US3338755A - Production of phosphate coatings on metals - Google Patents

Description

United States Patent New York No Drawing. Filed Sept. 3, 1963, Ser. No. 306,332 9 Claims. (Cl. 148-615) The present invention broadly relates to a method and composition for producing phosphate coatings on ferrous and'zinc surfaces, and more particularly to an improved solution and method for forming adherent phosphate coatings on zinc and ferrous surfaces employing cold processing conditions.

The treatment of metal surfaces with aqueous solutions containing vmetal phosphates so as to produce a phosphate coating thereon enhancing the adhesion of an overlying paint film to the metal surface and an improvement in the corrosion protection of the surface is well known and in widespread commercial use. In order to provide adherent phosphate coatings on metal surfaces consisting predominantly of either iron or zinc, it has been conventional in the past to employ an acid aqueous solution 1ncorporating metal phosphates dissolved therein of the socalled coating-type including zinc phosphate, manganese phosphate, and iron phosphate which precipitate on the surface of the metal being treated forming the phosphate coating thereon. Zinc phosphate constitutes the principal coating-type phosphate compound employed for forming such phosphate coatings, and this material has almost invariably been used in processing conditions wherein the surfaces of the metal to be coated are sprayed with the treating solution.

It is essential to achieve a commercially practical process to employ a treating solution composition which is operative to form a phosphate coating of the requisite weight within the shortest period of time possible. It is principally for this reason that zinc phosphate has been extensively employed due to the greater reactivity and coating speed in comparison to phosphate solutions containing iron or manganese phosphate. The reactivity or coating speed of such treating solutions has been increased by the inclusion of suitable accelerator agents commonly nitrates, nitrites or chlorates. It is also known that the reactivity of the solution and its coating speed increases with the temperature of the solution; however, for reasons which will become apparent later, it is desirable to use treating solutions at a temperature as low as possible.

Phosphate coating processes may be broadly divided into hot, cool, and cold, and whereas, for many years only hot processes were used, cool processes are now common. In cool processes, the solution temperature ranges from above about 100 F. to about 120 F. Hot processes employ temperatures above 120 F. and usually about 160 F. to about 180 F. Cold processes, on the other hand, are those carried out at ambient temperatures, that is, from about 40 R, up to about 100 F. but usually not above 70 F. In order to operate a cold process satisfactorily, it is known that the phosphating solution must be highly accelerated and must be more concentrated and have a higher pH than solutions yielding equivalent coating results when used at higher temperatures.

It is also known that phosphating is normally carried out by immersion of the workpieces in the treating solution or by spraying and that a coating speed-that is tolerable in an immersion process usually is too slow when employed in a spray process. Since spray-type processes are advantageously used to treat large articles carried by conveyors, it is a commercial expedient that the articles Patented Aug. 29, 1967 ice must be coated in as short a time as possible while being passed through a spray installation. It will be apparent that the use of a cold-type phosphating process is particularly desirable in a spray-type process in comparison to cool or hot-type processes since the loss of heat, and therefore the consumption of fuel, as well as vaporization of the solution is particularly high in such spray processes. The use of cold-type processes also provides for an improvement in working conditions, greater safety to equipment, and less sludge formation.

In spite of the advancements inthe technology of phosphate coatings and processes heretofore made, the adaptation of cold-type phosphating processes, particularly cold spray-type processes, has not received commercial acceptance due to the excessive time, required to effect a satisfactory coating in conventional spray installations which are of sufficient strength to withstand subsequent rinsing and drying without encountering incipient tarnishing of the metal susbtrate.

It is accordingly a principal object of the present invention to provide an improved phosphate coating solution and process which is adaptable to cold-type processing overcoming the disadvantages present in coating solutions and processes of similar type heretofore known.

Another object of the present invention is to provide an improved phosphate coating treating solution and method wherein the solution possesses a high degree of reactivity producing phosphate coatings of the requisite weight and strength under cold-type processing conditions enabling its use in spray-type processes achieving further process- 1ng economies as a. result of a substantial reduction in fuel requirements heretofore required in maintaining the treating solution at a controlled elevated temperature.

Still another object of the present invention is to provide an improved phosphate coating solution and process for forming adherent phosphate coatings on metal surfaces comprised predominantly of iron and zinc and which solution and process are of simple and versatile operation, of simple control, and of efficient and economical use.

The foregoing and other objects and advantages of the present invention are based on the discovery that satisfactory crystalline phosphate coatings can now be produced on zinc surfaces and particularly on ferrous surfaces in a cold spray-type process employing temperatures below F. by using an'aque'ous phosphate solution containing a controlled proportion of zinc and manganese in addition to a controlled proportion of a nitratenitrite accelerator agent. The production of phosphate coatings in the cold having sufiicient strength to withstand subsequent rinsing and drying without incipient tarnishing in commercially satisfactory time periods is particularly surprising in view of the relatively slow coating speed of manganese phosphate itself. Other objects, features and advantages of the present invention will become apparent from the subsequent description and the example provided illustrative of a typical treating solution composition and conditions for applying the solution in accordance with the practice of the present invention. I The phosphate treating solution comprising the present mvention incorporates as the active coating constituents a controlled proportion of phosphate, zinc and manganese. The manganese content of the solution may comprise up to about 50% by weight of the Zinc content but is preferably controlled between about 5% and 20% of the total zinc content. During the use of the treating solution in a cold-type process, a depletion of the constituents including the zinc and manganese content in the solution occurs as a result of the deposition of these materials as part of the phosphate coating formed in addition to loss of the solution through drag-out on the surfaces of the workpieces.

Due to the gradual depletion of the solution, it is necessary to periodically replenish the solution to maintain the zinc and manganese contents within the appropriate range. Since it is desirable to replenish the treating solution with a suitable concentrate of the materials in the same proportion in which the zinc and manganese are consumed or depleted in order that the same mixture of chemicals can be employed, the zinc and manganese contents are preferably controlled within a ratio conventionally ranging from about 0.10:1 to about 0.13:1 parts manganese per part zinc. This ratio will vary within the aforementioned range depending on the specific processing conditions and resultant coating formed as well as the degree of drag-out of the solution on the surfaces of the workpieces.

The total concentration of zinc, manganese and phosphate in the treating solution is dictated by the pointage of the solution which in indicative of the total acidity of the solution and is defined in the art as the number of milliliters of N/ 10 sodium hydroxide necessary to neutralize 10 milliliters of the solution to a phenolphthalein end point. For the purposes of the present invention, satisfactory coatings are obtained when the treating solution has a total acidity of greater than 20 points up to about 100 points or greater. A total acidity in excess of 100 points conventionally is commercially uneconomical. The active coating constituents may conveniently be added to form a concentrate of the solution by employing basic compounds of zinc and manganese such as the oxide, hydroxide, carbonate, etc. The manganese is preferably added in the form of the sulfate. The phosphate constituent is preferably added as phosphoric acid although other techniques well known in the art can be employed for forming the solution.

The pH or free acidity of the treating solution is established by the free acid content thereof and is dictated by the equilibrium under the conditions of the solution between the free acid and zinc tertiary phosphate. The free acid concentration of the treating solution is defined in the art as the number of milliliters (or points) of N/ 10 sodium hydroxide required to neutralize a 10 milliliter sample of the solution to a bromophenol blue end point. It has been found that by controlling the ratio of the total acidity to the free acidity of the treating solution or the so-called total-free acid ratio within a range of about 50 to about 100, the benefits of the improved treating solution and method comprising the present invention are attained.

In addition to the zinc, manganese, and phosphate constituents in the treating solution, the solution also contains the nitrate ion in a ratio preferably ranging from about 0.2:1 to about 0.411 nitrate to phosphate (P In addition to the nitrate accelerator agent, the solution also preferably contains a second accelerator comprising the nitrite radical. The concentration of nitrite is preferably maintained within a concentration range of from about 0.015% to about 0.075% by weight calculated as sodium nitrite (NaNO As is known in the art, the

nitrite accelerator agent must be added to the coating solution either continuously or at regular intervals to maintain its concentration within the aforesaid range as a result of its depletion during the oxidation of the ferrous ions dissolved from an iron surface to the ferric state. The solution may also contain a very small amount of nickel to serve as a supplementary accelerator agent in accordance with the practice well known in the art.

In addition to the foregoing constituents, it has also been found that more consistent results are obtained if the phosphating solution also contains an aliphatic alphahydroxy acid such as tartaric acid, dihydroxy tartaric, tartronic, glycolic, lactic, dextro-malic, dihydroxy maleic, gluconic and citric acid or a suitable compatible water soluble salt thereof. The quantity of such alpha-hydroxy acid or salt preferably are incorporated within a range I]: of from about 0.02 to about 4.0 grams per liter calculated as tartaric acid.

In the treatment of ferrous surfaces to which the solution and process comprising the present invention are particularly applicable, the mechanism of the coating reaction includes the solution of iron on the metal surface into the solution in the ferrous form which in the presence of the nit-rite accelerator agent is oxidized to the ferric state. As the coating reaction progresses, the solution gradually becomes saturated with ferric iron. It has been found that the most consistent phosphate coatings are obtained when the concentration of the ferric iron in the solution is constant. Accordingly, it is preferred to start with a treating solution which is already saturated in ferric ions by adding such ions initially, conveniently, as ferric sulfate, for example. Normally, the addition of about 0.02 grams per liter ferric iron calculated as Fe serves this purpose. Conveniently, the ferric ion required is included in the mixture of chemicals employed to prepare the treating solution.

The formation of phosphate coatings employing the treating solution as hereinabove described, is achieved under cold-type conditions within a temperature ranging from about 40 F. to less than 100 F. and usually F. or less. The surfaces to be coated may be preliminarily subjected to a cleaning treatment wherein the surfaces thereof are cleaned to remove dirt and any protective film of mineral oil or other extraneous contaminants to assure the formation of phosphate coatings of high quality. The cleaning phase of the treating process is conducted in a manner well known in the art so as not to render the surface passive to the coating solution. Any one of a variety of suitable cleaning techniques can be satisfactorily employed of the types well known in the art which preferably are conducted under cold conditions. For example, the surfaces may be cleaned with an organic solvent such as kerosene to remove the organic contaminants on the surfaces of the metal articles to be coated. Preferably, the surfaces are cleaned in the cold with an alkali cleaner in the conventional sequence in which the cleaned surface is rinsed twice prior to the application of the phosphate treating solution thereto.

A cleaning composition suitable for cold-type cleaning which is preferred, is free or substantially free of condensed phosphates and has a pH less than 12 and includes an emulsified organic element. An alkali cleaner having a pH less than 12 has little detergent action in the cold even if a considerable proportion of a surface active agent is present but sufiicient detergency at a pH less than 12 can be obtained by incorporating the emulsified organic solvent in the cleaner.

Ferrous metal surfaces are sometimes also protected against corrosion with a mixture of a mineral oil and a drying or semi-drying oil. In order to effect satisfactory cleaning of such surfaces, it has been found that the pH of an alkali cleaner to be used should not be less than about 11 and conventionally within a range of between about 11 and 12.

It is also contemplated that the surfaces of the metal can be cleaned with an aqueous acid solution of zinc phosphate capable of producing a phosphate coating and containing in addition either a surface active agent or an emulsified organic solvent. The use of such an aqueous acid phosphate cleaner effects a conditioning of the metal surface and it is believed that the formation of the phosphate coating is initiated in the cleaning zone serving as nuclei on which the principal coating is formed during the subsequent coating phase. After the cleaning step employing the aqueous acid phosphate cleaning solution, the treating solution is applied without any intervening water-rinse steps.

Irrespective of the particular precleaning operation performed on the surfaces to be coated, the phosphate coating solution is thereafter applied in the cold for a period of time sufiicient to form a coating of the requisite weight.

As hereinbefore set forth, due to the high reactivity of the phosphate coating solution and the good strength of the coating formed during the coating phase of the process, the process and solution comprising the present invention are particularly applicable to spray-type operations and this constitutes the preferred form although immersion type coating wherein the articles are immersed in the solution may also be employed. Following the coating step, the coated metal surfaces are water rinsed and are thereafter preferably subjected to a final rinse with a dilute solution of chromic-phosphoric acid effecting the removal of all unreacted chemicals from the surface and effecting an improvement in the corrosion resistance of the phosphate coating formed. Conventionally, final rinse solutions of the types well known in the art, contain up to about 0.1% by volume of chromicphosphoric acid.

In order to further illustrate the improved phosphate treating solution and the method comprising the present invention, the following example is provided. It will be understood, however, that the example is included for illustrative purposes and is not intended to be limiting of the scope of the invention as set forth inthe subjoined claims.

Example A series of steel test panels were treated so as to form a phosphate coating thereon in a process comprising seven steps. The first step comprised a cleaning step wherein an aqueous cleaning solution was employed containing 5 grams per liter of a cleaning composition comprising 616 parts by weight sodium metasilicate (Na SiO .5H O), 300 parts by weight sodium metaborate (Na B O .8H O), 50 parts by weight of a coating refining ingredient containing 1.5% titanium and commercially sold under the trademark Parcolene Z, 10 parts by weight Alkanol I-ICS (fatty alcohol-ethylene oxide condensate), 15 parts by weight of nonyl phenol-ethylene oxide condensate, 8-10 moles ethylene oxide; and 9 parts by weight nonyl phenol-ethylene oxide condensate, 1-2 moles ethylene oxide; and 10 grams per liter of an emulsion comprising white spirit having a flash point of 115 F., 30% of a 90/ 160 naphtha, and the balance water and emulsifying agent. The foregoing cleaning solution was sprayed on the steel panels for a period of 1 minute at a temperature of 70 F.

Following the cleaning step, the panels were rinsed in a spray of water at a temperature of 70 F. for a period of /2 minute. The third step consisted in a second rinsing by spraying the panels with fresh water at a temperature of 70 for a period of minute.

The fourth step of the process comprises spraying the panels with a phosphating treating solution prepared in accordance with the present invention for a period of 1 minute and at a temperature of 70 F. The phosphate treating solution was prepared by diluting a concentrated mixture of the chemicals and thereafter adding some zinc carbonate to the diluted mixture to diminish the free acidity thereof. The resultant diluted solution employed for spraying the panels had the following composition.

Ingredient: Grams/ liter Mn 0.9 Zn 7.6 Ni 0.05 Ferric ions 0.02 so 1.3 P0 19.6 N0 4.6 Tartaric acid (C H O 1.0

In addition to the constituents as set forth in the foregoing table, the solution also contained sodium nitrite in a concentration of 0.5 gram per liter. The total acidity of the solution was 30 points and the free acidity was 0.5 point.

After passing through the spray phosphate zone, the steel panels were again sprayed with a rinse water for /2 minute at a temperature of 70 F. to effect a rinsing of the surfaces thereof and were then subjected to a final rinse step comprising spraying them with a solution for a period of /2 minute at 70 F. containing 0.03 gram per liter phosphoric acid (H PO and 0.03 gram per liter chromate (CrO After the final rinse treatment, the panels were oven dried at 250 F.

Throughout the course of the prolonged operating run, replenishments of the phosphate treating solution were periodically made by adding concentrated mixtures of the chemicals and sodium nitrite was added continuously. An examination of the panels processed in accordance with the sequence as hereinbefore described indicated that excellent phosphate coatings were obtained on the surfaces of the panels Weighing about milligrams per square foot.

By way of comparison, a standard zinc phosphate solution accelerated by nitrate and nitrite was used used employing the same spray processing equipment for coating a similar series of steel panels. The steel panels were processed employing phosphate solutions and rinse solutions at a temperature of 110 F. and the resultant panels were found to have excellent phosphate coatings on the surfaces thereof in an amount of about milligrams per square foot.

Representative panels obtained from the method employing the improved zinc-manganese phosphate solution and the conventional zinc phosphate solution were painted with a single coat of a black, stoving, oil-modified alkyd paint and were subjected to performance tests. These tests showed that the adhesion of the paint film on the panels treated at 70 in accordance with the practice of the present invention was superior to that obtained on the panels treated at 110 F. with the conventional zinc phosphate solution and, moreover, that the spread or undercutting of rust from a scratch in .the panel in a saltspray test was less on the panels treated in accordance with the present invention.

When the conventional zinc phosphate solution was employed at a temperature of 70 F. in the spray process instead of at 110 F., the resultant steel treated panels were all stained to such an extent that it was not thought worthwhile to paint any of them and subject them to performance tests.

While it will be apparent that the preferred embodiments of the invention disclosed are well calculated to fullfill the objects above stated, it will be appreciated that the invention is susceptible to modification, variation and change without departing from the proper scope or fair meaning of the subjoined claims.

What is claimed is:

1. The method of forming a phosphate coating on zinc and ferrous surfaces in the cold, comprising the steps of applying an aqueous acid solution at a temperature of from about 40 F. up to about 100 F. to the clean surfaces of the articles to be coated, said solution consisting essentially of zinc, manganese and phosphate in a concentration to provide a total acidity of from about 20 points to about 100 points and a ratio of total acidity to free acidity within the range of about 50 to 100:1; said manganese being present in the proportion of from about 5% up to about 50% by weight of the zinc present, said solution further containing as an accelerator agent the nitrate radical present in the ratio of from about 0.2 to about 0.4 parts nitrate per part phosphate and the nitrite radical present in the solution in a concentration of from about 0.015% to about 0.075% by weight calculated as sodium nitrite, and continuing the application of said solution until a coating results.

2. The method as claimed in claim 1 wherein the aqueous acid phosphate solution is at a temperature of about 70 degrees Fahrenheit when it is applied to the ferrous surface.

7 3. The method as claimed in claim 1 wherein the aqueous acid phosphate solution also contains an aliphatic alpha-hydroxy acid, present in an amount from about 0.02 to about 4.0 grams per liter, calculated as tartaric acid.

4. The method as claimed in claim 3 wherein the aqueous acid phosphate contains an acceleratnig amount of nickel ions.

5. The method as claimed in calim 1 wherein the aqueous acid phosphate solution also. contains ferric iron in an amount sufficient substantially to saturate the said solution.

6. The method as claimed in claim 1 wherein the manganese is present in the aqueous acid phosphate solution in a proportion of from about 5 percent to 20 percent by weight of the zinc present.

7. The method as claimed in claim 6 wherein the aqueous acid phosphate solution also contains an aliphatic alpha hydroxy acid, present in an amount of from about 0.02 to about 4.0 grams per liter, calculated as tartaric acid.

8. A phosphating solution consisting essentially of zinc, manganese and phosphate in a concentration to provide a total acidity of from about 20 points to about 100 points and a ratio of total acidity to free acidity within the range of from about 50 to 100:1, the manganese being present in the proportion of from about 5 percent up to about 50 percent by weight of the zinc present, said solution further containing, as an accelerating agent, the nitrate radical present in a ratio of from about 0.2 to about 0.4 part of nitrate per part of phosphate, and the nitrite radical present in the solution in a concentration of from about 0.015 percent to about 0.075 percent by weight, calculated as sodium nitrite.

9. The phosphating solution as claimed in claim 8 wherein there is also present an aliphatic alpha-hydroxy acid in an amount of from about 0.02 to about 4.0 grams per liter, calculated as tartaric acid and an accelerating amount of nickel ions.

References Cited UNITED STATES PATENTS OTHER REFERENCES Chemical Age, Corrosion Protection, Sept. 1, 1951, Metallurigica Section 295, 296, 300.

Schuster et al., Korrosion und Metallschutz, vol. 20 #5, 1944, pp. 153, 154, 155, 156, 157, Translation pp. 110 and 16.

ALFRED L. LEAVITT, Primary Examiner.

RALPH S. KENDALL, Examiner.

Claims (1)

1. THE METHOD OF FORMING A PHOSPHATE COATING ON ZINC AND FERROUS SURFACES IN THE COLD, COMPRISING THE STEPS OF APPLYING AN AQUEOUS ACID SOLUTION AT A TEMPERATURE OF FROM ABOUT 40*F. UP TO ABOUT 100*F. TO THE CLEAN SURFACES OF THE ARTICLES TO BE COATED, SAID SOLUTION CONSISTING ESSENTIALLY OF ZINC, MANAGANESE AND PHOSPHATE IN A CONCENTRATION TO PROVIDE A TOTAL ACIDITY OF FROM ABOUT 20 POINTS TO ABOUT 100 POINTS AND A RATIO OF TOTAL ACIDITY TO FREE ACIDITY WITHIN THE RANGE OF ABOUT 50 TO 100:1; SAID MANGANESE BEING PRESENT IN THE PROPORTION OF FROM ABOUT 5* UP TO ABOUT 50* BY WEIGHT OF THE ZINC PRESENT, SAID SOLUTION FURTHER CONTAINING AS AN ACCELERATOR AGENT THE NITRATE RADICAL PRESENT IN THE RATIO OF FROM ABOUT 0.2 TO ABOUT 0.4 PARTS NITRATE PER PART PHOSPHATE AND THE NITRITE RADICAL PRESENT IN THE SOLUTION IN A CONCENTRATION OF FROM ABOUT 0.015% TO ABOUT 0.075% BY WEIGHT CALCULATED AS SODIUM NITRITE, AND CONTINUING THE APPLICATION OF SAID SOLUTION UNTIL A COATING RESULTS.