Electrochemical Study On The Iocalized Corrosion Behavior Of High Strength Aircraft Aluminum Alloys

The corrosion behaviors of pure aluminum, LY12 (AA2024) and LC4 (AA7075) in different accelerated localized corrosion solutions were studied using electrochemical impedance spectroscopy (EIS) and electrochemical noise (EN) techniques in conjunction with the scanning electron microscopy (SEM) and metalloscope. Some conclusions of this dissertation were drawn as follows:The high concentration of chloride ion in solution inhibits the repassivation of a metastable pit. The continuous oscillation of the electrode potential may be made up of several parallel and series pitting corrosion, indicating that Cl~- ion can improve pitting corrosion. All EN experiments of pure aluminum, LY12 and LC4 show the pitting corrosion takes on randomicity and indeterminacy. Impedance technology is a useful method for pitting corrosion study. EIS of Pitting corrosion on pure aluminum, LY12 and LC4 takes on different characteristic in Nyquist and Bode plot. We can obtain more detail information of pitting corrosion from the parameter variety of the corresponding equivalent circuit. The study also shows the corrosive of Cl" ion is much higher than other aggressive ion. From the SEM images, the pitting corrosion mainly occurs around and/or along constituent particles containing high atomic weight elements.For intergranular corrosion, Nyquist and Bode shape could be used for judgement of intergranular corrosion sensitivity on aluminum alloy, while parameters Rt and Clf/Cdl could provide conveniently more corrosion information, like degree of localized corrosion, corrosion rate. EIS of different aged state LY12 and LC4 present different behavior. State Td of LC4 alloy takes on a single capacitive arc during whole immersion time, while corresponding metalloscope image indicates the pitting corrosion is main corrosion type in intergranular corrosion accelerated solution. The EIS above real axis of the state T3, T6 of LY12 and Ts of LC4 alloy takes on two capacitive loops behavior, while high frequency is corresponding to charge/discharge behavior of origin interface of corrosive electrode, low frequency is corresponding to new interface attacked by aggressive ions. From immersion test, SEM image and EIS, We also put forward a new parameter Clf/Cdl, which is useful tool to estimate degree of localized corrosion,, For exfoliation corrosion, LY12 and LC4 alloy have similar electrochemical behavior. At the beginning of immersion in EXCO solution, theNyquist plot is mainly comprised of the least two depressed capacitive arc in the high-mediate frequency range and an inductive arc in the low frequency range. In a short immersion period, charge transfer resistance gradually decrease, illustrating the acceleration of the corrosion rate, whereas the proton concentration decreases steeply, indicting the cathodic process is pre-dominant. Then the corrosion rate decreases gradually corresponding to the exhausting of proton ion.When exfoliation begins to occur, the low frequency inductive arc disappears and the Nyquist plot is mainly composed of two capacitive arcs, the high-mediate one originates from the old surface of the corroding electrode while the low frequency loop from the newly formed interface by exfoliation corrosion. The exfoliation is developed from pitting corrosion through intergranular corrosion to general corrosion at the end.With the change of corrosion type of LY12-T3 from pitting corrosion through intergranular corrosion to exfoliation corrosion, the maximum relative energy defined in EDP will change from the region of crystal series coefficients with smaller scales through middle scales to larger scales. Under the experimental conditions, the characteristics of EDP obtained from wavelet analysis technique can be used as "fingerprints" of EN signal and can be used to differentiate the corrosion type.The EIS of aluminum alloy LC4-Ts in standard EXCO solution at open circuit potential was well analyzed by the proposed reaction model, which indicates the electrochemical reaction of aluminum dissolution can be divided into three stages through the EIS character and variety of pH value of EXCO solution in different time. From the mathematic deduction, we think the low frequency inductive loop is associated to Hydrogen evolution at the first 3 Oh immersion when the localized corrosion is mainly pitting corrosion. At the middle of immersion, two capacitive loops can be seen which indicates film formation with chloride ions on the working electrode surface. At the last the 36h immersion, the inductive loop takes on the low frequency again, which indicated the corrosion reaction changes and corrosion product, loose Al(0H)3, is covered partly on the surface.We also developed electrochemical noise analysis software, ENAN, by butterfly Fast Fourier Transform algorithm using Vision C++ language. The software ENAN could analyze the electrochemical noise data that were obtained by electrochemicalworkstation of the Labs. At the same time, the software also calculated the parameter of power density spectroscopy (SPD) by linear least square method and the criterion Se and Sg of pitting corrosion. The software was easy to operate and had a good reproducibility of the analysis result. The value of parameter Se and Sg could characterize the severity and trend of pitting of corroding material, so we could use the ENAN software to estimate the pitting corrosion quickly and rightly. This dissertation also provides wavelet analysis program based on matlab soflware. The results export as ASCII mode file, which is convenient to mathematic and plot software process.