Pickling process based on nitric acid
A limitation of alkaline pickling is that with certain aluminium alloys containing copper or zinc a grey or black deposit can sometimes originate that is difficult to rinse off. To remove this deposit these alloys are then immersed in nitric acid. Aluminium is barely affected by nitric acid. This means pickling can take place for several hours at ambient temperature if the concentration of nitric acid is not too high. The big advantage of this process is that a piping system can be cleaned in circulation, for example.
The other pickling processes have a too short pickling time to be useable in practice.
Pickling process based on nitric acid – hydrofluoric acid
This is a very fast smoothing pickling process that gives an attractive and uniform mat effect.
– Temperature: ambient temperature
– Time: 1 – 10 minutes
If the aluminium contains silicon as an alloy element this is also the appropriate pickling method.
Pickling process based on phosphoric acid
This pickling agent is only effective if the oxide skin is very thin. Thicker oxide layers require the addition of fluoride to the phosphoric acid solution. Pickling in phosphoric acid causes a phosphate coat to form that can serve as a primer for a coat of lacquer or paint. The phosphate coat is a conversion coating with improved adhesive properties.