钢制螺栓紧固件具有优异的机械性能和较低的成本，被广泛应用于塔架、桥梁、海洋平台和压力容器等设备零部件的连接中[1,2]。尽管它们具有诸多优点，但由于服役场景的复杂性和严酷性，其应用也存在诸多安全隐患。除了螺栓结构的疲劳、应力开裂和其他类型的应力机械故障外，腐蚀是降低其承载能力并危及结构完整性的又一大关键因素，特别是在如海水和海岸大气等含盐量较高的恶劣环境中，螺栓的腐蚀问题更为严重[3]。近年来，许多与螺栓紧固件腐蚀相关的故障和事故被相继报道，例如汽车事故、起重机故障、桥梁、建筑和平台崩塌、石油和天然气管道泄漏等。由于螺杆和螺母的耦接部位不可避免地存在缝隙，可以导致接触区域内外的传质过程受到限制，从而在离子浓度上产生显著的差异，并导致缝隙内部电解质缺氧等后果[4,5]。此外，螺杆和螺母的耦接将紧固件结构分为阳极区和阴极区两部分。曝露于大气环境并拥有充足O2供应的区域作为阴极区，而O2扩散较为困难的闭塞部位作为阳极区。具有供氧差异的螺纹表面不同区域之间会形成腐蚀原电池，它将影响腐蚀产物的性质、腐蚀模式和腐蚀动力学等，如果不加控制，则可能导致螺栓紧固件的突然失效，从而威胁整个结构体系。
用于制备钢制紧固件的主要材料通常为碳钢和不锈钢两种。碳钢属于活性金属，腐蚀速率较快，且在其表面通常生长出一定厚度的锈层。尽管碳钢对缝隙腐蚀有一定的抵抗力，但仍然容易受到其他形式的腐蚀攻击。例如在诸如土壤、大气和深海等环境中，都有碳钢紧固件腐蚀的相关报道，其腐蚀受到金属表面状态、溶液组分、pH、温度、CO2含量、残余应力和电偶作用等多种因素的影响[6~8]。不锈钢属于钝性金属，其表面自生一种纳米级的钝化膜，使其具有良好的耐蚀性[9]。然而，在Cl-存在的情况下，缝隙腐蚀是影响不锈钢紧固件的主要问题。Cl-会改变钝化膜的结构、形态和保护性，从而影响不锈钢的腐蚀热力学与动力学[10~12]。Muzghi等[13]在对曝露于阿拉伯湾海洋大气环境中的不同材质紧固件的一项失效分析研究中指出，碳钢螺栓均匀腐蚀减薄量较大，腐蚀极为严重，而不锈钢螺栓发生了点蚀、缝隙腐蚀和严重的应力腐蚀开裂等局部腐蚀。Elshawesh等[3]研究表明，在含盐、CO2和H2S的侵蚀性服役环境中，多级水泵的碳钢螺栓发生严重的电偶腐蚀和缝隙腐蚀，致使其在螺杆-螺母接触的螺纹区域发生开裂。Aziz等[14]研究了完整尺寸碳钢螺栓长期曝露于侵蚀性环境中的服役行为，表明点蚀和缝隙腐蚀是该环境中的主要腐蚀类型。Yang等[15]对曝露于海洋和工业大气环境中的1Cr17Ni2高强度螺栓的腐蚀行为进行了研究，表明γ-FeOOH和α-FeOOH是螺栓表面锈层的主要成分。Shah等[16]研究了螺栓扭矩和腐蚀对螺栓紧固件结构机械行为的作用，表明扭矩和腐蚀的交互作用是影响螺栓接头的腐蚀行为和承载能力的另一个主要因素。
以往对螺栓紧固件腐蚀的研究通常集中在环境、机械和冶金等因素的相互作用上[15,17,18]。关于螺栓紧固件的电偶行为与缝隙腐蚀耦合作用的研究较少。尤其是针对于钢制紧固件上具有两种不同电化学属性区域间的耦合腐蚀机制尚缺少清晰的认识，这对评估钢制紧固件服役寿命、开发耐蚀螺栓是至关重要的基础。近年来，本研究团队针对碳钢和不锈钢两种材质紧固件进行了系统的腐蚀机制研究，揭示了紧固件的几何设计尺寸、腐蚀产物和阴/阳极区域面积等是影响紧固件腐蚀动力学规律的主要因素，同时阐明了碳钢和不锈钢两种不同材质紧固件在腐蚀机制上的本质性差异，为机械设备连接选材与服役寿命评估提供了重要的依据[19~21]。本文将从腐蚀行为、腐蚀产物、电偶腐蚀和腐蚀机制等4个方面分别进行阐述。
1 螺栓紧固件腐蚀模拟装置的搭建
为了研究碳钢和不锈钢紧固件腐蚀机制的差异，在实验室搭建了螺栓紧固件结构腐蚀模拟装置，如图1所示。螺栓紧固件曝露螺纹区域的腐蚀行为可通过“曝露区域”样品进行模拟，与螺母接触的螺纹区域的腐蚀行为可通过“接触区域”样品进行模拟。为了模拟螺杆和螺母通过电连接产生的电偶腐蚀行为，将“曝露区域”样品与“接触区域”样品通过零阻电流计进行耦接，并可测试其电偶电流。螺杆和螺母组装模型的剖面如图1a所示。图1b所示为电化学模拟装置，其中，区域A和B分别表示为“曝露区域”和“接触区域”。两个电极的表面积均为2.0 cm2。采用厚度为350 μm的聚四氟乙烯垫片作为缝隙形成器。Ag/AgCl电极和Pt片分别用作参比电极和辅助电极[19]。
图1
2 缝隙腐蚀行为的差异
图2
图2
C45碳钢螺栓紧固件在0.06 mol/L NaCl溶液中浸泡28 d前后的宏观形貌图[19]
Fig.2
Macroscopic images of C45 carbon steel bolt and nut assembly before immersion (a, b) and after 28 d of immersion in 0.06 mol/L NaCl solution (c-f): (a) coupled bolt with nut, (b) uncoupled bolt, (c) coupled bolt with nut, (d) uncoupled bolt, (e) region A exposed to the bulk solution, (f) region B (bolt and nut contact) [19]
图3
图3
304不锈钢螺栓紧固件在腐蚀前后的表面形貌图 [20]
Fig.3
Surface morphologies of the 304 stainless steel threaded specimen[20]: (a) as-received specimen, (b) exposed region after corrosion test, (c) crevice region after corrosion test, (d) enlarged view of the crevice region, (e) sampled area at the flank, (f) sampled area at the root
由以上分析可知，碳钢紧固件的曝露部位腐蚀严重，缝隙内部腐蚀轻微，腐蚀形式以均匀腐蚀为主。不锈钢紧固件曝露部位腐蚀轻微，而缝隙部位腐蚀严重，腐蚀形式以点蚀为主。可见，两种材料紧固件的腐蚀存在着腐蚀机制上的差别，需要从腐蚀产物和电偶极化机制上寻找本质原因。
3 腐蚀产物的差异
碳钢和不锈钢分别属于活性金属和钝性金属，在腐蚀产物组分上存在着较大的差别。碳钢的腐蚀产物通常为较厚的锈层，锈层的存在可以为体系提供额外的IR降，从而影响紧固件体系的腐蚀。由图4可见，C45碳钢紧固件曝露区域的锈层厚度为60 μm左右，接触区锈层的厚度为15 μm左右。曝露区域表面的锈层可以分为较为疏松的外锈层和相对致密的内锈层，而接触区域只有相对致密的内锈层。通过比较两个区域锈层厚度可以发现，曝露区域中形成的腐蚀产物明显多于接触区域。图5所示为C45碳钢紧固件曝露区域和接触区域表面形成的腐蚀产物的相组成。由图可见，C45碳钢螺栓紧固件表面腐蚀产物的主要相组成为β-FeOOH、α-FeOOH、γ-FeOOH和Fe3O4。从XRD谱可知，曝露区域形成的腐蚀产物与接触区域有所不同[21]。在图5a中，曝露区域的腐蚀产物相中最突出的峰值对应于γ-FeOOH。同时，也检测到了β-FeOOH、α-FeOOH和Fe3O4的衍射峰。环境中Cl-的存在有利于β-FeOOH的形成。相比于γ-FeOOH和β-FeOOH，Fe3O4和α-FeOOH更具有热力学稳定性[22]。由图5b可见，在接触区域腐蚀产物相组成中，Fe3O4和β-FeOOH的衍射峰较强，同时，也可以检测到较弱的α-FeOOH和γ-FeOOH的衍射峰。在Fe2+和Cl-含量较高的环境中，β-FeOOH更容易生成并稳定存在，接触区域中较低的O含量更有利于Fe3O4的形成[20~24]。
图4
图5
图6
图7
通过腐蚀产物对比分析可以看出，对于碳钢和不锈钢两种材质的螺栓紧固件来说，曝露区域腐蚀产物的厚度和保护性能均优于接触区域。由于缺少O2的供应，不锈钢紧固件接触区的钝化膜极薄，对基体几乎起不到保护作用。碳钢紧固件曝露区域和接触区域的锈层具有一定的厚度，可以进一步阻挡传质过程，并为结构引入额外的IR降，这也是两种紧固件腐蚀产物最重要的差别。
4 电偶极化作用的差异
紧固件特殊的几何结构将螺栓曝露区域 (O2供应充足) 和接触区域 (O2供应不足) 分为了阴极区和阳极区。阴极区表面发生的氧还原过程会对阳极区起到一定的电偶极化作用，这将是螺栓腐蚀失效的一个加速步骤。
图8
图9a所示为在零阻电流计联通的模式下，304不锈钢螺栓紧固件曝露区域和接触区域的电偶电流密度随浸泡时间的变化趋势[20]。在浸泡初期 (5 d以内)，电偶电流密度大幅度地增大，表明曝露区域电极对接触区域电极的电偶极化作用在不断地增强。在浸泡初期，曝露区域和接触区域表面的钝化膜不完善，二者电位差异较小，电偶极化作用较弱。随着腐蚀时间的延长，由于氧供应充分，曝露区域的钝化膜不断完善，保护性能逐渐变强。由于缺少O2的供应，接触区域的钝化膜保护能力较差，并且随着缝隙内部不断地酸化，钝化膜性质愈发劣化。随着曝露区域和接触区域的电位差不断地变大，电偶极化作用也相应地增强。另外，与碳钢紧固件接触区域不同，不锈钢表面没有较厚的锈层产生过多的IR降，因此，不锈钢螺栓阴/阳极区域之间的电偶极化作用没有额外的消耗，一直处于较强的状态。由图9b可见，接触区域304不锈钢的维钝能力和点蚀电位均下降，这与接触区域表面钝化膜性质变差，更易诱发点蚀直接相关。
图9
图10
5 腐蚀机制的差异
通过以上分析可知，不论是碳钢还是不锈钢紧固件，在耦合情况下，宏观上都存在阳极区域与阴极区域之间的分离。螺纹的接触区域主要发生阳极过程，螺纹的曝露区域发生阴极过程。在这两个区域之间的氧浓差极化的作用下，二者之间存在电偶电流。
图11
图12示意性地描述了304不锈钢紧固件在含Cl-环境中，曝露区域和接触区域表面形成的钝化膜以及涉及到的各种反应过程。对于304不锈钢螺栓紧固件系统，材料表面为几纳米厚的钝化膜，无额外的IR降消耗电偶极化作用。在曝露区域，由于供氧充分，表面钝化膜结构完整且稳定，构成腐蚀的阴极性区域；而接触区域则因为缺氧而成为阳极性区域。因此，在两部分区域之间的电偶电流持续作用于接触区域。由于接触区域狭窄的缝隙结构阻碍了传质过程，因此，该区域内的阳极溶解产物金属阳离子不断地聚集。为了维持电中性，更多的Cl-迁移到缝隙中，进一步的水解导致缝隙内溶液的酸化，从而形成自催化效应，破坏钝化膜的保护性，加速缝隙内部的腐蚀，腐蚀形式以点蚀为主。因此，其防护措施应以阻断缝隙口传质为主，发展强钝性的紧固件材料或开发螺纹密封包覆技术是解决该问题的有效手段。
图12
6 结论与展望
(1) 在含Cl-的海洋腐蚀环境中，碳钢和不锈钢紧固件表现出截然相反的腐蚀特征，碳钢紧固件的曝露部位腐蚀严重，缝隙内部腐蚀轻微，腐蚀形式以均匀腐蚀为主。不锈钢紧固件的曝露部位腐蚀轻微，而缝隙部位腐蚀严重，腐蚀形式以点蚀为主。
(2) 碳钢紧固件的曝露区域和接触区域表面锈层的厚度分别为60和15 μm，腐蚀产物的相组成均为β-FeOOH、α-FeOOH、γ-FeOOH和Fe3O4。不锈钢紧固件表面覆盖有纳米级的钝化膜，曝露区域的钝化膜较厚且具保护性。接触区域的钝化膜极薄，金属离子含量较低，保护性极差。
(3) 对于碳钢紧固件，由于接触区域表面覆盖的锈层产生了额外的IR降，紧固件阴极区对阳极区的极化作用被大幅度削弱。因此，表现出接触区域腐蚀较轻的腐蚀现象。对于不锈钢紧固件，阴极区域对阳极区域的电偶极化作用随着阴、阳极区域电位差的增大而不断地增强。因此，表现出接触区域点蚀更为严重的腐蚀特征。
(4) 碳钢螺栓紧固件的腐蚀防护措施应以保护曝露部位为主，例如发展高性能耐候钢螺栓。不锈钢螺栓紧固件的腐蚀防护措施应以阻断缝隙口传质为主，例如发展强钝性的紧固件材料或开发螺纹接触区密封包覆技术等。
参考文献
Axial force measurement of the bolt/nut assemblies based on the bending mode shape frequency of the protruding thread part using ultrasonic modal analysis
[J].
Stress-corrosion cracking and galvanic corrosion of internal bolts from a multistage water injection pump
[J].
Determination of bimetallic corrosion risk using an electrochemical method
[J].
Electrochemical behavior of iron within crevices in nearly neutral chloride solutions
[J].
Corrosion of metal construction structures
[J].
Laboratory and field testing of bolting systems subjected to highly corrosive environments
[J].
Numerical simulation of galvanic corrosion between carbon steel and low alloy steel in a bolted joint
[J].
Low energy SIMS characterisation of ultra thin oxides on ferrous alloys
[J].
Effect of chloride and sulfate ions on crevice corrosion behavior of low-pressure steam turbine materials
[J].
Corrosion of low carbon steel in atmospheric environments of different chloride content
[J].
Synergistic effect of 3-amino-1,2,4-triazole-5-thiol and cerium chloride on corrosion inhibition of AA2024-T3
[J].
Fastener corrosion in Arabian Gulf offshore installations
[A].
Rock bolt corrosion-an experimental study
[J].
Atmospheric corrosion behaviour and degradation of high-strength bolt in marine and industrial atmosphere environments
[J].
Ultrasonic monitoring of corroding bolted joints
[J].
Structural uses of stainless steel-buildings and civil engineering
[J].
Materials science: share corrosion data
[J].
Corrosion behaviour of carbon steel fasteners in neutral chloride solution
[J].
Synergistic effect of crevice corrosion and galvanic coupling on 304SS fasteners degradation in chloride environments
[J].
The synergistic effect of crevice corrosion and galvanic coupling on the degradation of 304 stainless steel fasteners exposed to a chloride environment was investigated using electrochemical and surface analysis techniques. Results obtained revealed that the crevice region of the fastener showed lower corrosion resistance due to decreased surface passivation induced by the crevice geometry and low oxygen conditions, while the exposed region with a higher oxygen supply exhibited improved corrosion resistance. Coupling of the exposed and crevice regions of the fastener demonstrated accelerated corrosion attributable to galvanic effect. The driving force for the galvanic effect on the fastener was linked to three (3) key factors: (I) the potential difference established between the different regions on the fastener surface due to uneven oxygen distribution, (II) the difference in the solution environment, and (III) the difference in surface properties (active-passive surface). Morphological studies showed localised corrosion occurring mainly in the crevice.
Insights into the characteristics of corrosion products formed on the contact and exposed regions of C1045 steel bolt and nut fasteners exposed to aqueous chloride environments
[J].This study investigated the characteristics of corrosion products formed on the contact and exposed regions of C1045 steel bolt and nut fasteners exposed to aqueous chloride environments. The corroded surface morphology, rust compositions, and corrosion kinetics of the bolt specimen were studied by visual observation, optical microscopy (OM), scanning electron microscopy (SEM), X-Ray diffractometry (XRD), micro-Raman, electron probe micro-analyser (EPMA), and potentiodynamic polarization techniques. Results obtained showed a variation in corrosion kinetics, morphology, and composition of the rust layer which were driven by differential aeration and concentration effects. Due to the availability of sufficient dissolved oxygen, the oxyhydroxide compound, lepidocrocite (γ-FeOOH) was detected in the outer rust layer in the exposed region, whereas the inner rust layer was composed of magnetite (Fe<sub>3</sub>O<sub>4</sub>). However, the oxygen-deficient contact surface revealed the presence of akaganeite (ß-FeOOH) and magnetite (Fe<sub>3</sub>O<sub>4</sub>) as dominant oxide phases. The most stable phase, goethite (α-FeOOH) was also detected in the rust formed in both regions, though in significantly low amounts. Furthermore, owing to variation in environmental conditions, the amount and density of the rust layer varied in the different regions. The estimated corrosion stability values for the different regions revealed that the corrosion products formed on the steel surfaces were non-protective, suggesting the need for specific surface treatment as a protective measure.
Effect of corrosive media on galvanic corrosion of complicated tri-metallic couples of 2024 Al alloy/Q235 mild steel/304 stainless steel
[J].Galvanic corrosion of tri-metallic couples is more complicated than that of bi-metallic couples. In this study, the effect of the pH of corrosive media on the galvanic corrosion of 2024 Al alloy/Q235 mild steel/304 stainless steel tri-metallic couples was investigated using potentiodynamic polarization, scanning electron microscopy, scanning vibrating electrode technique and a multi-channel galvanic corrosion meter. The results show that 2024 always acts as the only anode in 3.5 wt% NaCl at pH 5.56, 9.72 and 12.0, while both Q235 and 2024 act as anodes at pH 2.39 in the initial stage and then the role of Q235 changes at longer coupling time, which can be attributed to the effect of pH on the surface film of 2024. It is also found that the galvanic current density of a tri-metallic couple is the superposition of two bi-metallic couples when cathodic reactions are controlled by the diffusion of oxygen, otherwise it is smaller than that of the sum of two bi-metallic couples. The localized corrosion instead of uniform corrosion of anodic metal is accelerated by galvanic corrosion.
FTIR spectroscopic semi-quantification of iron phases: a new method to evaluate the protection ability index (PAI) of archaeological artefacts corrosion systems
[J].
