Brass Plating [PDF]

Zitiervorschau

Brass Plating By Stan Olander, EPI, Inc. New Berlin, WI

Brass plating primarily is a decorative finish. However, there are some engineering uses, such as brass-plated steel wire, that promote adhesion to rubber in steel-belted tires and as an anti-galling coating. Brass also is plated on bearing surfaces. Bright decorative brass finishes are produced by first plating with bright nickel for brightness, followed by a brass flash plate for 35-90 sec. Such finishes are used in wire goods, decorative lamps, furniture hardware and builder’s hardware. Heavy brass deposits (0.0003-0.0006 inch) are used for finishes that will be buffed, burnished, antiqued and/or oxidized. Some of the brass plate will be removed with antiquing and oxidizing processes and, therefore, the minimum thickness for such processes is 0.0003 inch. Heavy brass deposits are not as bright as brass plated over bright nickel. To obtain bright finishes with heavy brass deposits, they must be buffed or burnished. Addition agents can refine the grain of the brass so that the amount of burnishing or buffing is greatly reduced. Alloy and Bath Compositions The most widely used plate is yellow brass, which contains 70% copper and 30% zinc. A composition for a yellow brass plating solution is shown in Table I. This solution is used for both rack and barrel plating. The most important as-pects of brass plating are the ratio of copper to zinc and the level of free cyanide. The former should be maintained between 2.3:1 to 2.6:1 with 2.3:1 being ideal. The latter should be at a level equal to zinc metal. Higher concentrations than those shown in Table I may be used if the ratio of ingredients is maintained. Alloys with more than 80% copper are red in color and include the commonly used 85/15 and 80/20 alloys. They are called red brass finishes. A formula for red brass is listed in Table II. A 90/10 alloy with zinc is used as an architectural bronze finish because of the color. The substitution of tin for zinc will produce an alloy with a color close to that of gold and is frequently used in the architectural trades. The very desirable yellow-green brass is controlled by grain size. The slight green cast is developed with grain refiners. Efficiency of a brass bath can be reduced by using copper metal concentrations as low as 1.5-2 oz/gal. Lower efficiency increases throwing power.

Table II - Solution Composition for Red Brass Copper cyanide............................ Zinc cyanide................................. Sodium cyanide............................ Sodium carbonate......................... Ammonia ....................................

6 oz 1 oz/gal 10 oz/gal 4 oz/gal 0:5-1 pint/100 gal

Zinc cyanide is the preferred salt. Zinc oxide is not recommended. Color control of the plate is achieved by varying the ratio of copper to zinc as well as the cyanide level. The level of cyanide has many effects on the bath. High concentrations of cyanide improves the anode dissolution and reduces anode polarization, but high cyanide content also lowers the plating efficiency and causes poor plating in low-current-density areas. Too high of a cyanide concentration will stop the plating. Higher levels of cyanide are used when producing the red brass 85/15 and 90/10 alloys. Since the cyanide concentration in the baths is very important, additions of sodium cyanide should be made slowly and carefully after chemical analysis. The addition of ammonia to a bath will affect the alloy composition and thus the color of the plate. One half pint of ammonia per 100 gal of solution will help to produce a more yellow color. The addition of ammonia on a daily basis should not be required. When a brass solution is operated properly, ammonia is generated by the decomposition of the cyanide at the anode. An addition of am-monia may be required when the brass plating bath has not been used for a few days. The ammonia is driven off by heat and evaporation. When ammonia is used to control the color, the alloy composition will be constantly changing because the ammonia is constantly evaporating. Relying on ammonia to control color may produce different alloys and colors on the same part due to varying current densities. Increasing current density increases the copper content. Ammonia additions should be made only after the chemical balance is in order. To reduce excess ammonia raise the temperature of plating solution to 140F and use air agitation to speed ammonia evaporation.

Carbonate is an important ingredient in a brass bath. It is necessary to buffer a new brass bath with 4 oz/gal of carbonate. Buffering stabilizes the color. As the bath is used, carbonate level will increase slowly, and at 15-16 oz/gal it can become detrimental to the performance of the bath with polarization of the anodes and development of a dull plate. Excess carbonates are removed by freezing out the carbonate crystals. This can be done by a one time chilling below 40F for 2-3 hr, or it can be done with a continuous chilling process. Anodes Anodes should be of the same composition as the alloy being plated. Yellow brass is plated with anodes containing 70% copper and 30% zinc whereas red brass finishes are produced with 80% copper and 20% zinc anodes. The red brass compositions will require higher levels of zinc cyanide in the solution. The purity of anodes is very important. Lead and tin are the most frequently found impurities in the brass anodes and they should be kept at a concentration of less than 0.03%. The anodes may be cast, rolled or wrought. The three types of anodes may be mixed in use. For barrel plating, it is recommended that brass nugget anodes be used in order to produce enough anode area and reduce polarization of the anodes. The anode to cathode ratio should be 2:1. Anode Baskets Steel baskets may be used, but corrosion of the steel will produce a ferrocyanide complex, which is very difficult to waste treat. Dragin of chlorine into the brass bath will lead to Copper cyanide............................ 10 oz/gal rapid corrosion of the steel and formation of ferrocyanide. Sodium cyanide............................ 16.6 oz/gal The ferrocyanide produces a white precipitate when it Zinc cyanide................................. 1.5 oz/gal combines with zinc in the solution. This precipitate should be Sodium hydroxide......................... 6 oz/gal removed by continuous filtration with a bath turnover of a Temperature.................................. 150-170F minimum of one time per hour. It can also be removed by being allowed to settle to the bottom of the tank and then decanting off the solution. Titanium baskets are recommended. Table III-Solution Composition for High Speed Brass

Agitation Air agitation is recommended in a rack brass bath because it increases the allowable current density and plating speed. The development of carbonates in a brass bath are due to the decomposition of bath components. Rod agitation is also used in a rack tank. Still tanks are discouraged. Temperature The temperature of the bath should be maintained between 90-105F, which is sufficient to generate ammonia at the anode and reduce the necessity for adding ammonia. Higher temperatures will increase the efficiency of the bath but will also drive off ammonia faster than it is generated at the anode, leading to frequent additions of ammonia. Higher temperatures (above 100F) favor copper deposits causing a redder plate because the copper plates out faster than the zinc. Impurities Metallic impurities are most often introduced to the bath from impure anodes. They are removed by dummy plating if the anodes are replaced. Nickel is a frequent impurity with a brass bath due to the dragin of nickel plating solution. Nickel contamination is evident only at the very highest current densities where it plates out as a silvery deposit. High-current dummy plating will remove nickel. Also, improved rinsing and drain time can reduce dragin of nickel plating solution. Chlorine contamination of the brass solution can come from dragin of nickel as well as acid solutions. Organic impurities from cleaners and inhibited acids as well as oils from equipment will affect color of the deposits, causing off-color brown deposits. Carbon treatment will remove organic impurities. The use of a proper wetting agent in a brass bath will help to produce a cleaner plate when organic impurities are present. Bath Analysis Analyses for copper, zinc and caustic in the bath are straightforward, and the methods used for titrating for these components are well known in the industry. Determining cyanide and free cyanide levels is critical in maintaining proper operation of the bath. Free cyanide is calculated as follows. Total cyanide in the bath is determined with a silver nitrate titration procedure. Free cyanide in oz/gal is then determined by using the total cyanide concentration in oz/gal minus three times the zinc metal in oz/gal. Free cyanide should be maintained at 2-2.5 oz/gal with metal concentrations in Table I. Free cyanide concentration will affect grain size. pH The pH of the solution is maintained between 10.2-10.7. pH is raised by adding caustic soda and reduced by adding sodium bicarbonate. Small additions of ammonia have very little, if any, effect on the pH. At a pH higher than 10.7, the plate will

begin to go red. Current Density Brass is typically plated at a current density of about 20 asf. However, current densities as high as 90 asf may be used with the composition given in Table I, if vigorous air agitation is used. If the chemical composition is maintained per Table I, a uniform color and alloy composition will be deposited over a wide range of current densities. If the current range for uniform color is very narrow, the solution is out of balance. Plating Various Metals Special preparations are usually not required for plating on most metals. Brass plates very well over zincated aluminum and may be used as a base for nickel plating. The solution will plate slower on cast and malleable irons than on steel. Brass may be plated directly on zinc die castings with a low caustic concentration and if sufficient thickness is used. If the brass plate diffuses into the die casting, a white deposit will develop. If diffusion becomes a problem, then the die casting should first be given a copper strike. High-Speed Plating The composition of the bath given in Table I will produce economical brass plating. If a higher plating speed is desired, it is best to use higher concentrations of the components in Table I rather than going to the high-caustic, high-speed plating baths. High-caustic plating baths are very difficult to control because of the caustic level, and they are run at a higher temperature, which necessitates continuous additions of ammonia. Table III contains a formula for a high-speed bath. Passivation Bright brass should be protected by a passivating agent that reduces cyanide spotting out and improves the performance of clear lacquer topcoats. Cyanide-Free Alkaline Brass Plating Non-cyanide brass plating solutions have been widely investigated. A non- cyanide bath has not been commercialized. Table I - Solution Composition for Yellow Brass Plating Component

Optimum

Range

Copper metal, as copper cyanide

4.0 oz/gal (5.6) oz/gal

3.5-4.5 oz/gal (4.9-6.3) oz/gal

Zinc metal, as zinc cyanide

1.7 oz/gal (3.0) oz/gal

1.5-2.0 iz.gak (2.7-3.6) oz/gal

Free cyanide (calculated)

1.7 oz/gal

1.5-2.5 oz/gal

Caustic Soda (naOH)1

0.0 oz/gal

0.0-0.2 oz/gal

Soda Ash (NaCO3)

4.0 oz/gal

4.0-12.0 oz/gal

Grain refiner

1% by volume

Anode corroder/bath stabilizer

5% by volume

Wetting agent

0.1% by volume

(1) Caustic soda is not added when making up a new solution because some is formed as a byproduct during makeup. Operating Conditions Optimum

Range

pH (electrometric)

10.4

10.2-10.7

Temperature

105F

90-120F

Agitation

Air, cathode rod 15 ft/min or by barrel rotation

Anode to cathode ratio

2:1

Voltage

Barrel 9-12 v

Rach 2-6v

Filtration

Continuous with minimum of one (1) per hour Equipment

.

Anodes

Rolled or extruded brass

Anode baskets

Titanium

Tank

Mild steel is permissible, however, a lined tank is preferred to avoid stray currents. An unlined tank should not be used as an anode

Filter material

Compatible with alkaline cyanide solutions

Heating elements

Mild steel permissible

Ventilation

Forced ventilation required