【作者】 蒋全通；

【导师】 张奎；

【作者基本信息】 北京有色金属研究总院 ， 材料科学与工程， 2014， 博士

【摘要】 镁合金的主要服役环境为大气环境,因此镁合金大气腐蚀最为普遍。研究镁合金的大气腐蚀行为,了解不同环境中镁合金的腐蚀特性和腐蚀规律,对合理选用防护措施,延长设备和构件的使用寿命,减少经济损失,有重要的理论意义和广泛的应用价值。目前镁合金大气腐蚀的研究手段单一,可靠性差。本论文就镁合金大气腐蚀研究方面的不足,通过分析水溶性无机盐、相对湿度、C02浓度、氧化膜致密度系数(PBR)对不同状态EW75镁合金腐蚀行为的影响规律,结合大气腐蚀模拟实验和大气暴露实验,揭示腐蚀机理,为开发新型耐蚀镁合金以及确定防护工艺提供了可靠的科学依据。设计研制镁合金大气腐蚀模拟实验箱,通过控制箱体内的温度、湿度、腐蚀气体浓度、无机盐种类与浓度,保持实验周期不变,模拟真实大气环境对EW75镁合金腐蚀行为的影响,有效改进传统盐雾实验的不足,可以更加准确的掌握合金在自然大气环境中的服役状态。研究了大气环境中代表性水溶性无机盐Ca(NO3)2、NaCl、(NH4)2SO4对EW75合金腐蚀行为的影响规律。EW75合金的平均腐蚀失重顺序为：Ca(NO3)2<NaCl<(NH4)2SO4。在Ca(NO3)2环境中,Ca2+离子半径为99pm,N03-离子半径为121pm,均能穿过腐蚀产物膜到达基体,Ca2+与OH-反应生成难溶性中强碱Ca(oH)2,导致基体表面薄膜液pH值增大,形成Mg(OH)2/Ca(OH)2保护膜,因此腐蚀程度较低。在NaCl环境中,C1-离子半径为130pm,被腐蚀产物膜吸附后代替薄膜液晶格中H2o、OH-或02-的位置,降低了腐蚀反应活化能；同时C1-会加速电子传输过程,导致腐蚀电流密度增大,因此腐蚀比较严重。在(NH4)2S04环境中,NH4+双水解产生H+,H+离子半径仅为1.2pmm,极易穿过表面钝化膜；同时Mg(oH)2钝化膜反应形成可溶性MgS04,导致裸露出新的基体,因此合金腐蚀最严重。由于NH4+离子半径为148pm,S042-离子半径为295pm,因此在(NH4)28o4环境中合金表面形成腐蚀坑的尺寸较大。研究了相对湿度和Co2浓度对EW75合金腐蚀行为的影响规律,探索难溶性腐蚀产物膜对基体的防护机理,进行双因素相关度分析。RH=40%时,随着Co2浓度增加,腐蚀失重速率和力学性能损失量均增大,但合金表面腐蚀程度相对轻微；RH=70%时,Co2浓度增大,合金腐蚀加剧,但腐蚀程度远大于RH=40%的样品。RH=90%时,随着C02浓度增加,合金腐蚀程度出现先增大后减小的趋势(拐点处C02=800ppm)。相同C02浓度下合金腐蚀顺序为：RH40%<RH90%<RH70%。A4RH40%+2000ppm)样品腐蚀产物主要为Mg(OH)2和MgCO3·3H2O, B4(RH70%+2000ppm)样品为MgCO3·3H2O和MgCO3·5H2O, C2(RH90%+800ppm)样品为Mg7(CO3)4(OH)2·5H2O, C4(RH90%+2000ppm)样品为Mg7(CO3)5(OH)4·24H2O。腐蚀速率受腐蚀产物膜的结构、厚度、稳定性、渗透性等因素控制,难溶性腐蚀产物对基体具有一定保护作用。影响因子相对湿度和C02浓度发生交互作用,取检验水平为a=0.0001,FA=0.10997531>F0.0001(2,6),FB=0.08750816>Fo.0001(3,6),2个因素影响均十分显著,且相对湿度的影响大于Co2浓度。EW75合金生产、存储及服役过程中,为了减小Co2的腐蚀作用,应该尽可能降低环境相对湿度(人工除湿)。模拟EW75合金在北京站和青岛站腐蚀不同时间。模拟北京站时效态合金的腐蚀失重速率曲线为：Y=A+B,*X+B2*X2+B3*X3, A=-5.60973, Bi=1.04291, B2=-0.01687,B3=1.17699×10-,相关系数为0.99251；模拟青岛站曲线为：Y=A+B1*X+B2*X2+B3*X3,A=-4.79713, B,=1.20479, B2=-0.01924, B3=1.31504×10-4,相关系数为0.99161,其中1≤x≤72。拟合曲线的系数均接近于1,相关性良好。进行EW75合金北京站和青岛站大气暴露实验。北京站腐蚀12个月,时效态、挤压态、固溶态挂片样品的平均腐蚀失重速率分别为42.4444,19.1833,18.4444g.m-2；时效态、挤压态、固溶态拉伸样品的平均剩余抗拉强度分别为365,306,290MPa。青岛站腐蚀12个月,时效态、挤压态、固溶态挂片样品的平均腐蚀失重速率分别为55.9334,25.7833,22.0389g-m-2;时效态、挤压态、固溶态拉伸样品的平均剩余抗拉强度分别为347,305,275MPa。2个站点不同状态的EW75合金大气暴露腐蚀程度顺序均为：北京站<青岛站。分析2个站点的大气暴露实验的腐蚀产物。时效态合金经过6个月大气暴露,2个站点腐蚀产物都含有Mg(oH)2和MgS04,但北京站MgS04峰强度高于青岛站,青岛站产物中出现MgC12衍射峰；经过12个月大气暴露,2个站点的Mg(OH)2衍射峰减弱,MgS04和MgC12衍射峰的增强,北京站出现MgS04"4H20衍射峰。合金在大气环境中的腐蚀过程,由水蒸汽吸附、腐蚀气体和水溶性无机盐在水膜中的溶解、合金表面电化学反应、表面干湿交替组成,这四个过程在合金表面交替发生,相互促进,最终使基体发生严重腐蚀。进行时效态AZ80、ZK60、EW75与铍青铜形成电偶对的大气电偶腐蚀实验。AZ80、ZK60. EW75电偶腐蚀的平均失重速率为3.0667g·m-2,5.6833g·m-2,6.5667g·m-2,其顺序为：AZ80<ZK60<EW75。根据PBR理论,通过公式计算镁合金中常见金属间化合物被氧化后形成的氧化膜致密度系数。Mg12Nd和Mg24Y5氧化膜PBR在1-2之间,能对合金基体起到保护作用,Mg3Gd和MgNi2氧化膜PBR在2-3之间,存在压应力,Mg17Al12,MgZn2, Mg3Sb2,MgCu2,Mg2Ca,Mgl2Ce,MgAg,Mg2Si氧化膜PBR小于1存在张应力,对合金基体保护作用有限。通过T5、T6态EW75合金氧化膜对基体的作用,论证PBR理论。T5、T6态氧化膜厚度分别为0.6μm和10μm,PBR分别为1.1692和1.3440,氧化膜主要元素组成为O、Mg、Y、Nd、Gd。氧化膜致密性良好,腐蚀性介质很难透过接触基体。在实际合金开发过程中,设计成分使合金中形成氧化后PBR大于1的金属间化合物,便可以对合金直接加工成型,然后再进行热处理,该过程中合金表面形成的保护性复合氧化膜,能对减少防护工艺和降低成本起到重要的作用。更多还原

【Abstract】 Magnesium alloys as structural materials are widely application in the atmospheric environment, resulted that the corrosion phenomenon is widespread. In order to prolong life of the magnesium alloys, atmospheric corrosion behaviors in different environments were researched to select reasonable protections. However, the current research methods are simple and unreliable, which are lack of many important influence factors. Thus, in this study, effects of inorganic salts, relative humidity, carbon dioxide and PBR on the corrosion behaviors of EW75alloys were researched by atmospheric exposure and accelerated corrosion experiments. The results of this investigation may guide the development of new corrosion-resistance alloys and find wider applications for magnesium alloys in atmospheric environments.The atmospheric corrosion experiment equipment was designed to simulate the corrosion behaviors of EW75alloys by controlling the temperature, relative humidity, carbon dioxide, inorganic salts. This research improved the shortage of the traditional salt fog testing effectively, which will analyze the atmospheric corrosion conditions for magnesium alloys precisely.The effect of NH4SO4, NaCl and Ca(NO3)2on corrosion behaviors of EW75alloys were investigated. The weight loss and residual mechanical properties indicated that the corrosion order was NH4SO4>NaCl>Ca(NO3)2. The radiuses of Ca2+and NO3-are99and121pm, respectively. They will pass through the product films to Mg substrate easily to form the alkaline Ca(OH)2, which will protect the alloy from corrosion. The Cl-(130pm) adsorbed into the product films to instead the location of H2O, OH-and O2-, reducing the corrosion reaction activation energy. Furthermore, Cl-accelerated the electron transfer process and increased the corrosion current density, so the corrosion in NaCl was serious. NH4+hydrolyzed to produce H+(1.2pm), which reached the a-Mg substrate easily and caused serious corrosion. Additionally, the Mg(OH)2changed to the soluble MgSO4, so the corrosion degree in (NH4)2SO4was the most serious. The radiuses of NH4+and SO42-are 148and295pm, respectively, therefore corrosion pits on the surface of alloy in (NH4)2SO4were relatively larger.The effects of relative humidity and carbon dioxide on the corrosion behaviors of EW75alloys were researched. When relative humidity equaled40%, both the weight and mechanical strength were obviously reduced with increasing of carbon dioxide. The EW75alloys corrode only a little area on the surface. When the relative humidity equaled70%, the corrosion degree was larger. When the relative humidity equaled90%, the corrosion degree increased and then decreased when the concentration of the carbon dioxide was more than800ppm. The specimens could be arranged in corrosion rates order:RH40%<RH90%<RH70%. The corrosion products on the surface of A4specimen were Mg(OH)2and MgCO3·3H2O, while the B4were MgCO3·3H2O and MgCO3·5H2O. The corrosion products of C2and C4were Mg7(CO3)4(OH)2·5H2O and Mg7(CO3)5(OH)4·24H2O, respectively. Corrosion rates were depended on the structure, thickness, stability and permeability of the product films, which played an effective protection to the matrix. When the test level a equaled0.0001, FA=0.10997531>F0.0001(2,6), FB=0.08750816>F0.0001(3,6). Both the relative humidity and concentrations of carbon dioxide played an important influence on the corrosion behaviors of EW75alloys. In order to control the corrosion of CO2for EW75alloys during production, storage and application, relative humidity should be reduced by artificial dehumidification.Simulation analysis of EW75alloy on the corrosion behavior in Beijing and Qingdao sites, the specimens could be arranged in decreasing order of weight and mechanical strength:Qingdao>Beijing. The corrosion data curves were established according to weight loss, Beijing site:Y=A+B,*X+B2*X2+B3*X3, A=-5.60973, B1=1.04291, B2=-0.01687, B3=1.17699×10-4, correlation coefficient equaled0.99251; Qingdao site: Y=A+B1*X+B2*X2+B3*X3, A=-4.79713, B1=1.20479, B2=-0.01924, B3=1.31504×10-4, correlation coefficient equaled0.99161. Correlation coefficients were close to1, demonstrated that the outcomes of model fitting were both true.The atmospheric corrosion behaviors of EW75alloys in typical land (Beijing) and marine (Qingdao) environments were investigated. The corrosion rates for T5, T4and extrusion specimens at Beijing sites were42.4444,19.1833and18.4444g·m-2, respectively. The corrosion rates for T5, T4and extrusion specimens at Qingdao sites were55.9334, 25.7833and22.0389g·m-2, respectively. The test sites could be arranged in decreasing order of corrosion for EW75alloys:Qingdao>Beijing.Peak-aged EW75alloy exposure for6months, corrosion products contained Mg(OH)2and MgSO4in Beijing and Qingdao sites. However, the characteristic peak of MgSO4in the corrosion products at the Beijing site was higher than these at the Qingdao site, while the characteristic peak of Mg(OH)2was of little difference. The characteristic peaks of MgCl2were found in the corrosion products of Qingdao site. After12months exposure, the characteristic peaks of Mg(OH)2reduced while MgSO4and MgCl2enhanced, and the characteristic peaks of MgSO4·4H2O appeared in Beijing site. The corrosion process for EW75alloys in atmospheric environment included that adsorption of water vapor, dissolution of the corrosive gas and inorganic salts, electrochemical reactions on the surface of a-Mg substrate and dry-wet alternate process. The four processes occurred alternately and accelerated corrosion of alloys.The corrosion behaviors of EW75, AZ60and ZK80connected copper-beryllium alloys in Beijing site were researched. The corrosion weight loss of AZ80, ZK60and EW75galvanic coupling were3.0667,5.6833and6.5667·gm-2, respectively. The weight loss order was arranged as:AZ80<ZK60<EW75.The value of PBR for common intermetallic compounds of magnesium alloys are calculated through the formula. The results show that the PBR of Mg12Nd and Mg24Y5are both between1to2, which will protect to magnesium alloys substrate from corrosion; the PBR of Mg3Gd and MgNi2are2to3; Mg17Al12, MgZn2, Mg3Sb2, MgCu2, Mg2Ca, Mg12Ce, MgAg, and Mg2Si are less than1, which will not play an effective protection.PBR theories were demonstrated by the oxide films formed on the surface of T5and T6alloys. The thicknesses of oxide films were0.6and1.0μm, while the PBR were1.1692and1.3440, respectively. The main elements consisted of O, Mg, Y, Nd, Gd. Compactness of the oxide films were excellent, which played an effective protection from corrosion. The formation for protective oxide films (PBR>1) will simplify the protection processing and reduce the costing in actual development.更多还原