Theoretical Prediction on the Effect of Acidity on Stability of Passivation Solution

Abstract

For theoretically predicting the stability of the passivation solution with changing acidity, based on the equilibria of precipitation-dissolution, complexation-dissociation and ionization-hydrolysis, and considering the effect of ionic strength, the equations of the material balance and the proton balance for all the substances in the passivation solution were established.By solving the multivariate high order equations, content of all the substances in passivation solution were obtained under a certain material ratio, the acidity value was calculated.The stability of the passivation solution was predicted by judging whether there was precipitation in the passivation solution at this acidity through the solubility product constant.Comparing the theoretical calculation with the experimental result, it could be found that in the range of pH=2.3～3.0, the calculated value of pH of passivation solution was in good agreement with the experimental data, and the error was less than 10%.With the decrease of acidity, the passivation solution prepared in the experiment became unstable when pH≥2.5 and resulting in cloudiness and precipitation, which was generally consistent with the precipitation phenomenon at pH ≥3.0 in calculated prediction.With the decrease of the concentration of the passivation solution, the theoretical pH value of the passivation solution was closer to the experimental data, and the error between the theoretical pH value and experimental pH value was smaller when the precipitation occurred.Therefore, the established dissolution and precipitation model of passivation solution could reflect the influencelaw of the acidity on the stability of the passivation solution well.According to this concentration prediction model, the precipitation process of different insoluble salts caused by the dynamic increase of acidity during the continuous dissolution of aluminium in passivation solution could be calculated and the model could be used to infer the interface layer structure between aluminium alloy and passivation layer, which would be of guiding significance for understanding the passivation mechanism of aluminium alloy.

Keywords

• passivation solution|acidity; stability|precipitation|dissolution|theoretical calculation|passivation layer|interface structure