US2901409A

US2901409A - Anodizing magnesium

Info

Publication number: US2901409A
Application number: US602059A
Authority: US
Inventors: Long Herbert K De
Original assignee: Dow Chemical Co
Current assignee: Dow Chemical Co
Priority date: 1956-08-03
Filing date: 1956-08-03
Publication date: 1959-08-25
Anticipated expiration: 1976-08-25

Description

United States Patent'C) o 9 Claims. (Cl. 204-56) The invention relates to baths for and method of treating light metal articles particularly those of magnesium and the magnesium-base alloys to produce thereon a coating which affords protection against corrosion. It more particularly concerns an improved anodic coating method by which articles of magnesium and the magnesium-base alloys are given a hard strongly adherent coatingaflording protection against corrosion. The term magnesiumbase alloy used herein means an alloy of magnesium in which the magnesium content is at least 80 percent by weight.

Magnesium and the magnesium base alloys, like other structural metals, oftentimes are subjected to'cor-rosive conditions in use and numerous methods of treatment have been proposed heretofore in efforts to provide the surface of these metals with corrosion resistant coatings to prevent the underlying metal from being attacked. Yet, in spite of these efforts, no highly corrosion resistant coatings have become commercially available. As a consequence, it is a desideratum of the art to provide coatings for magnesium and its alloys having improved resistance to corrosion in comparison with available coatings. Accordingly, it is an object of the invention to provide a bath for and method of coating magnesium and its bath, there is dissolved in the bath Patented Aug. 25, 1959 the bath or by the addition of a strong mineral acid not involving the foregoing radicals, for example, sulfuric acid, nitric acid, or hydrochloric acid. However, it is generally preferable to rely upon the chemical compounds comprising the aforesaid radicals, which are used in. formulating the bath, to render the bath sufliciently acid, as will become apparent on considering the various com binations of chemical compounds which are suitable for the purpose as in the following examples of such compounds.

For supplying the fluorine or fluoride radical ofthe a suitable soluble fluoride. Since the bath must also contain sodium as well as ammonium, it is advantageous to use either sodium fluoride or ammonium fluoride since these chemical com.

pounds contribute either sodium or ammonium as the case may be as well as the fluoride radical. The fluorides used remaining constituents of the bath.

advantageously maybe in the form of acid salts, for ex ample, NaFHF and NH FHF, which can serve to acidify the bath, as indicated above, as well as, provide the desired fluoride radical. Hydrofluoric acid may be used to provide-the fluoride radical. Insofar as the fluoride radical is concerned, its concentration should be at least 6 percent by weight of the bath and may be as high as that producing a saturated solution in the presence of the In this connection,

it will be understood that two or more fluorides may be alloys which fulfills the foregoing need. Other objects,

tained in a short time. The coatings are nominally of a tan to an olive green color. In addition tostrongly ,resisting corrosion, as from the atmosphere and salt water, the

coatings are strongly adherent, hard, and resistant to (lam age by abrasion. The coatings also form an advantageous base on which to apply paint, varnish, enamel, and lacquer 5 since the coatings are free from the alkali, usually present in conventional protective coatings applied to magnesiumbase alloys, which contributes to premature paint failures.

The invention then consists of the improved anodizing bath composition and method of anodizing articles of, magnesium and the magnesium-base alloys herein fully described and particularly pointed out in the claims.

- In carrying outthe invention, the articles to be coated are anodized in a bath which consists of a slightly acid aqueous solution, the'pH being not over 4 but preferably about 0.5 to 2, containing the radicals ammonium, fluoride, Phosphate, sodium, and hexavalentchrornium in solution. The necessary acidity may be derived generally from the chemical compounds which are dissolved. in the,

bath to contribute to it the essential radicals referred to above, namely, ammonium, fluorine, phosphate, sodium, and hexavalent chromium, as when acids or acid salts having the foregoing radicals in combination are used in used at the same time to achieve the desired fluoride concentration as for example by using both sodium fluoride (or bifluoride) and ammonium fluoride (or bifluon'de). The maximum concentration of the fluoride radical which can be attained is largely limited by the necessary pres ence of the sodium radical which brings about precipitation of sodium fluoride when its solubility is exceeded. Undissolved fluoride may be present in admixture with the bath without detriment, although it is pr eferable to use only suflicient fluoride in the presence of the dissolved sodium radical to provide at least 6 percent of dissolved fluoride without attaining, saturation lwith .,res.p.ect to. I

sodium or other fluoride. Forsupplying the sodium radical, there is dissolved in the bath a suitable soluble sodium compound, for

example Na SO although thesodium compound used may also at the same time provide a portion of the desired fluoride content of the bath, as already indicated, as when the sodium salt used is sodium fluoride, or a portion of the desired hexavalent chromium con tent as when the sodium salt used comprisesa soluble, hexavalent chromium compound or a portion or all the desired phosphate content as when the sodium salt. used comprises a soluble phosphate. The following sodium compounds in addition to those already mentioned may be used appropriately to provide all or a :part of the "necessary sodium radical: Na Cr O ZH O,

Na P O- NilzHzPgOq-6H20, NaHFz, NaNI-I HPO .4H 0, N3.H2PO4.H20, Na HPO .7I-I O, Nagclo Na CO NaOH. The sodium content of the bath should exceed:- about 0.3 percent and maybe as high as 3 or 5 percent by weight. In the lower ranges of the sodium content, it is apparent that if the sodium is derived from sodium fluo ride additional fluoride would need to be supplied by some other soluble fluoride to produce a sutficient fluoride concentration in the bath to meet the minimum fluoride requirement of 6 percent. 1 Since the bath must also contain the ammonium radical, thewfluoride may advantageously be added as the: normal or acid ammonium fluoride and thereby furnish part. or all of the fluoride requirement as well as part or all of the NH, requirement which is to be at least 3.2 percent and maybe as much as 15 percentby weightf'of the bath. Other ammonium compounds 'may;bie used for supplying the necessary ammonium radical, for example, those which furnish other needed radicals besides NH such as the ammonium phosphates and the ammonium chromates. Ammonium hydroxide may be used as the ammonium source in which case it becomes acidified in the bath in view of the necessity to maintain the pH of the bath between 0.5 and 4.

The phosphate content of the bath calculated as P is to be between 2 and 10 percent by weight and may be derived from the alkali metal and ammonium orthoand pyrophosphates and orthoand pyrophosphoric acids, and, as already mentioned, some of these phosphates may contribute some or all of the required amount of the sodium radical as well as more or less of the required NH Among the phosphates which may be used as the source of phosphates are: H PO H4P2'O'1, NaH P O NH4H2PO4, (NH4)3PO4, NB3PO4, and N212PO4, and NaH PO The hexavalent (sexivalent) chromium content of the bath, which is to be 0.3 to percent by weight of the bath, may be derived from. any soluble hexavalent chrom-ium compound, as for example, Nflgcrzoq, CrO Na CrO and (NH CrO As in the case of the other compounds used in the bath, the hexavalent chromium compounds used may also furnish part or all of the sodium and a part of the ammonium radical requirements of the bath.

In anodizing articles of magnesium and the magnesium-base alloys in accordance with the invention, the anodizing bath prepared as above described, so as to contain the desired concentration of each of the radicals NH F, P0 Na, and Cr, and a PH of 0.5 to 4, is maintained at a temperature between 150 F. and the boiling point of the bath during the anodization. A preferred operating temperature is 155 to 165 F. The article to be anodized is immersed in the bath which is then electrolyzed using the article as the anode. A.C. or D.C. may be used, A.C. being preferred, although its use requires about 30 percent more time to complete the coating than D0.

form at applied voltages less than those producing a characteristic sparking or arcing phenomenon which occurs between the surface of the article and the bath when the voltage attained during anodization exceeds about 70 to 75 volts. Accordingly, as the anodization proceeds, the applied potential is progressively increased to and beyond the sparking voltage and thereafter is maintained at a value which continuously produces a substantial current flow with sparking until the desired thickness of coating is obtained. After the sparking voltage is attained, the color changes from grey to tan for:

light thickness of the coating and finally to an olivegreen for the thicker coatings. Effective thicknesses of coating are 0.0012 to 0.0015 inch, but other thicknesses may be produced. The voltage thus applied need not exceed about 120 volts. The magnesium-base alloys containing a relatively small amount, for example, less than 5 percent of non-magnesium metal require the appli-' cation of a higher voltage than the magnesiunnbase alloys containing a relatively higher content, for example,-

10 percent of non-magnesium metal. With these alloys,

good coatings are obtained with to volts. The du-- ration of the anodization after the sparking voltage is reached is not sharply critical and may be from 5 to 30' minutes or more depending upon the current density used and the coating thickness desired. coatings have been obtained using from 250 to 350 ampere minutes per square foot of surface anodized with current densities of upwards of 2 amperes per square foot of anodized surface. A larger number of ampere minutes, for example 600, does not appear to yield more The voltage applied to the article is determined by 40 corrosion resistant coatings. For example, there may Table I Anodizing Bath Composition Operating Conditions Compounds Used Radicals: Calculated Total Wt. Percent BathNo. I. H 7 Current Final pH Density, Time, Voltage Temp., Wt. PerampJ Min. Attained F.

cent Formula NH; F P0 N a Or sq. ft.

20 Nrnnri. 2 10 NaNH4HPO .4HiO 7.1 18. 3 4. 5 2. 3 2.8 2 20 15 13.0.--

8 NfliCl20L2H20--- 20 HF 3 g 7.1 13.3 5.77 2.3 2.8 1-2 20 15 100 D.C..- 160 20 4 7 6 3 16. 6 5. 77 1.85 2. 5 2 20 20 100 DD.-- 180 3 20 7 1g 6. 3 13.3 8.2 1.2 2.8 2 20 20 90 D.O..-. 160

A.C.=A1ternating current of 60 cycles. D.O.=Direct current.

Satisfactory be used a'current density of 15 amperes per square foot for 20 minutes. This time includes the time taken to reach the voltage at which sparking occurs (that is 70 to 75 volts) which is usually not more than one to three minutes at such current densities and the balance of the time is taken up in the anodization while sparking during which the hard corrosion resistant coating of the invention is produced.

The practice of the invention is illustrated in the exainples set forth in TableI showing various anodizing bath compositions and conditions under which anodizations in accordance with the invention were made.

In Table I, the anodizations with each bath were made with test panels of a magnesium-base alloy sheet 0.04 inch thick having the nominal composition of 3 percent Al, 1 percent Zn, 0.3 percent Mn, the balance magnesium. The panels anodized in baths 1 to 7, inclusive, were pickled before anodization in 7 percent aqueous nitric acid solution for 30 seconds at 76 F. The panels used in baths 8 and 10, inclusive, were anodized without a previous cleaning. In each instance, the panels on being anodized developed a uniform coating of more or less olive green color which was hard and strongly resistant to corrosion as determined by a salt solution spray test. In testing the corrosion resistance of the panels, they were placed in a spray chamber with one side facing upward at an angle of 15 percent to the horizontal and subjected to a spray of 20 percent sodium chloride solution (water solution) at 95 F. produced by air atomization of the solution onto the panels. The spraying was continued for 200 hours. The top face of each sprayed panel was examined to determine the proportion of the area, if any, which was attacked by the spray after 24 hours and after 200 hours of the spray treatment. The results of these examinations are set forth in Table II.

Table 11 Salt Spray Corrosion Test: Percent of Area Bath No. Attack at 95 F.

24 hours 200 hours 1 1 25 2 none 8 3 none 4 4 none 6 5 12 8 none 1 none 7 9 a t 10 none 1 1 Not observed.

Among the advantages of the invention are that the coating obtained is uniform in appearance and hides the underlying metal; the coating is hard and strongly resists abrasion; the coating possesses extreme resistance to corrosion; the bath effectively cleans the work as it is being anodized, thereby obviating the need for the usual pickling prior to the anodization; the coating provides an enduring base for paint, varnish, lacquer, and enamel.

This application is a continuation-in-part of my copending application Serial No. 332,495, filed January 21, 1953 (now abandoned).

I claim:

1. An anodizing bath consisting of an aqueous solu tion of water-soluble inorganic compounds yielding in the equeous solution the radicals: ammonium, fluoride, phosphate, sodium, and hexavalent chromium, the amount by weight of the ammonium radical being between 3.2 and 15 percent, that of the fluoride radical being at least 6 percent, that of the phosphate radical calculated as P being between 2 and 10 percent, that of the sodium radical being between 0.3 and percent and that of the hexavalent chromium radical being between about i. lected from the group consisting of the orthoand pyrophosphates.

3. An anodizing bath consisting of a solution in water of ammonium bifiu'ciride, a sodium phosphate selected from the group consisting of the orthoand pyr'ophosphates, and a soluble hexavalent chromium compound, in proportions such as to provide in the bath from 3.2 to 15 percent of the ammonium radical, at least 6 percent of the fluoride radical, from 2 to 10 percent of the phosphate radical calculated as P0 from 0.3 to 5 percent of the sodium radical, and from 0.3 to 5 percent of hexavalent chromium, and sufiicient mineral acid to give the solution a pH of 0.5 to 4.

4. A bath according to claim 3 in which the sodium phosphate is sodium dihydrogen orthophosphate.

5. A bath according to claim 3 in which the sodium phosphate is sodium dihydrogen pyrophosphate.

6. An anodizing bath consisting of a solution in water of chemical compounds consisting of ammonium fluoride, a phosphate selected from the group consisting of orthoand pyrophosphoric acid, the sodium, ammonium, hydrogen, and dihydrogen phosphates and pyrophosphates; and a hexavalent chromium compound selected from the group consisting of chromic acid. and the sodium chromates and dischromates; the amount of the said chemical compounds being sufficient to produce in the bath a sodium radical concentration of 0.3 to 5 percent, an ammonium radical concentration of 31.2 to 15 percent, a fluoride radical concentration of at least 6 per cent, a phosphate radical concentration calculated as P0,; of between 2 and 10 percent, and a hexavalent chromium radical concentration of between 0.3 and 5 percent, and a sufficient amount of a mineral acid to give the solution a pH of 0.5 to 4.

7. The method of producing a corrosion resistant coating upon an article of magnesium or the magnesiumbase alloys which comprises anodizing the article in an aqueous solution at a temperature between F. and the boiling point of the solution at a voltage sufficient to produce sparking between the surface of the article and the solution, the solution consisting of water in which is dissolved water-soluble inorganic compounds yielding the radicals ammonium, fluoride, phosphate, sodium, and hexavalent chromium in amount sufiicient to produce in the solution an ammonium concentration between 3.2 and 15 percent, a fluoride concentration of at least 6 percent, a phosphorous concentration calculated as P0 between 2 and 10 percent, a sodium concentration between 0.3 and 5 percent, and a hexavalent chromium concentration between 0.3 and 5 percent, and suificient mineral acid to give the solution a pH of 0.5 to 4 percent, the anodizing being continued for a time sufiicient to give the surface of the article a tan or green color.

8. The method of producing a corrosion resistant coating upon article of magnesium or the magnesium-base alloys which comprises anodizing the article in an aqueous solution at a temperature of at least 150 F. at a voltage suflicient to cause sparking at the surface of the article, the said solution consisting of water in which is dissolved a soluble ammonium compound in amount sufficient to produce a concentration of ammonium of 3.2 to 15 percent, a soluble fluoride in amount sufiicient to produce a concentration of fluoride of at least 6 percent, a soluble phosphate in amount sufiicient to produce a phosphorous concentration calculated as P0 of 2 to 10 percent, a soluble hexavalent chromium compound in amount sufiicient to produce a hexavalent chromium concentration of 0.3 to 5 percent, a soluble sodium compound in amount sufiicient to produce a sodium concentration 7. of '013 to 5 percent; and a suflicient amount of a mineral acid to give the solution a pH of from 0.5 to 4, the anodizing being continued for a time sufficient to give the surface of the article a tan or green color.

9. An anodizing bath consisting of a solution in Water of ammonium bifluoride, orthophosphoric acid and sodium dichroinate, in proportions such as to provide in the bath from 3.2 to 15 percent of the ammonium radical, at least 6 percent of the fluoride radical, from 2 to 10 percent of the phosphate radical, P0 from 0.3 to 5 percent of the sodium radical, from 0.3 to 5 percent of hexavalent chromium, and sufficient mineral acid to give the solution a pH of 0.5 to 4.

References Cited in the file of this patent UNITED STATES PATENTS 2,414,090 'Buzzard Jan. 14, 1947 FOREIGN PATENTS 543,726 Great Britain Mar. 10, 1942 907,465 France July 2, 1945 Dedication 2,901,409.Herbert K. DeLong, Midland, Mich. ANODIZING MAGNE- SIUM. Patent dated Aug. 25, 1959. Dedication filed Aug. 9, 1974, by the assignee, The Dow Ohemz'eal Company. Hereby dedicates to the Public the remaining term of said patent.

[Ofiieial Gazette December 24, 1974.]

Claims

1. AN ANODIZING BATH CONSISTING OF AN AQUEOUS SOLUTION OF WATER-SOLUBLE INORGANIC COMPOUDS YIELDING IN THE AQUEOUS SOLUTION THE RADICALS; AMMONIUM, FLUORIDE, PHOSPHATE, SODIUM, AND HEXAVALENT CHROMIUM, THE AMOUNT BY WEIGHT OF THE AMMONIUM RADICAL BEING BETWEEN 3.2 AND 15 PERCENT, THAT OF THE FLUORIDE RADICAL BEING AT LEAST 6 PERCENT, THAT OF THE PHOSPHATE RADICAL CALCULATED AS PO4 BEING BETWEEN 2 AND 10 PERCENT, THAT OF THE SODIUM RADICAL BEING BETWEEN 0.3 AND 5 PERCENT AND THAT OF THE HEXAVALENT CHROMIUM RADICAL BEING BETWEEN ABOUT 0.3 AND 5 PERCENT, AND SUFFICIENT MINERAL ACID TO GIVE THE SOLUTION A PH BETWEEN 0.5 AND 4.