US2526544A

US2526544A - Method of producing a metallic coating on magnesium and its alloys

Info

Publication number: US2526544A
Application number: US778283A
Authority: US
Inventors: Long Herbert K De
Original assignee: Dow Chemical Co
Current assignee: Dow Chemical Co
Priority date: 1947-10-06
Filing date: 1947-10-06
Publication date: 1950-10-17
Anticipated expiration: 1967-10-17

Description

Patented Oct. 17, 1950 METHOD OF PRODUCING A METALLIC COATING ON MAGNESIUM AND ITS ALLOYS Herbert K. De Long, Midland, Mich, assignor to The Dow Chemical Company, Midland, Mi ch a corporation-gof'Delaware V H No Drawing. Application October 6, 1947, Serial No. 778,283

9 Claims.

The invention relates to the surface treatment of articles of magnesium and magnesium base alloyscontaining upwards of 85 per centof magnesium. I Itmore particularly concerns a surface treatment method for-the aforesaid metals which promotes the adherence thereto'of metallic coatings, particularly those produced by electrodeposition.

Articles of magnesium and its alloys are among the most difiicult ofall common structural metals to provide with an adherent metallic coating of another metal. A number of methods have'been proposed heretofore by'which coatings of certain metals maybe provided on articles of magnesium and its alloys. Howeveigthe coatings obtained are not wholly satisfactory either from the standpoint .ofiadhesion, appearance, orprotection afforded the underlying metal, and, in some instances, the coating procedures are complex and difficult to perform. It is a desideratum in the art to provide a method of treatment by which a magnesium or magnesium-base alloy article maybe given an adherent coating of another metal, and particularly an adherent metallic coating which is either or both protective and decorative.

Accordingly, it isthe principal object of the invention to .provide 'a method fulfilling this need. Aparticular object is to provide a method of treating the surface of an article of a magnesium or magnesium-base alloy, whereby adherent electroplates of other metals thereon may be obtained. Other objects and advantages of the invention will appear as the description of the invention proceeds.

Briefly the method contemplated by the invention comprises treating the cleaned article of magnesium or magnesium-base alloy with an alkaline-aqueous solution containing pyrophosphate and zinc is solution, whereby a zinc coating is depositedupon the metal. The zinc coated surface obtained is used as a base for theretention of other metallic coatings particularly those deposited thereon by electrodeposition, as from an alkaline cyanide aqueous. electroplating bath. These electroplates may be used as the base for still other metallic coatings, .such as those which cannot be deposited directly on zinc or magnesium.

.Priorto carrying out the method, the surface of the articleis cleaned in a manner to leave the surface free from contamination and in a condition whereby the metal will react with the zincing solution, of dissolved pyrophosphate and zinc, to bring about the deposition of the zinc coating. The surface of the article may be pre- 2 pared for zincing by a mechanical cleaning such as machining, wire brushing, buffing, and the like, or by a chemical treatment with solvents or .pickling agents, the method used beingchosen .in accordance with the nature of the surface to be cleaned and smoothness desired. While mechanical cleaning usuallyleaves the surface in a condition capable of reacting with the zincing solution, chemical treatment, such as the conventional pickling operationsleavean oxidic film which interferes with .the action of the zincingsolution. 1

I have found that the oxidic film, however formed, should be removed before proceeding with the method of the invention. For the removal of the film or coating, the article may be immersed in a film removing solution such as a 2 per cent solution of acetic-acid in Water or a 1 percentsolution of hydrochloric-acid in-water. Still bet ter results as regards the adhesion of the zinc 1 coating subsequently to be applied are obtained an article of magnesium and its alloys in' an aqueous solution of aluminum chloride has ,spe-- cial advantages and is more fully set forthin my copending application, Serial, No. 778,282, filed October 6, 1947 issued'as Patent No.'2,499,269 on February 28, 1950.

In carrying out the method of the invention, the article to be zinc coated is treated with an aqueous zincing bath having a pH between about 8 to 11 and comprising zinc and pyrophosphate is solution as aforesaid, as by immersing the ar-.

ticle in the bath, the article and treating solution being maintained in contact until a, coating of metallic zinc is deposited on the surface of the article. Formation of the coating is visually evident by its uniform grey color and appears to result from the reducing action of the metallic magnesium of the surface of the article on the dissolved zinc in the zinc-pyrophosphate solution, whereby the reduced zinc is deposited in situ, the reduction proceeding'according to the:

A number of formulations of the ziric-pyrophosphate bath have been found effective to The aluminum deposit the zinc coating of which the following are illustrative:

Example 1 Grams Sodium pyrophosphate (NGAPZO'!) 10 Zinc fluoride (ZnF2) 1.5

Water sufficient to make 100 grams of solution The pH of the solution is about 9.6.

Example 2 Grams Sodium pyrophosphate 10 Zinc sulfate (ZnSOa'lHzO) 4 Sodium fluoride (NaF) 0.5

Water sufficient to make 100 grams of solution The pH of the solution is about 10.2.

Example 3 Grams Sodium pyrophosphate 10 Zinc fluoride 1 Ferricfluoride (FeFa) 0.01

Water suflicient to make 100 grams of solution I The pH of solution is about 9.8.

Example 4 Grams Potassium pyrophosphate (K4P207) 10 Zinc fluoride 0.5 Sodium fluoride 2 Water sufficient to make 100 grams of solution The pH of the solution is about 9.6.

Example 5 Grams Sodium pyrophosphate 6 Zinc cyanide (Zn(CN)2) 1 Sodium fluoride 2 Water sufficient to make 100 grams of solution The pH of the solution is about 10.6.

Water su fficient to make 100 grams of solution The pH of the solution is about 10.6.

.-The foregoing solutions are used preferably at their boiling temperature, the article to be treated being immersed therein for about 1 to 30 minutes or longer, although 3 to 7 minutes immersion is preferred. The coating obtained is generally not over 0.0001 inch thick and cannot be peeled off.

Other alkali metal pyrophosphates may be used, such as those of lithium, rubidium, and cesium. If desired, the pyrophosphate in the bath may be provided by adding pyrophosphoric acid to the bath and suflicient alkali of the alkali metal family to raise the pH to the alkaline side, thereby in eifect producing an alkali metal pyrophosphate in solution as an active ingredient. The zinc in solution may be derived from various soluble zinc compounds, in addition to those in the foregoing examples, such as zinc acetate, zinc borate, zinc bromide, and zinc chloride.

The concentration of the pyrophosphate and zinc in solution, effective for producing the Zinc coating, does not appear to be critical and a wide variation in their concentrations is permissible. In the case of the pyrophosphate, the concentration of P201 may be from about 0.6 to 27.0 grams, and the concentration of Zn from about 0.1 to 4.0

4 grams per 100 grams of solution, although other proportions may be used. Pyrophosphoric acid and various alkalis, such as an alkali metal hydroxide or carbonate, may be used to adjust the pH of the zinc plating solution to the desired value, if necessary.

While the presence of dissolved fluoride, derived from a soluble metal fluoride (preferably an alkali metal fluoride), in the Zinc plating bath is preferred because it reduces the time of treatment and. results in better adhesion of the zinc, its presence is not essential and may be omitted, if desired, provided the bath contains the ions of pyrophosphate and zinc and the pH is between about 8 and 11. A suitable fluorine concentration is between about 0.25 and 8 per cent, although other concentrations may be used.

Although the pyrophosphate-zinc bath is used preferably while at or about the boiling point, lower temperatures may be used but the rate of zinc deposition is slower. At 20 to 25 C., for example, a zinc coating is obtained which appears to reach a maximum thickness, in about 30 minutes; while at the boiling point or near 100 C., a similar coatingis obtained in about 1 minute.

In forming the coating, there is a mild gassing at the surface of the immersed article especially at first and this gassing diminishes as the coating action proceeds. On attaining its ultimate thickness, the reaction appears to cease and little or no gassing is observed. The article need not be left in the bath beyond the stage when gassing ceases or nearly ceases,

Another indication that the Zinc coating op-v eration has proceeded to a limiting extent is that the surface undergoing plating becomes uniformly grey in appearance. It is to be understood that useful coatings are obtained in less time than that required for the coating action to come to an apparent end, as when gassing ceases, or nearly ceases, or when the article becomes uniformly grey.

Immersion of the article in the. zinc plating solution is the preferable way of maintaining contact between the solution and. the surface to be coated, but other ways of maintaining contact may be used such as spraying the solution onto the surface of the article. After the treatment, the plated article is thoroughly rinsed in Water.

The zinc coating obtained by the foregoing method, referred to hereinafter sometimes as a contact coating, has utility as a protection and decoration, and, if desired, the coated article need not be further treated except to dry it. However, the zinc contact coatin has the unique advantage of being capable of receiving adherent deposits of other metals in similar manner to that of zinc itself. For example, the contact coating before drying may be electroplated in conventional cyanide alkaline plating baths suitable for electroplating zinc. Various metals may be thu deposited from their alkaline'electroplating baths, e. g. silver, copper, gold, cadmium, tin, and various combinations of these metals. 1

In providing an electroplating over the contact I coating, the coated article is preferably given a strike from a cyanide platin bath, for example, a copper strike from an alkaline copper plating bath, such as a conventional copper cyanide.

Grams Copper cyanide (CuCN); 41.3 Sodium cyanide (NaCN)' 50.8 ,Sodium carbonate (NazC'O'sl 30.0 Rochelle salt (KNaCzH4O64H2O') 45.0

Other conventional alkaline cyanide plating baths may be used, suchasgthose of theaforementioned. metals, althoughqforz the purposeof obtaining a strike, the copper plating bath is preferred. In forming: a:.copp e deposit using the above bath, it ,isoperated,.preferablyj at a temperature of about 150 F. A currentdensity of about 5m 10. amperes; per square. footof surfacewis maintained .for about .3 minutesorlonger and then raised to about 15 to 20 amperes per square: foot for 2 minutes longer. This treatment results in coating the zinced surface with a layer of copper that adheres well and covers -tl e zin c. If desired, the copper plating operation may be continued for a longer time to produce a heavier coating and other current densities and platingtemperature may be used as known in the electroplatingart.

Having applied a strike of copper or other metal coating .over the contact coating, it is possible tov further. electroplate the article with any other metal that is. capableof being electrodeposited upon the strikecoating. Fo example, I have found that cadmium, chromium nickel, silver, and zinc or combinations of metals can be electroplated inconventional manner on the electroplated'contact coating.

Among the advantages-of the method are that by either contact coating the magnesium (or magnesium alloy) article with zinc or electroplating the zinc contact coated article, it is possible to provide a finish coating of any metal that is capable of electrodeposition from a conventional plating bath. As already indicated, for example, the cyanide plating bath may be used for electroplating over zinc (deposited by contact coating or plating as described or electrolytically over the contact coating) an of the various metals that can be plated on zinc from such baths. Furthermore, by choosing as the metal to be deposited from the cyanide bath one over which a plating may be deposited from an acid plating bath, for example, the metal copper, all the metals which are capable of being applied as electroplates from either a conventional acid or alkaline plating bath, may be deposited upon articles of magnesium and its alloys as a finish coating.

For example, metals such as chromium and nickel which cannot be plated directly on magnesium or zinc, because of the acid nature of their plating baths, can be readily electroplated as a finish coating on a magnesium and magnesium alloy article from the conventional electroplating baths of these metals after the article is zinc contact coated according to the invention and plated from an alkaline bath with a metal resistant to acid plating baths.

I claim:

1. In a method of forming an adherent metallic coating upon the surface of an article of magnesium and its alloys, the step which comprises treatin the surface of the article with an al- .mide, zinc chloride, zinc cyanide, zinc fluoride,

zinc sulfate, the concentration of the pyrophosphate radical being between 0.6 and 27 grams and that of the. zinc being between 0.1 and ligrams per I00 grams of .the solution, whereby a coating of'metallic zinc is deposited in situ on the surface of the article.

2. In a method of forming an adherent metallic coating upon the surface of. an article .of magnesiumand its alloy the step which comprises treatin the surface ofithe article withan alkaline aqueous solution having a pH. between about '8' and l1 and consisting essentially of water, an. alkali metal pyrophosphate, and a water-soluble zinc saltfiselectedj from the group consisting of zinc acetate, zinc ,borate, zinc bro.- mide, zinc chloride, zinc cyanide, zinc, fluoride, zinc sulfate, the concentration. of the pyrophosphate radical being; between 0;6' and 27 grams and thatof flthezinc being between 0.1 and'4 grams per grams of the solution and, the temperatureof the-solutionbeing between about 20 C. andits atmospheric boiling point, whereby a coatingof metallic,zinc-isdepositedin situ on the, surface of the. article.

3'. In a method of forming an'adherent'metallic coating upon the. surface of. an. articlev of magnesiumand its. alloys, the step whicncomprises treatingthe surface of the, article. with an, alkaline aqueous solution. having,a pI-Ii be.-

twe'enaboutfif.ancLll and consisting essentially of water, an alkali. metal pyrophosphate, a water-soluble. zinc. salt selfectedfromvv the. group consistin of zinc. acetate; zincborate, zincbronide, ,zinc. chloride, zinc cyanide;,.zinc. sulfate, the concentration of the pyrophosphate radical being between 0.6 and 2'7 grams and that of the zinc being between 0.1 and 4 grams per 100 grams of the solution, and a water-soluble metal fluoride in amount between about 0.25 and 8 grams per 100 grams of the solution, whereby a coating of metallic zinc is deposited in situ on the surface of the article.

4. In a method of forming an adherent metallic coating upon the surface of an article of magnesium and its alloys, the step which comprises treating the surface of the article with an alkaline aqueous solution having a pH between about 8 and 11 and consisting essentially at water, an alkali metal pyrophosphate in amount sufiicient to furnish 0.6 to 27 grams of pyrophosphate radical per 100 grams of the solution, zinc sulfate in amount sufficient to furnish 0.1 to 4 grams of zinc per 100 grams of the solution, and a water-soluble metal fluoride in amount suflicient to furnish between 0.25 and 8 grams of fluoride per 100 grams of solution.

5. In a method of forming an adherent metallic coating upon the surface of an article of magnesium and its alloys, the step which comprises treating the surface of the article with an alkaline aqueous solution having a pH between about 8 and 11 and consisting essentially of water, an alkali metal pyrophosphate in amount sufficient to furnish 0.6 to 27 grams of pyrophosphate radical per 100 grams of the solution and zinc fluoride-in amount suflicient to furnish 0.1 to 4.0 grams of zinc per 100 grams of solution,

whereby a coating of metallic zinc is deposited in situ on the surface of the article.

6. In a method of forming an adherent metallic coating upon the surface of an article of magnesium and its alloys, the step which comprises treatin the surface of the article with an alkaline aqueous solution having a pH between about 8 and 11 and consisting essentially of water, an alkali metal pyrophosphate in amount suflicient to furnish 0.6 to 27 grams of pyrophosphate radical per 100 grams of the solution, zinc fluoride in amount sufficient to furnish 0.1 to 4.0 grams of zinc per 100 grams of solution, and ferric fluoride in amount sufficient to furnish 0.25 to 8 grams of fluoride per 100 grams of solution, whereby a coating of metallic zinc is deposited in situ on *the surface of the article.

7. In a method of forming an adherent metallic coatin upon the surface of an article of magnesium and its alloys, the step which comprises treating the surface of the article with an alkaline aqueous solution having a pH between about 8 and 11 and consisting essentially of water, an alkali metal pyrophosphate in amount sufficient to furnish 0.6 to 27 grams of pyrophosphate radical per 100 grams of the solution, zinc fluoride in amount sufiicient to furnish 0.1 to 4.0 grams of zinc per 100 grams of solution, and. sodium fluoride in amount suflicient to furnish 0.25 to 8 grams of fluoride per 100 grams of solution, whereby a coating of metallic zinc is deposited in situ on the surface of the article.

8. In a method of formin an adherent metallic coating upon the surface of an article of magnesium and its alloys, the step which comprises treating the surface of the article with an alkaline aqueous solution. having a pH between about 8 and 11 and consisting essentially of water, sodium pyrophosphate in amount sufficient to provide 0.6 to 27 grams of pyrophosphate radical per 100 grams of the solution,

zinc cyanide in amount sufficient to provide 0.1 to 4.0 grams of zinc per grams of the solution, and sodium fluoride in amount suflicient to provide 0.25 to 8 grams of fluoride per 100 grams of the solution, whereby a coating of metallic zinc is deposited in situ on the surface of the article.

9. In a method of forming an adherent metallic coating upon the surface of an article of magnesium and its alloys, the step which. comprises treating the surface of the article with an alkaline aqueous solution having a pH between about 8 and 11 and consistin essentially of Water, sodium pyrophosphate in amount sufficient to furnish 0.6 to 2'7 grams of pyrophosphate radical per 100 grams of the solution, zinc cyanide in amount sufficient to furnish 0.1 to 4 grams of zinc per 100 grams of the solution, sodium fluoride in amount 'sufiicient to furnish 0.25 to 8 grams of fluoride per 100 grams of the solution, and ferrous oxalate, whereby a coating of metallic zinc is deposited in situ on the surface of the article.

HERBERT K. DE LONG.

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

UNITED STATES PATENTS Number Name Date 744,170 Darlay Nov. 17, 1903 1,607,676 Jirotka Nov. 23,1926 1,627,900 Hewitson May 10, 1927 1,801,629 Kenaga Apr. 21, 1931 2,297,241 Perner Sept. 29, 1942 2,313,756 Loose Mar. 16, 1943 2,419,190 Wagoner Apr. 15', 1947 OTHER REFERENCES Iron Age, September 20, 1888, page 42

Claims

1. IN A METHOD OF FORMING AN ADHERENT METALLIC COATING UPON THE SURFACE OF AN ARTICLE OF MAGNESIUM AND ITS ALLOYS, THE STEP WHICH COMPRISES TREATING THE SURFACE OF THE ARTICLE WITH AN ALKALINE AQUEOUS SOLUTION HAVING A PH BETWEEN ABOUT 8 AND 11 AND CONSISTING ESSENTIALLY OF WATER, AN ALKALI METAL PYROPHOSPHATE, AND A WATER-SOLUBLE ZINC SALT SELECTED FROM THE GROUP CONSISTING OF ZINC ACETATE, ZINC BORATE, ZINC BROMIDE, ZINC CHLORIDE, ZINC CYANIDE, ZINC FLUORIDE, ZINC SULFATE, THE CONCENTRATION OF THE PYROPHOSPHATE REDICAL BEING BETWEEN 0.6 AND 27 GRAMS AND THAT OF THE ZINC BEING BETWEEN 0.1 AND 4 GRAMS PER 100 GRAMS OF THE SOLUTION, WHEREBY A COATING OF METALLIC ZINC IS DEPOSITED IN SITU ON THE SURFACE OF THE ARTICLE.