Treatments for steel. In the previous post we talked about some of the most common cleaning systems: degreasing and sandblasting. Today, we will cover other.
The acid pickling.
Before pickling, perform a thorough cleaning. To eliminate any trace of insoluble substance, using special bath. For example?
Ferritic and Martensitic steels annealed stage
- Bath at 50 – 60° C or at ambient temperature (for very long dive times):
– Nitric acid 52% (36° Bé) litre 100
– Hydrofluoric acid 65% litre10
– Water litre 900
- For quick pickling or materials where the cleanup is difficult, it is possible through bath with a temperature close to boiling:
– Soda 20% by weight
– Potassium permanganate 5% by weight
– Water 75% by weight
Low carbon austenitic steel
- Bath at 50 – 60° C or at ambient temperature (for very long dive times):
– Nitric acid 52 % (36°Bé) litre 100
– Hydrofluoric acid 65 % litre 20
– Water litre 900
NB: when there is a risk of intergranular corrosion, it is a good idea to keep as little time as possible materials in pickling baths.
The pickling with molten soda.
Sometimes it’s shall be carried out before the acid pickling. In order to make easier the removal of calamine.
- 15 minutes of immersion in molten soda bath heated to 450° C with addition of sodium nitrate or potassium nitrate to 5-20%. At the end of the dive, cool immediately and energetically in water, then move on to acid pickling.
- a few minutes of immersion in molten soda bath heated to 370-380° C with sodium hydride to 1-2%. At the end of the dive, cool immediately and energetically in water, then move on to acid pickling.
The decontamination/passivation.
It’s takes place with the immersion of stainless steel products in nitric acid. The thin pollutants are mainly due to:
- rubbing with non stainless steels
- cold cuts
- iron brushes
- abrasive wheels
The acid has the property of dissolving these contaminating particles which often trigger localized corrosion. Transforms ferrous residues can yield cold either take a decontamination bath, at room temperature and dive for a few hours, consisting:
- Nitric acid 52% (36° Bé) litre 250
- Water litre 750
- Thorough cleaning
This treatment is not recommended for stainless steels with addition of Sulphur and it is replaced by cleaning with special pulps. When you request must be carried out immediately after the pickling and flushing or after polishing. For diving material you can use the same bath described in decontamination.
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