US2777785A - Composition for and method of treating metals as well as the treated product - Google Patents

Description

United States Pate 2,777,785 Patented Jan. 15, 1957 COMPOSITION FOR AND METHOD OF TREATING METALS AS WELL AS THE TREATED PRODUCT NoDrawing. ApplicationJuly 30, 1953, Serial-No. 371,427

The present invention relates to metal coating and particularly to such coating which resistance of the metal and/or the adhesion of paint, varnish, lacquer, and ,other'organic finishes to the metal.

Among the objects of the present invention is the novel coating methods as well as the resulting articles and treating baths for effecting such coating.

The above, as well as additional objects of the present invention -w'll be more completely understood from the following description of several of'it exemplifications'.

According to the present invention, an extremely ef- :fective corrosion-resistant .and organic finish-retaining coating is formed on metals by applying to the surface of ing is efiected. The resulting coating should weigh at least mgs. per square foot of metal surface to pro vide the desired results. Although the reducing agent can be applied to the film after the chromicacid, it has been found unexpectedly advantageous to include the repolyethylene glycols rolled steel against corrosion for a considerable periold of In an atmosphere of 99% relative humidity. at 125 F. black plate coated in accordance'withthepresent When uncoated, practically completely corroded and entirely useless before '24 hours of exposure. However, the chronlic acid'bath itself will gradually undergo deterioration, apparently by vreason of the gradual reduction of the ehrornic acid. When a precipitate appears in a bath, it can no longer be used commercially inasmuch as the precipitate deposits on the metal in spots and renders the final coating .non uniform. It has been discovered that the life of the bath is greatly prolonged if the bath also contains a negative catalyst such as a dissolved man- A very effective distribution of, the

film 15 obtained at low rolling speeds when the rubber At higher Natural or other resilient materials such as plasdrying to a temperature above 212 F. Above 350 F. very little additional advantage isobtained. At drying temperatures of 450 to 550 F. the coating appears to be adversely affected, depending upon the particular reducfected as flaming, that is, passing the filmed metal directly through gas flames. Inasmuch as oxidizing actionis preferably sharply restricted during the drying of the-present invention, it is better when flaming the metal to pass it'through the reducing ,portion'of thegas flame rather thanthe oxidizing portion. Flaming is generally bet-terthan-the use of an air oven, because the flaming effects a considerable portion of its heating by radiation so At drying temperatures. of 212 aqueous coating film is converted to coating in about two to five-seconds.

is a very light grayish, and slightly greenish, more or less transparent layer that is very adherently held on the metal and does not scratch off very easily. The folding of a sheet of metal back on itself so as to produce a sharp crease does not loosen the coatingeither on the inside or outside of the crease.

The corrosion resistance of the coating can be further improved by quenching the coated metal from the drying temperature down at least 25 F. in a liquid such as water or oil. This treatment lengthens about 50% the period in which the coated metal can be exposed to a hot humid atmosphere without showing corrosion. liven better corrosion resistance is obtained by quenching in water containing up to about 1% chromic acid. The lacquer adhesion of the coating is also improved in water quenching where about or more of chromic acid is dissolved. At lower concentrations of chromic acid the lacquer adherence is slightly diminished.

A particularly efiective coating operation is one that ends with a drying step performed at 250 F. followed by a quenching down to 200 F. in a /z% chromic acid solution in water.

The above type of coating operation is effective with all metals including ferrous metals such as iron, steel, both low carbon as well as high carbon, stainless steel, aluminum, copper, nickel, magnesium, tin, Zinc and alloys of these metals with each other. With ferrous metal, however, and particularly readily corrodible forms of this metal, the present invention is particularly valuable. A very desirable use of the present invention is in the manufacture of metal containers such as those used for oil, foods including milk as well as dried foods, and chemicals. For this purpose cold-rolled sheet or strip carbon steel containing from about 0.05 to 0.2% carbon is coated in the manner pointed out above and then formed into a container such as the usual crimped seam food can, after which a layer of lacquer can be applied to the outside of the can surface. An application of the lacquer coating in this manner helps to seal the crimped joints against leakage. If desired, however, the lacquer can be applied to the inside as well as to the outside, or alternatively only to the crimped areas, preferably before the crimping is effected so that the lacquer has a gasket-like leak-reducing effect on the joint. Such leakage reduction can also be provided with other, plastic materials such as resins that are not in lacquer form.

In the coating of ferrous metals additional improvements can be made in accordance with the present invention by first etching the metal with a nitric acid solution in water before the chromic acid treatment. The etching should be as uniform as practicable and a metal removal of at least about 100 milligrams per square foot of metal surface is very effective. There appears to be no upper limit for the amount of metal removal that will provide the desired increase in corrosion resistance and organic film adhesion. However, an etch of more than about 1000 milligrams per square foot will tend to leave a roughened surface that does not make a good appearance. Furthermore, in the interest of economy the etch is preferably kept down to not more than about 450 milligrams per square foot. In use the etching bath accumulates a relatively large amount of dissolved iron and may then deposit a rust-like coating on the metal, especially when the dissolved iron is in the ferric form. Before this build up, however, the nitric acid treatmentcan be at any desired concentra tion, time, and temperature so long as it effects a uniform metal removal. A bath temperature of about 120 F. appears to be a practical upper limit for this ose. An etching bath that deposits a rust-like coating can be used providing this coating is not so tenaciously held that it cannot be brushed otf as by means of felt pads. or fiber-bristle brushes.

The effect of etching appears to be to form an invisible amount of reactive iron oxide on the surface of the metal, and in the subsequent chromic acid solution the iron oxide appears to react with the chromic acid before as well as during reduction of the chromic acid to form the desired coating.

The best types of final coating. in accordance with the present invention are actually. water repellent. An appreciable drying operation of at least five seconds duration at 250 F. to 300 F. forms such a coating. More vigorous drying, as at temperatures around 500 F., produces a coating that is not quite fully water repellent but still highly suitable forcorrosion resistance and lacquer adherence. Drying at temperatures below about 500 F. gives more immediate ,water repellancy if the dry coating is kept exposed to th'e 'atmosphere for at least a half minute before the coated metal is coiled or otherwise covered. There is very little difference in the corrosion resistance and organic film adhesion between the partially and completely water repellent coating. In fact the partially water repellent coating will npon standing gradually become completely water repellent.

The time necessary to subject the coated metal to the above drying or curing conditions varies with the thickness of the metal, Where plates or other relatively thick metals are used more heat is generally required to reach the desired temperature.

Maximum corrosion resistance is obtained when the least possible water soluble materials are included in the final coating. For this reason the nitric acid etching should preferably be followed by a rinse to wash off surplus nitric acid as well as any water soluble iron compounds. Also the chromic acid bath should be substantially free of substances that will not be converted into completely water insoluble form by the process. The best reducing agents are accordingly those that do not introduce water soluble material to any appreciable degree into the final coating. Although phenol and potassium iodide are listed above as suitable reducing agents the final coatings they produce are not quite as protective as the coating produced with other reducing agents. Similarly any reducing agent is best used in an amount that is stoichiometrically no larger than is oxidizable by'the chromic acid to be reduced. It is possible to reduce only about half of the chromic acid. However, coatings with up to about 5% of water soluble material are still of outstanding utility in accordance with the present invention, as for example to make containers for dry foods, oils or similar materials.

The nitric acid treatment is sometimes effected under conditions in which ferrous metal becomes passive. When this takes place, the etching either does not occur or is only spotty over the surface of metal being treated. The passivating effect is serious with high concentrations of nitric acid butwill be troublesome even with lower concentrations of this acid where the etching bath has picked up a sizeable amount of dissolved iron.

According to the present invention, the etching bath is advisedly preceded by an activating treatment. Examples of such treatments are the contacting of the metal with a liquid non-oxidizing etchant such as solutions of acids including:

HCl

H3PO3 plus a small amount of HNOa Citric acid Acetic acid Trichloroacetic acid Tartaric acid and even tannic acid A clip in an aqueous solution of any of the above materials at concentrations as low as 4 by weight will so aifect the metal that it will not show any passivity to introduce water soluble materials into the dual coat- F. or at 70 F. in a NaHSOs in water will prevent passivity. A 2.3% solution of HCl in water will also produce the desired results with a /2 second dip at 35 F. If the activation step causes etching of the metal. ment can be performed either before or during the nitric acid treatment.

Where the metal to be coated is not perfectly clean, it should be subjected to a careful cleaning operation before the nitric acid etch, or Where no such etch is used, before contact cleaning can be of any kind such as degreasing with or- Without limiting the invention in any way, the following specific examples are given of elfective coating operations.

Example 1 A strip .of full bright finish sheet steel (black plate) ten mils thick is subjected to the following treatment:

A. Cathodically clean in an aqueous solution containing 16 grams of KOH per liter at 140-160 F. at'a current density of i6 amperes per square foot of cathode for 10 seconds.

B. Cold water rinse.

C. Clean anodically in an aqueous solution containing 16 grams of KOH per liter at Mil-160 F. at a current density of 16 amperes per square foot of anode for 10 seconds.

D. Cold water rinse.

E. Flood with 2.3 percent HNO; in water at 80 F. for

five seconds.

F. Cold water rinse (flood).

G. Flood with an aqueous solution containing 3.2 percent CrOg and 2 percent glycerine at 80 F. for two seconds.

H. Roll between smooth rubber rolls wetted with the same solution.

I. Dry by flaming both sides of the resulting strip with a reducing gas flame for-twoseconds, the strip reaching a temperature of 250-270 F.-

1. Permit the strip to cool in the Example 2 Same as Example No. 1, except in G an aqueous solution containing 6.5 percent CrOa and 2 percent glycerine is used.

Example 3 Same as Example No. 1, except in G an aqueous solution containing 5 percent CrOa and 2 percent cane sugar is used.

Example 4 Same as Example No. 1, except in G an aqueous solution containing 10 percent CrOs and 3 percent cane sugar is used.

Example 5 Same as Example N0. 4, except in I, dry by flaming one side of the strip with a reducing gas flame.

The strip produced in Examples 2 to 5 inclusive behaved about the same as that of Example 1.

Example 6 Same as Example 1 except that between steps I and J, the strip was quenched in an aqueous solution containing 0.5% CrOa and held at 200 F.

Example 7 A sheet of No. 7 finish black plate 11 mils thick is treated as follows:

A. Immerse in an aqueous solution containing 3.0%

HNO: for 5 seconds at 75 F.

B. Cold Water rinse.

C. Immerse in an aqueous solution containing 5.0%

CIO: and .5 cane sugar and held at F.

D. Roll through coarse rubber rolls.

E. Dry between infra-red lights for 10 seconds at 250- F. Cool by standing the sheetin the air.

Example 8 Same as Example No. 7, except in C an aqueous solution containing 2.5% CrOs and .4% glycerine is used.

Example 9 Same as Example No. 7, except in a solution containing 5% CrO3, 1.3% glycerine and .5% HaSOris used.

Example 10 Same as Example No. 7, except in C a solution containing 5% CIOs and 4% manm'tol is used.

Example '11 Same as Example No. 7, exceptin C a solution containing 5% CIOs and 2% carbowax 1500 is used. bowax 1500 is a high molecular polyethylene glycol, formula CI-I2OH(CH2OCH2)JCCH2QH where xis about 30.)

- Example 12 C a solution conglycerine is used at a tempera- Same as Example No. 7, except in taining CIO: and 1% ture of 180-l90,F.

V I Example 13 Same as Example No. 1 except omit forced drying step I -insteaddry by letting stand in air.

' Example 14 A sheet of 28 gauge full bright finish steel (black plate) is subjected to the following treatment:

A: Treat by -immersing 1n an aqueous solution containing 2.5% HNOa for 5 seconds at 70 F.

B. Cold water rinse.

C. Treat by immersing 2.5% CrQa and .7 70 F. Y

D. Roll between coarse rubber rolls.

E. Dry between infra-red lights for 15 seconds the metal temperature reaching 240 F.

F. Quench in water held at 200 F.

in an aqueous solution containing triethanolamine for 3-5 seconds at Example 15 Same as Example No. 14 except in C an aqueous solution of 2.5% CIOs and 2% triethanolamine for 3-5 seeonds at 180-l9 0 F. is used.

Example 16 Same as Example 14 except in C an aqueous solution of 2.5 CrOz and 1.5% hydroxylamine sulfate is used.

Example 17 Same as Example No. 14 except in C an aqueous solution of 2.5 C1'O3 and 2% phosphorous acid is used.

Example 18 Same as Example 14 except in C an aqueous solution of 2.5% CrOa and 3% potassium iodide is used.

Example 1 9 Same as Example No. 14 except in C an aqueous solution 2.5% C103 and, .5 %,,phenol used.

Example 20 Same as Example No. 14 except in C an aqueous solution of 2.5% CIOs and .5% hydroquinone used.

Example 21 Same as Example No. 14 except in C an aqueous solution of 20% CrOs and 10% triethanolamine at a temperature of 130 F. is employed.

Example 22 A sheet of 26 gauge full bright finish steel (black plate) is subjected to the following treatment:

A. Treat by immersing in an aqueous solution containing 2.5% HNO: for 5 seconds at 75 F.

B. Cold water rinse.

C. Treat by immersing in an aqueous solution containing 5% C1303 for 3-5 seconds at 180 F.

D. Roll between coarse rubber rolls.

E. Dry between infra-red lights for 7 seconds.

F. Hold in fumes from a boiling aqueous solution of formaldehyde (37%) for seconds.

G. Dry between infra-red lights for 10 seconds the sheet temperature reaching 220 F.

H. Cool by standing in air.

Example 23 Same as Example No. 22 except in step C use a temfumes from boiling perature of 75 E, and in step F ethyl alcohol are used.

Cit

Example 24 A. Clean 28 gauge black plate cathodically in an aqueous solution containing 16 grams KOH per liter using a current density of 15 amperes per square foot of cathode at 140-160 F. for 10 seconds.

B. Cold water rinse.

C. Clean anodically in an aqueous solution containing 16 grams KOH per liter using a current density of 15 amperes per square foot of cathode at 140-160 F. for 10 seconds. I

D. Cold water rinse.

E. Flood with anaqueous solution containing 10% CrOz.v

and 3% cane sugar, at F. for 2 seconds.

FL Roll through smooth rubber rolls wetted with the solution mentioned in E.

G. Dry the sheet by flaming both sides of the sheet with reducing gas flames.

H. Cool by standing in air.

Example 25 except in E, a solution conglycerine is used.

Same as Example No. 24, taining 6.5% CrOa and 2% Example 26 The product of this example has a very satisfactory corrosion resistance whereas an identical metal finished the same process but omitting the in accordance with reducing agent in step E had an unacceptable corrosion 5% CrOs resistance.

. Example 27 Same as Example No. '26, except that in C" there is used a 5% citric acid solution in water at F. for

10 seconds instead of the HCl solution.

Example 28 Same as Example No. 26, except that in C there is used a 10% HzSOi solution .in'water at F. for 10 seconds instead of the HCl solution.

Example 29 Same as Example 26, except that in C there is used an aqueous solution having 3% ferric sulfate at 106 F. for 5 seconds instead of the HCl solution.

Example 30 Same as Example No. 26, except that in C there is used an aqueous solution having 3% oxalic acid at 160 F. for 10 seconds instead of the HCl solution.

Example 31 I Same as Example No. 26, except that in (2" there is used an aqueous solution of 39% NaOH and 2% NaNOs at 280 F. for 15 minutes in lieu of the HCl treatment.

v Example 32 I Treating of aluminum metal, Type 38 /2 H:

A. Immerse 25 mil thick sheet aluminum type 3S- /2 H in an aqueous solution containing 2% KOH for 1% minutes at F. H B. Water rinse at 70 F.

C. Immerse the rinsed metalgin an aqueous solution containing 10% HNOs for 1-minute at130"-;F.

D. Water rinse at 70 F.

E. Immerse the resulting metalinan aqueous solution containing 2.5% CrOs and 2% HsPOs for 5 seconds at 75 F.

F. Dry between infra-red lights for 1% minntesthemetal reaching 200 F.

G. Cool by standing in air.

The product had a thin grey coating and was not attacked during 60 seconds immersion a solution containing 1% KOH at 85 F. A product made the same way except that the reducing agent'in step E is omitted, is attacked at the end of 25 seconds under the same conditions. I

Example 33 A. A strip of 28 gauge black plate having a bright electrolytic tin coating weighing 300'milligrams per square foot of metal surface, is cleaned anodically in an aqueous solution containing 1% NazQOa at 70 F. for

15-20 seconds using a current density of 1 5.arnperes per square foot of anode.

B. Water rinse at 70 F. C. Immerse in an aqueous solution containing 0.2%

H2504 for l to 2 seconds at 75 F.

D. Water rinse at 70 F. E. Dip in an aqueous bath containing 20% .CIOs and triethanolamine at 70 F. for5 seconds.

F. Roll between coarse rubber rolls. G. Dry between infra-red lights for 1 minute, the metal temperature reaching 250-300 F.

H. Cool by standing in air.

A water repellent, insoluble film showing very good lacquer adherence and corrosion preventionis produced by this process.

Example 34 28 gauge black plate having an electrogalvanized-coating Weighing 200 milligrams per, square foot ofsurface is treated as follows.

A. Swab with trichloroethylene.

B. Rinse for seconds in an aqueous solution containing 4 ounces per gallon of trisodium phosphate at 180 F.

C. Water rinse at 70 F.

D. Immerse at 70 F. in an aqueous solution containing 10% CrOs and 3% triethanolaminefor 5 seconds.

E. Roll between coarse rubber rollers.

F. Dry between infra-red lights for 1 G. Cool by standing in air.

The resulting article has a water repellent, insoluble film of excellent protective properties.

minutes at Examp e 35 Same as Example 34 except that the final cooling, instead of by standing in air, is elfected by queue in tap water containing 0.1% CrQs and held at 200 F., the

contact time in the quenching solution being 2 seconds. Erample 36 The black plate of Example lis treatedas follows;

E. Brushed to assure that adherent reaction products orcontaminants are removed. F. Passed through a water solution containing 236% CrOs, 0.9% sucrose, 0.1% KMnQr-atflfi t Fr, and thenpassedbetween smooth rubber rollers.

The coated metals of Examples 35 and 36 are even more corrosion resistant and show better organic finish adhesion than the products of Examples 34 and 1, respectively.

Example 37 Same as Example 24 except that instead of the black plate, a sheet of 26 gauge A. I. S. I. type 430 stainless steel is used, the chromic acid bath containing 3% CrOa and 1 /2% cane sugar and the curing being elfected by a. 5 second pass through an incandescent gas burner assembly, the metal reac ga temperature of 300 F.

The final coated metal shows an excellent retention of paint, lacquer and varnish, and is well protected against pitting in salty atmospheres. The above coating of the 430 stainless steel is further improved by a preliminary etching as in an aqueous solution containing by Weight 8% HNO: and l /z% HP at F. forZminutes. This etching is effected just before the treatment With the CrOa-cane sugar solution.

In the above examples all percentages are by weight. Any of the products made in accordance with these exarrples has a very satisfactory corrosion resistance and lacquer adhesion. Similar very effective results are obtained when other metals as listed above are treated with the present process. All manners of various metals including cold-rolled and hot-rolled steels and irons also show the advantages of this invention.

The coatings and coating processes described above are highly suited for protection against corrosion as Well as to improve the adhesion of paints, lacquers, varnishes, etc. In some cases, however, these coatings may show a tendency to brittleness caused by sudden impact, as for example when a coated metal is subjected to a high speed stamping operation. This tendency to brittleness can be considerably reduced by using coatings Weighing not more than about 50 milligrams per square foot of metal surface, as Well as by decreasing the ratio of reducingagent to the chromic acid. A good ratio for this purpose is about the lower limit of the range suitable to form a coating that gives the effective corrosion resistance. By way of example, an aqueous solution containing 3% chromic acid and 0.75% sucrose produces a coating that is extremely resistant to mechanical shocks and can be stampedat any desired stamping speed without showing significant damage.

A feature of the present invention is that the coatings it produces are all of relatively low cost and therefore can be Widely used with a minimum of difnculty. The cost of the coating can even be further reduced by recovering nitric acid from the etching bath after it become exhausted. Thi exhausted bath generally contains a relatively large amount (about 50 to 60 grams per liter) of ferrous iron as Well as a small amount, usually less than 10 grams per liter, of ferric iron. The exhausted bath also usually contains as much as 4 or more grams per liter of HNOa, as well as additional nitrate ion content corresponding to that of the ferrous and ferric iron.

Much if .not all of the nitrate ion content of the exhaustedbath can be directly-recovered for reuse by merely diminishing the dissolved iron concentration as by means of.cation exchange treatment. Thus a sulfonated polystyrene type cation exchange resin having activating groups which e essentially,entirely +SOsl-I groups, such as those i 11 through this bed. The solution emerging from the iron exchanger has an iron content about A to li that of the incoming solution, after a contact time of only about one to two minutes. Much if not all ofthe ferric iron in the incoming solution appears to be unafiected and is carried right through in the ion exchange treated solution. On. reuse some of the ferric iron remaining in the reused bath appears to become converted to ferrous iron, so that the build-up of ferric iron is not as serious as would otherwise be expected. The ion exchange resin. after it has become saturated, that is lost much of its ability to remove dissolved iron, can be regenerated with sulfuric acid or other strong acid of any desired concentration such as for example a by weight solution in water.

For uninterrupted operation, it is preferred to have two batches of ion exchange beds, one through which the etching bath is passed, while the other is being regenerated. As soon as a batch of iron exchange resin becomes exhausted, a simple switching operation can then be used to immediately switch the etching solution to the freshly regenerated batch, and to begin the regeneration of the exhausted bath.

it is not necessary to wait until the etching bath is completely exhausted before it is subjected to a recovery operation. Thus the recovery can be eifected continuously while the etching bath is being used, as by continuously circulating the etching bath through a batch of the ion exchange material. in this way the etching bath can be maintained much more uniform in composition so that less supervision and control is needed.

Furthermore, by withdrawing the excess dissolved iron before it has built-up to a very high concentration, the increase in nitric acid content produced by the ion exchange is kept to a minimum and any attack of ion exchange material by the resin is also minimized.

The ferric iron content of the etching bath appears to be present as some type of complex ion that is not removed by a cation exchange material. However, by withdrawing the ferrous iron through an exchange bed before the dissolved iron content gets too high, the total ferric iron build-up in the bath is kept to very small amounts.

An additional technique for recovering usable values from the exhausted etching bath is merely boiling the exhausted bath. Free nitric acid is thereby distilled ofi and at the same time any ferric nitrate is hydrolyzed to give off the nitrate iron bound in this fashion. After all possible nitric acid has been recovered in this manner, the residue can be oxidized as by blowing with air to convert the ferrous iron content to ferric iron. After oxidation, the material can again be boiled to distill ofi further amounts of nitric acid. Alternatively the blowing or similar oxidation can be carried out before any distillation, so that only one distillation step is needed.

Both of the above recovery techniques can be used together if desired. By way of example, the bath can be rejuvenated with cation exchange resin until the ferric iron content becomes excessive. At that time the bath may become very viscous and difficult to use, but it can be boiled to distill over all its free nitric acid as well as nitric acid formed by the hydrolysis of its ferric nitrate content. As substantially all of the ferrous nitrate can be removed by ion exchange before this distillation, substantially all of the original nitric acid will be recovered.

As indicated above the proportion of reducing agent to chromic acid in the coacting bath is such that the chromic acid is not completely reduced. Where sucrose is the reducing agent a proportion within the range of from 1 part of chromic acid for 1 part of sucrose, to about 7 parts of chromic acid for -1 part of sucrose by weight is suitable. With triethanolamine a range of from Z'parts of chromic acid for 1 part of reducing agent, to about 10 parts of chromic acid for 1 part reducing With other reducing agents agent by weight is suitable. proportions giving about the same results are used.

In general the preliminary etching in an aqueous solution of nitric acid is very effective with plain carbon steels or low alloy steels, thatis, alloys containing not over about 5 to 6% of total alloying ingredients. Steels having higher alloy contents tend to be immune to etching by such a solution and may require the addition of supplemental etching materials such as the hydrofluoric acid referred to above.

Thepresent application is in part a continuation of copending applications, Serial No. 277,286 filed March 18, 1952, and Serial No. 278,481 filed March 25, 1952.

As many apparently widely different embodiments of this invention may be made without departing from the spirit and scope hereof, it is to be understood that the invention is not limited to the specific embodiments hereof except as defined in the appended claims.

What is claimed is:

l. A stabilized solution consisting essentially of chromic acid, a reducing agent for the chromic acid, and water, the reducing agent being one that at 80 F. coexists in dissolved condition with the chromic acid for at least about one day without producing a precipitate, the

solution also containing about 0.01 to 0.5% of a dissolved manganese compound.

2. The combination of claim -1 in which the manganese compound is KMnOr.

3. The combination of claim 1 in which the manganese I compound is MnCOa.

at least 25 F.

'4. An aqueous metal coating solution containing essentially /2 to 30% CrOz, to 25% of an organic polyalcohol reducing agent, and about 0.01 to 0.5% of a dissolved manganese compound.

5. An aqueous solution containing C103, 1.5% of a sugar, and 0.1% KMIIOI.

6. A method for protectively coating ferrous metals which comprises applying to the metal an aqueous solution of chromic acid, drying the solution on the metal by heating the metal to a temperature at least as high as 212 F. while reducing at least half but not all the chromic acid with a reducing agent to form a dried coating weighing at least 10 milligrams per square foot of. coated surface, and liquid quenching the dried metal by essentially 3 7. The invention of claim 6 in which the quenching liquid is water.

8. The invention of claim 6 in which the quenching liquid is water containing 0.05 to 1 percent CrOa.

9. The invention of claim 6 in which the drying is effected at about 250 F. and the quenching liquid is held at about 200 F.

10. The invention of claim 6 in which the reducing agent is an organic polyalcohol.

11. The product produced by the method of claim 6.

12. The product produced by the method of claim 8.

13. A method for forming a corrosion-resistant, lacquer-adhering surface on ferrous metal, which method comprises contacting the metal with an aqueous nitric acid bath having a concentration of from 1 to 20 percent HNOs by weight for a period of from 2 to seconds at a temperature of 60 to F., rinsing the contacted metal, applying to the rinsed metal a film of a bath consisting essentially of water having dissolved in it about /2 to '30 percent CrOs, from about A to 25 percent of a reducing agent for the chromic acid, said reducing agent being one that co-exists in the bath with the chromic acid without producing a precipitate at the contacting temperature for at least one day, and about 0.01 to 0.5 percent of a manganese compound, and then drying the filmed metal by heating to cause the reducing agentto be completely oxidized by the chromic acid in the film.

14. The invention of claim 13 in which the reducing h agent is an organic polyalcohol.

15. The invention ofclaim 13'in which the drying is carried outat a temperature of from 250 to 450 F.

16. The invention not is liquid quenche least 25 F.

References Cited in the file of this patent UNITED STATES PATENTS 13 of claim 15 in which the dried prod- 2,315,564 d from the drying temperature by at 2,393,663

14 Thompson et a1 Apr. 6, 1943 Thomas et a1 Jan. 29, 1946 Thomas et a1. Jan. 13, 1948 Hempel Dec. 26, 1950 Watson July 10, 19511 FOREIGN PATENTS Great Britain Jan. 21, 1944 Great Britain Mar. 21, 1947 Great Britain Feb. 3, 1948

Claims (1)

1. A STABILIZED SOLUTION CONSISTING ESSENTIALLY OF CHROMIC ACID, A REDUCING AGENT FOR THE CHROMIC ACID, AND WATER, THE REDUCING AGENT BEING ONE THAT AT 80*F. COEXISTS IN DISSOLVED CONDITION WITH THE CHROMIC ACID FOR ATLEAST ABOUT ONE DAY WITHOUT PRODUCING A PRECIPITATE, THE SOLUTION ALSO CONTAINING ABOUT 0.01 TO 0.5% OF A DISSOLVED MANGANESE COMPOUND.