Electroplating of copper-zinc alloys, or brass is obtained with cyano-alkaline baths and is more difficult than the single deposition of copper or zinc from the corresponding cyanide solutions.
Usually, this alloy (yellow) consists of 70% copper and 30% zinc. Depending on the higher or lower copper content, it may tend to reddish or pale yellow.
Brass plating is directly used to finish polished iron or, better, bright nickel plated iron, or as an intermediate layer for subsequent nickel plating (for example. brass plating of aluminium or zinc-based die-cast alloys), or to facilitate rubber adhesion to steel, in all probability by chemical bond with sulphur or sulphur compounds contained in rubber itself.
Brass plating is, however, mostly used in the field of decorative ornaments for the home, clothing, hinges, handles, lamps, toys, etc.
To achieve greater uniformity of colour, brass electroplating requires a constant control of the baths, from the chemical, temperature, and current density point of view. Brass plating is obtained with cyanide alkaline baths containing complex copper cyanide, K2Cu(CN)3 or Na2Cu(CN)3, complex zinc cyanide, K2Zn(CN)4 or Na2Zn(CN)4, free cyanide and other compounds such as carbonates, free alkali, ammonia, sodium arsenite, etc..
In practice, to obtain a given brass deposit under given conditions of temperature and current density, the person in charge of the electroplating must pay maximum attention to ensure that the composition of the solution is kept consistent. Fortunately, the colour of the coating acts as a marker, and then this task is facilitated.
Often, deposits may look blistered and speckled; in addition, a lot of mud can form in the bath. Also, that is an indication of an unsuitable electrolysis operation and, in particular, of an altered content of free cyanide, as well as of a pH of the bath due to which the simultaneous deposition of zinc is irregular. In brass plating baths, zinc electroplates at pH values usually not used in cyano-alkaline galvanization; For this reason, it is difficult to maintain the electrolyte within the pH limits compatible with the good electroplating of the alloy.
For good operation of the brass plating baths, it is necessary that the copper and zinc dissolve from the anodes (which are, in fact, made of brass) in the same proportion in which they are deposited at the cathode; otherwise, the solution must be corrected with the appropriate additions. Therefore, while the anodes must be made of the same kind of brass (or very close) as the brass that must be deposited, the contents of the two metals in the solution can be different.
Brass with a given colour contains a precise percentage of the components. However, even if the colour is the same, the percentage in electrolyte brass and in cast or laminate brass, is not. For example, in order to imitate the colour of cast brass containing 65% copper and 35% zinc, brass must be electroplated with 75-80% copper, using anodes with just 75-80% copper.
The basic components of brass plating baths are copper and zinc complex cyanides, as well as free cyanide. Therefore, the content of copper, zinc and free cyanide is what makes a certain type of brass plating.
Needless to say, baths also contain carbonate, ammonia, and some kind of hydrate, formed by the action of the carbon dioxide in the air, by oxidation and hydrolysis of cyanide.
In addition to basic components, the baths sometimes contain other ingredients: strong acids alkali salts, glycerine, sodium sulphite or bisulphite, ammonium chloride, etc.. These products, however, are not indispensable and the advantages they bring to electroplating do not compensate for the complications they often provoke.
When a bath contains approximately the same amount of copper and zinc in the form of double cyanides, the finest brass colouring is produced, since, using the common anodes, it has about 70% copper and 30% zinc. If the copper-zinc ratio in the bath is 1: 7, the brass deposit becomes whitish, while if it is 2: 5, the deposit is light yellow and slightly greenish.
Even stirring has a strong influence on the type of brass that is electroplated; the colour ranges from yellow-green to red depending on more or less stirring of the objects.
The baths that are not stirred, however, offer the advantage of giving more regular deposits. Precise colouring is difficult to obtain for objects brass plated in mass inside galvanic barrels: when all the other conditions are unchanged, the higher the speed of the galvanic barrel, the whiter the deposit is.
Similar conditions occur when the electrolyte is made to circulate around the objects using relatively high current density.
The tanks are the same as those used for cyanide copper plating; the use of tanks coated with rubber or incorruptible synthetic resin is preferable.
The anodes consist usually of cast brass plates, at least 5 mm thick, containing 70-80% pure copper and 20-30% pure zinc. Cast brass is certainly to be preferred to laminated brass. Anodes are suspended to the anodic bars by means of brass parts, hardened and folded on the bars themselves so as to obtain perfect contact.
Before use, when they are still new, they must be treated with coarse sand (e.g. by means of a sandblasting machine), or immersed for a short time, first in dilute nitric acid, and then rinsed in water and immersed in diluted sulphuric acid. After that, they can still be treated with a solution of 40% nitric acid, and then rinsed well.
Even during inactivity of the bath, the anodes can be immersed in the solution.
The person in charge of the electroplating must often observe the appearance of the anodes during the electroplating process, so as to deduce important data on the operation of the bath. For example, when anodes are covered with a rather thick mud, it means that the bath lacks free cyanide, or that it contains inappropriate substances; the lack of free cyanide is, on the other hand, also confirmed by a blue-green bath and by extremely slow electroplating. In this case, the anodes should be brushed while still in the bath, with a long wire or vegetable fibre brush, and then, a bit of potassium or sodium cyanide should be added, stirring, in order to restore the pale yellow colour of the bath and dissolve the mud.
Brass plating baths do not usually require continuous filtration or frequent purification with activated carbon. This purification is carried out when the baths are so contaminated as to completely alter the electroplating. Purification by activated carbon is carried out at a temperature of 50 °C with 5-10 g/l of carbon and a few g/l of diatomaceous earth or asbestos fibres, which facilitate the filtration. The filtering apparatus should be completely made of ferrous material (steel, cast iron, etc.).
The new baths work better after a certain period of electrolysis.
Usually, brass plating baths should be reinforced with 2 kg of sodium cyanide per 10 000 dm² of coated surface.