US2513237A - Method of protection of magnesium and magnesium-base alloys - Google Patents

Description

Patented June 27, 1950 UNITED snares PATENT "OFFICE METHOD .QF PRDTECTION 0F MAGNESIUM AND'MAGNESIUM-BASE :ALLOY S Herbert Manfred Freudiditflean iirasch,

Nanterre, France 'No Drawing. Application Eluly 23, 1941, Serial "No. 763,159. Tn'FranceSeptember 22, 1938 :91Clalms; (o1. zoo-5m The present invention concerns the treatment of objects made of magnesium metal-or alloys composedprincipa-lly (e. .g. "-85% or-more) of magnesium, for forming said objects a or coating which can protect said articles from corrosion. As is we'l1 known, magnesium in the pure state and to a somewhat less extent alloys which contain magnesium associated ialloyed) with small amounts (up to (1 0% or 1'5'%, more or less) of other common metals, e. g. Mn, Zn, A1, rare earth metals, etc, are "prone to become 'oxidizedor corroded by continued exposure to damp air, air containing traces of acid vapors, air-containing moisture and salts, or sea water, river water, lake water, etc, and efforts havedongbeen made to protect such magnesium or "magnesium alloys, from such oxidation and corrosion. The present invention covers such protection.

Hereinafter, the term magnesimn-base metal will be used to cover such metal and alloys-as discussed above.

The objects may be cast, forged, machined, rolled, etc. and maybe of any appropriate sizes and may be completely made, or not, as desired.

Thearticles (unlessalready wholly clean) should 1 first be degreased, cleaned, washed and dried, which steps may be carried out according to proc- 'esses already known.

The present invention is in' part a continuation of my earlier application Ser. No. 297,191, filed Sept. 29, 1939, (which has become abandoned).

The present process involves an alternating current electrolytic treatment, in a bathof proper acidity, consisting =principal'lycf an aqueous solution of manganese dichmmate, :tree zfrom substances capable .of attacking the magnesium in such a way as to "bring any magnesium .eom-

pounds into the coating being formed ontheobjects treated, and/or to bring any .substantial' amount of magnesium compounds .intosolution in the electrolytic bath. .Coatingsof the type set forth herein, which contain magnesium com pounds (oxide or salts) are usually porous and only insecurely attached to .the object treated,

are generally weak and .offer onlyslight protection. This is because MgO is to a slight extent capable of being dissolved by water, especially water containing usual amounts of carbon dioxide in solution. Most so-called" insoluble magnesium salts are *likewise subject to attack by water especia'lly 'water containing usual amounts ofcarbon dioxide inrsolution.

It is important, in sorder to :avoid chemical .atrtack "of the magnesium, with consequent intro- .duction of such magnesium :compoimdszinto 5 eleotrolyteand/or into the coating being formed on the object-to avoid the presence in the electrolytezof any substantial amounts of acids which would chemically attack magnesium, to dissolve ithe'sa-me, such acids as sulphuric, "phosphoric, nitric, hydrochloric, and many of the common organic acids, anysubstantial amounts, thus being excluded *from the electrolyte, as :well :as the salts of such racids. But more slight traces of such acids, even of the strong mineral acids, do 2110 appneciableharm, and hence can .be tolerafted. Hence it :is not necessary, in "making up the electrolyteto-use thesubstances (iCrOaMnCGa, M1102, etc.) .in'amabsolutely chemicallypurestate, :but the amounts of impurities therein, especially sulphates, .:halides,.-etc., should not begreat. The definite addition of substantial amounts of sulphates, phosphates, nitrates, halides, etc, of courseis avoided.

.It is of :course understood that while I preferably use "two articles or objects of magnesium- :base metal as the two electrodes, and these two articles .are preferably alike and have equal surface areas, it is permissible to usesuch a magnesium object as one electrode with an insoluble electrode of electrically conductive material such as carbon, with the production of equally good results. i

In carrying .outthe process, the electrolyte is 'a solution of manganese dichromate, MnCrzOq. This solution should have a pH between 2 and '5, and preferably between 3 and 4. A pH of close to 3:5 is preferable and produces the best results.

solutionis preferably made by dissolving 10 to 100 grams of chrome anhydride, *CrOz in a liter of water, and adding manganese carbonate thereto,- to give the desired pH. The CO2 of the carbonate is liberated into the atmosphere. Hence the carbonic acid does not act to injure themagnesiu-m. 'Manganese' dioxide, preferably in a freshly precipitated state can be similarly dissolved in the chromic acid solution, preferably heated. A part of the oxygen content of-the 'dioxide is liberated during the reaction. Manganese oxalate can be used in a similar manner, this also liberating CO2 (chromic acid being a strong oxidizer). In this latter case, solution is preferably eiiected, =hot. Mixtures of the manganese compounds can also he used. Manganese carbonate is the preferred compound to use.

Manganese carbonate, MnCCDa is readily obtainable, is relatively :cheap sand is easily and promptly \dissolved rbyachromic acid solutions and the action of the .chromic acid "thereupon can substantially 10 about 15 minutes.

.10 range 5 to 7 parts (preferably 5.5 tov 6.5,) of.man-

ganese carbonate to 10 parts Ci'Os. This-same proportion (2.4 to 3.4 parts of the element manganese to 10 parts CIOs, and preferably 2.6't0f 3.1 parts of the element manganese to 10' parts CI'Os) will be observed using the other manganese. compounds named above as the source of.,man-

ganese ions.

Normal manganese chromate MnC'rOris not readily soluble in water. Manganese dichromate MnCIzOv is sufficiently soluble. It will be'under stood that the ratio of manganese compound to C10; as set forth herein is about the proper proportion for the formation of the MnCrzO-z, with perhaps'a slight excess of CrO3. The electrolyte may contain between 2.66% and 12% (e. g. 4% to l0%)' of manganese bichromate (based'on the whole electrolyte). Without limiting the invention' to the details, I- give thefollowing' examplesr Two electrodes (each a machined plate-like object of magnesium metal), of approximately equal size and equal surface area are immersed in a bath made from 7.5'parts CIO3 and 4.7 parts MIlCOa in .100 parts of solution. This gives a solution of manganese bichromate, MnCrzOv (perhaps combined with some water). The two electrodes were connected to the A. C. circuit, at 15 volts, and an alternating current of 4 amperes per square decimeter was allowed to flow through the cell for The electrolyte was at room temperature (about l8-20 0.), throughout said treatment. At the end of this period, the electrodes were, removed from the solution, wellwashed in running water, dried and examined. It was found that the objects had a strongly adhering deposit covering their surfaces, which was dark brown, nearly black in color, composed of CrQz and MmO4. This coating offered good pro- .-It .is also possible to somewhat vary the proportion of MnCOs and C1O3 used in making the electrolyte, to give pH values between 2 and 5.

The coatings are less good as the pH of the electrolyte is above or below the optimum of 3.5. With lower pH, the coatings are somewhat more porous. This can be somewhat offsetby agood painting. The coatings can also be somewhat improved by a subsequent boiling in sodium silicate solution or by treatment in hot molten parafiin, as shown in my copending patent applications.

The strength-of the alternating current can vary between 1.5 and 12 amperes at a voltage between'4 and 40 volts. The duration of the .A. C. electrolytic treatment can vary substantially. A treatment for 15 to minutes is usually sufiicient to give a sufficiently thick coating.-

I also give the following specific examples which have shown favorable results.

(a) gr. CrOa, 65 gr. 1VII1CO3, 1 liter of Water. This gives a substantially 12% solution of manganese bichromate. A. C. treatment at 15 volts, 4 amperes per square decimeter of surface of magnesiumor, magnesium wbase alloy object treated, continued 15 minutes at 15-20 C.

(b) 20 gr. CrOa, 12 gr. MnCOa, 1 liter of water. This gives a substantially 2.66% solution of, manganesebichromate. A. C. at 18 volts, 3 amperes, per s q. decimeter of Mg. surface, continued 15 20 minutes at 15 to 20 C.

Iclairn 1. A process of protecting magnesium-base metalobjects in an electrolyte which consists essentially of; an. aqueous solution which is the liquid reaction product made by dissolving at least one substance selected from the group consisting of manganese carbonate, manganese dioxide and manganese oxalate, which substance contains from. about 2.4 to about 3.4 parts by weight of the element manganese, in an aqueous solution of chromic acid alone, containing the equivalent of10 parts by weight of chromic anhydride in about 100 to 1,000 partsby weight of water, and

applying saidliquid reaction product, while the latter is free from more than traces of acids which are capable'of chemically attacking metallic magnesium and dissolving same, and free from more than traces of salts of'such acids, and while such solution has a pH value between about 2 and about ;5, as an electrolytic bath, in which is placed a pair of electrodes, at least one of which is such a magnesium-base -metal object and the other electrode being. formed of an electrically conductive material which is insoluble in said electrolyte, and. applying to such electrodes an alternating electric current of between 1.5 and 12 amperes per square decimeter of surface of said magnesium-base metal object being treated, at a voltagebetween 4 and 40 volts, at about atmospheric temperature, and continuing the treatment, until a good protective film substantially ,free of magnesium compounds has been formed on said magnesium object.

2. ,A process of protecting magnesium-base metal objects which comprises dissolving a substance. selected from the. group consisting of manganese carbonate, manganese dioxide and manganese oxalate, which substance contains about 2.6 toabout 3.1 parts of the element manganese, in an aqueous solution of chromic acid -alone,.containing the equivalent of 10 parts of chromic anhydride, and applying said solution, while the latter is free from more than traces of acids which are capable of chemically attacking metallic magnesium and dissolving same, and free from more than traces of salts of such acids, andwhile such solution has a pH value of about '3.5,' as an electrolytic bath in which is placed a pair electrodes, at least one of which is such a magnesium-base metal object and the other electrodebeing'formed of an electrically conductive materialwhich is insolube in said electrolyte, and applying to such electrodes an alternating electric current of about 4 amperes' per square decimeter oi surface of said magnesium-base metal object being treated, at a voltage of between 10 and 20 volts, at about room temperature, and continuing the treatment until a good protective filmhas been formed on said object.

' 3. A'proce ss as claimed in claim 1 in which a pair of said magnesium-base objects, having about equal surface areas, constitute the two electrodes in said electrolytic bath.

4. A process which comprises applying an alternating electric current to an electrolytic cell containing a pair of electrodes in an electrolyte, at least one of said electrodes being an object composed of magnesium-base metal, and the other electrode being an electrically conductive material which is insoluble in said electrolyte, and in which said electrolyte consists essentially of an aqueous solution of manganese dichromate having a pH value between about 2 and about 5.

which solution is free from more than traces of acids which would be capable of chemically at tacking metallic magnesium and dissolving same, and free from more than traces of salts of such acids, the said alternatin electric current being between about 1.5 and 12 amperes per square decimeter of surface of said magnesium-base metal object being treated, and at a voltage between about 4 and about 40 volts, and -continuing said treatment until a good protective film has been formed on said object.

5. A process as in claim 4, in which the pH of the electrolyte is about 3.5.

6. For protecting magnesium base articles against corrosion, the step of subjectingsuch an article, as an electrode, to alternating current electrolysis in a bath consisting essentially of an aqueous solution of manganese bichromate, and which bath has a pH between 3 and 4, which bath is substantially free of acids which can chemically attack metallic magnesium and substantially free of the salts of such acids.

7. For protecting magnesium base. articles against corrosion, the step of subjecting such an.

article, as an electrode, to alternating current electrolysis in a bath consisting essentially of an aqueous solution of manganese bichromate of about 4% to about 10% concentration, and which 8. For protecting magnesium base articles against corrosion, the step of subjecting such an article, as an electrode, to alternating current electrolysis in a bath consisting essentially of an aqueous solution of manganese bichromate of about 2.66% to'about 12% concentration, and which bath has a pH between 3 and 4, which bath is substantially free of acids which can chemically attack magnesium and substantially free of the salts of such acids.

9. In protecting magnesiumbase metal articles, by A. C. electrolysis in an acid chromate solution, the steps of forming the electrolyte by heat ing maganese oxalate in-a liquid consisting essentially of water and chromic acid until carbon dioxide is liberated, the ratio of chromic acid to manganese oxalate being such as to leave an aqueous solution containing manganese bichromate, and applying alternating electric current to electrodes, at least one of which is magnesium-base metal, while immersed in said aqueous solution as electrolyte.

HERBERT MANFRED FREUD on: JEAN FRASCH.

REFERENCES CITED The following references file of this patent:

UNITED STATES PATENTS are of record in the OTHER REFERENCES Metaux & Corrosion, vol. 14, No. 166 (June 1939), pages 83 through 87.

Claims (1)

1. A PROCESS OF PROTECTING MAGNESIUM-BASE METAL OBJECTS IN AN ELECTROLYTE WHICH CONSISTS EXSENTIALLY OF AN AQUEOUS SOLUTION WHICH IS THE LIQUID REACTION PRODUCT MADE BY DISSOLVING AT LEAST ONE SUBSTANCE SELECTED FROM THE GROUP CONSISTING OF MANGAESE CARBONATE, MANGANESE DIOXIDE AND MANGANESE OXALATE, WHICH SUBSTANCE CONTAINS FROM ABOUT 2.4 TO ABOUT 3.4 PARTS BY WIEGHT OF THE ELEMENT MANGANESE, IN AN AQUEOUS SOLUTION OF CHROMIC ACID ALONE, CONTAINING THE EQUIVALENT OF 10 PARTS BY WEIGHT OF CHROMIC ANHYDRIDE IN ABOUT 100 TO 1,000 PARTS BY WEIGHT OF WATER, AND APPLYING SAID LIQUID REACTION PRODUCT, WHILE THE LATTER IS FREE FROM MORE THAN TRACES OF ACIDS WHICH ARE CAPABLE OF CHEMICALLY ATTACKING METALLIC MAGNESIUM AND DISSOLVING SAME, AND FREE FROM MORE THAN TRACES OF SALTS OF SUCH ACIDS, AND WHILE SUCH SOLUTION HAS A PH VALUE BETWEEN ABOUT 2 AND ABOUT 5, AS AN ELECTROLYTIC BATH, IN WHICH IS PLACED A PAIR OF ELECTRODES, AT LEAST ONE OF WHICH IS SUCH A MAGNESIUM-BASE METAL OBJECT AND THE OTHER ELECTRODE BEING FORMED OF AN ELECTRICALLY CONDUCTIVE MATERIAL WHICH IS INSOLUBLE IN SLAID ELECTROLYTE, AND APPLYING TO SUCH ELECTRODES AN ALTERNATING ELECTRIC CURRENT OF BETWEEN 1.5 AND 12 AMPERES PER SQUARE DECIMETER OF SURFACE OF SAID MAGNESIUM-BASE METAL OBJECT BEING TREATED, AT A VOLTAGE BETWEEN 4 AND 40 VOLTS, AT ABOUT ATMOSPHERIC TEMPERATURE, AND CONTINUING THE TREATMENT UNTIL A GOOD PROTECTIVE FILM SUBSTANTIALLY FREE OF MAGNESIUM COMPOUNDS HAS BEEN FORMED ON SLAID MAGNESIUM OBJECT.