US3141797A - Phosphating process - Google Patents

Description

United States Patent 3,141,77 PHOSPHATENG PROCESS Howard Pelrar and Leo Donald Barrett, Cleveland, Ohio, assignors to The Lubrizol Corporation, Wicidifie, ()hio, a corporation of ()hio No Drawing. Filed Sept. 7, 1961, Ser. No. 136,440 Claims. (Cl. 148-'6.15)

This invention relates to phosphated metal surfaces and more specifically to a method of treating iron and its alloys prior to phosphating. Still more specifically, it relates to a method of improving the corrosion-resistance of phosphated metal surfaces by pretreating the metal with an alkaline solution of a permanganate. The permanganate pretreatment not only improves the corrosion-resistance of phosphated surfaces but also improves the bonding characteristics of these surfaces with respect to other coatings.

The widespread use of inorganic phosphate compositions provides a means of preparing protected metal surfaces. It is well known that phosphoric acid or the acid salts thereof react with metal surfaces to form phosphate coatings. Since these phosphate coatings prevent corrosion of the metal, phosphating has become an important step in preparing protective undercoatings for many of the well known siccative coatings such as paint.

It has now been further discovered that metal surfaces, especially ferrous surfaces, can be put into better condition for phosphating by treating said surfaces with certain permanganate compositions. This permanganate conditioning is believed to be a way of uniformly activating the metal surface so that it will react more rapidly and completely with any subsequently applied phosphate composition.

The process of pretreating and phosphating metal surfaces, in accordance with the teachings of this invention, is a means of preparing improved protective coatings on many kinds of metal surfaces for many different purposes. One purpose, for example, is in the field of processing metals. Here, when an unprotected metal article is covered with an organic coating the slightest break in the coating will enable corrosion to take place, not only at the exact point of the break but also in the surrounding area. Thus, when paint is applied to bare metal and then scratched ofi, corrosion will occur rapidly not only at the scratch but also under the paint adjacent to the scratch. On the other hand, if a phosphate primer prepared according to this invention is used, it will protect the painted metal surface by preventing the spread of corrosion beyond the scratched area of the painted surface.

Still another purpose of this invention is to provide a method of phosphating thin metal sheets so that they can be formed into various shapes without rupturing the protective coating. The rupturing resistance of these coatings appears to be due, for the most part, to the tenacity with which they are bonded to the pretreated metal surfaces. It has been noted that when light guage metal is phosphated without a permanganate pretreatment, the resulting coatings can not withstand the abuse of a forming operation to the same extent as the phosphate coatings tinuous process of uniformly conditioning metal surfaces prior to phosphating.

A still further object of this invention is to provide a method of preparing phosphated metal surfaces useful not only as protective undercoatings but also for forming desirable bases which have improved bonding characteristics with respect to other coatings.

These and other objects will become apparent from a further and more detailed description of the invention.

More specifically, it has been discovered that substantial improvement in the corrosion-resistance of phosphated metal surfaces can be obtained by first treating such metal surface with an alkaline solution of a metal permanganate and then phosphating said permanganate-treated surface with an aqueous phosphating solution.

Heretofore, it was usually necessary to subject the metal surface to a cleaning operation before it was phosphated. In the instant process, where the metal is pretreated with permanganate, the cleaning operation frequently is unnecessary. In instances where it appears to be desirable to clean the metal, however, it may be accomplished either mechanically or chemically. Any good metal cleaner that will remove dirt, oil, grease, etc., from the metal surface may be used. Alkaline cleaners are especially useful and these include solutions of alkaline materials such as caustic soda, phosphates, silicate, borax, and the like. Vapor degreasing, sandblasting, grinding, brushing, and the like are also suitable methods of cleaning metal surfaces for the purposes of this invention.

The alkaline cleaners are especially useful. The temperature of such solutions, in use, ranges from about 68 F. to 210 F. while the cleaning time and concentration of solution are adjusted accordingly, i.e., a high operating temperature allows a short cleaning time as well as the use of a less concentrated solution. A specific metal cleaning process may consist of spraying the surface with a solution of 5.67 parts of sodium metasilicate, 3.77 parts of sodium tripolyphosphate and 0.56 part of an alkylphenyl polyether alcohol in 1300 parts of water. In such instance the aqueous solution is maintained at a temperature of approximately 170 F. and the metal surface is sprayed for a period of about 45 seconds. The cleaned metal surface then is rinsed with water.

The cleaned metal surface may be dipped or sprayed with an aqueous alkaline solution containing a metal permanganate. The metal permanganate is preferably an alkali metal permanganate such as potassium, sodium, or lithium. The alkalinity of the cleaner generally is im parted by a metal hydroxide, particularly potassium and lithium hydroxides. In addition to the alkali metal hydroxides, such alkalizers as sodium silicate, sodium carbonat e, sodium phosphate, etc., may be used.

The combined concentration of permanganate and alkalizer in aqueous solution may range from about 0.1 to 25 percent by weight with optimum concentrations ranging from about 1 to 5 percent by Weight. At these concentrations, the relative proportions of the perman ganate to alkalizer, respectively, may range from about 1:05 to 1:4. The essential purpose of the alkalizer is to maintain a basic solution. In order to pretreat a metal surface successfully, the permanganate solutions should have a pH greater than 8 and preferably a pH greater than 11.

The permanganate solution may be applied to the metal surface by dipping or spraying said surface with a solution at temperatures ranging from about 68 F. to 210 F. for a period of at least about 30 seconds. Preferably, the temperature of the solution may range from F.210 F. with the period of treatment ranging from about 1 to 60 minutes. The period of treatment together with the temperature and concentration of the permanganate bath may be adjusted to give optimum results. In dipping the metal surface in permanganate, for example, maximum eifectiveness is obtained with bath temperatures of about 160 F.200 F. with periods of treatment ranging from about 5-15 minutes. On the other hand, in spraying operations maximum effectiveness is obtained by spraying for about /2 to 5 minutes at a temperature of about 170 F.- 190" F.

After the permanganate treatment, the metal surface is optionally rinsed in water and then coated with a phosphating composition. The phosphate coatings may be applied to the permanganate treated metal surface by using any of the coating solutions and techniques known in the art such as those disclosed, e.g., in U.S. Patents Nos. 1,206,075; 1,247,668; 1,305,331; 1,485,025; 1,610,- 362; 1,980,518; and 2,001,754. The techniques employed include, for example, rolling, brushing, dipping, or spraying the metal with a phosphating solution at a temperature varying from about 68 F. to about 210 F. Generally, best results are obtained when the phosphating solution is sprayed on the metal at a temperature within the range of from about 140 F. to about 210 F. If desired, however, the aqueous phosphating bath may be used at higher temperatures, e.g., 225 F., 250 F. or 300 F. by employing superatmospheric pressure.

The phospating compositions preferred for the purpose of this invention form amorphous or micro-crystalline integral coatings on the permanganate-treated metal surface. These amorphous or micro-crystalline coatings are formed on metal surfaces by treating them with aqueous inorganic phosphating solutions having an acid number within the range of from about 5 to about 100 and containing as essential ingredients from about 0.05 to about 1.0 percent by weight of zinc ion, from about 0.25 to about 2.0 percent by weight of phosphate ion, from about 0.25 to about 6.0 percent by weight of nitrate ion, and from about 0.1 to about 3.0 percent by weight of a cation selected from the group consisting of lithium, beryllium, magnesium, calcium, cadmium, strontium, and barium. While the above compositions are preferred for preparing micro-crystalline coatings on a pretreated metal surface, a particularly preferred composition comprises a solution having an acid number within the range of about 5 to 50 and containing as essential ingredients from about 0.05 to about 0.6 percent by weight of zinc ion, from about 0.25 to about 1.5 percent by weight of phosphate ion, from about 1.0 to about 4.0 percent by weight of nitrate ion, and from about 0.1 to about 0.8 percent by weight of calcium ion. Some illustrative examples of these preferred phosphating baths are given in Table I, with other operable embodiments being disclosed in copending US. patent application, Ser. No. 373,449, filed August 10, 1953, by John A. Henricks and owned by the assignee of the present application. Said application matured into US. Patent 3,090,709, granted May 21, 1963.

Table I Solution Solution Solution Solution A B C D Ion, wt. percent:

The acid number referred to above is an indication of the acidity of the phosphating solution. It represents the number of milliliters of 0.1 normal sodium hydroxide solution required to neutralize a 10 milliliter sample of the phosphating solution in the presence of phenolphthalein as an indicator. Generally an acid number of from about to about 100, and preferably from about 5 to about 50 is required to obtain phosphating solutions which 4; are capable of providing commercially satisfactory coating weights and coating speeds. Coating weights suitable for the purposes of this invention will generally range from about to about 600 mg./ft. However, a preferred range is from about to about 300 mg./ft.

The above phosphating solutions can be prepared conveniently by dissolving zinc dihydrogen phosphate in water to supply the essential zinc and phosphate ions, adding calcium nitrate and, optionally, ammonium nitrate to supply the calcium, nitrate and ammonium ions, and finally adjusting the acidity of the solution by the addition of phosphoric acid and/or nitric acid. Alternatively, the solutions can be made by dissolving zinc nitrate, calcium nitrate, calcium phosphate, and ammonium nitrate in water, and then adjusting the acidity of the solution by the addition of phosphoric acid and/ or nitric acid. In place of the ammonium ion, any of the alkali metals, particularly sodium, may be used.

The ions of the bath used in the method of this invention may be derived from a variety of compounds and it appears to be of no consequence whether or not these ions come from different salts or acids. Regardless of the identity of the salts which provide the nitrate ion, calcium ion, zinc ion, and phosphate ion, the resulting bath is eifective to serve the purpose of this invention. It is only essential that these salts or acids be used in amounts which provide the necessary concentration of the several ions discussed above.

Solution A in Table I, for example, is formulated by dissolving 32 grams of calcium nitrate, 6.8 grams of 75 percent phosphoric acid, 17.5 grams of zinc nitrate, and 7.9 grams of ammonium dihydrogen phosphate, in sufficient amount of water to make 1 liter of solution.

Although the above phosphate coating compositions are preferred, particularly the calcium-containing phosphate baths, there are many other inorganic phosphate coating compositions which, as indicated earlier, may be improved by being applied over a permanganate treated metal surface.

After the pretreatment and subsequent phosphating operation, the coated metal surface is usually water-rinsed and then optionally treated with an aqueous solution of an oxidizing or passivating agent. These agents include, e.g., phosphoric acid, chromic acid, mixtures of phosphoric and chromic acid, and any of their metal salts or mixtures thereof, particularly the alkali or alkaline earth metal salts. Although the acid rinse is not essential, it is desirable, however, since it tends to seal the phosphate coating and to improve its bonding characteristics with any subsequently applied coating. A typical acid or salt rinse consists essentially of 0.01 to 1.0 part by weight of chromic acid or the calcium salt thereof per 1000 parts by weight of water.

A preferred rinse consists essentially of an aqueous solution containing chromate ions, phosphate ions and calcium ions. This rinse may be prepared by dissolving a calcium base, e.g., CaCO in an aqueous solution of chromic acid and phosphoric acid. The concentration of such rinse solutions will vary depending upon the length of time and the temperature at which the phosphated metal surface is treated. It is important that the time, temperature, and concentration of the rinse be adjusted so that no appreciable amount of phosphate coating is removed. After the acid and/ or salt rinse, the phosphated metal surface is dried by any means which will remove moisture without dehydrating the phosphate coating. In instances where speed is required, electric .heaters may be used for this purpose.

The following examples illustrate specific modes of carrying out the process of the present invention:

Example 1 Several 4-inch x 12-inch ferrous metal panels were cleaned by spraying them for 45 seconds at F. with a cleaning solution made by dissolving'5.67 parts of sodium metasilicate, 3.77 parts sodium tripolyphosphate, and 0.56 part of an alkylphenyl polyether alcohol in 1300 parts of water. The cleaned panels were water-rinsed and then pretreated by dipping them into a two percent by weight aqueous solution of equal parts of potassium permanganate and sodium hydroxide for about 15 minutes at a temperature of about 170 F. After the pretreatment, the metal panels were again water-rinsed and then spray-phosphated for 70 seconds at 160-165 C. with a phosphating bath made by diluting, in sufficient water to yield a 2 percent aqueous solution, a phosphating composition consisting of 33.6 parts of zinc nitrate, 31.6 parts of calcium nitrate, 34.4 parts of monosodium phosphate, and 0.4 part of kerosene.

After the phosphating operation was completed, the panels were water-rinsed, sprayed for about 30 seconds at 70 F. with a 0.01 percent aqueous solution of chromic acid, and then dried.

Example 2 A set of two steel panels was processed in the same manner set forth in Example 1. Another set of two steel panels was similarly processed, except that the alkaline permanganate treatment was omitted.

Subsequently both sets of panels were painted with a good, commercial oil base paint. The painted panels were line-scribed with a sharp instrument down to the bare metal. The scribed panels were then subjected to the salt spray test described in ASTM Procedure B117- 54T. In this test, metal panels are exposed for a predetermined time to the corrosive atmosphere of a mist or fog of 5 percent aqueous sodium chloride solution at 95 F. Corrosion generally starts at the scribed portion of the panel and then creeps under the paint, causing it to blister and flake from the metal panel. The loss of paint is reported as the average loss from each side of the scribe in thirty-seconds of an inch.

After 120 hours exposure in the salt spray test, the two panels which had received the alkaline permanganate treatment and phosphating treatment of this invention showed paint losses, respectively, of only 0.1 and 0 thirtyseconds of an inch. On the other hand, the two panels which had not received the alkaline permanganate treatment prior to phosphating showed paint losses, respectively, of 8 and 9.2 thirty-seconds of an inch.

Example 3 This example was carried out principally to determine the relative effectiveness of calcium-containing and calcium-free zinc phosphate baths in the practice of this invention. It was also desired to determine what effect was produced by eliminating either the alkaline permanganate treatment step or the phosphating step of this invention.

A steel panel (A) was processed in the same manner set forth in Example 1, except that the phosphating solution employed contained 0.74 part of P0 ion, 0.36 part of N0 ion, 0.21 part of Zn ion, 0.89 part of Na ion, 0.35 part of Ca ion, and 97.45 parts of water. The phosphating solution had an acid number of 11.6.

A second steel panel (B) was processed in the same manner as A, except that the calcium ion was omitted from the solution.

A third steel panel (C) was processed in the same manner as B, except that the alkaline permanganate treatment was eliminated.

A fourth steel panel (D) received only the cleaning and alkaline permanganate treatment steps set forth in Example 1. It received no phosphate treatment.

All four panels were painted with a good, commercial oil-base paint, scribed, and then subjected to the salt spray test described in Example 2.

The test results are shown in Table II.

6 TABLE II Paint loss in Panel: thirty-seconds of an inch D Complete loss of paint.

A comparison of panels A and B indicates that a calcium-containing zinc phosphate bath is more effective than a similar, but calcium-free, zinc phosphate bath in the practice of this invention.

The results on panels C and D indicate that neither the alkaline permanganate treatment step nor the phosphate treatment step of this invention is in itself effective to prevent severe paint loss. The results also point to a definite synergistic action between the permanganate and phosphate treatment steps.

The use of alkaline solutions of metal permanganates, as taught by this invention, is to be distinguished from the use of oxidizing agents in phosphating baths. It is old in the art for example to use nitrates, sulfites, permanganates, chromates, perborates, and the like in phosphating baths. These oxidizing agents accelerate the rate at which phosphate coatings are formed on a metal surface. Most phosphating baths have a pH of about 2 to 6, since it is diflicult, if not impossible, to phosphate in an alkaline medium. In this invention, however, the pretreatment of the metal surface with permanganate must take place in an alkaline medium. Ordinarily the pH of the permanganate bath is greater than 8 and preferably greater than 11. During a continuous process of pretreating metal surfaces, the concentration and basicity of the bath may be maintained by the addition of more permanganate and alkaline material. This addition can be made, periodically, according to the requirements indicated by an analysis of the bath.

Some discussion is also in order regarding the mode of applying the alkaline permanganate solution to the metal surface. In any commercial treatment of metal surfaces with liquids, spraying provides advantages with respect to time and space which are not realized in dipping operations. Still more important with respect to the present invention is the observation that in spraying the permanganate solution, the quantity of permanganate required is smaller in comparison to the quantity required to obtain the same degree of treatment by dipping. The reason for this difference, while not completely understood, is believed to be due to the additional amount of oxygen obtained from the air which tends to accelerate the rate at which the metal surface is oxidized.

Although spraying is an advantage, it was heretofore considered commercially impractical to spray alkaline solutions of metal permanganates because of their absorption of CO from the air to form carbonates. It is this CO absorption which reduces the effectiveness of the permanganate bath by decreasing its basicity. It has now been discovered, however, that the basicity of the bath can be maintained by simply adding more alkaline material and that the mere presence of these carbonates has no adverse effect on the permanganate treatment. In fact, just so long as the solution remains basic, the alkaline material may be almost completely converted by the CO to the carbonate without substantially impairing the operability of the bath. Thus, for purposes of this invention, an acceptable pretreatment of metal surfaces can be obtained by spraying the alkaline permanganate solutions.

A typical example comprises spraying a metal surface with a two percent by Weight aqueous solution of a 1:1 mixture of potassium permanganate-sodium hydroxide at a temperature of about -170 F. for a period of about /2 to 5 minutes. Any type of nozzle such. as a flood jet, Whirl Jet, or V-Jet may be used with pump pres- .4 sures ranging from about to 20 pounds per square inch. These nozzles should be positioned to give complete and uniform coverage of the metal surface.

The alkaline permanganate treatment step of this invention has also been found to be useful in preparing metal surfaces to receive porcelain enamels. In such applications the permanganate treatment step of this invention has been found to improve both the appearance and the adhesion of the porcelain enamel coating. For example, an excellent, adherent porcelain enamel coating may be applied to a steel plate by treating said plate with alkaline potassium permanganate solution in the manner set forth in Example 1 and then subsequently acid-pickling, nickelplating, and porcelainizing the permanganate-treated plate.

What is claimed is:

1. A method of preparing an improved phosphated metal surface which comprises:

(A) treating a ferrous metal surface with an aqueous solution having a pH greater than 8 and consisting essentially of Water, an alkalizer, and a metal permanganate at a temperature within the range from about 68 F. to 210 F. for a period of from about 30 seconds to 60 minutes, the combined concentration of said metal permanganate and said alkalizer in said aqueous solution being within the range of from about 0.1 to 25% by weight and the relative proportions by weight of said metal permanganate to said alkalizer being within the range from about 1:05 to 1:4; and thereafter;

(B) phosphating said treated ferrous metal surface of A with an aqueous phosphating solution at a temperature within the range from about 68 F. to about 210 F., said aqueous phosphating solution containing as essential ingredients from about 0.05 to about 1.0% by Weight of zinc ion, from about 0.25 to about 2.0% by weight of phosphate ion, fromabout 0.25 to about 6.0% by weight of nitrate ion, and from about 0.1 to about 3.0% by weight of a cation selected from the group consisting of lithium, beryllium, magnesium, calcium, cadmium, strontium, and barium.

' 2. A method in accordance with claim 1 wherein the metal permanganate of A is an alkali metal permangamate.

3. A method in accordance with claim 2 wherein the alkali metal permanganate is potassium permanganate.

4. A method in accordance with claim 1 wherein the alkalizer of A is sodium hydroxide.

5. A method in accordance with claim 1 wherein the metal permanganate and alkalizer of A are, respectively, potassium permanganate and sodium hydroxide, and wherein the aqueous phosphating solution of B contains as essential ingredients from about 0.05 to about 0.6% by weight of zinc ion, from about 0.25 to about 1.5% by weight of phosphate ion, from about 1.0 to about 4.0% by weight of nitrate ion, and from about 0.1 to about 0.8% by weight of calcium ion.

References Cited in the file of this patent UNITED STATES PATENTS 1,911,726 Tanner et al May 30, 1933 2,296,844 Glasson Sept. 29, 1942 2,329,065 Lum Sept. 7, 1943 2,335,868 Lodeesen 2 Dec. 7, 1943 2,743,204 Russell Apr. 24, 1956 2,743,205 Condon Apr. 24, 1956 3,046,165 Halversen et al July 24, 1962 FOREIGN PATENTS 440,215 Great Britain Dec. 23, 1935 310,756 Germany July 1, 1922

Claims (1)

1. A METHOD OF PREPARING AN IMPROVED PHOSPHATED METAL SURFACE WHICH COMPRISES: (A) TREATING A FERROUS METAL SURFACE WITH AN AQUEOUS SOLUTION HAVING A PH GREATER THAN 8 AND CONSISTING ESSENTIALLY OF WATER, AN ALKALIZER, AND A METAL PERMANGANATE AT A TEMPERATURE WITHIN THE RANGE FROM ABOUT 68*F. TO 210*F. FOR A PERIOD OF FROM ABOUT 30 SECONDS TO 60 MINUTES, THE COMBINED CONCENTRATION OF SAID METAL PERMANGANATE AND SAID ALKALIZER IN SAID AQUEUS SOLUTION BEING WITHIN THE RANGE OF FROM ABOUT 0.1 TO 25% BY WEIGHT AND THE RELATIVE PROPORTIONS BY WEIGHT OF SAID METAL PERMANGANATE TO SAID ALKALIZER BEING WITHIN THE RANGE FROM ABOUT 1:0.5 TO 1:4; AND THEREAFTER; (B) PHOSPHATING SAID TREATED FERROUS METAL SURFACE OF A WITH AN AQUEOUS PHOSPHATING SOLUTION AT A TEMPERATURE WITHIN THE RANGE FROM ABOUT 68*F. TO ABOUT 210*F., SAID AQUEOUS PHOSPHATING SOLUTION CONTAINING AS ESSENTIAL INGREDIENTS FROM ABOUT 0.05 TO ABOUT 1.0% BY WEIGHT OF ZINC ION, FROM ABOUT 0.25 TO ABOUT 2.0% BY WEIGHT OF PHOSPHATE ION, FROM ABOUT 0.25 TO ABOUT 6.0% BY WEIGHT OF NITRATE ION, AND FROM ABOUT 0.1 TO ABOUT 3.0% BY WEIGHT OF A CATION SELECTED FROM THE GROUP CONSISTING OF LITHIUM, BERYLLIUM, MAGNESIUM, CALCIUM, CADMIUM, STRONTIUM, AND BARIUM.