Pickling
In the technical field, pickling is the treatment of solid bodies to change the surface with a stain . This pickling can u. a. serve to protect the surface against oxidation (in the case of metal) or mold (in the case of wood and fabrics), but also to color the surface. Mordants are used for the production of mordant dyes .
Metal and plastic processing
In the case of metal and plastic surfaces, the pickling is mainly done by etching with aggressive chemicals, usually acids or alkalis . The process is used, among other things, in electroplating to remove applied metal layers or to get an oxide-free surface. The process is often supported by electrical current ( electrolysis ).
Pickling of ferrous materials
Pickling is an important process step in the production of cold-rolled steel strips. The purpose is to remove the solid burn-off , the so-called scale , created on the surface by the hot-rolling process. There are two types of pickling plants: continuous pickling and push pickling. In the continuous pickling line, the strip , which is usually wound in coils, is unwound, straightened, the end of the previous and the beginning of the new strip are welded after a cut to form an "endless" strip and conveyed through hydrochloric acid or sulfuric acid baths . In a push pickling line, the coils are unwound one after the other, pushed through the system and wound up.
Due to the selective attack of scale, the sulfuric acid process requires mechanical pretreatment, which as a rule consists of stretch-bending with a degree of stretching of up to 2.5%. The sulfuric acid process is usually only operated in steelworks which, in connection with the coke production, have sulfuric acid production (hard coal contains sulfur).
After pickling, the steel strips are rinsed in a water cascade and usually made alkaline, i. In other words, they are immersed in an alkaline solution (e.g. sodium hydroxide solution (NaOH)) to neutralize the acid so that no acid residues stick to the surface. Residual acid would inevitably corrode the tapes . Pickling also harbors the risk of hydrogen embrittlement of the steel.
Before winding up the stained strip, it is possible to oil the surface and trim the strip edges ("trim").
Pickling of aluminum
Pickling of aluminum is often used as a pretreatment for adhesive joints, which are difficult with anodized or bright-rolled aluminum. Similar to cold-rolled steel strips, the continuous pickling method is also popular for aluminum. The pickling process uses z. B. a mixture of 27.5 wt .-% concentrated sulfuric acid and 7.5 wt .-% sodium dichromate (Na 2 Cr 2 O 7 · 2H 2 O) and the remainder (65 wt .-%) water. A slightly simpler and less toxic process for pickling aluminum for adhesive bonds is to use caustic soda .
Particular attention must be paid to whether the aluminum end product is used for construction or food. Depending on this, some chemicals may not be allowed in pickling.
In electroplating, aluminum is often pickled with a mixture of nitric acid and hydrofluoric acid . Treatment with hydrofluoric acid, which has been largely banned in Germany, serves to dissolve the silicon that is worked into the part during the production of aluminum and cannot be electroplated.
The pickling times are a few seconds to a few minutes, or during continuous pickling, the aluminum is sprayed with the chemical on both surfaces with spray / jets and cleaned in the immediately following phase.
Pickling of other metals
The pickling of copper , bronze , brass , tombac or gunmetal using chromic acid mixtures is also known as burning or yellowing .
Yellow pickling for gold alloys
1 part concentrated nitric acid, 1 part concentrated sulfuric acid, 1 part dist. Water. The mixture is heated to 80 ° C. When used, copper and its oxides as well as parts of the silver dissolve in the alloy. A rich yellow color is created.
Various electroless and power-assisted pickling processes are used in electroplating. The reason is usually the activation of the base metal for further coating. These activations are different for each base metal / alloy. Often, even small differences in the alloy components can require different pickling processes.