Electroplating, tin, nickel, zinc, grease removal, rust & oxide removal and more...

When the scratches have disappeared, polish further with a cloth or felt disc. on which a paste is smeared, which consists of grease and a very fine polishing agent. Less hard polishing agents are used here, such as tripel, Viennese lime, polishing red (dead head) etc.

Removing grease

All greases can be removed with the help of organic solvents. It makes no difference whether they are saponifiable or unsaponifiable. Petrol, low or higher boiling, is used for this purpose, but nowadays more and more non-flammable chlorinated hydrocarbons such as trichlorethylene or carbon tetrachloride, possibly also a mixture of these with petrol, are used. However, it is not possible to completely remove the fat in one operation; after all, some of the solution always remains on the workpiece and when the solvent evaporates, there are always traces of grease on the metal; these residues must then be washed away with clean solvent. To avoid this, devices have been constructed in which the solvent is boiled. The vapors are condensed in a cooler and the refluxing solvent then rinses the objects completely clean. In this case, a non-flammable solvent must be used. Today, trichlorethylene is commonly used, which is not as easily decomposed by trace moisture as carbon tetrachloride.

If the objects to be degreased contain aluminium, zinc, tin or lead, the methods described above with alkaline solutions cannot be used. Lye and potash in particular should be avoided completely, as the zinc and aluminum dissolve very easily in them. During cathodic electrolytic cleaning, caustic always forms and the metals dissolve in it, while zinc is sometimes deposited on the side, as a result of which the layers applied later can easily come off. In this case, the object must be made an anode for a short time, so that the foreign metal is dissolved again.

In addition, it is possible to purify the objects electrolytically by making them an anode in certain solutions. Here the surface is cleaned because the top layer goes into solution, so it is pickled; the dirt will then come loose on its own. This method is often used for brass and copper.

A simple degreasing solution may consist of a 6 pc solution of calcined soda or a 3 pc solution of sodium metasilicate. 1% soap and 1% sodium hydroxide are then added to both solutions. The soap is not used for electrolytic cleaning.

For degreasing very large quantities, the largest amounts of grease are usually removed by boiling in a strong lye solution, the last residues are then dissolved electrolytically. Here, a treatment of 3 to 4 minutes is sufficient.

After degreasing, the objects are carefully rinsed with clean water.

After pickling, the objects must be rinsed carefully and immediately afterwards they are hung in the electroplating bath, with the current already switched on. The latter is necessary as in some acid baths the solution would immediately begin to dissolve some of the metal. The time the object is exposed to air between rinsing and electrolysis should be as short as possible. The cleaner the metal is, the more sensitive it is to oxidation.

In many cases, cleaning can be simplified. For example, if the metal is particularly well polished to a high gloss, pickling can often be omitted; the oxides are then already completely gone, while further oxidation has been prevented by the fats of the polishing paste. This grease is then removed with lye or solvent and electroplating can be carried out immediately afterwards. This method is often possible in chrome plating, as the highly oxidizing chromic acid burns small residues. Sometimes, however, it gives rise to serious errors.

It is not necessary to pickle on high-gloss polished brass if you add a little potassium cyanide (heavy poison) to the degreasing liquid; this potassium cyanide completely dissolves the oxide residues. However, this method cannot be used with copper.

In certain cases, pickling with acid can be the cause of serious errors. With iron, carbon can form on the surface, which prevents the layers from bonding, or hydrogen is absorbed, which makes the metal layers very brittle and also does not adhere and can be completely peeled off after some time; this used to be very common, especially in nickel plating. In recent years it has been learned to avoid these errors by completely removing the gas after pickling by electrolysis in strong sulfuric acid using the object as anode at 12 volts. They start with 5 amps per square decimetre and after 30 seconds to 10 minutes the current strength drops to virtually zero, the gas is then completely gone. Although the metal becomes passive, this does not affect the adhesion of the electrolytically applied metal layer.

Another method consists of pickling with solutions containing chromic acid or bichromates. Solutions of chrome plating that have become unusable are extremely suitable for this purpose.

About the electroplating process (end 1930s)

While metal is dissolved in an electrical element, the metal can be separated from a saline solution by passing an electric current through it. Electroplating is based on this fact, i.e. the deposition of a layer of metal on another metal or on a surface made conductive.

In the year 1836, De la Rive saw that the copper, which deposited the usual elements on the copper plate at the time, could be detached from it and then microscopically showed exactly all the irregularities of the substrate, so it was a negative print. In the course of several years, the ground operations for electroplating and galvanic reproduction were now discovered. Jacobi and Spencer invented the multiplication of objects by means of electroplating, in 1840 Elkington built a factory for silver plating in Birmingham, in 1840 making conductive by means of graphite powder was discovered, in the course of the following years nickel plating, gilding and silver plating with cyanides discovered.