US2512563A

US2512563A - Method of electrolytically coating magnesium and its alloys

Info

Publication number: US2512563A
Application number: US708923A
Authority: US
Inventors: Long Herbert K De
Original assignee: Dow Chemical Co
Current assignee: Dow Chemical Co
Priority date: 1946-11-09
Filing date: 1946-11-09
Publication date: 1950-06-20
Anticipated expiration: 1967-06-20

Description

depending upon the extent to which metal loss or dimensional changes due to the pickling can be tolerated.

Solution A To be used when dimensional changes must be a minimum:

Chromic acid (Cros) pounds-.. 1.5 Sodium nitrate (NaNOs) do 0.25 Magnesium fluoride (MgFz) ounce 0.3

Dissolve in sufficient water to make 1 gallon of solution The oiland grease-free articles to be cleaned are immersed in the solution at 75-85 C. forV Solution B To be used Where small dimensional changes can be tolerated:

Glacial acetic ac'id pint-- 0.9 Sodium nitrate pound 0.25

AWater to -make l gallon of solution The solution is used at room temperature, the articles to be cleaned being immersed therein for a few minutes until cleaned.

Although most articles can be suitably pickled by either of the foregoing pickling solutions with due regard for the amount of metal loss involved, die castings are generally best pickled in solution A for a short time, usually 15 to 30 seconds.

Ordinary fabricated articles of magnesium and alloys which have not received any special treatment and which are free from oil and grease may be satisfactorily prepared for the coating treatment according to the invention by conventional mechanical methods, such as machining, hurling, and sand blasting.

The bath used for carrying out the coating treatment according to the invention may be prepared in various Ways depending upon the choice of ingredients, and the pH adjusted to between about 10 and l12 by the addition, if necessary, oi an alkaligsuch as sodium hydroxide. In the preferred embodiment of the invention, a borate, silicate, and phenol are used in aqueous solution at a pH between about l and 12. An example of a formulation including the aforementioned types of ingredients and which is illustrative of a preferred practice of the invention is the following:

In compounding this solution, a portion of the water needed may be placed in a tank and the requisite amount of phenol and sodium hydroxide added with vigorous agitation, then the sodium meta borate and sodium meta silicate are added together with suiiicient additional water to make up the required proportions. Sodium phenate may be used in place of phenol and sodium hydroxide. The tank may be of any suitable material, e. g. magnesium.

In operating the bath, its temperature is maintained preferably between about '70 and 85 F., although somewhat higher or lower temperatures may be used. A water coil may be used in the tank to control the temperature, if necessary.

The articles to be coated (after suitable preparation as described if cleaning be required) are suspended in the solution in the tank. The articles are connected to one lead of a source of alternating current as the electrolyzing potential, the other lead being connected to the solution through a magnesium electrode immersed in the solution or preferably through additional articles to be coated which are also suspended in the solution so as to function as the other electrode. In using two lots of articles as the pair of electrodes, it is desirable to match their surface areas, at least approximately, so that the current density of the one electrode (or lot of articles) will be equal to the current density of 'the other electrode (or lots of articles) at least approximately. The A. C. source should be capable of supplying up to about 150 volts in gradual incrementsf'iso that both the current passing through the Work and the voltage` across the electrodes may be subject to appropriate regulation. A.

After immersingin the bath the articles to be coated, the electrolyzing potential is applied beginning with a4 low voltage, i. e. in the range 0 to 35 volts, so as to produce a current flow through the work equal to from about 10 to l5 amperes per square foot of exposed surface of the articles connected to one of the two leads of the source of electrolyzing current. In the beginning of the electrolysis, the resistance to the passage of current is small so thatl a fairly small voltage suiiices to produce the required current density, Once the current begins to ow an electrically resistant coating begins to form on the articles and becomes increasingly resistant to current ow as `the electrolysis is continued. Thus, in order to maintain the required current density as the coating is forming, the voltage applied to theelectrodes is increased in accordance with the increase in resistance so that a more or less constant current flow is maintained (preferably in the range of about 10 to 15 amperes per square foot of exposed electrode area as aforesaid) until the desired coating is obtained.

This may be ascertained in various ways, as by xing the duration of the electrolyzing treatment. For example, the treatment may be allowed to run for from about 5 to 20 minutes. In the shorter time ofV the foregoing range, the

Vcoating obtained is relatively thin and is useful where only a light coating is required. In the middle of the range, usually the lmost desirable coatings are obtained. In the longer time of the range, the coatings tend to deteriorate. Another Way to determine duration of the treatment is to terminate the electrolysis when the voltage required to maintain the proper current density reaches the range of to 150 volts. Undue prolongation of the electrolysis results in a deterioration of the coating, as alreadi7 indicated, and this fact may be used also as a guide in determining when the electrolysis should be discontinued. Periodic examination of the coating as it forms' will reveal when its maximum hardness and abrasion resistance has been attained. It is usually preferable to end the electrolysis at a voltage of about to 120. After the electrolysis treatment has run for the desiredlength of time,"the current is cut o and 5 then the articles are removed from the solution and rinsed with water, preferably heated to near boiling, so as to facilitate subsequent drying of the rinsed articles.

Although the foregoing example is typical of a preferred bath composition in which to perform the electrolysis, other proportions and ingredients may be used withinthe purview of the invention. In general, the concentration of the borate may be from about 80 grams per liter up to and including saturating amounts, the concentration of the silicate between about 5 and 100 grams per liter, and the concentration of the organic compound between about 1 and 50 grams per liter. The preferred proportions are respectively 150 to 300 grams per liter, l0 to 50 grams per liter, and 5 to 20 grams per liter.

While the bath may be operated without either the silicate or the organic compound, the presence of silicate increases the hardness of the coating, while the presence of the organic compound permits higher voltages to be attained for the same current density and more uniform, dense, and harder coatings result.

The following table sets forth further examples of the practice of the invention and the abrasion resistance of the coatings produced, the abrasion resistance being measured by the Haueisen abrasimeter. In using this device, the abrasive was 200 mesh aloxite and the air pressure 12 ounces per square inch.

diethylene glycol, mannitol, phenol, sodium phenate, the pH of the solution being between about 10 and l2 and the electrolyzing potential being raised gradually from an initial value of between and about 35 volts so as to produce a current density between about and 40 amperes per square foot of surface of the article during the electrolysis and discontinuing the electrolysis when the appiied voltage reaches a value between 80 and 150.

2. The method of producing an abrasion resistant coating upon an uncoated article 'of magnesium and its alloys containing at least 80 per cent of magnesium which comprises anodically electrolyzng the article in an aqueous solution the dissolved constituents of which consist essentially of an alkali metal borate in amount between 150 and 300 grams per liter, an alkali metal silicate in amount between and 50 grams per liter, and from 5 to 20 grams per liter of a water-soluble organic compound selected from the group consisting of methyl alcohol, ethyl alcohol, ethylene glycol, diethylene glycol, mannitol, phenol, sodium phenate, the pH of the solution being between about 10 and 12 and the electrolyzing potential being raised gradually from an initial value of between 0 and about 35 volts so as to produce a current density between about 5 and 40 amperes per square foot of surface of the article during the electrolysis and Table Bath Composition: Grams per 100 Grams of Bath (Balance Being Water) Example Voltage Abrasicn N o, Organic Compound *md I-Ileslcse NazBOzAHzO NanSOaHzO Kind Amount 100 12. 5 24 5 100 19. 4 24 7 sod1um phenate 0.5 110 30.7 24 5 I 4 110 36. 1 24 5 9. 7 120 40. 5 24 5 0.5 140 48. 9 24 5 2 130 84. 1 24 5 l 130 104. 5 24 5 2 140 112. 7

Referring to the table, it will be observed that columns 2 to 5, inclusive, give the composition (in weight per cent) of the aqueous solution or bath in which the magnesium alloy articles were anodically electrolyzed. The duration of the electrolysis in each example was 10 minutes and the voltage was applied gradually at a constant rate of increase from 0 at the beginning of the electrolysis until the electrolysis was terminated at the voltage set forth in column 6. Each of the coatings obtained was hard, light grey in color, uniform in appearance and possessed the abrasion resistance set forth in column 7.

I claim:

1. The method of producing an abrasion resistant coating upon an uncoated article of magnesium and its alloys containing at least 80 per cent of magnesium which comprises anodically electrolyzing the article in an aqueous solution the dissolved constituents of which consist essentially of an alkali metal borate in amount between 80 grams per liter and that producing saturation, an alkali metal silicate in amount between 5 and 100 grams per liter, and from 1 to grams per liter of a water-soluble organic compound selected from the group consisting of methyl alcohol, ethyl alcohol, ethylene glycol,

discontinuing the electrolysis when the applied voltage reaches value betwPfn and 150.

HERBERT K. DE LONG.

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

UNITED STATES PATENTS OTHER REFERENCES Hackhs Chemical Dictionary, 2d edition, 1937, page 737.

Ser. No. 268,694, Berthier (A. P. C.), published June 8, 1943.

Claims

1. THE METHOD OF PRODUCING AN ABRASION RESISTANT COAING UPON AN UNCOATED ARTICLE OF MAGNESIUM AND ITS ALLOYS CONTAINING AT LEAST 80 PER CENT OF MAGNESIUM WHICH COMPRISES ANODICALLY ELECTROLYZING THE ARTICLE IN AN AQUEOUS SOLUTION THE DISSOLVED CONSTITUENTS OF WHICH CONSIST ESSENTIALLY OF AN ALKALI METAL BORATE IN AMOUNT BETWEEN 80 GRAMS PER LITER AND THAT PRODUCING SATURATION, AN ALKALI METAL SILICATE IN AMOUNT BETWEEN 5 AND 100 GRAMS PER LITER, AND FROM 1 TO 50 GRAMS PER LITER OF A WATER-SOLUBLE ORGANIC COMPOUND SELECTED FROM THE GROUP CONSISTING OF METHYL ALCOHOL, ETHYL ALCOHOL, ETHYLENE GLYCOL, DIETHYLENE GLYCOL, MANNITOL, PHENOL, SODIUM PHENATE, THE PH OF THE SOLUTION BEING BETWEEN ABOUT 10 AND 12 AND THE ELECTROLYZING POTENTIAL BEING RAISED GRADUALLY FROM AN INITIAL VALUE OF BETWEEN 0 AND ABOUT 35 VOLTS SO AS TO PRODUCE A CURRENT DENSITY BETWEEN ABOUT 5 AND 40 AMPERES PER SQUARE FOOT OF SURFACE OF THE ARTICLE DURING THE ELECTROLYSIS AND DISCONTINUING THE ELECTROLYSIS WHEN THE APPLIED VOLTAGE REACHES A VALUE BETWEEN 80 AND 150.