摘要
为提升建筑用钢的耐久性,通过电沉积法在钢表面形成层状双氢氧化物(LDH)晶核,再通过水热反应让LDH晶核生长成形成致密的Mg‑Al‑NO3‑LDH膜,分析了电沉积电位对钢基底表面LDH膜生长的影响,讨论了LDH膜的形成机理.结果表明,当电沉积电位为-1.4 V(相对于Ag/AgCl电极)时,LDH纳米片聚集堆叠致密,形成的LDH膜结构最为密实,实现了对钢基底良好的锈蚀防护.
滨海环境中钢筋锈蚀是影响钢筋混凝土结构耐久性的主要因素之
本文提出了电沉积-水热法,先采用电沉积法在钢表面形成晶核,然后通过水热法让LDH晶核继续生长,最终在钢基底表面原位生长出致密的Mg‑Al‑NO3‑LDH膜,通过分析电沉积电位对钢基底表面LDH膜生长的影响,得出了最适宜的电沉积电位并评估了LDH薄膜的耐腐蚀性能.
钢基底样品铁含量(质量分数)99.20%,碳含量0.15%,其他元素含量为0.65%,尺寸为15 mm×15 mm×2 mm.先用质量分数为20%的盐酸溶液对原始钢片进行超声清洗,以去除表面的氧化物和污渍.清洗3 min后,取出钢片,用无水乙醇清洗并在空气中干燥,备用.
制备方法为电沉积-水热法,包括电沉积和水热两个阶段.电沉积阶段在一个传统的三电极系中进行,Ag/AgCl电极和铂片电极分别作为参比电极和对电极,钢基底作为工作电极.采用50 mL Mg(NO3)2·6H2O浓度为0.045 mol/L和Al(NO3)3·9H2O浓度为0.015 mol/L的混合溶液作为电沉积阶段的电解液.将预处理好的钢片进行电沉积:在固定时间300 s内,对其施加-1.1 V的电势(相对于Ag/AgCl电极).
水热阶段的反应液是由45 mL Mg(NO3)2·6H2O浓度为0.12 mol/L和Al(NO3)3·9H2O浓度为0.04 mol/L的混合溶液组成,并向该溶中滴加NH3·H2O,将溶液的pH值调整至9;然后,将电沉积后的样品与水热溶液一同置于反应釜聚四氟乙烯内衬中,经90 ℃水热处理12 h后,将样品取出,洗涤,并充分干燥,即可在钢基底表面原位制备Mg‑Al‑NO3‑LDH膜,并命名为MH1.在25 ℃和pH=3的条件下,
用美国Quanta TM 250 FEG型扫描电子显微镜(SEM)观察所制备样品表面的微观结构和形貌.使用德国Bruker D8 Advance型 X射线衍射仪(XRD)分析样品的晶体结构(Cu Kα射线,扫描角度5°~75°,扫描速率10(°)/min).使用美国PerkinElmer Spectrum 100型傅里叶变换红外光谱仪(FTIR)分析样品的官能团和化学键.试验时需将所制备样品的粉末从钢基底表面刮下来,然后将其与纯KBr粉末混合压制成型,在4 000~400 c
25 ℃时,Mg和Al的Pourbaix图显示:不同的电沉积电位下,Mg和Al在电解液中的表现形式不同,这也就会导致不同的反应结果出
(1) |

图1 空白样品和不同电沉积电位钢片表面的SEM图像
Fig.1 SEM images of blank sample and steel surfaces after electrodeposition at different potentials

图2 空白样品和不同电沉积电位钢表面的XRD图谱和拉曼光谱
Fig.2 XRD patterns and Raman spectra of blank sample and steel surfaces after electrodeposition at different potentials
由
由
以上结果表明,电沉积是LDH原位生长的前提条件,这决定了能否形成完整的有良好结晶度的LDH薄膜.

图3 不同电沉积电位-水热反应后钢表面的SEM图像
Fig.3 SEM images of steel surfaces after electrodeposition‑hydrothermal treatment at different potentials
(1)对于样品MH1,水热反应后钢表面仍然是由片状组合成的球型颗粒,但是片状的形貌结构与水热反应前相比差异较为明显,聚集在一起的球状颗粒也能很好地覆盖在钢基底表面.此时片状结构的结晶度更高,是典型的球状LDH纳米片结构.
(2)对于样品MH2,片状组成的球形结构更为明显,底部的LDH纳米片也能相互堆叠,结晶良好. 对于样品MH3,水热处理后的表面形貌变得平整和光滑,整个钢基底表面都被细小的LDH片状结构覆盖,这些LDH片状结构聚集成一层薄膜. 对于样品MH4,水热反应后钢基底表面的形貌与样品MH3类似,也是由微小的LDH片状纳米片聚集组成一层薄膜.对于样品MH5和MH6,钢基底表面的形貌也与水热处理前的形貌类似,均覆盖着球状的LDH纳米片,但是此时LDH纳米片的数量更多,更密集,相互挤压产生孔隙.对于样品MH7和MH8,球状LDH结构的数量逐渐减少.电沉积电位为-1.3、-1.4 V时的LDH薄膜最为密实,LDH纳米片聚集堆叠致密.

图4 不同电位电沉积-水热反应后钢表面的XRD图谱和FTIR图谱
Fig.4 XRD patterns and FTIR spectra of steel surfaces after electrodeposition at different potentials

图5 不同电沉积电位制备Mg‑Al‑NO3‑LDH膜的Nyquist图和Bode图
Fig.5 Nyquist and Bode plots for the preparation of Mg‑Al‑NO3‑LDH films at different electrodeposition potentials
因此,可以认为Mg‑Al‑NO3‑LDH在钢基底上的生长过程分为2个主要阶段:(Ⅰ)电沉积阶段中水的电解使LDH晶核在钢表面原位生成;(Ⅱ)在水热阶段中,LDH晶核结晶生长,形成LDH薄膜.
为了探究Mg‑Al‑NO3‑LDH薄膜的腐蚀抑制性能,把空白样品和带有LDH薄膜的钢基底浸泡在3.5%(质量分数)的NaCl溶液中,然后采用电化学阻抗光谱测量其抗腐蚀性能.
由
一般来说,EIS谱低频处的阻抗模量越大,涂层的耐蚀性越
采用
为了进一步研究不同电沉积电位下所形成LDH薄膜的耐腐蚀性,还进行了动电位极化试验.空白钢片和不同电沉积电位LDH样品在3.5%NaCl溶液中的动电位极化曲线如

图6 空白钢片和不同电沉积电位LDH样品在3.5%NaCl溶液中的动电位极化曲线
Fig.6 Polarization curves of blank steel and LDH samples deposited at different potentials in 3.5% NaCl solution
可以通过
(2) |
式中:icorr,0和icorr,1分别代表空白钢样品和LDH膜样品的腐蚀电流密度,μA/c
Type | Ecorr/mV | icorr/(μA·c | η/% |
---|---|---|---|
Blank | -708.57 | 38.11 | |
MH4 | -425.93 | 3.63 | 90.5 |
由
为确定Mg‑Al‑NO3‑LDH膜能够适用于钢筋混凝凝土被海水侵蚀的服役环境,进行了LDH膜在饱和Ca(OH)2和3.5%NaCl混合溶

图7 在饱和Ca(OH)2 /3.5% NaCl混合溶液中的样品MH4和空白样品的Nyquist图和Bode图
Fig.7 Nyquist and Bode plots of sample MH4 and blank sample immersed in saturated Ca(OH)2/3.5% NaCl mixed solution
由
(1)通过电沉积-水热法处理,在钢基底上成功制备了Mg‑Al‑NO3‑LDH薄膜,以保护钢基底免受腐蚀.生长过程分为两个主要阶段:电沉积阶段中水的电解使层状双氢氧化物(LDH)晶核在钢表面原位生成和水热过程中LDH晶核结晶生长形成LDH薄膜.
(2)在-1.1~-1.8 V(相对Ag/AgCl电极)的电位范围内均可成功在钢表面制备LDH膜.但是当电沉积电位太低时(-1.1、-1.2 V),LDH膜的孔隙太多,缺陷明显.当电沉积电位太高时(-1.5~-1.8 V),由于电沉积过程中氢气生成较多且附着在钢基底表面,阻碍了LDH晶核的沉积,导致LDH膜在水热后生长不够致密,不能很好的覆盖钢基底. -1.3、-1.4 V电位下制备LDH膜的结晶度较好,其中-1.4 V电位下制备的LDH膜生长更致密,防护效果最好.
(3)在3.5%NaCl溶液中,电沉积电位为-1.4 V样品的抗腐蚀性能最好,抗腐蚀效率可达90.5%,在饱和Ca(OH)2和3.5%NaCl的混合溶液中也有较好的稳定性,浸泡3 d样品的阻抗模量为是空白样品浸泡40 min后的2倍.
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