不同镁含量铝基牺牲阳极材料的组织与性能研究

材料保护　2004年5月第37卷第5期不同镁含量铝基牺牲阳极材料的组织与性能研究丁振斌1,孔小东2,朱梅五2(1.海军驻武汉四三八厂军事代表室,湖北武汉430060;2.海军工程大学金工教研室,湖北武汉430030)[摘　要]　为了探讨Mg 对铝基牺牲阳极的影响,在研制高效铝基阳极的基础上,运用电化学测试和扫描电镜技术,考察了不同Mg 含量对合金结晶形态和显微组织的影响。

结果表明:合金元素Mg 的含量主要影响合金的树枝晶形态和第二相粒子的性质、形态、数量和分布,进而影响阳极表面溶解的均匀性和电流效率,Al -Zn -In -Sn -Mg -S i 系合金阳极中Mg 的最佳含量应在2%左右。

本研究为进一步开发高效铝基合金阳极提供了理论参考。

[关键词]　镁;铝基牺牲阳极;显微组织;材料性能[中图分类号]　TG 174.2 [文献标识码]　A [文章编号]　10011560(2004)05005002 [收稿日期]　200311250　引　言牺牲阳极的阴极保护是防止金属腐蚀的有效措施之一,铝基牺牲阳极因其电化学当量小、单位重量的输出电量大且资源丰富、施工简单而获得了广泛的应用。

Mg 作为一种重要的合金元素对铝基牺牲阳极的影响表现得较为突出,大多数高效铝基牺牲阳极都将Mg 作为一种合金添加元素。

作者主要从微观角度研究不同Mg 含量的Al -Zn -In -Sn -Mg -S i 系合金的结晶形态和显微组织,与其宏观性能对比,进而探讨合金元素Mg 的作用,为进一步开发高效铝基合金阳极提供理论参考。

1　试验材料与方法1.1　试验材料试验材料为海军工程大学研制的Al -Zn -In -Sn -Mg-S i 系阳极,该系列合金的成分为Zn 0.03%,In 0.03%,Sn 0.03%,S i 0.5%,Mg 1%3%,余量为Al 。

试验材料共分为A ,B ,C 3组,Mg 含量分别为1%,2%,3%。

1.2　试样制备用于电化学性能测试的试样加工成Φ16mm ×48mm 的圆柱,每种阳极取3个平行试样。

在试样的一个端面从中心钻孔并攻丝。

试样用丙酮去除油污后称重,阳极引线为带螺纹的细铜棒,与阳极拧紧后用石蜡封样,留出14cm 2(长28mm )的圆柱工作面。

用于金相分析的试样加工成Φ16mm ×10mm 的圆柱,每组平行试样3个,根据需要采用多种磨光、抛光、侵蚀相互结合的方法进行[1]。

1.3　电化学性能测试采用国家标准G B/T 17848-1999推荐的恒电流法测定铝阳极的电流效率,并在试验后用目视法观察阳极表面腐蚀情况。

主要试验条件如下:阴阳极面积比为60∶1,阳极工作电流密度1mA/cm 2,试验时间15d ,试验介质为人造海水,其化学成分配比见表1[2]。

表1　人造海水的化学组分 g/L化合物NaCl M gCl 2Na 2S O 4CaCl 2NaHCO 3浓度24.530 5.200 4.090 1.1600.201化合物H 3BrO 3SrCl 2NaF K Cl K Br 浓度0.0270.0250.0030.6950.1011.4　金相分析利用X J G-05光学金相显微镜观察铝阳极合金的显微组织并摄取显微组织照片,用定量金相法测量第二相粒子的显微特征参数。

利用JX A -8800R 扫描电镜观察合金的相组成、第二相的形态与分布等,并结合电子探针分析确定第二相的成分、可能的结构以及主要、重点元素的分布情况等。

2　结果与讨论2.1　Mg 含量对合金结晶形态的影响合金阳极都是采用铸造方式生产,合金的结晶形态主要表现为树枝晶的大小(枝干厚度、枝晶间隔)和均匀程度。

图ac 分别为A ,B ,C 3组阳极低倍显微组织。

可以看到,随着Mg 含量的增加,合金枝晶间隔减小,枝晶逐渐变得粗大。

在树枝晶结晶时,前沿会出现低熔点组分偏析,以至于造成树枝晶接二连三的分枝,在树枝晶周围产生一层较薄的偏析层,从而使树枝晶彼此接近地长大。

树枝晶的枝晶间隔除受合金冷却速度影响外,还受到合金成分的影响。

随着合金中Mg 含量的增加,合金的凝固范围变宽,枝晶长大就越充分,因此间隔就越小。

合金铸造时形成的树枝晶的均匀粗细与阳极表面溶解状况密切相关。

具有均匀细小树枝晶的铝阳极,其表面溶解状态较为均匀,具有粗大放射状的树枝晶和微观组织不均匀的阳极,其表面溶解也不均匀[3]。

本次试验中,A 和B 两组阳极表面溶解均匀,而C 组阳极表面溶解很不均匀,试验结果很好地印证了该项理论。

图　铝阳极显微组织及形貌2.2　Mg 含量对合金显微组织的影响图d f 分别为A ,B ,C 3组阳极SE M 照片。

从中可以看出,铝阳极合金的显微组织为α固溶体基体上弥散分布着第二相。

基体呈树枝状结晶,树枝晶或粗大,或细小,有的呈细小网状分布,有的则具有明显的方向性;第二相或呈点状,或呈枝丫状,有的在晶界析出,呈网状分布,有的在晶内析出,弥散分布。

进一步的电子探针分析(见表2)表明,图中点状相为富镁相,枝丫状的第二相为富铁相。

表2　铝阳极第二相成分分析 %元素A质量分数原子分数B质量分数原子分数C质量分数原子分数Mg K 5.408.5312.7917.5115.8921.27Al K 49.1269.9250.4462.1850.5360.94Si K 0.240.33 5.41 6.41 2.46 2.85Fe K --0.300.18 2.18 1.27Zn K 24.0514.1321.8011.9025.5512.72In L 21.047.04 2.410.70 1.710.49Sn L0.150.056.851.921.680.46根据三阳极第二相的成分鉴定和铝-镁-锌三元合金相图分析可以确定,B 和C 两组阳极中的富镁相主体为T 相(Al 2Mg 3Zn 3):A 组阳极中的富镁相主体为η相(MgZn 2);T 相和η相相对铝基体都是阳极相[4],因而作为活性点在有氯离子存在的介质中形成点蚀源,促进阳极的腐蚀溶解。

Mg 含量不仅对合金第二相的组成产生影响,而且还能影响第二相的形态、大小和数量。

由定量金相学的计算结果(见表3)可以看出,Mg 含量为2%的B 阳极具有数量最多、尺寸适当、形状最规则的第二相粒子。

具有大小均匀、形态规则、数量适中的第二相粒子的铝阳极表现出较好的综合电化学性能[3],研究结果也说明了这一点。

表3　不同Mg 含量阳极的第二相粒子特征参数和性能试样编号第二相粒子特征参数λ/mm L 3/mmQ 电流效率/%表面溶解状态A 0.3400.012269.887.25均匀B 0.1330.013295.188.66均匀C0.2050.013225.382.73不均2.3　合金元素Mg 对阳极活化的影响Mg 在铝阳极合金中,除少量以固溶形式存在外,随着含量的升高,多以化合物形式出现,例如Al 2Mg 3Zn 3、MgZn 2等,它们相对基体为阳极相,周围易发生点蚀成核,是点蚀萌生的敏感位置。

该阳极的活化溶解主要归因于第二相粒子的“优先溶解-脱落”[5]:阳极刚开始溶解时,其表面分布的第二相粒子与Al 2O 3膜构成腐蚀电池,作为阳极型的第二相粒子发生溶解,且越接近Al 2O 3膜,溶解越快,使基体铝暴露出来;裸露的基体铝与Al 2O 3膜构成新的电偶,因两者电位差大,铝将大量活化溶解,此时第二相转化为阴极,溶解停止;第二相粒子因周围铝基体的溶解而脱落,自行溶解。

第二相粒子的“优先溶解-脱落”作用虽然增加了活化点,改善了阳极的表面溶解状态,但由于第二相粒子的脱落将引起电流效率的损失,所以应适当控制第二相粒子的数量。

3　结　论(1)随着Mg 含量的增加,阳极合金枝晶间隔减小,枝晶逐渐变得粗大,阳极溶解越不均匀。

(2)Mg 含量影响阳极合金第二相粒子的组成、形态、数量。

含Mg 2%的阳极具有数量最多、尺寸适当、形状最规则的第二相粒子,表现出较好的综合电化学性能。

(3)Mg 的化合物是点蚀萌生的敏感位置,阳极的活化溶解主要归因于第二相粒子的“优先溶解-脱落”。

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Naval Agency in The N o.438Shipyard,Wuhan430060;2.Metal Processing Laboratory,Naval University of Engineering,Wuhan 430033,China).Cailiao Baohu2004,37(5),50～51(Ch).In flu2 ence of magnesium content on the microstructure and crystalline state was investigated based on prepared highνefficiency aluminumνalloy sacrificed anodes by electrochemical methods and SE M.Experi2 mental results indicate that the content of alloying element Mg has an effect on the dendrite m orphology and the characteristics,m orpholo2 gies,quantity and distribution of the second phase particles,then af2 fects the surface diss olved characteristic and efficiency of anodes.The best content of magnesium is2%in AlνZnνInνSnνMgνS i alloy anode.K ey w ords:magnesium;aluminumνalloy sacrificed anode;mi2 crostructure;material performanceT rouble Shooting and Process Improvement in R oll Zinc P lating of Lamp H olderDONG X ingνhua(Wuhan Institute of C omm odity Hardware Indus2 try,Wuhan430064,China).Cailiao Baohu2004,37(5),52～53 (Ch).F or uninterrupted production of large roll zinc plating product line,various impurities will increase to s ome extent through continual。