椰油酸二乙醇酰胺对钢在三氯乙酸溶液中的缓蚀性能

图1 非离子表面活性剂椰油酸二乙醇酰胺 (CDEA，C₁₁H₂₃CON (CH₂CH₂OH)₂) 的分子结构式

Fig.1 Chemical molecular structure of the nonionic surfactant of coconut diethanolamide (CDEA, C₁₁H₂₃CON(CH₂CH₂OH)₂)

将2.5 cm×2.0 cm×0.05 cm的钢片试样依次采用粗、中、细的砂纸对其表面进行打磨，丙酮脱脂处理后称取初始质量 (精度0.1 mg)，悬挂浸没于一定温度下250 mL空白或含有不同浓度CDEA的0.10 mol/L Cl₃CCOOH溶液中，恒温6 h后取出，清洗、冷风吹干并称量，得到钢片腐蚀前后的质量失重值△m (g)，并计算得出钢片的腐蚀速率 (v) 和缓蚀率 (η_w)^[15,16]。

电化学实验采用PARSTAT2273电化学工作站进行，采用操作软件Powersuite和三电极系统进行测试：饱和甘汞电极 (SCE) 为参比电极；铂电极 (s=10² mm²) 为对电极；环氧树脂灌封的冷轧钢片 (10 mm×10 mm) 为工作电极。将工作电极钢表面处理后浸泡在250 mL待测溶液中约30 min，待测试体系稳定后进行电化学实验；动电位极化曲线测试参数：扫描区域-0.25~+0.25 V，扫描速率为0.5 mV/s。电化学阻抗谱 (EIS) 的测试参数：10⁵~10^-2 Hz，交流激励幅值为±10 mV。

将处理好的钢片试样分别在Zeiss Sigma 300扫描电子显微镜 (SEM)、SPA-400 SPM unit原子力显微镜 (AFM) 和OCA20接触角测量仪进行SEM、AFM的表面形貌检测和接触角测试。

采用环法 (铂环) 在JYW-200A自动表面张力仪上进行表面张力测试，其分辨率为0.01 mN/m。在PE38电导率仪平行3次实验测试各溶液的电导率，采用梅特勒-托莱多标准装置 (1413 μs/cm) 进行校准。

2 结果与讨论

2.1 CDEA对冷轧钢的缓蚀性能 (失重法)

图2a为20~50℃下，冷轧钢在0.10 mol/L Cl₃CCOOH溶液中的腐蚀速率 (v) 随CDEA浓度 (c) 的变化曲线。未添加缓蚀剂0.10 mol/L Cl₃CCOOH溶液中冷轧钢的腐蚀速率分别为：12.23 (20 ℃)、27.63 (30 ℃)、35.84 (40 ℃) 和38.77 g·m^-2·h^-1 (50 ℃)。当添加CDEA后，各温度下的v急剧下降，直到当CDEA浓度超过30 mg/L时，v不再随缓蚀剂浓度的增加而明显变化。此外，随着温度的升高，腐蚀速率整体上移，说明升温加快了钢片表面的析氢腐蚀。

图2

图2 20~50 ℃时0.10 mol/L Cl₃CCOOH溶液中腐蚀速率 (v) 和缓蚀率 (η_w) 与CDEA浓度 (c) 的变化曲线

Fig.2 Relationship between corrosion rate (v) (a) and inhibition efficiency (η_w) (b) with changing CDEA concen-tration (c) in 0.10 mol/L Cl₃CCOOH solution at 20-50 ℃

图2b为20~50 ℃下，缓蚀率 (η_w) 与CDEA浓度 (c) 的关系曲线。显然，当 CDEA浓度为10~30 mg/L时，η_w急速增大，但随后的CDEA浓度处于30~100 mg/L较大浓度范围时，η_w基本平稳。这可能是由于低浓度下的CDEA分子在钢表面的吸附量随浓度增加而急剧增多，但当CDEA浓度大于30 mg/L后缓蚀剂分子在钢表面的吸附量逐渐趋于饱和，故缓蚀性能随CDEA的添加而增长缓慢逐渐趋于平衡值。20和30 ℃两个温度下的CDEA的缓蚀性能相当，说明低温有利于CDEA抑制钢在Cl₃CCOOH中的腐蚀。当温度继续升高后，缓蚀率明显下降，这可能与高温时钢表面的腐蚀程度急剧上升，从而导致CDEA分子难以吸附在钢表面，或会使已在钢表面吸附的缓蚀剂分子发生脱附所致。各温度条件下，50 mg/L CDEA的缓蚀率 (η_w) 分别为97.1% (20 ℃)、97.1% (30 ℃)、93.7% (40 ℃)、85.4% (50 ℃)，表明CDEA在0.1 mol/L Cl₃CCOOH中对钢具有较好的缓蚀性能。

2.2 CDEA在钢片表面的吸附等温式

利用Langmuir吸附模型对失重法实验数据进行拟合^[18]。

\frac{c}{θ} = \frac{1}{K} + c

(1)

式中，c为CDEA的物质的量浓度 (mmol/L)；θ为CDEA在钢表面的覆盖度，其值约等于缓蚀率；K为吸附平衡常数 (L/mmol)。

20~50 ℃下的c-c/θ的线性关系见图3，相关参数列于表1。由表可知，线性相关系数r²(0.9953~0.9980)、直线斜率 (0.97~1.08) 都在1附近，说明在0.10 mol/L Cl₃CCOOH溶液中，CDEA在钢表面符合Langmuir吸附模型。20和30 ℃下，拟合直线的斜率更靠近1，可能和CDEA分子在钢片表面吸附后相互之间的排斥力微弱有关，故低温时更能有效覆盖在金属表面^[19]。K值随着温度的升高而下降，说明随温度的升高，CDEA在钢表面吸附能力减弱。

图3

图3 不同温度下0.10 mol/L Cl₃CCOOH溶液中c/θ-c关系

Fig.3 Relationship of c/θ-c in 0.10 mol/L Cl₃CCOOH solu-tion at different temperatures

表1 c/θ-c线性拟合参数

Table 1 Linear regression parameters between c/θ and c

T / ℃	r²	Slope	K / L·mg^-1
20	0.9999	1.01	0.95
30	0.9994	1.01	0.68
40	0.9719	0.97	0.48
50	0.9801	1.08	0.32

2.3 CDEA在钢片表面的吸附热力学参数

0.10 mol/L Cl₃CCOOH溶液中CDEA分子在钢片表面的标准吸附Gibbs自由能 (ΔG⁰) 通过下式计算^[20]：

K = \frac{1}{ρ_{s o l v e n t}} e x p (\frac{Δ G^{0}}{R T})

(2)

式中，R为气体常数，取值8.314 J·K^-1·mol^-1，T为热力学温度 (K)，ρ_solvent为H₂O的质量浓度，约为1.0×10⁶ mg/L^[20]。

为深入探讨CDEA分子在缓蚀体系中在钢表面的吸附过程，利用Van't Hoff方程式^[21]分析吸附平衡常数 (K) 和温度 (T) 的关系：

l n K = \frac{- Δ H^{0}}{R T} + D

(3)

式中，D为不定积分常数。图4为不同温度下0.10 mol/L Cl₃CCOOH溶液中lnK-1/T的线性关系，具有良好的线性关系 (r²=0.9949)，通过斜率 (-ΔH⁰/R) 计算出标准吸附焓 (ΔH⁰)。标准吸附熵 (ΔS⁰) 根据热力学方程计算^[22]：

图4

图4 不同温度下0.10 mol/L Cl₃CCOOH溶液中lnK-1/T关系

Fig.4 Relationship between lnK-1/T in 0.10 mol/L Cl₃CCOOH solution at different temperatures

Δ S^{0} = \frac{Δ H^{0} - Δ G^{0}}{T}

(4)

吸附热力学参数列于表2，ΔH⁰＜0，说明CDEA在钢表面的吸附行为是放热反应，即CDEA分子吸附在钢表面后，会在缓蚀体系中释放能量。标准吸附自由能ΔG⁰＜0，即CDEA分子在钢表面的吸附行为是自发的。同时，20 kJ/mol＜|ΔG⁰|＜40 kJ/mol，说明CDEA在钢片表面上的吸附行为是物理吸附和化学吸附共同作用的混合吸附类型^[22]。ΔS⁰＞0,表明CDEA分子在钢表面的吸附过程是熵增的过程，即缓蚀剂CDEA分子在钢表面的吸附时驱替了吸附在钢表面的水分子，由于水分子脱附引起的混乱度增大幅度大于CDEA分子在钢表面的吸附而引起的混乱度，故总熵增加^[23]。

表2 0.10 mol/L Cl₃CCOOH溶液中CDEA在钢表面的吸附热力学参数

Table 2 Adsorption thermodynamic parameters of CDEA on steel surfacein 0.10 mol/L Cl₃CCOOH solution

T / ℃	ΔG⁰ / kJ·mol^-1	ΔH⁰ / kJ·mol^-1	ΔS⁰ / J·mol^-1·K^-1
20	-33.54	-28.18	18.28
30	-33.84	-28.18	18.67
40	-33.03	-28.18	15.49
50	-34.08	-28.18	18.26

2.4 钢在Cl₃CCOOH溶液中的腐蚀动力学参数

冷轧钢在Cl₃CCOOH溶液中的腐蚀速率 (v) 与温度 (T) 可通过Arrehnius公式进行拟合^[24]：

l n v = \frac{- E_{a}}{R T} + l n A

(5)

式中，E_a为活化能，A为指前因子。图5a为不同温度下0.10 mol/L Cl₃CCOOH溶液中lnv与1/T的线性关系，通过斜率 (-E_a/R) 和截距 (ln A) 可求算出E_a和ln A。E_a和ln A与CDEA的浓度 (c) 的关系见图5b。从式 (5) 可知，钢的腐蚀速率取决于E_a和A两个参数。从图5b中可以看出，E_a和ln A随着CDEA的添加呈上升趋势并逐渐趋于平缓。E_a的上升可能是吸附在钢片表面的CDEA分子抑制了钢片表面活性点起到缓蚀作用所致，E_a越高，说明腐蚀反应越难以进行，这是因为腐蚀源必须获得大于E_a的能量变成活化状态，越过能垒变成腐蚀产物^[24]。因此，E_a随CDEA的添加而增大并趋于平缓，Cl₃CCOOH溶液越难以对钢进行腐蚀，进一步表明CDEA在Cl₃CCOOH溶液中对冷轧钢具有良好的腐蚀抑制作用。

图5

图5 不同温度下0.10 mol/L Cl₃CCOOH溶液中lnK-1/T关系E_a和ln A与c关系

Fig.5 Relationship between lnK-1/T (a) relationship between E_a and ln A with c (b) in 0.10 mol/L Cl₃CCOOH solution at different temperatures

2.5 钢在含CDEA的Cl₃CCOOH溶液中的动电位极化曲线

图6为钢片在不含或含一定浓度CDEA的0.10 mol/L Cl₃CCOOH溶液中的极化曲线。从图6可知，添加CDEA后，阴极、阳极极化曲线分别负移、正移，说明CDEA既有效抑制了阴极的析氢腐蚀又同时抑制了阳极的溶解腐蚀，故CDEA为混合抑制型缓蚀剂。

图6

图6 20 ℃时钢片在不含或含一定浓度CDEA的0.10 mol/L Cl₃CCOOH溶液中的动电位极化曲线

Fig.6 Potentiodynamic polarization curves of CRS in 0.10 mol/L Cl₃CCOOH solutions without and with CDEA at 20 ℃

用Tafel直线外推法拟合电化学试验数据，结果列于表3，极化曲线法的缓蚀率 (η_p) 由下式计算^[16]：

η_{p} = \frac{I_{c o r r (0)} - I_{c o r r (i n h)}}{I_{c o r r (0)}} \times 100 %

(6)

式中，I_{corr (0)}、I_{corr (inh)} 分别为钢电极在不含和含一定浓度CDEA在0.10 mol/L Cl₃CCOOH溶液中的腐蚀电流密度 (μA/cm²)。

表3 20 ℃时钢在空白或含CDEA的0.10 mol/L Cl₃CCOOH溶液中的极化曲线参数

Table 3 Potentiodynamic polarization parameters for coldrolled steel in 0.10 mol/L Cl₃CCOOH solutions without and with CDEA at 20 ℃

cmg·L^-1	E_corrmV	I_corrμA·cm^-2	b_cmV·dec^-1	b_amV·dec^-1	η_p%
0	-444	732	-307	167	---
10	-423	165	-235	91	77.5
50	-435	92	-217	90	87.4
100	-420	37	-209	67	95.0

如表3所示，I_corr在0.10 mol/L Cl₃CCOOH溶液中高达732 μA/cm²，冷轧钢表面遭受了剧烈的腐蚀，但添加CDEA后I_corr急速下降，且随缓蚀剂浓度的增加而下降，当添加100 mg/L CDEA时，I_corr降至37 μA/cm²。η_p随c的增加而增大，100 mg/L时高达95.0%，与失重法的缓蚀率 (η_w=97.4%) 较为吻合且规律一致。此外，添加CDEA后，腐蚀电位 (E_corr) 最大正移24 mV，改变幅度较小；进一步表明CDEA在Cl₃CCOOH溶液中为混合型缓蚀剂，并可推断其缓蚀机理为“几何覆盖效应”^[25]。在0.10 mol/L Cl₃CCOOH溶液中，随着CDEA的添加，阴极斜率 (b_c) 增大，而阳极斜率 (b_a) 减小。这可能是由于CDEA分子在钢电极表面吸附覆盖后，造成缓蚀剂的电位随电流密度的变化规律而改变。

2.6 钢在含CDEA的Cl₃CCOOH溶液中的EIS

图7a为钢片在不含或含有不同浓度CDEA的0.10 mol/L Cl₃CCOOH溶液中的Nyquist图。未添加CDEA的Cl₃CCOOH溶液Nyquist图由高频区的大段容抗弧和低频区的小段感抗弧组成，高频区的容抗弧表明钢的腐蚀为电荷转移控制，而感抗弧则是与Cl₃CCOO^-在电极表面的吸脱附引起的不平衡状态有关^[15,16]。值得注意的是，当添加CDEA后，低频区的感抗弧部分几乎很难分辨。各条件下的高频区的容抗弧不是一个完整的半圆，说明电极腐蚀过程中存在因电极表面粗糙、不均匀引起的频率弥散效应^[26]。此外，容抗弧半径随CDEA的添加而增大，说明电极表面的阻抗增加，即钢片表面难以腐蚀，腐蚀速率降低，从而缓蚀效果增强。

图7

图7 20 ℃时冷轧钢在不含或含CDEA的0.10 mol/L Cl₃CCOOH溶液中的EIS谱

Fig.7 Nyquist plots (a) and Bode modulus and Bode phases (b) of the corrosion of CRS in 0.10 mol/L Cl₃CCOOH solutions without and with CDEA at 20 ℃

图7b为Bode模量图和Bode相位角图，在lg f=-3~1范围内，Bode模量值 (lg Z) 随着CDEA浓度的增加而增大，在CDEA浓度为100 mg/L时达到最大，说明CDEA对冷轧钢在Cl₃CCOOH溶液中的缓蚀作用随浓度的增加而增强。从Bode相位角图中看出，未添加缓蚀剂的Cl₃CCOOH在中频区内有一个相位峰，而在低频区有一微弱的相位谷，说明EIS含有两个时间常数，而在含有CDEA的缓蚀体系中则仅有一个相位峰，且相位角进一步增大，说明缓蚀剂分子大量覆盖在钢电极表面^[27]。

分别采用等效电路图8a和b对不含和含有EDEA的Cl₃CCOOH溶液中的EIS图谱进行拟合，结果列于表4。表中R_s为缓蚀体系中的Cl₃CCOOH溶液电阻，R_t为腐蚀过程中的电荷转移电阻，R_L为电感电阻，CPE为常相位角原件，n为弥散效应系数。EIS法的缓蚀率 (η_R) 以下式计算^[15]：

η_{R} = \frac{R_{t (i n h)} - R_{t (0)}}{R_{t (i n h)}} \times 100 %

(7)

式中，R_t(inh)、R_t(0) 为含有CDEA、空白Cl₃CCOOH溶液中电极腐蚀过程中的电荷转移电阻 (Ω·cm²)。

图8

图8 不含和含有EDEA的Cl₃CCOOH溶液等效电路图

Fig.8 Equivalent circuit diagram without R_s (CPER_t(LR_L)) (a) and with EDEA R_s(CPER_t) (b)

表4 20 ℃时冷轧钢在不含或含CDEA的0.10 mol/L Cl₃CCOOH中的EIS参数

Table 4 EIS parameters for the corrosion of cold rolled steel in 0.10 mol/L Cl₃CCOOH solutions without and with different concentrations CDEA at 20 ℃

c / mg·L^-1	R_s / Ω·cm²	R_t / Ω·cm²	R_L / Ω·cm²	L / H·cm²	CPE /μF·cm^-2	n	χ²	η_R / %
0	8.6	5.8	10.6	17.4	3.89×10^-4	0.98	3.4×10^-3	---
10	10.9	28.2	---	---	3.0×10^-4	0.83	2.4×10^-3	79.3
50	10.5	181.0	---	---	2.3×10^-4	0.78	4.5×10^-3	96.8
100	10.78	293.1	---	---	3.0×10^-4	0.68	2.6×10^-3	98.0

由表4所示，数据拟合卡方值χ²较小，表明所用等效电路图拟合出的理论数据和试验数据较为一致。n＜1，说明电极/溶液界面存在频率弥散效应，在0.10 mol/L Cl₃CCOOH溶液中，随着CDEA的添加，n值降至0.68，偏离1的幅度较大，说明缓蚀体系中频率弥散效应强度增大，可能与缓蚀剂CDEA分子在电极/溶液界面吸附、脱附过程有关。CPE整体呈下降趋势，可能由于缓蚀剂CDEA分子挤走了吸附在钢表面的水分子，随CDEA浓度的增加，缓蚀剂吸附层厚度增加所致。空白的Cl₃CCOOH溶液在低频区存在电感参数R_L和L，可能与溶液中酸根离子或水分子在电极表面的“吸附/脱附”过程引起的扰动有关^[28]。R_t在0.10 mol/L Cl₃CCOOH缓蚀体系中仅为5.83 Ω·cm²，添加CDEA，其值显著增大，且随CDEA浓度的添加而增大，说明CDEA对钢在Cl₃CCOOH溶液中具有较强的缓蚀效果。当溶液中CDEA为100 mg/L时，η_R达98.0%，与失重法所得数值 (97.4%) 基本吻合。

2.7 SEM微观形貌分析

图9为不同条件下的钢表面SEM微观形貌。图9a为刚打磨好的钢片，表面较光滑，砂纸打磨的痕迹清晰可见，并存在不规则划痕。图9b为在0.10 mol/L Cl₃CCOOH溶液中浸泡6 h后的钢片表面形貌，表面十分粗糙，大量白色腐蚀颗粒不均匀的附着在钢片表面，难以阻止酸介质对钢片的持续腐蚀。图9c为含100 mg/L CDEA的0.10 mol/L Cl₃CCOOH溶液中浸泡6 h后的钢片表面形貌，较图9b表面平整、光滑，钢片表面留有少许腐蚀痕迹，并可观察到打磨划痕，腐蚀程度大幅度减弱，说明CDEA在Cl₃CCOOH溶液中对钢片具有较强的腐蚀抑制作用。

图9

图9 钢片表面的SEM形貌

Fig.9 SEM microscopic images of steel sheet surfaces: (a) before corrosion; (b) after corrosion in 0.10 mol/L Cl₃CCOOH solution for 6 h; (c) after corrosion in 100 mg/LCDEA+0.10 mol/L Cl₃CCOOH solution for 6 h

2.8 3D-AFM微观形貌分析

图10为钢片在不同条件下的3D-AFM微观形貌、幅度和相位图。图10a为钢片刚打磨好的3D-AFM形貌，表面伴有打磨留下的红色“沟壑”划痕；从图10b可见钢表面有少量白色物质。通过观察可见，少量白色颗粒和白色条状物体覆盖在红色基体上，如图10c。图10d为0.10 mol/L Cl₃CCOOH溶液中浸泡6 h后的钢表面形貌,表面凹凸不平，较为粗糙，图10e显示钢表面产生了大量“砂浆”状的腐蚀产物，从图10f可见，腐蚀产物覆盖在整个钢表面，只见零星少量的红色基体。说明Cl₃CCOOH溶液对冷轧钢产生了较强烈的腐蚀。图10g为钢片含有100 mg/L CDEA的0.10 mol/L Cl₃CCOOH溶液中浸泡6 h后的表面形貌，较图10d相比表面平整许多，伴有少许白色“山峰”，从图10h可见包状产物大量吸附在钢表面，形成较厚的吸附膜层。由图10i可知，少许“盐粒”状白色小颗粒均匀分布在基体上，红色基体清晰可见，说明Cl₃CCOOH对钢的腐蚀程度降低。AFM微观形貌测试进一步说明了缓蚀剂CDEA对钢在Cl₃CCOOH溶液中具有较强的缓蚀性能。

图10

图10 冷轧钢表面的3D-AFM轻敲振幅和轻敲相位图

Fig.10 3D-AFM (a, d, g), tapping amplitude and tapping phase images (b, c, e, f, h, i) of CRS surfaces: (a-c) before corrosion; (d-f) after corrosion at 20 ℃ in 0.10 mol/L Cl₃CCOOH solution for 6 h; (g-i) after corrosion at 20 ℃ in 0.10 mol/L Cl₃CCOOH solution containing 100 mg/L CDEA for 6 h

3D-AFM的表面粗糙度参数详见表5，R_a为平均粗糙度，R_q为均方根表面粗糙度，R_max为表面最大起伏度。据表5，刚打磨好的钢片表面的R_a、R_q、R_max最小，说明表面较为平整光滑。但在0.10 mol/L Cl₃CCOOH溶液中浸泡6 h后，钢表面的R_a、R_q、R_max增大，说明Cl₃CCOOH溶液中的Cl₃CCOO^-、H⁺大量吸附在钢片表面，对钢产生了严重的腐蚀，大量的腐蚀物质吸附在钢表面。值得注意的是，在Cl₃CCOOH溶液中添加CDEA后，R_a、R_q、R_max进一步增大，这可能由于体系中缓蚀剂分子在浸泡后的钢片表面形成了起伏较大的吸附膜层，或吸附膜层较薄的钢表面发生严重腐蚀，故表面粗糙度相对较大。

表5 3D-AFM的表面粗糙度参数

Table 5 Surface roughness parameters of 3D-AFM of CRS

CRS	R_a / nm	R_q / nm	R_max / nm
Before immersion	3.63	4.39	33.8
Cl₃CCOOH	10.6	13.3	102
Cl₃CCOOH+CDEA	12.3	15.7	131

2.9 钢表面接触角测试

图11为钢表面不同条件下的接触角测试图。图11a所示刚打磨好的钢表面接触角为锐角 (66.76°)，说明钢表面具有亲水性，Cl₃CCOOH溶液较容易湿润钢表面而产生腐蚀。图11b所示在Cl₃CCOOH溶液中浸泡6 h后的表面接触角减小至54.55°，这可能与钢表面遭受了剧烈腐蚀有关，钢表面亲水性进一步增强，故Cl₃CCOOH溶液进一步渗透对钢会产生持续不断的腐蚀。图11c为钢在含有100 mg/L CDEA的0.10 mol/L Cl₃CCOOH溶液中浸泡6 h后的表面，接触角增大至100.66°，即钢表面由亲水性转变为疏水性，有利于降低Cl₃CCOOH溶液对钢表面的腐蚀。接触角测试进一步说明缓蚀剂CDEA对钢在Cl₃CCOOH中具有较好的腐蚀抑制作用。

图11

图11 钢片表面在不同条件下的接触角测试

Fig.11 Contact angel measurements on steel surfaces with different conditions: (a) before corrosion; (b) after 6 h corrosion in 0.10 mol/L Cl₃CCOOH at 20 ℃; (c) after 6 h corrosion in 0.10 mol/L Cl₃CCOOH solutions containing 100 mg/L CDEA at 20 ℃

2.10 溶液表面张力分析

图12为常温时，不同条件下溶液表面张力 (σ) 与CDEA浓度 (c) 的关系曲线。从图12可以看出，随缓蚀剂CDEA的添加，各条件溶液表面张力急速减小后趋于平缓；这是由于表面活性剂含有两种基团 (疏油基和憎水基)，憎水基倾向于从水中逸出，吸附在水表面，紧密排列在水面上所致^[29]。CDEA水溶液的σ-c曲线在50 mg/L附近出现了拐点，可视为临界胶束浓度 (CMC)。与CDEA水溶液相比，在Cl₃CCOOH介质中的CDEA溶液的表面张力曲线整体下移，且CMC降至30 mg/L，故在酸介质中的CDEA的表面活性较高。当钢片腐蚀浸泡6 h后，Cl₃CCOOH介质中的CDEA溶液的表面张力较浸泡前溶液表面张力大，可能由于缓蚀剂CDEA分子由从溶液中转移到钢片表面，形成吸附膜，降低了Cl₃CCOOH溶液中的缓蚀剂CDEA浓度而导致。

图12

图12 表面张力 (σ) 与CDEA浓度 (c) 的关系

Fig.12 Relationship between surface tension (σ) and CDEA concentration (c)

2.11 溶液电导率分析

图13为不同条件下溶液电导率 (к) 随浓度 (c) 变化的曲线图。由图13可知，CDEA水溶液的电导率非常小，这可能与非离子表面活性剂CDEA在溶液中几乎不会发生电离有关；且随CDEA浓度的增加而增大。在Cl₃CCOOH介质中的CDEA溶液电导率急剧增大，这主要是由于Cl₃CCOOH的酸性较强，在水溶液中会电离生成H⁺和Cl₃CCOO^-所致，故使溶液的电导率急剧增加。从图13可见，钢片浸泡前后的Cl₃CCOOH介质中的CDEA溶液的缓蚀体系，电导率随CDEA浓度增加呈“峰”状，在缓蚀剂CDEA浓度为50 mg/L时达到最大，分别为41.99 ms/cm和41.49 ms/cm。这可能是由于高浓度的CDEA一定程度抑制了Cl₃CCOOH在水溶液中的电离所致。

图13

图13 溶液电导率 (к) 与CDEA浓度 (c) 的关系曲线

Fig.13 Relationship between the electrical conductivity (к) of solution and CDEA (c)

未添加缓蚀剂的Cl₃CCOOH溶液，在钢片腐蚀浸泡6 h后的电导率有所降低，这主要是由于H⁺与钢表面发生腐蚀反应被消耗所致；但当腐蚀介质中添加CDEA后，电导率几乎和浸泡前的相等，这表明CDEA有效抑制了溶液中H⁺与钢片反应的消耗，即CDEA在冷轧钢在Cl₃CCOOH溶液中表现出良好的缓蚀性能。

2.12 钢片在Cl₃CCOOH溶液中的腐蚀及CDEA缓蚀机理探讨

常温下，Cl₃CCOOH的酸电离平衡常数 (K_a) 为0.22，在H₂O中发生电离：

C l_{3} C C O O H \to C l_{3} C C O O^{-} + H^{+}

(8)

实验中，可观察到钢片表面析出大量气泡，钢片在0.10 mol/L Cl₃CCOOH溶液 (PH计算值为1.13) 中发生析氢腐蚀。依据文献^[30]，阴极反应 (2H⁺+2e^-→H₂↑) 机理含有如下过程：

F e + H^{+} \leftrightarrow (F_{e} H^{+})_{a d s}

(9)

(F e H^{+})_{a d s} + e^{-} \leftrightarrow {(F e H)}_{a d s}

(10)

{(F e H)}_{a d s} + H^{+} + e^{-} \leftrightarrow F e + H_{2}

(11)

根据冷轧钢在甲酸 (HCOOH)^[31]和乙酸 (CH₃COOH)^[32]中的反应机理，羧酸根离子会参与阳极反应，故在此推测Cl₃CCOO^-会参与阳极反应，拟定按如下反应机理：

F e + C l_{3} C C O O^{-} \to (F e C l_{3} C C O O^{-})_{a d s}

(12)

(F e C l_{3} C C O O^{-})_{a d s} \to (F e C l_{3} {C C O O)}_{a d s} + e^{-}

(13)

(F e C l_{3} {C C O O)}_{a d s} \to (F e C l_{3} C C O O^{+}) + e^{-}

(14)

F e C l_{3} C C O O^{+} \to F e^{2 +} + C l_{3} C C O O^{-}

(15)

综上，在Cl₃CCOOH溶液中，产生的Cl₃CCOO^-和H⁺导致了冷轧钢的腐蚀。缓蚀剂CDEA在Cl₃CCOOH溶液中的缓蚀机理推测如下：CDEA分子结构中包含亲水的极性基团 (-OH、-CONH₂、-N H₂) 和疏水的极性基团 (-C₁₁H₂₃)。冷轧钢表面吸附了大量的亲水极性基团，而疏水的极性基团在缓蚀体系中阻止溶液对钢表面的腐蚀，从而有效提高了钢表面的耐腐蚀能力。同时，CDEA在酸性介质中发生质子化反应而带正电荷，与阳极腐蚀反应过程的中间产物 (FeCH₃COO^-)_ads通过静电引力作用吸附在钢片表面，Cl₃CCOOH与钢片表面的接触面积随之减少，从而抑制钢表面的腐蚀。此外，Fe的空d轨道与CDEA分子结构中N、O的孤对电子形成配位键，发生化学吸附^[16]。从而有效抑制了Cl₃CCOOH对钢片的腐蚀。

3 结论

(1) CDEA在0.10 mol/L Cl₃CCOOH溶液中能有效抑制钢的腐蚀，低温时，CDEA浓度为20 mg/L时缓蚀率可达95%以上，且排序为：20 ℃≈30 ℃＞40 ℃＞50 ℃。CDEA在钢表面的吸附与Langmuir吸附模型吻合，吸附过程中混乱度增大，释放热量到环境中，为化学吸附和物理吸附相结合的混合吸附类型。表观活化能和指前因子均随CDEA的添加而明显增大。

(2) CDEA为通过“几何覆盖效应”作用的混合抑制型缓蚀剂。Nyquist图的高频区容抗弧半径随CDEA的添加而增大，电荷转移电阻增大，而常相位角元件下降。SEM和AFM进一步证实了CDEA对钢在0.10 mol/L Cl₃CCOOH溶液中具有较强的腐蚀抑制作用。加入CDEA后钢表面接触角由锐角增大至钝角，由亲水性变为疏水性。当钢片在含CDEA的Cl₃CCOOH溶液中腐蚀浸泡后，表面张力增大，而电导率几乎没有变化。

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用失重法和电化学法研究了盐酸介质中,稀土铈(IV)离子和钼酸钠对冷轧钢的缓蚀协同效应.研究结果表明,钼酸钠和稀土Ce4+对冷轧钢都有一定的缓蚀作用,但最大缓蚀率不超过42%.通过吸附模型的讨论发现,在Langmuir吸附模型的基础上进行校正后的模型能更合理地解释实验结果,并求出了三个吸附参数.通过对比实验,发现稀土铈(IV)离子和钼酸钠对冷轧钢产生了明显的缓蚀协同效应,最大缓蚀率可达90%左右.

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