Study On The Anodizing And Corrosion Behavior Of AZ31 Magnesium Alloys

Magnesium is one of the most abundant elements in the earth, which alloys possess many advantageous properties that make them promising materials for a lot of applications, especially for aerospace components, automobiles, computer parts, 3C equipments mobile phones and handheld tools and so on. The excellent properties included are as follows: high strength / weight ratio, high thermal conductivity, high dimensional stability, good electromagnetics, good machinability and being easily recycled. However, magnesium alloys also possess some disadvantageous properties including poor corrosion and wear resistance, high chemical reactivity, while the former has been the bottleneck for their widespread use in many applications. One way to prevent corrosion is to coat the base metals. Coatings can protect the base materials by provide a barrier, which contains corrosion inhibiting chemicals, between the interface of the base metal and the aggressive medium. Researchers have developed many coating processes to enhance the corrosion protection on the magnesium alloys, two classics are HAE and D0W17 processes, both of which can produce a two phase coating of c.a. 30μm in thickness on the base magnesinum metals. But till now most treatments, including HAE and DOW 17 processes, have chromate/ phosphate/ fluoride in the electrolyte, these compounds do harm to human beings and the environments. So researchers began to developed new processes that are not harmful.Based on the advantages of magniseum alloys and their potentially widespride apphcations, this dissertation trys to obtain a novel environmently friendly anodizing technique of AZ31 magniseum alloys, and ascertain the corrosion mechanisms of nude AZ31 magniseum alloys and their corresponding anodized films. The main conclusions of this dissertation are drawn as follows:A novel anodizing process using 50 Hz ac voltage for AZ31 magnesinum alloys has been developed and studied. The anodizing electrolyte is an alkaline borate solution containing a new kind of organic additive, which possesses no harmful compounds like chromate, phosphate and fluoride. It was found that the formation of the anodic films wasalways coupled with sparking and oxygen evolution, which intensity changed with the additive. The structure, the corrosion resistance and the morphology of the anodic films were mainly depended on the anodizing voltage, time and additives. The results also shown that, the clay-like anodic film may be an alternative for traditional ceramic-like anodic film for magnesium alloys protection.The influences of the main anodizing technological parameters including the anodizing voltage, the anodizing time and the additives on the structure and anti-corrosion performance were ascertained using potentiodynamic polarization method in conjunction with the scanning electron microscopy (SEM) and X-ray diffraction (XRD) techniques. The anodizing technological parameters have been optimized and the smooth clay-like anodic film with high corrosion resistance was obtained. The optimized anodized film is uniform and with high anti-corrosion performance and high adhesion to magnesinum alloy substrate, meanwhile its thickness and surface color can be manipulated by anodizing time and additives respectively. X-ray diffraction patterns show that the composition of the optimized anodic film is mainly MgO (Periclase) and little magnesium.The corrosion processes of nude AZ31 magnesium alloys and their anodized films in neutral O.lmol/L NaCl solutions were investigated using electrochemical noise (EN) technique and electrochemical impedance spectroscopy (EIS) method in conjunction with the scanning electron microscopy (SEM) and X-ray diffraction (XRD).During the corrosion process of nude AZ31 magnesinum alloys, the complex plane plot of EIS always contains a low-frequency inductive loop, which may be originated from the pitting corrosion. At the beginning of corrosion, there exist two capacitive arcs above the real axis of Nyquist plot, the high-frequency arc may be attributed to the original discrete film formed in the air, while the capacitive arc in the intermedial frequency domain may be originated from the corrosion reaction. With the prelongation of corrosion time, the two capacitive arcs evolved into only one arc at last, the reason may be that, the original discrete film has been dissolved due to the corrosion process and the electrode surface has been entirely covered by a loose corrosion-product layer. Meanwhile, EN analyzed results showed that, electrochemical noise signal is composed of distinct types of events that can be classified according to their scales, i.e. their time constants. Atthe beginning of corrosion, the maximun relative energy is defined in the low-frequency (crystals with high scale) region due to the diffusion processes of aggressive ions to the defects of the original discrete film, which is formed in the air, while the D crystals of small timescales also possesses high relative energy due to the pitting corrosion. With the prelongation of immersion time, the structure of corrosion product layer changes (local departion, growth) due to the influences of H2 and aggressive solutions, the accumulated position of the maximun relative energy changes synchronously.During the holistic corrosion process of anodized AZ31 magnesinum alloy films, the characteristics of Nyquisy plot change from the onset two capacitive arcs and one inductive loop, through one capacitive arc and one inductive loop to two capacitive arcs and one inductive loop again. The former two capacitive arcs in the high frequency and the intermedial frequency domains may be attributed to the oxide film and the thermodynamic effect caused film refining respectively, while the latter two capacitive arcs may be originated from the disabled thin discrete oxide film including corrosion products layer (high frequency arc) and the corrosion reaction (low frequency arc) respectively.