about 0.1-0.5 by weight of orthophospho'ric acid.

16. The process of claim 15 in which the composition contains additionally about 0.5-1.5% by weight of acetic acid.

17. The process of phosphatizing a metal capable of being phosphatized which comprises immersing the 5391C. for between about 2- and minutes at lift- C. in a composition consisting essentially of trichlorethylene containing about by Weight of oleyl acid phos: phate and about 0. 1-0.5 by weight of orthophosphot'ic acid.

18. The process of claim 17 in which the composition contains additionally about 0.5.1.5 by weight of acetic acid. 1 7

19. The methodof solubilizing phosphoric acid in a chlorohydrocarbon ,degreasing solvent which comprises supplying saidacid to said solvent in the presence of be ween b utlan l' 10% y e gh of an acid alkyl phosphate in which the alkyl radical contains 6-18 carbon atoms. ,20. The method of claim 19 in which the chlorohydroe carbon; degreasing solvent is a member of the group consisting of trichlorethylene and perchlorethylene and the alkyl radical- ,a member of the group consisting of c ylan o y vra i a si I References Cited in the tile of this patent UNITED STATES PATENTS Great Britain Nov. 10, 1954

Claims (1)

1. A UNIFORM, SUBSTANTIALLY WATER-FREE COMPOSITION OF MATTER SUITABLE FOR PHOSPHATIZING METALS CONSISTING ESSENTIALLY OF A CHLOROHYDROCARBON DEGREASING SOLVENT CONTAINING BETWEEN ABOUT 1 AND 10% BY WEIGHT OF AN ACID ALKYL PHOSPHATE IN WHICH THE ALKYL RADICAL CONTAINS BETWEEN ABOUT 6 AND 18 CARBON ATOMS AND AN EFFECTIVE PHOSPHATIZING QUANTITY OF ORTHOPHOSPHORIC ACID.