STAINLESS STEEL PICKLING PROCESS
In stainless steel production process oxides are formed on the stainless steel surface during annealing and hot rolling. Under these oxides layers chromium is depleted and the steel has lost a significant amount of corrosion resistance. Thus removal of the oxide scale is necessary. Best performance and quality is reached by using chemical cleaning of the surface. This is done with an acid pickling process, where mixtures of hydrofluoric acid (HF) and nitric acid (HNO3) are used to remove the oxides and to enrich the chromium depleted areas again. This site should give an overview of the chemical reactions during pickling process.
PICKLING WITH NITRIC AND HYDROFLUORIC ACID
Nitric acid is an oxidizing acid by itself not being suitable to pickle stainless steel – it is used in combination with hydrofluoric acid. This combination allows good control of pickling rates by varying the ratio of the two acids. The nitric acid dissolves oxides and metal from the surface, while the hydrofluoric acid forms stable complexes with the metal ions.
REACTION MECHANISM FOR IRON
(1) 2 Fe + 2 HNO3 + 4 H+ → 2 Fe2+ + 2HNO2 + 2 H2O
(2) 2 Fe2+ + HNO3 + 2H+ → 2 Fe3+ + HNO2 + H2O
(3) 2 HNO2 → NO + NO2 + H2O
(4) Fe3 + 3F– → FeF3
The Fe and Cr is dissolved by the nitric acid from steels’ surface. In the next step metals are oxidized from two valent to three valent state.
To keep the reaction sequence (1-2) ongoing it is necessary to remove some reaction products. HNO2 decomposes to nitric oxide and nitrogen dioxide, which are gaseous and will escape to the atmosphere (3).
The three valent iron will form a stable complex with the fluorides (4). The types of formed fluoride complexes are depending on the pickling process conditions.
Nickel largely does not form fluorides, it reacts with nitric acid and stays in solution, giving green color to the pickling acid.
(5) NiO + 2 HNO3 → Ni(NO3)2 + H2O
In this way parts of the acids are transformed to chromium-, nickel- and iron salts during the pickling process. After pickling the acid needs to disposed leaving behind a problematic waste causing high disposal costs and environmental problems. SUSTEC has developed the REGMAX process being able to recover total waste acid including metals, enabling a closed acid loop for pickling. No waste is generated and no new resources for pickling are used.