| Magnesium, used as the function material in negative electrode of electrochemical power, possesses several characteristics of high theoretical specific capacity(2.22 A·h/g), low density, great material abundance and environmentally friendly that rank it as one of the highlights in the research of energy field. A key obstacle on the way of R&D of the usage of Mg alloy in primary batteries is the “delayed action effect” caused by the passive film formed on the electrode surface, which is owing to the higher chemical and electrochemical activity of Mg alloy and its faster corrosion rate. AZ91, AZ61 and AZ31 are the earlier employed magnesium alloys in the field of primary battery, however the researchers mainly focus on their electrochemical behavior and corrosion resistance.For improving the performance of primary battery, the systematic investigation was conducted on the electrochemical characteristics, corrosion film-form process, surface film structure and delayed action of AZ Mg alloys in electrolyte mainly composed of magnesium sulfate. The main contents are as follows:(1) The LSV, CV and EIS techniques were employed to study the general corrosion and pitting corrosion behavior of AZ Mg alloys, and to build the relationship of the factors for film breakdown with electrochemical behavior. With increasing concentration of Mg SO4, the polarization performance of AZ63 Mg alloy weakened and the range of passivation narrowed down. An increase of Mg SO4 concentration gave rise to an enhanced polarization performance of AZ31 B Mg alloy, and a decreased corrosion rate. For higher scanning rate, a more noble value of passivation-activation transformation potential and a lower polarization current appeared. When the initial potential scan shifted to a more negative value, the corrosion current became lower. The electrochemical impedance spectra of AZ Mg alloys were characterized by two capacitive loops, namely a high-frequency capacitive loop induced by charge transfer and a low and medium-frequency impedance loop caused by film resistance respectively, the later of which often comes out a certain extent of deflection because of “dispersion effect”. For longer immersion time, low-frequency inductive loop disappeared and both of the resistances for charge transfer and film increased.The potential scanning speed, types of electrolyte and cathodic polarization had significant influence on the pitting corrosion behavior of AZ Mg alloys. When the potential sweep rate decreased from 3.3 m V/s to 0.17 m V/s, the area of pitting hysteresis loop increased successively and the breakdown potentical(Eb100) shifted to more negative values. Mg SO4 concentration of 1.5 mol/L might be more prone to coming about pitting corrosion. When the cathodic polarization potentical turned negative, Eb100 diminished, and the area of pitting hysteresis loop increased, signifying a bigger tendency for pitting corrosion occurrence.(2) The structure of the surface film on AZ63 Mg alloy was amorphous, presenting a morphology of gravel granular material embedded on magnesium oxide base film, which mainly contains Mg O、Mg(OH)2 and sulphate or carbonate of magnesium. Magnesium oxide base film grew preferentially on α phase of AZ alloys, then it occured on α phase close to the edge of β phase, and finally formed on β phase. The sites of α phase provide active sites of mosaic structure on the surface film to form chemically complex basic carbonate, hydroxyl magnesium aluminum sulfate salt. Mg Al2(SO4)4·22H2O and Mg5(CO3)4(OH)2·8H2O were observed in the surface film for more than 24 h of immersion, while for the immersion time of less than 12 h, there were mainly Mg(OH)2 and Mg CO3.The surface film on AZ31 B Mg alloy showed a monolayer crack structure, mainly containing the elements of C, O, Mg and S. When increasing immersion time, the dried mud flat slice grew rapidly with an average grain size of 80 μm and crack width of about 20 μm. Slice layer or clubbed S-O-Mg corrosion products accumulated on the surface film after 12 d of immersion. The surface film initially formed and grew stably after 9 h of immersion, the various micrographs of which could be described by SKP potential maps. With increasing immersion time, volt potential moved to positive direction, the potential dip between the surface film and magnesium substrate narrowed, and surface film gradually became continuous and dense.(3) During the immersion period, the width of the crack in the surface film increased progressively, embedded particles increased and the grain size became bigger. Holes and tunnels occured after corrosion on local region, large areas of the edge of magnesium matrix fell off, giving rise to collapse of the structure. The structure change of the surface film under free corrosion condition could be summarized as “slow film dissolution, significantly increased crack―partial film repair―repeated film dissolution and repair―structural damage and performance loss”.After discharge of AZ Mg alloys, the diameter of the capacitive loop reduced significantly, simultaneously the breakdown of the film led to the low-frequency inductive loop. According to the potential variation, the potential-time curve could be generally divided into three stages, namely sharp decrease, continuous negative shift and stable attenuation. After a long time for large current discharge, the surface micro-cracks grew in number, mosaic structure fell off, and multiple base film deletion occured, resulting in bulky tunnels filled with loose corrosion products, which mainly consisted of Mg(OH)2 and Mg SO4. The structure variation of the surface film for the discharge process could be generalize as “rapid film dissolution, crack micronization―continuous film dissolution, product clastic accumulation―complete base film dissolution, mosaic structure fall off ”.(4) The voltage delay of magnesium corrosion film was under the influence of discharge current, immersion time, concentration of the electrolyte and the addition agent. The delay time of AZ63 Mg alloy was shortened with increasing the concentration of magnesium sulfate, which had little effect on the delay time of AZ31 B Mg alloy. When added with potassium bromide, the delay time of AZ63 Mg alloy was reduced to 1 s, but the delay time of AZ31 B Mg alloy changed little. The difference lied in the different structure of the surface film on the two alloys. Slightly shorter delay time of AZ31 B Mg alloy appeared when the additon of Na F was 70-80 mmol/L. For AZ63 Mg alloy, greater current density would make for more negative stable potential and increased potential dip. Current density had particularly significant impact on the delay time of AZ31 B Mg alloy. The immersion time had more influence on the delay time of AZ63 than that of AZ31 B. Prolonged immersion time would lead to enhanced potential dip of magnesium alloy, more positive stable potential and increased delay time.(5) The electrochemical behavior of magnesium electrode in mixed solutions and the effect of surface film structure on the delayed action were investigated separately. ?The morphology of the surface film on AZ63 Mg alloy in the mixed solutions(magnesium perchlorate-magnesium sulphate) was similar to that observed in single magnesium sulphate. The passivation range extended with increasing volume concentration of magnesium sulphate, while the dissolution rate of surface film decreased. The delay time of magnesium electrode would be shortened to 0.5 s when the volume ratio(magnesium perchlorate/magnesium sulphate) was 2:1and 6:1. ?The EIS of AZ Mg alloys immersed in the mixed solutions(magnesium nitrate-magnesium sulphate) all consisted of two continuous capacitive loops. It would obtain shorter delay time than single salt at the volume ratio of 67:33, where the stable potential was more negative and no obvious pitting corrosion was observed. ? The delay time of AZ31 B Mg alloy in the mixed solutions(sodium nitrite-magnesium sulphate) presented a trend that firstly increased and then decreased with increasing immersion time. The charge chansfer resistance and film resistance increased with prolonged immersion time.(6) Current pulse would change the structure and composition of surface corrosion film on AZ Mg alloys, as well as the phenomenon of voltage delay. After current pulse, the surface film was shocked and developed into different sizes of corrosion holes. The component of the film on AZ Mg alloys after current pulse and constant-current discharge was basically the same. The results of XPS revealed that the main composition was Mg(OH)2 and Mg SO4. It had been discovered that the stable potential decreased, and the potential dip was lower than that observed before pulse. The pulse height of 50 m A or width of 100 ms would contribute to shorten the delay time. The optimum condition for double pulse was first pulse stage(5 m A-100 ms)―interval(1 s)―second pulse stage(25 m A-50 ms), under which the delay time was nearly 0 s, and the potential dip would be controled within 2.5 V. |
PDF Full Text Request