海洋环境中铜绿假单胞菌对增材制造Al-Mg-Sc-Zr合金腐蚀行为影响研究

图1 Al-Mg-Sc-Zr合金成形过程示意图

Fig.1 Schematic diagram of forming process for the Al-Mg-Sc-Zr alloy

1.2 P. aeruginosa 细菌的培养

P. aeruginosa细菌(MCCC 1A00099)购买自中国海洋微生物菌种保藏管理中心自然资源部第三海洋研究所，并在实验室用含KNO₃的Luria-Bertani培养基(LB-NO₃)进行了富集培养。LB-NO₃培养基成分如下^[36]：NaCl (5 g)、KNO₃ (10 g)、酵母提取物(5 g)、胰蛋白胨(10 g)、去离子水(1 L)。根据ASTM D1141-98标准^[37]，模拟海水的化学成分如下：NaCl 24.53 g、MgCl₂ 5.20 g、Na₂SO₄ 4.09 g、CaCl₂ 1.16 g、KCl 0.695 g、NaHCO₃ 0.201 g、KBr 0.101 g、H₃BO₃ 0.027 g、SrCl₂ 0.025 g、NaF 0.003 g、去离子水1 L。利用1 mol/L的NaOH溶液将LB-NO₃培养基的pH调节在7.0到7.2之间。接种前，培养基在121 ℃下高压灭菌20 min。实验前，P. aeruginosa菌株在恒温震荡培养箱中活化12 h。14 d实验期间，在荧光显微镜下利用血球计数板对溶液中的细菌数量进行计数。

1.3 电化学测试

所有电化学测试均在Corrtest电化学工作站(CS310X)上完成。电化学测试采用三电极系统，其中工作电极为Al-Mg-Sc-Zr合金(暴露面积为1 cm²)，对电极为方形铂片(10 mm × 10 mm × 1 mm)，参比电极为饱和甘汞电极(SCE)。实验分为两组，分别为无菌组和接菌组，无菌组为10%LB-NO₃培养基+ 90%模拟海水，接菌组为10%含P. aeruginosa菌种的LB-NO₃培养基+ 90%模拟海水。所有电化学测试均在开路电位(OCP)波动小于1 mV后进行。电化学阻抗谱(EIS)测试的扫描频率范围为10⁵~10^-2 Hz，正弦电位扰动为10 mV，分别在第1、3、5、7、10和14 d对Al-Mg-Sc-Zr合金的EIS谱图进行了连续监测。浸泡14 d后，对Al-Mg-Sc-Zr合金进行了动电位极化曲线测试，扫描速率为0.5 mV/s，扫描范围为-300 mV至1000 mV (vs. OCP)。为了实时监测细菌生长代谢对试样腐蚀行为的具体影响，所有试样均在原模拟液中进行电化学测试。EIS测试过程中，P. aeruginosa不会死亡或生长代谢异常，这是因为EIS测试仅施加了10 mV的正弦扰动电位，该电位对试样表面及溶液中P. aeruginosa细胞的生长基本无影响。然而，极化曲线测试施加的过负或过正电位可能会对P. aeruginosa的生长代谢产生影响。为了避免该影响，在第14 d实验结束时进行了极化曲线测试，因此不会影响14 d实验期间P. aeruginosa的生长代谢。

1.4 表面形貌观察及成分分析

浸泡14 d后，通过PHENOM XL扫描电子显微镜(SEM)观察试样在非生物和生物介质中的腐蚀产物和生物膜形貌。在形貌观察前，将生物介质中的试样浸泡于含4%戊二醛的磷酸盐缓冲液(PBS) (pH在7.0至7.2之间)中8 h以固定生物膜，随后依次用25%、50%、75%和100%的无水乙醇溶液进行脱水处理。利用DAPI染色剂对试样表面的生物膜进行染色，在黑暗环境中染色20 min，并利用荧光显微镜观察P. aeruginosa细胞在试样表面的分布情况。DAPI染色剂可以穿透细胞膜完整性丧失的死细胞或固定细胞，与DNA结合后发出蓝色荧光。通过SEM配备的能量色散光谱仪(EDS)对试样表面的腐蚀产物膜和生物膜的元素组成进行了分析。通过K-Alpha型X射线光电子能谱仪(XPS)对试样表面腐蚀产物中Al、Mg、N和P的化学价态进行了表征。首先，利用SEM找到具有典型腐蚀产物的测量位置，随后利用XPS对该位置进行了表征。去除试样表面腐蚀产物后，利用NPFLEX-1000白光干涉仪对试样表面的点蚀形貌进行了观察，并对点蚀坑进行了测量。

2 结果与讨论

2.1 Al-Mg-Sc-Zr合金的金相及SEM图像

图2为Al-Mg-Sc-Zr合金经Keller试剂(95 mL去离子水+ 2.5 mL HNO₃ + 1.5 mL HCl + 1.0 mL HF)刻蚀后的金相图像以及SEM二次电子图像。由图2a，b可见，熔池沿着XOZ方向为层叠状排列，呈明显“鱼鳞状”，且熔池内部较为明亮，而熔池边界部分较暗。可以推断，熔池边界处为等轴晶区，内部为柱状粗晶区，这主要是因为等轴晶区的晶粒细小，晶界更多，同时存在较多的析出相，通过化学试剂侵蚀后呈现暗色，而柱状晶的晶粒相对粗大，晶界较少，侵蚀后呈现亮色。图2c，d也证实了熔池内部为粗晶区，而边界为等轴晶。

图2

图2 Al-Mg-Sc-Zr合金的金相图像及SEM二次电子图像

Fig.2 Metallographic images (a, b) and SEM secondary electron images (c, d) of the Al-Mg-Sc-Zr alloy

2.2 pH和细菌生长曲线

14 d实验后，对无菌和接菌环境中溶液的pH以及P. aeruginosa的细胞浓度进行了连续监测，如图3a所示。无菌组中溶液的pH在一定范围内波动较小，而接菌组中溶液的pH随着时间增加逐渐下降。此外，由图3a中可见，P. aeruginosa细胞的数量随时间增加，在第5 d达到峰值，随后逐渐下降。

图3

图3 测试溶液的pH值、细胞含量变化以及试样表面的细菌荧光图像

Fig.3 Changes in pH value and cell counts of the test solution (a) and fluorescence images of P. aeruginosa cells on the Al-Mg-Sc-Zr alloy surfaces (b)

当有机碳源充足时，P. aeruginosa可以优先利用有机碳源(如乳酸)作为其生长所需的能量来源，而硝酸盐会作为电子受体，总反应如下所示^[36]：

C H_{3} C H O H C O O^{-} + 2 N O_{3}^{-} \to

C H_{3} C O O^{-} + 2 N O_{2}^{-} + C O_{2} + H_{2} O

(1)

5 C H_{3} C H O H C O O^{-} + 4 N O_{3}^{-} + 4 H^{+} \to

5 C H_{3} C O O^{-} + 2 N_{2} + 5 C O_{2} + 7 H_{2} O

(2)

2 C H_{3} C H O H C O O^{-} + N O_{3}^{-} + 2 H^{+} \to

2 C H_{3} C O O^{-} + N H_{4}^{+} + 2 C O_{2} + H_{2} O

(3)

从上述反应可以看出，P. aeruginosa会消耗质子并产生OH^-，导致pH升高。因此，与无菌组相比，接菌组中溶液的pH偏高。随着腐蚀的进行，溶液pH缓慢下降，这可能是由于腐蚀过程中释放的Al³⁺发生如下水解反应^[2]：

A l \to A l^{3 +} + 3 e^{-}

(4)

A l^{3 +} + 2 H_{2} O + C l^{-} \to A l {(O H)}_{2} C l + 2 H^{+}

(5)

14 d后，利用DAPI染色剂对接菌溶液中的试样表面进行染色，荧光形貌如图3b所示。可以看出，P. aeruginosa可附着在铝合金表面，呈现不均匀分布状态，可能会对Al-Mg-Sc-Zr合金的腐蚀产生不利影响。

2.3 电化学测试结果

图4为Al-Mg-Sc-Zr合金在14 d实验期间连续监测的OCP和EIS结果。如图4a所示，在接菌环境中，该合金的OCP随时间呈上升趋势，而在无菌环境中该合金的OCP则先正移后负移，最后又缓慢上升。接菌环境中Al-Mg-Sc-Zr合金的OCP值小于无菌环境，表明P. aeruginosa导致该合金的腐蚀倾向增大。图4b和c分别为无菌和接菌环境中Al-Mg-Sc-Zr合金的Nyquist图，利用图中所示等效电路模型来拟合该合金在无菌和接菌环境中的EIS谱图，其中R_s代表测试溶液的电阻，Q_f和R_f分别代表腐蚀产物膜或生物膜的电容和电阻，Q_dl和R_ct分别代表双电层的电容和电阻。拟合得到的相应参数被列于表1中。图4d为Al-Mg-Sc-Zr合金的极化电阻R_p (对于电容型EIS，R_p = R_ct + R_f)随时间变化的趋势。在接菌环境中，Al-Mg-Sc-Zr合金的R_p值随时间不断增大，这可能是由于其表面腐蚀产物与生物膜的持续累积所致。而在无菌环境中，铝合金的R_p值随时间先增大后减小，最后又缓慢上升。这可能是因为前期合金表面形成了保护性的钝化膜，但在海水中该钝化膜可能随后被破坏。海水中的Cl^-可以穿透氧化膜的薄弱区域，引发局部腐蚀，导致在第3至5 d OCP急剧负移。最后随着腐蚀产物的积累，减缓了Cl^-的扩散，使腐蚀速率降低，电位缓慢正移。

图4

图4 14 d实验期间无菌组与接菌组中合金的OCP值和EIS数据

Fig.4 OCP values (a) and Nyquist diagrams of the Al-Mg-Sc-Zr alloy in the sterile (b) and inoculated media (c), and the R_ct + R_f values of the Al-Mg-Sc-Zr alloy from curve-fitting of the EIS spectra (d), and equivalent circuit models used for curve-fitting of the EIS spectra displayed in Fig.4b and Fig.4c during the 14 d experiment

表1 Al-Mg-Sc-Zr合金在非生物和生物介质中EIS谱的电化学参数

Table 1 Electrochemical parameters derived from the EIS spectra for the Al-Mg-Sc-Zr alloy in the abiotic and biotic media

Condition	t / d	R_s / Ω·cm²	Q_f / 10^-6 S·cm^-2·s ⁿ	R_f / 10³ Ω·cm²	Q_dl / 10^-6 S·cm^-2·s ⁿ	R_ct / 10³ Ω·cm²
Control	1	8.388	7.136	12.21	0.500	767.1
	3	8.697	6.516	6.624	0.795	12360
	5	8.741	7.168	5.129	1.088	998.6
	7	8.425	9.302	6.481	0.875	485.3
	10	8.702	15.17	3.384	0.826	614.9
	14	8.664	14.59	2.380	1.000	1834
P. aeruginosa	1	7.523	11.35	33.30	1.348	42.66
	3	8.014	9.817	0.0204	6.085	171.5
	5	8.386	9.704	0.0345	6.827	243.6
	7	8.292	9.527	0.0349	7.767	238
	10	8.524	10.500	0.0442	8.158	386.6
	14	8.485	12.55	0.0589	7.692	791.3

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在无菌和接菌环境中浸泡14 d后，测量了Al-Mg-Sc-Zr合金的动电位极化曲线，如图5所示。与无菌组相比，该合金在接菌环境中的阳极和阴极反应都被显著加速，拟合后的参数见表2。由表2可知，P. aeruginosa的存在使得腐蚀电位发生负移，且该合金在接菌环境中的腐蚀电流密度约为无菌环境中的3.3倍。以上数据表明，该合金在接菌环境中的腐蚀速率明显大于无菌环境，即P. aeruginosa可显著加速Al-Mg-Sc-Zr合金的腐蚀。从二者极化曲线上的阳极区来看，两者均出现了明显的钝化现象，并且几乎在同一电位下发生钝化膜破裂现象，这说明P. aeruginosa的存在并未显著改变该合金钝化膜的破裂电位，但在含P. aeruginosa溶液中钝化电流密度较高，说明P. aeruginosa可能降低铝合金表面膜的保护性。P. aeruginosa可能通过调控局部化学微环境或电化学界面过程，从而加速钝化膜溶解及失效后的金属溶解过程。

图5

图5 试样在无菌和接菌环境中浸泡14 d后的动电位极化曲线

Fig.5 Potentiodynamic polarization curves of Al-Mg-Sc-Zr alloy after 14 d of immersion in the sterile and inoculated media

表2 试样在无菌和接菌环境中通过拟合动电位极化曲线所得到的相关电化学参数

Table 2 Relative electrochemical parameters of Al-Mg-Sc-Zr alloy calculated from potentiodynamic polarization curves after 14 d of testing

Condition	E_{corr vs. SCE} / V	I_corr / 10^-8 A·cm^-2
Control	-1.098	2.08
NRB	-1.147	6.78

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2.4 微观形貌表征

图6为14 d实验后，无菌和接菌环境中Al-Mg-Sc-Zr合金表面的SEM形貌以及EDS面扫描结果。对比图6b和d中形貌可见，该合金在无菌环境中几乎观察不到腐蚀的痕迹。然而，接菌环境中合金表面存在大量球状腐蚀产物，且周围分布着P. aeruginosa细菌细胞。图6b1和b2中的面扫描结果表明，无菌环境下合金表面均匀分布着Al(94.98%)和Mg (5.02%)，这属于合金本身的元素成分。图6d1~d6的面扫描结果表明，合金表面的球状腐蚀产物主要由Al (67.49%)和O (27.69%)组成。此外，合金表面还分布着微量的N (0.09%)和P(0.41%)，这两种元素可能由P. aeruginosa代谢产生。值得注意的是，接菌环境中合金表面还存在一些Cl，可能会对合金腐蚀产生不利影响。图中结果表明，生物膜不均匀覆盖在金属表面，导致局部电化学腐蚀差异。此外，Cl在腐蚀产物中可能如上述水解反应方程式(5)所示，Cl^-与Al³⁺发生络合作用，和P. aeruginosa协同加速了钝化膜的破裂。

图6

图6 浸泡14 d后试样表面的SEM形貌与EDS面扫描元素分布图

Fig.6 Morphologies of surface films: (a, b) SEM images and (b1, b2) EDS mapping results from (b) after 14 d of immersion in the sterile media; (c, d) SEM images and (d1-d6) EDS mapping results from (d) after 14 d of immersion in the inoculated media

图7a和b分别为14 d实验后，Al-Mg-Sc-Zr合金在无菌和接菌环境中去除腐蚀产物后的二维和三维形貌。由图7a可见，试样在无菌环境中没有明显的腐蚀坑，只能看到一些打磨导致的划痕。而从图7b可以看出，合金在接菌环境中发生了明显的点蚀行为，经测量得出最大坑深约为3 μm，这进一步证明了P. aeruginosa可促进该合金的局部腐蚀。

图7

图7 去除腐蚀产物后试样表面腐蚀形貌

Fig.7 2D and 3D color images of the Al-Mg-Sc-Zr alloy surfaces after removal of corrosion products in the sterile medium (a) and in the inoculated medium (b)

为进一步确定腐蚀产物成分，利用XPS对Al-Mg-Sc-Zr合金表面腐蚀产物的元素价态进行了分析。在无菌环境中，Al 2p谱图主要在74.9和72.9 eV处出现2个峰，分别对应于样品表面的Al₂O₃与Al (图8a1)；在Mg 1s谱图中主要在1303.5和1303.9 eV处出现2个峰，分别对应于样品表面的Mg与MgO (图8a2)；在N 1s谱图中主要在400.1和399.7 eV处出现2个峰，这可能对应于一些吸附在金属表面有机物中的N元素(图8a3)；P 2p谱图主要在133.8 eV处出现1个峰，对应于HPO $_{4}^{2 -}$ (图8a4)。在接菌环境中，Al 2p谱图主要在74.9和74.2 eV处出现2个峰，分别对应于样品表面的Al₂O₃与Al(OH)₃ (图8b1)；在Mg 1s谱图中主要在1303.9 eV处出现1个峰，对应于样品表面的MgO (图8b2)；在N 1s谱图中主要在403、400和399 eV处出现3个峰，分别对应于样品表面的NO $_{2}^{-}$ 、-NH₂和-NH $_{3}^{+}$ ，其中NO $_{2}^{-}$ 是由于P. aeruginosa细胞发生了硝酸盐还原反应所造成(图8b3)。P 2p谱图主要在134.1 eV处出现1个峰，对应于H₂PO $_{4}^{-}$ ，这可能是局部酸性增强，由HPO $_{4}^{2 -}$ 转化而来(图8b4)。综上所述，在无菌环境中仍可以检测到基体金属的特征峰，而在有菌环境中合金表面几乎都是腐蚀产物。以上XPS结果进一步说明P. aeruginosa能加速Al-Mg-Sc-Zr合金的腐蚀，与电化学结果一致。

图8

图8 浸泡14 d后Al-Mg-Sc-Zr合金表面的XPS图谱

Fig.8 High-resolution XPS spectra Al 2p (a1), Mg 1s (a2), N 1s (a3) and P 2p (a4) for Al-Mg-Sc-Zr alloy in the sterile medium; Al 2p (b1), Mg 1s (b2), N 1s (b3) and P 2p (b4) for Al-Mg-Sc-Zr alloy in the inoculated medium

2.5 P. aeruginosa 诱导Al-Mg-Sc-Zr合金的腐蚀机理

根据上述实验结果，提出了P. aeruginosa诱导Al-Mg-Sc-Zr合金腐蚀的可能机理(图9)。如图9所示，P. aeruginosa通过分泌胞外聚合物(EPS)附着于铝合金表面，形成分布不均的生物膜，生物膜内部的局部pH微环境可能有显著差异。Hollmann等^[38]分析了铜绿假单胞菌生物膜中的实时三维pH变化，表明菌落核心内环境具有高度酸性(pH约为3.5~4.5)，而这些菌落的外缘则表现出略微酸性的pH (5.5~6.0)。因此，生物膜下方的局部酸化可能促进氧化铝的溶解(图9II)，这一解释与Kolics等^[39]报道的结果一致，在0.1 mol/L NaCl溶液中，当pH低于4时，由于活性金属的溶解和氧化膜的不稳定，纯铝表面的纳米级Al₂O₃层显著减少。此外，在有菌环境中，Al-Mg-Sc-Zr合金表面监测到的Cl^-可能被吸附在生物膜中，与P. aeruginosa协同破坏了氧化膜。另外，生物膜的粘附性结构还会阻碍氧气扩散，导致生物膜内部与外部形成氧浓度梯度。氧浓度差异促使铝合金表面形成阳极(低氧区)和阴极(高氧区)，引发局部电偶腐蚀，加速点蚀的发展(图9中的III)。

图9

图9 P. aeruginosa诱导Al-Mg-Sc-Zr合金的腐蚀示意图

Fig.9 Schematic diagram of P. aeruginosa induced corrosion of Al-Mg-Sc-Zr alloy

从图中还可看出，在实验初期，当有机碳源充足时，P. aeruginosa可从有机碳源中获取生命活动所需的能量。根据生物催化阴极硝酸盐还原(Biocatalytic cathodic nitratereduction，BCNR)理论^[40]，当环境中有机碳源匮乏时，P. aeruginosa生物膜可以催化硝酸盐还原反应的进行，并从Fe中获取能量进行自身代谢。在本文中，试样表面生物膜下的P. aeruginosa细胞能以Al-Mg-Sc-Zr合金中的Al、Mg元素以及微量Fe元素作为能量来源，产生的电子可用于细胞内部的硝酸盐还原反应，该氧化还原反应产生的能量可维持细菌的生命活动以及生长代谢，从而显著恶化合金的腐蚀性能(图9IV)。此外，生物膜可以作为有机碳源的扩散屏障，导致膜下与金属基体接触的固着细胞处于饥饿状态，只能将金属作为唯一能量来源，这会显著加速其腐蚀过程。从图3a看，腐蚀可能很快就进入到图9IV的模式。

3 结论

(1) EIS结果显示，合金在接菌条件下的阻抗值远小于无菌条件，表明细菌显著降低了合金的腐蚀稳定性。此外，与对照组相比，细菌显著促进了合金的阳极溶解，腐蚀电流密度增大约3.3倍。

(2) 无菌条件下合金在模拟海水中具有优异的耐蚀性，表面基本无明显点蚀坑；而在接菌条件下，P. aeruginosa可在合金表面形成生物膜，加速点蚀损伤。

(3) P. aeruginosa能从合金的Al、Mg元素中获取电子，在细胞内部用于硝酸盐还原反应从而获取生存所需能量，同时异质生物膜会导致氧浓差电池的形成，加速了局部腐蚀的扩展，并恶化了合金的腐蚀性能。

参考文献

原文顺序

文献年度倒序

文中引用次数倒序

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[J]. Corros. Sci., 2025, 250: 112884

[38]

Hollmann

, Perkins

, Chauhan

V M

, et al.

Fluorescent nanosensors reveal dynamic pH gradients during biofilm formation

[J]. npj Biofilms Microbiomes, 2021, 7: 50

\n Understanding the dynamic environmental microniches of biofilms will permit us to detect, manage and exploit these communities. The components and architecture of biofilms have been interrogated in depth; however, little is known about the environmental microniches present. This is primarily because of the absence of tools with the required measurement sensitivity and resolution to detect these changes. We describe the application of ratiometric fluorescent pH-sensitive nanosensors, as a tool, to observe physiological pH changes in biofilms in real time. Nanosensors comprised two pH-sensitive fluorophores covalently encapsulated with a reference pH-insensitive fluorophore in an inert polyacrylamide nanoparticle matrix. The nanosensors were used to analyse the real-time three-dimensional pH variation for two model biofilm formers: (i) opportunistic pathogen\n Pseudomonas aeruginosa\n and (ii) oral pathogen\n Streptococcus mutans\n. The detection of sugar metabolism in real time by nanosensors provides a potential application to identify therapeutic solutions to improve oral health.\n

[39]

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, Besing

A S

, Baradlai

, et al.

Effect of pH on thickness and ion content of the oxide film on aluminum in NaCl media

[J]. J. Electrochem. Soc., 2001, 148: B251

[40]

T Y

Theoretical modeling of the possibility of acid producing bacteria causing fast pitting biocorrosion

[J]. J. Microb. Biochem. Technol., 2014, 6: 068