基礎(chǔ)知識——陰極保護(hù)與腐蝕工程
基本而重要的概念
The basic yet important concepts
以下內(nèi)容旨在總結(jié)任何陰極保護(hù)或腐蝕工程師在其職業(yè)生涯中都會用到的一些基本概念和定義。陰極保護(hù)和腐蝕建模解決方案都依賴于這些概念。
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.
氧化與陽極
Oxidation & Anode
氧化是指原子或分子失去一個或多個電子，形成帶正電的離子。這種現(xiàn)象發(fā)生在原子或分子釋放電子時。隨著電子的釋放，原子或分子的負(fù)電荷減少。發(fā)生氧化的位置（通常是電極）通常被稱為陽極。
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.
還原與陰極
Reduction & Cathode
還原是指原子或分子獲得一個或多個電子，從而形成帶負(fù)電荷的離子。這種現(xiàn)象發(fā)生在原子或分子獲得電子的任何時候。隨著獲得電子，原子或分子的負(fù)電荷增加。發(fā)生還原反應(yīng)的位置（通常是電極）通常稱為陰極。
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.
電化學(xué)電池
Electrochemical Cell
電解質(zhì)：電解質(zhì)是一種能夠?qū)щ姷碾x子化溶液。電離：除了氧化和還原反應(yīng)中可能產(chǎn)生的離子外，由于離子化分子的解離，電解質(zhì)中還可能存在離子。陽離子是帶正電的離子，陰離子是帶負(fù)電的離子。這些離子是載流電荷。因此，電離度較高的電解質(zhì)具有較高的電導(dǎo)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.
圖 1 - 電化學(xué)電池原理 - 示意圖
Figure 1 - Electrochemical Cell Principles - Schematic Overview
腐蝕電池
Corrosion Cell
與電化學(xué)電池類似，腐蝕電池由四部分組成：陽極、陰極、電解質(zhì)和金屬路徑。
腐蝕發(fā)生在腐蝕電池內(nèi)部。腐蝕是一個涉及電子和離子流動的電化學(xué)過程。腐蝕（或金屬損失）發(fā)生在陽極，而陰極則未觀察到腐蝕（因此陰極受到保護(hù)，不會發(fā)生腐蝕）。
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).
圖 2 - 腐蝕電池原理 - 示意圖
Figure 2 - Corrosion Cell Principles - Schematic Overview
極化
Polarization
在電化學(xué)領(lǐng)域，尤其是在腐蝕和/或陰極保護(hù)領(lǐng)域，（金屬的）極化是指由于電流通過而導(dǎo)致的電位偏離穩(wěn)定狀態(tài)。在分析腐蝕行為時，人們通常將開路電位稱為“自由腐蝕電位”，而極化是指電位相對于該參考電位的偏移。
在電化學(xué)電池中，極化同時發(fā)生在陽極和陰極，導(dǎo)致兩者之間的電位差降低。陽極和陰極之間的電位降低會導(dǎo)致腐蝕電流降低，從而降低腐蝕速率。
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.
陰極保護(hù)
Cathodic Protection
陰極保護(hù)旨在將陽極區(qū)域和陰極區(qū)域之間的電位差降至零。事實(shí)上，在腐蝕表面上，陽極區(qū)域和陰極區(qū)域之間存在電位差，從而驅(qū)動腐蝕電流。降低該電位差最終會降低腐蝕電流。
在陰極保護(hù)裝置中，腐蝕實(shí)際上并未被消除，而是從待保護(hù)結(jié)構(gòu)轉(zhuǎn)移到系統(tǒng)的陽極。這樣，陰極保護(hù)裝置將待保護(hù)結(jié)構(gòu)轉(zhuǎn)換為直流電路的陰極。
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.
圖 3 - 埋地管道陰極保護(hù)系統(tǒng)安裝 - 示意圖
Figure 3 - Galvanic Cathodic Protection System Installation for Buried Pipeline - Schematic View
由于涉及電流，因此必須了解陰極保護(hù)只有當(dāng)金屬暴露于電解質(zhì)（水、土壤、混凝土等）時才會發(fā)生。它在大氣中不起作用。
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.
犧牲陽極陰極保護(hù)系統(tǒng)
Sacrificial Anode Cathodic Protection System
犧牲陽極陰極保護(hù)系統(tǒng)（也稱為電流保護(hù)系統(tǒng)或 SACP）是一種基于異種金屬腐蝕的陰極保護(hù)裝置。其基本安裝方式是將陽極與其所保護(hù)的系統(tǒng)直接連接。
此類裝置的優(yōu)點(diǎn)在于無需任何外部電源，維護(hù)要求低，成本也較低。另一方面，其電流輸出較低，可能需要多個陽極來保護(hù)大型且涂層較差的結(jié)構(gòu)。犧牲陽極的安裝環(huán)境也可能成為限制因素，因?yàn)樵撓到y(tǒng)在高阻電解液中效果不佳。
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.
外加電流陰極保護(hù)系統(tǒng)
Impressed Current Cathodic Protection System
外加電流陰極保護(hù)系統(tǒng) (ICCP) 除了陽極外，還涉及外部電源。在該系統(tǒng)中，電源推動電流從陽極經(jīng)電解液流向待保護(hù)結(jié)構(gòu)。由于使用外部電源，陽極材料可以相對惰性，這與犧牲陽極不同。
使用外部電源可以靈活地滿足各種電壓和電流要求，并克服與高電阻電解液相關(guān)的挑戰(zhàn)。另一方面，應(yīng)特別注意避免過度保護(hù)，從而避免潛在的涂層損壞或氫脆。
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.
圖 4 - 埋地管道 ICCP 安裝 - 示意圖
Figure 4 - ICCP Installation for Buried Pipeline - Schematic View
涂層
Coating
在腐蝕控制領(lǐng)域，涂層通常指保護(hù)涂層，即在金屬結(jié)構(gòu)上添加一層材料，用于防止腐蝕的出現(xiàn)和/或發(fā)展。
保護(hù)涂層是腐蝕防護(hù)體系中不可或缺的支柱。使用涂層保護(hù)埋地或水下結(jié)構(gòu)可以減小陰極保護(hù)裝置的體積（從而降低成本），因?yàn)橹恍璞Ｗo(hù)裸露的金屬表面。值得一提的是，應(yīng)定期檢查涂層狀況，以確保涂層狀況良好，因?yàn)橥繉悠茡p（也稱為漏點(diǎn)）可能會加速腐蝕。
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).
電阻率
Resistivity
電阻率是指單位長度和單位截面積的導(dǎo)體的電阻。電解質(zhì)電阻率的常用測量單位是歐姆-厘米。腐蝕和陰極保護(hù)中涉及的電解質(zhì)包括土壤和液體（水）。電解質(zhì)的電阻率差異很大。有些電解質(zhì)的電阻率低至 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).
圖 5- 電阻率 - 基本公式
Figure 5- Resistivity - Basic Formulas
電導(dǎo)率
Conductivity
電導(dǎo)率是電阻率的倒數(shù)，它表征了材料支持電流流動的能力。高導(dǎo)電性材料并不一定意味著高腐蝕活性。高導(dǎo)電性材料僅反映了該材料由于高離子含量密度而具有的導(dǎo)電能力。
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.
腐蝕速率
Corrosion Rate
腐蝕速率通常以微米/年為單位，量化了陽極側(cè)排出的物質(zhì)量。腐蝕速率可根據(jù)法拉第定律計(jì)算，即金屬的重量損失與金屬的消耗速率、電流大小以及金屬暴露于電流的時間呈函數(shù)關(guān)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.
酸堿度 (pH)
Acidity and Alkalinity (pH)
電解質(zhì)（土壤或任何水性介質(zhì)）的酸堿度確實(shí)會影響腐蝕速率。因此，了解目標(biāo)體系周圍的酸度至關(guān)重要。
酸度是由氫離子 (H+) 或氫氧根離子 (OH-) 的存在決定的。當(dāng) H+ 離子過量時，介質(zhì)即為酸性介質(zhì)。酸度強(qiáng)度根據(jù) pH 值來衡量。pH 值定義為氫離子濃度 [H+] 以 10 為底的負(fù)對數(shù)。
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+]
對于大多數(shù)金屬而言，當(dāng)pH值低于約4時，腐蝕速率會加快。pH值在4到8之間時，腐蝕速率與pH值基本無關(guān)。當(dāng)pH值高于8時，環(huán)境進(jìn)入所謂的鈍化狀態(tài)，腐蝕速率會降低。
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
陰極保護(hù)裝置中常用的整流器主要有三種類型：
? 恒流：在電路中施加恒定電流，直至達(dá)到最大額定輸出電壓
? 恒壓：在電路中施加恒定電壓，直至達(dá)到最大額定輸出電流
? 恒電位：通過改變電流和電壓來施加恒定電位
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
直流干擾
DC Interference
直流干擾，也稱為雜散電流干擾，是指電流流經(jīng)不應(yīng)構(gòu)成電路一部分的結(jié)構(gòu)時產(chǎn)生的電氣干擾。
由于腐蝕速率和相關(guān)的金屬損耗與從金屬結(jié)構(gòu)流向電解液的電流量成正比，因此直流干擾在腐蝕預(yù)防和管理方面是一個重要的威脅。
任何傳導(dǎo)電流的系統(tǒng)，如果與電解液有兩處或多處接觸，都可能成為雜散電流的來源。如果金屬結(jié)構(gòu)跨越了這兩處或多處接觸點(diǎn)之間的電壓差，就會在該結(jié)構(gòu)上產(chǎn)生電流。因此，在電流離開金屬結(jié)構(gòu)的位置就會發(fā)生腐蝕。
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.
交流干擾
AC Interference
交流干擾是指在金屬結(jié)構(gòu)上產(chǎn)生的交流電流和電壓。交流感應(yīng)電流或電壓通常通過三種機(jī)制產(chǎn)生：靜電耦合、電磁感應(yīng)或電阻耦合。相關(guān)的雜散電流也會引起腐蝕，盡管金屬損耗小于等效的直流電流放電。
另一方面，交流雜散電流的幅度通常很大（在電力線故障期間可達(dá)數(shù)千安培），因此需要提出具體的建議，以減輕交流電和雷擊對金屬結(jié)構(gòu)的影響。
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.
聲明：
-文章轉(zhuǎn)載自ELSYCA，由愛澤工業(yè)翻譯，如有侵權(quán)，請聯(lián)系刪除！
-如有偏頗，歡迎指正！
滬公網(wǎng)安備31011002006738號
