基础知识——阴极保护与腐蚀工程

基本而重要的概念

The basic yet important concepts

以下内容旨在总结任何阴极保护或腐蚀工程师在其职业生涯中都会用到的一些基本概念和定义。阴极保护和腐蚀建模解决方案都依赖于这些概念。

The below content aims at summarizing some of the essential concepts and definitions that any cathodic protection or corrosion engineer will use during his/her career. Cathodic protection and corrosion modeling solutions rely on these concepts.

氧化与阳极

Oxidation & Anode

氧化是指原子或分子失去一个或多个电子，形成带正电的离子。这种现象发生在原子或分子释放电子时。随着电子的释放，原子或分子的负电荷减少。发生氧化的位置（通常是电极）通常被称为阳极。

Oxidation refers to the loss of one or more electrons from an atom or molecule resulting in a positively charged ion. This phenomenon occurs any time electrons are released by an atom or molecule. As an electron is released, the atom or molecule decreases in negative charge. The location (commonly an electrode) where oxidation occurs is conventionally named an anode.

还原与阴极

Reduction & Cathode

还原是指原子或分子获得一个或多个电子，从而形成带负电荷的离子。这种现象发生在原子或分子获得电子的任何时候。随着获得电子，原子或分子的负电荷增加。发生还原反应的位置（通常是电极）通常称为阴极。

Reduction refers to the gain of one or more electrons from an atom or molecule resulting in a negatively charged ion. This phenomenon occurs any time electrons are gained by an atom or molecule. As an electron is gained, the atom or molecule increases in negative charge. The location (commonly an electrode) where reduction occurs is conventionally named a cathode.

电化学电池

Electrochemical Cell

电解质：电解质是一种能够导电的离子化溶液。电离：除了氧化和还原反应中可能产生的离子外，由于离子化分子的解离，电解质中还可能存在离子。阳离子是带正电的离子，阴离子是带负电的离子。这些离子是载流电荷。因此，电离度较高的电解质具有较高的电导率。

Electrolyte: the electrolyte is an ionized solution capable of conducting electricity. Ionization: in addition to ions that may be produced in oxidation and reduction reactions, ions may be present in the electrolyte due to the dissociation of ionized molecules. Cations are positively charged ions and anions are negatively charged ions). These ions are current-carrying charges. Therefore, electrolytes with higher ionization have greater conductivity.

图 1 - 电化学电池原理 - 示意图

Figure 1 - Electrochemical Cell Principles - Schematic Overview

腐蚀电池

Corrosion Cell

与电化学电池类似，腐蚀电池由四部分组成：阳极、阴极、电解质和金属路径。

腐蚀发生在腐蚀电池内部。腐蚀是一个涉及电子和离子流动的电化学过程。腐蚀（或金属损失）发生在阳极，而阴极则未观察到腐蚀（因此阴极受到保护，不会发生腐蚀）。

Similarly to an electrochemical cell, a corrosion cell is made of four parts: anode, cathode, electrolyte, and metallic path.

Within a corrosion cell, corrosion occurs. Corrosion is the electrochemical process that involves the flow of electrons and ions. The corrosion (or metal loss) occurs at the anode while no corrosion is observed at the cathode (the cathode is therefore protected from corrosion).

图 2 - 腐蚀电池原理 - 示意图

Figure 2 - Corrosion Cell Principles - Schematic Overview

极化

Polarization

在电化学领域，尤其是在腐蚀和/或阴极保护领域，（金属的）极化是指由于电流通过而导致的电位偏离稳定状态。在分析腐蚀行为时，人们通常将开路电位称为“自由腐蚀电位”，而极化是指电位相对于该参考电位的偏移。

在电化学电池中，极化同时发生在阳极和阴极，导致两者之间的电位差降低。阳极和阴极之间的电位降低会导致腐蚀电流降低，从而降低腐蚀速率。

In the context of electrochemistry and in particular corrosion and/or cathodic protection, polarization (of a metal) refers to the potential deviation from a stabilized state due to the passage of current. When analyzing corrosion behavior, one often refers to the open-circuit potential as the “free corroding potential” and polarization refers to the shift in potential from this reference.

Within an electrochemical cell, polarization occurs both at the anode and the cathode and results in lowering the potential difference between the two. A lower potential between the anode and the cathode leads to a lower corrosion current and therefore reduced corrosion rate.

阴极保护

Cathodic Protection

阴极保护旨在将阳极区域和阴极区域之间的电位差降至零。事实上，在腐蚀表面上，阳极区域和阴极区域之间存在电位差，从而驱动腐蚀电流。降低该电位差最终会降低腐蚀电流。

在阴极保护装置中，腐蚀实际上并未被消除，而是从待保护结构转移到系统的阳极。这样，阴极保护装置将待保护结构转换为直流电路的阴极。

Cathodic protection aims at reducing the potential difference to zero in-between an anodic and cathodic area. Indeed, on a corroding surface, there is a potential difference between anodic and cathodic areas that drives corrosion current. Reducing this potential difference ultimately reduces corrosion current.

In a cathodic protection installation, the corrosion is in fact not eliminated but transferred from the structure to be protected towards the anode of the system. Doing that, a cathodic protection installation transforms the structure to be protected to a cathode of a direct current circuit.

图 3 - 埋地管道阴极保护系统安装 - 示意图

Figure 3 - Galvanic Cathodic Protection System Installation for Buried Pipeline - Schematic View

由于涉及电流，因此必须了解阴极保护只有当金属暴露于电解质（水、土壤、混凝土等）时才会发生。它在大气中不起作用。

As current is involved, it is important to understand that cathodic protection can only take place when a metal is exposed to an electrolyte (water, soil, concrete, etc). It does not work within the atmosphere.

牺牲阳极阴极保护系统

Sacrificial Anode Cathodic Protection System

牺牲阳极阴极保护系统（也称为电流保护系统或 SACP）是一种基于异种金属腐蚀的阴极保护装置。其基本安装方式是将阳极与其所保护的系统直接连接。

此类装置的优点在于无需任何外部电源，维护要求低，成本也较低。另一方面，其电流输出较低，可能需要多个阳极来保护大型且涂层较差的结构。牺牲阳极的安装环境也可能成为限制因素，因为该系统在高阻电解液中效果不佳。

A sacrificial anode cathodic protection system (also known as a galvanic protection system or SACP) is a cathodic protection installation that relies on dissimilar metal corrosion. A basic installation consists in a direct connection between the anode and the system the anode is protecting.

This type of installation has the advantage of not requiring any external current source and has minimal maintenance requirements as well as reduced cost. On the other hand, the current output is low and numerous anodes might be required to protect large and poorly coated structures. The environment in which sacrificial anodes can be installed can also be a limitation as this system is poorly effective in high-resistive electrolytes.

外加电流阴极保护系统

Impressed Current Cathodic Protection System

外加电流阴极保护系统 (ICCP) 除了阳极外，还涉及外部电源。在该系统中，电源推动电流从阳极经电解液流向待保护结构。由于使用外部电源，阳极材料可以相对惰性，这与牺牲阳极不同。

使用外部电源可以灵活地满足各种电压和电流要求，并克服与高电阻电解液相关的挑战。另一方面，应特别注意避免过度保护，从而避免潜在的涂层损坏或氢脆。

An impressed current cathodic protection system (ICCP) involves an external power source in addition to an anode. With this system, the power source pushes current to flow from the anode to the structure to be protected via the electrolyte. Because an external power source is used, the material of the anode could be relatively inert, in opposition to a sacrificial anode.

The use of external sources offers the flexibility to cover a wide range of voltage and current requirements and overcome the challenges related to high-resistive electrolytes. On the other hand, specific care should be taken to avoid overprotection and therefore potential coating damage or hydrogen embrittlement.

图 4 - 埋地管道 ICCP 安装 - 示意图

Figure 4 - ICCP Installation for Buried Pipeline - Schematic View

涂层

Coating

在腐蚀控制领域，涂层通常指保护涂层，即在金属结构上添加一层材料，用于防止腐蚀的出现和/或发展。

保护涂层是腐蚀防护体系中不可或缺的支柱。使用涂层保护埋地或水下结构可以减小阴极保护装置的体积（从而降低成本），因为只需保护裸露的金属表面。值得一提的是，应定期检查涂层状况，以确保涂层状况良好，因为涂层破损（也称为漏点）可能会加速腐蚀。

In the context of corrosion control, coating typically refers to protective coating, in other words, a layer of material added on a metallic structure to prevent the apparition and/or growth of corrosion.

Protective coating is an essential pillar of the corrosion prevention arsenal. Protecting buried or submerged structures with coating reduces the size (and therefore cost) of the cathodic protection installation as only the exposed metal surface should be protected. It is worth to mention that a regular coating inspection should be done to ensure that the coating conditions are maintained as accelerated corrosion can occur where the coating breaks (also known as holidays).

电阻率

Resistivity

电阻率是指单位长度和单位截面积的导体的电阻。电解质电阻率的常用测量单位是欧姆-厘米。腐蚀和阴极保护中涉及的电解质包括土壤和液体（水）。电解质的电阻率差异很大。有些电解质的电阻率低至 30 Ω-cm（海水），而高至 500,000 Ω-cm（干沙）。

Resistivity is the resistance of a conductor of unit length and unit cross-sectional area. The common unit of resistivity measurement for an electrolyte is ohm-centimeter. Electrolytes dealt with in corrosion and cathodic protection include soils and liquids (water). Electrolyte resistivities vary greatly. Some electrolytes have resistivities as low as 30 Ω-cm (seawater) and as high as 500,000 Ω-cm (dry sand).

图 5- 电阻率 - 基本公式

Figure 5- Resistivity - Basic Formulas

电导率

Conductivity

电导率是电阻率的倒数，它表征了材料支持电流流动的能力。高导电性材料并不一定意味着高腐蚀活性。高导电性材料仅反映了该材料由于高离子含量密度而具有的导电能力。

Reciprocal of the resistivity, the conductivity of a material characterizes the ability to support current flow. Highly conductive material should not be automatically associated with high corrosion activity. A highly conductive material only reflects the ability of this material to conduct current due to a high ion content density.

腐蚀速率

Corrosion Rate

腐蚀速率通常以微米/年为单位，量化了阳极侧排出的物质量。腐蚀速率可根据法拉第定律计算，即金属的重量损失与金属的消耗速率、电流大小以及金属暴露于电流的时间呈函数关系。

Most of the time expressed in micron per year, the corrosion rate quantifies the amount of material that is discharged at the anode side. Corrosion rate can be computed based on Faraday’s law, expressing the weight loss as a function of the consumption rate of the metal, the current flow and the time this metal is exposed to the current.

酸碱度 (pH)

Acidity and Alkalinity (pH)

电解质（土壤或任何水性介质）的酸碱度确实会影响腐蚀速率。因此，了解目标体系周围的酸度至关重要。

酸度是由氢离子 (H+) 或氢氧根离子 (OH-) 的存在决定的。当 H+ 离子过量时，介质即为酸性介质。酸度强度根据 pH 值来衡量。pH 值定义为氢离子浓度 [H+] 以 10 为底的负对数。

The acidity or alkalinity of an electrolyte (soil or any aqueous medium) does impact the corrosion rate. That is the reason why an understanding of the acidity surrounding the system of interest is important.

Acidity is driven by the presence of hydrogen (H+) or hydroxyl (OH-) ions. A medium is qualified as an acid medium when there is an excess of H+ ions. Acidity strength is qualified according to the pH scale. The pH is defined as the negative logarithm to the base 10 of the hydrogen ion concentration [H+].

pH = –log [H+]

对于大多数金属而言，当pH值低于约4时，腐蚀速率会加快。pH值在4到8之间时，腐蚀速率与pH值基本无关。当pH值高于8时，环境进入所谓的钝化状态，腐蚀速率会降低。

For the majority of metals, the corrosion rate increases below a pH of about 4. In between a pH value of 4 and 8, the corrosion rate is fairly independent of the pH value. Above a pH of =8, the environment becomes what is called passive and corrosion rates are observed to be decreasing.

整流器

Rectifier

阴极保护装置中常用的整流器主要有三种类型：

› 恒流：在电路中施加恒定电流，直至达到最大额定输出电压

› 恒压：在电路中施加恒定电压，直至达到最大额定输出电流

› 恒电位：通过改变电流和电压来施加恒定电位

Commonly used in cathodic protection installations, there are three main types of rectifiers:

› Constant current: imposes a constant current over the circuit up to the maximum rated output voltage

› Constant voltage: imposes a constant voltage over the circuit up to the maximum rated output current

› Constant potential: imposes a constant potential by varying the current and voltage

直流干扰

DC Interference

直流干扰，也称为杂散电流干扰，是指电流流经不应构成电路一部分的结构时产生的电气干扰。

由于腐蚀速率和相关的金属损耗与从金属结构流向电解液的电流量成正比，因此直流干扰在腐蚀预防和管理方面是一个重要的威胁。

任何传导电流的系统，如果与电解液有两处或多处接触，都可能成为杂散电流的来源。如果金属结构跨越了这两处或多处接触点之间的电压差，就会在该结构上产生电流。因此，在电流离开金属结构的位置就会发生腐蚀。

DC Interference, also known as stray current interference, refers to an electrical disturbance issued by an electric current flowing on a structure that is not supposed to be part of the electrical circuit.

As the corrosion rate and related metal loss are proportional to the amount of current being discharged from a metal structure towards the electrolyte, DC interference is an important threat in the context of corrosion prevention and management.

Any system conducting electrical currents having two or more contact with the electrolyte could be a source of stray current. If the voltage difference in-between these two or more points is crossed by a metallic structure, a current will be created on this structure. As a consequence, corrosion will occur at the location where the current leaves the metallic structure.

交流干扰

AC Interference

交流干扰是指在金属结构上产生的交流电流和电压。交流感应电流或电压通常通过三种机制产生：静电耦合、电磁感应或电阻耦合。相关的杂散电流也会引起腐蚀，尽管金属损耗小于等效的直流电流放电。

另一方面，交流杂散电流的幅度通常很大（在电力线故障期间可达数千安培），因此需要提出具体的建议，以减轻交流电和雷击对金属结构的影响。

AC interference refers to the generation of AC current and voltages induced on metallic structures. AC-induced currents or voltages are typically generated according to three mechanisms: electrostatic coupling, electromagnetic induction or resistive coupling. The related stray currents can as well cause corrosion although the metal loss is than an equivalent amount of DC current discharge.

On the other hand, the magnitude of AC stray current is often large (up to thousands of amperes during power line fault) leading to specific recommendations to mitigate alternating current and lighting effects on metallic structures.