ELECTROLYSIS OF SALT SOLUTION USED FOR ETCHING DIFFERENT METALS
Dangers and inconveniences
| The use of Sodium Chloride (brine or salt solution) as an electrolyte in galv-etch or any type of electro etching is often advocated, and assumed to be safe, convenient and economical. But there are complex chemical reactions involved that are not as safe as the solutions recommended for etching Copper, Zinc and steel described in Green Prints and this website. They result from the separation of the Chlorine ions and the Sodium ions in the water solution, and their reactions with the metal ions of the anode plate or in solution. There are secondary reactions between the solutions produced which result in more or less toxic and unstable precipitations. For nearly all metals there are large volumes of insoluble precipitations produced, which can block lines and prevent even etching, and require frequent filtering of the electrolyte. There is always copious production of hydrogen which is explosive and to which many people are allergic. The process of electrolysis is not reversible, in other words, metal cannot be deposited on a plate or object by making it the cathode instead of the anode. The filtered solution cannot be used indefinitely as its chemistry and concentration can change over time, and must be disposed of safely.
For a printmaker, it is inadvisable to risk the contact with these solutions and gases, to have the problem of the unpredictability of the results of using a gradually changing electrolyte and to be faced with the necessity for disposal of spent solutions. It is extremely inconvenient to have large volumes of precipitate, and to have to a high level of ventilation over the etching tank to remove the gases. Remember that, for example, using a Zinc Sulphate electrolyte with Zinc plates, or Copper Sulphate with Copper plates, results in an unchanging solution that only requires occasional filtering, reversibility, no precipitate, and no gases. I have been using the same solutions since 1991, only filtering and topping them up very occasionally. Printmakers are not chemists and cannot be expected to take the extraordinary precautions that are required in a laboratory to isolate gases or dangerous chemicals or to know when the solutions they are using are changing and becoming unusable. The principle of precaution should be applied, and only tried and tested methods should be used.
saline sulphate etch
The use of a mixture of salt and Copper Sulphate called “Saline Sulphate etch” is promoted as a alternative to using “Bordeaux Etch” which is pure Copper Sulphate for etching zinc, iron, and aluminium without needing to use electricity. Note that under certain circumstances Copper Chloride (CuCl2 ) and Sodium Sulphate (NaSO4) are produced when adding salt to Copper Sulphate. Copper Chloride is a very toxic chemical. Sodium Sulphate is known as “Glaubers Salt” once used as a laxative, and it is also a Eutectic salt, whose solid melting point of between 40°C and 50°C makes it useful for storage of heat. See below for precise details of the circumstances and the proportions of NaCl and CuSO4 that can produce the reaction.
- Test setup for electro-etching salt solution with different metals. Laboratory power supply, time switch, glass jar with salt solution - 175 gms. salt/2.3 litres distilled water, electrodes of each metal tested in turn. Fresh salt solution for each test
General description of processes
|The descriptions below show in detail, the processes of the electrolysis of Sodium Chloride,( 'salt solution or brine), and the reactions with the Chlorine gas that occur when different metals are etched. The secondary reactions between the metal Chlorides produced and the caustic soda that is always generated whatever metal is etched, are described and illustrated. The hazards and ill effects of the chemicals that are produced is listed. Some of these reactions can be found with Saline Sulphate Etch if it is used electrolytically in the passive Galv-On process or by mistake as an electrolyte for etching copper.
Electrolysis of Sodium Chloride solution with carbon electrodes produces Chlorine gas at the anode and hydrogen gas at the cathode, having a solution of Sodium Hydroxide (caustic soda). If the anode (+ve) is inert (for instance platinum or carbon) it will not react with the Chlorine ions, and the gas will be given off as bubbles and can be separated and collected. The positive Sodium ions will react with water rather than with the negative metal of the cathode (explained below) releasing hydrogen as gas. The water (H2O) loses an ion of hydrogen leaving HO which combines with the Sodium to make Sodium Hydroxide (Caustic Soda).
If the anode is a metal plate to be etched then the -ve Chlorine ion combines with the etched +ve metal ion (charged by the current) to produce a Chloride, and most of the Chlorine is not released as a gas. For example, a Copper anode plate will be etched by loss of one Copper ion which bonds to two Chlorine ions, making Copper (II) Chloride (CuCl2 ). Then the soluble Sodium Hydroxide will react with the soluble Copper Chloride to make insoluble Copper Hydroxide, and soluble Sodium Chloride (by exchange of ions). Being insoluble, the Copper Hydroxide will be seen first as a slight yeloow green cloudiness in the solution, and then in time will sink to the bottom as a solid powder precipitate. Similar reactions are produced by Zinc, Iron, or Aluminium used as anodes, producing Zinc(II)Chloride, Iron(III)Chloride, or luminium(III)Chloride, and then reacting with the caustic soda, to leave Zinc Hydroxide (grey), Iron Hydroxide (black), or Aluminium Hydroxide (white). In practice, depending on the current and area of metal surface, a foam can be produced on the surface, which is hydrogen in bubbles of the soluble metal Chloride and caustic soda (see illustration). Also while the current flows the formation of the precipitate seems to be slightly inhibited, so at that stage the electrolyte is a mixture of more or less toxic solutions. After the current is turned off, the precipitate forms more rapidly if the foam is stirred into the electrolyte and then the liquid clears slowly. Because of the precipitate a plate flat on the bottom of a tray with a grid cathode would etch very unevenly. Even with a vertical Zinc anode the Hydroxide forms a very hard layer on the etched surface which inhibits further etching after a while. Etching steel plates produces very varied results depending on the other metals that are alloyed with pure iron.
Stainless steel should not be used as an anode, because it is very resistant to being etched, or in other words, it will not combine easily with the Chlorine, which is either given off as a gas, or reacts with the caustic soda to produce Sodium Chlorate, which is a weed killer now banned for use in the European Union.
(This page is not complete but the PDF version of it may be downloaded below to read and to print if required. This contains further illustrations and details of the chemistry involved.)
© Cedric Green -last update