Papers by Author: Di Li

Abstract: The kinetics of film-forming process of anodizing on AZ91D Mg alloy have been studied through analyses of voltage-time and thickness–time curves. The surface morphology, structure, composition and valence of element, phase constituent of anodic films have been analyzed by SEM, EDS, XPS and XRD respectively. The results show that the film-forming process can be divided into four stages: I - formation of a dense layer; II - formation of a porous layer; III - fast growth of the porous layer; IV - slow growth of the porous layer. The growing process of the dense film is characterized by the rapidly formed intact and imperforate oxide film; there are few micro-cracks in partial film formed on α phase, on the contrary, intact film is formed on β phase; the forming of porous film is based on the particulate sparking product with micro-porous structure. There are two kinds of pores in the anodic film; the first one is circular or ellipse, while the second one is the irregular gaps. The dense layer is composed mainly of MgO and small amount of Al2O3, while the porous layer mainly consists of MgSiO3 and Mg3B2O6. We propose a possible model about growing anodizing film on AZ91D Mg alloy based on the experimental results.

Abstract: Rare earth chemical conversion coating of Mg alloys was studied. Corrosion and electrochemical behavior in chloride environment were investigated with tests of evolution of hydrogen and electrochemical measurements. The surface morphologies and composition of rare earth conversion coating were studied through SEM, EDAX and XPS. The results showed that rare earth conversion coatings could improve corrosion resistance and their corrosion resistance was comparable with that of chromate coatings (HB/Z5078278). This result was further proved by Polarization and EIS.

Abstract: By analyzing morphology, weight gain, pitting depth, potential and AC impendence in corrosion course, initial pitting behavior of LY12 aluminum alloy in simulated marine atmosphere was studied. Results indicated initial pitting of LY12 was affected much by Cl- sedimentation and atmosphere humidity, little by temperature below 35°C. Weight gain and pitting depth increased with rising of Clsedimentation quantity, humidity and temperature. Correlation of weight gain and time fitted Model Boltzman. In initial pitting course of LY12, open circuit potential changed little and Nyquist graph had the characteristic of adsorbent resist system.

Abstract: A novel environmental protective water based metallic coating was developed for aluminum alloys, which mainly contains metal flake, silicate and silane. The coating's properties were investigated by neutral salt spray test, micro-hardness testing, adhesion test and electrochemical technique etc. Meanwhile the coating's surface and microstructure was observed by scanning electron microscopy (SEM). Furthermore, the film forming matter was examined by Fourier transform infrared spectroscopy (FTIR) test. Results showed that an excellent adhesive, heat-resisting, protective coating for aluminum alloy could be achieved by this technique. An interpenetrating polymer network (IPN) was formed in the coating by means of cross linking reaction of organosilicone and inorganic silicate. In thesis, the film forming mechanism and protection of coating were also discussed.

Abstract: Magnesium and its alloys have many applications for theirs excellent physical and mechanical properties. Due to magnesium’s active chemical properties and difficulties in direct painting, a novel environmental protective water based coating was developed, which mainly contains metal flake, silicate, silane and sodium polyphosphate etc. The properties and structure of the coating were investigated by adhesion test, hardness test, heat resistance test, neutral salt spray test and Scanning Electron Microscopy (SEM) et al. Meanwhile, the effect of sodium polyphosphate was also examined by hydrogen evolution test and electrochemical test. Furthermore, the film forming mechanism of the coating was explored by Fourier Transform Infrared spectroscopy (FTIR). Results showed that the coating had a stepped structure which could achieve extraordinary adhesion, 350°C temperature heat resistance, HV0.50/30 210 hardness and 300 hours anti-corrosion time in salt spray test. In the coating, the addition of sodium polyphosphate could greatly improve the corrosion resistance of the coating because of the barrier film formed by sodium polyphosphate. The mechanism of interpenetrating polymer network (IPN) was preliminary put forward based on the results of FTIR.

Abstract: The different chrome-free chemical conversion coatings were prepared on AZ91D magnesium alloy and SEM was used to observe the surface. The corrosion behaviors of die-casting AZ91D magnesium alloy with different coating in chloride environment were investigated by hydrogen gas evolution in immersion test, salt spray test and electrochemical measurement. The results showed that the corrosion resistance of magnesium alloy treated with two-step chemical conversion and sealing was the highest and was comparable with that of chromating, and followed by two-step chemical conversion treatment without sealing. The one-step chemical conversion treatment had the lowest corrosion resistance. Polarization and EIS characteristics in 5% NaCl solution depended on the type of surface treatment and correlated well with the order of corrosion resistance. Contrasted with one-step chemical conversion coating, the coating of two-step chemical conversion with sealing on magnesium alloy had more excellent corrosion resistance because the |Z| increased, the corrosion area was decreased and the time of the first pitting occurrence was prolonged. It is expected that this experiment can provide a foundation for designing more super chemical conversion coating to replacing the chromate conversion.

Abstract: Influence of SO2 concentration on initial corrosion of aluminum was studied in simulated marine atmosphere. Variation of initial corrosion morphology, relation between weight gain and time in the course of initial corrosion, corrosion speed (token with maximum pitting depth and weight gain) were analyzed and discussed. Primary results is list below: In marine atmosphere, SO2 sedimentation above 0.1 mg/100cm2 can accelerate Al pitting with low Cl- sedimentation. Both corrosion weight gain and pitting depth increase obviously. For pitting surface of LY12 with SO2 and Cl- sedimentation, corrosion potential change little, Nyquist Graph was single arc of capacitance resist with retractile real part, Rr can’t reflect corrosion degree.