本研究采用绿色、简便的水热法从可再生的荔枝叶中制备了生物质CDs，并详细研究了其在HCl溶液中的腐蚀防护性能。研究表明，所制备的CDs含有丰富的含氧和含氮官能团，在HCl溶液中呈现出高达97.70%的缓蚀率。此外，我们通过吸附等温线和腐蚀形貌分析，揭示了其缓蚀机理。本研究证明了生物质基CDs在腐蚀防护中具有良好的缓蚀性能，为推动绿色高效缓蚀剂的开发提供了新的思路。
1 实验方法
碳钢是工程和日常生活应用中最广泛使用的金属之一，具有低成本、高强度和耐磨等优点。本文采用Q235钢，其含量(质量分数，%)为：C 0.18，Si 0.3，Mn 0.6，Cr 0.25，P 0.045，S 0.045，Cu 0.25，Ni 0.25，Fe余量。实验前，使用400至2000级不同粒度的砂纸依次打磨Q235钢试样，然后用去离子水洗涤和乙醇脱脂，最后在冷气流下干燥。使用AR级别36%~38%的盐酸稀释至1 mol/L浓度。荔枝叶从深圳大学校园收集，并在洗净、干燥和粉碎后使用。
将2 g干荔枝叶和100 mL去离子水密封在150 mL聚四氟乙烯内衬的不锈钢高压釜中，加热到200℃处理12 h。自然冷却至室温后，将棕黄色混合物在10000转下离心10 min，然后通过0.2 μm的滤膜去除残留的树叶大颗粒。随后，通过旋转蒸发和真空冷冻干燥，得到棕黑色的固态荔枝CDs粉末，产率约为5%。采用透射电子显微镜(TEM，FEI Talos F200S)观察所制备CDs形貌。采用Fourier变换红外光谱仪(FTIR，Thermo Scientific Nicolet6700)对CDs基团进行观测。
将15 mm × 15 mm × 3 mm的Q235钢试样浸泡在不同浓度的CDs中的1 mol/L盐酸溶液中96 h，每24 h冲洗干燥后迅速称重3次。测试前，所有试样均通过精密电子天平称重3次，精度为0.0001 g。失重实验在室温下进行。根据公式计算腐蚀速率和缓蚀效率(η)：
式中，ΔW为样品3次称重的平均失重，g；A为样品的比表面积，cm2；t为浸泡时间，h；vcorr为不加CDs溶液的腐蚀速率，mg·cm-2·h-1；
将CDs直接加入1 mol/L HCl中，分别形成CDs浓度为25、50、100和200 mg/L的HCl溶液。采用三电极系统(暴露面积为1 cm²的Q235钢作为工作电极(WE)、饱和甘汞电极作为参比电极(RE)、Pt片作为对电极(CE))，在PARSTAT 4000 + 电化学工作站上进行电化学阻抗谱(EIS)和动电位极化(PDP)测试。所有测试均在室温下完成。测试前，开路电位(OCP)应达到稳定状态。EIS测量范围为105~10-2 Hz，振幅为10 mV；PDP扫描电位范围为± 250 mV，速率为1 mV/s。所有电化学实验均进行3次以确保重现性。EIS数据采用ZSimDemo 3.30软件拟合和分析。
取出试样后，进行清洗和干燥，并存放于真空烘箱中进行形态观察。采用Zeiss Sigma 300型扫描电子显微镜(SEM)和自带的能谱仪(EDS)分析试样表面的二维形态和元素组成。通过FTIR获取试样表面的化学状态。
2 结果与讨论
2.1 CDs的表征分析
图1
图2
图3
图3
1 mg/mL的CDs水溶液UV-Vis光谱以及用不同激发波长测试得到的PL光谱
Fig.3
UV-Vis absorption spectrum (a) and PL spectra with different excitation wavelengths (b) of 1 mg/mL CDs aqueous solution. The inset photo-graphs shown in Fig.3a are 1 mg/mL CDs aque-ous solution taken under sunlight and a 365 nm UV light
2.2 CDs的缓蚀效果研究
为了研究CDs作为Q235钢在1 mol/L HCl中缓蚀剂的效果，进行了失重实验、EIS和PDP测量。图4描述了不同CDs浓度下，在室温下不同浸泡时间内Q235钢在1 mol/L HCl中的腐蚀速率和CDs的缓蚀效率(η)情况。与空白溶液相比，加入CDs后，无论CDs的浓度或浸泡时间如何，腐蚀速率都大大降低。随着CDs浓度的增加，腐蚀速率呈显著下降趋势(图4a)。当CDs的含量为200 mg/L时，在24 h浸泡后，Q235钢的腐蚀速率相对于空白溶液下降了86.06%(从4.59 mg·cm-2·h-1降至0.64 mg·cm-2·h-1)。更重要的是，随着浸泡时间的延长(图4b)，CDs的η从86.06%升至92.74%，表明CDs作为长期缓蚀剂具有出色的潜力。失重实验的所有结论均证明了CDs的高效缓蚀性能和长期缓蚀潜力。
图4
图4
Q235钢在含不同浓度CDs的1 mol/L HCl溶液中浸泡不同时间的腐蚀速率和缓蚀率随时间的变化曲线
Fig.4
Variations of corrosion rate (a) and inhibition efficiency (b) vs immersion time of Q235 steel in 1 mol/L HCl solution with different contents of CDs
如图5a所示，在最低频率(0.01 Hz)处的阻抗模量与碳钢电极的腐蚀速率成反比，随着CDs浓度的增加呈现上升趋势，表明其对Q235钢表面的防腐性能逐渐增强[25,26]。值得注意的是，在200 mg/L CDs的HCl溶液中，|Z|0.01 Hz值达到353.22 Ω·cm2，比空白溶液高出95.51%。无论CDs浓度如何，所有Bode相位角曲线均呈现出单个峰值，对应一个时间常数。但是，随着CDs含量的增加，峰值逐渐上移，揭示了碳钢/溶液界面的电容响应增强，这归因于CDs的吸附[27]。正如图5b所示，典型Nyquist图中出现了抑制性的电容半圆，其直径随着CDs的加入而显著增大，表明CDs能够在低至25 mg/L的浓度下在碳钢表面形成有效的保护膜[14]，而CDs浓度的增加则提高了保护膜的致密性。此外，所有电容半圆形状均相似，表明CDs未能改变Q235钢电极的电化学特性[28]，与Bode图的结果相一致。
图5
图5
Q235钢在含不同浓度CDs的1 mol/L HCl中进行电化学测试结果
Fig.5
Bode (a) and Nyquist (b) plots of Q235 steel in 1 mol/L HCl solution and its equivalent circuit shown in Fig.5b
表1 Q235钢浸泡在不同溶液中的等效电路拟合参数
Table 1
C mg·L-1 | Rs Ω·cm2 | Rp Ω·cm2 | CPEdl μF·cm-2 | χ | γ |
|---|---|---|---|---|---|
| Blank | 1.65 | 15.83 | 193.850 | 0.04042 | - |
| 25 | 1.513 | 39.69 | 76.892 | 0.06159 | 59.41% |
| 50 | 1.796 | 94.81 | 71.187 | 0.04857 | 83.30% |
| 100 | 1.912 | 161.37 | 41.835 | 0.0634 | 90.19% |
| 200 | 5.451 | 353.22 | 37.127 | 0.00697 | 95.51% |
图6
图6
Q235钢在含不同浓度CDs的1 mol/L HCl中的PDP曲线
Fig.6
PDP curves of Q235 steel in 1 mol/L HCl solution with different concentrations of CDs
其中，I0corr和Icorr分别为无和有缓蚀剂的腐蚀电流密度。随着CDs浓度的增加，βa和βc均减小，同时阴极和阳极区域的Icorr均向更低值移动，说明对阴极反应和阳极反应都有抑制作用，表明CDs是作为一种混合型缓蚀剂发挥作用。同时，Icorr值随着CDs浓度的增加呈现出下降趋，且IE值呈上升趋势。当CDs含量为200 mg/L时，IE值达到最大值97.70%。上述结果与失重实验和EIS分析结果一致。
表2 PDP测试所得电化学参数
Table 2
C mg·L-1 | Ecorr mV/SCE | Icorr μA·cm-2 | -βc mV·dec-1 | βa mV·dec-1 | IE |
|---|---|---|---|---|---|
| Blank | -468 | 891.10 | 168 | 179 | - |
| 25 | -479 | 254.21 | 147 | 152 | 71.47% |
| 50 | -491 | 154.21 | 124 | 130 | 82.69% |
| 100 | -494 | 31.28 | 106 | 129 | 96.49% |
| 200 | -496 | 20.47 | 104 | 111 | 97.70% |
失重实验、EIS和PDP测试结果表明，生物质基CDs具有高效的缓蚀性能和潜在的长期缓蚀潜力，这可能归因于CDs吸附在Q235钢表面形成的有效保护膜。相应缓蚀机理将在下文中进一步分析。
2.3 吸附等温分析
图7所示的吸附等温线可以提供关于CDs和Q235钢表面相互作用的关键信息，以探究其缓蚀机理。根据以下方程：
其中，θ为表2所示IE对应的表面覆盖度，Kads和Cinh分别为吸附平衡常数和缓蚀剂浓度，拟合结果的线性相关值R2为0.9989。结果表明，CDs缓蚀剂的吸附过程符合Langmuir吸附等温线。此外，通过以下方程确定吸附自由能(ΔG
图7
图7
依据电化学测试数据计算所得的Langmuir吸附曲线
Fig.7
Langmuir adsorption isotherm of CDs on the surface of Q235 steel
2.4 腐蚀形貌分析
为了更好地了解CDs的缓蚀机理，采用SEM、EDS和FTIR对Q235钢在1 mol/L HCl中浸泡96 h时的腐蚀形貌进行了综合分析。如图8所示，原始Q235钢表面非常光滑，有少量微弱的划痕和可忽略的凹坑，而在1 mol/L HCl中浸泡96 h后，Q235钢表面表现出严重的损伤，表面有明显的大坑和小裂纹。加入200 mg/L的CDs后，腐蚀明显减轻，小裂纹和大坑消失，这可能是由于CDs在钢表面形成了吸附膜。为了验证这一假设，用EDS和FTIR分别测量了相应的表面元素组成和化学状态[34]。从图8的EDS能谱中看出，在钢的表面检测出多种元素。其中，抛光后的Q235钢各元素所占的质量比例分别为C (26.29%)、O (3.82%)和Fe (69.77%)。而在1 mol/L HCl溶液中，Q235钢表面的能谱和元素含量变化较大。由于Fe的溶解，Fe峰强度降低(42.81%)。此外，由于Q235钢表面的氧化，O的强度(39.89%)增加。然而，当200 mg/L CDs存在时，Q235钢在1 mol/L HCl溶液中的EDS谱与抛光后的不同，其Fe含量的下降幅度比1 mol/L HCl溶液中的样品稍小，为55.61%。这些数据表明，CDs的加入明显延缓了Q235钢的腐蚀[8,16]。此外，如图9所示，CDs粉末和暴露在含有200 mg/L CDs的1 mol/L HCl中的Q235钢表面具有多个相同的特征峰，如C-H，O-H/N-H，C-N，C=O和C-O。与此同时，这些峰的强度有很大的变化，并且它们的峰值有所偏移，CDs的C=C峰无法在Q235钢表面上观察到，这归因于CDs与Q235钢表面形成了配位键[35]，这进一步提供了CDs吸附在Q235钢表面的有力证据。
图8
图8
Q235钢的SEM和EDS图像
Fig.8
SEM (a-c) and EDS (d-f) images of Q235 steel samples: (a, d) original state, (b, e) immersion in 1 mol/L HCl without CDs for 96 h, (c, f) immersion in 1 mol/L HCl with 200 mg/L CDs for 96 h
图9
图9
浸泡96 h前后Q235钢表面及CDs材料的FTIR谱
Fig.9
FTIR spectra of CDs as well as Q235 steel surface before and after 96 h immersion in 1 mol/L HCl with 200 mg/L CDs
2.5 CDs的缓蚀机理
结合上述电化学实验、吸附等温线和腐蚀形貌的分析，本文对CDs的缓蚀机理给出了一个合理的解释，来说明CDs与Q235钢表面的相互作用。Q235钢在不含CDs的HCl溶液中的主要反应为：阳极反应为Fe-2e- = Fe2+，阴极反应为2H+ + 2e- =H2↑，通过引起钢的快速溶解和H+还原，从而对钢表面产生严重的腐蚀。同时，钢表面带正电荷，因此水合Cl-易于通过静电相互作用附着在钢表面上[36]。而在掺入CDs后，它们立即在HCl溶液中质子化；其次，所附的水合Cl-起到相互连接桥的作用，以促进质子化CDs在金属表面上通过静电相互作用的物理吸附，从而与H+形成竞争关系[37]。吸附等温线研究表明，除物理吸附外，化学吸附也起着重要作用。由FTIR(图2)所表征的CDs含有丰富的含氧和含氮官能团，可以通过O与N原子的孤对电子与Fe原子的未占据3d轨道之间形成配位键化学吸附在钢表面。最终，通过电化学表征、EDS (图8)和FTIR(图9)证实了CDs成功地在钢表面形成保护膜。在保护膜的作用下，可有效避免腐蚀介质与金属的直接接触，从而同时抑制阴极析氢过程和阳极金属溶解过程。此外，随着CDs含量的增加，CDs可以更有效地吸附在钢片表面，形成更致密的保护膜，进一步减轻HCl的侵蚀。综上所述，CDs在钢表面形成的保护膜，主要依赖于化学和物理吸附机制，这一过程是其发挥缓蚀作用的主要原因。
3 结论
(1) 以荔枝叶为原料，采用绿色、简便、一步水热法制备CDs，避免了有害元素的使用和有毒气体的产生。所制备的以sp2 C为核，表面含O、N官能团的环保型CDs在HCl溶液中具有良好的长期分散稳定性，为其作为酸性缓蚀剂的应用奠定了坚实的基础。
(2) 当CDs浓度为200 mg/L时，其缓蚀率高达97.70%，是一种有效的碳钢缓蚀剂。
(3) 本文研究结果有助于提高人们对生物质基CDs优异的缓蚀性能的认识，并为制备绿色高效的缓蚀剂开辟了新的途径。
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