US2487137A

US2487137A - Producing coatings on metal

Info

Publication number: US2487137A
Application number: US773304A
Authority: US
Inventors: George R Hoover; Cox Norman
Original assignee: Armco Inc
Current assignee: Armco Inc
Priority date: 1947-09-10
Filing date: 1947-09-10
Publication date: 1949-11-08
Anticipated expiration: 1966-11-08

Description

Patented Nov. 8,1949

PRODUCING COATINGS ON METAL George R. Hoover and Norman Cox. Middletown, Ohio, assignors to Armco, Steel Corporation, a

corporation of Ohio No Drawing. Application September 1 0, 1947,

. Serial No. 753,30?"

This application is a continuation-in-part of application Serial No. 437,414, having the same title, and filed April 2, 1942, which in turn is a continuation-in-part of our previous application, Serial No. 415,080, filed October 15, 1941, under the same title, both of which are now abandoned.

The invention relates to the production of phosphate coatings on metals, which coatings are effective either in rendering the surface of the metal passive to oxidation, or in providing a bond or tooth for the reception of paints, enamels, and the like, or both. In general, treatments for these purposes are set forth in such patents as Green 2,082,950, Tanner et al. 1,888,189, Thompson et al. 1,869,121, Green 1,651,694, and Thompson et al. 2,001,754. For convenience hereinafter we shall refer to the production of such coatings as ings of zinc and zinc alloys and as applied to coatings which are high in or consist of aluminum, than treatments hitherto found in the art.

It is an object of our invention to provide a Bonderizing method and solution in which the activity of the phosphating solution is very greatly stepped up, while the quality of the formed coating is fully maintained. It is our object to produce coatings which are physically strong and tightly adherent, which are devoid of a coarse, granular or rough consistency, which are not soft and chalky, and which, while furnishing an excellent tooth for paints and enamels, are inert with respect thereto. It is an object of our invention not only to provide Bonderizing solutions of high activity, but solutions having uniform coating producing effects, so that a better bond or tooth for paints and enamels can be provided with a thinner over-all coating.

Still more specifically, it is an object of our invention to provide a very substantial acceleration in the speed of action of Bonderizing solutions by means of the addition of substances containing fluorine, without at the same time producing coatings containing significant amounts of fluorine, since it has been found that coatings containing fluorine have a detrimental effect upon superimposed coatings of paints and enamels.

The objects of our invention which have been set forth above, and other objects which will be set forth hereinafter or will be apparent to the 4 Claims. (0!. 1489-615) skilled worker in the art upon reading these specifications, we accomplish by that certain procedure, composition and treatment, of which we shall describe an exemplary embodiment.

Our invention is applicable to usual Bonderizing solutions as previously taught in the art, modified by the additions hereinafter set forth. It is also applicable to Bonderizing solutions departing in proportions of ingredients from those hitherto known.

In the development of the Bonderizing art, it was early found that a solution containing phosphoric acid or a phosphate could, under certain circumstances, produce upon the surface of iron a passivating phosphate coating. Such solutions were extremely slow; and it was ascertained that the addition of iron to the solution would accelerate its action. Th use of metals other than iron, such for example as nickel, copper, manganese, cobalt, and the like, were found to be helpful in speeding up and controlling the action; and the use of an oxidizing agent, such as a nitrate, helped in the matter of control by tending to remove active hydrogen involved in the process. A content of zinc is also conventional, especially when Bonderizing galvanized products, and the zinc may be added, if desired, as zinc nitrate.

In general, the usual Bonderizing solutions are described in terms of their content of free acid and total acid (as determined by titration with different indicators) nitrate, zinc (or other metal-- lic accelerator) and iron, usually as ferrous iron.

The acid concentration in points is a measure arrived at by titrating a 10 ml. sample of the solution with 0.1 normal sodium hydroxide. The number of points is equal'to the number of milliliters of sodium hydroxide necessary to produce thecolor change of the indicator. The indicator used for determining free acid" was methyl orange, and that used for determining total acid was phenolphthalein.

' A conventional Bonderizing solution had the following characteristics:

Total acid points 30 Free acid do 4 to (i Nitrate per cent 0.30 to 0.65 Zinc do 0.30 to 0.60 Iron do 0.15 to 0.30

It was taught in Patent No. 1,869,121 that the ratio of free acid to total acid should range between l:6 and 1:15 for best results.

An iron sheet or article was immersed in such a bath for a suflicient period of time and usually at an elevated temperature, after which it was rapidly dried. The time duration of the treatment usually had to be of the order of several minutes. Such solutions were effective on the zinc surfaces of galvanized iron; but within commercial time limitations they were not satisfactorily effective on the surfaces of alloys of zinc and aluminum, and on aluminum surfaces.

Hays and Hoover in Patent 2,346,302 taught that they could improve the action of Bonderizing solutions on zinc surfaces, by making that action more uniform so that thinner coatings could be employed, and hence speeding up the process, by increasing the nitrate content to between .65 and 1.10% and cutting down somewhat on both the free acid and total acid. Acceptable coatings could be formed with immersions not exceeding two minutes, and in some instances as short as twenty seconds. y

In accordance with the present invention, we add to Bonderizing solutions fluorine bearing compounds in small amounts, and in accordance with the teachings hereinafter set forth, we achieve a marked and useful acceleration of the action of the bath such that we are able to produce coatings in substantially less than one minute, and in instances in as little as five seconds or less. Coating produced in accordance with our invention contain less than 0.01% fluorine and do not affect the life of paints and enamels as shown by the standard accelerated paint-life tests.

The nature of the fluorine bearing compounds is of the greatest importance; and the compounds employed by us consist in hydrofluoric acid and its salts.

Complex fluorine bearing compounds such as the fluo-silicates, fluo-borates, fluo-titanates, fluo-vanadates, and the like should not be used. These are relatively very much less active as accelerators, and when used in proportions to provide useful acceleration, result in coatings which contain fluorine, and detrimentally affect the life'and adherence of coatings of paints and enamels. This is believed to be due to the fact that when these substances ionize in solution, they form complex fluorine bearing ions which, joining with the base metal or metals in the bath, go over into the coating. The use of such complex fluorine bearing compounds in conjunction.

with hydrofluoric acid or its salts is likewise ineffective, since the coatings formed will be high in fluorine. In addition, the complex fluorine bearing compounds have a strong tendency to produce sludge in the bath.

We employ substances capable of ionizing to yield the simple fluorine ion, without the production of complex ions in which fluorine is joined with other elements. These conditions can be met, so far as we know, only through the use of hydrofluoric acid or its salts. In accordance with our invention we employ baths containing from approximately 015% to .80% by weight of fluorine. These limits may be exceeded in either direction, but below about 015% of fluorine, the acceleration falls off rapidly, while above about .80% the increase in acceleration is less than proportionally great and the formed coatings begin to pick up fluorine.

As indicated above, the fluorine maybe added in the form of hydrofluoric acid and its salts. Hydrofluoric acid has the effect of increasing both the free acid and total acid in the bath.

Acid salts of hydrofluoric acid, such as ammonium bifluoride, also increase the total and free acid content, but the latter to a lesser extent.

The normal salts of hydrofluoric acid, such for example as sodium or potassium fluorides, or the fluorides of other metals or of ammonium, have the effect of increasing the total acid content of the bath, but not its free acid content. The range of quantities of fluorine in the bath given above is easily calculated for any fluorine comfree to total acid of from substantially 1:2 to

1:10, with a preferred range lying between 1:4 and 1:8. If the free acid is increased so that its ratio to total acid is 1 to less than approximately 2, the bath is likely to have an action in which pickling predominates over coating. If the free acid is decreased so that the ratio is l to more than 10, substantially, soft coatings are likely to be produced, and the baths have an enhanced tendency to form sludges. Within the limits set forth, baths with high acid content tend to be more vigorous and rapid in their action.

Ingeneral, we have found it advantageous to increase the content of nitrate with an increase in the fluorine content. In other words, within the ranges set forth herein, the quantity of nitrate should be roughly proportional to the quantity of fluorine. With an increase in the nitrate content, the iron in the bath appears to remain largely ferrous,'in the absence of external Free acid points 1.5 to 10.0 Total acid I do 16.0 to Nitrate per cent .30 to 2.65 Zinc do .30 to 2 Ferrous iron, from less than do .002 to .30 Fluorine do .015 to .80

An exemplary solution for Bonderizing by dipping, and effective for the sheets or articles hereinafter set forth is as follows:

Free acid points 3 to 4 Total acid do 25 to 30 Nitrate per cent .90 to 1.10 Zinc d0 .40 to .50 Ferrous iron do .08 to .25 Fluorine do .015 to .80

The materials to be Bonderized are treated with the solution at room temperature or above, but

perferably in the range of 120? to 210 F. for

lengths of time required to build up the desired weight of coating, which by reason of the enhanced uniformity of attack, may be a very thin coating. The time of immersion is brief, usually not above one minute as a maximum, and may be as short as five to fifteen seconds. The materials lined, are operative for iron, mild steel, galvanized iron or steel and metals coated with zinc and its alloys and aluminum and its alloys.

The solutions will Bonderize chromium steels substantially free of nickel.

The rapidity of the action'of our baths will vary somewhat with the metallic surfaces being treated. For'surfaces of aluminum and aluminum alloys high in aluminum, together with surfaces of iron, we prefer to operate in the higher portions of the ranges set forth, i. e. with greater amounts of free and total acids and greater amounts of fluorine. An alloy zinc with aluminum, where the aluminum content is small in' proportion to the zinc, is an intermediate material for which best results are obtained by operations in the midsections of the ranges given above. vanized materials, i. e. materials coated with substantially pure zinc, are easier to Bonderize and may, with advantage, be treated in the lower portions of the ranges set forth above.

Modifications may be made in our. invention without departing from the spirit of it.

Having thus described our invention, what we in the process and equivalent to 30% to 2.65%

nitrate, fluorine, and an acid phosphate .of a metal which will deposit as a phosphate coating, water being the solvent, said solution having a ratio of free acid to total acid of 1 :2 to 1:10, a free acid content from substantially 1.5 to points, a total acid content vfrom substantially 16.0 to 65 points, and in which the fluorine in the solution results from the addition of a material from the class consisting of hydrofluoric acid and its salts. the amount of fluorine present being substantially 0.015% to 0.80% by weight.

2. The solution of claim 1 in which the stated fluorine content results from the addition of from substantially 025% to substantially 1.2% of ammonium bifluoride.

3. An accelerated phosphate coating solution capable of yielding substantially fluorine-free paint receiving coatings on iron, zinc and aluminum surfaces and consisting essentially of an oxidizing agent adapted to remove hydrogen in the process and'equivalent to 30% to 2.65% nitrate, fluorine, and an acid phosphate of zinc,

aiamav water being the solvent, said solution having a ratio of free acid to total acid of 1:2 to 1:10, a free acid content from substantially 1.5 to 10 points, a total acid content from substantially 16.0 to 65 points, containing ferrous iron from substantially .002% to .30%, in which the zinc constitutes substantially .30 to 2%, and in which the fluorine in the solution results from the addition of a material from the class consisting'of hydrofloric acid and its salts, the amount of fluorine present being substantially 0.015% to 0.80%

Ordinary galby weight.

4. A process of producing substantially fluorinefree phosphate coating on iron, zinc and aluminum surfaces, which comprises immersing articles having such surfaces in a solution consisting essentially of an oxidizing agent adapted to remove hydrogen in the process and equivalent to .30% to 2.65% nitrate, fluorine, and an acid phosphate of a metal which will deposit as a phosphate coating, water being the solvent, said solution having a ratio of free acid to total acid of 1:2 to 1:10, a free acid content from substantially 1.5 to 10 points, a total acid content from substanstially 16.0 to 65 points, and in which the fluorine in the solution results from the addition of a material from the class consisting of hydrofluoric acid and its salts, the amount of fluorine present being substantially 0.015% to 0.80% by weight, maintaining said solution at temperatures between substantially 120 and substantially 210 F., and withdrawing the articles from said solution after a treatment time of substantially 5 seconds to substantiaily 1 minute.

GEORGE R. HOOVER. NORMAN COX.

REFERENCES CITED The following references are of record in th file of this patent:

UNITED STATES PATENTS Number Name Date 1,911,726 Tanner et al. May 30, 1933 2,203,670 Buzzard June 11, 1940 2,234,206 Thompson Mar. 11, 1941 2,312,855 Thompson Mar. 2, 1943 2,314,565 Thompson Mar. 23, 1943 FOREIGN PATENTS 7 Number Country Date 522,681 Great Britain June 25, 1940 728,364 France Apr. 11, 1932 728,411 France Apr. 12, 1932