Pickling

It is very important to clean stainless steel if it becomes contaminated with carbon steel, such as through grindings or welding splashes. This can also be done mechanically by blasting or grinding, but pickling is usually preferable, especially for larger surfaces or objects. A distinction is made in the United States between descaling, which involves the removal of thick annealing skin and mill scale, and acid cleaning, where the stainless steel itself is not touched. In order to allow the pickling acids to work evenly, grease and other contaminants must be removed by an appropriate cleaning method before the pickling process is started. An alkaline cleaning agent is normally used for degreasing. There are many formulas for pickling agents, but only a few are used on a wide scale. The type of stainless steel plays a significant role in determining the selection of the right pickling agent. An agent that works very effectively for one particular type of material may be too aggressive for a different type of stainless steel. 

• Pickling agents based on nitric acid and hydrofluoric acid (the most commonly used)
• Pickling agents based on sulfuric acid 
• Pickling agents based on hydrochloric acid (this leads to the formation of iron(III)chloride: risk of pitting corrosion!) 
• Pickling agents based on iron(III)sulphate (commonly used with the manufacture of stainless steel sheeting) 

There are different pickling methods: immersion into a bath, electrolytic polishing in a bath, drum pickling, circulating the pickling agent, spraying with a jet pipe, and finally, manual pickling by applying pickling paste.   

Passivation

During the pickling process the original oxide film is removed. It is as if the material is stripped, with the result that the passive film disappears. This layer of film has to be replaced – and this is done by passivation. Passivation is often carried out with the help of diluted nitric acid or hydrogen peroxide with citric acid, as these are oxidizing reacting agents that quickly restore the thickness of the chromium oxide film. Tests have shown that the passivation of stainless steel can also be done successfully with the use of dematerialized water. The water is sprayed powerfully onto the object that has been pickled, so that any remaining acid deposits are removed. Passivation in this way turns out to be sufficient. The tests have even revealed that corrosion levels are unaffected – that is good to know, given the user-unfriendly and environmentally burdensome acid treatment. In fact, passivation occurs in the air if the material has been properly rinsed, which will guarantee optimum passivation.