Etching of metals. The etching process, methods and equipment
Etching is the process by which a part of a metal is removed from the surface by chemical means. This method is used for final processing of parts, in the preparation of the workpiece before coating (electroplating), as well as for the creation of various drawings, ornaments and inscriptions.
The essence of the method
Metal etching involves thorough surface treatment. The product is applied a protective coating, which is washed in the place of the picture. Then use either acid or electrolyte bath. Unprotected places are destroyed. The longer the exposure time, the deeper the etching of metals. The picture becomes more expressive and clear. There are various ways of producing engravings (inscriptions): the image itself can be directly etched or the background can be. Often such processes are combined. Also used and multi-layer dressing.
Depending on the material used to destroy the material’s surface, the following etching methods are distinguished.
1. Chemical method (it is also called liquid). It uses special solutions based on acids. Thus, alloys are applied to the alloys.
2. Electrochemical etching of metal - involves the use of an electrolyte bath. It is filled with a special solution. Also often with the use of lead salts, which prevent over-etching. This method has several advantages. First, the pattern is clearer, and the time required to complete the process is significantly reduced. Moreover, such metal processing is economical: the amount of acid used is much less than with the first method. Another undoubted advantage is the absence of harmful gases (the mordant does not contain caustic acids).
3. There is also an ion-plasma method (the so-called dry). In this case, the surface is minimally damaged. This method is used in microelectronics.
Basically, this treatment is used to remove scale and various oxides.This procedure requires careful adherence to the technology, since over-etching of the base metal is undesirable. The process uses both a chemical method and electrolyte baths. For the preparation of solutions used hydrochloric acid, sulfuric acid. All parts require thorough surface degreasing. Even a small fingerprint can ruin a piece. As a protective coating used varnish based on rosin, turpentine, tar. However, it should be remembered that the components are flammable substances, so the preparation of varnish requires great concentration and caution. After the processing of the metal is completed, the etching process itself takes place. Upon its completion, the item must be cleaned of varnish.
Mordants used for steel
Very often, a solution of nitric acid is used to pickle steel. Also used salt, tartaric (with a small addition of nitrogen). Solid steel grades are etched with a mixture of nitric and acetic acids. Glyphogen is a special liquid based on water, nitric acid and alcohol. The surface is treated with such a composition for several minutes.Then washed (solution of wine alcohol in purified water), quickly dried. This is a pretreatment. Only after such manipulations are the blanks placed in the pickling solution. Cast iron is well treated in a solution of sulfuric acid.
Etching non-ferrous metals
Copper and alloys based on it are etched with sulfuric, hydrochloric, phosphoric or nitric acids. The process is accelerated by solutions of chromates or nitrates. The first stage is the removal of scale, then the brass is directly etched. Aluminum (and its alloys) are etched in a solution of caustic alkali. For cast alloys, nitric and hydrofluoric acids are used. Spot welded blanks are treated with phosphoric acid. Titanium alloys are also etched in two stages. First - in caustic alkali, then in a solution of sulfuric, hydrofluoric, nitric acids. Etching of titanium is used to remove the oxide film before electroplating. Molybdenum is treated with a solution based on sodium hydroxide and hydrogen peroxide. In addition, metals (such as nickel, tungsten) are etched using water, hydrogen peroxide, and formic acid.
Etching printed circuit boards
There are several ways to dress boards. In the first case, water and ferric chloride is used. It can be made independently. For this, iron filings are dissolved in hydrochloric acid. The mixture is kept for some time. Also etching of printed circuit boards is carried out using nitric acid. The whole process takes about 10 minutes. At the end of the fee must be thoroughly wiped with baking soda, as it perfectly neutralizes the remains of caustic substance. Another composition for dressing includes sulfuric acid, water, hydrogen peroxide (tablets). It takes much more time to etch the boards with such a composition: hot water, salt, copper sulphate. It is worth noting that the temperature of the solution should be at least 40 degrees. Otherwise, the etching will take longer. It is possible to etch the boards using DC. Glassware, plastic containers can be used as dishes for this process (it does not conduct current). Fill the container with a solution of edible salt. That he is an electrolyte. As the cathode, you can take a copper (brass) foil.
Etching process for other materials
Such a type of glass processing as etching is currently widespread. Used vapors of hydrofluoric acid, hydrogen fluoride. First, acid polishing of the surface is carried out, then drawing is applied. After these manipulations, the product is placed in a bath with pickling solution. Then the glass is thoroughly washed and cleaned of the protective coating. As the latter, you can use a mixture based on beeswax, rosin, paraffin. Etching glass with hydrofluoric acid is used to give it a haze. There is also the possibility of color etching. Silver salts give the surface yellow, red, blue hues, copper salts - green, black, red. Sulfuric acid is added to hydrofluoric acid to produce a clear, shiny pattern. If deep dressing is necessary, the process is repeated several times.
Etching metals is a rather unsafe activity that requires a lot of concentration. This is due to the work with aggressive materials - acids and their mixtures.First of all, for this process, you must correctly choose a room with good ventilation. Ideally, a fume cupboard will be used during dressing. If this is not available, then you must take care of a respirator to avoid inhalation of harmful fumes. Wear rubber gloves and an apron when handling acids. At hand should always be baking soda, which - if necessary - will be able to neutralize the action of acid. All pickling solutions must be stored in special containers (glass or plastic). Do not forget about the stickers, which will indicate the composition of the mixture, the date of preparation. There is another rule: cans with acids should not be put on high shelves. Their fall from a height is fraught with serious consequences. The artistic etching of the metal is not complete without the use of nitric acid, which is quite caustic. In addition, in some mixtures it can be explosive. Most often nitric acid is used for sterling silver. Etching solutions are prepared by mixing acids with water.It is also worth remembering that in all cases acid is added to water, and not vice versa.