ZnAlCe-NO2 水滑石@硅烷涂层的氯离子捕获和响应缓蚀行为

图1 水滑石及掺杂LDH-NO $_{2}^{-}$ 的硅烷溶液溶胶凝胶涂层的制备示意图

Fig.1 Schematic illustration of the preparation of layered double hydroxide (LDH) and silane-based sol-gel coatings modified with NO $_{2}^{-}$ -intercalated LDH

1.4 表征

采用扫描电镜(SEM，Axia ChemisEM HiVac)和场发射扫描显微镜(FE-SEM，Zeiss Sigma 300)对合成的水滑石、制备的涂层和浸泡实验后碳钢表面形貌进行了表征，并用EDS分析了表面元素的分布。采用Fourier变换红外光谱仪(FT-IR，Perkin-Elmer Spectrum Two)分析不同水滑石的化学成分的差别，使用KBr微球在450~4000 cm^-1的范围内进行测试。采用X射线衍射仪(XRD，Ultima IV)，在5~80°范围内，以10 (°)/min的扫描速率对水滑石的晶体结构进行了测定。在35~800 ℃的N₂气氛下，以10 ℃/min的升温速率(TG/DTG，TG209F1 Libr)对不同水滑石的热稳定性进行了分析。使用X射线光电子能谱仪(XPS，ESCALAB Qxi，USA)测定了不同水滑石基缓蚀剂粉末的表面化学组成和元素价态。使用IC离子色谱仪(IC，DIONEEX ICS-6000)，对合成的水滑石缓蚀剂的Cl^-吸附和NO $_{2}^{-}$ 释放性能进行分析。

使用电化学工作站(Gamry Interface 1010E)来分别对不同的样品进行电化学测试(电化学阻抗谱和极化测试)。对于水滑石粉末样品，以Q235碳钢(工作电极)、铂片(对电极)和KCl饱和甘汞(Hg/Hg₂Cl₂)电极(参比电极)置于含有NaCl溶液(0.05 mol/L)的电解池中，构成常规的三电极体系。然后将合成的水滑石基缓蚀剂以2 g/L的浓度，加入电解池中，进行电化学测试。样品的测试面积为1 cm²，扰动电压设置为10 mV。在10⁵~10^-2 Hz的频率范围内测量了电化学阻抗谱。此外，用ZSimpWin软件对EIS的测试结果进行拟合。在测试过程中，为防止干扰，将电解池放置在Faraday笼中，所有测试至少进行3次，以减小实验误差。对于极化测试，实验装置与EIS测试相同，测试范围为相对于开路电位-0.2~1.2 V，扫描速度0.5 mV/s。

对于涂层样品，电化学测试以涂层样品为工作电极，其他步骤与水滑石粉末样品相同。并且，使用扫描振动电极测试系统(SKP，Princeton Applied)对划伤后的涂层样品进行分析，研究涂层的自修复作用。

2 结果和讨论

2.1 水滑石的组成和结构形貌分析

通过SEM对ZnAlCe-NO $_{2}^{-}$ LDH样品的表面形貌进行了分析(图2a)。锌铝水滑石具有类八面体的片层结构，多层堆叠。而加入Ce³⁺后合成的水滑石仍能看出较明显的片层结构，但形貌更不规则，且尺寸变大，这可能是由于Ce³⁺相较于Al³⁺半径更大。并且，在添加Ce³⁺后合成的水滑石片层上能看到少量的颗粒物，这可能是由于合成水滑石过程中，Ce³⁺氧化生成了一部分CeO₂颗粒物，附着在水滑石片层上。从ZnAlCe-NO₂ LDH的能谱图中也能确认合成的物质具有Zn、Al、Ce、N等ZnAlCe-NO₂ LDH的特征元素。

图2

图2 ZnAl-LDH和ZnAlCe-NO₂ LDH的SEM图、ZnAlCe-NO₂ LDH的EDS能谱图，不同水滑石样品的XPS谱图及XRD图，FT-IR图，TG和DTG图以及不同NaCl浓度下，Q235碳钢在空白溶液和添加ZnAlCe-NO₂ LDH溶液中的腐蚀电位和低频模值的变化图，ZnAlCe-NO₂ LDH水滑石样品的氯离子吸附、缓蚀剂释放曲线

Fig.2 SEM images of ZnAl-LDH and ZnAlCe-NO₂ LDH, EDS spectra of ZnAlCe-NO₂ LDH (a), XPS spectra of different hydrotalcite samples (Zn 2p) (b), (Al 2p) (c), (N 1s) (d), (Ce 3d) (e) XRD images of different hydrotalcite samples (f), FT-IR figures (g), TG and DTG figures (h), Q235 carbon steel in blank solution and ZnAlCe-NO₂ added under different sodium chloride concentrations variation of corrosion potential (i) and low-frequency modulus (j) in LDH solution, chloride ion adsorption and corrosion inhibitor release curve (k) of ZnAlCe-NO₂ LDH hydrotalcite samples

2.2 水滑石的成分晶型和热稳定性分析

XRD分析(图2f)表明，所有LDH样品均保持层状结构特征(出现(003)(006)等晶面衍射峰^[19])，但Ce³⁺掺杂导致特征峰强度降低且(012)等晶面峰位偏移(Ce³⁺ (0.114 nm)与Al³⁺ (0.0675 nm)的离子半径差异引发晶格畸变)。通过Bragg方程2dsinθ = nλ，根据衍射角计算得出晶体中各晶面的间距，即ZnAl-NO₃ LDH和ZnAlCe-NO₂ LDH的层间距d₍₀₀₃₎分别为0.894和0.888 nm,一部分NO $_{2}^{-}$ 替换了层间NO $_{3}^{-}$ 使ZnAlCe-NO₂ LDH样品的层间距变小。

FTIR光谱(图2g)显示出类水滑石化合物的特征谱带：3393 cm^-1 (O-H伸缩振动)、1627 cm^-1 (H₂O弯曲振动)、1387 cm^-1 (NO $_{3}^{-}$ 伸缩振动)及M-O晶格振动带(400-700 cm^-1)^[20,21]，金属氧化物(M-O)的晶格振动峰(400-700 cm^-1)^[26]。Ce³⁺掺杂引起特征峰位移，且ZnAlCe-NO₂ LDH在1274 cm^-1处出现NO $_{2}^{-}$ 特征峰^[22,23]，证明了Ce³⁺和NO $_{2}^{-}$ 在水滑石中的成功嵌入。

TGA曲线(图2h)显示：35-200 ℃对应水滑石物理吸附水和层间水分子受热脱水，200-300 ℃为硝酸根及部分层间羟基受热分解，300 ℃以上层板羟基进一步受热分解。202 ℃ (NO $_{3}^{-}$ 分解)和414 ℃(NO $_{2}^{-}$ 分解)的双吸热峰印证层间阴离子置换成功合成了ZnAlCe-NO₂ LDH^[24,25]。

2.3 水滑石的化学组成分析

通过使用高分辨率XPS光谱技术进一步鉴定了两种水滑石样品的表面化学组成和结合状态(图2b~e)。Zn 2p_3/2 (1020 eV)和Zn 2p_1/2 (1043 eV)峰证实Zn²⁺存在(图2b)；Al³⁺配位环境改变使Al 2p峰向低能偏移(图2c)；Ce 3d (图2e)谱中882.15/900.6 eV (Ce³⁺)、888.65/916.45 eV (Ce⁴⁺)及卫星峰(884.5/903 eV)证实Ce³⁺部分取代Al³⁺，部分氧化为CeO₂^[38]。样品存在Ce³⁺和Ce⁴⁺的混合价态，一部分Ce³⁺替换了ZnAlCe-NO₂水滑石主层板中的Al³⁺，一部分Ce³⁺被氧化为CeO₂。综合分析表明：Ce³⁺成功掺入主层板，NO $_{2}^{-}$ 有效置换层间NO $_{3}^{-}$ (图2d)，且存在Ce³⁺/Ce⁴⁺双价态共存体系。

2.4 水滑石的Cl^- 吸附缓蚀剂释放性能

在25 mL浓度分别0、0.05、0.1、0.2、0.3和0.6 mol/L的NaCl溶液中添加0.05 g的ZnAlCe-NO₂ LDH，在开路电位30 min稳定后，来观察开路电位(OCP)(图2i)和电化学阻抗谱低频模值(|Z|_{0.01 Hz})(图2j)的变化。ZnAlCe-NO₂ LDH (2 g/L)在0~0.6 mol/L NaCl体系中呈现显著浓度依赖性响应。开路电位与低频阻抗(|Z|_{0.01 Hz})随Cl^-浓度升高迅速衰减，0.2 mol/L时达到临界阈值(|Z|_{0.01 Hz} = 3.2 × 10³ Ω·cm²，OCP = -412 mV)，此后阻抗响应趋于稳定。

使用IC离子色谱仪，在50 mL 0.03 mol/L NaCl溶液中加入1 g的该插层化合物(ZnAlCe-NO₂ LDH)，考察其Cl^-的吸附和缓蚀剂NO $_{2}^{-}$ 的释放特性(图2k)。水滑石样品对Cl^-具有非常敏感的响应，在第1 h内Cl^-快速吸附和NO $_{2}^{-}$ 快速释放，随后达到化学平衡计算得到每克水滑石样品吸附的Cl^-量为75.50 mg，释放的缓蚀剂量为21.16 mg。

2.5 水滑石的缓蚀性能

2.5.1 电化学分析

(1) 电化学阻抗谱分析　在0.05 mol/L的NaCl溶液中进行电化学测试，图3中的裸钢指Q235碳钢浸泡1h后测得的数据(在1 h时Q235碳钢已经发生腐蚀)，图3a~c是添加2 g/L ZnAlCe-NO₂ LDH的样品测得的电化学数据。频率为0.01 Hz时的低频模值(|Z|_{0.01 Hz})可以直接反映耐蚀性，一般情况下，样品的防护性能越好，|Z|_{0.01 Hz}值越高；同时，样品Nyquist图中对应的圆弧半径越大，表明耐蚀性能越好^[27,28]。由图可见，在初始浸泡时(即浸泡1 h)，空白样品(裸Q235钢)的低频模值|Z|_{0.01 Hz}为2.68 × 10³ Ω·cm²，添加ZnAlCe-NO₂ LDH的|Z|_{0.01 Hz}为1.38 × 10⁴ Ω·cm²。其初始|Z|_{0.01 Hz} (1.38 × 10⁴ Ω·cm²)较空白组(2.68 × 10³ Ω·cm²)提升5倍，且48 h内阻抗值持续高于空白组1个数量级，24 h达峰值1.46 × 10⁴ Ω·cm² (图3a)。等效电路拟合(表1)证实：ZnAlCe-NO₂体系R_ct值较空白高1个量级(24 h达最大值)，CPE_f值最低(24 h：1.24 μF/cm²)，表明其表面钝化膜最致密^[27,28]。

图3

图3 碳钢在添加ZnAlCe-NO₂ LDH溶液中的Bode和Nyquist图。碳钢在NaCl (0.05 mol/L)溶液与添加ZnAlCe-NO₂ LDH的NaCl (0.05 mol/L)混合溶液中浸泡不同时间后的极化曲线图及EIS拟合的等效电路模型

Fig.3 Bode plots (a, b) and Nyquist plots (c) of carbon steel in ZnAlCe-NO₂ LDH-containing solution. Polarization curves of carbon steel after immersion in 0.05 mol/L NaCl blank solution and 0.05 mol/L NaCl solution containing ZnAlCe-NO₂ LDH for different durations: 24 h (d) and 48 h (e). Equivalent circuit models (a, b) for EIS fitting (f)

表1 Q235碳钢在NaCl混合溶液中浸泡不同时间的EIS拟合参数

Table 1 Fitting parameters from EIS spectra of Q235 carbon steel soaked in NaCl mixed solution for different time

Sample	Time	R_s / Ω·cm²	CPE_f / Y₀ (Ω^-1·cm^-2·S ⁿ )	n	R_f / Ω·cm²	Q_dl / Y₀ (Ω^-1·cm^-2·S ⁿ )	n	R_ct / Ω·cm²	R_w/ Ω·cm²
Carbon steel	1 h	110.5	-	-	-	4.039 × 10^-4	0.78	1.876 × 10³	5.954 × 10^-3
ZnAlCe-NO₂ LDH	1 h	112.9	-	-	-	2.006 × 10^-4	0.82	1.468 × 10⁴	8.688 × 10^-3
	6 h	111.2	1.871 × 10^-4	0.83	125.3	8.09 × 10^-5	0.83	1.162 × 10⁴	1.133 × 10^-3
	12 h	101.1	2.234 × 10^-4	0.81	134.8	7.022 × 10^-5	0.90	1.793 × 10⁴	5.202 × 10^-3
	24 h	90.6	2.861 × 10^-4	0.79	120.1	6.857 × 10^-5	0.98	1.915 × 10⁴	5.750 × 10^-3
	48 h	111.2	2.718 × 10^-4	0.80	78.08	1.470 × 10^-4	0.94	1.410 × 10⁴	1.600 × 10^-3

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XPS与EIS关联分析表明：LDH通过Cl^-/NO $_{2}^{-}$ 离子交换释放缓蚀剂，NO $_{2}^{-}$ 促使γ-Fe₂O₃钝化膜形成，Ce³⁺/CeO₂₂协同增强膜层稳定性^[29]。

图3f中的等效电路模型分别对初始时刻和不同浸泡时间下的界面电极结构进行模拟^[29]。其中，R_s是溶液电阻，R_f和CPE_f分别代表膜电容和膜电阻；R_ct和Q_dl代表双电层电容和电荷转移电阻；W是带电离子或电子在溶液中扩散所引起的电阻^[30]。在初始时刻腐蚀已经发生，但还未有大量锈迹生成，以图3f1来拟合这一阶段，在后续浸泡中，水滑石分解的Ce³⁺生成的氧化物和腐蚀产物(Fe的氧化物)会在基体的表面沉积，且释放的缓蚀剂离子会在表面生成钝化膜，因此以图3f2来拟合。拟合后的数据如表1所示，一般来说，CPE_f的大小与基体表面保护膜阻隔性能有关CPE_f值越小，膜层的阻隔性能越好，而R_ct通常用来描述腐蚀速率，反映界面处电化学反应的动力学，R_ct越大基体的腐蚀速率越慢，腐蚀程度越小^[31]。从表中数据可知，添加ZnAlCe-NO₂ LDH的样品溶液，R_ct呈现先增大后减下的变化趋势，初始时刻的R_ct值比空白溶液大一个数量级，并在24 h时刻达到最大值。且添加ZnAlCe-NO₂ LDH的样品溶液的CPE_f值在24 h时具有最小值。说明在24 h水滑石在碳钢表面作用生成的保护膜最致密，阻隔性能最好。

(2) 极化曲线测试

动电位极化曲线评价了碳钢在不同水滑石/氯化钠溶液中的缓蚀性能。NaCl(0.05 mol/L)溶液为空白组(命名为KS)，添加2 g/L ZnAlCe-NO₂ LDH的NaCl (0.05 mol/L)混合溶液(命名为S/LDH-NO₂)。(图3d、e)表示S/LDH-NO₂体系击穿电位达+65.6 mV，腐蚀电位正向偏移501 mV，腐蚀电流密度(I_corr)降至0.32 μA/cm² (缓蚀效率97.57%)，钝化区扩展显著。说明水滑石的加入可以促进钝化膜的形成，增强局部抗氯化物腐蚀的能力。根据I_corr、阴极Tafel斜率(β_c)、腐蚀电位(E_corr)等电化学动力学参数，按照公式(1)确定不同水滑石型缓蚀剂的缓蚀效率^[32,33]。

η = \frac{I_{c o r r, 0} - I_{c o r r}}{I_{c o r r, 0}} \times 100 %

(1)

其中，I_{corr, 0}为碳钢在NaCl (0.05 mol/L)溶液中浸泡后的腐蚀电流密度，I_corr为碳钢在S/LDH-NO₂中浸泡后的腐蚀电流密度。碳钢在不含LDH溶液中的腐蚀电流密度最大，说明钢试件处于高速腐蚀状态。添加LDH-NO₂的样品，I_corr降低明显，缓蚀效率更高。

2.5.2 碳钢浸泡24 h后的表面性质分析

浸泡实验是为了评定金属腐蚀行为、金属防蚀措施的有效性或环境的腐蚀性所进行的实验^[34]。以KS表示空白0.05 mol/L NaCl溶液，取ZnAlCe-NO₂ LDH样品与0.05 mol/L的NaCl溶液配置2 g/L LDH/NaCl混合溶液(S/LDH-NO₂)，将Q235碳钢分别放入空白0.05 mol/L NaCl溶液、S/LDH-NO₂中24 h，并记录碳钢表面的腐蚀状况。表面形貌与成分分析(图4a、b)显示：S/LDH-NO₂体系碳钢表面无明显腐蚀(仅残留LDH片层)。XRD (图4d)证实含LDH体系在2θ =11.74° (d₍₀₀₃₎ = 0.75 nm)出现Cl^-插层特征峰，层间距收缩(Δd≈0.12 nm)印证Cl^-/NO $_{2}^{-}$ 离子交换^[36]。FTIR(图4c)进一步检测到1340 cm^-1处NO $_{2}^{-}$ 特征峰^[23]，与XRD共同揭示LDH-Cl^-/NO $_{2}^{-}$ 动态交换机制。吸附实验表明Cl^-交换后LDH仍保持结构稳定性((003)等特征峰存续)(图4e)，(003)由初始0.89 nm (NO $_{3}^{-}$ )降至0.77 nm (Cl^-)，与腐蚀界面检测值一致^[24,35~37]。该系列表征数据与电化学行为高度耦合，完整论证LDH通过Cl^-捕获/缓蚀剂释放实现腐蚀抑制的协同机制。

图4

图4 碳钢分别在0.05 mol/L NaCl溶液、S/LDH-NO₂溶液中浸泡24 h后的显微形貌；碳钢分别在0.05 mol/L NaCl空白溶液和S/LDH-NO₂溶液中浸泡24 h后的FT-IR图和XRD图谱及ZnAlCe-NO₂LDH样品在NaCl溶液中离子交换前后的XRD图谱

Fig.4 High-resolution micrographs of carbon steel after 24 h immersion in (a) 0.05 mol/L NaCl solution and (b) S/LDH-NO₂ solution. FT-IR spectrum (c) and XRD pattern (d) of carbon steel immersed in blank 0.05 mol/L NaCl solution and S/LDH-NO₂ solution for 24 h. XRD patterns of ZnAlCe-NO₂-LDH before and after ion exchange in NaCl solution (e)

2.6 水滑石掺杂溶胶凝胶涂层的分析

2.6.1 溶胶凝胶膜层截面组成分析

将空白溶胶凝胶涂层、ZnAlCe-NO₂ LDH掺杂的溶胶凝胶涂层分别命名为SC、SC/NO₂-LDH。涂层性能分析表明SC/NO₂-LDH涂层(厚度≈10 μm)较空白组(SC)实现显著结构优化。SEM显示SC表面存在机械缺陷(图5a)，而SC/NO₂-LDH因LDH片层嵌入形成微纳粗糙界面，有效弥合涂层裂纹(图5b)。EDS证实涂层含Zn、Al、Ce特征元素，其Si-O-C网络由GPTMS/TEOS缩合构建，与LDH形成有机-无机协同结构(图5c、d)。该结构特性为后续提升涂层屏障性能奠定基础。

图5

图5 空白溶胶凝胶涂层、ZnAlCe-NO₂ LDH掺杂的溶胶凝胶涂层的表面SEM图谱、空白溶胶凝胶涂层截面及能谱图及ZnAlCe-NO₂ LDH掺杂的溶胶凝胶涂层截面及能谱图

Fig.5 SEM patterns (a), (b) of blank sol gel coating, ZnAlCe-NO₂ LDH-doped sol gel coating, cross-section and energy spectrum (c) of blank sol gel coating, ZnAlCe-NO2 LDH doped sol gel coating cross-section and energy spectrum (d)

2.6.2 溶胶凝胶涂层的耐蚀性分析

采用电化学方法研究了不含LDH和含LDH硅烷涂层在0.05 mol/L NaCl溶液中的防腐性能。图6a1和b2是涂覆空白溶胶凝胶(SC)、图6b~d涂覆了掺杂1 mg/mL (图6b1和b2) (SC/NO₂-LDH₁)，2.5 mg/mL ((图6c1和c2) SC/NO₂-LDH_2.5)，10 mg/mL (图6d1和d2) (SC/NO₂-LDH₁₀))的ZnAlCe-NO₂ LDH的3种硅烷涂层的低碳钢试样在0.05 mol/L NaCl中浸泡6 h内的Bode和Nyquist图。由Bode图可见，随着浸泡时间的增加，除了SC/NO₂-LDH_2.5 (图6c1和c2)硅烷涂层，所有样品的低频模值(|Z|_{0.01 Hz})和Nyquist曲线的圆弧半径都呈逐渐减小的趋势，低频模值(|Z|_{0.01 Hz})可以直接反映涂层耐腐蚀性能的好坏^[45,46]，说明随着浸泡时间的增加，侵蚀性离子逐渐进入涂层内部，破环了涂层的腐蚀屏障作用^[39,40]。SC/NO₂-LDH_2.5初始低频阻抗模量达9.72 × 10⁵ Ω·cm² (较空白组高29.6倍)，6 h后仍保持9.23 × 10⁴ Ω·cm² (衰减率仅5%)，显著优于其他掺杂体系，所有LDH溶胶-凝胶涂层都比空白溶胶凝胶涂层高大约4倍，这说明LDH颗粒在胶凝胶涂层中的掺杂，提高了涂层的完整性和阻隔性能^[40]。

图6

图6 在0.05 mol/L NaCl溶液中浸泡不同时间后的不同的涂层的Bode图和Nyquist图模型a及等效电路(R(Q(R(QR))))

Fig.6 Bode plots (a1-d1) and Nyquist plots (a2-d2) of different coatings after immersion in 0.05 mol/L NaCl solution for varying durations: SC (a1, a2), SC/NO₂-LDH₁ (b1, b2), SC/NO₂-LDH_2.5 (c1, c2), SC/NO₂-LDH₁₀ (d1, d2). Equivalent circuit Model a: R(Q(R(QR))) (e)

使用ZsimpWin和模型a对上述图中的电化学阻抗谱数据进行拟合分析(图7a~d)。其中，R_s代表溶液电阻、R_c代表涂层电阻、R_ct代表电荷转移电阻、Q_c和Q_dl分别代表涂层电容和双层电容的恒相元件^[42,43]。

图7

图7 EIS拟合得到的R_c、Q_c、R_ct随浸泡时间的变化曲线和低频模值的变化曲线，开路变化曲线及不同涂层样品在NaCl溶液中浸泡6 h后的极化曲线图

Fig.7 Variation curves of EIS-fitted parameters with immersion time: (a) coating resistance (R_cR_c), (b) coating capacitance (Q_cQ_c), (c) charge transfer resistance (R_ctR_ct). Low-frequency modulus variation curve (d) and open-circuit potential variation curve (e). Polarization curves of different coating samples after 6 h immersion in NaCl solution (f)

一般来说，R_c越大，涂层的抗渗性越好^[30]。阻渗电阻R_c(6 h：2.85 × 10⁴ Ω·cm²)较空白高1个量级，过量掺杂(10 mg/mL)则使R_c降低76% (图7a)，电容值Q_c稳定在3.2 × 10^-9 F/cm²，而高掺杂体系Q_c增幅达320% (图7b)，佐证其孔隙率控制优势^[44]。一般来说，电荷转移电阻(R_ct)是由于金属基体表面电子的转移而形成的电阻，与腐蚀过程有关^[31]。低频模值(|Z|_{0.01 Hz})可以直接反映涂层耐腐蚀性能的好坏。随着浸泡时间增加，R_ct和|Z|_{0.01 Hz}都呈逐渐减小的趋势(图7c、7d)，这说明随着电解液(NaCl)的渗透入侵，腐蚀正逐渐加速，涂层逐渐恶化失去保护作用，其中空白溶胶凝胶涂层防护作用最差。对于SC/NO₂-LDH_2.5涂层，R_ct值随浸泡时间先减小，后增大，在4 h时达到最小值1.113 × 10⁵ Ω·cm²，整体具有最高的电荷转移电阻。(SC/NO₂-LDH_2.5)硅烷涂层的低频模值(|Z|_{0.01 Hz})随浸泡时间整体变化不大，屏障性能几乎没有破环。说明2.5 mg/mL水滑石掺杂的溶胶凝胶涂层(SC/NO₂-LDH_2.5)具有最好的腐蚀防护作用，而少量水滑石掺入，不能完全弥补硅烷涂层形成过程中产生的缺陷，少量的ZnAlCe-NO₂ LDH的氯离子的吸附和缓蚀剂的释放量都有限，不能起到很好的防腐效果。过量水滑石物质的掺杂，则会影响溶胶凝胶涂层的网状结构，导致产生更大的缺陷如涂层内部的孔隙甚至裂纹，最终影响涂层的耐腐蚀性能开路电位(图7e)监测显示LDH涂层初始电位(-0.36~-0.40 V)显著高于空白组(-0.46 V)，且SC/NO₂-LDH_2.5在6 h浸泡后电位降幅最小(维持-0.38 V)，印证其阻隔性能优势^[38]。极化曲线(图7f)验进一步研究了添加水滑石的溶胶凝胶涂层样品在0.05 mol/L NaCl溶液中浸泡6 h时的腐蚀防护情况，SC/NO₂-LDH_2.5使腐蚀电位正移109 mV，阳极电流密度降低2个量级。其性能提升源于LDH层间NO $_{2}^{-}$ 可控释放，诱导γ-Fe₂O₃钝化膜形成。Ce³⁺/Zn²⁺协同沉积增强膜层致密性。Cl^-捕获与结构缺陷修复双重作用。该研究明确2.5 mg/mL为最佳掺杂浓度，过量LDH (10 mg/mL)因团聚效应使涂层孔隙率增加43%，导致屏障性能劣化。

2.6.3 掺杂溶胶凝胶涂层的自修复性能分析

扫描Kelvin探针技术(SKP)研究溶胶凝胶涂层的腐蚀状况以及在涂层和金属衬底之间的界面反应情况^[47,48]。SKP微区电位分析(图8)揭示划伤后的SC及SC/NO₂-LDH_2.5涂层在0.05 mol/L NaCl溶液中浸泡30/90 min后，电位分布差异显著。SC体系30 min时电位差达765 mV (299~-466 mV) (图8a)，90 min扩大至879 mV (图8b)；而SC/NO₂-LDH_2.5体系电位差始终低于500 mV (0~-450 mV) (图8c、d)，证实了掺杂LDH所负载缓蚀剂对腐蚀介质的响应释放能够有效抑制微区局部腐蚀。结合EIS和极化数据，该现象源于LDH通过Cl^-/NO $_{2}^{-}$ 动态响应交换，释放出NO $_{2}^{-}$ ，促进了缺陷处基体表面γ-Fe₂O₃钝化膜原位修复。CeO₂纳米颗粒及溶解释放的Zn²⁺/Ce³⁺形成复合氢氧化物层，阻断腐蚀介质渗透^[47,48]。LDH结构缺陷修复降低涂层孔隙率(较空白减少68%)。该机制使SC/NO₂-LDH_2.5在损伤状态下仍维持稳定界面电位，自修复效能随时间延长提升37%。

图8

图8 空白硅烷涂层(SC)和溶胶凝胶涂层(SC/NO₂-LDH_2.5)在NaCl溶液中浸泡30 min和90 min后测得的SKP图

Fig.8 SKP (Scanning Kelvin Probe) maps of (a, b) blank silane coating (SC) and (c, d) sol-gel coating (SC/NO₂-LDH_2.5) measured after immersion in 0.05 mol/L NaCl solution for (a, c) 30 min and (b, d) 90 min

2.7 掺杂溶胶凝胶涂层的腐蚀防护机理

根据以上实验数据可知，相较于空白溶胶凝胶涂层，ZnAlCe-NO₂ LDH微纳米容器的掺杂显著提升了溶胶凝胶涂层的防护性能。这是因为空白溶胶凝胶涂层由于自身的固有缺陷，H₂O、O₂和Cl^-等可通过这些缺陷连续渗入涂层，涂层中的缺陷逐渐联通，直至基体表面，导致H₂O、O₂和Cl^-等在涂层和碳钢界面大量聚集，从而发生涂层剥离，产生大量FeOOH等腐蚀产物(图4a、b、图9a)。

图9

图9 掺杂溶胶凝胶涂层的腐蚀防护机理

Fig.9 Corrosion protection mechanism of doped Sol-Gel coatings

当溶胶凝胶涂层中掺杂纳米容器ZnAlCe-NO₂ LDH后，纳米容器首先通过物理填充作用不仅显著提升了涂层致密性(图6) (R_c提升40%，Q_c降低68%)，其二维片层结构所产生的迷宫效应还延长了腐蚀介质的扩散路径(图9b)；同时，在局部腐蚀产物水解的酸性环境刺激下ZnAlCe-NO₂ LDH会发生智能响应，主层板释放的Zn²⁺/Ce³⁺形成Zn(OH)₂及Ce基钝化层修复表面缺陷(图5)。

从热力学视角看，LDH层间NO $_{2}^{-}$ 与环境Cl^-交换过程与弹簧高能状态向低能状态自发变化类似(图9b)：在较高温度(65 ℃)合成的ZnAlCe-NO₂ LDH具有较大的层间距(图2f)，处于高势能，类似于弹簧受力伸长的状态，当遇到侵入涂层中的Cl^-时，由于Cl^-半径(0.181 nm)明显小于NO $_{2}^{-}$ 半径(0.25 nm)，二者交换后能显著减小主层板间距(图4e)，处于低势能，类似于伸长的弹簧收缩的状态，因此，溶胶凝胶涂层对Cl^-的智能响应是ΔG < 0的自发过程，释放出来的NO $_{2}^{-}$ 作为氧化性缓蚀剂使涂层缺陷局部地区发生钝化(图9)，从而显著提升了涂层的防护性能(图7和8)。值得注意的是，LDH层间通道通过尺寸筛分效应(SO $_{4}^{2 -}$ 等大尺寸阴离子较大的水合半径导致显著的空间位阻效应，需克服更高的水合能壁垒(ΔG > 0)，因此不会与层间阴离子发生交换响应)确保对Cl^-的特异性捕获(图2a)，这种精准的离子交换行为与结构强化效应共同构筑了动态防护体系。

3 结论

(1) 通过离子色谱法验证了ZnAlCe-NO₂ LDH能够固定自发智能捕捉Cl^-的同时释放NO $_{2}^{-}$ ，具有优异的离子交换能力。每克水滑石样品吸附的Cl^-量为75.50 mg，释放的缓蚀剂为21.16 mg。

(2) 在0.05 mol/L的NaCl溶液中，ZnAlCe-NO₂ LDH对碳钢表现出优异的缓蚀性能，缓蚀效率可达97.57%。

(3) 相较于空白溶胶凝胶涂层，2.5 mg/mL添加量的ZnAlCe-NO₂ LDH纳米容器能显著提升溶胶凝胶涂层耐腐蚀能力，原因是纳米容器的加入不仅降低了溶胶-凝胶涂层的孔隙率，而且能够对侵入涂层的Cl^-智能响应，在捕捉侵入Cl^-的同时，释放层间NO $_{2}^{-}$ ，和主层板溶解产生的Ce³⁺共同对涂层局部缺陷的基体表面进行钝化缓蚀。

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