What’s the difference between warm pickling/NOX and cold pickling?

The main purpose with pickling is most often to remove welding oxides. With hot pickling, also called NOX-pickling (pickling acid containing nitric acid HNO3 and hydrofluoric acid HF), the acid pushes into the natural crevasses beneath the oxide, attacking mainly the chromíum depleted steel, forming nitrous gases. The gases crack the oxide layer. In other words, the nitric acid does not dissolve the oxide.


Advantages of warm pickling

Hot pickling creates a similar result over the whole surface. It’s not just oxides that are removed, giving an even smooth steel grey surface. With cold pickling, stains or an uneven result can occur, since primarily the oxides are removed and remaining surface is left untouched. With warm pickling, about 5 µm (or 0,005 µm) is removed across the whole surface. This means all impurities, free iron and other contamination from the manufacturing process are removed. This decrease the risk of corrosion and discolouration appearing later.

Hot pickling also has a degreasing effect because of the high temperature. High temperature gives a faster and more efficient pickling with shorter lead time.


Analysis equipment, drying fans and rinsing steps

We have our own analysis equipment for quick analysis of the pickling baths. This is especially important when special demands are made regarding polishing and pickling results. It also lessens the risk of stops in the process waiting for analysis results. There is a risk of dry stains appearing after pickling. To avoid this, a careful multi-step rinsing process is necessary. Which is why we have several rinsing steps with different purities. We also have access to high-purity water. This is very important when there are demands for extreme cleanliness.


Another factor that affects the result is the steel alloy: the higher the alloy, the more time is needed.


Calamo has several different pickling baths to meet our customers’ different needs and requirements.