Technics Of Aluminum Alloy Free-chromate Passivation And Mechanism Analysis

The passivation treatment can improve the corrosion resistance of aluminium alloy effectively. The chromate passivation is one of the importmant methods to improve the corrosion resistance of aluminium alloy. Because of the toxicity and environmental pollution for chromate treatment, the low or non-toxic passivation treatment becomes imperative. The exploitation of chromate-free passivation which has better practicability and environmental friendly is the developing direction.This paper mainly studied the rare earth passivation for LY12 aluminum alloy. The optimum techniques were obtained by single factor experiment and orthogonal test. The chemical dipping method was as following: the dipping temperature 55℃, CeCl3·7H2O concentration 0.05mol/L, the dipping time 20min, H2O2 concentration 30ml/L. The electrochemistry method was as following: the electrolysis temperature 40℃, CeCl3·7H2O concentration 0.045mol/L, the electrolysis time 9min, current density 1.1mA/cm2, H2O2 concentration 30ml/L.The neutral salt spray experiment and electrochemistry test were adopted to study the corrosion resistance of the different passive film. The results revealed that the corrosion resistance of cerium-based passive film by electrochemical way was more better than the film by chemical way, which was confirmed the superiority the electrochemical way. The corrosion resistance of aluminum alloy was obviously improved.The results revealed that the surface of the film by cathodic electrolysis method distributed the oxide and hydroxide of cerium homogeneously. The coating consisted of Ce,Al,O by XRD and EDS energy spectroscope analysis. No crystalline substance was found on the surface of aluminum alloy. The Cerium-based passive film was non-crystalline material. According to the results abtained the mechanism of forming Cerium-based passive film by bipolar theory was advanced.The neutral salt spray experiment and electrochemistry test were adopted to studied the corrosion resistance of rare earth passive film and chromate passive film. The results revealed that corrosion resistance of cerium-based passive film is close to the corrosion resistance of chromate passive film.