US2303242A - Metal coating - Google Patents

Description

Patented Nov. 24, 1942 UNITED STATES PATENT OFFICE Robert a. Tanner, Detroit, and Robert v.

Harris,

Highland Park, mm. assignora to Parker Bust Proof Company, Detroit, Mich.

No mum. Application November 19, 1938,.

- Serial No. 241,393

4am (cl. 148-65) This invention relates the production of paint-holding and corrosion-retarding coatings on ferrous surfaces, and relates more especially to producing such coatings by a simpler and more flexible method than is used at the present time in the rustprooflng art.

The usual method of rustprooflng articles, preparatory to painting, involves reacting the sur-.

face for a period with an aqueous acid phosphate solution whereby a coating of insoluble phosphates is formed on the surface. After the production of the coating, the processing solution remaining on the surface is rinsed off with water and the articles then given a rinse in a hot solution of chromic acid. This final rinse has been of a combination of comparatively insoluble phosphates of zinc and iron..

One function of the chromic acid rinse is to dissolve out a part of the zinc phosphate, leaving a coating that is comparatively richer in iron phosphate because this is the more insoluble of the two.

The final chromic acid rinse, therefore, results in a film on the ferrous articlewhich contains iron phosphate, zinc phosphate, hexavalent chromium, and some trivalent chromium resulting from reduction of some chromic acid caused by its dissolving action.

Before a ferrous article can be processed in phosphate solutions it must be cleaned. This is usually done commercially by washing the surface with a hot alkaline solution followed by a water rinse. The strength of the cleaner as well as the time of washing must be carefully regulated, otherwise the phosphate coatingwill be the coating as the articles pass along on a conveyor.

The object of the present invention is to provide a novel method of treating ferrous surfaues by a simplermeans than used with the common phosphate method.

It is a further object to provide a treating solution with which a greater latitude of cleaning conditions can be 'used without encountering the formation of objectionably large phosphate crystals in the coating.

the form of a phosphoric acid and the necessary chromic acid, either as such or as an alkali chromate or dichromate, since the addition of either of these will result in the formation of chromic acid in the presence of the phosphoric acid.

The phosphoric acid and chromic acid should be present under conditions where a definite etching of the metal surface will occur. It is essential that the solution applied be retained permanently on the surface and be not rinsed off or otherwise removed. Whether or not the solution will etch the metal surface depends both on the concentration of the phosphoric acid and the hexavalent chromium, and on the ratio existing between the two.

It is well known that chromic acid in certain amountshas a decided inhibiting action on the,

attack of phosphoric acid on ferrous surfaces. Since we desire etching, the correct conditions of the solution must be selected. For example, if a concentration of ortho-phosphoric acid of .5% is used, a ratio of chromic acid to phosphoric acid of .35 to 1 would not cause etching of metal;

therefore, the objects of the invention could not formed of objectionably large crystals which are V be attained, but if this ratio is reduced to .25 to 1 a definite reaction with the metal occurs resulting in a protective film of iron phosphate, trivalent chromium, and possibly some hexavalent chromium which has not been reduced. Any ratio of .25 to 1 or less will therefore result in etching and the formation of the desired products of the reaction.

To give a further example, if a concentration of ortho-phosphoric acid of 4% is used, a ratio of chromic acid to phosphoric acid of .25 to 1 will prevent etching, but a ratio of .15 to 1 allows it to proceed as desired. In speaking of etching,

'rust and we mean visible etching. The ratios of phosphoric and chromic should be adjusted so that visible etching of the surface is obtained. .In carrying out the process the metal to be treated is first cleaned by any suitable means. Such methods as solvent wiping, or vapor degreasing are usually employed for small articles or where there is a limited amount of production. Alkali cleaning is ordinarily adopted for production lines where auto bodies or sheet metal parts, or similar large objects are to be treated. In this case, the cleaning operation is followed by a water rinse to remove the alkali.

Large objects suchas structural members of buildings, in addition to having the grease removed, usually have considerable loose scale and cleaning. operation the metal is ready to receive the treatment of the present invention.

If there is any residual surface it may be diiiicult to readily obtain uniform wetting and so in order to avoid poor results it is usually advisable to incorporate in the treating solution a wettin agent. There is a wide selection of these materials suitable for the purpose. Sodium lauryl sulfate, known commercially as Gardinol or Duponol (trade names) is very effective. Also, sodium lauryl benzene sulphonate and certain of the free sulphonic acids, one of which is known commercially as Novonacco NN (trade name) are effective.

Where the amount of chromic acid present is not so high as to cause objectionable reaction. the use of. alcohol is permissible. Tertiary butyl alcohol is among the least reactive and is therefore more preferable than some of the lower alcohols.

Although there is some difference in the emgrease or oil on the A specific example showing the use of the procass is as follows: A cold rolled steel article is this also must be removed. After the cleaned of oil and grease by wiping with naphtha and then drying the surface with clean cloths. A formula containing 10.0% phosphoric acid, 1.5% chromic acid, and .5% Duponol WA paste wetting agent is then applied to the surface by means of a brush. When the surface has been uniformly wetted the article is placed in an oven and dried for 3 minutes at 300 F. It is then removed and allowed to cool and is ready for the application of any desired paint finish without further treatment of the article.

If desired, the metal, the'solution, or both may be heated prior to the rustprooflng operation in order to facilitate the chemical action and hasten the drYi 8..

Although many concentrations and many different proportions of the phosphoric acid and hexavalent chromium may be employed, in all cases it is necessary that the prevailing conditions shall allow etching of the ferrous surface to take place by the phosphoric acid present.

Any of the phosphorous acids may be substituted for the ortho-phosphoric acid. Among those are phosphorous acid, meta and pyro phosphoric acid, and hypo-phosphorous acid. The anhydrides, such as phosphorous pentoxide, can

also be used since these materials immediately become acid upon addition to aqueous solution.

For the purpose of economy in shipping, the materials may be in concentrated form and diluted to the proper strength before use. In case it is desired to use a wetting. agent or alcohol, it is best to add these after dilution since they will react with the chromic acid when in concentrated form.

ciency of different wetting agents, .596 to 1% is usually suiiicient. amounts up to 5% or over may be required.

Numerous methods may be employed for the application of the tr ting solution. It is important in any event that the surface be 1miformly wetted with the solution so that the etching willbeuniformover the entiresurface. The solution may be applied with a brush "or by means of a spraygumor the article canbemomentarily dipped in a quantity of the solution.

After the wetting operation, the solution is allowed to dry upon the treated surface. The resulting film will contain iron phosphate, trivalent chromium an possibly some hexavalent chromium.

With the weaker solutions air drying is usually sufficient, but with higher concentrations it may beexpedienttoresortto aforceddrying'operation. After dryin .thetreatedarticleisreadyto be finished with paint, enamel. lacquer andlso forth without further operations.

, The following are examples of solutions that may be employed in carryin out the invention.

If alcohol is employed.

. Duponol (tram- P chromic mark) acid acid WA pasta tutti! The method of rustproofing according to this invention makes it possible to treat ferrous surfaces where it would be impractical with the usual coating procedure. The solutions can be used on stationary objects such as bridges, structural members of buildings, large oil tanks. truck bodies and so forth, there being no necessity for large immersion tanks or spray chamhers. Heat can be dispensed with through the entire procedure which is impossible with the described phosphate methods. The resulting coating, however, compares favorably with the phosphate coatings when used as a paint base.

It is understood that the examples given above are merely illustrative and are not meant to limit the invention. The scope of the invention is to be measured only by the following We have discovered that in some instances the addition of a wetting agent to a solution containing chromic and phosphoric acids will cause the solution to etch the surface of themetal, whereas without the wetting agent, no etching ,would occur. The exact reason for this is not eniu'rely clear but it is understood that such solutims, conditions, and coatings obtained in this way are within the scope of the invention.

What we claim is:' 1. A ferrous metal object having on a surface thereof an integral coating having a'composition resulting from drying onto said surface an aqueous solution containing phosphoric acid and chromic acid which will visibly etch the ferrous 2. A ferrousmetal object having on. 9. surface thereof an integral coating having a composition resulting from drying onto the said surface an aqueous solution containing chromic acid and phosphoric acid, the ratio between them being not more than .15 part of chromic for each part of phosphoric.

3. The method of producing a corrosion-resistant paint-holding coating integral with the 5 metallic surface which comprises drying onto a ferrous surface an aqueous solution containing chromic acid and phosphoric acid which will visibly etch the ferrous surface;

4. The method oi producing a corrosion-resistant paint-holding coating integral with the metallic surface which comprises drying onto a ferrous surface a film of an aqueous solution containing chromic acid and phosphoric acid,

inwhich the ratio of chromic to phosphoric is not more than .15 to 1.

- ROBERT R. TANNER.

ROBERT V. HARRIS.