【作者】 陈琳；

【导师】 陈昌国；

【作者基本信息】 重庆大学 ， 化学工程与技术， 2015， 博士

【摘要】 镁电势较负,理论比容量高达2.22 A·h/g,具有密度小、来源丰富、环境友好等优点,可作为功能材料应用于化学电源的负极,是目前能源领域研究的重点之一。但镁合金较高的化学及电化学活性致使负极的腐蚀速率快,金属表面在电极反应过程中形成的钝化膜造成滞后效应,严重限制了镁合金在一次电池领域中的应用。AZ91、AZ61、AZ31合金是较早应用在电池领域的镁合金,然而近年来人们主要针对其腐蚀行为及耐蚀性能进行了研究。为此本论文对AZ镁合金在以硫酸镁为主的电解液中的电化学特性、腐蚀成膜过程、表面膜结构和电压滞后的进行了系统研究。结果表明:(1)采用LSV、CV、EIS技术研究AZ镁合金的全面腐蚀和点蚀行为,建立起镁负极腐蚀膜的破坏因素与电化学特性的关联性。Mg SO4浓度增大后AZ63合金的极化性能减弱,钝化范围收窄;AZ31B合金的极化性能增强,腐蚀速率下降。扫速增大致使钝化-活化转变电位正移,镁电极的极化电流变小。随起始扫描电位负移,腐蚀电流变小。AZ镁合金的阻抗谱由电荷传递引起的高频容抗弧和膜电阻引起的中低频阻抗弧构成,后者通常由于“弥散效应”而发生偏转。延长浸泡时间会使低频感抗弧消失,电荷转移电阻Rct和膜电阻Rf均增大。电位扫描速度、电解质溶液、阴极极化对镁合金点蚀有显著的影响。扫描速率从3.3 m V/s下降至0.17 m V/s时,AZ63合金的点蚀滞后环面积依次增加,击穿电位Eb100负移;Mg SO4浓度为1.5 mol/L时AZ31B合金最容易发生点蚀;阴极极化电位变负,击穿电位减小,滞后环的面积增加,镁合金点蚀的倾向变大。(2)AZ63合金表面膜为非晶结构,呈现基膜上镶嵌碎石状颗粒形貌,主要成分包括Mg O、Mg(OH)2和镁的硫酸盐、碳酸盐。氧化镁基膜优先在AZ镁合金的α相上生长,然后在靠近β相边缘的共晶α相上出现,最后在β相上形成氧化物薄膜。α相为表面膜的镶嵌结构提供了反应活性位,该处形成了成分复杂的碱式碳酸盐和硫酸镁铝盐,它们存在于浸泡24 h及以上时间的表面膜中,而浸泡时间低于12 h的表面膜镶嵌结构中主要存在Mg(OH)2和Mg CO3。AZ31B合金表面膜为单层龟裂结构,主要含C、O、Mg、S元素。延长浸泡时间使平直干泥状层片迅速长大,平均粒径80μm,裂纹宽度约20μm,浸泡12 d后表面堆叠着片状硫氧镁腐蚀产物。AZ31B合金在Mg SO4溶液中浸泡9 h后,表面膜初步形成并开始稳定生长,能够使用SKP电势分布图来描述表面微观形貌的变化,此后伏打电位逐渐正移,表面膜与镁基质间的电位降缩小,膜层变得连续致密。(3)自腐蚀过程中表面膜上裂纹宽度逐渐增大,镶嵌颗粒数量增多、粒径变大;局部区域腐蚀后出现孔洞和坑道,边沿部分的镁基质大面积脱落,引发结构塌陷。自然腐蚀状态下表面膜的结构变化可概述为“膜缓慢溶解、裂纹显著增多—膜原位部分修复—膜反复溶解、修复—结构破坏、性能丧失”。AZ镁合金放电后容抗弧的直径明显减小,同时膜破损引发低频感抗。表面膜的电位变化分为急剧下降、持续负移、平稳衰减3个阶段。经过长时间大电流放电,表面微裂纹增多,镶嵌结构脱落,基膜多处发生缺失,留下体积较大的蚀坑,其中填充着疏松的腐蚀产物,主要成分为Mg(OH)2和Mg SO4。放电状态下镁表面膜的结构变化可概述为“基膜迅速溶解、裂纹微细化—基膜持续溶解,产物碎屑堆叠—基膜完全溶解,镶嵌结构脱落”。(4)镁负极腐蚀膜的电压滞后受硫酸镁浓度、添加剂、电流密度和浸泡时间的影响。AZ63合金的滞后时间随硫酸镁浓度增加而缩短,而硫酸镁浓度对AZ31B合金的滞后时间影响不大。添加KBr后,AZ63合金的滞后时间缩短至1 s,但AZ31B合金的改变很小,这种差异是由两种合金表面膜的结构不同所引起。当添加70-80 mmol/L Na F后AZ31B合金的滞后时间略有缩短。提高放电电流可使AZ63合金的稳定电位正移,电位降增大,电流密度对AZ31B合金滞后时间的影响特别显著。浸泡时间对AZ63合金滞后的影响比AZ31B合金大得多,镁电极的电位降随浸泡时间延长而增大,稳定电位变正,滞后时间增加。(5)研究了镁负极在混合电解液中的电化学行为,以及表面膜结构对电压滞后的影响。①在(高氯酸镁-硫酸镁)混合液中AZ63合金表面膜形貌与单一硫酸镁中相似。随硫酸镁体积浓度增大,镁电极的钝化范围扩大,表面膜的溶解速率下降。当(高氯酸镁/硫酸镁)的体积比为2:1和6:1时,镁负极的滞后时间缩短至0.5 s。②在(硝酸镁-硫酸镁)混合液中AZ镁合金的阻抗谱均由两个连续的容抗弧构成,当体积比为67:33时可以得到比单盐滞后时间更短,稳定电位更负,且电极表面无明显点蚀的膜层。③AZ31B合金在(亚硝酸钠-硫酸镁)混合液中的滞后时间总体呈现先增加后减小的趋势,随着浸泡时间的延长,电荷转移电阻和膜电阻均增大。(6)电流脉冲可干预AZ镁合金表面腐蚀膜的结构和成分,改善镁负极的电压滞后现象。电流脉冲后,AZ镁合金表面膜被冲击形成了大小不同的蚀坑,其成分与恒流放电后基本相同,XPS结果表明腐蚀产物的主要成分为Mg(OH)2和Mg SO4。加载脉冲使AZ镁电极的平衡电位降低,电位降远小于脉冲前,脉冲高度50 m A或脉冲宽度100 ms有利于减少AZ镁合金的滞后时间。最佳双电流脉冲条件为:第1阶段脉冲(5 m A-100 ms)—间歇(1 s)—第2阶段脉冲(25 m A-50 ms),此时不仅滞后时间近似为0,而且脉冲电位降能控制在2.5 V以内。更多还原

【Abstract】 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.更多还原