US2318656A - Coated metal article and method of making same - Google Patents

Description

Patented May 11, 1943 COATED METAL ARTICLE AND METHOD OF MAKING SAltIE John S. Thompson, Detroit, Micln, assignor to Parker Rust Proof Company, Wayne County,

Mich.

No Drawing. Original application April25, 1941, Serial No. 390,337. Divided and this application September 14, 1942, Serial No. 458,333

13 Claims.

This invention relates to an article having a surface of zinc, or an alloy of zinc, and having on said surface a coating of paint, enamel, lacquer, or similar siccative material bonded to the surface by a thin, continuous, chemically formed, visible, adherent coating containing oxide of the metal of the surface, and to a method of producing such an article.

This application is a division of my application of the same title filed April 25, 1941, Serial No. 390,337, which in turn is a continuation in part of my former application for Coated article and method of making the same, Serial No. 260,741, filed March 9, 1939, which in turn was a continuation in part of prior applications, including application Serial No. 45,792, filed October 19, 1935, and this application is also a continuation in part of my former applications Serial No. 97,298, filed Aug. 21, 1936; of applicatlon No. 42,667, filed Sept. 28, 1935, in which is retained claims specific to the use of compounds of hexavalent chromium in the original coating solution: of application No. 42,669, filed Sept. 28, 1935; of application No. 42,670, filed Sept. 28, 1935, issued Mar. 17, 1942, as U. 8. Patent No. 2,276,353, and in which is retained claims specific to the use of the SlFs radical in the coating solution; and of application No. 158,706, filed Aug. 12, 1937, which in turn was a continuation in part of said application 97,298; and is generic in some respects to my joint application with Edward S. Hanson, Serial No. 85,335 filed June 15, 1936, which issued July 29, 1941, as U. S. Patent No. 2,250,508, which contains specific claims to coating zinc in solution containing certain organic acids.

The bonding coating to which the invention relates is a :thin coating as distinguishing from a relatively thick phosphate or similar coating of salts of the metal being coated, although some of such salts may be present in the thin coating. It is a visible coating as distinguished from a coating of oxide, or the like, which may be sufllcient to passivate the metal to some extent, .but leaves the surface with its original metallic appearance.- While the coating may be somewhat variable in thickness, it is preferably substantially continuous so that it bonds the 'siccative coat to all portions of the metal surface. It may contain compounds of a metal other than the metal of the surface, but:if so, these compounds are given their final form by chemical reaction at the surface of the-work in connection with the oxidation of some of the metal of the surface so that the coating is integrally bound to the surface of the metal.

The bonding coat is produced by a dilute aqueous solution of an etching acid for the metal being treated, and an oxidizing agent for the metal being treated. The solution may also contain a compound of a metal which is capable of forming an insoluble oxide which may constitute a part of the coating. 1

A wide variety of etching acids may be employed, but the successful production of the coating depends upon employing the acids in such a strength that there will be a promntreaction with the metal surface being treated, but that this etching action shall not be too violent. Various oxidizing agents may be employed which are compatible with the etching-acid in the solution, the amount of the oxidizing agent being such that the metal of the surface will be oxidized promptly as it. is etched. By the phrase an oxidizing agent for the metal being treated is meant an agent that acts as an oxidizing agent in facilitating the formation in the coating of a substantially insoluble oxide of the metal being treated. The etching and oxidizing action should be sumciently active so that a satisfactory coating can be produced in not over five minutes. In actual operation, usually one minute is sufficient time. This rapid reaction produces a different coating than that which could be produced by subjecting the surface to a prolonged treatment, such as a half hour or more, in an acidulated solution containing an oxidizing agent.

The action of the solution can be expedited frequently, and sometimes the quality of the coating can be improved, by the inclusion in the solution of a compound that is soluble in the solution but contains a metal which may form an oxide substantially insoluble in the solution.

This is true of substantially all of the metals in the electromotive series below magnesium and' above hydrogen, and includes'some below hydrogen, such as arsenic for example.

The oxidizing agent and acid, when properly proportioned and employed upon a ferrous surface, result in producing a gray oxide which may darken in colorrto approach a black. 'This is a different character of oxide from that produced by ordinary rusting, for example, and appears more like the oxide produced on iron at temperatures above those used for producing. a blue oxide on iron. I

A chemically produced coating of the character described improves the surface of the metal as a base for a slccative coat; but-the adherence of the siccative coating and the resistance to corrosion of the completed article is generally enhanced to a marked degree by an intermediate treatment of the chemical coating with a dilute solution of a compound of hexavalent chromium before the siccative coating is applied.

Having indicated the general nature of the invention, specific examples of the manner of carrying out the invention will be given in detail.

Example 1.-To one gallon of water is added 20 cc. of 75% ortho-phosphoric acid and 20 grams of sodium chlorate.- When used at 100 F. and sprayed onto the metal to be coated through the use of a mechanical spraying machine this solution is found to coat iron or steel in less than one minute. 0.002% or more silver in the form of silver sulfate; 0.002% copper, or more, added in the form of copper sulfate; 0.002 manganese, or more, added in the form of manganese sulfate or manganese dihydrogen phosphate, may be added to the solution with beneficial results.

Example 2.-Increasing the phosphoric acid in Example 1 to 30 cc. gives somewhat better results under the conditions there described. When used at 170 F. for one minute, 20 cc. of phosporic acid to 5 grams of potassium or sodium chlorategave very good results.

Example 3.-Using a solution of exactly the same composition as is shown in Example 1, a coating is produced on a zinc surface in one minute.

Example 4.-A solution is made up composed of one gallon of water, 20 cc. of 75% ortho-phosphoric acid and 8 cc. 30% hydrogen peroxide. An adherent coating is obtained on iron or steel when treated with this solution for one minute at 100 F. The addition of 1 gram of sodium arsenate to this solution results in a blue-black coating in one minute.

Example 5.A solution composed of one galion of water, 20 cc. of 95% sulfuric acid and 20 grams of sodium chlorate produces an adherent oxide coating on iron or steel in one minute when used at 100 F. and applied by means of a mechanical spraying machine. when 5 grams aluminum sulfate were added to this solution a coating resulted on iron which proved to be even more corrosion resistant. 1

Example 6.--A solution composed of one gal- 5 ion of water, 20 cc. of 95% sulfuric acid, 20, grams of potassium chlorate and 2.5 grams of chromic sulfate produces an oxide coating on iron or steel in one minute when applied to the metal surface by means of a mechanical spraying machine at 100 F. Other chromic saltsmay be substituted for the, chromic sulfate in the same proportion. In place of the chromic salt may be used 18 grams of zinc dihydrogen phosphate. 0.7 gram of M003, 0.5 gram of basic lead carbonate or 1.6 grams of sodium arsenate.

Example 7.A solution composed of 1,000 cc.-

water, 10 cc. 75% phosphoric acid and 2 grams sodium bromate produces a uniform coating on iron or steel in 1 minute at 100? F. when the metal is immersed in the solution.

Example 8.An adherent coating is formed upon iron and steel when subjected to a spray of a solution containing cc. of 50% hydrofluoric acid and 20 grams of sodium chlorate per gallon of water at 100 F.

Example 9.A solution containing 20 cc. of 75% ortho-phosphoric acid and 10 grams of quinone per gallon of water satisfactorilycoats iron or steel surfaces in one minute when applied by means of a mechanical spray machine.

Example 10.The treatment of iron or steel surfaces in a solution containing grams of 5 acetic acid, 20 cc. of 75% ortho-phosphoric acid and 20 grams of sodium chlorate per gallon of water results in an adherent oxide coating in one minute.

Example 11 .-A steelpanel was processed for five minutes in a solution of 18 cc. H1804 and 18 grams mm in a gallon of water. A soft, dark gray, coating resulted on the surface.

Example 12.A steel object was immersed five minutes in a boiling solution of 36 grams sodium 15 chromate, 18 cc. sulfuric acid, and 36 grams nickel sulfate in a gallon of water. A soft black coating was obtained. A more adherent coating was produced by the addition of sodium nitrate.

In general, solutions given as suitable for treat- 20 ing iron may be employed in treating zinc, although some changes in proportions are advisable because of the differences in the readiness with which iron and zinc, respectively, react with the various chemicals.

A few specific examples for treating zinc will be given.

Example 13.A dark coating was produced on galvanized iron by immersion for three minutes a at 125 F. in a solution containing 20 grams of 0 NaClOa and 20 cc. sulfuric acid per gallon and 0.05% of ferric iron, introduced as sulfate.

Example 14.-A dark coating was produced on galvanized iron by immersion for two minutes at 125 F. in a solution containing 20 grams NACIO: and 20 cc. sulfuric acid per gallon and 0.04% Ni, introduced as carbonate.

Example 15. -A dark coating was produced on galvanized iron by immersion at 125 F. in a solution containing 20 grams NaClOa and'20 cc. sulfuric acid per gallon and 0.02% cobalt introduced as sulfate.

The time, temperature and strength of in gredlents required varies with the nature of the zinc surface being treated. A galvanized surface on which bright spangles appear is not coated as readily as a comparatively dull surface. The above examples are given by way of illustration and not as formulae which must be followed ac curately, since wide variations in proportions of ingredients, as well as substitutions, may be made within the scope of the general principles disclosed herein.

While many different accelerating metals may 5 be employed in producing oxide coatings on iron or zinc, it is found that, in coating iron, the presence of zinc in the solution aids in the prompt formation of a coating, and the presence of aluminum or trivalent chromium results in exceptionally good results under paint. In coating, zinc, iron, nickel and cobalt have proven to be exceptionally good.

Apparently the iron oxide in the least partially magnetic.

coatings is at This is difllcult to ascertain for certain when iron is coated, because the iron itself is magnetic, but when zinc is properly coated in a solution containing iron, the magnetic quality of the coating is easily demonstriated.

Other etching acids, or mixtures thereof, may be substituted in the above solutions. Slight variations in the proportions'may be advisable with different acids, although in general quite wide variations in the proportions of acids are 75 permissible.

Other oxidizing agents, including strong agents such as persulfate to weak agents such as sulflte. may be used. Here, likewise, some variation in proportion is advisable according to the nature of the oxidizing agent employed, but quite an excess of oxidizing agent, over that necessary to produce the coating, may be used without harmful results.

Of course, the acid and oxidizing agent should be chosen so that they will both remain in solution and function as an etching acid and an oxidizing agent.

When an acid is employed which is capable of forming both soluble and insoluble salts of the metal being treated and/or of any accelerating metal in the solution, there is likely to be some of the insoluble salt in the coating. With inorganic acids, the insoluble salt generally contains less acid in proportion to the metal than does the soluble-salt. For example, phosphates of iron increase in solubility as the proportion of acid to iron in the salt is increased. Probably this accounts for the observed fact that the proportion of the salt to oxide in the coating increases as the acid in the solution becomes more dilute.

The following data as to variations in treating solutions and resultant coatings is provided to assist in proportioning the solutions so as to obtain a coating containing a material amount of the oxide of the metal being coated.

Tapas:

Temper- Solution per gallon 8mm Time F. Jilinuics (l) 5 cc. 75% H1PO4+m g. NaC-lO: 100 2 (2) 8 O0. 75% HsP04+20 g. NaClOa. 100 2 (3) 12 cc. 75% HaPOr-i-N a. N'aC-lOa 100 2 (4) 14 cc. 75% HsPOi-i-Z) E. NaClO; 100 2 (5) 16 cc. 75% HaPO4+20 g. NaClOg. 100 2 (6) 18 cc. 75% H5P04+20 g. NaClO 100 2 (7) 18 cc. 76% H3POl+m g. NaCl i 1m 2 (8) 30 75% HaPOH-ZO g. NaClO: l!) 2 (9) 30 cc. 75% HsPOr-HO g. NaClOa 160 2 TAaLu-Continued Coating analysis Fe Fe PO.

. Per cent Per can! Per cent 4,50 38. 23 17. 61 15. 04 i7. 48 6. M 14. 76 40. 58 4. 34 9. 93 45. 80 3. 82 9 84 48. 03 3. 50 3. 84 47. 49 1. P6 4. 16 ii. 75 25. 33 2. 79 48. 68 1.58 l7. l i0. 02 11.30

The trend is obviously that as the amount of phosphoric acid in solution increases the percentage of phosphate in the coating decreases, while the percentage of ferric iron increases. The chart also shows that where the temperature is raised the amount of phosphate inthe-coating increases surprisingly. The remainder of the coatings shown in the chart is oxide.

In the light of the foregoing description, iron or zinc may be provided with a chemical coating in accordance with this invention. This coating is not recommended as the final finish, but it is recommended that it be covered with a suitable siccative coat, such as paint, lacquer, or enamel. Also, an intermediate treatment of the chemical coating before the application of the siccative coating is recommended for best results.

In nearly every instance, added corrosion resistance and better paint-holding qualities are obtained by a treatment of the. metal article, after being coated in accordance with the foregoing description, in a solution of a compound of hexavalent chromium. For this purpose. a solution containing as little as 2 grams of C20: per gallon of water is satisfactory; For convenience, this solution will be referred to as a rinse solution. The rinse solution may be applied to the coated surface in any suitable manner, but it is usually best to apply it in the same manner as that used for applying the coating solution. It may be used at any temperature from room temperature up to boiling. The time of treatment may vary'with the strength of the solution, the temperature at which it is applied, the character of the first coating, etc., but a treatment for one minute is usually ample to produce the desired results.

Where CrOa is employed as the hexavalent chromium compound, 2 grams per gallon will produce good results, but this amount can be increased materially without harmful results. From 2 grams to 20 grams CrOa per gallon is recommended. Too strong a solution may leave discolored streaks, and the solution reacts with the first coating and dissolvessome compounds from some of the coatings quite rapidly. Some reaction is desirable and apparently necessary to obtain the best results, but it is preferred to so limit the strength of the solution and the time I and temperature of its application that the appearance of the first coating is not materially altered.

When the treatment with the hexavalent chromium solution is quite vigorous or prolonged, the remnant of the solution may be rinsed oil before the siccative coating is applied, but generally it is more. convenient and satisfactory to let the remnant of the solution clinging to the work dry thereon, and good results are obtained by drying under heat.

Usually the major part of the chromium remaining on the coating from this treatment is reduced in situ to the trivalent form. Usually some of the chromium remains in hexavalent form, but this constitutes a minor amount of the chromium, and good results are obtained where all of the chromium is reduced by the time the siccative coat is thoroughly dried.

A solution of C10: is preferred for the rinse solution, but it does not have to be a pure solution of CrOa or of hexavalent chromium compounds. In processing, not only chromates and dichromates, but other impurities are usually built up in the solution, but it is not necessary to purify the solution until it is evident that the contamination has some effect upon the coating. Any'solution containing chromic acid may be employed in the rinse provided, of course, that care be taken that the other ingredients of the rinse solution do not injuriously affect thecoat-.

portions is permissible, but it is recommended for commercial purposes to stay within the proportions of 1 cc. to 4 cc. of the acid to 2 grams of chlorate or its equivalent.

For replenishing purposes, a similar solution may be provided. When phosphoric acid is employed, phosphates formed by etching the metal being coated are deposited as sludge and to some degree enter the coating, so that the acid is depleted more rapidlythan'is the oxidizing agent. In coating iron with a solution of phosphoric acid and chlorate, it has been foundnecessary to replenish the solution with from 2 to 6 cc. of phosphoric acid to each gram of chlorate, and therefore a replenishing solution for such a treatment should be made up within those proportions.

Initially, the phosphate formed by etching the surface is soluble, and soluble phosphates may be added deliberately, as indicated in Examples 1 and 6. The presence of such phosphates in the solution tends to increase the amount of phosphates in the coating, but a sufficient amount of a strong oxidizing agent, such as hydrogen peroxide, chlorate, or permanganate, will result in a material amount of oxide in the coating even with considerable increase of soluble phosphate over that indicated in the examples.

While numerous variations in the character of solutions employed and in their manner of application have been described above, it will be understood that these are by way of illustration;

and do not attempt to set forth all variations which are possible within the broad principles of this invention and within the scope of the appended claims.

What I claim is:

1. An article with a surface containing zinc carrying a chemically bound coating which is predominantly oxide and contains zinc oxide, and which also contains hexavalent chromium.

2. An article with a surface containing zinc and having on said surface a thin continuous adherent protective and bonding coating chemically bound to the surface and being predominantly oxide, containing zinc oxide and also containing triy-alent chromium reduced in situ, said chemically bound coating being covered with a siccative coating.

3. An article of metal selected from the class consisting of zinc and its alloys and having formed thereon a thin continuous adherent protective and bonding coating composed essentially of an oxide of zinc and of an oxide of another metal below magnesium and above hydrogen'in the electromotive series, said coating being formed by reaction of the metal of the article with an acidulous aqueous solution of an oxidizing agent for said metal and a salt of said other metal, said coating also containing trivalent chromium reduced in situ, and having a siccative coating covering said coating formed by reaction of the metal.

4. An article having a surface of zinc and having formed on said surface a thin continuous adherent protective and bonding coating composed-essentially of oxides of iron and zinc, said coating being formed by reaction of the metal of the article with an acidulous aqueous solution of an oxidizing agent for said metal and an iron salt, and having a siccative coating covering said coating formed by reaction of the metal.

5. An article having a surface of zinc and having formed on said surface a thin continuous adherent protective and bonding coating composed essentially of oxides of nickel and zinc,

said coating being formed by reaction of the metal of the article with an acidulous aqueous solution of an oxidizingagent for said metal and a nickel salt, and having a siccative coating covering said coating formed by reaction of the metal.

6. An article having a surface of zinc and having formed on said surface a thin continuous adherent protective and bonding coating composed essentially of oxides of cobalt and zinc, said coating being formed by reaction of the metal of the article with an acidulous aqueous solution of an oxidizing agent for said metal and a cobalt salt, and having a siccative coating covering said coating formed by reaction of the metal.

7. An article havinga surface of zinc and having formed on said surface a thin continuous adherent protective and bonding coating.composed essentially of oxides of iron and zinc, said coating being formed by reaction of the metal of the article with an acidulous aqueous solution of an oxidizing agent for said metal and an iron salt, the iron oxide being present principally in the form of the magnetic oxide, and having a siccative coating covering said coating formed by reaction of the metal.

8. An article having a surface of zinc and having formed on said surface a thin continuous adherent protective and bonding coating composed essentially of oxides of iron and zinc, said coating being formed by reaction of the metal of the article with an acidulous aqueous solution of an oxidizing agent for said metal and an iron salt, said coating also containing trivalent chromium reduced in situ and having a siccative coating covering said coating formed by reaction of the metal.

9. The method of coating a surface of metal of the class consisting of zinc and its alloys, which comprises treating said surface with an aqueous solution containing an etching acid for the metal being treated and an oxidizing agent for the metal being treated, proportioning the acid and oxidizing agent to form an oxide of the metal of the surface as it is etched at a rate to produce a thin continuous visible adherent predominantly oxide coating on said surface in not more than five minutes, continuing the treatment until said continuous coating is produced, thereafter treating said chemical coating with a dilute solution of a compound of hexavalent chromium, and thereafter covering said continuous chemically formed coating with a siccative coating.

10. The method of coating a zinc surface which comprises treating said surface with an aqueous solution of an etching acid for zinc. an oxidizing agent for zinc, and a soluble compound of a metal between magnesium and hydrogen in the electromotive series, proportioning the acid and oxidizing agent to form an oxide of the zinc of said surface as it is etched and to'produce a thin continuous visible adherent predominantly oxide coating on said surface in not more than five minutes, said coating containing oxide of zinc and also containing metal from said compound in the solution, continuing the treatment until said continuous coating is formed, thereafter treating the continuous chemically formed coating with a dilute solution'of a compound of hexavalent chro-' mium, and thereafter applying a siccative coating to said continuous chemical coating.

11. The method of coating a zinc surface which comprises treating said surface with an aqueous solution of an etching acid for zinc, an oxidizing agmt for zinc, and a soluble compound of iron, proportioning the acid and oxidizing agent to form an oxide of the zinc of said surface as it is etched and to produce a thin continuous visible adherent predominantly oxide coating on said surface in not more than five minutes, said coating containing oxides of zinc and iron, continuing the treatment until said continuous coating is formed, thereafter treating the continuous chemically formed coating with a dilute solution of a compound of hexavalent chromium, and thereafter applying a siccative coating to said continuous chemical coating.

12. The method of coating a zinc surface which treating said surface with an aqueous solution of an etching acid for zinc, an oxidizing agent for zinc, and a soluble compound of cobalt, the acid and oxidizing agent to form an oxide of the zinc of said surface as it is etched, and to produce a thin continuous visible adha'ent predominantly oxide coating on said surface in not more than 5 minutes, said coating oxides of zinc and cobalt, continuing comprises treating said surface with an aqueous solution of an etching acid for zinc, an oxidizing agent for zinc, and a soluble compound of nickel, proportioning the acid and oxidizing agent to form an oxide of the zinc of said surface as it is etched and to produce a thin continuous visible adherent predominantly oxide coating on said surface in not more than five minutes, said coatin containing oxides of zinc and nickel, continuing the treatment until said continuous coating is formed, thereafter treating the continuous chemically formed coating with a dilute solution of a compound of hexavalent chromium, and thereafter applying a siccative coating to said continuous chemical coating.

JOHNS. THOMPSON.