US2313756A - Method of electroplating magnesium - Google Patents

Description

Patented Mar. 16, 1943 METHOD OF ELECTROPLATING MAGNESIUM William S. Loose, Midland, Mich, assignor to The Dow Chemical Company, Midland, Mich., a corporation of Michigan No Drawing. Application March 1, 1939, Serial No. 259,287

2 Claims.

The invention relates to methods of electroplating metals. It more particularly concerns an improved electroplating bath for electroplating metals and a method of producing electroplated metal coatings, particularly upon readily oxidizable metals such as aluminum, magnesium,

zinc, and alloys thereof in which an aforesaid metal as predominant constituent.

Aluminum, zinc, and their alloys are diflicult, and magnesium and its alloys practically impossible, to electroplate with an aqueous electroplatingbath apparently because such metals are relatively high in the electromotive series and tend to enter solution or form oxidic coatings whileimmersed in the bath, preventing the electrodeposited metal from properly adhering to the article being plated. Articles of magnesium and its alloys, for example, cannot be plated by known aqueous electroplating baths which are suitable for heavy metals because of the tendency for the magnesium of the article to readily dissolve in such baths and also becausemagnesium and its alloys spontaneously form an oxidic coating when immersed in aqueous baths. if these be slightly alkaline. In acid plating baths the oxide does not form, but instead the metal continuously passes into solution during the immersion. As a result, no commercially useful electrodepositcd coating can be obtained.

The principal object of the invention is to provide a method of producing electroplated coatings upon readily oxidizable metals. A particular object isto provide a method of electroplating magnesium and its alloys. v

Other objects and advantages will become apparent as the description proceeds.

I have discovered that by first suitably etching the article of readily oxidizable metal to be plated and then subjecting it to the action of an aqueous solution containing the ions of the metal to be deposited, together with a soluble fluorine componentyand"applying a suitable potential to t material, is cleaned, it necessary, of adhering dirt, scale-oxide,- or grease in conventional manneras by wire brushing and subsequent treatment in alkaline detergent solutions such as those of sodlumcarbonate and sodium phosphate, or organic degreasing solvents such as carbon tetrachloride.

After the article has been cleaned, it is then ready to be treated according to the first step in the process. the action of an etchant so as to produce a slightly roughened or pitted surface which serves to hold in place the electroplated deposit to .be subsequently produced. Apparently the effective etchants are those which act mainly on the grain boundaries of the metal crystals Various methods may be employed to produce the necessary etch, as by immersing the article in a pickling bath containing an agent which dissolves the metal, together with an agentwhich tends to produce a protective coating upon the metal. 'The. protective coating which forms during the treatment does not completely cover the metal, but instead permits the etching agent to act largely upon the uncovered portions (these presumably being the grain boundaries), producing a slight pitting eifect. After the article has been so treated, it is then, if necessary, subjected to a further treatment to remove any coating which may have been deposited during the etching step. If desired, etching solutions may be used which produce an etch without leavmg a protective coating which has to be removed, obviating the necessity for further treatment before electroplating. If the etching solution produces a protective coating, however, it is usually necessary to follow such etching treatment with another treatment in a fairly concentrated aqueous solution of chromic acid or similar agent to remove the film or protective coating produced.

The composition of the etchingbath to use may be varied to suit the metal article to be treated. The following are illustrative examples of suitable etching solutions to employ:

Example 1Etchant for magnesium, zinc and their alloys A relatively dilute aqueous solution containing from about 0.1 to 2 percent of hydrofluoric acid.

Etch in the solution for l to 10 minutes at room temperature.

In this the article is subjected to.

Example 3]Etchant for magnesium and. its

- alloys Etch for 30 minutes in a boiling 10 per cent aqueous solution of a soluble chromate salt such as sodium dichromate.

Example !-Eichan't for magnesium and its H jalloils Etch roiabout 5 to so minuts at about 50 c. in amaqiieous solution containing per cent of 1 dichromate and 2. per cent of mono-so- Example 5-Etchant for magnesium, zinc and their alloys Dissolve 1.5 pounds of sodium bichromate and 1.5 pints of 68 per cent nitric acid in water to make 1 gallon of solution. Etch in the solution for to 2 minutes at room temperature.

Example 6--Etchant for magnesium, zinc and their alloys Dissolve 1 pound of chromic acid and 0.9 pint of 68 per cent nitric acid in water to make 1 gallon of solution. Etch in the solution for to 2 minutes at room temperature.

Somewhat superior electroplates upon articles of magnesium and its alloys result when prior to the etching treatment'of the Examples 2 to 6, inclusive, above the articles are first subjected to the action of hydrofluoric acid alone as by immersion for A to 10 minutes in aqueous hydrofluorlc acid containing from about 0.25 to 50 per cent of anhydrous hydrofluoric acid.

Example 7-Etchaut for aluminum and its alloys Mix together 3 pints of nitric acid (68 per cent by volume) and 1 pint of hydrofluoric acid (50 per cent by volume). Etch in the mixture for 1 to 3 minutes at room temperature.

Example 8Etchant for zinc and its alloys Etch in a 10 per cent aqueous hydrochloric acid solution at room temperature until gas bubbles (hydrogen) begin to be evolved, and then remove the article from the solution.

After having produced an etched surface upon the article and removed, if necessary, any adhering protective film resulting from the action of the etching bath, the article may be carefully rinsed, preferably first. with hot water and then with cold, but not dried, and then while still wet subjected to the electroplating operation. In some instances. as when the article is made of extruded metal which may show markings of the die used in its making when subsequently plated, the article is preferably first given an additional etching or treatment in hydrofluoric acid solution of from 10 to 50 per cent concentration. The electroplating operation is carried out subsequently by electrolyzing the article as cathode in an aqueous solution containing a soluble salt of the metal to be deposited, together with a fluorine component.

For nickel plating, such baths may be simple or complex according to the selection of the compounds employed in preparing the bath. Simple baths may be obtained by dissolving nickel carbonate in aqueous hydrofluoric acid solution, preferably in proportion such as to form the compound NiFz-HF. A suitable concentration of nickel and of fluorine in such a bath is about 9 and 7 grams per liter, respectively. By adding potassium hydroxide to the bath until the pH is raised to about 4 its throwing power is increased.

Theme of boric acid in nickel plating, is an advantage and is illustrated in the following grams per liter of ammonium fluoride (prefer-- ably 60 grams per liter) in aqueous solution makes a good nickel plating bath. If boric acid is also added, then the proportions may be as follows: nickel sulphatel0 to 60 grams per liter, ammonium fluoride-10 to grams per liter, and boric acid-l0 to 40 grams per liter.

Best results are obtained when the pH values of these baths are between about 5 and 5.5.

For nickel plating magnesium and its alloys in general, the more com lex plating baths are preferred to the simpler o y es, best results being obtained when the constituents of the bath are present in such proportions as could form complex fluo compounds. This is obtainable by using a soluble salt of the metal to be deposited and a complex fluo compound as by employing a soluble alkali metal or ammonium fluoride and an acid which forms therewith a complex fluo compound or such a fluo compound itself as sodium fluoborate, fluosilicate, etc. in the bath. The proportions and concentrations of such compounds in the plating baths may be varied, but in general the soluble fluo compound and the soluble fluoride together should preferably be present in stoichiometrical excess of the soluble compound of the metal to be deposited.

For example, for the above-mentioned nickel plating bath, from 4 to 20 grams of nickel metal (from a salt thereof), from 12 to 50 grams of fluorine (from the soluble fluoride), and from 10 to 40 grams of boric acid per liter of solution should be used. The preferred proportions are 6 to 12 grams of metal to be deposited, 20 to 30 grams of fluorine, and from 12 to 24 grams of boric acid per liter ofsolution. a The pH of the plating bath may be between about 4 and 6.5, and preferably between 5 and 5.5, as above mentioned. This may be obtained by employing hydrofluoric acid in place of or in addition to the soluble fluoride used. If desired, some other acids, e. g. sulphuric acid, may be used to ad-' just the pH of the bath.

As a further and general guide to the effective relative proportions of the ingredients for the complex type of plating bath, reference may be had to the following equation illustrating its application to a nickel plating bath:

According to the foregoing equation, the relative proportions of nickel, fluorine and boric acid to satisfy theequation are 58.79, 152, and 123.6 grams, respectively, and these same proportions are generally satisfactory for use in the plating bath. Similar equations can be written for other" metals of the nickel family, e. g. cobalt, and for other acids forming with a soluble fluoride other complex fluo salts, such as nickel fluosulphonate, nickel fluosilicate, etc. Similar proportions can be used when depositing a combination of metals, for example, an alloy of cobalt and nickel.

Also, instead of employing a soluble salt of the metal to be deposited, together with a soluble fluoride and an acid forming therewith a complex fluo acid, a complex fluo salt of the metal may be used, such as nickel borofluoride (Ni(BF4):)., nickel fluosilicate (NiSlFc), etc., the pH of the bath being adjusted, if necessary, by adding, for example, suitable amounts of hydrofluoric acid when the-pH is too high or sodium hydroxide when the pH is too low. I

The following are further illustrative examples of plating baths: I

Example 9 A nickel plating bath: nickel sulphate (NiSO4-6H2O)40 grams, boric acid (H3803)- 20 grams, potassium fluoride (KF'2H2O)80 grams, dissolved in' water to make 1 liter of solution. Add hydrofluoric acid in sufllcient amount to reduce the pH to between 5 and 5.5.

Example 1 A nickel plating bath: nickel sulphate (NiSQ4-6H:O)--60 grams, boric acid (H3BOa)-. 30 grams, ammonium fluoride (NHiF) -70 grams, dissolved in water to make 1 liter of solution.

Add hydrofluoric acid in sufllcientf amount to' reduce the pH to between 5 and 5.5.

' Example 11 A nickel plating bath: nickel carbonate (NiCOs)--88 grams, hydrofluosilicic acid (HaSiFs-H2D)82 grams of 28 per cent aqueous solution, potassium fluoride (KF'2H2O)100 grams, dissolved in water to make 1 liter. Add hydrofluoricacid in suflicient amount to reduce the pH to about 5.

Example 12 A nickel alloy plating bath: nickel sulphate (NiSO4-6HzO)-30 grams, cobalt sulphate tained at the temperatures between about C. and'65" C. or higher. Similar temperatures may be used with the other baths. The current density to employ varies somewhat with the bath composition. I have found that in general a'current density of about 5 to or amperes per square foot'produces good results, but other current densities may be used. The duration of the, electrolysis depends, of course,'upon the thickness of the coating to be produced. It usually requires 3Q minutes or more to obtain satisfactory coverage, After the coating has been deposited tothe desired thickness, the arti- I cle is removed from the plating bath-and then immediatelywashed in-coldwater to remove any of the liquid of the electroplating bath. After such washing, the article may be rinsed in hot water and then allowed to dry.

After producing a nickel or nickel alloy deposit, as described, other metals may then be electrolytically deposited, if desired, directly on the deposit in conventional manner., This is particularly advantageous in the case of articles of magnesium and its alloys which cannot be conventionally directly electroplated with other metals. For example, after depositing nickel on an article of magnesium, as described, cadmium. chromium, copper, zinc, silver, etc. can be deposited thereon in conventional manner.

Attention is directed to a continuing application- Serial No. 411,697, filed September 20, 1941, in which there is claimed a plating bath described in the foregoing specification.

Other modes of applying the principle of my invention mayjbe employed instead of those .ex-

plained, ;change being made as regards the a method hereindisclosed, provided the step or steps stated by any of the following claims or the equivalent of such stated step or steps be the electroplating bath should be maintained below that at which the complex fluo acid therein,

ii any, tends to decompose. In the case of the fluoboric acid bath, good electroplates are obemployed. I e v I I therefore particularly point out and distinctly claim asmy invention:

1. In a method of depositing an electroplate of a nickel group ,metal uponarticles of magnesium and magnesium-base alloys, the steps which consist in subjecting the articlejto the action of an acidified aqueous etching solution containing a solublechromateto produce a protective coating on the article, then subjecting the article to the action of-a boiling aqueous solution of chromic' acid, and then electrolyzing the article as cathode in an aqueous solution containing a soluble fluoride. an acid forming with the'fluorlde a'complex fluo acid, and a soluble salt of the metal to be deposited.

2. In a method of depositing an electroplate upon articles of magnesium and magnesiumbase alloys, the steps which consist in subjecting the article to the action of an aqueous solution containing from 5 to 30 per cent of sodium bichromate and from 5 to 30 per cent of nitric acid, and then to the action of a heated aqueous solution containing up to 5 percent of chromic acid, and then electrolyzing the so-treated article as cathode in an aqueous solution containing a soluble nickel salt, analkali metal fluoride,-

and boric acid, said solution having a pH value between about 4.5 and moss.