US3397093A - Treatment of metal surfaces - Google Patents

Description

United States Patent 3,397,093 TREATMENT OF METAL SURFACES Anthony J. Oswald, Addlestone, Surrey, and Dennis Brian Freeman, Harrow, Middlesex, England, assignors to Hooker Chemical Corporation, Niagara Falls, N.Y., a

corporation of New York No Drawing. Filed May 19, 1964, Ser. No. 368,738

9 Claims. (Cl. 148-616) ABSTRACT OF THE DISCLOSURE A process for treating metal surfaces wherein the surface is contacted with an aqueous acidic phosphate coating solution containing at least one additive selected from the group consisting of nonionic surface active agents and emulsified organic solvents, said contact being sufficient to effect cleaning of the surface, and thereafter, without an intervening water rinse, the thus-cleaned surface is contacted with a second aqueous acidic phosphate coating solution, which solution is substantially free of the aforesaid additives, until a phosphate coating is formed on the surface.

This invention relates to improvements in treating metal surfaces to produce corrosion resistant coatings thereon and more particularly relates to an improved composition and process for cleaning and coating metal surfaces to prevent corrosion thereof.

It has been known for many years to produce crystalline phosphate coatings on metal surfaces, such as those consisting predominantly of iron or zinc, by treating these surfaces with an acid aqueous solution containing zinc phosphate and a coating accelerator. In such processes the work may be contacted with the phosphate solution by immersion or spraying. Any visible rust or scale is normally removed before treatment in such a phosphate coating solution, for example by treatment with a pickling solution consisting largely of phosphoric acid. Even after such pickling, however, the surface of the work is still commonly contaminated with grease or other dirt, and before the surface is contacted with the phosphate coating solution it is generally cleaned by spraying with an alkaline solution containing one or more detergents. Such a cleaning solution can remove grease deposits from the surface very effectively, but it should not be carried over into the phosphating zone, and therefore the surface is rinsed with water between cleaning and phosphating. For example, in a spray coating process two separate rinsing zones are commonly used, fresh water being used in the second zone. Rinsing after the coating has been formed is also usual, and often after this rinsing, coated surfaces are sprayed with a solution containing the chromate ion. The conventional spraying plant therefore comprises six zones. In view of the space and capital investment for such processing equipment, it is desirable to reduce the number of stages or zones used.

It is, therefore, an object of the present invention to provide a method of cleaning and coating metal surfaces to provide corrosion protection, which method is ac complished in fewer steps than has heretofore been possible.

Another object of the present invention is to provide a novel cleaning and coating method which is particularly adapted for use in so-called cold and cool spray phosphating processes.

A further object of the invention is to provide novel cleaning and coating compositions for use in such processes.

These and other objects of the invention will become apparent to those in the art from the description of the invention which follows.

Pursuant to the above objects, in the method of the present invention, metal surfaces are cleaned by spraying them with a zinc phosphate coating solution containing an additive selected from the group consisting of nonionic surface-active agents, emulsified organic solvents and mixtures thereof. After this cleaning step, the surfaces are immediately sprayed with a zinc phosphate coating solution. It is found that no intermediate rinsing between cleaning and coating is necessary when the cleaning solution described above is used. Although these zinc phosphate solutions adequately clean grease and other dirt from the metal surfaces, they are generally not suitable for the removal of visible rust or scale from ferrous surfaces. Therefore, a surface bearing such rust or scale is desirably subjected to pickling or otherwise treated for rust or scale removal before it is treated in accordance with this invention. The zinc phosphate solutions used in the present invention differ from pickling solutions, the former commonly having a pH of from 2.5 to 4, whereas pickling solutions commonly have a pH below 1. Moreover, it is well-known in the art that zinc phosphate solutions of this type operate at a pH at or slightly above equilibrium, or in other words, under conditions such that zinc tertiary phosphate is in equilibrium with free phosphoric acid and primary zinc phosphate, or under somewhat less acid conditions when the solution is supersaturated with primary zinc phosphate.

By way of explaining the mechanism of the present process, it appears probable that the formation of the phosphate coating is initiated, at least on the cleaner parts of the surface, in the cleaning zone and the crystals formed act as nuclei on which the coating proper may form in the second spray zone. Whether this is the case or not, it has been found that for best results the composi tion of the cleaning solution, with regard to its coacting ingredients, should be the same as that of the coating solution used after cleaning in the second spray zone. This method of operation has the additional advantage that the solution in the cleaning zone can be discarded as it becomes dirty and may be replaced by used solution from the second spray zone, which is itself replaced by fresh solution. If desired, the solution in the second spray zone may also contain a surface-active agent to insure the removal of any grease which may not have been removed in the first zone. If such an agent is not present, then before the phosphate-coating solution is used to replace the cleaning solution in the first zone, a surface-active agent should be included or an organic solvent should be emulsified in it.

The non-ionic surface-active agents suitable for use in the present invention are usually those organic compounds having an oxygenated side chain, for example, a polyoxyethylene side chain. Included in these are the essentially straight chain polymerized alkylene oxide condensates having more than five alkylene oxide groups in straight line configuration, e.g., higher fatty acid esters of polyoxyethylene alcohols, ethylene oxide-propylene oxide block copolymers of molecular weight between about 1000 to about 20,000, and so forth. More specifically, these include the ditertiary acetylenic glycols, ethylene oxide nonylphenol, polyethylene glycol tertiarydodecylthiourea ethers, fatty acid esters of higher polyglycols and the like. A preferred non-ionic surface active agent for use in the present invention is the condensation product of a rosin alcohol with 20 moles of ethylene oxide, terminated by a benzyl ether group. Typically, the non-ionic sulrface active agents are present in an amount up to about 5 grams per liter of solution, with amounts from about 0.1 to about 2.0 grams per liter being preferred.

The emulsified solvents which may be used in the present invention are known in the art. These materials may be classified as petroleum solvent materials, used as such or diluted with relatively inexpensive petroleum distillates, and the lower toxicity chlorinated solvents. The former group includes materials such as naphtha, paint thinner, various petroleum fractions, including kerosene, diesel oil and the like. The latter group includes materials such as trichloroethylene, perchloroethylene, methyl chloroform, methylene chloride, ethylene dichloride, o-dichlorobenzene, propylene dichloride, and the like. Additionally all of these materials may be used in conjunction with other solvents, such as alcohols, glycols, glycol ethers, phenols, cresylic compounds, pine oil and the like. As is known in the art, to these solvents will be added a suitable emulsifying agent. Generally, an emulsifiers of the anionic, cationic, and non-ionic type may be used so long as they provide the requisite emulsification of the solvent and do not have an adverse or detrimental effect on the solvent, the other components of the cleaning and/or coating solution or the metal being treated. It will, of course, be appreciated that where the solvent, such as kerosene is used in conjunction with one of the non-ionic surface active agents, as indicated hereinabove, the use of an additional emulsifier, to provide emulsification of the solvent, may not be necessary. Typically, the emulsified solvent, such as the preferred kerosene, will be used in amounts up to about 20 grams per liter of solution, with amounts within the range of about 1 to about grams per liter being preferred.

The method of cleaning and then forming a phosphate coating without an intermediate rinse according to the invention is particularly useful when the phosphate coating is formed in the cold, that is to say at a temperature of 40 to 100 F., though usually not exceeding 70 degrees Fahrenheit. When strong conventional alkaline cleaners are used instead of the solutions of the present invention, there is often difliculty, apparently because the metal surface is passivated, and the coating solutions do not operate satisfactorily at temperatures of 100 degrees Fahrenheit or less.

When an emulsified solvent, e.g., kerosene, is used in the cleaning solution it is particularly advantageous to work in the cold because volatilization of the solvent in the cold is then low, thus reducing waste and the risk of fire. When the process is carried out at higher temperatures, e.g., as in a cool process at about 100-120 degrees Fahrenheit or in a hot process at about 120-180 degrees Fahrenheit, a surface-active agent is preferred over an emulsified solvent. At such higher temperatures the use of emulsified kerosene or other solvent to improve the detergent power, is generally less important.

The phosphate coating and cleaning solutions used in the present invention may be characterized as being acid aqueous solutions incorporating metal phosphates, such as zinc iron or manganese phosphate, dissolved therein. Preferably, because the desired coating weights are obtained in a shorter time, the solutions used contain zinc phosphate. Also, suitable accelerators, such as bromides, nitrates, nitrites or chlorates, may also be incorporated in the solutions. Additionally, the solution used for forming the coating may advantageously contain both zinc and manganese phosphate, the manganese content being up to 50 percent by weight of the zinc content, as described in a copending application Ser. No. 306,332, filed Sept. 3, 1963, now US. Patent 3,338,755. Accordingly, this solution may also be used as the basis for the cleaning solution in the first zone.

As in conventional spray processes, surfaces cleaned and coated in accordance with this invention are generally then passed through two rinsing Zones, the first of which is a water-rinse and the second a rinse with a conventional dilute solution of chromic acid, with or without some phosphoric acid.

The invention may be used with advantage when the coated surface is painted with a water-thinned paint, in particular, a water-thinned primer. In this case the second rinsing step after the coating may be effected with water or with an aqueous solution having a pH between 7 and 11 as described in a copending application Ser. No. 178,247, filed Mar. 8, 1962, now US. Patent 3,154,438. Particularl good results have been achieved when a water-thinned primer is applied by electrophoresis.

It has been found that by using the process of the present invention, excellent phosphate coatings are obtained, particularly in cold spray applications. Moreover, it is found that the overall length of the processing line, even when a painting zone is included, is less than that of the conventional lines presently used. The resulting reduction in initial capital investment and processing time, thus makes the use of the present process very attractive.

In order that those in the art may better understand the present invention and the manner in which it may be practiced, the following specific examples are given. These are intended to be merely exemplary of the preferred embodiments of the invention and are not to be taken as limiting the invention.

Example 1 Steel panels were treated in four zones. In the first zone they were sprayed for 1 minute at Fahrenheit with a solution of the following composition:

Kerosene 5 The surface-active agent was a condensation product of a rosin alcohol with 20 moles of ethylene oxide terminated by a benzyl ether group.

The total acidity of this solution was 20 points and the free acidity was 0.4 point. The total acidity given above is the number of ml. of N/ 10 NaOH (points) needed to neutralize a 10 ml. sample of the solution to a phenol-phthalein end-point whereas the free acidity is the number of ml. of N/10 sodium hydroxide (points) required to neutralize a 10 ml. sample to a bromophenol blue end-point.

In the second zone the panels were sprayed for 1 minute at 70 degrees Fahrenheit in a solution of the same composition except that the surface-active agent and the kerosene were omitted.

In the third zone the coated panels were sprayed for /2 minute at 70 degrees Fahrenheit with water in order to rinse them, and in the fourth zone they were sprayed for /2 minute at 70 degrees Fahrenheit with a solution containing 0.03 gram per liter CrO and 0.03 gram per liter H PO The panels were then oven-dried at 250 degrees Fahrenheit, and excellent uniform crystalline phosphate coatings were obtained with a coating weight of 150 mg./sq. ft.

Example 2 Steel panels were treated similarly in four zones in what may be called a cool rather than a cold process. The panels were first sprayed for 1 minute at degrees Fahrenheit in a solution of the following composition:

G./l. Zn 4.5 P0 13.6 No 14.0 Tartaric acid 0.3 NaNO 0.3 Non-ionic surface-active agent 0.5

The surface-active agent was the same as in the first example. The total acidity was points and the free acidity 1.2 points.

In the second zone the panels were sprayed again at 115 degrees Fahrenheit for 1 minute in a solution of the same composition as in the first zone except that the surface-active agent was omitted.

In the third zone the panels were rinsed by spraying with water for /2 minute at 70 degrees Fahrenheit, and in the last zone theye were rinsed at the same temperature for /2 minute by a solution containing 0.03 g./l. CrO and 0.03 g./l. H PO Excellent crystalline coatings were obtained f 170 mg./sq. ft. The quality of the coatings was as good as that of panels coated by a conventional process carried out in six zones.

Example 3 A zinc phosphate coating solution containing manganese was formulated according to the procedure of the example in Ser. No. 306,332, now US. Patent 3,338,755. To this was added the non-ionic surface active agent used in the previous examples in an amount of about 0.5 gram per liter. The cleaning and coating process of Example 1 is carried out using this composition for both the cleaning solution in the first zone and the coating solution in the second zone. In each instance, the solutions are :at 80 degrees Fahrenheit. Using this procedure, there were obtained on the steel panels treated excellent coatings of 150 milligrams per square foot. The coated panels were subjected to a 5 per-cent salt spray test for 384 hours, as described in the American Society for Testing and Materials (ASTM) Standard Method B1l7-61, the standard 100% humidity test for 504 hours, the standard knife blade adhesion test and the standard impact test, both concave and convex. In each instance excellent results were obtained with little or no evidence of coating failure on any of the panels.

Example 4 Example 5 The procedure of Example 4 was repeated with the exception that the coating solution used in the second zone also contained kerosene in a proportion of about 5 grams per liter. Using this procedure, there was obtained a coating weight of about 165 milligrams per square foot on the treated panels, which coating :gave substantially the same results when subjected to the test of Example 3 as did the coatings of Example 3.

While there have been described various embodiments of the invention, the compositions and method described are not intended to be understood as limiting the scope of the invention, as it is realized that changes therewithin are possible and it is further intended that each element or component recited in any of the following claims is to be understood as referring to all equivalent elements or components for accomplishing substantially the same results in substantially the same or equivalent manner, it being intended to cover the invention broadly in whatever form its priniciple may be utilized.

What is claimed is:

1. A process for treating metal surfaces which comprises contacting the metal surface with a first aqueous acidic phosphatin-g solution selected from the group consisting of zinc, iron and manganese phosphating solutions, said solution having a pH of from about 2.5 to 4 and containing at least one additive selected from the group consisting of non-ionic surface active agents and emulsified organic solvents, said additives being present in amounts of 0.1 to 5 grams per liter and amounts of l to 20 "grams per liter, respectively, maintaining said first solution in contact with the metal surface for a period sufficient to effect cleaning thereof, thereafter, without an intervening water rinse, contacting the thus-cleaned surface with a second aqueous acidic phosphating solution having a pH of from about 2.5 to 4, said second phosphating solution having the same composition as said first phosphating solution but being substantially free of the additives selected from the indicated group, and maintaining said solution in contact with the metal surface for a period sufiicient to form a phosphate coating.

2. The process as claimed in claim 1 wherein the first and second phosphate solutions are zinc phosphate solutions.

3. The process as claimed in claim 2 wherein the zinc phosphate solutions also contain manganese phosphate, said manganese content being up to about 50% by Weight of the zinc content.

4. The process as claimed in claim 3 wherein both the first and second phosphating solutions are at a temperature from about 40 to degrees Fahrenheit.

5. The process as claimed in claim 4 wherein the nonionic surface active agent additive is present in an amount of from about 0.1 to 2 grams per liter and the emulsified organic solvent additive is present in an amount from about 1 to 10 grams per liter.

6. The process as claimed in claim 2 wherein after the phosphate coating is formed on the metal surface, the thus-coated surface is rinsed with water and, thereafter, the thus-rinsed surface is rinsed with a dilute solution of chromic acid.

7. The process as claimed in claim 2 wherein after the phosphate coating is formed on the metal surface, the thus-coated surface is rinsed with water, the water-rinsed surface is then rinsed with an alkaline solution having a pH between 7 and 11 and thereafter, the rinsed surface is painted with a water-thinned paint.

8. The process as claimed in claim 7 wherein the waterthinned paint is applied by electrophoresis.

9. The process as claimed in claim 5 wherein the first phosphating solution is discarded when dirty and is replaced by used second phosphating solution, after adding thereto at least one additive selected from the group consisting of non-ionic surface active agents and emulsified organic solvent, said additive being added in amounts of 0.1 to 5 grams per liter and amounts of 1 to 20 grams per liter, respectively, and wherein said used second phosphating solution is itself replaced by fresh phosphating solution which is substantially free of the additive selected from the indicated group.

References Cited UNITED STATES PATENTS 1,949,713 3/1934 Gravell 148-615 2,209,291 7/1940 Witty l48-6.15 2,470,136 5/ 1949 Bramberry 148--6.15 2,479,423 8/1949 Snyder l48--6.15 2,744,555 5/1956 Nicholson et al. 1486.15 2,800,422 7/ 1957 Piccinelli 148-6.15 2,809,906 10/1957 Baecker 148-6.15 2,857,298 10/ 1958 Smith 1486.15 3,007,817 11/1961 Cavanagh et a1. 148-6.15

RALPH S. KENDALL, Primary Examiner.