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通讯作者:

崔学军(1978—),男(汉),教授,博士;研究方向:金属材料使役行为与表面工程;E-mail:cxj_2046@163.com

中图分类号:TG174.451

文献标识码:A

文章编号:1007-9289(2020)02-0037-10

DOI:10.11933/j.issn.1007-9289.20200217001

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目录contents

    摘要

    为进一步增强微弧氧化(MAO) 处理后镁合金的耐蚀性能,通过涂装聚苯胺改性环氧树脂( PMER) 制备了 MAO/ PMER 复合涂层,并利用扫描电子显微镜(SEM)、电化学工作站、中性盐雾试验箱等表征手段研究了涂层的结构、 耐蚀性能及其长期腐蚀行为。 结果表明:MAO 涂层样品在盐雾试验第 5 d 时已经出现明显的腐蚀条纹,而复合涂层样品在至少 15 d 后才发生明显的鼓泡现象;与 MAO 涂层样品相比,复合涂层样品的腐蚀电流密度下降了 3 个数量级,极化电阻提高了 2 个数量级,PMER 涂层显著提高了 MAO 涂层对镁合金基体的腐蚀防护能力,并且随着 PMER 涂层厚度从 40 μm 增加到 110 μm,复合涂层的腐蚀防护能力变得更佳。 在 30 天的中性盐雾腐蚀过程中,复合涂层样品的阻抗值在 10~ 1010Ω·cm2内波动,而 MAO 样品在 10 d 后阻抗值降低至 200 Ω·cm2左右。 MAO 涂层易受氯离子破坏而导致涂层失效,而 PMER 涂层不仅起到优异的物理屏障作用,还具有低一定的缓蚀及自修复能力,这将能有效地提高对镁合金 MAO 的后长效防腐寿命。

    Abstract

    To further improve the corrosion resistance performance of micro-arc oxidation (MAO) coated magnesium alloys, the MAO/ polyaniline modified epoxy resin (PMER) composite coatings was fabricated. The structure of the composite coating was investigated by scanning electron microscope ( SEM), their long-term corrosion behavior and corrosion resistance perform- ance were studied by electrochemical workstation and neutral salt spray test chamber. The results show that the MAO coated sam- ples appear corrosion stripe on the 5th day of the salt spray test, while the composite coated samples appear obvious bubble until the 15th day later. Compared with the MAO coated samples, the corrosion current density of the composite coated samples de-crease by about three orders of magnitude, and the polarization resistance increase by about two orders of magnitude. It indicatsthat the PMER layer significantly improved the corrosion protection ability of MAO coatings to the AZ31B magnesium alloy, and the corrosion protection ability of composite coatings become better with the increase of thickness of PMER layer from 40 to 110 μm. In 30 days of neutral salt spray corrosion process, the impedance value of the composite coated samples fluctuates in the range of 109-1010Ω·cm2, while MAO coated samples only remaines around 200 Ω·cm2after 10 days of salt spray corrosion. MAO coatings are easily damaged by chloride ion, which will fail to protect AZ31B magnesium alloy. The PMER layer not only has excellent physical barrier effect, but also has excellent corrosion inhibition and self-repair ability, which can effectively in- crease the long-term anticorrosion life of magnesium alloy after MAO process.

  • 0 引言

  • 镁合金不仅具有高的比刚度和比强度,密度低,尺寸稳定性好,而且还有较好的抗冲击性能和优良的机械加工性能,但镁合金较差的耐蚀性能严重地限制了其应用的范围[1-4]。 微弧氧化(Micro-arc oxidation, MAO)技术因其工艺简单、低污染、且能形成耐蚀耐磨的陶瓷质氧化膜而得到企业界和科研工作者的广泛关注[5-6]。 但在MAO过程中,击穿放电使得内部的熔融氧化物和气体向外逸出,导致MAO涂层具有微观多微孔结构[7-8]。 这种多微孔结构特征大大降低了MAO涂层的耐蚀性能,因此,必须对MAO涂层进行封孔或涂装处理。 蒋百灵等[9]研究了MAO+石蜡封孔、MAO+喷涂聚苯乙烯泡沫银粉漆封孔、MAO+电泳漆等一系列的封孔工艺,但只有MAO+电泳漆复合涂层达到中性盐雾试验240 h不腐蚀的耐蚀性要求。 段红平等[10]研究了AZ91D镁合金MAO后封孔处理的复合涂层的电化学腐蚀行为,结果表明,MAO涂层对腐蚀介质的物理屏蔽作用较差,而复合涂层可以极大地抑制腐蚀介质的转移或扩散过程。 可见,复合涂层能极大地提高镁合金MAO后的长效防腐寿命。

  • 聚苯胺,由于其原料易得、合成方法简单及良好的环境稳定性,是当前应用于防腐领域最有前景的聚合物之一。 但是纯聚苯胺涂层的附着力较差,极易从金属表面剥离甚至脱落,不能为金属基体材料提供长期有效的防护。 将聚苯胺作为填料加入到有机树脂中制备成涂料,结合聚苯胺的氧化还原可逆性和有机树脂的良好耐化学药品性能及高粘附性,不仅能提高涂层的耐蚀性能,还能在金属表面生成保护性氧化膜,并且聚苯胺的存在还能促进有机树脂的固化,提高涂层的力学性能[11]。 Baghzadeh M R等[12] 在盐雾环境下研究了纳米聚苯胺掺杂环氧涂料,发现涂层的耐蚀性能显著增强,比普通的环氧树脂涂层的效果好很多,且附着力也有所增强。 Cupta G等[13]发现掺杂态聚苯胺改性环氧树脂( Polyaniline modified epoxy resin,PMER)涂层在服役过程中促进界面处金属氧化膜及缺陷处掺杂酸-金属络合物形成,通过对金属基体钝化以及掺杂粒子的缓蚀作用实现对金属基体的多重防护。 张颖君等[14] 在镁合金表面制备了一种掺杂态的PMER涂层,即使是缺陷的PMER涂层,也可以在镁合金表面形成一层保护层,从而使镁合金表面具有一定的缓蚀及自修复效应。 但是由于聚苯胺的掺杂酸种类和掺杂度等的多样性以及生成氧化物膜的致密性等差异,这都将导致聚苯胺涂层的性能大有不同,并且聚苯胺的防护机制也还没有定论,这些因素都进一步限制了聚苯胺涂层的发展和应用。

  • 文中在前期研究的基础上,利用PMER对AZ31B镁合金表面的MAO层进行后续封孔涂装处理,进而制备复合涂层,重点研究MAO/PMER复合涂层对镁合金长效防腐的失效行为与机理,为聚苯胺的应用研究提供实验支撑和理论依据。

  • 1 试验与方法

  • 基体材料为AZ31B镁合金板材( 50 mm ×50 mm×2 mm),依次对试样进行碱洗除油→用水磨砂纸进行打磨(粗糙度依次为320、400、600、800、1000、1200 号) →水洗→冷风吹干→酒精超声→除油除垢→冷风吹干,用保鲜膜包裹待用。采用MAO-III型电源通过恒流控制模式进行MAO处理, 电流密度0.05 A/cm2, 占空比为30%,频率300 Hz,处理时间为25 min。 其基础电解液由15 g/L Na2 SiO3 、10 g/L KOH和3 g/LNaF组成, 样品作为阳极, 石墨板( 100 mm ×100 mm×5 mm)作为对电极。

  • 环氧树脂(E44)和聚酰胺(651)由中国蓝星公司提供。 聚苯胺-氟化氢粉末的掺杂度为10%,采用球磨机将聚苯胺-氟化氢粉末分散在环氧树脂中。 以聚酰胺为固化剂,以二甲苯和丁醇的混合物为溶剂制备PMER涂料。 把MAO镁合金试样水平放置在桌面上,将PMER涂料刷涂在试样的上面,再通过涂布器把涂料从样品的一侧刮到另一侧,每涂抹一次将得到50 μm厚的漆膜,重复此步骤完成不同厚度涂层的制备,最后把涂好的试样放置在通风处固化48 h。 用涂层测厚仪测量涂层厚度,涂层厚度在±4 μm内取整数值方便描述与研究,测得的涂层厚度如表1所示。

  • 表1 样品的固化前后厚度

  • Table1 Thickness of samples before and after curing

  • 依据国家标准GB/T 10125-2012 在5%NaCl溶液中进行中性盐雾试验,溶液pH值控制在6.5~7.2 之间,试验温度(35±2)℃,盐雾沉降率在1~3 mL/80 cm2∙h之间。 为确保涂层样品测量的可靠性,使用3 块并行涂层样品进行测试。

  • 利用VersaSTAT3F电化学工作站考察空白及涂层样品在3.5%NaCl溶液中的腐蚀行为。采用标准三电极体系,试样暴露区域1 cm2 作为工作电极,Pt电极作为对电极,AgCl电极作为参比电极,并使用VersaStudio配套软件进行测量和拟合。 先对样品进行电位-时间曲线测试,当开路电位波动稳定在±10 mV后,测量交流阻抗谱。交流阻抗谱正弦波信号相对于开路电位±30 mV,频率10-2~105 Hz;最后测量极化曲线,参数设置:开始电位为开路电位± 0.5 V,扫描速率1 mV/s。 为研究涂层样品的长期腐蚀失效行为,将涂层样品浸泡在3.5%NaCl溶液中,并每隔5 d测量一次开路电位和交流阻抗谱。

  • 2 结果与讨论

  • 2.1 表面/截面形貌

  • 图1(a)为MAO涂层的表面形貌。 MAO涂层中的主要物相为MgO、MgF2、Mg2 SiO4[15]。从图1(a)可见,MAO涂层表面随机分布着大量的微孔,孔径大小在0.1~2 μm之间。 这些微孔是MAO过程中的放电通道,部分微孔被内部熔融氧化物封闭而形成盲孔,但仍存在部分放电通孔而降低涂层的腐蚀防护性能。 图1( b) 为MAO/PMER复合涂层的表面形貌,其中PMER涂层的厚度为80 μm。 从图1( b) 可以看出,MAO/PMER复合涂层表面只有极其少量微米级别的孔洞,这主要是因为可溶性涂料在配制和涂覆过程中会不可避免地产生少量气泡,在固化过程中,由于涂料黏度增加,气泡不会闭合从而形成气孔残留在涂层内部或表面。

  • 图1 AZ31B镁合金涂层的表面形貌

  • Fig.1 Surface morphologies of the coated AZ31B Mg alloy

  • 图2 为不同厚度复合涂层样品的截面形貌。PMER涂层的厚度由涂层测厚仪测出,MAO涂层的厚度为3 μm左右,PMER涂层固化后的厚度依次为40、60、80、110 μm。 从图2(a)与图2(b) 中可以看出PMER涂层厚度为40 μm和60 μm的样品中PMER涂层与MAO涂层结合比较紧密。 图2(c)与图2(d)中可以看出PMER涂层厚度为80 μm和110 μm的样品中的PMER涂层与MAO涂层结合比较差,层间出现了一条缝隙。 随着涂层厚度增加,涂层附着力有所降低,高聚物涂层在固化过程中收缩程度不同会产生相应的应力,外层涂层可通过向内收缩降低应力,而内层涂层由于界面阻挡,会产生应力集中现象,当内应力大于附着力时,涂层还会脱落[16]。 随着PMER涂层厚度的增加,缝隙的尺寸也有所增大,这对镁合金的耐蚀性是有所影响的。

  • 图2 不同厚度MAO/PMER涂层样品的截面形貌

  • Fig.2 Cross-section morphologies of MAO/PMER composite coated samples with different thicknesses

  • 2.2 中性盐雾试验

  • 表2 为涂层样品在中性盐雾箱中经过不同时间腐蚀所拍摄的表面宏观照片,图中的白色箭头处为复合涂层样品PMER涂层的鼓泡部分。MAO涂层在盐雾试验第5 d时,样品局部就出现了细长的腐蚀条纹,当盐雾时间超过15 d后,样品表面开始出现了大量的腐蚀凹坑,这表明MAO样品已经经历了严重的腐蚀。 对于40 μm厚的MAO/PMER复合涂层样品,在盐雾试验初始阶段,涂层表面并没有发生明显的变化;当盐雾时间超过15 d后,涂层表面才开始出现鼓泡,随着盐雾时间的增长,气泡也开始逐渐变大,并且随着PMER涂层厚度的增加,涂层表面出现鼓泡的时间得到延长。 而对于110 μm厚的复合涂层样品,在38 d时,PMER涂层中间才出现了大的鼓泡。 MAO涂层只能达到盐雾时长96 h不腐蚀,而复合涂层能使雾时长至少达到336 h不腐蚀。这表明PMER涂层显著提高了镁合金MAO后的腐蚀防护性能,并且随着PMER涂层厚度增加,样品的腐蚀防护能力达到更佳。

  • 2.3 极化曲线

  • 图3 所示为涂层样品浸泡2 h后测得的极化曲线图,在开路电位±10 mV范围内用塔菲尔外延法获得的样品自腐蚀电位 E cor、腐蚀电流密度 i cor 及极化电阻 R p 的拟合结果如表3 所示。与只经过MAO的样品相比,复合涂层样品的自腐蚀电位都明显正移,腐蚀电流密度也都下降了3 个数量级左右;极化电阻也都提高了2 个数量级左右。 测试结果表明:与MAO涂层相比,复合涂层具有更小的腐蚀倾向,腐蚀速率也都极大地降低,样品的腐蚀防护性能得到显著提升,并且随着PMER涂层厚度增加,样品具有更加优异的耐蚀性能。 这是因为镁合金MAO涂层表面的孔洞被PMER所填充,以及由于PMER涂层的致密性及阻挡效应,导致溶液与MAO涂层之间腐蚀介质的运输受阻,扩散速率变得缓慢,从而使MAO涂层的溶解反应和阴极去极化反应明显减弱[17]

  • 表2 不同盐雾时间AZ31B镁合金涂层样品的表面宏观形貌

  • Table2 Surface macromorphologies of the coated AZ31B Mg alloy with different salt fog time

  • 图3 AZ31B镁合金浸泡2 h的极化曲线

  • Fig.3 Polarization curves of the coated AZ31B Mg alloy after immersion for 2 h

  • 表3 塔菲尔极化曲线拟合电参数

  • Table3 Electrical parameters fitted by Tafel polarization curve

  • 2.4 电化学阻抗谱

  • 图4 为涂层样品浸泡2 h后的Nyquist图。由图4 可知,所有样品经过短时间浸泡整个扫描区域都表现为不完整的大直径容抗弧,可以类似为纯电容行为[18]。 与只经过MAO的样品相比,复合涂层样品的容抗弧直径高出MAO样品4 个数量级,并且随着PMER涂层厚度的增加,容抗弧直径也显著升高。 测试结果表明:在较短时间内,MAO涂层和复合涂层都能有效阻挡氯离子渗入涂层内部,从而避免腐蚀的发生,并且随着涂层厚度增加,涂层的机械屏蔽作用更加显著。

  • 图4 AZ31B镁合金涂层样品浸泡2 h后的Nyquist图

  • Fig.4 Nyquist plots of the coated AZ31B Mg alloy after immersion for 2 h

  • 2.4.1 MAO涂层样品的长期腐蚀行为

  • 图5 为MAO涂层样品长效腐蚀3 个阶段的Nyquist图。 氯离子渗透穿过了MAO涂层, 使MAO涂层及基体与水发生反应生成Mg(OH)2与H2[19-20]。 反应方程式可能如下:

  • MgO+H2O=Mg(OH)2
    (1)
  • Mg+2H2O=Mg(OH)2+H2
    (2)
  • 从图5( a) 可以看出,在2 h和1 d时样品的电化学阻抗谱都表现为不完整的大直径容抗弧,随着浸泡时间的增长,容抗弧的直径有所降低。 这是腐蚀的第一阶段,由于水分子先大量接触在MAO涂层表面,涂层表面的部分氧化镁由于水化作用生成少量的氢氧化镁固体,这会使MAO涂层变得膨松,从而阻碍氯离子的进一步扩散,反应方程式如式( 1) 所示;在另一方面,氯离子也开始从MAO涂层孔洞处渗入涂层内部,氯离子易吸附在MAO涂层表面及孔洞处,破坏MAO涂层的镁离子与氧原子的结合,使得MAO涂层的溶解占优势, 从而使MAO涂层受到破坏。 在这一阶段,MAO涂层的耐蚀性是有所降低的,但仍能有效地阻止腐蚀的发生。 在这一阶段,MAO涂层表面几乎没有任何变化。

  • 从图5( b)可以看出,在3~6 d时的样品的电化学阻抗谱都表现为较为完整的大直径容抗弧,并且随着时间的增长,容抗弧直径显著降低, 在第7 d时,出现了两个连续较小的容抗弧,高频容抗弧来自于镁合金基体和溶液界面的双电层,低频容抗弧来自镁合金基体表面的MAO涂层。这是腐蚀的第二阶段,氯离子已经在MAO涂层逐渐形成渗透通道,随着浸泡时间的增长,此时的MAO涂层对镁合金基体保护作用已经越来越小,在第7 d时,镁合金基体已经吸附着少量氯离子。 在这段时间内,MAO涂层会逐渐失效,MAO涂层的耐蚀性会随浸泡时间增长发生显著变化。在这一阶段,MAO涂层局部会出现一些细小的黑点但不明显。

  • 从图5(c)可以看出,在9 d后,所有样品的电化学阻抗谱都变为较为完整的容抗弧和不规则的感抗弧。 这是腐蚀的第三阶段,此时镁合金基体表面的MAO涂层已经形成大量渗透通道,在氯离子的作用下,镁合金基体已经发生了大量点蚀,大量点蚀产生的氢氧化镁固体和氢气,使涂层表面变得弛豫,在电化学阻抗谱中表现为感抗弧,反应方程式如式(2)所示。 直到点蚀暴露出镁合金基体,形成大阳极小阴极,此时腐蚀反应将变得更加剧烈,氢氧化镁的摩尔比迅速增加,在这一阶段,大量的黑点形成的腐蚀条纹将不断拓展[21-23]。 在盐雾试验中可看到这些较为明显的腐蚀现象。

  • 图5 MAO样品长期腐蚀的3 个阶段的Nyquist图

  • Fig.5 Nyquist plots of three stages of long-term corrosion of MAO Samples

  • 图6 为MAO涂层样品在3.5%NaCl溶液中浸泡不同时间测得的低频区的阻抗值,图中虚线部分为阻抗值在5~30 d的放大图。 由图6 可知:MAO涂层随着浸泡时间的增长,阻抗值变化较为明显,5 d内阻抗值从105 Ω·cm2下降到了103 Ω·cm2,10 后阻抗值只维持在200 Ω·cm2左右,表明MAO涂层已经完全失去了防护效果。这能更加直观地说明只经过MAO的镁合金样品,耐蚀性较差,MAO涂层比较容易受到氯离子破坏而完全失效。

  • 图6 MAO样品的阻抗值随时间变化的关系图

  • Fig.6 Diagram of impedance of MAO samples changing with time

  • 2.4.2 MAO/PMER复合涂层样品的长期腐蚀行为

  • 图7 为复合涂层样品在3.5%NaCl溶液中浸泡30 d各个时间段所测得的交流阻抗图谱,在30 d内,复合涂层样品全部表现为不完整的大直径的容抗弧,容抗弧的直径在30 d内不断波动,但在大部分时间内容抗弧相互重叠。 这是因为在前期,复合涂层具有良好的机械屏蔽作用和附着力,氯离子难以渗入复合涂层与镁合金基体接触[24]。 随着浸泡时间的增长,少量氯离子会扩散至MAO涂层表面,氯离子易对MAO涂层产生破坏形成渗透通道与镁合金基体接触。 试验中使用的是本征态的聚苯胺掺杂氢氟酸,在酸性条件下,很容易触发聚苯胺的氧化还原反应,在镁合金基体表面进行阳极反应形成钝化保护膜,而形成的钝化膜又具有良好的缓蚀作用,从而更好地保护镁合金基体[25-26]。 随着浸泡时间的进一步增长,氯离子又会从其他渗透通道与镁合金基体表面接触,PMER涂层中的聚苯胺掺杂的氢氟酸还会释放氟离子,这些氟离子易与镁离子结合形成耐蚀性能极其优异的氟化镁附着在MAO涂层表面,因此PMER涂层还能使MAO涂层具有一定的自修复行为,能够抑制氯离子所引起的局部的微区腐蚀;但是当大量氯离子渗入MAO涂层内部,PMER涂层难以对MAO涂层进行及时修复,腐蚀产物会在基体界面大量堆积,由于PMER涂层的致密性,腐蚀产物中的氢气难以逸出,PMER涂层还会表现为鼓泡,这将导致涂层的附着力降低,同时涂层还会因为局部膨胀产生一定的内应力,当内应力大于附着力时,涂层还会脱落,从而使复合涂层完全失效。 在盐雾试验中也能看到PMER涂层明显的鼓泡现象。

  • 图7 不同厚度的复合涂层样品长时间浸泡的Nyquist图

  • Fig.7 Nyquist plots of composite coated samples with different thicknesses soaked for a long time

  • 图8 为复合涂层样品在3.5%NaCl溶液中浸泡不同时间测得的低频区的阻抗值。 由图8可知,在30 d内,样品的阻抗值相差不大,并且阻抗值随时间维持在109~1010 Ω·cm2内波动变化,这能更加直观地说明复合涂层的自修复效应,在30 d内,涂层表面都没有出现明显的鼓泡现象;PMER涂层在110 μm时阻抗值变化较为明显,这可能是涂层厚度过厚,涂层间的结合力较差,复合涂层界面处更容易堆积大量氯离子,而使得PMER涂层难以及时修复MAO涂层表面,所以涂层的阻抗值变化比较明显;与MAO涂层相比,复合涂层不仅具有更加优异的腐蚀防护性能,还具有一定的缓蚀及自修复效应,这将能极大地增加镁合金MAO后的长效防腐寿命。

  • 图8 复合涂层样品的阻抗值随时间的变化

  • Fig.8 Changing of impedance of composite coated samples with time

  • 3 结论

  • (1) 与MAO涂层相比,聚苯胺改性环氧树脂涂层显著提高了MAO涂层对镁合金基体的腐蚀防护能力,并且随着聚苯胺改性环氧树脂涂层厚度从40 μm增至110 μm,复合涂层的腐蚀防护能力变得更佳。

  • (2) MAO涂层能对腐蚀介质起到物理屏障作用,但微观多孔的结构为腐蚀介质提供通道而只能在短时间有效防止镁合金基体腐蚀;而聚苯胺改性环氧树脂涂层不仅封闭微孔增强MAO层对腐蚀介质的物理屏障作用,还具有一定的缓蚀自修复效应,这极大地提高了MAO涂层对镁合金的防腐性能。

  • (3) MAO涂层的多孔结构易使氯离子渗入基体表面而使镁合金表面发生大量点蚀,最后迅速发展成为全面腐蚀而导致涂层完全失效;而聚苯胺改性环氧树脂涂层封闭微孔阻挡氯离子渗入,并通过自修复行为抑制微区腐蚀,但随着氯离子长时间的侵蚀作用以及腐蚀产物氢气的聚集,涂层将发生鼓泡甚至脱落,从而使复合涂层完全失效。

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