US2342738A - Corrosion resistant coating for metal surfaces - Google Patents

Description

Feb. 29, 1944. G, w; JERNSTEDT v 2,342,738

` coRRosIoN RESISTANT comme FORMETAL sURFAcEs Filedsept. 17. 1941 Patented Feb. 29, 1944 CORBOSION RESISTANT COATING FOR METAL SURFACES George W. Jernstedt, Bloomfield, N. J., assigner to Westinghouse Electric & Manufacturing Company, East Pittsburgh, Pa., a corporation of Pennsylvania Application September 17, 1941, Serial No. 411,141

7 Claims.

This invention relates to the art of producing corrosion resistant coatings on; the surface of iron, zinc, other metals, and alloys. It is particularly useful in the production of phosphate coatings by a rapid and economical process. Such coatings are of primary importance in the preparation of the metal surface for the reception of a final finish of paint, enamel, lacquer, or the like.

The object of the invention is to expedite the production of protective phosphate coatings on metal surfaces by applying an activating pretreatment to the metal surfaces.

Other objects of the invention will,'in part, be

obvious and will, in part, appear hereinafter.

' For a fuller understanding of the nature and objects of the invention, reference may be had to the following detailed description taken in conjunction with the accompanying drawing, in which:

Figure 1 is a schematic view of one form of process for accomplishing the invention; and

Fig. 2 is a fragmentary sectional view of a metallic-member whose surface has been subjected to the treatment herein described.

It is a fairly extensive practice to treat metallic surfaces with compositions which will, under appropriate conditions, deposit upon the surface a protective phosphate coating. These coatings have been produced by subjecting the metal surface to a solutionr containing phosphates and phosphoric acid. The time of treatment of the metal surface usually has been prolonged, but within recent years the time during which a satisfactory phosphate surface coating has been secured has been reduced to a matter of a fraction of an hour, for example, 15 or 20 minutes.

It has been discovered that the time necessary for a reaction between metalsurface and the phosphate coating composition whereby a suc cessful and satisfactory protective coating has been produced on the metal surface depends to a great extent upon the condition of the metal surface. By condition of the metal surface" is meant more than simply cleanliness or freedom from foreign substances. Under certain conditions the metal adjacent the surface may have imparted to it improved chemical characteristics whereby it is activated so that its reaction with the coating composition is greatly accelerated. The metal surfaces so treated will react with great rapidity with the phosphate compositions applied thereto so that a highly satisfactory and complete protective coating is secured in a matter of one minute or less. It

has further been noted that the activated metal surface acquires a protective phosphate coating of the most desirable type. that is, the surface is uniformly covered with a fine crystalline material whose individual grains cannot be separated by inspection with the naked eye. The corrosion resistance of the coating upon activated surfaces is greatly improved over that secured by the prior art treatment. In addition the activating process and subsequent steps take less time than previously required.

Furthermore, the activating pretreatment for metal surfaces has been phenomenally 'successful in producing coatings of the highest quality upon zinc, galvanized iron, and zinc-plated metal. It is Well known to those skilled in the art that zinc and zinc coatings have not been treated with uniformly satisfactory results with phosphate solutions. Zinc metal surfaces frequently acquire only a spotty, non-uniform coating, and in some cases the phosphate coating is practically negligible. By activating the zinc metal surface, the phosphate compositions react therewith to produce protective coatings of even more resistance to corrosion than those which are produced upon ferrous base metals.

In order to secure protective coatings which have high corrosion resistance upon metals, it is not necessary to modify the normal phosphate solutions for application to the activated metal surface. The commercial solutions for producing phosphate coatings will in many instances be found to be satisfactory in effecting a phosphate coating upon the metal of the type specified. In some cases, however, it has been found that the phosphate solutions may be modified in order to increase the quality or the speed with which a coating is obtained.

According to this invention, cleaned metal surfaces are subjected to an activating pretreatment by applying to the metal surface a solution of di-alkali phosphate. A solution of di-sodium phosphate has been found to be productive of satisfactory results.

The di-sodium phosphate sait which is employed for activating the metal surface should contain a small proportion of ferric iron addition in order to achieve the best results. The simplest way of securing this small quantity of ferric iron is to employ a raw material such as the USP di-sodium phosphate which has been prepared from raw material containing ak small quantity of iron. In the preparation of the salt. the ferric iron will be present in the final crystals in sumcient quantity to produce the desired resuits. It has been found through experimentation that from 0.01% to 0.001% of ferrie iron in the di-sodium phosphate is most desirable. Smaller quantities of ferrlc iron are insutlicient for rapid and satisfactory activation, while larger quantities do not appear to improve the activating reaction.

If ferrie iron in the proportions mentioned is lacking in the di-alkali phosphate as prepared, ferrie iron components may be added to the salt to produce a material suitable for use. For example, ferric lactophosphate was added to a disodium phosphate solution which did not produce satisfactory activation, and upon such addition the results were improved considerably. However, it has been found that ferrie iron present as an impurity in the di-alkali phosphate is the most convenient method of securing the presence of this quantity of ferrie iron. 'Ihus in crystallizing di-sodium phosphate from solution the less pure crystals should be selected.

'I'he di-sodium phosphate, with the minor amount of ferrlc iron ranging from 0.01% to 0.001%, is dissolved in water. It has been found that well water containing a large quantity of lime, magnesium, and other impurities is an undesirable solvent for this pllrDOSe. In most instances, s.` fairly pure water which has been subjected to ltration and the like may be used as a solvent without exercising any deleterious effect due to the presence of harmful impurities. It has been found that approximately 0.001% of calcium, magnesium, or other alkaline earth metal found in thewater wil be harmful to the disodium phosphate. In some instances it may be desirable to employ distilled water in order to avoid any question of the effect of dissolved impurities in the water upon the activating chemicals. v

Solutions of dialkali phosphate in water ranging from 1 to 2% have produced the most consistent and best results upon the metal surface. However from 0.1% to saturated solutions of the phosphates in water will produce the activating pretreatment.

The activating solution may be applied to the clean metal surfaces. either by dipping or spraying the solution over the surface. In the one production set up, 120 gallons of solution were successfully used as a spray on an average of 5,0004

plates per day for several days, each plate having a surface area of about one square foot. Other methods of applying the solution to the metal may be employed, providing the surface is in contact with the solution for a period of time of approximately one minute or less. In many cases it has been found that the time of contact of metal surface and solution produced optimum results with only 20 or 30 seconds contact. In this short period of time, a reaction has been set up upon the metal surface whereby the metal is rendered extremely active with respect to subsequent phosphate coating solutions. 'I'he activating treatment does not appear to produce any protective coating or to impart any pronounced physical change at the surface. Apparently a certain degree of passivity, particularly as is found associated with zinc surfaces, has been greatly reduced so that the metal surface becomes chemically more active. l if the activating di-sodium phosphate solution is in contact with the metal surfaces for longer periods of time than those above indicated and 2 to 5 minute periods may be employed for the pretreatment without any harm.

No detrimental result occurs.

saunas After the metal surface has been activated, the memberissubiected totheaction ofaphosphate coating solution of the type known to the art, or a solution modified to secure results superior to those of known composition. The phosphate solutions are varied in accordance with the metal being treated. that is, ferrous base metal surfaces are treated with one type of phosphate composition, while zinc is treated with another type of composition. The following examples aare given as indicative of the various modes of applying the invention: Steel stampinss such as automobile bodies, fenders, electrical appliances, and the like. after being subjected to the activating pretreating are immersed in a coating solution prepared in accordance with the following formula:

Formula No. I

Ounces Zim` 6 Phosphoric acid 16 Water 16 After the ingredienis in the formula have been dissolved, then the solution is diluted with water in the amount of one ounce of the solution to one gallon of water. The diluted solution may beusedimmediatelyforthepxoductionofthe phosphate coating on ferrous surfaces. This solutionmaybepa'eparedinsomecaseswithzinc oxide in place of zinc. In order to expedite the reaction the solution is preferably kept hot and the time for immersion of the iron member therein will range from one minute up to 5 minutes or more. After rinsing, the coating metal may be provided with a final protective coating of organic nish, such as paint, lacquer, enamel, varnish, or the like.

In order to provide for a much more rapid production of the protective phosphate coating, particularly as compared to the use of the relatively slow reacting solution of Formula I, the following solution is suggested:

Formula No. II

Cupric nitrate ounce-- Water to make one gallon. Y

'Ihe concentrated solution of the ingredients of Formula No. II is diluted with water to provide a solution having the concentration of from 2 to 4% of the above formula. This solution reacts very rapidly with a ferrous metal surface and produces uniform te coatings. The presence of the sodium nitrate is for the purpose of providing an oxidizing agent in contact with the metal surface. since it has been found that the metal surface produces small quantities of nascent hydrogen during the reaction with the composition, and unless the hydrogen is oxidized or removed, will ultimately form gas bubbles adherent to the metal surface which will prevent groupconsistingof zincl etalswiththepliosphatecom-A metal. There is some evidence of the fact that the phosphate is a ferro-ferrie phosphate. Whatever the composition may be, the protective nature of the phosphate coating is greatly improved by the previous application of the activating solution of the invention.

It should be remembered that since the protective coatingsA are so thin due to the rapid formation thereof they are not intended to long withstand the effects of the elements and moisture by themselves. It is well known in the art that phosphate coatings immediately after drying should be coated with an inorganic finish in order to achieve the maximum resistance to corrosion. However, in the case ywhere the activating pretreatment is used, while organic finishes are intended to be applied to produce the final coating having maximum corrosioni resistance, the time of application of the organic nnish is not as critical as was heretofore required. For example, it has been noticed that steel members which have been coated with prior art protective phosphate material will often show small rust spots within a day or more. Apparently the protection was not absolutely satisfactory, and the prompt application of organic inisheswas necessary. However, in the case where the activating pretreatment is followed, the resistance to rusting is greatly improved.

For the purposes of indicating the comparative resistance to rusting of phosphate coated articles, the following is given: Steel instrument bases treated to produce thereon the phosphate coating composition, part of the bases having been subjected to activating pretreatment and a part lacking such treatment, were placed within a steam clr est, kept at a temperature of 140. In 164 .hours the steel bases which were not treated with the activating solution exhibited large rust areas over most of their surfaces at portions where machining had been performedI and at points where the drawing stresses were a maximum the rusting was pronounced. A similar instrument base which was preactivated and thereafter treated to produce a phosphate coating showed no signs of rusting and had only a few dark spots on the surface and was considered by comparison to be still resistant to corrosion over substantially all of its surface.

In the case where zinc metal, either as die casting or zinc plating or galvanized metal, is to be -provided with a phosphate coating upon its surface, a slightly modified procedure is necessary. Referring to Fig. l of the drawing, there is illustrated an activating pretreatment tank I containing the di-sodium phosphate solution l2. The zinc metal is subjected to the activating pretreatment, preferably by spraying within the tank I 0 after the surface has been thoroughly cleansed by a vapor degreasing process or the like. In about a minute or less, the zinc surface will become chemically activated and may be transferred to the container I4 in which is la composition of the following type:

Formula No. III

Percent Iron 0.03 to 0.30

Zinc phosphate 0.04 Sodium nitrate 0.2 to l phosphoric acid to comprise 1:8 of total phosphate content.

It has been discovered that to produce the nest grained and best protective coatings that fil 7 protective coatings.

the concentration of iron in the solution should be near saturation or approximately 0.3%. A convenient way of securing the maximum concentration 'of iron is to suspend steel wool or iron filings within the solution in a porous receptacle.

Furthermore, the reaction is greatly expedited by an arrangement to heat the solution to a temperature of from 140 to 190 F., by means of a steam pipe I0 passing through the tank I4. Other methods of heating the solution I6 are obvious. When the activated zinc metal surface issuspended within the solution of Formula No. III, the surface will emit a great quantity of gaseous bubbles. These bubbles are produced for a time period of approximately 40 seconds at the termination of which it will be noted that a velvety appearing, line crystalline protective coating has been produced over the entire zinc surface. 'I'he process is substantially complete upon the cessation of bubbling, but 5 or 10 seconds longer in the solution may be beneficial. Thereafter, it may be desired to rinse the metal members in the rinsing tank 20, though this is not necessary.

For improved results it has been found that subjecting the coated zinc metal to a chromic acid provides a desirable sealing treatment for the phosphate coating. The solution of chromic acid 24, in a concentration of 12 ounces per 100 gallons, is kept within the tank 22 through which solution the articles may pass by means of an automatic conveyor or the like. Only a few seconds in the solution 24 is necessary to secure the desired results. In some cases the chromic acid solution remaining upon the surface of the zinc metal may be dried by passing the articles through a drying oven 26. Alternatively, it has been found that by heating the chromic acid solution 24 in tank 22 to a temperature of about 190 F. or even higher, that suilicient heat is stored in the metal so that after removal from the solution, drying is automatically accomplished. 1

The chromic acid sealing treatment may be advantageously applied to all types of phosphate Steel and iron members having a phosphate coating can be treated similarly to the zinc metal with beneilcial results.

An inspection of the treated zinc member 33 subjected to the treatment shown in Fig. l shows a uniform, bluish-gray, velvety appearing coatlng. This coating is shown at 32 in Fig. 2. With the naked eye, no separate crystals may be seen on any portion of the member. The coating is uniformly deposited over all v,recesses and othei` i' irregularities of the metal surface of member 30.

While occasionally by prior art methods protective coatings may be secured that have somewhat of the appearance of those produced by this process, the test of corrosion resistance indicates a remarkable difference in the quality of 'the coating. The following test is given as indicin Fig. 1, and placed within a steam chest operating at 140 F., 14 days elapsed before any sample showed a sign of corrosion. Generally, a month in the steam chest passed before the samples showed any extensive degree of corrosion.

In each of the above three tests, the bare metal surface. without any organic finish, was under test. In a subsequent series of corrosion tests wherein lacquer coatings were applied to untreated zinc, to zinc treated with phosphate solution alone, and to zinc treated to the complete process of Fig. 1, an almost equivalent proportionality in resistance to corrosion occurred, that is, the samples with the underlying phosphate coating produced by the present process lasted approximately 28 times as long before any failure of the coating was observed.

Comparative tests with protective phosphate coatings on zinc plate, the phosphate coatings being produced by the process illustrated in Fig. l. which is one form of the invention of the present case, withstood the corrosive effects in steam chests and in salt spray baths for a period of time which is greatly superior to that of the other types of phosphate coatings,

While the present invention is limited to an activating pretreating of di-alkali phosphate containing a predetermined amount of ferric iron, lit has been found that in some cases the life of the solution may be greatly increased by the employment of a chemical complex formed from di-alkali phosphates. The action of hydroxy organic acids such as citric acid with di-alkali phosphates, such as di-sodium phosphate, produces a complex which has a much more consistent hydrogen iron concentration and also retains its activating characteristics for a greater period of time. This modified type of activating pretreatment is disclosed in the copending patent application of G. W. Jernstedt and J. C. Lum, Serial No. 407,886, filed August 22, 1941, and assigned to the same assignee as this application.

It will be seen that by this process not only has the total time of production of a phosphate coating been greatly reduced, but that a superior type of coating with greatly enhanced corrosion resistance has been secured. I'he characteristics of the coating are such that application of organic finishes is expedited and overall durability is increased over that heretofore thought the maximum.

Furthermore, it should be mentioned that the process may be economically applied to existing installations employing tanks or other equipment containing detergent solutions for cleaning the metal surfaces prior to the application of phosphate solution thereto. Generally, the detergent solution tank may be used for the purpose of containing the activating solution which may be applied in the same sequence of operations as previously. Accordingly, it would not require an extensive change in the process or apparatus heretofore used to take advantage of the activating pretreatment herein disclosed.

The presence of metals other than ferrie iron in phosphates for preactivation of metal surfaces is disclosed in the copending patent application of G. W. Jemstedt, Serial No. 416,552, filed October 25, 1942, and assigned to the assignee of the instant application.

Since certain changes may be made in the above invention and different embodiments of the invention may be made without departing from the scope thereof. it is intended that all matter contained in the above described disclosure or taken in connection with the accompanying drawing shall be interpreted as illustrative and not in a limiting sense.

I claim as my invention:

1. The method of treating the surface of zinc and ferrous metals to provide for corrosion resistance which comprises, in combination, applying to the member a di-alkali metal phosphate solution containing 0.001% to 0.01% of ferrie iron based on the amount of the dialkali metal phosphate to Iprovide an activating pretreatment and thereafter applying to the pretreated member a solution comprising iron phosphate, free phosphoric acid and an oxidizing agent to produce a crystalline corrosion resisting phosphate coating thereon.

2. The method of treating the surface of zinc and ferrous metals to provide for corrosion resistance which comprises, in combination, applying to the member di-sodium phosphate solution containing ferrie iron to provide an activating pretreatment and thereafter applying to the pretreated member a solution comprising iron phosphate, free phosphoric acid and an oxidizing agent to produce a crystalline corrosion resisting phosphate coating thereon.

3. The method of treating the surface of zinc and ferrous metals to produce a corrosion resisting coating thereon which comprises, in combination, applying to the metal surface a to 10% di-alkali metal phosphate solution containing from 0.001% to 0.01% ferrie iron based on the amount of the dialkali metal phosphate to provide for an activating pretreatment of the surface and applying to the activated surface a solution of iron phosphate, free phosphoric acid, and an oxidizing agent to produce a tlne crystalline corrosion resisting phosphate coating on the metal.

4. The method of treating the surface of zinc metal to provide for corrosion resistance which comprises, in combination, applying to the zinc surface a V4% to 5% di-alkali metal phosphate solution containing 0.001% to 0.01% ferrie iron based on the amount of the dialkali metal phosphate to activate the surface for subsequent treatment, applying to the activated Zinc Surface a solution containing zinc, iron phosphate, free phosphoric acid, and a nitrate to produce a ilne crystalline, corrosion resisting, phosphate coating capable of retaining organic finishes and sealing the phosphate coating by applying chromic acid.

5. The method of treating the surface of ferrous metal to provide for corrosion resistance which comprises, in combination, applying to the ferrous metal surface a V4% to 5% solution of di-alkali phosphate, the solution containing from 0.01% to 0.001% of ferrie iron based on the weight of theldialkali phosphate to activate the surface for subsequent treatment, applying to the activated ferrous surface an aqueous solution of iron phosphate, an oxidizing agent, free phosphoric acid, and one or more metals selected from the group consisting of zinc, copper, and manganese, the aqueous solution producing a tenaciously adherent, ne grained protective phosphate coating on the ferrous metal and applying chromic acid to the protective phosphate coating to provide for sealing.

6. The method of activating the surface of zinc and ferrous metals to provide for a rapid formation of a corrosion resisting phosphate coating on the surface of the metal when subjected to phosphosphate solutions, which comprises applying to the metal surface an aqueous solution containing from V4% to 5% of cli-alkali phosphate and 0.01% to 0.001% of ferrie iron based on the 5 amount of dialkaliphosphate.

GEORGE W. JERNSTEDT.

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