When pure iron and aluminum encounter concentrated sulfuric acid or concentrated nitric acid, the atoms on the surface react with the acid to produce a dense oxide film, which prevents the internal material from continuing to react with the concentrated acid and plays a protective role. This is called passivation.
The chemical cleaning of the citric acid equipment system is to use some chemicals themselves to react slowly with FeO, and the resulting product has a small solubility of ferric citrate, which is easy to precipitate. The aqueous solution of the product to remove various sediments in the system, and when using citric acid as a cleaning agent, to form a good anti-corrosion protective film on the surface of the cleaning solution, generally including ammonia to adjust the pH value of the solution to 3.5 ~ 4.0.
Citric acid is alkali washing, pickling, rinsing and passivation several processes. Trivalent acid, which is complex with ammonia in three forms. Different pH values of citric acid cleaning is a kind of pickling, can avoid chloride, the degree of dissociation of citric acid is different. Stress - corrosion fragmentation at pH 3.0 ~, especially suitable for large fertilizer plants.