US2465247A

US2465247A - Composition for and method of producing phosphate films on metals

Info

Publication number: US2465247A
Application number: US705373A
Authority: US
Inventors: Byron V Mcbride
Original assignee: Westinghouse Electric Corp
Current assignee: Westinghouse Electric Corp
Priority date: 1946-10-24
Filing date: 1946-10-24
Publication date: 1949-03-22
Anticipated expiration: 1966-03-22
Also published as: CH268172A; FR954618A; BE476601A

Description

March 22, 1949. B. v. M BRIDE 2,465,247

COMPOSITION FOR AND METHOD OF PRODUCING PHOSPHATE FILMS ON METALS Filed Oct. 24, 1946 Fly].

Fig. Z.

I I I 16 v4 WITNESSES: INVENTOR Patented Mar. 22, 1949 COMPOSITION FOR AND METHOD OF PRO- DUCING PHOSPHATE FILMS ON METALS Byron V. McBride, Irwin, Pa., assignor to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Application October 24, 1946, Serial No. 705,373

4 Claims.

This invention relates to the treatment of ferrous, zinc and cadmium metal surfaces by means of adherent paste-like compositions to produce protective phosphate coatings on predetermined portions of the metal surfaces.

Heretofore, in the art of producing protective phosphate coatings on metal surfaces, it has been quite difiicult to treat only predetermined portions of the metal surfaces. Phosphate compositions, as known in the art today, are so fluent that regardless of their method of application, they can not be confined to predetermined sources unless the metal surfaces which it is desired not to treat are masked off. Spraying of the phosphate compositions on the metal surfaces results in running and dripping of the phosphate composition indiscriminately and no adequate control of application confining the solution to certain areas is possible. Dipping or other modes of application of the relatively fluid phosphate compositions obviously will result in all the surfaces being subjected to the action of the phosphate coatings. For many purposes, it is necessary that the protective phosphate compositions be applied to only predetermined portions of the metal surfaces without running, dripping or other contamination of the rest of the surfaces of the particular member being treated. No phosphate composition known has been found suitable or adequate for controlled treatment of metal surfaces.

The object of the present invention is to provide a paste-like adherent coating composition which may be applied to predetermined portions of surfaces of members to produce thereon protective phosphate films without undesirable action on other portions of the surfaces.

A further object of this invention is to produce a paste-like adherent coating composition which, when applied to metal surfaces, will produce thereon protective phosphate films and a readily removable powderedlayer on drying.

A still further object of the invention is to provide for treating metal surfaces to produce thereon protective phosphate films by applying pastelike coating compositions.

Other objects of the invention will in part be obvious and will in part appear hereinafter.

For a better understanding of the nature and objects of the invention, reference should be had to the following detailed description and drawing, in which:

Figure 1 is a vertical cross section through a member treated in accordance with the invention; and

Fig. 2 is likewise a vertical cross section showing a further step in the process of treatment,

In accordance with the invention, paste-like compositions that are relatively permanently stable have been produced for application to ferrous, zinc and cadmium metal surfaces whereby to produce protective phosphate coatings therein at any predetermined portion of the surface. After-treatment of the metal surfaces is easy and highly protective phosphate films on the metal surface are obtained. The paste-like coating compositions are produced by combining a finely divided porous silica aerogel with an aqueous solution capable of producing phosphate films when applied to the metal surfaces. Outstanding results have been obtained when from 10 to 45 parts by weight of a porous silica aerogel, finely divided to the extent that at least about 50% passes through a 180 mesh screen, is combined with 100 parts by weight of an aqueous phosphate solution. The aqueous phosphate solution should contain essentially from 25% to by weight of a phosphorous compound capable of supplying phosphate ions to react with the metal and an oxidizing agent, the balance of the solution being water. The composition is relatively paste-like so that when applied by means of brushing or even by spraying, the composition remains at the point of application to the metal as an adherent coating that does not flow or drip. Furthermore, the paste-like nature of the composition is relatively permanent and it may be stored for prolonged periods of time without undue separation and it may be employed without continual stirring. Other advantages of the composition will be set forth hereinafter.

The porous silica aerogel may be produced in various ways, the composition disclosed in the Patent 2,093,454 to Kistler or the product sold to the trade as Santocel 45 or Santocel 58 are suitable for use in the present invention. The porous silica aerogels have a dry bulk condensity of from 6 to 9 pounds per cubic foot. Their pH varies from 4.5 to 6.5. The silica aerogels are composed of from to of SiOz, the balance being volatile matter and impurities such as alkali sulphates. It has been found that the best pastelike compositions of the present invention are secured if at least 50% of the silica aerogel passes through a mesh screen.

The porous silica aerogel has been found to possess new and unexpected properties enabling the production of paste-like compositions entirely different from those obtainable by the use of other finely divided inert inorganic materials.

For example, tests have shown that diatomaceous earth will not form a paste. When combined with a phosphate solution, the diatomaceous earth will settle out in a few seconds. Fuller's earth will not form a paste and it also settles out in a few seconds. Finely powdered ceramic frits do not make a good paste and when combined with the phosphate solution had been found to be detrimental to the production of a satisfactory phosphate coating on ferrous metal surfaces. Finely divided titanium dioxide similarly results in a poor, spotty phosphate film on the metal surfaces. A further defect encountered with compositions embodying many of the finely divided inorganic powders other than silica aerogel has been that such compositions when applied to metal surfaces and dried produce a strongly adherent powder layer, which is undesirable since it requires excessive scrubbing and brushing to remove, the powdered flayer. Thus bentonite, when combined with a phosphate solution, forms layers extremely adherent to the metal surfaces on drying and the removal of the bentonite powder entails a considerable amount of hand scrubbing. For instance, a stream of compressed air at30 lbs. per square inch pressure would not remove any of these coatings. By contrast, the porous silica aerogel compositions of this invention produce smooth protective phosphate films onthe metal and on drying a loose fluffy layer of aerogel is present that may be removed by a simple rinsing in water, a light brushing with a brush or cloth, or a stream of compressed air.

Corrosion tests in salt spray and steam chests show that protective phosphate films produced byapplications of the silica aerogel compositions of this invention are superior to those secured with other compositions embodying ceramic frit, infusorial earth, asbestine, calcium carbonate and the like.

. .Examples of the paste-like compositions of the present invention are:

Example I 1 /2 pounds of the porous silica aerogel sold to thetrade as No. 58 micropulverized Santocel is admixed with one gallon of a composition composed of the following ingredients.

Phosphoric acid 84% pounds 5 Manganese phosphate do 1% Sodium nitrate do A Cupric nitrate ounces 4 Water to makeone gallon The composition resulting on admixture of the ingredients was a smooth paste that could be readily brushed upon metal surfaces or applied by'means of a spray gun. The composition adhered to the metal surfaces without running. After {application to the metal surfaces, the coating air dried in about 15 minutes and produced asmooth uniform protective phosphate film on the metal surface and an upper layer of loose and fluffy silica aerogel. The layer of silica aerogelcould be removed by brushing with a soft bristle'brush or dipping in water or directing against it a spray of water or by a blast of compressed air.

' Ea'ample II "'14i parts by weight of a porous silica aerogel was admixed with an aqueous solution having the following composition:'

Parts Zinc dihydrogen phosphate 14 Copper nitrate Nitric acid 1 Water 49 Methylcellulose (water soluble) 1 Wetting agent /g Orthophosphoric acid to give a pH of from 1 to 2.

The wetting agent was a sodium alkyl aryl sulfonate sold to the trade under the name Nacoonal. The composition was a smooth, thick paste that could be applied to steel panels as a thin uniform coating that adhered to the surfaces to which it was applied. The composition formed protective phosphate films in a few minutes and on air drying a fluffy, nonadherent layer of silica aerogel resulted.

"Steam chest and salt spray tests indicate that the phosphate film is considerably more corrosion resistant than the film secured by use of the same phosphate solution without the silica aerogel.

' Paste-like compositions were prepared with as little as 10 parts by weight of the silica aerogel to parts by weight of the aqueous phosphate solution. The paste was readily applied by spraying. Thicker pastes that could be readily applied by brushing or spraying were prepared with as much as 45 parts by weight of the silica aerogel to the liquid phosphate solution.

The use of a water soluble cellulose ether in the proportion of one-half to 5% of the weight of the phosphate solution improved the stability of the paste. Suitable cellulose ethers are water soluble methyl cellulose and ethyl cellulose formulations.

A small amount of wetting agent enables the prompt admixture of the silica aerogel in aqueous phosphate solution. A wetting agent in the amount of from 0.1% to 3% of the weight of the phosphate solution is beneficial. Suitable wetting agents are the sulfonates and sulfates of long carbon chain organic compounds.

The phosphate solutions may contain free phosphoric acid and acidic phosphate compounds. The pH of the phosphate solutions will vary from 0.5 to 6. The oxidizing agent may be a nitrate, a nitrite or other compound capable of oxidizing hydrogen which results from the reaction of the metal and the phosphate solution.

Referring to Fig. 1 of the drawing, there is illustrateda metal sheet l2 whose upper surface I4 is to be treated to provide a protective phosphate coating thereon. The paste-like silica aerogel composition of the present invention is applied as a coating I6 to the upper surface of the member I 2. For most purposes a coating of a thickness of inch to 1 3' of an inch in thickness is adequate. At room temperature, a protective phosphate film will be produced on the surface I4 in a matter of a few minutes. If desired, the treated metal surfacecan be put in an oven in order that the coating react more promptly and the silica aerogel form a dried layer. Even surfaces that are slightly oily can be coated with the composition and a satisfactory ,phosphate film produced. I

A single application of the composition has been found to produce heavier and more uniform phosphate films thanthe aqueous phosphate solution without the silica aerogel would be capable of producing. Vertical and overhead surfaces can be easily treated without running or dripping occurring. The phosphate films produced on 5. such vertical or overhead surfaces are equal in quality to that produced on upper horizontal surfaces.

It is believed that the porous silica aerogal retains the phosphate solution in such a manner that it is released to the metal surface during the chemical reaction With the metal at a uniform rate over the entire surface. Apparently, none of the phosphate solution is retained within the body of the porous silica aerogel and the silica aerogel dries readily. After the reaction of the phosphate solution with the metal surface has been completed and any water present has evaporated into the atmosphere, the silica aerogel is not bonded appreciably to the phosphate film and is present as a light fiufiy layer that may be removed with a single stroke of a soft bristle brush, by dipping in water or with a stream of compressed air. No scrubbing or other involved manual labor is necessary to remove the layer of dried silica aerogel.

In order to seal off the protective phosphate coating and enhance its protective properties, it is desirable to apply thereto water soluble hexavalent chromium compound such as chromic acid or sodium dichromate. A paste-like chromium composition may be prepared by admixing 100 parts by weight of the silica aerogel with from to 200 parts by weight of the hexavalent chromium compound and sufiicient water to produce a fiowable paste. The chromium composition so produced may be applied to the phosphate film in the manner shown in Fig. 2 of the drawing. The metal surface M of the member l2 carrying the protective phosphate film I8 is covered with a coating 20 of the paste-like chromium containing composition. The coating 28 may be dried in about minutes and a simple brushing or dipping in water will remove the dried silica aerogel which forms and is in a loose powdery state. The inhibitive effect of the chromium derived from layer has been found to be far superior to that secured with the conventional dilute chromic acid rinse common in the art today. A suitable hexavalent chromium composition is as follows:

Example III Porous silica aerogel oz 13 Sodium dichromate oz 13 Water gal- 1 The composition forms a smooth paste that may be brushed on and it will not drip or spread undesirably.

The protective phosphate films produced by means of the paste-like coating composition of the present invention are so superior to those produced by other compositions that it is desirable to employ the compositions even when treating all the surfaces of a metal member. The use of the paste-like com-position is also advantageous in cases where only a small amount of metal is being treated since expensive installations, such as dip tanks, conveyors and the like, are unnecessary.

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 disclosure shall be interpreted as illustrative and not in a limiting sense.

I claim as my invention:

1. A stable paste-like coating composition suitable for use in producing protective phosphate coatings on ferrous, zinc and cadmium metal surfaces composed of from 10 to 45 parts by weight of a porous silica aerogel finely divided to the extent that at least-about 50% passes through a mesh screen, and 100 parts by weight of an aqueous "phosphate solution having a pH of from 0.5 to 6, the phosphate solution composed essentially of from 25% to 70% by weight of a phosphorus compound capable of supplying phosphate ions to react with the metal and an oxidizing agent, and the balance of the phosphate solution being water, the porous silica aerogel rendering the paste-like nature of the composition relatively permanent, the composition being sufficiently fiowable to be applied as a thin smooth coating to the metal surfaces which upon drying forms a relatively nonadherent silica aerogel and a smooth underlying protective phosphate film on the metal.

2. A stable paste-lik coating composition suitable for use in producing protective phosphate coatings on ferrous, zinc and cadmium metal surf-aces, composed of from 10 to 45 parts by weight of a porous silica aerogel finely divided to the extent that at least about 50% passes through a 100 mesh screen, from 0.1 to 3 parts by weight of a wetting agent, from to 5 parts by weight of a water soluble cellulose ether, and 100 parts by weight of an aqueous phosphate solution having a pH of from 0.5 to 6, the phosphate solution composed essentially of from 25% to 70% by weight of a phosphorus compound capable of supplying phosphate ions to react with the metal and an oxidizing agent, and the balance of the phosphate solution being water, the porous silica aerogel rendering the paste-like nature of the composition relatively permanent, the composition being sufficiently flowable to be applied as a thin, smooth coating to the metal surfaces which upon drying forms a relatively nonadherent silica aerogel layer and a smooth underlying protective phosphate film on the metal.

3. In the process of producing protective phosphate coatings on ferrous, zinc and cadmium metal surfaces the steps comprising applying a paste-like layer of a coating composition composed essentially of from 10 to 45 parts by weight of a porous silica aerogel finely divided to the extent that at least about 50% passes through a 100 mesh screen, and 100 parts by weight of an aqueous phosphate solution having a pH of from 0.5 to 6, the phosphate solution composed essentially of from 25% to 70% by weight of a .phosphorus compound capable of supplying phosphate ions to react with the metal, and an oxidizing agent, and the balance of the phosphate solution being water, drying the coating, removing the relatively nonadherent layer of silica aerogel produced on drying, and exposing a smooth protective phosphate film on the metal surfaces.

4. In the process of producing protective phosphate coatings on ferrous, zinc and cadmium metal surfaces the steps comprising applying a paste-like layer of a coating composition composed essentially of from 10 to 5 parts by weight of a porous silica aerogel finely divided 'to the extent that at least about 50% passes through a 100 mesh screen, and 100 parts by weight of an aqueous phosphate solution having a pH of from 0.5 to 6, the phosphate solution composed essentially of from 25% to 70% by weight of a phosphorus compound capable of supplying phosphate ions to react with the metal, and an oxidizing agent,

7 and thebalance of the phosphate solution bein water, drying the coating,removing the relative ly rionadhererit layer 'of silica aerogel produced on drying, exposing -a smooth protective 'phosphate film on the metal surfaces, applying to the phosphate film a, paste-like coating of a second composition composed of 100 parts by weight of the silica aerogel and from 10 to 200 parts by weight of a water soluble hexavalent chromium compound and sufiicient water to produce a flowable paste, drying the applied coating of the second com-position whereby a relatively nonadherent layer of silica aerogel is produced, and removing the last-mentioned layer of silica aerogel.

BYRON V. MCBRIDE.

8; REFERENCES CITED The following references are of record inthe file of this patent: V

UNITED STATES PATENTS Number Tanner Nov. 1'7, 1942