Yellowing of electrophoretic aluminium profiles: yellowing caused by nitric acid contamination of oxidation tank liquor
In order to achieve better ash removal effect, adding a certain proportion of nitric acid in the neutralization tank is justifiable, but if the control of water washing after neutralization is not good, nitric acid will be brought to the oxidation tank, when the nitrate in the oxidation tank reaches a certain concentration, it will have a certain impact on oxidation, and even cause the yellowing of electrophoretic profiles. In the process of oxidation, nitrate entering the pore of the oxide film will etch the oxide film, corrode the barrier layer of the oxide film, deepen the pore of the oxide film, and then change the structure of the pore. This corrosion has two effects on the oxide film:
(1) The barrier layer of the oxide film becomes thinner, and the bonding tightness between the oxide film and the aluminium matrix becomes worse, which results in the decrease of the adhesion of the oxide film.
(2) Under normal washing conditions, it is difficult to remove sulfate radicals from the membrane pore. The electrophoretic profiles produced under this condition also have yellowing phenomenon. How to avoid this yellowing phenomenon? Add neutralizer in the pure water tank before ironing bath, adjust PH value 8-9.5, wash for 2-3 minutes, neutralize sulfate radical in oxide film pore with amine radical, then electrophoresis production, there will be no yellowing phenomenon.