中国腐蚀与防护学报, 2024, 44(1): 107-118 DOI: 10.11902/1005.4537.2023.012

研究报告

稀土铈改性石墨烯/水性环氧树脂复合涂料涂装技术研究

陈施润, 陈文革,, 钱颖, 张辉

西安理工大学材料科学与工程学院 西安 710048

Preparation and Perfromance of Rare Earth Cerium Modified Graphene Oxide / Waterborne Epoxy Resin Composite Coating

CHEN Shirun, CHEN Wenge,, QIAN Ying, ZHANG Hui

School of Material Science and Engineering, Xi'an University of Technology, Xi'an 710048, China

通讯作者: 陈文革,E-mail:wgchen001@263.net,研究方向为功能材料

收稿日期: 2023-01-19   修回日期: 2023-03-03  

基金资助: 陕煤联合基金.  2019JLM-2
西安市科技计划项目.  21XJZZ0042

Corresponding authors: CHEN Wenge, E-mail:wgchen001@263.net

Received: 2023-01-19   Revised: 2023-03-03  

Fund supported: Shaanxi Coal Industry Group United Fund of China.  2019JLM-2
Xi'an Scinece and Technology Planning Project.  21XJZZ0042

作者简介 About authors

陈施润,女,1999年生,硕士生

摘要

将制备的稀土铈改性石墨烯/水性环氧树脂复合涂料,以喷涂、滚涂和刷涂的方式在Q235钢表面进行涂覆,研究其组织结构及防腐性能。结果表明,稀土铈或CeO2以纳米颗粒状化学负载在石墨烯表面,改性石墨烯以小片层均匀分布在环氧树脂中。复合涂料涂覆于Q235钢表面后存在片层状石墨烯的堆叠结构。截面断口呈现河流状“银纹”。不同涂装方式制备的复合涂层的防腐机理为,喷涂法依靠高速运动的气体将雾化的涂料液滴冲击到基体表面并快速聚集、铺展成膜,在净化基体表面的同时提高涂层与基体的结合。滚涂法依靠线棒涂布器、刷涂法依靠软毛刷使涂层与基体结合,涂膜中的涂料分子易堆叠,表面易形成微孔。测得涂层附着力为0~1级,硬度达2H,面粗糙度~2 μm,腐蚀速率为1.565 × 10-4~5.889 × 10-3 mm/a。其中,喷涂法制备的涂层综合性能最好。

关键词: 稀土改性 ; 石墨烯 ; 水性涂料 ; 涂装技术

Abstract

The prepared rare earth cerium modified graphene/waterborne epoxy resin composite coating was applied on the surface of Q235 steel by spraying, rolling and brushing respectively, then of which the structure and corrosion resistance in 3.5%NaCl solution were studied. The results showed that as nano particles, rare earth cerium or CeO2 was chemically deposited on the surface of graphene, while the modified graphene was uniformly distributed in epoxy resin as small lamellae. The applied composite coating on Q235 steel presents a stacked structure of graphene lamellae, while the fractured surface of the coating presents a pattern of river-like silve stripes. The formation of the composite coating applied by different methods may be described as that: the spraying method relies on the high-speed moving gas to impact the atomized coating droplets onto the surface of the substrate and quickly gather and spread them into a film, which improves the combination of the coating to the substrate while purifying the surface of the substrate. The roller coating method relies on the wire rod coater, and the brush coating method relies on the soft brush to combine the coating with the substrate. Thus molecules in the coating are easy to stack, and mocropores may easy form on the surface. The adhesion of the coating was measured to be 0-1, the hardness was 2H, and the surface roughness was ~2 μm. The corrosion rate is 1.565 × 10-4-5.889 × 10-3 mm/a. It is found that the coating applied by spraying method has the best comprehensive performance.

Keywords: rare earth modification ; graphene ; water based coating ; decorate method

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本文引用格式

陈施润, 陈文革, 钱颖, 张辉. 稀土铈改性石墨烯/水性环氧树脂复合涂料涂装技术研究. 中国腐蚀与防护学报[J], 2024, 44(1): 107-118 DOI:10.11902/1005.4537.2023.012

CHEN Shirun, CHEN Wenge, QIAN Ying, ZHANG Hui. Preparation and Perfromance of Rare Earth Cerium Modified Graphene Oxide / Waterborne Epoxy Resin Composite Coating. Journal of Chinese Society for Corrosion and Protection[J], 2024, 44(1): 107-118 DOI:10.11902/1005.4537.2023.012

水性涂料因挥发性有机化合物(VOC)含量低、涂层附着力强、成本低及在潮湿环境中可直接涂覆施工等优点,成为目前主要的防腐手段[1]。但水性涂料对基材的表面清洁度要求更高,并且水的蒸发潜热大,涂层的表干时间长,表面张力大。同时,水性涂料固化后残存的亲水基团会导致涂层层间附着力差,涂层的耐水性、耐腐蚀性变差,容易出现闪蚀与透蚀等现象[2,3]。因此,急需对水性涂料进行改进。

石墨烯具有小尺寸效应、二维片层结构与疏水性,可作为纳米填料来提高涂料的防腐性能[4, 5]。但石墨烯在涂料中分散性和润湿性差会严重影响涂层的防腐性能,所以需要对石墨烯表面进行改性。稀土元素(RE)电负性低、活性大,不仅可以洁净石墨烯表面还可以形成RE-C键或混合杂化使其状态更稳定[6,7]。Zong等[8]在7050高强铝合金表面加入氧化铈(CeO2)和石墨烯制备涂层。涂层微孔尺寸明显降低,结构致密,耐磨性较好,粗糙度最低(1516.03 nm),且此时的自腐蚀电位最大,自腐蚀电流最小,耐腐蚀性最佳。Liang等[9]将稀土氧化物(La2O3和Y2O3)分散在羟基磷灰石(HA)-氧化石墨烯(GO)复合涂料中制备涂层。与纯HA-GO涂层相比,添加RE后的HA-GO复合涂层组织均匀,Y2O3与HA形成了无序的网状结构,La2O3形成了“海胆”状结构,均提高了HA-GO复合涂层的耐蚀性。Alam等[10]制备了聚吡咯(Py)/石墨烯纳米片(GNS)/稀土离子(RE3+ = La3+、Sm3+、Nd3+)/十二烷基苯磺酸(DBSA)纳米复合涂层。同时,含有GNS和RE元素的聚吡啶涂层具有最高的电荷转移电阻(Rct)和最低的双层电容(Cdl),耐腐蚀性能最好。这是因为涂层基体中纳米颗粒分布均匀,有利于在钢表面形成均匀的钝化膜。Zhang等[11]在碱性镀液的Cu表面制备了Fe镀层和Fe-rGO复合镀层。结果表明,GO提高了复合镀层组织的致密性,使得Fe基镀层的耐腐蚀能力大幅提高。Ye等[12]将硅烷化苯胺三聚体(SAT)和石墨烯合成的SAT-G加入纯环氧涂层中,结果表明加入0.5% SAT-G的复合涂层长期耐蚀能力显著提高。Xiao等[13]将聚苯胺(PANI)分散在GO中制备PAGO复合材料,加入0.5% PAGO的锌基水性涂料涂层具有更好的阴极保护作用和阻隔性能,涂层的耐蚀性明显提高。

优质的涂料是实现高性能涂层的基础,但涂料只是涂层的半成品,涂层性能才是最终评价涂料的标准。然而涂层性能的好坏不仅取决于涂料本身的质量,更大程度取决于涂装的工艺过程与条件[14]。董佳晨等[15]在除油打磨、不做处理、泛锈与全锈4种Q235钢表面制备涂层。对比看出,打磨后基体的粗糙度最高,涂层比不做处理和生锈表面的附着力都强。生锈基体涂层由于锈层的存在,腐蚀介质更容易通过涂层,涂层附着力下降。王浩伟等[16]采用机械、脱脂、热碱清洗以及络酸盐钝化4种方式对铝合金表面进行处理。其中,热碱清洗能够为涂层提供较好的极性附着表面,对涂层附着力的提高更为显著。Zhang等[17]利用喷涂法制备了碳纳米管(CNTs)/环氧树脂复合涂层。结果表明,随着CNTs含量的增加,涂层表面粗糙度由(2.98 ± 0.51) μm增加到(25.45 ± 0.44) μm,涂层表现出超疏水性,且具有高的耐腐蚀性和电导率。李娟等[18]利用刷涂法制备氟碳漆/CNTs防腐导电涂层,结果表明复合涂层在300℃以下极为稳定,复合涂层中形成了网状导电结构。董英豪[19]利用手工涂刷制备磷酸镁水泥涂层。结果表明涂层的线性极化电阻(Rp)基本维持在2.0 × 105 Ω·cm2Rct始终保持在8 × 104 Ω·cm2左右,粘结强度为4.8 ± 0.7 MPa,表明涂层具有较好的防腐作用。

综上所述,目前国内外主要开展石墨烯改性及改性石墨烯复合涂料的制备与性能研究,而有关涂料涂装工艺的研究较少。本文以稀土铈改性石墨烯/水性环氧树脂复合涂料为基础,采用喷涂(Spray)、滚涂(Roll)及刷涂(Brush)3种涂装方式将复合涂料涂覆至Q235钢板表面。系统研究不同涂装方式制备的复合涂层的微观结构变化与防腐性能差异,并探讨不同涂装方式制备的涂层的防腐机理,旨在获得最佳的涂装方式。

1 实验方法

将GO粉末加入去离子水中,配制2 mg/mL的氧化石墨烯水溶液40 mL,磁力搅拌30 min、超声分散1 h,得到氧化石墨烯分散液。将GO分散液与173 mg的七水氯化铈混合,磁力搅拌3 h、室温静置24 h、120℃水热12 h。经真空抽滤、去离子水洗涤至中性后,将混合液与40 mL的去离子水混合,-70℃下冷冻2 h,真空条件下(≤1 Pa)干燥48 h,N2气氛下750℃保温1 h,得到稀土铈改性石墨烯粉末(CeO2@rGO)。将30 mg的润湿分散剂、525 mg CeO2@rGO、50 mL去离子水混合,磁力搅拌30 min、超声分散1 h,得到改性石墨烯分散液。将改性石墨烯分散液、70 g水性环氧树脂乳液搅拌混匀,超声分散30 min后,加入35 g固化剂搅拌混匀,即得到稀土铈改性石墨烯/水性环氧树脂复合涂料(CeO2@rGO/EP coating),此时复合涂料呈均一黑灰色,粘度~18 s(涂-4杯粘度计)。

将规格为25 mm × 25 mm × 2 mm的Q235钢板用砂纸打磨至1200#,无水乙醇超声清洗,吹风机吹干。然后分别用喷涂、滚涂、刷涂将制备得到的环氧涂料与复合涂料均匀涂敷于钢板上,均采用单方向、单次涂覆,室温干燥24 h。喷涂法采用空气喷枪均匀涂覆于钢板表面;滚涂法采用线棒涂布器推动涂料在钢板上刮涂均匀;刷涂法采用软羊毛刷蘸取涂料涂覆在钢板表面。喷涂法制备的环氧涂层和复合涂层的厚度分别为(61 ± 2.3) μm和(34 ± 1.6) μm,滚涂法分别为(60 ± 0.76) μm和(64 ± 1.4) μm,刷涂法分别为(94 ± 2.0) μm和(90 ± 3.5) μm。

采用XRD-7000型X射线衍射仪(XRD)表征涂料的基本物相组成,用Cu Kα靶材,电压为40 kV,电流为40 mA,扫描角度范围5º~90º,扫描速率8º/min。采用Thermo Scientific Nicolet Is5型红外光谱仪(FT-IR)表征涂料的键合情况,测试波数范围4000~400 cm-1,扫描次数32,分辨率4 cm-1。采用JEM-3010型透射电镜(TEM)表征涂料的界面及形貌。采用TESCAN VEGXMU型扫描电子显微镜(SEM)表征涂料和涂层的微观形貌。采用LEXT OLS4000型激光共聚焦显微镜(LSCM)表征涂层表面粗糙度。

涂层与基体附着力按GB/T 9286-1998[20]划格法测量,附着力等级分为0~5级。涂层硬度按GB/T 6739-2006[21]铅笔硬度计测量,测试铅笔为三菱铅笔(9B-9H)。采用Dataphysics OCA20型接触角测试仪测定涂层与基材的静态水接触角,采用ZDY方法[22]计算涂层的表面能,见 式(1)。将制备的涂层涂覆在自制的三电极体系上,暴露面积为1 cm2,在3.5%NaCl电解质溶液中浸泡2 h,待开路电位(OCP)平稳后(5 min内变化小于10 mV),用CHI600E电化学工作站测量OCP、极化(Tafel)曲线和电化学阻抗谱(EIS)。

γs=γlg21+sin2θ+cosθ

式中,γs为固体表面能,mJ∙m-2γlg为去离子水的表面张力,γlg = 72.8 mJ·m-2θ为表观接触角。

2 结果与讨论

2.1 显微组织结构观察与分析

图1为稀土铈改性石墨烯/水性环氧树脂复合涂料的组织结构。图1a为改性石墨烯的TEM图。可见,GO的片层非常薄,呈半透明薄纱状,片层上均匀的附着一层纳米颗粒,片层上有部分褶皱,但仍有大面积的平展部分。这说明经过超声处理后,氧化石墨烯片层已被充分分散在环氧树脂中。图1b为改性石墨烯的高分辨电镜(HREM)图。可见石墨烯片层的晶格条纹,估计原子层数约为5~7层。还能非常清晰观察到石墨烯表面纳米粒子的晶格条纹,其快速傅里叶变换(FFT)结果准确对应为立方氧化铈结构,表明石墨烯上原位生长的纳米粒子有非常好的结晶性。通过Digital Micrograph软件算出CeO2@rGO的晶粒尺寸平均仅有(1.7 ± 0.5) nm,形状为不规则的多边形。图1c为纯水性环氧树脂涂料及稀土铈改性石墨烯/水性环氧树脂复合涂料的XRD谱。可见,稀土铈改性石墨烯在2θ = 28.6°、33.1°、47.4°和56.4°处出现属于CeO2 (111)、(200)、(220)、(311)4个晶面的衍射峰(JCPDF No.34-0394)。对于环氧涂料,其在10°~50°之间出现了由固化环氧树脂分子散射所引起的宽而弥散的衍射峰[23, 24],表明了环氧树脂的无定形性质。而复合涂料与环氧涂料的衍射峰类似,并未出现稀土铈改性石墨烯的特征峰。对此主要是石墨烯的添加量较少,低于X射线衍射检测极限[25,26]。其次,石墨烯在涂料中的剥离与随机排列的程度越高,石墨烯衍射峰强度降低[27~29]图1d为纯水性环氧树脂涂料、稀土铈改性石墨烯/水性环氧树脂复合涂料及氧化石墨烯的FT-IR图谱。可见,3390 cm-1左右的峰为O-H的吸收峰,2970与2870 cm-1为CH3中C-H键的对称与反对称伸缩振动吸收峰;2930 cm-1是CH2中C-H键的不对称吸收峰。1606与1511 cm-1处的峰对应苯环骨架-C=C-的特征吸收峰;1296、1247 cm-1与1117、1037 cm-1处的吸收带属于芳香醚键与脂肪醚键C-O-C的对称与不对称伸缩振动吸收峰。830 cm-1处属于苯环中C-H键的面外弯曲振动,560 cm-1处属于C-C键的面外弯曲振动[30]。添加稀土铈改性石墨烯后的水性环氧树脂的特征峰位置未发生明显变化,表明改性石墨烯的引入未改变水性环氧树脂的结构,两者之间属于物理结合[31]

图1

图1   稀土铈改性石墨烯和含改性石墨烯水性环氧树脂复合涂料的微观结构以及XRD和FT-IR分析

Fig.1   TEM (a) and HREM (b) images of CeO2@rGO, XRD patterns (c) and FT-IR spectra (d) of three coatings. The insets in Fig.1b show the enlarged image of the local area marked by the box, and electron diffraction pattern of CeO2


图2为稀土铈改性石墨烯/水性环氧树脂复合涂料本身表面和截面的SEM图。灰黑色为涂料基体,亮白色褶皱颗粒为暴露出的石墨烯颗粒。复合涂料表面由片层状与不规则颗粒组成(图2ab),片层边缘模糊,断面趋向于一个连续而完整的平面。局部放大区域可见河流状银纹,并且在这些银纹中可以看到连续的片层结构。复合涂料折断后的断面可见部分树脂在受力过程中被拉出基体呈现河流状银纹与连续纵向条纹(图2cd)。这表明石墨烯能够吸附高分子形成高分子包裹的结构单元,提高涂料的致密性,促使涂料体系更为完整。同时使得复合涂料在受力过程中产生大的剪切带,吸收断裂能,提高复合涂料的韧性[32, 33]

图2

图2   稀土铈改性石墨烯/水性环氧树脂复合涂料的SEM图

Fig.2   SEM surface images (a, c) and fracture sections (b, d) of the CeO2@rGO/EP composite coating


图3为不同涂装方式制备的稀土铈改性石墨烯/水性环氧树脂复合涂层在钢基体上的表面和截面SEM图。可见,复合涂层表面出现较多凸起结构(图3a~c)。从局部放大照片可见(图3a1~c1),基体磨痕被遮盖,涂层呈现出灰色背景和黑点两个明显特征。灰色背景表明石墨烯在环氧基体中能够实现良好的分散[28],而由于石墨烯大的比表面积与范德华力,部分石墨烯在环氧基体中发生团聚,形成不同大小的黑点。综合可知凸起结构为树脂包裹的团聚石墨烯。从局部放大SEM图可见,喷涂涂层相比滚涂法和刷涂法涂层,表面的凸起数量最多,分布最为均匀。对此认为主要与涂装方式有关,即部分团聚的石墨烯颗粒会在涂布器和羊毛刷的作用下被带出基体表面,而这些较重的涂料颗粒仍能够在压缩空气作用下被涂覆在基体表面。因此喷涂法制备的涂层的成分均匀性高于滚涂法和刷涂法。喷涂涂层截面比较光滑致密(图3a2),涂层与基体之间的结合紧密,裂缝的数量与长度较少,框选区域可见涂层中属于石墨烯的“蝉翼状”片层结构。滚涂涂层截面粗糙(图3b2),涂层中的片层结构不明显。刷涂涂层截面致密(图3c2),涂层与基体的结合紧密,在涂层上方存在明显的片层堆积结构。总的来说3种涂装方式制备的涂层与基体的结合均较为紧密,表明石墨烯的引入能够增强涂层与基体的结合。此外石墨烯能够作为涂层中的强化相,提高涂层的硬度,使得涂层截面在磨削后更为光滑。由图可见,采用不同涂装方式制备的涂层厚度具有较大差异,这是因为喷涂过程中,压缩空气对涂料分子施加冲击力,使得涂料分子间的堆积密度增加,涂层的致密性增加。而滚涂法依靠线棒涂布器施加压力,刷涂依靠羊毛刷施以压力。

图3

图3   喷涂、滚涂和刷涂3种涂装方式制备的复合涂层的表面和截面SEM图

Fig.3   SEM surface images (a, b, c), corresponding enlarged images (a1, b1, c1) of the local areas marked by the blocks, and cross sections (a2, b2, c2) for the CeO2@rGO / EP composite coatings prepared by spraying (a, a1, a2), roller painting (b, b2, b2) and brush painting (c, c1, c2), respectively


2.2 性能测试结果与分析

表1为不同涂装方式制备的稀土铈改性石墨烯/水性环氧树脂复合涂层在钢基体上的各项性能测试结果。可见,喷涂法与刷涂法制备的涂层附着力较优,附着力为0级;滚涂法制备的涂层附着力次之。在一定厚度范围内,涂层厚度增加会延长固化时间,涂层的流平性能越好,涂层内部缺陷数量减少,涂料与基材之间的有效附着点与区域增多,涂层结合强度增加[34]。结合截面SEM图可知,喷涂涂层厚度最小,附着力等级较高。对此主要是喷涂过程中,压缩空气使得涂料液滴对基体有冲击力的作用,雾化的涂料液滴与基体的接触更为充分。同时,压缩空气有助于净化基体表面,从而提升涂层附着力。而滚涂与刷涂法中,涂布器对基体施加的力较小,涂料液滴不能充分接触基体,因此附着力等级提升不明显,但与喷涂涂层接近。不同涂装方式制备的涂层硬度均为2H。涂层硬度与基体的交联程度有关,交联度愈高,涂层硬度越高[35]。而对于相同成分的涂料,在相同温度、长时间固化后,涂层的交联程度接近。因此不同涂装方式的涂层硬度接近。

表1   喷涂、滚涂和刷涂3种涂装方式制备的复合涂层的性能测试结果

Table 1  Determined properties of the composite coatings prepared by spraying, roller painting and brush painting

Decorate methodAdhesionHardnessWater contact angleSurface roughness /μmSurface energy / mJ·m-2
SprayingLevel 02H62.0° ± 1.8°2.7 ± 0.265.7 ± 0.7
Roller paintingLevel 12H65.3° ± 1.6°2.2 ± 0.264.4 ± 0.9
Brush paintingLevel 02H61.8° ± 1.6°2.0 ± 0.165.8 ± 0.5

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表1所示,采用3种涂装方式制备的涂层的水接触角均小于90°,涂层属于亲水表面。其中,水接触角刷涂<喷涂<滚涂。根据润湿理论,固体表面润湿性与其表面粗糙结构和化学组成有关。对于粗糙表面,一般采用图4中Wenzel模型[36]和Cassie模型[37]进行说明。其中,Wenzel模型更适合于接触角低于90°,表面表现出一定的“粘附性”的材料[38]。Cassie模型更适合于接触角高于90°,表面表现出一定“光滑性”的材料。虽然Wenzel模型和Cassie模型分别考虑了液体完全润湿表面和液体完全不润湿表面,是浸润现象的两个极限状态。但实际上,对于粗糙固体表面上的液滴,由于表面的凹凸不平,通常在固液界面之间会截留部分气泡,即会造成部分接触为固-液-气三相接触[39]。对此考虑图4c中Wenzel模型和Cassie模型的中间状态[40],接触角公式如 式(2)和(3)所示。此时凹坑的形状对接触角产生较大影响,随着微孔深度的增大,液体的浸润深度增大,润湿性减小。

图4

图4   几种润湿模型示意图

Fig.4   Schematic diagrams of the wetting models: (a) Wenzel model, (b) Cassie model, (c) intermediate state of Wenzel model and Cassie model


cosθ*=rcosθ

式中,θ*为表观接触角,粗糙表面测得接触角;θ为本征接触角,理想光滑固体表面接触角;r为粗糙度因子,r ≥ 1。

cosθ*=fs+πaxa+b2cosθ+fs-1

式中,fs为接触面凸出固体面积Ss与表观接触面积Sp之比,fs < 1;x为液体在凹坑中的浸润深度;a为凹坑直径;b为凹坑间距。

图5为3种涂装方式制备的稀土铈改性石墨烯/水性环氧树脂复合涂层在钢基体上的三维图像。可见,复合涂层表面出现由树脂包裹团聚石墨烯引起的粗糙结构(图5a~c),其中喷涂涂层表面突出的颗粒数较多,分布较为均匀,并且颗粒之间分布有微粗糙结构,表面粗糙结构均匀。喷涂涂层表面的石墨烯颗粒高度最高(图5a1~c1),而滚涂法与刷涂法涂层由于涂装方式的差异与涂层厚度增加,涂层表面突出的石墨烯颗粒的数量与高度减小,粗糙结构的均匀性下降。喷涂与滚涂涂层表面的粗糙度更大,凹坑深度更高,水接触角较大。而刷涂涂层的粗糙度下降,但润湿角上升,对此可以通过涂层表面自由能进行解释,结果如表1所示。一般的固体表面能越高,表面亲水性越强[41]。不同涂层之间的表面能相近,表面能滚涂<喷涂<刷涂,表面能与涂层润湿性之间表现出一致性。

图5

图5   喷涂、滚涂和刷涂3种涂装方式制备的复合涂层的三维图像

Fig.5   Three dimensional images (a, b, c) and sectional heights (a1, b1, c1) of the composite coatings prepared by spraying (a, a1), roller painting (b, b1) and brush painting (c, c1)


图6为不同涂装方式制备的环氧涂层与复合涂层在3.5%NaCl溶液中稳定2 h后测试的极化曲线。可见,Q235钢基板的腐蚀电位为-0.596 V,腐蚀电流密度为66.278 μA·cm-2。在涂覆涂层后,试样的自腐蚀电位(Ecorr)正移,自腐蚀电流密度(Icorr)明显下降,即基体的腐蚀倾向与腐蚀速率下降。对不同的涂装方式,在环氧涂层中,自腐蚀电位表现出滚涂<喷涂<刷涂,自腐蚀电流密度喷涂>滚涂>刷涂,对图6公式(4)[42]计算得出刷涂涂层的腐蚀速率为1.35 × 10-2 mm/a,远低于喷涂与滚涂层。在复合涂层中,腐蚀电位表现出滚涂<刷涂<喷涂,腐蚀电流密度滚涂>刷涂>喷涂。计算出喷涂涂层的腐蚀速率为1.565 × 10-4 mm/a,远低于滚涂与刷涂涂层。对比两种涂层,相同涂装方式制备的复合涂层的腐蚀电位高于环氧涂层,腐蚀速率下降一个数量级,表明改性石墨烯的引入能够提高涂层的阻隔性能,降低涂层的腐蚀倾向与腐蚀速率,基体金属得到了有效的保护。厚度较小的喷涂复合涂层腐蚀速率远低于较厚的刷涂涂层,且喷涂法制备的复合涂层腐蚀速率下降幅度最高。对此主要是由于喷涂作用下涂层的致密度、涂层与基体的结合力更高,因此对腐蚀介质的阻挡作用更强。其次,可能是由于涂层中的稀土氧化物发挥了缓蚀作用。由图1d可知,GO表面含有大量的活性氧官能团,如羟基,羧酸和环氧基。这些官能团的电离使得氧化石墨烯片带负电,Ce3+会与这些官能团发生静电相互作用,而紧密吸附在GO表面。另外,在制备过程中,GO表面的-OH和-CO等官能团会再次解离,在一定条件下与表面吸附的Ce3+发生反应形成CeO2。而3384 cm-1对应的O-H,1296、1247 cm-1与1117、1037 cm-1对应的C-O-C在复合涂料附近的峰均减弱且产生了红移,说明稀土氧化物(CeO2)活性高、与氧结合能力强,Ce对石墨烯改性时,易和氧元素反应形成配位键,生成新的官能团,而不会破坏石墨烯的sp2结构,降低石墨烯的界面能及表面能(~64.397 mJ·m-2),提高了石墨烯的分散性[43]。当腐蚀介质中的H2O、O2和Cl-沿着涂层中的微孔和缺陷逐步向涂层界面扩散渗透时,表面修饰有氧化铈的石墨烯在涂层中能均匀分散在环氧基体中,并与基体紧密结合,作为屏障减缓介质侵入。腐蚀介质要绕过石墨烯填料才能向基体渗透,其渗透路径被延长,从而大幅度降低涂层的腐蚀速率。

CR=3.269×10-3×MmIcorrnρ

式中,Icorr为腐蚀电流密度,μA·cm-2Mm为基体金属相对原子质量,Mm(Fe) = 55.85 g/mol;n为金属失去电子数,n(Fe) = 2;ρ为基体金属密度,ρ(Fe) = 7.85 g/cm3

图6

图6   喷涂、滚涂和刷涂3种涂装方式制备的环氧涂层与复合涂层的极化曲线

Fig.6   Polarization curves of the epoxy and composite coatings prepared by spraying, roller painting and brush painting


图7为不同涂装方式制备的环氧涂层与复合涂层的EIS及其等效电路拟合。从图7a可以看出,采用不同涂装方式制备的环氧涂层与复合涂层的阻抗谱均呈现两个容抗弧,不同涂装方式涂层在高频区的容抗弧大小不同。在环氧涂层中,高频区容抗弧半径喷涂<刷涂<滚涂,在复合涂层中滚涂<刷涂<喷涂。这表明在环氧涂层中滚涂与刷涂涂层的防护性能较好,复合涂层中喷涂涂层的防护性能较好,其中喷涂复合涂层容抗弧半径最大。对Bode图,中低频区(f < 0.1 Hz)反应了涂层的腐蚀过程,|Z|f = 0.01 Hz值越大,涂层防护性能愈佳[44]。从图7b可以看到,不同涂层低频阻抗模量不同。其中,喷涂环氧涂层的阻抗模值最低,喷涂复合涂层的阻抗模值最高,其余涂层的阻抗模值相近。这表明喷涂环氧涂层的防护性能低于滚涂和刷涂涂层,复合涂层中滚涂与刷涂涂层的防护性能低于喷涂涂层。腐蚀介质对涂层的渗透过程可以用等效电路来模拟,一般涂层体系浸泡初期与中期的等效电路如图7cd所示,其中,Rs为溶液电阻;Rc为涂层电阻,表示涂层中孔隙阻力,涂层孔隙越少或越小,涂层越致密,涂层的保护效率更好。Rct为电极反应的电荷转移电阻,反映涂层下方金属表面对电子转移反应的阻力,其与涂层下的腐蚀速率成反比。Rt = Rc + Rct可反映涂层在腐蚀过程中的保护效果[45]Qc为涂层电容,表征腐蚀介质渗透到涂层中的量,数值越小代表涂层抗介质渗透能力越强,Qdl表征涂层失效面积大小,值越大表明水在涂层中或形成分层的程度越大[46]。浸泡初期,随着浸泡时间的增加,电解质溶液逐渐沿着涂层缝隙向钢基底渗透,阻抗谱低频段出现了代表扩散特征的Warburg阻抗[47,48],涂层电阻逐渐减小,涂层电容逐渐增大。原因可能是随着界面区腐蚀反应的不断进行,越来越多的腐蚀产物在界面局部腐蚀区域不断聚集、沉积而使得腐蚀反应为传质过程所控制,代表界面腐蚀反应的低频半圆被Warburg扩散阻抗所掩盖。这段时间的涂层劣化过程可称为基底金属腐蚀发展与涂层失效阶段。表现在Bode图中,就是|Z|-f的曲线向着低频方向移动,曲线逐渐下降。浸泡中期,由于涂层电阻与涂层电容相并联(图7d),两者并联后的复合原件的阻抗值主要显示阻抗值小的元件的阻抗特征,即电容的阻抗。对图7b按Burg公式(见 式(5))[49]进行归一化处理,其中,涂层导纳(Y0)与阻抗(Z)互为倒数,可知喷涂复合涂层具有最高的阻抗模值。利用Zview软件选用图7d等效电路进行拟合,最终涂层电化学参数拟合结果如表2所示。结果表明在环氧涂层中,刷涂与滚涂涂层具有低的涂层电容(~1.231 × 10-5 Ω-1·cm-2·s n )和高的涂层电阻(~5.2906 × 104 Ω·cm2),表明较厚的滚涂与刷涂环氧涂层中腐蚀介质的浸入较少。复合涂层中,喷涂涂层具有低的涂层电容(~1.159 × 10-6 Ω-1·cm-2·s n )和高的涂层电阻(~35.5498 × 104 Ω·cm2)。对比环氧涂层与复合涂层,复合涂层电容(Cc)的降低与电阻(Rt)的增加,表明石墨烯的引入能够提高涂层的屏蔽性,减少腐蚀介质的浸入。而其中,喷涂复合涂层的Cc最小,表明喷涂复合涂层中浸入的腐蚀介质最少,涂层的致密性最高。综上所述,喷涂复合涂层防腐性能更优。

图7

图7   喷涂、滚涂和刷涂3种涂装方式制备的环氧涂层与复合涂层的电化学阻抗谱及等效电路拟合图

Fig.7   Nyquist (a) and Bode (b) plots and equivalent circuit diagrams (c, d) for the epoxy and composite coatings prepared by spraying, roller painting and brush painting in the initial (c) and mid (d) soak


Qc=Y0Rc1-n1n

式中:Y0为涂层导纳,Y0 = 1Z = ΔIΔE Ω-1·cm-2·s nn为经验常数,0 ≤ n ≤1。

表2   涂层电化学阻抗谱拟合参数

Table 2  Fitting parameters of electrochemical impedance spectra of the epoxy coating and the composite coatings

CoatingCcRc / Ω·cm2CdlRct / Ω·cm2
Y0 / Ω-1·cm-2·s nnY0 / Ω-1·cm-2·s nn
Epoxy coatingSpraying8.141 × 10-50.58329716.64.085 × 10-100.87712969.20
Roller painting1.231 × 10-50.49629218.96.201 × 10-100.88023687.00
Brush painting6.564 × 10-60.57978536.42.719 × 10-100.88124553.40
Composite coatingSpraying1.159 × 10-60.5823299051.084 × 10-90.89725593.25
Roller painting5.026 × 10-60.48179522.61.921 × 10-100.93019172.20
Brush painting8.307 × 10-60.462340574.89.006 × 10-110.95919946.80

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2.3 机制探讨

综合上述复合涂层的微观组织结构与性能,提出不同涂装方式制备涂层的防腐机理,如图8所示。有机涂层主要是通过涂层对腐蚀介质的屏蔽作用实现基体金属的防护,涂层的致密性、涂层与基体的结合力会对涂层的防护性能产生影响。喷涂过程中(图8a),一定粘度、呈水滴状的涂料分子在高速运动气体介质的带动下,冲击并快速聚集在基体表面。随后,近球形水滴状的涂料分子在基体上迅速铺展成膜,因而最初涂膜中涂料分子间具有高的堆叠密度。充分雾化的涂料液滴,在冲击力作用下能够与基体充分接触,提高涂层的结合力。此外,高速流动的空气能够对基体进行二次清洁,清除基体残存杂质,提高涂层结合力。因此,喷涂法能够在净化表面的同时实现涂层与基体的紧密结合。并在随后多次的气体冲击下,涂膜进一步加厚和致密化,从而使基体得到较为理想的保护。而对于滚涂与刷涂法(图8bc),线棒涂布器与羊毛刷对涂料施加的外力较小,涂膜中的涂料分子堆叠程度低。并且直接滴加的涂料液滴与基体的接触充分性低于喷涂法,涂料分子与基体接触面积小于喷涂法。此外基体上残存的杂质难以在工具的作用下带离基体,从而影响涂层的结合力。综上,喷涂法对涂层性能的提升幅度最高,是制备复合涂层的最佳涂装方式。

图8

图8   喷涂、滚涂和刷涂3种涂装方式制备的复合涂层的防腐机制示意图

Fig.8   Schematic diagram of anti-corrosion mechanism of the composite coatings prepared by spraying (a), roller painting (b) and brush painting (c)


3 结论

(1) 稀土铈改性石墨烯/水性环氧树脂复合涂料宏观上呈均一黑灰色,涂料粘度~18 s。微观上,石墨烯以小片层均匀分布在环氧树脂中。石墨烯与环氧树脂之间为物理结合。

(2) 稀土铈改性石墨烯/水性环氧树脂复合涂层表面分布有树脂包裹的团聚石墨烯颗粒。涂层中存在片层状石墨烯的堆叠结构。截面断口具有河流状“银纹”,以及不规则的片层与颗粒相互叠加的形貌特征。

(3) 喷涂、滚涂和刷涂3种涂装方式制备的复合涂层的附着力为0~1级,硬度均达2H,面粗糙度~2 μm。涂层表面亲水。其中,喷涂法是复合涂料最佳的涂装方式,腐蚀速率1.565 × 10-4 mm/a,耐蚀性能最佳,其防护机理是雾化的涂料液滴在高速运动气体的带动下,冲击基体表面并快速聚集、铺展成膜,在净化基体表面的同时提高涂层与基体的结合。在后续多次的气体冲击下,涂膜进一步加厚与致密化,片状堆叠的石墨烯也促进涂层的耐腐蚀性。

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