US2497988A - Indium plating - Google Patents

Description

Patented Feb. 21 1950 INDIUM PLATING Ealing, London, and John David Frederick Green,

Ernest Salmon,

London, England, assignors to Vandervell Products Limited, London, England N Drawing.

Application July 11, 1945, Serial No.

604,524. In Great Britain May 22, 1943 Section 1, Public Law 690, August 8, 1946 Patent expires May 22, 1963 6 Claims; ((1204-45) This invention relates to indium plating and particularly in its relation to bearing surfaces.

Indium has been electrodeposited in various ways and has become of importance in recent years when the plating has been applied on top of a bearing material such as lead, lead-copper, lead-bronze and cadmium alloys, etc. to fortify the bearing metal against corrosion due to the acids formed by oxidation of the lubricating oil.

The main difiiculty experienced in practice is the provision of a stable plating bath.

Attempts have been made to stabilise an indium cyanide bath by additions of saccharidespreferably a high grade dextrose, and while these attempts have been partially successful there has always been the disadvantage that the bathshad to be made up extremely carefully and aged for some 48 hours during which time the solution darkened and a dark brown sludge precipitated which contained indium. The plating rate oi these known solutions started at about 15 mg./amp'./min. (milligrams, per ampere, per minute) and fell ofi rapidly to about 5-8 mg./amp./min. More recently attempts have been made to develop a successful acid bath based on the employment of indium sulphate. A bath of this character, however, also suffers from the serious disadvantage that it isdiflicult to dissolve the indium metal in sulphuric acid except by prolonged refluxing and the chemical control required is very delicate, particularly in respect of its pH value. The solution not being alkaline it has no cleaning properties unlike the older type of bath. Moreover, the cathode efiiciency is only about 50%. The acidity is a disadvantage also from the point of view of corrosion of th parts of the articles not required to be plated. A further disadvantage is that unless adequate precautions are taken to agitate the bath thoroughly, the plate is stratified.

We have by research discovered a new type of indium cyanide dextrose plating bath which overcomes the disadvantages inherent in the processes in prior use and the primary characteristic of our invention is the use in the bath of additions of alkali metal hydroxides such, for example, of sodium or potassium hydroxide.

The success of our new bath depends, however, partly on the procedure for making up the solution, employing the before-mentioned hydroxide additions made to stabilise the bath. By these means it is possible to prepare a stable concentrate suitable for renewing a depleted bath.

Thus our invention provides means whereby it is possible to prepare stable concentrates and baths of the cyanide, alkali hydroxide type which operate. at very high cathode eificiencies approaching 100% i. e. having a plating rate of some 20-23 mg./amp./min.

The following procedure will explain by way of example the method of manufacture of a stable indium-potassium cyanide-potassium hydroxide concentrate. For the best results the use of pure chemicals is recommended.

According to this procedure g. indium metal are dissolved in an excess of concentrated hydrochloric acid and the solution is evaporated until crystallisation of indium chloride begins. It is preferred that the indium chloride shall not be allowed to evaporate to dryness before it is used in the next stage of the process. The indium chloride which has crystallised is then dissolved in distilled water and the volume made up to about 100 ccs.

A second solution is then prepared by dissolving 110-150 gms. of potassium cyanide in 200 to 250 ccs. of cold distilled water and to this is added a cold solution of 30 g. dextros in 50 ccs. distilled water. A third solution isprepared by dissolving 50-100 gms. of potassium hydroxide in 50-100 ccs. distilled water and cooling thoroughly.

article would be serious it is The first solution is then added to the second drop by drop (forinstance from a burette) at the rate ofabout 10 ccs. per minute with constant stirring, the temperature being maintained preferably below 30 C. and in any case below 10 C.

When all of the first solution has been added to the second the third solution can be added in the normal way. The resulting concentrate will be of clear golden yellow colour and will contain approximately gms. per litre indium, 60 gms. per litre dextrose, 200-300 gms. per litre potas sium cyanide, 100-200 gms. per litre potassium hydroxide.

This concentrate can be used as a plating solu tion asit is, but as drag out losses 1. e. losses consequent on adhesion of the liquid to the plated preferable to dilute to at least 1 litre when the plating rate will still be high (about cathode efficiency) and drag out losses considerably less important. At some further sacrifice in speed of plating the solution may be still further diluted up to a maximum of some five or six litres when plating will be slow but losses of indium metal due to dragout will be very much less important.

In fact the plating rate and drag out can be balanced at will to suit the particular work in hand.

These baths have 'the'great advantage that they are completely stable and at the same time are usable over a very wide range of composition, voltage, current density and temperature and can be adjusted to operate at rates of deposition approaching 100% cathode efficiency.

The sodium and potassium baths are distinguished from the earlier cyanide dextrose baths which have been suggested by additions of the corresponding alkali metal hydroxide and it appears likely that for a sodium bath containing say 30 g./1. indium, 25 g./l. sodium hydroxide will be sufficient though we prefer 30 g./l. .minimum. In the case of potassium 35 g./l. potassium hydroxide appears to be suficient thoughwe prefer 4o g./l. minimum. If solutions are operated with lower concentrations of indium lower concentrations of alkali metal hydroxide will he satisiactory. It is to be understood that the scope of our invention is not to be limited by these minimum figures the dominant feature of the invention being the principle of stabilising the bath by additions of alkali metal hydroxide in quantities suihcient to result in a stable bath.

In practice we prefer not to operate the baths at concentrations so'close to the minimum and it will be found that the baths will operate between the following Wide limits:

The ratio of dextrose to indium is preferably at least 1:1 but should not be below 1:4 and in any case dextrose should be at least 5 g./l.

Potassium Potassium cyanide -500 Potassium hydroxide 8-450 Sodium Sodium cyanide 8-200 Sodium hydroxide 6-200 Additions of sodium and potassium cyanide and sodium and potassium hydroxide can be 'increased to the limit of their solubility so far as we have at present ascertained, though there is little advantage in so doing and at extremely high concentrations of alkali hydroxide some deposition of indium hydroxide appears to take place on the anode, while the current is passing. The cathode current density may 'be up to at least 20 amperes per square decimetre. The temperature may be from room temperature or even lower up to the boiling point. Anodes may be of an inert conducting material such as mild steel, carbon or platinum though it-ispossible with solutions of certain compositions within the given range to operate with a soluble indium anode particularly at elevated-temperatures. It is to be understood that though the concentrations of each constituent may vary throughout the given range, the concentrations of the several constituents are not independent of each other and must be varied in due relation.

The deposits produced are smooth white and adherent. The baths have the advantage that if for any reason they are seriously contaminated or the metal is required it can be recovered by heating th solutions and plating out.

It is to be understood that the details may be considerably varied without departing from the principles enumerated above and suitable saccharides other than dextrose may be used.

Examples of suitable stable baths prepared according to the principles of our invention:

80 g./1. ini- Sodium Indium, 30 g./l.

Dextrose, 30 g./l.

Total NaCN (sodium cyanide), g./l. (initially) Free NaCN, 20 g./l. (maintained) Total NaOI-I (sodium hydroxide), 80 g./l. (initially) Free NaOH, 40 -g./l. (maintained) Room temperature (20 C.)

Current density, 2 amperes per square decimetre Inert anode Rate of deposition, 19 mg./amp./min.

Cathode efiiciency, 78%

The same solution at 80 C. at the same current density gave an efiiciency of 95%.

Since these baths are largely used for plating thin flash coatings for subsequent diffusion into a basis metal or .alloy the use of the higher current densities referred to will be advantageous.

What we claim and desire to secure by Letters Patent is:

1. An aqueous indium cyanide dextrose electrolytic bath for electrodepositing indium consisting of from 5 to 40 grams per liter of indium,

5 to 40 grams per liter of dextrose with a ratio by Weight of dextrose to indium of 1 to 1, a cyanide of the group consisting of sodium cyanide and potassium cyanide of from 1 to 5 times the weight of indium present and the corresponding hydroxide of the above cyanide in quantities of from 6 to grams per liter.

2. .An aqueous indium cyanide dextrose electrolytic bath for electrodepositing indium consisting of .from .5 to 40 grams per liter of indium, 5 to 4.0 grams per liter of dextrose with a ratio by weight of dextrose to indium of 1 to 1, sodium cyanide in amounts 1 to 5 times the weight of indium present and 6 to 135 grams per liter of sodium hydroxide.

3. An aqueous indium cyanide dextrose electrolytic bath for electrodepositing indium consisting of from 5 to 40 grams per liter of dextrose with a ratio by weight of dextrose to indium of l. to 1, potassium cyanide in amounts 1 to 5 times the weight of indium presentand 6 to 135 grams .per liter of potassium hydroxide.

4. A method of preparing a stable aqueous in- .dium cyanide dextrose electrolytic bath which consists of adding indium trichloride in small quantities to .a solution composed of dextrose and a cyanide of the group consisting of potassium cyanide and sodium cyanide, cooling and mixing the solution by rapid agitation during the addition to keep the temperature below 30 to 40 C. and subsequently rapidly adding the hydroxide of the metal whose cyanide was included in the first solution.

5. A method of preparing a stable aqueous indium cyanide dextrose electrolytic bath which consists of adding indium trichloride in small quantities to a solution composed of dextrose and potassium cyanide, cooling and mixing the solution by rapid agitation during the addition to keep the temperature below 30 to 40 C. and subsequently rapidly adding potassium hydroxide.

6. A method of preparing a stable aqueous in- DAVID FREDERICK GREEN. JOHN ERNEST SALMON.

:0 Metal Finishing,

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

UNITED STATES PATENTS OTHER REFERENCES Transactions of the American Electrochemical Society, volume 65, pages 377-380 (1934).

Iron Age, Dec. 19, 1940, pages 35-3S.

July 1944, pages 405-407. Chemical Age, Oct. 6, 1945, pages 314 and 315.

Claims (1)

1. AN AQUEOUS INDIUM CVANIDE DEXTROSE ELECTROLYTIC BATH FOR ELECTRODEPOSITING INDIUM CONSISTING OF FROM 5 TO 40 GRAMS PER LITER OF INDIUM, BY WEIGHT OF DEXTROSE TO INDIUM OF 1 TO 1, A CYANIDE OF THE GROUP CONSISTING OF SODIUM CYANIDE AND POTASSIUM CYANIDE OF FROM 1 TO 5 TIMES THE WEIGHT OF INDIUM PRESENT AND THE CORRESPONDING HYDROXIDE OF THE ABOVE CYANIDE IN QUANTITIES OF FROM 6 TO 135 GRAMS PER LITER.