US3272664A - Composition and method for coating metal surfaces - Google Patents

Description

United States Patent C 3,272,664 CUMPOSlTiON AND METHOD FOR COATING METAL SURFACES lLouis Schlossberg, Oak Park, Earl Eugene Logue, Birmingham, and Paul J. Carrier, Detroit, Mich., assignors to Detrex Chemical Industries, Inc., Detroit, Mich, a corporation of Michigan No Drawing. Filed July 25, 1963, Ser. No. 297,694

6 Claims. (Cl. 148-617) This invention relates generally to compositions and methods for producing conversion coatings on ferrous metal surfaces. More particularly this invention relates to compositions and methods for providing what are known as amorphous or micro-crystalline coatings of exceptional quality and weight composed predominantly of iron phosphate-iron oxide mixed crystals.

Conversion phosphate coatings applied from aqueous phosphate solutions have previously been known and used in the metal treating art. Numerous processes have been proposed, yielding coatings of widely varying properties. While the compositions and processes of the prior art have provided coatings which are satisfactory in some cases, it has been found that there are disadvantages inherent in such compositions and processes, particularly under conditions of actual commercial use where rela tively heavy coatings are desired. Included among such disadvantages are the formation of appreciable amounts of sludge, the necessity of utilizing a run-in period or initial aging, the difiiculty of consistently forming a heavy coating, and the formation of excessive amounts of undesirable loose deposit on the surface of the coating, which in the industry is referred to as dust.

Therefore, it is a primary object of this invention to produce a premium quality conversion coating of the type which is known in the art as an iron phosphate coating and which possesses the superior attributes now expected of such premium coatings for paint adhesion, salt-spray resistance and humidity resistance, while at the same time consistently providing heavy coatings.

It is a further object of this invention to provide a process of producing such coatings without the formation of appreciable sludge.

It is a further object to provide an improved process wherein the run-in period or initial aging of a fresh bath can be eliminated or greatly abbreviate-d and still achieve results comparable to those from a well-aged, smoothly operating bath.

It is a still further object to provide an improved composition and process which produces consistently heavier coatings than do presently available processes, under normal operating parameters, thus assuring the user of a Wider safety factor for meeting the coating weight requirements of his customers.

A further object is to provide an improved liquid concentrate which facilitates handling and additionally offers less hazard because of a greatly reduced oxidizer content, while improving the coating activity of the solution.

A still further object is to provide a process which precludes the necessity for seeding the process solution with almost immeasurable but necessary amounts of ferrous ion in order to achieve heavy coating weights on steel in the order of 45 milligrams per square foot and higher.

A further object is to provide an improved composition and process which produces substantially less dust on coated parts than is now possible. This has the decided advantage of reducing handling problems and contamination in subsequent operations.

Other objects will become apparent from the description of the composition and process set forth below.

3,272,664 Patented Sept. 13, 1966 ICC? It has been known in the prior art that dihydrogen phosphates of the alkali metals, including ammonium, may be caused to produce phosphate coatings on metals, under proper operating conditions, by the addition of an oxidizing radical, such as the C10 ion, provided at least 0.5% of oxidizing radical up to about 4% oxidizing radical is included in the bath. It has also been known and established that a reduction in total chlorate ion by as little as 0.1% below the minimum of 0.5% results in a working bath which does not perform effectively, either the desired quality and degree of coating or the general operability of the coating bath. Other agents have been proposed in place of chlorate, such as nitrite, bromate, and sulfite ions. However, effective results in the use of such compounds have been limited to amounts at least equivalent in activity to 0.5 by weight of chlorate ion.

In the development of the present invention, we have discovered a hitherto unsuspected synergistic relationship between chlorate and dinitrobenzene sulfonic acid (specifically 2,4 dinitrobenzene sulfonic acid or its sodium salt) when used in combination in dihydrogen phosphate coating baths, Which makes possible a highly functional combined total additive content substantially below 0.5%. This synergistic relationship is evidenced by the fact that the addition of either compound separately to dihydrogen phosphate coating baths in an amount corresponding to 0.4% and substantially less gives unsatisfactory results, whereas the addition of a combination of the two compounds in an amount totaling 0.4%, or even less, provides phosphate coating results which are measurably superior to phosphate coatings produced through the use of 0.5 or more of chlorate ion both as to quality and coating weight. As a result of this synergistic action, very superior and unexpected coatings are achieved at greatly reduced over-all concentrations, so that it is possible, in the practice of the present invention, not only to obtain greater coating weights than were previously possible but also :to avoid some of the disadvantages which apparently stem from the greater total amounts of oxidizing agents previously found to be necessary, particularly where heavy coatings are desired.

The following examples are set forth as illustrative of the present invention, but it should be noted that the specific formulations of concentrates and working solutions hereinafter set forth are intended merely as examples of the present invention. The relative amounts of the various ingredients may be varied over a wide range, within which the benefits of the synergistic relationship of the additive compounds are obtained. However, for the formation of heavy coatings mgs. per sq. ft. or higher) essentially free of dust, the preferred range of chlorate ion is from 0.3% to 0.4% and the range of dinitrobenzene sulfonate is from 0.002% to 0.4%. In the examples the sodium salts of the phosphate, chlorate and nitrobenzene sulfonate ingredients have been employed. For each ingredient any of the other alkali metal salts may be used, but the sodium salts are preferred as a matter of economy. The pH range is preferably from about 4.2 to about 6.

Water 56.50

To prepare a working solution containing 0.38% of C10 ion, 5.25% of the above concentrate was added to 94.75% of water. The pH was adjusted with soda ash to approximately 5.0. The solution was then aged by spray processing steel to the extent of 20 square feet of steel surface area per gallon of solution. The chlorate ion level was maintained at 0.38% at all times.

Standard mild steel panels were then spray coated with this solution at normal operating conditions of one minute spray exposure, at 160 F. and 10 psi. After spray coating, several panels were checked for coating weight using a standard chromic acid stripping technique. The coating weight was 43 mgs. per square foot.

Several of the phosphate coated panels were also painted by immersion in a standard epon base appliance paint, the average paint thickness after baking being about 0.4 mils. The painted panels were subsequently scribed and then submitted to salt spray test exposure. Failure, which was recorded when one-quarter inch peel was evident from the scribe line and/or when heavy blisters occurred, took place in six days. This salt spray performance Was rated as poor.

Example 2 A working solution was prepared essentially identical to that of Example 1 except that the sodium chlorate was omitted and in its place 0.4% of 2,4 dinitrobenzene sodium sulfonate was added. The solution was pH adjusted and aged in the same manner, as in Example 1. Then standard steel panels were spray coated, painted, scribed and tested as in Example 1. The coating weight was 78 mgs. per sq. ft. The time to failure in the salt spray cabinet was 13 days. This salt spray performance, though superior to that of Example 1, was not sufliciently good. Moreover, panels phosphated with this working solution showed excessive amounts of undesirable dust residues.

Example 3 A concentrate was made up as follows:

Percent by weight Monosodium phosphate 25.00 Phosphor acid, 75% 9.25 Sodium chlorate 9.25 2,4-dinitrobenzene sodium sulfonate 0.50

Water 5600 To prepare a working solution containing 0.37% of C10 ion and 0.025% of 2,4 dinitrobenzene sodium sulfonate, 5.1% of the above concentrate was added to 94.75% of water. The solution was then pH adjusted, aged and panels treated as in Example 1. It was found that the coatings produced were of a very high degree of quality, fine grained, and essentially free of dust. The coating weight was 72 mgs. per sq. ft. Salt spray performance was excellent, no failure taking place even after 17 days of exposure.

Example 4 A concentrate was prepared to conform to the composition taught in US. Patent No. 2,609,308, as follows:

Percent by weight Monosodium phosphate 25.00 Phosphoric acid, 75% 9.25 Sodium chlorate 12.30 Water 53.45

To prepare a working solution containing 0.5% C10 ion, 5.25% of the above concentrate was added to 94.75% of water. The solution was pH adjusted and aged in the same manner as Example 1. Standard panels were spray phosphated, painted, scribed and tested as in Example 1. The coating weight obtained was 50 mgs. per sq. ft. Salt spray results indicated only 12 days to failure.

Example A working solution was prepared essentially identical to that of Example 3 except that the solution contained 0.38% C ion and 0.05% 2,4 dinitrobenzene sodium sulfonate rather than 0.38% C10 ion and 0.025% 2,4 dinitrobenzene sodium sulfonate. The solution was then pH adjusted and aged and steel panels were spray coated, painted and scribed as in Example 1. The coating weight was 70 mgs. per sq. ft. The time to failure in the salt spray cabinet was over 17 days. Coatings were of excellent quality and free of dust.

In the practice of the present invention, the working solution is applied to the metal surfaces by either spray or immersion. The invention is particularly useful in what are known as power spray washers through which the metal work is conducted on a conveyor system. Time cycles of approximately 30 seconds to seconds duration are satisfactory for spray processes, while immersion applications require from about 3 to 5 minutes of processing time. The preferred operating temperature is approximately -165 F., although somewhat lower or higher temperatures may be used for specific purposes.

While greatly improved results are achieved in practicing the present invention through a broad range of concentrations of synerigstic additives, we have found that the particular value of the present invention resides in the fact that heavy coatings in the order of 70 milligrams per square foot can be achieved having enhanced paint adhesion characteristics and at the same time the total concentration of synergistic additives may be held to levels substantially lower than heretofore employed.

It will be understood that substitution of equivalent materials and variations in the amounts of the materials employed are included within the present invention, as defined in the claims set forth below.

Having described our invention, we claim:

1. A solution for producing phosphate coatings on ferrous metal surfaces consisting essentially of water, an alkali metal acid phosphate, an alkali metal chlorate and an alkali metal dinitrobenzene sulfonate synergistically cooperative with said chlorate.

2. A solution for producing phosphate coatings on ferrous metal surfaces consisting essentially of water, an alkali metal acid phosphate, an alkali metal chlorate and an alkali metal dinitrobenzene sulfonate in an amount synergistically cooperative with said chlorate to cause said solution to produce heavy, substantially dust free coatmgs.

3. The solution of claim 2 in which the alkali metal chlorate is present in the range from 0.30% to 0.4% and the alkali metal dinitrobenzene sulfonate is present in the range from 0.002% to 0.4%.

4. A solution for producing phosphate coatings on ferrous metal surfaces consisting essentially of water, sodium acid phosphate, sodium chlorate and 2,4 dinitrobenzene sodium sulfonate synergistically cooperative with said chlorate.

5. The process of producing a phosphate coating on a ferrous surface comprising applying to the ferrous surface a solution consisting essentially of water, alkali metal acid phosphate, alkali metal chlorate and an alkali metal dinitrobenzene sulfonate synergistically cooperative with said chlorate.

6. The process of producing a phosphate coating on a ferrous surface comprising applying to the ferrous surface a solution consisting essentially of water, alkali metal acid phosphate, alkali metal chlorate and 2,4 dinitrobenzene sodium sulfonate in an amount synergistically cooperative with said chlorate to cause the formation of a heavy phosphate coating.

References Cited by the Examiner UNITED STATES PATENTS 2,375,468 5/1945 Clifford et al. 1486.15 2,609,308 9/1952 Gibson et al. 1486.15 X 2,657,156 10/1953 Hyams et al. 1486.17 X

ALFRED L. LEAVITT, Primary Examiner.

R. S. KENDALL, Assistant Examiner.

Claims (1)

1. A SOLUTION FOR PRODUCING PHOSPHATE COATING ON FERROUS METAL SURFACES CONSISTING ESSENTIALLY OF WATER, AN ALKALI METAL ACID PHOSPHATE, AN ALKALI METAL CHLORATE AND AN ALKALI METAL DINITROBENZENE SULFONATE SYNERGISTICALLY COOPERATIVE WITH SAID CHLORATE.