Pretreatment in electroplating
The pretreatment in electroplating is used to create a perfect surface for the subsequent electroplating . This is done mechanically - by grinding and polishing , as well as chemically by pickling baths ( acid baths ) and degreasing - a distinction is made here between anodic degreasing, cathodic degreasing (both electrolytic degreasing) and decoction degreasing. Furthermore, ultrasonic baths are used for pretreatment in electroplating .
Grinding and polishing
Grinding and polishing are used to remove scratches and scores in the base material; so they should create a smooth surface.
Pickling baths
The pickling baths are acid baths, which mostly consist of hydrochloric or sulfuric acid . In rare cases, nitric , phosphoric and hydrofluoric acid are also used (here, however, attention must be paid to the dangerousness of these acids! - dangerous gas development can occur, e.g. with nitric acid the nitrous gases )
The acid in electroplating u. a. Rust and scale removed from the part to be electroplated.
Degreasing
Degreasing be used to deep-drawing fats / oils, release agents (usually silicones ), corrosion protection oils , flux , dust, grinding, drilling and slide grinding oils and metal oxide / scale from the surface of the components to be removed.
When iron workpieces strongly used to alkaline degreasing, in other metals, in particular with aluminum and its alloys, as well as non-ferrous metals , a weak alkaline degreasing is used. But the most important thing about the degreasers are the surfactants they contain. They have the ability to bind oils and fats to themselves and bring them into solution.
Decoction degreasing
The degreasing is an aqueous strong alkaline solution at a temperature around 70-80 ° C
Electrolytic degreasing
Cathodic degreasing
During cathodic degreasing, the goods are switched as the cathode . The electrolyte consists of sodium hydroxide (NaOH) or potassium hydroxide (KOH). This creates hydrogen , which in its gaseous state is able to penetrate under the oil or fat layer and literally blast it off. To support the cleaning effect, auxiliaries, such as complexing agents , e.g. B. gluconates or triethanolamine , phosphates , silicates and surfactants are added.
Anodic degreasing
The same electrolytes are used for anodic degreasing as for cathodic degreasing. However, oxygen is formed on the material here. In addition, metallic impurities and often the base material are dissolved electrolytically.
In electroplating technology, cathodic degreasing is more effective than anodic degreasing, since twice as much hydrogen as oxygen is formed with the same amount of electricity. Due to the risk of hydrogen embrittlement , an anodic circuit is sometimes prescribed. In some cases the polarity of the degreasing process is reversed, in most cases it is first connected to the cathode and then reversed to an anodic circuit towards the end.
Anodically connected electrolytes with special chemicals “stripper solutions” are used for demetallization (removal of old deposits, e.g. nickel plating, etc.). However, electricity is not always required to remove deposited metal layers. There are stripper solutions that also work without electricity, but then usually require higher temperatures to remove the layer (s). Many deposits can also be removed with concentrated acids (acid mixtures) or alkalis, e.g. B. Chromium or zinc with hydrochloric acid.