The dissolution of metals in the pickling solution is accompanied by the transfer of electrons on the metal atoms, so the phenomenon of electrochemical corrosion is the main phenomenon. Metal atoms lose electrons as a result of the formation of metal ions. For example, titanium loses electrons in reducing acids and becomes trivalent titanium ions.
The pickling of the processed titanium material is performed in a mixed acid solution containing hydrofluoric acid or fluoride. During acid washing, hydrogen ions may be reduced to form atomic or molecular hydrogen. Atomic hydrogen tends to diffuse into the metal, which may cause the metal hydrogen to become brittle.
When titanium is pickled in a nitric acid solution containing fluoride ions, titanium is oxidized and nitric acid is reduced. The final product is the formation of titanium fluoride complex, water and colorless nitric oxide.
If exposed to the atmosphere, nitric oxide is rapidly oxidized into brown toxic nitrogen dioxide when it encounters oxygen in the air, and volatilizes.
Therefore, when pickling, try to expel the nitrogen dioxide gas quickly through ventilation. If H202 is added to the pickling solution at about 0.5%, NO can be oxidized in the solution and small-escape from the liquid surface, so that air pollution can be appropriately reduced.
When the titanium wire is pickled in a nitric acid solution containing 5% -7% fluoride ions, if the mass fraction of oxidant nitric acid is less than 15%, the overpotential of various reduction reactions in the solution determines the hydrogen ions. Opportunities for reduction, and after being reduced to hydrogen atoms, it is sucked into the interior of titanium at a fixed ratio, diffuses to some parts of the alloy and aggregates, or generates hydrides of titanium and causes hydrogen embrittlement. Therefore, in order to prevent hydrogen absorption during acid pickling of titanium, the ratio of nitric acid to hydrofluoric acid should always be kept greater than 5. However, if the content of nitric acid is too large, the reaction will be slow, the production efficiency will be reduced, and the production of smoke from the director will make the operation difficult.
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