US2987428A

US2987428A - Metal coating composition and method of coating steel

Info

Publication number: US2987428A
Application number: US721340A
Authority: US
Inventors: Ferdinand P Heller; Roger L Beamon
Original assignee: Amchem Products Inc
Current assignee: Henkel Corp
Priority date: 1958-03-14
Filing date: 1958-03-14
Publication date: 1961-06-06
Anticipated expiration: 1978-06-06
Also published as: BE576317A

Description

U te S a e Pate 2,987,428 METAL COATING COMPOSITIONAND METHOD OF COATING STEEL Ferdinand P. Heller, Philadelphia, and Roger L. Beamon, Lansdale, Pa., assignors to Amchem Products, Inc., a corporation of Delaware No Drawing. Filed Mar. 14, 1958, Ser. No. 721,340 3 Claims. (Cl. 148-615) This invention relates to the art of, coating metal and especially to the provision of improved compositions useful for this purpose as well as to the provision of an improved method for coating steel. More particularly the invention relates to the art of applying a substantially amorphous pyrophosphate coating on metal surfaces without depositing thereon any substantial overburden of powder.

As is known to those skilled in the art attempts have been made to increase the corrosion resistance of metal surfaces and prepare them for the subsequent application of a final siccative finish by treating them with acid pyrophosphates. However, the aqueous acid pyrophosphate solutions heretofore employed for this purpose involve certain inherent disadvantages or difliculties. One of these arises by virtue of the rapid reversion of acid metal pyrophosphates to orthophosphates when disso'lvedin aqueous solutions even when the solutions are employed at ordinary living or room temperatures. Furthermore, if pyrophosphate solutions which are relatively stable at room temperature are employed they still lead to difficulties of a similar nature because, when heated, they change readily to the ortho form. Still further, additional difliculties are encountered with the use of pyrophosphates arising from the fact that more free acid is produced from pyrophosphates by hydrolysis than is thecase where 'a similar amount of a corresponding 'orthophosphate is employed.

In efforts to overcome the difiiculties referred to a number of proposals have been made by those skilled in the art, one of which is represented by U.S. Patent No. 2,067,- 007. This patent suggests that the problems referred to can be overcome through the use of so-called balanced solutions containing a pyrophosphate salt of certain metals such as zinc acid pyrophosphate. Unfortunately, how- 'ever, solutions of this type tend to form copious quantities of sludge as well as to produce powdery and nonadherent coatings on the surfaces of the metal being treated. In order to avoid decomposition of the pyrophosphate the patent just mentioned recommends that, in addition to pyrophosphate, the solultion should contain dissolved orthophosphate. q I

The difiiculties referred to which have been encountered heretofore in maintaining balance (i.e., maintaining a proper relationship between free and total" acidity) as well as those involved in maintaining suitable ortho to pyro phosphate ratios have prevented the process from achieving commercial'success. This is emphasized by the fact that the solutions of the prior art required: 1) virtual saturation with respect to a solid, heavy, metal pyrophosphate, (2) definite control'of the free acid content of the pyrophosphate solution and (3) the simultaneous presence of orthophosphate and pyrophosphate to insure solution stability. 7

In sharp contrast to the foregoing the present invention is not dependent upon any of these criteria for its success. Rather it consists in the provision'of means for applying a new and superior type of pyrophosphate coating from solutions which 'are simple in composition and easy to maintain in operating condition. In other words, our invention makes it possible toernp loy a composition and process which are almost completely free of the problems incident to the variables of concentration, temperature, etc. as will further appear.

Patented June 6,- 1961 ice With the foregoing in mind the objects of the invention will be better understood and they may be said to include the provision of a phosphating solution which remains substantially free of sludge and which is capable of producing uniform power-free and substantially amorphous pyrophosphate coatings on the surfaces of steel as well as the provision of a solution and a method which are capable of producing very smooth and exceptionally flexible pyrophosphate coatings on the surfaces of metal objects. A concomitant object consists in the provision of a process which is capable of forming pyrophosphate coatings on metal surfaces which coatings permit the subsequent application of appreciably thinner layers of a final siccative surface finish having a higher degree of gloss than is ordinarily obtainable through the application of a surface finish over a conventional phosphate coating.

Additional objects of our invention include the provision of a process which is easily controlled and which does not require the use of so called balanced phosphate solutions as well as the provision of a phosphate solution which can be simply and inexpensively prepared and easily controlled and replenished.

A still further object is the provision of solutions which do not form objectionably large quantities of insoluble sludge or encrustations on the equipment used.

How the foregoing together with such other objects which may appear hereinafter are attained will now be described.

Our invention is based upon the discovery that compositions containing, as their essential coating ingredients, pyrophosphate selected from the class consisting of sodium, potassium, ammonium and lithium pyrophosphate, ferric ion and oxidizing accelerating agents which are capable of oxidizing ferrous iron to ferric iron, are ideally suited for the formation of an apparently'amorphous pyrophosphate coating on metal surfaces without the pres ence of zinc, manganese or other so-called coating metal phosphates. The solutions and process of the invention yield unusually smooth, uniformly coated and substantially amorphous coatings without the formation of a substantial overburden of powder.

In general the solutions of our invention contain from 0.05 to 12 or 13% of a pyrophosphate depending upon the type chosen and limited by the maximum solubility of the pyrophosphate in aqueous solutions. However, the preferred concentration of the pyrophosphate has been found to be from about 1 to 5% since entirely satisfactory coatings, which are smooth, uniform in appearance and amorphous in character, are readily obtained from solutions containing the preferred amount of coating producing ingredient. At concentrations of less than 1% the length of time required to produce the desired coating is increased and at concentrations of less than 0.05% no useful coating is produced in reasonable treating times. No increase in the speed of the coating reaction is obtained if concentrations are greater than 5%, although no detrimental effects are encountered at higher concentrations. Therefore, in the interest of economy, we generally prefer to use not more than 5% of the pyrophosphate.

The minimal amount of ferric iron which must be incorporated into the pyrophosphate bath in order to insure production of the desired amorphous coating is 0.0008 part for each part of pyrophosphate calculated as P 0 Above this quantity, we have found that the desired coating results can be obtained where an amount of ferric iron is included in the bath all the way up'to the limit'of this solubility which, in these pyrophosphate solutions, is approximately 0.06 part for each part or no harm. Use of less than 0.0008 part ferric iron for each part P may result in no coating being formed on the surface of the work when the pyrophosphate solution is applied in a spray process.

The accelerator used in the pyrophosphating solution must be selected from the class ofoxidizing agents which are capable of oxidizing ferrous to ferric iron. Typical examples are chlorates, nitrites and per oxygen compounds and they should be added in an amount chosen so as to give an efiect equivalent to from 0.2 gram to 10 grams of chlorate per 100 milliliters of coating solution. The preferred accelerator is chlorate although entirely satisfactory coating results are obtained from the use of one or a combination of the accelerators previously mentioned. Use of less than 0.2 gram of chlorate or equivalent per 100 mls. of coating bath is not efiective and results in a non-uniform coating of the metal surface. Conversely, use of more than 10 grams/100 mls. of coating bath or equivalent amount of other accelerator oflers no additional advantage over those obtained at the 10% level and at times may actually be detrimental.

Other accelerators such as nitrate, may be used in conjunction with the primary accelerators previously specified and where used the amount of primary accelerator may be reduced. For instance, as little as 0.2 gram of NaNO per liter of coating solution may be used when 7 grams/liter of NaNO are used.

The pH of the bath is also important and, in general, should not be so low that the solution contains free pyrophosphoric acid. We have found that a pH range of 3.2 to approximately 6.0 produces acceptable amorphous pyrophosphate coatings. However, optimum results are attained where the pH ranges from 3.8 to 5.5 for which reason we prefer to operate within this range. Use of a pH below 3.2 results in the formation of free pyrophosphoric acid which is undesirable in the coating solution. A pH in excess of 5.5 often results in thin and powdery coatings if indeed any coating at all is produced on some steels while above a pH of 6.0 no coating at all is produced.

The temperature at which the coating solution may be used ranges anywhere from room temperature to 120 F. Useful coatings have been produced at temperatures as low as 40 to 50 F. Higher temperatures result in somewhat heavier coatings. The use of temperatures in excess of 120 F. has no deleterious effect upon the quality of the coatings produced but higher temperatures cause a degradation of the pyrophosphates to the ortho form which results in a needless and costly waste of coating chemical as the orthophosphates are inert in the coating solution of the present invention. 7

Where precleaning has been borderline, it is possible to incorporate in the coating bath relatively small amounts of surface active agents which insure uniformity of coating in such instances.

In general the types of surface active agents which may be used in this invention may be of the class of Well known organic detergents of good wetting and emulsifying power. Examples of wetting agents suitable for use in this invention are the nonionic detergents such as the derivatives of polyethylene glycols, polyether alcohols and substituted polyglycol esters sold under the trade names Igepal (Allied Chemical & Dye Corporation), Lavapon (Rohm & Haas Company) and Nonic 300 (Pennsylvania Salt Manufacturing Company);

quaternary ammonium compounds, whichare cationic, sold under the'trade name Hyamine (Rohm & Haas Company; and anionic detergents such as sulfonated hydrocarbons and fatty alcohol sulfates sold under theof the pyrophosphating solution 'to be satisfactoryfor Car our purposes. In addition there are others which would be useful-the criteria being that they should have adequate surface tension depressant values and emulsifying power.

Examples of several pyrophosphate coating solutions which fall under the purview of this invention are listed below by way of illustration although they are not intended to limit the scope of the invention except as defined in the appended claims.

Example 1 7 Grams Disodium dihydrogen pyrophosphate 20 Sodium chlorate 5 Ferric chloride (FeCl .6H O) 0.5

Water, to make 1 liter.

The bath of Example 1 has a pH of 4.2 and produces exceptionally smooth and amorphous pyrophosphate coatings ranging in weight from 40 to mg./sq. ft. when operated at F. and using a S-minutes dip process.

Example 2 V Grams Dipotassium dihydrogen pyrophosphate 20 Sodium nitrite 5 Ferric nitrate (Fe(NO .9H O) 1 Lavapon WS-2 1 7 1 Water, to make 1 liter.

Reputed to be an alkyl aryl polyether alcohol with 10 ethylene oxide groups.

The solution of Example 2 had a pH of 4.5 and produced a completely uniform amorphous pyrophosphate coating on steel surfaces when applied as a spray process using a 2-minute contact period at 75 F.

Water, to make 1 liter.

The pH of the bath of Example 4 is 4.5. 7 The coating solutions of this invention as represented by the foregoing examples may be controlled by pointage titration. Pointage is'commonly defined in the art as the number of m1s. N/10 sodium hydroxide which are required to titrate a 10 ml. sample of the coatingbath to a permanentphenolphthalein endpoint. Replenishment of a coating bath is readily accomplished on the basis of the titration value determined from the control techniques and is effected by additionsof the same chemicals whichwere used in the original makeeup'of the bath in amounts as dictated by the control analyses..

In carrying out the improved process of the present invention, the surface to be coated should first be cleaned. This step, of itself, forms no part of thepresent invention and may be performed by any conventional cleaning procedure. For instance, grease and dirt may be removed by meansof alkali cleaners or an emulsion of a grease solvent. Rust may be removed by. pickling in acid solution. The cleaned work, either wet or dry, may then be treated with a solution of the-present invention of which the foregoing are typical examples although many other formulations may be adopted.

The coating may be formed on the work by immersing the surface to be coated in the solution, or by flowing, spraying or brush painting the solution on the surface. Also the solution may be wiped on by means of a cloth wet with the solution or by any other convenient technique which will permit the solution to act upon the work until the desired coating is produced.

Following the application of the pyrophosphate coating the treated metal surfaces may be given a final acidulated rinse using a dilute solution of chromic and/or phosphoric acid. Although this particular step forms no part of the invention and is not necessary in order to secure useful corrosion resistant and lasting coatings, it has been observed that a final passivation of the pyrophosphate coated surfaces tends to increase the overall corrosion resistance as well as to increase the adherence of the final siccative finish.

Inasmuch as the chemical constituents in the pyrophosphate coating solutions are depleted during use, it is necessary to replenish the coating baths at periodic intervals in order to maintain the proper balance of coating materials. We have found that solid admixtures of the coating chemicals may be used advantageously for both initial solution make-up and for replenishing during operation. Admixtures of this type may contain from 75-85% of a pyrophosphate, from 13 to approximately 25% sodium chlorate or an equivalent amount of an agent capable of oxidizing ferrous to ferric iron such as nitrite or per oxygen compound and from 0.0008 part to 0.06 part of ferric iron for each part of P 0 Specific examples of solid admixtures of coating chemical constituents which have been found to be useful both for initial solution make-up and for replenishment are shown below but are presented by way of illustration only and are in no way intended to limit the scope of the invention.

In certain industrial operations which employ electrical metering devices such as special automated equipment, it is frequently desirable and convenient to employ a concentrated solution of the desired constituents in preparing and replenishing the treating baths of our invention. This aids in maintaining a homogeneous coating batha matter which is of importance where such electrical metering devices for special metering equipment are employed. Make-up and replenishing solutions of this type may be prepared from solid admixtures of the coating chemicals as represented in Examples 5 and 6 above and a typical concentrated solution is prepared by dissolving 1.04 pounds of the solid mixture of Example 5 in 1 gallon of water.

We claim:

1. A composition for use in preparing and replenishing an aqueous metal coating solution; said composition consisting essentially of pyrophosphate from the class which consists of sodium, potassium, ammonium and lithium phosphates; accelerating agent from the class which consists of chlorates, nitrates and per oxygen compounds; and ferric iron: the quantity of pyrophosphate being from to the quantity of accelerating agent being equivalent to from 13 to approximately 25% of sodium chlorate, and the quantity of ferric iron constituting the balance on a basis of 0.0008 part to 0.06 part for each part of pynophosphate present calculated as P 0 2. The method of coating steel which comprises treating its surface with an aqueous solution consisting essentially of the following:

Quantity Pyrophosphate from the class consisting of sodium, potassium, ammonium and lithium pyrophosphates from 0.05% to 13% by weight. Ferric ion from 0.0008 part for each part of pyrophosphate calculated as P 0 to saturation of the bath. Accelerating agent from the class which consists of chlorates, nitrites and peroxygen compounds equivalent in eflfect to from 0.2 gram to 10 grams of chlorate per mls. of coating solution.

the solution being maintained at a pH of from 3.2 to 6 and at a temperature not exceeding F.

3. The method of claim 2 wherein the solution is maintained at a pH of from 3.8 to 5.5.

References Cited in the file of this patent UNITED STATES PATENTS 2,067,007 Darsey Jan. 5, 1937 2,514,149 Amundsen July 4, 1950 2,744,555 Nicholson et al. May 8, 1956 $2,758,949 Ley et a1. Aug. 14, 1956 2,856,322 Parson et al. Oct. 14, 1958 2,891,884 Rausch et a1. June 23, 1959 FOREIGN PATENTS 517,049 Great Britain Jan. 18, 1940 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No, 238L428 June 6 1961 Ferdinand P. Heller et ale It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below Column 1, line 58 for solid heavy metal read solid-heavy-metal column 2, line 5 for "power free" read powder iree line 69 for this read its --=a Signed and sealed this 31st day of October 1961,

(SEAL) Attest:

ERNEST W. SW'IDER DAVID L. LADD Attesting Officer Commissioner of Patents USCOMM-DC

Claims

2. THE METHOD OF COATING STEEL WHICH COMPRISES TREATING ITS SURFACE WITH AN AQUEOUS SOLUTION CONSISTING ESSENTIALLY OF THE FOLLOWING: QUANTITY PYROPHOSPHATE FROM THE CLASS CONSISTING OF SODIUM, POTASSIUM, AMMONIUM AND LITHIUM PYROPHOSPHATES FROM 0.05% TO 13% BY WEIGHT. FERRIC ION FROM 0.0008 PART FOR EACH PART OF PYROPHOSPHATE CALCULATED AS P2O7 TO SATURATION OF THE BATH. QUANTITY ACCELERATING AGENT FROM THE CLASS WHICH CONSISTS OF CHLORATES, NITRIES AND PEROXYGEN COMPOUNDS EQUIVALENT IN EFFECT TO FROM 0.2 GRAM TO 10 GRAMS OF CHLORATE PER 100 MLS. OF COATING SOLUTION.