US3178319A - Phosphate coating compositions and methods of making and using the same - Google Patents

Description

d. W65. Pate iofi ce 3 17:; 319 PHOSPHATE COAThJG COMPOSITIONS AND METHODS on MAKING AND USING m John Arthur Henricks, Baltimore, Md., assignor to Geraldine D. Henricks, Baltimore, Md. No Drawing. Filed June 5, 1958, Ser. No. 739,986 2. Claims. (Cl.'148 6.15)

This invention is a continuation-impart of application Serial Number 271,930, filed February 16, 1952, and now abandoned.

The prior art zinc phosphate systems are based on zinc acid phosphate baths that coat out tertiary zinc phosphate mixed with iron phosphate as the iron surface is attacked by the free phosphoric acid of the bath. By incorporating ammonium ions equivalent to the zinc ions of the system, some zinc ammonium phosphate is co-deposited with the iron and zinc to produce a mixed phosphate coating that is finer in grain size and superior to the prior art coatings in protective value.

The saturation effect of the ammonium ion can be shown by the following tables which illustrate the increasing coating weights obtained in a fixed zinc phosphate bath, to which increasing increments of monoammonium phosphate are added. In the phosphating art, the baths have been controlled by titrating a 10 ml. sample of the bath with 0.1 normal sodium hydroxide, and calling the number of ml. required, the bath points. 7 In the following tables, a 10 point bath is one having a 0.1 normality in total acid, meansured by a phenolphthalein end-point. The free acid likewise represents the ml. of 0.1 normal sodium hydroxide consumed at the initial methyl orange end-point.

TABLE I.-EFFECT OF MONO-AMMONIUM PHOS- PHATE SATURATION ON RATE OF COATING IN CHLORIDE FREE BATH Bath composition 2% solution of secondary zinc orthophosphate 10 points phosphoric acid10 points Z11(NO2)2.6H2O No zinc chloride 20 point bath Moles of Free to Weight of Ammonium Total Grams Coating, ng/ Phosphate Acid per Liter sq.ft., 2-10 Added to Ratio of Iron min.

Bath

0.0 5. 6 l. 7 336 0.1 5. 1. 2 528 0. 2 6. 5 l. 8 680 0.51 6. 8 l. O 7 92 0.8 6. 4 1.5 904 Rumours-The ammonium phosphate free panel had the darkest color while the other panels had almost the same gray color. The grain size was rather constant, fine and dense.

TABLE II.EFFECT OF MONO-AMMONIUM PHOS- PHATE SATURATION OF COATING IN CHLO- RIDE BATH I Bath composition 2% solution of secondary zinc phosphate composition 10 points phosphoric acid with 5 points zinc chloride and 5 points of zinc mitrate 20 point bath Kansans-The gray color and fine grain size was about the same in all cases. f

It can be seen that the ammonium phosphate additions greatly increase the coating rate of these baths, and convert a dark granular coating to a finedense grey coating.v

It is an object of this invention to compound novel and highly soluble zinc phosphate concentrates based, upon zinc salts, phosphoric acid, and ammonium phosphate.

It is an object of this invention to keep these ionized salt baths in equilibrium against metathesis by incorporating a minor amount of zinc (ii-hydrogen phosphate and ammonium nitrate with a major proportion of the highly ionized coating salts.

It is also an object of this invention to utilize the principle of ammonium ion saturation to operate a zinc phosphate bath at room temperature. 7 i

It is another object of this invention to use ammonium ions in hot nitrite accelerated zinc phosphate baths to eliminate the nitrous fumes by reducing the nitrous acid to nitrogen by reaction with ammonium ion.

Ammonium ion is known to be a precipitating agent for zinc phosphate, and is the. basis for the preferred method of gravimetric zinc analysis. During the phosphate coating Operation the free acid adjacent to the iron surface is neutralized by the metal, and the resultant ferrous phosphate is co-deposited with the zinc phosphate to form the coating. At the higher pH brought about by neutralizing the free acid with the metal, the excess ammonium ion supersaturates the solution adjacent to the metal to lower the solubility of both the zinc and iron ions and thereby accelerates the coating action. Zinc, ammonium, and lithium cations expedite zinc phosphate coating by a saturating phenomenon and hence may be provided in these compositions in the form of their soluble nitrates or phosphates. 7

Since zinc ammonium phosphate is insoluble, these coating baths and concentrates must be based upon very soluble ionized salts of these three components. Zinc nitrate is my preferred zinc salt, since it has infinite water solubility and the nitrate anion is an excellent hydrogen depolarizer to expedite the coating action. Mono-ammonium phosphate is the preferred source of the amine phosphate, and is used with suflicient phosphoricacid to pre vent precipitation of zinc ammonium phosphate in the bath or in the concentrate. Minor amounts of zinc or ammonium chlorides or sulfates are used to eliminate sludge formation with these formulations. The chloride or sulfate ion eliminates sludge by increasing the iron salt solubility. Less than 5% by weight, based on the concentrate, of anhydrous zinc chloride or sulfate is required to eliminate sludge in a dip phosphate bath, although more is often used to take advantage of the economy of these salts.

I have found that mono-ammonium phosphate is unique in that it forms a completely soluble concentrate when dissolved in concentrated mixtures of phosphoric acid and zinc nitrate in water. When equimolar zinc nitrate solutions containing 16 to 18% zinc and 75% phosphoric acid are mixed to form a concentrate of about 1.58 specific gravity, one half to two moles of anhydrous mono-ammonium phosphate can be dissolved in this strong solution to yield a clear zinc phosphatizing concentrate having a specific gravity of between 1.62 and 1.70.'- When crystalline mono-sodium or mono-potassium phosphate isused in place of the mono-ammonium phosphate, no satisfactory phosphatizing concentrate can be prepared. -When mono-sodium phosphate is used in place of the monoammonium phosphate, the mixture of zincnitrate and PatentedApr. 13, 1965,

it crystallizes out in long needles containing zinc, when the solution cools. This indicates that the ammonium phosphate-and only mono-ammonium phosphate-forms a complex with the mixture.

From 1% to 5% by volume of these novel concentrates are diluted with water to make up a zinc phosphate coating bath. These baths can be heated up to boiling without precipitating tertiary zinc phosphateunlike the prior art zinc di-hydrogen phosphate baths which hydrolyze to form some insoluble zinc phosphate and free phosphoric acid when they are heated above 160 F. The elimination of this hydrolytic zinc phosphate sludge is a significant and important movement in the art.

While these ionized salt baths do not hydrolyze to precipitate zinc phosphate when heated, they do undergo a metathesis to form about 20% ammonium nitrate and zinc phosphate, or zinc ammonium phosphate. For example, when equal volumes of 0.20 normal zinc nitrate (equal to 20 points) solution is mixed with an equal volume of 0.20 normal mono-ammonium phosphate, the resulting phosphate bath is not 0.20 normal, but has only a 0.16 normality (equal to 16 points)a loss of about 20%. When such a solution is boiled, some zinc ammonium phosphate precipitates out, and some free acid is formed. Under average conditions, the free acid will account for 8% of the loss, and the precipitated zinc ammonium phosphate for the other 12% of the loss.

Due to the buffer action of the ammonium phosphate and the zinc ammonium phosphate inthe mixture, this phosphate bath stays about pH 3.0 instead of falling to pH 2.0, the pH of 0.01 normal nitric acid. While a zinc phosphate composition based on only zinc nitrate and ammonium phosphate is very satisfactory as a base for room temperature or spray phosphate baths; free phosphoric acid is used in all immersion phosphate compositions. When free phosphoric acid is utilized for bath formulation, no zinc ammonium phosphate precipitates out of the bath, even with continued boiling, however, a soluble zinc phosphate complex is formed, and again about 20% of the total normality, or points, are lost on mixing. For example, upon mixing equal volumes of 0.30 normal (30 point) phosphoric acid with 0.30 normal (30 point) zinc nitrate and 0.30 normal (30 point) monoammonium phosphate, a zinc phosphate immersion coating bath results that has only a 0.24 normality (equal to 24 points). These reactions are best shown by chemical equations, such as those on the accompanying Table III.

TABLE III.METATHESIS OF IONIZED PHOS- PHATING SALTS (1) Reaction between phosphoric acid and zinc nitrate:

This reaction shows a 22% point loss over the starting material.

(2) Reaction between mono-ammonium phosphate and zinc nitrate:

This reaction shows a 17% point 'loss over the starting material.

(3) Reaction between phosphoric acid, zinc nitrate, and ammon. phosphate:

This reaction shows a 20% point loss over the starting material.

4 (4) Reaction (3) buffered with zinc di-hydrogen phosphate:

This reaction shows only a 6% point loss over the starting material.

(5) Reaction of zinc chloride with phosphate chemicals: When zinc chloride is substituted for the equivalent zinc nitrate, in Equations 1, 2, and 3, the same magnitude of normality or point loss occurs as are shown for the mixtures using zinc nitrate.

It is apparent from Table III that zinc nitrate reacts with phosphoric acid to form some zinc acid phosphate and nitric acid, and with the mono-ammonium phosphate to form some zinc ammonium phosphate and ammonium nitrate. By including these reaction products in the same proportions in the concentrate, the phosphate baths made from this concentrate represent the same equilibrium obtained from baths made from the equivalent concentrate based on ionized salts alone.

I have found that metathesis can be reduced if about 25% of the zinc in the concentrate or in the coating bath is obtained from zinc di-hydrogen phosphate. When these formulae are compounded in this manner, there is little loss of strength, or normality, when the solutions are mixed. Thus, when equal volumes of 0.20 normal zinc di-hydrogen phosphate (20 points) 0.20 normal phosphoric acid (20 points), 0.20 normal zinc nitrate (20 points) and 0.20 normal mono-ammonium phosphate (20 points) are mixed, the resultant phosphating bath has a normality 0.19 (equal to 19 points) showing a loss of only about 6% from mixing.

While a simple mixture of zinc nitrate with ammonium phosphate and phosphoric acid is a very satisfactory phosphating composition, and a distinct improvement over the prior art; an equilibrium mixture with zinc dihydrogen phosphate is cheaper to manufacture and has greater stability against metathesis. When 20% to 25% of zinc di-hydrogen phosphate is added to mixtures of equimolar zinc nitrate, phosphoric acid, and mono-ammonium phosphate, the resultant concentrates give phosphatizing baths that are identical in performance to equivalent baths that are made from ionized salts. However, I .have found that baths that are based entirely upon zinc di-hydrogen phosphate produce coarse irregular zinc phosphate coatings, that have a slower coating rate and less corrosion resistance than the chemically equivalent baths made with the ionized salts.

For example, six baths were made up from difierent ratios of zinc nitrate, phosphoric acid, and ammonium phosphate; and compared with six baths ofidentical ion concentrations, but prepared from the equivalent zinc di-hydrogen phosphate, nitric acid and ammonium nitrate. The coating covering rate, judged by the free pore area, of the ionized salt baths averaged complete in 5 minutes and complete in 10 minutes immersion in these baths at 200 F. The coating covering rate of the zinc di-hydrogen phosphate baths averaged only 20% complete in 5 minutes, and 89% complete in 10 minutes immersion at 200 F. Bare panels from the ionized salt baths averaged 5.7 hours before the first rust spots, while bare panels from the chemically equivalent zinc di-hydrogen phosphate baths averaged only 2.1 hours before the first rust spots, when both sets were exposed in a 5% salt spray.

However, when the zinc di-hydrogen phosphate component is kept between 15% and 30% of the total concentrate, and the balance of the formula is derived from the ionized salts, then the coating performance of the mixtures is identical to that of the pure ionized salt formulations.

By utilizing zinc di-hydrogen phosphate and ammonium nitrate for about 25% of the concentrate, an appreciable cost saving in chemicals results, and a greater stability is imparted to the mixture by the buffer action of the zinc phosphate.

The zinc di-hydrogen phosphate component is made by reacting zinc oxide with 50% phosphoric acid and diluting the zinc phosphate reaction product with 60% ammonium nitrate solution. This product, in turn, is mixed with the concentrate based upon zinc nitrate, phosphoric acid and ammonium phosphate, as will be seen in the ensuing examples, to form the equilibrium mixture.

Different commercial phosphating methods are used to obtain different finishes, and to fit different equipment. Heavy phosphate coatings are used for cold drawing and as a protective base to hold inhibitive oils. These have a coating weight of 1000 mg. per sq. ft. or more. Light and preferably smooth coatings are used on articles to be lacquered or painted, preferably having a coating weight between 100 and 300 mg. per sq. ft. Phosphate coatings are applied by simple immersion in the coating bath or by spray application of the phosphating solution. The operation can be done at temperatures as high as the boiling point of the solution, or at room temperature. The work to be coated can be hung on appropriate hangers, or mechanically tumbled in perforated barrels, when the dip, or immersion, coating baths are used. These novel formulations can be adapted to the several methods of application, and can 'be modified to produce the desired thickness and texture of coating desired, as shown in the following examples. Certain basic relationships between the components of the formulae should be pointed out and discussed so that the mechanisms of formulation and solution modification will be understood before the examples are given. In a zinc phosphate system about by wt. of nitrate, based on the concentrate is sufficient to depolarize and accelerate the coating out of the 10-12% zinc contained in the concentrate. Any nitrate above 10% has only slight further acceleration, but will refine the coating grain size and produce a thinner and denser zinc phosphate coating. Chloride, as low as 1% of the concentrate, will eliminate iron sludge in a dip bath, and will produce a heavier and coarser zinc phosphate coating when used at higher concentrationswhen, for example, 50% of the zinc ion is supplied as chloride. Ammonium phosphate, as shown in Tables I and 11, increased coating weights by an ammonium saturation phenomenon. However, all of the weight increase shown is not due to ammonium ion alone, but is also due to phosphate ion increase. In a fixed bath formulation, an increase in phosphate will give an increase in coating weight. With adequate acceleration and ammonium ion, it is possible to double the coating weight by doubling the phosphate content of the bath. The phosphate balance for a given bath is obtained from a balanced mixture of phosphoric acid and ammonium phosphate, augmented in the equilibrium type bath, by -25% zinc di-hydrogen phosphate. For a dip, or immersion bath, up to 75% of the phosphate would be supplied as phosphoric acid, while up to 100% of a spray bath requirement would be supplied as mono-ammonium phosphate. -An immersion zinc phosphate will have a 5 to 1 ratio of total to-free acid, while it is desirable to maintain from 10 .up to to 1 ratio of total acid to free acid in a spray phosphate bath.

In salt baths low in phosphate, the nitrate, chloride or sulfate are correspondingly high and the rate of attack on the bare metal is high, resulting in the formation of a thin phosphate coating high in iron phosphate. Such baths that are starved for phosphate give deposits lower in corrosion protection than high phosphate baths.

In addition to controlling the balance of the inorganic phosphate components, the system can be modulated by galvanic accelerators such as nickel, copper or silver, which will be disclosed with the examples.

In order to more clearly illustrate the application of 6 this invention to the phosphate art, the following examples are given:

EXAMPLE I.EQUIMOLAR PRIMARY ZINC PHOSPHATE Percent 75% phosphoric acid, H PO 0.15 molar 20 Zinc nitrate, flake, Zn(NO 5H O, 0.15 molar 42 Water, to dissolve zinc nitrate 21 Mono-ammon. phosphate, NH I-I PO 0.15 molar 17 The zinc nitrate is dissolved in the water with heating and stirring means; The 75 phosphoric acid is then mixed into the zinc nitrate solution. The mono-ammonium phosphate is then dissolved in the mixture to make a zinc phosphate concentrate.

This concentrate in a 3% solution provides a simple sludgeless phosphating bath when heated up to 200 F. Clean work processed by a ten minute immersion in this bath received a 1200 milligram per square foot coating which was given a dilute hot 0.5% chromic acid rinse and a rust inhibiting oil coating that passed a 48 hour salt spray test.

EXAMPLE II.PRIMARY ZINC PHOSPHATE WITH CHLORIDE Percent Zinc nitrate, flake, Zn(No 5H O, 0.10 molar 29 Water, to dissolve zinc nitrate 14 75 phosphoric acid, H PO 0.15 molar 20 Zinc chloride, granular, ZnCl 0.10 molar 14 Mono-ammon. phosphate, NH H PO 0.20 molar 23 The zinc nitrate is dissolved in the water with heating and stirring means. The 75% phosphoric acid is then mixed into the zinc nitrate solution. The granular zinc chloride is next dissolved in the mixture, after which the mono-ammonium phosphate crystals are stirred in and tliisgolved. This concentrate has a specific gravity of This concentrate, diluted to a 3% sludge free solution, and heated to 200 F. gave a dense crystalline deposit of over 1000 mg. sq. ft. on clean low carbon steel in 10 minutes. The coating from this bath was coarser and darker in color than the coating from the Exp. I bath. The crystal size was coarsened by the chloride addition, while the coating weight was reduced by a reduction of the phosphate from 2x the zinc in Exp. I to a phosphate of 1.7x the zinc. This coating passed a 36 hour 5% salt spray when coated with rust inhibiting oil.

EXAMPLE III.EQUILIBRIUM BATH WITH ZINC ACID PHOSPHATE I Percent Zinc oxide, lead free, ZnO, 0.15 molar 12 75 phosphoric acid, H PO 0.40 molar 52 Water, to dilute acid 16 Ammonium nitrate, NH NO 0.15 molar 12 Water, to dissolve ammon. nitrate 8 The phosphoric acid is diluted with water and stirred rapidly as the fine zinc oxide powder is sifted into the acid and dissolved. The ammonium nitrate is dissolved in hot water to form a 60% solution with a specific gravity of 1.30. After all of the zinc oxide has dissolved in the diluted. phosphoric mixture, the ammonium nitrate solutron is added and mixed to give a 1.58 gravity, concentrate. This concentrate is used in blending, as below 25 parts of the zinc phosphate concentrate, shown above 75 parts of Example I equimolar zinc concentrate The two concentrates are mixed to form an equilibrium concentrate of 1.67 specific gravity. When diluted to a 3% phosphate bath, this concentrate gave coating results identical to those obtained in Exp. 1.

The zinc di-hydrogen phosphate and equimolar ammonium nitrate concentrate contains the metathesis products formed when Zinc nitrate, phosphoric acid, and ammonium phosphate are mixed. When used at 15 to 30% concentration, it can be mixed with 85 to 75% of the chloride formulae of Exp. II, the microcrystalline formula of Exp. IV, or any other ionized salt base formula, to obtain a balanced equilibrium concentrate. Such mixtures have a greater stability and buffer value, and lower the chemical cost of the product.

EXAMPLE IV.-MICROCRYSTALLINE ZINC PHOSPHATE This type of coating bath is disclosed in my co-pending patent applications 373,449 and 635,130 and are directed to obtaining smooth to amorphous zinc phosphate coatings.

Percent Zinc nitrate, flake, Zn(NO -5H O, 1.0 molar 28.0 Water, to dissolve zinc nitrate 12.0 75% phosphoric acid, H PO 1.6 molar 21.0 Mono-ammon. phosphate, NH H PO 0.4 molar 4.6 Calcium nitrate, beads, Ca(NO -4H O',

1.0 molar 23.6 Water, to dissolve calcium nitrate 10.8

The zinc nitrate is dissolved in the water with heating and stirring means. The 75 phosphoric acid is mixed with the zinc nitrate solution, and the granular monoammonium phosphate dissolved in the mixture. The calcium nitrate is dissolved in water, and the solution mixed with the zinc concentrate to obtain the finished product.

This concentrate is used at 5% concentration to obtain a 30 point amorphous phosphate bath, which is heated to \180 F. and used to dip coat clean sheet metal articles prior to painting.

A six minute immersion gave a 300 -mg./ sq. ft. slate like amorphous coating that withstood a 400 hour 5% salt spray when coated with a primer and a white appliance top-coat enamel. The lower zinc to phosphate ratio of 1 to 1, plus the high 2.0 molar nitrate to 1.0 molar phosphate all contribute to a thin coating mechanism. However, the smoothing action is obtained from calcium ion, augmented by the high acid content which causes a high percentage of iron ions to be formed at the solution interface, and included in the coating with the zinc and calcium.

EXAMPLE V.ZINC CHLORIDE BASE FORMULA Percent 75% phosphoric acid, H P 0.1 molar 20.3 Mono-ammon. phosphate, NH H 'PO 0.15 molar 17.9 Water to dissolve ammon. phosphate 17.8 Ammonium nitrate, NH NO 0.15 molar 12.4 Water, to dissolve ammon. nitrate 10.4 Zinc chloride, granular, ZnCl 0.15 molar 21.3

The 50% ammonium phosphate solution is mixed with the phosphoric acid, and the granular zinc chloride gradually added with stirring. The ammonium nitrate solution is added last, to avoid the formation of any nitrosyl chloride by reaction of the nitrate with the anhydrous zinc chloride granules during dissolving.

The resulting concentrate is diluted with water to a 3% phosphating bath. Low carbon steel bolts were phosphatized in a tumbling barrel immersed in the bath for twenty-five minutes at 205 F. The resulting dark crystalline coating averaged 1100 mg./ sq. ft. The bolts were given a /2% hot chromic acid rinse and coated with a rust inhibitive oil. These articles passed a 24 hour 5% salt spray.

This concentrate can be stablized by mixing parts of it with 25 parts of the zinc phosphate and ammonium nitrate concentrate of Exp. III. Thequality of phosphate coating obtained from this formulation based on zinc chloride alone, is not quite as high as that obtained from a total zinc nitrate -formulation, but the very appreciable cost savings justify this composition. While the nit-rate at 9% is only about half of that in the straight zinc formulation in Exp. 1., the coating action is reasonably rapid because there are two ammonium ions for every zinc ion in the formula.

I have found that urea, acetamide, methyl and ethyl amines, and the widely used ethanol amines, also give the ammonium ion saturation effect in these baths. While their greater cost preclude their substitution for ammonia, I wish to point out their equivalence and that they have certain valuable properties that could justify the expense of their substitution in whole, or in part, for the ammonium ion.

While numerous variations in the character of solutions employed and in their manner of application have been described above, it will be understood that these are 'by way of illustration and do not attempt to set forth all variations which are possible within the broad principles of this invention and within the scope of the appended claims.

What I claim is:

1. An improved method of formulating aqueous nonsludging zinc phosphate concentrates by mixing a zinc nitrate solution with from one-half up to two moles of 75 phosphoric acid per mole of zinc and then dissolving from one-half up to two moles of mono-ammonium phosphate into the resultant mixture which is then stablized against metathesis by incorporating 15% to 30% additional zinc and phosphate in the form of a concentrated zinc (ii-hydrogen phosphate solution and the whole thoroughly mixed into a homogeneous concentrate.

2. An improved method of formulating a smooth microcrystalline zinc phosphate concentrate by diluting the non-sludging and metathesis stable concentrate of claim 1 with a 40% to 60% calcium nitrate solution in the molar proportion of from 0.6 up to 1.0 mole of calcium per mole of zinc and then thoroughly mixing these components into a homogeneous concentrate.

References Cited by the Examiner UNITED STATES PATENTS 2,001,754 5/35 Thompson et al. 148-6.15 2,232,352 2/41 Verweij et al. 148-6.15 2,268,323 12/41 Martin et al. 148-615 2,293,716 8/42 Darsey 1486. 15 2,295,545 9/42 Cliiford et a1. 1486.1 5 2,304,299 12/42 Boyle et al. 148-6115 2,326,309 8/43 Romig 148-6.15 2,487,137 11/ 49 Hoover et al -l48-6.15 2,540,314 2/51 Amundsen 148--6.1;5 2,657,156 :10/53 Hyams et al. 1486.15

FOREIGN PATENTS 599,778 3/55- Great Britain. 741,050 11/55 Great Britain.

RICHARD D. NEVIUS, Pn'mary Examiner.

CLAUDE A. LE ROY, NATHAN MARMELSTEIN,

RAY V'JINDHAM, Examiners.

Claims (1)

1. AN IMPROVED METHOD OF FORMULATING AQUEOUS NONSLUDGING ZINC PHOSPHATE CONCENTRATE BY MIXING A ZINC NITRATE SOLUTION WITH FROM ONE-HALF UP TO TWO MOLES OF 75% PHOSPHORIC ACID PER MOLE OF ZINC AND THEN DISSOLVING FROM ONE-HALF UP TO TWO MOLES OF MONO-AMMONIUM PHOSPHATE INTO THE RESULTANT MIXTURE WHICH IS THEN STABILIZED AGAINST METATHESIS BY INCORPORATING 15% TO 30% ADDITIONAL ZINC AND PHOSPHATE IN THE FORM OF A CONCENTRATED ZINC DI-HYDROGEN PHOSPHATE SOLUTION AND THE WHOLE THOROUGHLY MIXED INTO A HOMOGENEOUS CONCENTRATE.