Materials Science Forum Vols. 519-521

Abstract: The corrosion susceptibility of friction stir welds in AA2024-T351 was found to vary with the weld processing parameters. Corrosion attack was investigated with in situ X-ray tomography, which showed how the penetration of corrosion into the interior of the structure varied with weld microstructure. The susceptibility to corrosion was related to the degree of overageing by comparing the corrosion behaviour to samples of the base alloy that had been aged at different temperatures. A systematic increase first in the anodic reactivity and then the cathodic reactivity of the overaged structures with temperature can be used to predict the location of the region of the weld with the highest susceptibility to corrosion. Similar investigations were made for a dissimilar weld between AA2024 and AA7010. Laser surface melting produces a thin homogeneous melted and rapidly solidified layer over the weld surface leading to a substantial improvement in corrosion resistance.

Abstract: The influence of microstructure on the electrochemical properties of an Al-5 wt% Cu binary alloy which was severely deformed by equal-channel angular pressing (ECAP) has been investigated in a borate-boric acid buffer solution containing Cl¯ ions at pH 8.3 and 25°C by potentiodynamic polarization test. The anodic polarization results showed that pitting potentials of ECAPed Al-Cu alloy samples were higher than that of the sample without ECAP and increased with repetitive ECAP passes. The pitting corrosion attack of Al-Cu alloy samples took place in the surroundings of Al2Cu (θ) phase. It is considered that the grain refinement of θ phase and the solid solution generated in Al matrix during ECAP process are responsible for the improvement of the corrosion resistance of ECAPed Al-Cu alloy.

Abstract: In this work we present deformation experiments of polymer-coated polycrystalline aluminium sheets. We observe that the straining is accompanied by the development of microstructural defects at the sample surface as well as in the interface between the metal and the different polymers. These defects are due to a variety of dynamical mechanisms which are essentially induced by bulk plasticity of the metal substrate. They micromechanically interact with the polymer coating and transfer some of the metallic roughness to the coating and to the surface.

Abstract: The properties of oxide formed on pure aluminium in acetate buffers under controlled potential, temperature, and pH are characterized by in situ electrochemical techniques and spectroscopic visual ellipsometry (VISSE). Monitoring the current density (cd) during transition between different applied potentials gives direct information about Faradaic changes in the oxide properties, while the ellipsometer monitors the changes in the actual thickness and refractive index of the film. Electrochemical impedance spectroscopy (EIS) reveals, in addition to film properties, information about the charge transfer processes and adsorption at steady-state conditions. VISSE and ex situ transmission electron microscopy (TEM) indicate the presence of a two layered oxide structure, consisting of the barrier and a porous hydrated layer. The thickness and density of these two layers are monitored in situ during growth and dissolution. The thickness of the barrier layer depends reversibly on the applied potential, while the thickness of the hydrated layer is less affected by the potential. The absorption sensitive parameter is correlated with the buffer concentration and therefore believed to be related to the acetate.

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: It is well known that the Al-Fe system intermetallic compound particles on the aluminum and its alloys are detrimental to the corrosion for aluminum materials. Trial and error efforts reveal that Al3Fe intermetallic compound particles exposed on aluminum are preferentially and selectively removed by electrochemically treatment in cathodic current. These treatment combined by two steps: the first step is selective dissolution of aluminum component in the Al3Fe intermetallic compound by electrochemical reaction. The second step is intensive removal of the iron-enriched intermetallic particles on which vigorous hydrogen bubbling has been took place. The electrochemical measurement and surface observation show that the aluminum and its alloys with free intermetallic compound particles treated by aboves have excellent corrosion resistance in a NaCl solution. It is also found that the intermetallic compound-free surface is preferable for the pretreatment surface for the surface finishing.

Abstract: Microstructure and corrosion behaviour of 6061 and 6013 sheet material were investigated in the naturally aged and peak-aged heat treatment conditions. Transmission electron microscopy did not reveal strengthening phases in the naturally aged sheet. In the peak-aged temper, β’’ precipitates were observed in alloy 6061, whereas both β’’ and Q’ phases were present in 6013- T6 sheet. Marked grain boundary precipitation was not found. Corrosion potentials of the alloys 6061 and 6013 shifted to more active values with increasing aging. For the copper containing 6013 sheet, the potential difference between the tempers T4 and T6 was more pronounced. When immersed in an aqueous chloride-peroxide solution, alloy 6061 suffered predominantly intergranular corrosion and pitting in the tempers T4 and T6, respectively. On the contrary, 6013 sheet was sensitive to pitting in the naturally aged condition, and intergranular corrosion was the prevailing attack in the peak-aged material. Both alloys 6061 and 6013 were resistant to stress corrosion cracking in the tempers T4 and T6.

Abstract: The morphology and propagation of corrosion pits on a 6056-T78 aluminum alloy in a sulfate and chloride-containing solution have been investigated and the influence of different parameters has been studied: the passivation potential for passive film growth, the pitting potential i.e. the applied potential during the pitting process and the time for pit propagation. The passivation potential did not influence the pit morphology and the pit propagation; it only influenced the pit density. On the contrary, it was found that the pitting potential and the time had a similar influence on the pit growth. For low pitting potentials or short times, the pit depth to pit radius ratio was high (about 0.7) whereas it decreased to a value close to 0.4 for higher pitting potentials or longer times.

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: The present work is part of a broader investigation of the effects of additives in zinc phosphate (ZPO) coating solutions that are designed for specific applications to Al and its alloys. ZPO conversion coatings improve the corrosion resistance of the Al substrate and increase the adhesion of paint. Coatings formed on 2024-T3 aluminum alloy, after dipping in ZPO coating baths containing Mn2+, have been studied by X-ray photoelectron spectroscopy (XPS), SEM, and scanning Auger microscopy (SAM). Variations are observed in coating morphology and composition as the amount of Mn2+ in the coating solution increases through the 0 to 2500 ppm range. Adhesion tests give information on the relative strength of the coating-substrate interaction at different microstructural areas.