In the previous article, we discussed some of the most common surface cleaning methods, including degreasing and sandblasting. Today, we will focus on additional treatments used for stainless steel and alloy steels.
Acid Pickling
Before pickling, the material must be thoroughly cleaned in order to remove any grease, oil, or insoluble residues from the surface. This operation is typically performed using specific chemical baths.
Ferritic and Martensitic Steels (Annealed Condition)
Pickling bath at 50–60°C or at room temperature (with longer immersion times):
- Nitric acid 52% (36° Bé): 100 litres
- Hydrofluoric acid 65%: 10 litres
- Water: 900 litres
For rapid pickling or for materials that are particularly difficult to clean, a hot alkaline bath close to boiling temperature may be used:
- Caustic soda: 20% by weight
- Potassium permanganate: 5% by weight
- Water: 75% by weight
Low-Carbon Austenitic Stainless Steels
Pickling bath at 50–60°C or at room temperature (with longer immersion times):
- Nitric acid 52% (36° Bé): 100 litres
- Hydrofluoric acid 65%: 20 litres
- Water: 900 litres
Important Note
When there is a risk of intergranular corrosion, the immersion time in pickling baths should be kept as short as possible.
Pickling with Molten Caustic Soda
In some cases, molten caustic soda treatment is carried out before acid pickling in order to facilitate the removal of mill scale (“calamine”).
Two common procedures are:
- immersion for approximately 15 minutes in a molten soda bath heated to 450°C, with the addition of 5–20% sodium nitrate or potassium nitrate
- immersion for a few minutes in a molten soda bath heated to 370–380°C, containing 1–2% sodium hydride
At the end of the treatment, the material must be rapidly cooled in water before proceeding with acid pickling.
Decontamination and Passivation
Decontamination and passivation treatments are carried out by immersing stainless steel products in nitric acid solutions.
Surface contamination is often caused by:
- contact with non-stainless steels
- cold cutting operations
- iron brushes
- abrasive grinding wheels
Nitric acid dissolves contaminating ferrous particles that may trigger localized corrosion and helps restore the protective passive layer of stainless steel.
A typical decontamination bath consists of:
- Nitric acid 52% (36° Bé): 250 litres
- Water: 750 litres
The material is immersed at room temperature for several hours, followed by thorough rinsing and cleaning.
Additional Notes
This treatment is generally not recommended for stainless steels containing sulphur, for which specific cleaning pastes or alternative treatments are preferred.
Whenever possible, passivation should be carried out immediately after pickling and rinsing, or after polishing operations, in order to maximize corrosion resistance and surface cleanliness.
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