Published · ANFA Chemical technical team
After hot rolling, annealing or welding, stainless steel carries a layer of oxide scale and, beneath it, a zone depleted in chromium. Both reduce corrosion resistance and spoil appearance. Pickling removes them and restores a clean, passive surface. Hydrofluoric acid (HF, CAS 7664-39-3) is a key ingredient in most industrial stainless steel pickling baths.
Why HF is used – the mixed-acid principle
The classic stainless steel pickling bath is a mixture of nitric acid and hydrofluoric acid ("mixed acid"):
- Hydrofluoric acid dissolves and loosens the chromium- and iron-rich oxide scale and forms stable fluoride complexes with dissolved metals such as iron and chromium.
- Nitric acid acts as an oxidiser, attacks the underlying metal at a controlled rate and helps the surface re-passivate.
Neither acid alone gives the same result on stainless steel scale, which is why the combination has been an industry standard for decades. Pre-treatments such as shot blasting or salt baths are often used before pickling on heavy scale.
What to control in the bath
- Free acid levels of HF and HNO3 – consumed as scale dissolves and must be replenished.
- Dissolved metal content – as iron builds up, fluoride is tied up and pickling slows; baths are regenerated or partially replaced.
- Temperature and immersion time – set according to steel grade and scale type; over-pickling causes roughening and metal loss.
- Rinsing – thorough rinsing after pickling prevents staining.
Exact concentrations and times depend on the steel grade, the scale and the line, and are defined by each plant's process specification.
Grade and packaging
For pickling, industrial-grade HF is used. ANFA supplies industrial hydrofluoric acid to GB/T 7744 in concentrations from 30% to 70% (typically 40 / 55 / 60%), in PE drums (25 kg, 200 kg), 1000 L IBCs and ISO tank containers. Our metal surface treatment blends are formulated for stainless steel pickling and related processes, and ammonium bifluoride or sodium bifluoride are used in some solid-based or paste formulations.
Safety and environment
- HF is corrosive and highly toxic by inhalation and skin contact. Use HF-specific PPE and keep calcium gluconate gel on hand.
- Pickling generates nitrogen oxide fumes and fluoride- and nitrate-bearing effluent; fume extraction and wastewater treatment are required under most regulations.
- Never store HF in glass, and keep it away from incompatible materials.
Need help matching a grade to your line? Contact us – SDS and COA are provided upon inquiry confirmation.