US2046061A - Coated metal - Google Patents

Description

Patented June 30, 1936 UNITED STATES PATENT OFFICE COATED METAL poration, New York,

New York N. Y., a corporation of No Drawing. Application January 10, 1935,

Serial No. 1,258

17 Claims.

This invention relates to coated metals; and it comprises an article of steel or iron carrying a thin black surface layer containing vanadium in combination with oxalic acid, said layer being hard, permanent, elastic enough to resist flexing to a marked degree and serving as a bonding agent between the metal and subsequently applied paint, cohering to both; and it further comprises a method of coating ferrous metal and preparing it for painting wherein the cleaned or pickled ferrous metal is exposed to the action of a solution containing tetravalent vanadium and oxalic acid and is thereafter washed and dried; all as more fully hereinafter set forth and as claimed.

In the metal finishing industry, it is now customary prior to painting to apply some type of a protecting coating to the clean metal; this preliminary coating serving as a basis for subsequent painting, varnishing and enameling. For the most part, it is difficult to secure completely protective coatings with ordinary types of paints and varnishes applied directly to clean naked iron or steel. One type of preliminary coating often used is that produced by the action of solutions of phosphoric acid or of acid phosphates, on iron. It contains ferrous phosphate and often other phosphates as well. A solution containing a manganous phosphate is sometimes used. When such a phosphate coating is washed and dried, it serves as a good basis for varnish, paint and lacquer. These coating materials cohere or bond better to the phosphated surface than they do to the naked metal. Besides phosphoric acid, other acids have been used as coating agents, the coatings consisting of metallic salts of the respective acids used.

A difficulty with most of these coatings of the prior art is that they do not resist flexing well; if coated sheet metal is bent at a sharp angle, there is a tendency to open flaws and crevices. Coatings satisfactory in most other respects are apt to be deficient in this; the coated metal often will not stand the manipulation necessary in making up various things; automobile bodies, for example. With a rupture in the coatings due to flexing, there is the possibility of what is called creeping corrosion; rust appears under the double coatings and spreads indefinitely. These coatings, while customarily called rustproof, are actually more in the nature of rust retardants. In damp locations, with any fracture or abrasion of the coating which exposes bright metal, rusting of the whole surface ultimatelv occurs; the rust, however, perhaps developing at 10 per cent of the rate at which the bright metal alone would rust. The coating is therefore a rust retardant.

We have found that coatings satisfactory as a protective agent for iron and steel and as a bond- 5 ing agent linking paint to iron or steel can be made with a solution containing vanadium oxalates; and that these coatings exhibit a further advantage in that they offer a considerable resistance to rupture by flexing. Sheet metal so 10 coated can be bent at a very sharp angle with no tendency for the coating to crumble, even at the inner edge of the bend. The coating improves the resistance of the sheet metal to sharp, heavy impact and is fairly resistant to high temperatures, even up to 300 C. In contradistinction to the usual types of coating, this may be described as rust-proofing; it is not merely rust retarding.

By immersing clean pickled sheet metal in an 20 acidified solution of a vanadium oxalate, a dark, firmly cohering coating is formed and, after washing and drying, the underlying bright metal is wholly protected. The solution is made by dissolving vanadium tetroxid with the aid of oxalic acid and the same chemicals are used in replenishment. A slight amount of oxalic acid in excess is employed but it is not necessary to have free acid present in greater amount than suffices to prevent hydrolysis of the vanadium oxalate. A 30 pH ranging from 1 to 3 serves well. The coating bath should be rather hot, somewhere between C. and 99 C. The time of treatment varies from 4 to 5 minutes to several hours. With 4 or 5 minutes treatment, the bath ordinarily gives extremely thin black coatings; but the thickness is sufiicient for some purposes. A- heavier type of coating is obtained with immersion for several hours. The color of the coating varies from a rich lustrous black to a dull black. It may be made decorative, in event no after-painting is used. With a rich lustrous black in the coating and a layer of transparent varnish, decorative effects are obtained.

In making a typical coating bath in the present invention, 166 parts of commercial vanadium tetroxid, V204, are dissolved in 10,000 parts of water with the aid of 260 parts of commercial oxalic acid. This bath contains the equivalent of 1.66 parts V204 to 100 parts of water, but higher and lower concentrations can be used. Good coatings can be produced with V204 concentrations ranging from 0.3 to 4.0 per cent. A bath s0 prepared may be used indefinitely long with occasional replenishment of the contained vanadim and oxalic acid. The'vanadyl oxalate may be used in the corresponding amount. Commercia i vanadium pentoxid, V205, may be used in replenishing the bath; it being reduced by oxalic acid to the vanadyl form. Additions of oxalic acid should be in the amounts required to keep the bath clear. Should a sludgeaof insoluble ferrous oxaiate or of insoluble reduced vanadium compounds form it can be cleared up by cautious additions or? strong commercial hydrogen peroxid solution in the right amount; Ferrous oxalate is converted into ferric oxalate. Weak solutions give an undesirable dilution and it is not advantageous to add enough 'peroxid to cause substantial oxidation in the bath itself. Bubbling air through the bath is sometimes expedient. An addition of V205 can sometimes be utilized in producing the limited amount ofoxidation wanted. The precise nature of the solute contained in the bathimade as described ante is not known and it changes after use in coating ferrous metal. Reduction appears to take place; trivalent vanadium appearing. Ferrous oxalate is also present. Ferrous oxalate although insoluble in water and in oxalic acid solutions, has a rather substantial solubility in a solution of vanadium oxalates. In the use of the bath with iron and steel, ferrous oxalate can be formed and go into solution. The presence of this ferrous oxalate in soiution is apparently desirable, up to concentrations of about, say, 0.5 per cent ferrous oxalate in the bath.

In forming the coating the iron metal is attacked and there is a production of ferrous oxalate, some of which becomes part of the coating. Some goes into the bath. In the coating, there are trivalent and tetravalent vanadium; there are derivatives of vanadium trioxidf'VzOs. Vanadium'forms a number of different oxids and they combine among themselves, so that it is possible that intermediate oxids between the trivalent and tetravalent are present, these'being in'combination, probably, with the oxalyl radical, although "vanadites of vanadium" or of ferrous iron may be present. In some instances the coating appears to contain a basic oxalate which may be a vanadyl oxaiate corresponding to the formula V204.VO C294.

As stated, the path always contains more or less iron and this iron is useful in forming and contributing the coatings. It has been found that good coatings may be obtained with baths in which the amount of iron present is greater than that incident to the formation and solution of ferrous oxalate. Small additions of ferrous oxalate are sometimes useful in bringing the bath to its steady state. Sometimes ferric oxalate is added. Good coatings can be made with a bath containing about equal proportions of vanadyl oxalate and ferric oxalate; say about 1.5 per cent or each.

The composition of the coating layer in all cases is complex but whatever its composition, it has the describedadvantageous properties. Its resistance to rusting is about fouftimes as great as that of similar commercial rustproofed metal produced by prior processes. In comparative tests of the prior material and the present material, with plates or panels each carrying three coats of lacquer, they were sharply scratched to expose bright metal and comparative observations made at weekly intervals showed about 4- fold resistance in the present material.

As stated ante, the coating is resistant to heat; and, as a matter of fact, it is sometimes, for

with less advantage.

special purposesgdesirable to heat it. Heatingmay be up to a low red heat, of even by direct impingement of a flame. The color changes from black to dark blue. Undoubtedly, the oxalate radical breaks up. The bluish coating left after heating, however, has substantiaiiy the same desirable physical and chemical properties, as well as the rust resistance, 'as an unheated similar coating. 1

The coating bath in the present invention is acid, having a pH between 1 and 3. Alkaline baths do not give the same results; a vanadiumcontaining, rust resisting coatingon ferrous metal. In an acid solutiomthe vanadium is present as a cation and a coating is formed containing vanadium without production of iron hydroxids; without rusting the metal. 1

As stated, the presence of iron compounds in the vanadium coating bath is often advantageous. Conversely, it is found that small additions of vanadium compounds improve the action of other oxalate coating baths. Iron is commercially coated by treatment with a solution of ferric ,oxalate and a small addition of a vanadium coma pound to this bath improves the results; it gives a better coating. The coating is darker and its rust resisting properties are considerably im-- proved. Iron can be coated in a solution of oxalic acid alone although its action isprather slow and an addition of a small amount of vanadium oxalate improves the results.

While we have particularly described the use of oxalic acid in making the bath, other organic acids may be used, but, as we at present think,

Solutions made with citric and tartaric acids give coatings which, made under the conditions described, are not as good as those made with oxalic acid. Phosphoric acid used as a substitute for oxalic acid in the directions given ante, produces a coating better than is given by tartaric orcitric acid under the same conditions. Good coatings can be made by these other acids, but their properties are somewhat different from those of the coating made with oxalic acid.

In all cases, the bath should be acid to the do scribedtextent, with a pH between 1 and 3. re preserving the desired acidity an addition of acid sodium oxalate is useful as a buffer salt.

What we claim is:-

1. As a new manufacture, a ferrous metal object having formed thereon a rustprooflng coherent surface coating containing oxalates and vanadium.

2. The method of rustproofing ferrous metals which comprises immersing such metal in a slightly acid solution containing vanadium as an oxalate.

3. The process of claim 2 wherein the slightly acid solutionI has a pH between 1 and 3.

4. The process of claim 2 wherein immersion is at a temperature between 50- and 99 C.

5. In the rustproofing of sheet metal, the process which comprises immersing a plurality of successive sheets in a replenished bath containing a vanadium oxalate, replenishment being by additions of oxalic acid and V204 and the tendency of both to deposit insoiuble products being corrected by occasional oxidative treatments.

6. In the process of claim 5, as an oxidative treatment, bubbling air through the solution.

'7. In the precess of claim 5, as an oxidative treatment, an occasional introduction of V205.

8.-In the process of claim 5, introducing hydrogen peroxid as an oxidative treatment.

9. In rustproofing ferrous metal articles and providing a protective coating, the process which comprises immersing such an article in a replenished bath containing vanadium and iron as oxalates with enough excess oxalic acid to keep the solution clear, withdrawing, washing and drying.

10. In the process of claim 9, the method of replenishment which comprises occasional additions of V204, of oxalic acid in the amount necessary to keep the solution slightly acid and of an iron oxalate.

11. As a rustproofing method for ferrous metal, the process which comprises forming upon a ferrous metal article a coherent surface coating containing oxalates and vanadium and afterwards heating the article to break up the oxalyl radical.

12. The process of providing ferrous metal objects with a rustproofing coating whichmomprises immersing such an object in a slightly acid solution of a vanadium salt, the acidity having a pH value of ,1 to 3.

13. As a new manufacture, a ferrous metal object having formed thereon a thin, black, hard, coherent surface coating of a vanadium compound, said coating being impervious and being resistant to rupture by flexing or impact.

14. The article of claim 13 wherein the coating containing vanadium has the characteristics of a baked vanadyl oxalate coating.

15. An article having a surface of ferrous metal having formed thereon a. protective coherent oxalate coating containing a substantial amount 10 of vanadium in addition to other metallic oxalates.

16. An article having a surface of ferrous metal having formed thereon a coherent protective coating containing a vanadium oxalate in addition to iron oxalate.

17. A process of providing ferrous metal objects with a rustproofing coating which comprises exposing such an object to the action of an acidulated solution containing a vanadyl salt of an acid of the group consisting of oxalic, citric, 20

tartaric and phosphoric acid.

LEO P. CURTIN. JAIWES M. PAYNE.