The Investigation On The Properties Of Aeronautic Aluminum Alloys Under Synergistic Action Of Environmental And Mechanical Factors

Super-duralium that is widely used for the longerons, frames, stringers of military and civil aeroplanes, is an important aeronautic material. Because fully avoiding of environmental erosion is difficult, the research on the corrosion damage of the super-duralium is still one of the affairs of the first magnitude.The damage of super-duralium caused in environment is mostly exfoliation corrosion. The variations of the surface state and environment conditions could change the procedure of exfoliation, therefore, the potentiometry and electric resistance methods in this paper are used to measure the exfoliation rate of Aluminum alloys in a constant temperature, the experimental results show that relations of natural corrosion potential vs time have a resemblance to those of corrosion rate vs time, both relations are all at the different change rates in the front and back of 120 minutes. From the varieties of the natural corrosion potential of Aluminum alloys, it is deduced that the gradual dissolution of natural passive state film of the alloys brings about high-speed enhancement in the corrosion rate of the alloys. Because of the place obstruction of corrosion products in the surface of the alloys, while corrosion time is at 120 minutes above, the corrosion rate descends to the stable value in 48 hours.In the action of external force, Aluminum alloys could bare out fresh metal, so there are galvanic couple between the fresh metal and rest parts passivated. Electrochemical tests show that the longer time is, the higher natural corrosion potential of the bare metal is. If corrosion time is long enough, the potential will go to a stable value. In the state of galvanic couple, the couple potential of the bare metal quickly shifts towards positive direction, and to approach the stable natural corrosion potential of Aluminum alloys after 40 seconds. On the basis of couple polarization theory, the mathematical expressions of couple potential and corrosion rate of fresh bare metal are inferred, the theory modeling relations of couple potential vs time is corresponding to experimental results.The couple theory above-mentioned is applied at the crack tip of Aluminum alloys, on ground of the theory, the experiment on the electrochemical action at the tip shows that the faster the strain rate is, the quicker the dissolution rate of the fresh surface of Aluminum alloys is. the faster the strain rate is, the quicker the increment rate of couple current vs time is, while strain is equal to 0.04 about, the couple currents show the trend of sharp enhancement.Corrosion affects not only the loading capacity of Aluminum alloys component, but also the strength. The composition and modality of the environment medium play animportant role in the crack growth of corrosion fatigue. The formation of 3.5% NaCl solution is almost similar to that of 3.5% NaCl thin electrolyte layer, just the mass transfer rate of the oxygen in the layer is more quicker than in the solution ?The test shows that the crack growth rate of wet-corrosion fatigue of LC4CS Aluminum alloy in the layer is quicker than in the solution, and also shows that morphology of fatigue fracture in the layer is evidently distinguished from in the solution. The corrosion resistance of Aluminum alloys is improved by anodic oxidation, if the fatigue behaviors are taken into account, the behaviors of Aluminum alloys by sulfuric acid anodizing is better than by chromic acid anodizing.Furthermore, the supposition of galvanic couple is drawn into the procedure of corrosion fatigue crack growth of Aluminum alloys, the relation of couple current vs loading time is determined with electrochemical methods. A formula in the theory is deduced which expresses the stress corrosion crack growth rate of LC4CS Aluminum alloy, the results of mathematical modeling preferably correspond with examination.