US2210850A - Method of finishing ferrous metal surfaces - Google Patents

Description

Patented Aug. 6, 1940 PATENT OFFICE METHOD OF FINISHING FERROUS METAL SURFACES Leo P. Cur-tin, Cranbury, N. J., assignor to Curtin- Howe Corporation, tion of Delaware No Drawing.

Application Ma New York, N. Y., a corpora- Serial N0. 206,311.

' 2 Claims.

This invention or discovery relates to methods of finishing ferrous metal surfaces; and it comprises a method of making a ferrous metal surface resistant to rusting and adapted to form a good bond with a finishing coat of lacquer, enamel, paint or varnish, wherein a ferrous metal object is provided with a thin adherent oxide coating which is advantageously of the magnetite which is treated with a chromating solution dried in place, the solution being advanof zinc dichromate; the oxide coating being formed on the metal during rolling or being produced by a short heating of cleaned metal; all as more fully hereinafter set forth and as claimed.

I have discovered that thin films of iron oxide on iron and steel, value, can be given high resistance to oxidation and rusting by a treatment with a dilute aqueous solution of certain reagents; soluble compounds of chromic anhydride being most advantageous. The solution is dried on or, less advantageously, washed off again. This chromating improves the resistance of the surface, and it has the further very great advantage of producing a surface which will take and retain siccative and other finish coatings, including vitreous enamels.

In a coated article produced in accordance with the present invention, the finish coating and the metal are spaced and united by a thin intervening oxide layer carrying a small amount of an oxygen compound of chromium.

When iron or steel surfaces, such as sheet iron, are exposed to elevated temperatures, a coating consisting of iron oxide is formed if air or other source of oxygen. is present. When formed in rolling mills, this coating is sometimes rather thick, and/is known as mill scale. In modern methods of producing sheet iron or steel, the coating is considerably thinner and more adherent, and may be less deeply colored, than ordinary mill scale. Mill scale is usually called magnetic oxide, and it approaches, but never quite reaches, the composition of magnetite. F8304: a part. of the oxide is FeO. The physical and chemical properties of the oxide film vary widely, depending upon time, rate of heating and cooling, and other factors.

Mill scale has but little protective value; it does not protect the underlying metal against the atmosphere. Nor does it take siccative coatings well. The coating produced in the old time Barlf method by exposing iron to steam at a red heat is also magnetic oxide, but it has considerable pro- 55 tectlve value. It does not, however, take paint while of no particular protective and varnish well. The modern continuous strip rolling mill is too rapid in action to permit the formation and retention of any substantial thickness of scale. The coating on this sheet metal is generally continuous, adherent and thin, being usually bluish in color. These coatings have some protective value. when of good character, but they do not take siccative coatings well. They make sheet metal more resistant to rustin than cleanwand bright metal. particularly if the oxide coating is treated with a suitable oil. Even with oil. however, protection is of short duration with the iron in outdoor use. that is, lacquer, enamel. paint or varnish. when applied directly to these oxide coatin s. do not give good results: the results being often not as good as those obtained by the direct application of the siccative coatings to clean. bright metal.

I have discovered that these oxide coatin s are rendered more rust resistant. and also capable of taking a siccatlve coating with good bond. if they are treated with a. solution of a suitable compound of chromic anhydride. On the whole. dilute solutions of z nc dichrom ate of about 1 per cent concentration give the best results. The solution is applied by sprayin or immersion and is dried in place. With hot. thin metal emerging from the strip mill. the heat of the metal helps. and may be sufficient. to dry the solution in place. Such a chromated surface takes all usual siccative coatmas and vitreous finishes, and gives a good bond therewith.

The bond between the coating and the underlying metal depends. however. upon the character of the oxide coating. And there is a considerable difference. as stated. in the haracter of these oxide coatings. With sheet metal emer in from the strip mill with an adherent oxide film of such a thickness as to b somewhat bluish in color. ood results are obtained. Most iron and steel articles other than these thin sheets do not generally carrv oxide coatings valuable for the present purposes. In such cases it is necessary to clean the surface, pickling if necessary. and then to oxidize it.

When clean iron is exposed at an elevated temperature to air. oxy en. steam or other gas capable. of iving up oxygen to the metal. a series of color effects are produced on the iron surface by the thin oxide fi m produced. The color depends principally on the temperature of the reaction, and to some extent on the time of treatment. The lighter colors are due to interference phenomena in very thin films. On the whole, the best results are obtained by stopping oxidation Siccative coatings.

Temperature I Color Yellow.

Bronze.

Light purple. Dark purple. Dark blue.

Pale blue.

Greenish blue.

' Pale greenish blue.

The yellow color developed at 240 C. requires at least 40 minutes to make its appearance. The same color may be produced in 5 minutes at 325 C. Likewise, the dark blue color produced in about 15 minutes at 300 C. is produced in 3 minutes at a temperature of 400 C. The greenish blue color produced at 350 C. in 10 minutes is produced at 500 C. in 2 minutes.

In the chromating treatment which I apply to these oxide films, rather dilute solutions of various chromates, dichromates or chromic acid itself maybe employed. Even trivalent chromium in the form of chromium acetate has been found to have some merit. For the present purposes, there are few other salts that have the virtues of the chromates. However, some results may be obtained with aluminum and lead acetates. The solution I have found most advantageous is one of Zinc dichromate in concentrations ranging from 0.25 per cent to 1.50 per cent. While concentrations outside this range have merit, the optimum effect. is obtained at concentrations near 1 per cent ZnCrzOv. Used in similar concentrations, cupric dichromate, cadmium dichromatef ammonium chromate and dichromate, and chromates of calcium and magnesium are almost as effective as zinc dichromate. A little free chromic acid with the dichromates is not objectionable. Likewise, ammonium chromate may be slightly more basic than is indicated by the formula without noticeable injury to the coating. Free chromic acid also has some merit, although it has an injurious effect when used in too high concentrations. A solution containing about 0.25 per cent CIOs gives good results.

I prefer to apply the chromic solution hot, at temperatures between 50 C. and C. A cold application is good but not quite so effective. I also prefer to dry the chromate solution on the oxide surface, without rinsing. Drying by heat is most advantageous. The chromic solution may also be applied to the hot metal coming from the hot rolling operation or from the annealing oven with the heat of processing, and this heat will help to dry off the solution.

The chromic solution may be applied by dipping or immersion. but I prefer to apply the solution as a spray. Time is not important, 5 or 10 seconds spraying or immersion being adequate. Any complete wetting of the oxidized ferrous surfacc with the chromating solution suffices.

It is believed that various chemical reactions take place between the oxides of zinc, iron and chromium present. in part resulting in the formation of various insoluble compounds, some of which are similar to minerals found in nature known as spinels. ZnQFezOz (Franklinite),

FEQCrzOs (chromite). and FeOFezO; (magnetite) are typical of these. All are highly inert, chemically. A coating so obtained adheres tenaciously to the metal and has excellent rustproofing and bonding properties.

In one embodiment of my invention, I spray hot, oxidized sheet iron, coming from the hot rolling process or from annealing ovens, with a hot 1 per cent solution of zinc dichromate. This is applied as a fine spray and in an amount no more than sufficient to completely wet the surface. The sheet is preferably allowed to remain flat during spraying. and sprays are located above and below to wet both surfaces. The sprays may be followed by light rolls to aid in completely spreading the chromate rinse. The metal is then allowed to dry without rinsing by means of its own specific heat, or with the aid of heat from an outside source. Considerable benefit is obtained by a'chromic treatment as described above when the dichromate wetting is followed by a water rinse. Much better results are obtained, however, by drying the chromic solution on the surface as described.

In another embodiment of the invention, clean cold rolled or pickled sheet iron is subjected, in the presence of air, to a temperature sufficient to produce the desired oxide coating. This may be done, on a batch basis, by placing the sheet iron in a furnace heated to the requisite temperature for a sufficient time to produce the coating desired. Or, the metal to be coated may be suspended from carriers and continuously carried through a furnace where the time and temperature are so regulated as to give the desired coating. In either case, the oxidized metal while still hot is dipped in or sprayed with a hot solu tion of 1 per cent zinc dichromate, which is dried on the surface. If desired, a siccative coat may now be applied.

As shown above, the color of the coating varies with time and temperature; and. in turn, the quality of the coating varies somewhat with color. For example, while the yellow coatings have some merit, when finished with the chromic treatment, they are not the equal in quality of the bronze, purple, blue and light greenish blue coatings. There is apparently insufficient oxide formation at the lower temperatures to provide a proper base or bond with the zinc dichromate, etc. At the other extreme, there is danger of overheating, which results in a non-adherent or semi-adherent oxide film; this is most undesirable.

As shown above, it is possible to get the oxide of a color characteristic of a certain temperature by heating the metal to a higher temperature for a shorter period of time. This is of great importance in the commercial application of the invention, because it is usually advantageous to raise the temperature or C. when the time of treatment can be cut down thcrcby to one-fifth, or less.

An oxide coating of good blue color is about equal to any of the others in the bronze to light greenish blue range. and this may be produced in 2 minutes at 450 C., working with 2? gage clean sheet iron. Light blue coatings of good quality may be produced at 600 or 650" C.. in a minute or even less. As stated above, there is considerable danger of getting non-adherent coatings at the higher temperatures and, for this reason, I prefer to carry on the oxide-coating operation in the range of 400 to 600 C. With 20 able combination of due care and regard to the treating time, however, perfectly adherent; coatings may be obtained at temperatures much above 600 C.

If the oxide coating is produced at 500,? C., or higher, any oil or grease present is usually destroyed and good coatings are obtained. It is generally the case, however, that all rustproofing, plating and other finishing operations on metal surfaces are most effective if the surface has previously been degreased and freed from foreign matter, and this is equally true of the present process.

Rustproofing and bonding coats obtained 'as above described are not only of especially high quality but are probably unequalled in the matters of cost and ease of application. Articles with siccative or baked vitreous enamel finishing coats can therefore be produced in accordance with my invention with an exceptionally favorhigh quality and low cost.

' copper, ammonia, calciu What I claim is: 1. The method of producing a rustproofing and bonding coat on a ferrous metal surface, which comprises oxidizing said surface with oxygen from a gaseous source at a temperature between 300 and 650 C. for a period sufficient to form thereon a thin adherent coating of iron oxides, wetting said coating with a dilute solution of a compoundv of chromic anhydride selected from the 'class consisting of chromic acidand soluble chromates and di-chromates of zinc,

cadmium and mag nesiurn, and drying said solution on the surface.

2. The method of claim 1, the step of applying a finish selected from the, class consisting of paints, varnishes, lacquers, enamels and vitreous coatings.

LEO P. CURTIN.

to which is added