Feature

Specialty Metals Plating—Part I


By Ronald J. Morrisey and Alfred M. Weisberg, Technic, Inc., Providence, R.I.

Reviewing the fundamentals of palladium-nickel, platinum and rhodium plating.

PALLADIUM AND PALLADIUM-NICKEL ALLOY PLATING
by Ronald J. Morrissey
Technic Inc., Providence, R.I.; www.technic.com

Palladium has been electroplated from a wide variety of systems, which can be broadly characterized as ammoniacal, chelated, or acid processes. Of these, the most numerous are the ammoniacal systems, in which palladium is present as an ammine complex, such as palladosamine chloride, Pd(NH3)4Cl2, or diaminodinitrite, Pd(NH3)2(NO2)2, which is known popularly as the P-salt. Some representative formulations are shown as follows:

P-SALT/SULFAMATE
Palladium as Pd(NH3)2(NO2)2, 10-20 g/L
Ammonium sulfamate, 100 g/L
Ammonium hydroxide to pH 7.5-8.5
Temperature, 25-35°C
Current density, 0.1-2.0 A/dm2
Anodes, platinized

PALLADOSAMINE CHLORIDE
Palladium as Pd(NH3)4Cl2, 10-20 gL
Ammonium chloride, 60-90 g/L
Ammonium hydroxide to pH 8.0-9.5
Temperature, 25-50°C
Current density, 0.1-2.5 A/dm2

Palladium electrodeposits are notably susceptible to microcracking induced by codeposition of hydrogen. For this reason, it is important to plate at current efficiencies as high as possible. Proprietary brightening and surfactant systems are available, which increase the range of current densities over which sound deposits may be obtained. Ammoniacal electrolytes, particularly at higher temperature and pH, tend to tarnish copper and copper alloys. Proprietary palladium strike solutions have been developed. In most cases, however, a nickel strike is sufficient.

Chelated palladium plating solutions contain palladium in the form of an organometallic complex. These solutions operate in the pH range of 5 to 7 and are in almost all cases proprietary. Requisite details may be obtained from the manufacturers.

Acid palladium plating solutions have been used for producing heavy deposits of very low stress. Such systems are ordinarily based on the chloride, although a proprietary sulfate solution brightened with sulfite has been reported. A representative formulation for the chloride systems is as follows:

ACID CHLORIDE
Palladium as PdCl2, 50 g/L
Ammonium chloride, 30 g/L
Hydrochloric acid to pH 0.1-0.5
Temperature, 40-50°C
Current density, 0.1-1.0 A/dm2
Anodes, pure palladium
Deposits from the acid chloride system are dull to semibright. Current efficiency is 97 to 100%. The plating solution itself is notably sensitive to contamination by copper, which can displace palladium from solution. Work to be plated in this solution should thus be struck with palladium or with gold.

PALLADIUM-NICKEL PLATING
Palladium readily forms alloys with other metals and has been plated in numerous alloy formulations. Of these, the most important commercially has been palladium-nickel, which can be deposited as a homogeneous alloy over a composition range from approximately 30% to over 90% palladium by weight. Current practice favors an alloy composition from approximately 75 to 85% wt. palladium. A formulation suitable for alloys in this range is as follows:

Palladium as Pd(NH3)4Cl2, 18-28 g/L (palladium metal, 8-12 g/L),
Ammonium chloride, 60 g/L
Nickel chloride concentrate, 45-70 ml/L (nickel metal 8-12 g/L)
Ammonium hydroxide to pH 7.5-9.0
Temperature, 30-45°C
Current density, 0.1-2.5 A/dm2
Anodes, platinized

Palladium-nickel alloy electrodeposits are notably less sensitive to hydrogen-induced cracking than are pure palladium deposits. They are, however, somewhat more susceptible than pure palladium to stress cracking upon deformation. As with pure palladium plating systems, various proprietary additives are available for brightening and stress control.
 

PLATINUM PLATING
by Ronald J. Morrissey
Technic Inc., Providence, R.I.; www.technic.com

Electroplating solutions for the deposition of platinum are generally similar to those employed for palladium; however, whereas palladium ions in solution are almost always divalent, platinum ions exhibit stable valences of 2+ or 4+. Divalent platinum ions can become oxidized to quadrivalent at the anode, particularly in alkaline solution. Such oxidation can lead to progressive, sometimes erratic, losses in current efficiency. For this reason it is often useful to separate the anode compartment in electroplating solutions of this type.

Dinitroplatinite Sulfate, Sulfuric Acid
For plating platinum directly onto titanium for fabricating anodes the dinitroplatinite sulfate formulation has been employed:
Platinum as H2Pt(NO2)2SO4, 5g/L
Sulfuric acid to pH 2
Temperature, 40°C
Current density, 0.1-1 A/dm2
Anodes, platinum

Chloroplatinic acid
An alternative acid formulation is based on chloroplatinic acid:
Platinum as H2PtCl6, 20 g/L
Hydrochloric acid, 300 g/L
Temperature, 65°C
Current density, 0.1-2 A/dm2
Anodes, platinum

Chloroplatinic Acid, Ammoniacal
In chloroplatinic acid platinum ions are quadrivalent rather than divalent, as in the dinitroplatinite sulfate. Plating formulations based on chloroplatinic acid can also be operated at neutral to alkaline pH:
Platinum as H2PtCl6, 10 g/L
Ammonium phosphate, 60 g/L
Ammonium hydroxide to pH 7.5-9
Temperature, 65-75°C
Current density, 0.1-1 A/dm2
Anodes, platinized

The alkaline formulation can be applied directly to nickel-based alloys without the use of a preplate. Both of the acid baths shown require a preplate, preferably gold, on most basis metals.
 

RHODIUM PLATING
by Alfred M. Weisberg
Technic Inc., Providence, R.I.; www.technic.com

Although several different electrolytic baths for rhodium plating have been proposed the only baths to achieve commercial significance are (1) phosphate for very white and reflective deposits; (2) sulfate for general jewelry and industrial deposits; and (3) mixed phosphate-sulfate for general decorative deposits.

DECORATIVE PLATING
The jewelry and silverware industries were the primary users of rhodium electroplates until quite recently. Although both the phosphate and sulfate baths gave bright white deposits the phosphate bath was preferred for soft-soldered jewelry, especially before the general adoption of bright nickel plating. Cold nickel did not always cover the soft solder, and the acid electrolyte attacked and dissolved some of the solder. Lead in a rhodium bath gave dull, dark deposits and destroyed its decorative white finish. Phosphoric acid attacked the solder less than sulfuric acid did, so phosphate rhodium was preferred. After the introduction of bright nickel most of the industry changed to sulfate because it could operate at a slightly lower rhodium concentration. The phosphate-sulfate solution was used because some considered the color to be a bit whiter or brighter.

The typical rhodium electroplate on costume or precious jewelry is 0.000002 to 0.000005 in. and is produced in 20 sec to 1 min at about 6 V in the following baths.

Phosphate Rhodium Bath
Rhodium as phosphate concentrate, 2 g/L
Phosphoric acid [85% chemically pure (CP) grade], 40-80 ml/L
Anodes, platinum/platinum clad
Temperature, 40-50°C
Agitation, none to moderate
Current density, 2-10 A/dm2

Sulfate Rhodium Bath
Rhodium as sulfate concentrate, 1.3-2 g/L
Sulfuric acid (95% CP grade), 25-80 ml/L
Anodes, platinum/platinum clad
Temperature, 40-50°C
Agitation, none to moderate
Current density, 2-10 A/dm2

Phosphate-Sulfate Rhodium Bath
Rhodium as phosphate concentrate, 2 g/L
Sulfuric acid (95% CP grade), 25-80 g/L
Anodes, platinum/platinum clad
Temperature, 40-50°C
Agitation, none to moderate
Current density, 2-10 A/dm2

Tanks for these baths should all be made of glass, Pyrex, plastic, or plastic-lined steel. If plastic is used it should be leeched once or twice with 5% sulfuric or phosphoric acid for 24 hr before the rhodium is added. In mixing a new solution distilled or deionized water should be used, and the acid should be added to the water carefully and mixed thoroughly before the rhodium concentrate is added. This will prevent precipitation of the rhodium.

Rhodium is, of course, plated out and also lost through drag-out. Because of the expense of rhodium the first rinse after plating should be a stagnant drag-out rinse, also contained in a glass or plastic tank. As water is lost from the plating solution it should be replaced with this drag-out rinse so that some of the “lost” rhodium is returned for reuse. Even with two drag-out tanks the actual amount of rhodium lost will be about 25 to 30% of the rhodium plated; therefore, rhodium should be replenished at the rate of 5 g/18 to 20 ampere-hours (A-hr) of flash plating. Because the drag-out is so high in jewelry plating sulfuric (or phosphoric) acid should also be replenished at the rate of 5 ml/18 to 20 A-hr. This recommended replenishment is only an average value. If possible it should be checked by analysis.

Bright nickel is the preferred base for decorative rhodium electroplates. It provides a bright base for rhodium and also prevents the rhodium solution from attacking a brass, copper, steel, lead, or tin base. All of these metals adversely affect the color and tarnish and corrosion resistance as well as the covering power of the rhodium solution. Nickel, of all the metals, has the least adverse effect on a rhodium solution. Baths can tolerate as much as 1 to 2 g/L and still give a satisfactory deposit. There are no truly satisfactory methods to purify a contaminated rhodium plating solution.

DECORATIVE BARREL PLATING
The usual decorative barrel finish is also 0.000003 to 0.000005 in. A variation of the sulfate-rhodium bath is always used. It is necessary, however, to reduce the metal concentration and to raise the acid concentration to get economical and satisfactory deposits. With many parts in the barrel it is necessary to plate quite slowly so that the parts have time to mix and be evenly exposed to the plating solution. This ensures that they are all plated to a similar thickness before more than 0.000005 in. is deposited. It is not advisable to slow the rate of plating by decreasing the current density (and voltage) because this may lead to nonadhering deposits over a bright nickel base. Therefore, the plating rate is best slowed by decreasing the cathode current efficiency by raising the acid and lowering the rhodium. A typical formulation for decorative barrel plating would be the following:

Rhodium as sulfate concentrate, 1 g/L
Sulfuric acid (95% CP grade), 80 g/L
Anodes, platinum/platinum clad
Temperature, 45-50°C
Current density, 0.5-2 A/cm2

ELECTRONIC/INDUSTRIAL PLATED RHODIUM
The emerging electrical/electronics industry in the 1950s and 1960s made considerable use of rhodium electrodeposits for many diverse uses, but it was particularly used on sliding and rotating contacts, printed circuit switches and commutators, and high-frequency switches and components.
There are many requirements for rhodium deposits of 0.000020 to 0.0002 in. over nickel or, occasionally, silver. These may be plated from the following solution:
Rhodium metal as sulfate concentrate, 5 g/L
Sulfuric acid (95% CP grade), 25-50 ml/L
Anodes, platinum/platinum clad
Temperature, 45-50°C
Current density, 1-3 A/dm2
Current efficiency, 70-90% with agitation; 50-60% without agitation

See the Metal Finishing Guidebook chapter on Decorative Plating for instructions on leeching the plating tank before use. It is preferable to use water jacket heating of the solution to prevent local overheating by an immersion heater or steam coil. Even a short exposure to temperatures over 160°F will result in chemical changes to the solution that will result in a permanent increase in stress of the deposit. The stress will be present even if the bath is later operated within the correct temperature range.
Because of the expense of the solution it is advisable to plate with as low a rhodium concentration as possible to achieve the desired plating thickness and finish. If some of the plating is to be 0.0002 in. and over it will be necessary to raise the rhodium concentration to 7 or 10 g/L.

Replenishment is based on ampere-hours plated and the cathode current efficiency. It is best determined by analytical control; however, an approximation would be to replenish 5 g of rhodium for every 5 to 10 A-hr of plating. The actual value will depend on the average thickness plated an the current density used.
The cathode current efficiency is quite low, even with agitation, and hydrogen gas bubbles will tend to cling to the work and leave imperfections. This effect may be minimized by adding a 1% solution of sodium lauryl sulfate to the bath. The rate of addition should be 1 to 5 ml of a 1% solution per gallon of the plating bath.

INDUSTRIAL BARREL PLATING
Not only the expense of rhodium but the high drag-out of barrel plating recommends the use of a low metal concentration. Coatings in the millionth inch range can be produced with as little as 1 g rhodium/L. Thicker deposits must use proportionally higher concentrations. Deposits of 0.000020 in. may be achieved with 2½ g/L; 0.000050 in. with as little as 3½ g/L; 0.0001 in. with as little as 4 g/L; and deposits of 0.0002 in. and over with 5 g/L. If the holes in the barrel are very small, and the parts have a high surface area, it will be necessary to use higher concentrations to compensate for poor solution transfer.
Otherwise, the formulations for barrel plating are the same:
Rhodium metal as sulfate concentrate, 2.5-5 g/L
Sulfuric acid (CP grade), 20 m/L
Anodes, platinum/platinum clad
Barrels, horizontal, submerged
Temperature, 45-50°C
Current density, 0.5-2 A/cm2

CARE OF RHODIUM SOLUTION
Contamination of the rhodium solution is the cause of most rhodium plating problems. The major contaminants are (1) organics, (2) rhodium basic salts, (3) rhodium complexes and (4) inorganics such as iron, lead/tin, copper, gold/silver, and nickel.

The most common contaminants are organics such as dust, dirt, adhesives from masking tape, stop-off paints and printed circuit board material, and organics from improperly leached plastic tanks. They are usually easily removed by batch-type carbon treatment. It is imperative that the carbon used be very low in acid-soluble residues. It is also important not to use a diatomaceous earth filter aid. If a single carbon treatment does not clean the solution a second treatment or a treatment with a carbon designed for the removal of very short chain organic molecules may be necessary. Carbon treatment will frequently eliminate stress brittleness and flaking of the deposit. It will also often cure finger staining or apparent tarnishing of the deposit.

Basic rhodium salts will precipitate from a rhodium solution and act as a contaminant if the pH of the bath rises above 2. The acidity of the solution should be controlled and never be allowed to fall below 25 ml/L. If plating is normally done at higher current densities of over 25 A/ft2 the acidity should be kept even higher. Levels of sulfuric acid of at least 50 ml/L are generally satisfactory. Phosphoric acid is not recommended for industrial plating baths.

Contamination and increased stress by unwanted rhodium complexes, as has been mentioned, can occur if the solution is overheated. Rhodium solutions should be indirectly heated and be thermostatically controlled.

Inorganic contaminants are usually introduced by the basis metal or base plates. The warm sulfuric acid electrolyte is extremely corrosive, and work should never be allowed to hang in the tank without current. Preferably, work should be connected to the negative power source before it is introduced into the rhodium tank. This may occasionally require a flying cathode bar or, in the case of barrel plating, a cathodic battery clamp and wire to be attached to the barrel before it is lowered into the tank. Of course dropped parts should immediately be removed from the bottom of the tank.

Copper, iron, tin, and lead, even after exhibiting a brief brightening effect in the parts per million range, will cause highly stressed heavy rhodium deposits. They will also cause dark and stained deposits and skip plating.

Most metallic impurities, theoretically, can be precipitated from a rhodium solution by potassium ferrocyanide; however, in practice the procedure is very difficult, time-consuming, and not very successful, especially with solutions used for heavy rhodium deposits. The best practice is to prevent metallic contamination.

The parameters that will tend to decrease the stress and brittleness of a rhodium deposit are the following:
 

  1.  Increased rhodium metal concentration
  2.  Increased sulfuric acid concentration
  3.  Increased temperature
  4.  Carbon treatment of the bath
  5.  Decreased inorganic contaminants

Low-stress rhodium proprietary baths are available that contain trace amounts of selenium and indium. Although the stress and attendant stress cracking are almost totally eliminated, the baths operate like conventional sulfate baths.
 

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