Chromate-free Conversion Coating On Aluminum Alloys

In this thesis, a deoxidizing/desmutting solution and a non-chromate conversion coating on aluminum alloys were researched. Based on the effect of the bisic ingredient HNO₃, activator NaHF₂ and oxidizer Fe₂(SO₄)₃ on aluminum alloys pretreatment, a deoxidizing/desmutting solution with good performance was developed in order to replace the expensive import products. Based on the experimental study of different compositions and reactive conditions, such as activator, oxidant, assistant salt, pH and temperature, together with its appearnnce and anti-corrosion property, a non-chromate conversion coating was developed. It exhibits a homogeneous appearance, no surface defects and strong adhesive bonding with the substrate of aluminum alloys. Compared with the result of 2 hours of neutral salt spray test for Al alloy LF21 substrate, The converted aluminum alloy could bear 9.5 hours of neutral salt spray test and showed that its corrosion resistence was greatly imporved.The formation mechanism of non-chromate chemical conversion coating was composed of two steps, which were chemical redox and precipitation reaction according to SEM, energy spectroscopy analysis and potential-time change of aluminum alloy surface during the film-forming process. This revealed that the film-forming were the result of dissolution of aluminum alloy surface, nuclearation, cystal growth. The energy spectroscopy analysis also indicated the film was mainly maken up of three elements, F, Al, Na, in which atomic ratio was 6:2.3:1.6. So, the major component of the film was chemical compound Na₃AlF₆. Conversion coating of different sizes particles completely covered on aluminum surface during the film formation, which further showed the correctness of the formation mechanism of chemical redox and precipitation reaction .