Electrochemical Corrosion Basics

Although emphasis in this Chapter has been placed on irons and steels, the electrochemical corrosion basics and the forms of corrosion described are applicable to all metallic materials. For more detailed information on the corrosion resistance of various metals and their alloys, the reader should consult the selected references listed at the conclusion of this Chapter, as well as Corrosion, Vol 13, of the ASM Handbook or Corrosion Understanding the Basics, published by ASM International in 2000. 

Electrochemical corrosion in metals in a natural environment, whether atmosphere, in water, or underground, is caused by a flow of electricity from one metal to another, or from one part of a metal surface to another part of the same surface where conditions permit the flow of electricity. 

For the flow of energy to take place, either a moist conductor or an electrolyte must be present An electrolyte is an electricity-conducting solution containing ions, which are atomic particles or radicals bearing an electrical charge. Charged ions are present in solutions of acids, alkalis, and salts. The presence of an electrolyte is necessary for corrosion to occur. Water, especially salt water, is an excellent electrolyte. 

The cell shown in Fig. 1 illustrates the corrosion process in its simplest form. This cell includes the following essential components (a) a metal anode, (b) a metal cathode, (c) a metallic conductor between the anode and the cathode, and (d) an electrolyte in contact with the anode and the cathode. If the cell were constructed and allowed to function, an electrical current would flow through the metallic conductor and the electrolyte, and if the conductor were replaced by a voltmeter, a potential difference between the anode and the cathode could be measured. The anode would corrode. Chemically, this is an oxidation reaction. The formation of hydrated red iron rust by electrochemical reactions may be expressed as follows  

During metallic corrosion, the rate of oxidation equals the rate of reduction. Thus, a nondestructive chemical reaction, reduction, would proceed simultaneously at the cathode. In most cases, hydrogen gas is produced on the cathode. When the gas layer insulates the cathode from the electrolyte, current flow stops, and the cell is polarized. However, oxygen or some other depolarizing agent is usually present to react with the hydrogen, which reduces this effect and allows the cell to continue to function. 

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Electrochemical corrosion is understood to include all corrosion processes that can be influenced electrically. This is the case for all the types of corrosion described in this handbook and means that data on corrosion velocities (e.g., removal rate, penetration rate in pitting corrosion, or rate of pit formation, time to failure of stressed specimens in stress corrosion) are dependent on the potential U [5]. Potential can be altered by chemical action (influence of a redox system) or by electrical factors (electric currents), thereby reducing or enhancing the corrosion. Thus exact knowledge of the dependence of corrosion on potential is the basic hypothesis for the concept of electrochemical corrosion protection processes. 

See the NACE Papers Oliver W. Siebert, Correlation of Laboratory Electrochemical Investigations with Field Applications of Anodic Protection, Materials Performance, vol. 20, no. 2, pp. 38-43, February 1981 Anodic Protection, Materials Performance, vol. 28, no. 11, p. 28, November 1989, adapted by NACE from Corrosion Basics— An Introduction. (Houston, Tex. NACE, 1984, pp. 105-107) J. Ian Munro and Winston W. Shim, Anodic Protection— Its Operation and Appheations, vol. 41, no. 5, pp. 22-24, May 2001 and a two-part series, J. Ian Munro, Anodic Protection of White and Green Kraft Liquor Tankage, Part I, Electrochemistry of Kraft Liquors, and Part 11, Anodic Protection Design and System Operation, Materials Performance, vol. 42, no. 2, pp. 22-26, February 2002, and vol. 42, no. 3, pp. 24-28, March 2002. 

Although iron corrodes under an immense variety of conditions, there are, basically, only two mechanisms involved, namely electrochemical corrosion and (hot gas) oxidation. 

Tower, Stephen. All About Electrochemistry. Available online. URL http //www.cheml.com/acad/webtext/elchem/. Accessed May 28, 2009. Part of a virtual chemistry textbook, this excellent resource explains the basics of electrochemistry, which is important in understanding how fuel cells work. Discussions include galvanic cells and electrodes, cell potentials and thermodynamics, the Nernst equation and its applications, batteries and fuel cells, electrochemical corrosion, and electrolytic cells and electrolysis. 

Magnesium Diazide (formerly called Magne-stum Azoimide or Magnesium Trinitride, Mg(Nj)2, mw 108.37, N 77.56% wh ppt sol in w, insoi in eth or tetrahydrofuran. The prepn of Mg diazide was attempted in 1898 by dissolving the merstl in di 1 HNj, but the product decompd on evapg the soln and was not isolated (Ref 1). Turrentine (Ref 2) studied the.electrochem corrosion of Mg in Na azide soln and obtd a wh flocculent ppt, probably basic magnesium azide, Mg(OH)N3, but he did not identify the product. Browne... 

Both processes of oxidation and reduction simultaneously occur at the metal-corrosive environment interface during an electrochemical corrosion. The basic processes occurring during the electrochemical corrosion of metallic materials are ... 

This chapter is coniined to analyze the complex aqueous corrosion phenomaion using the principles of mixed-potential, which in turn is related to the mixed electrode electrochemical corrosion process. This theory has been introduced in Chapter 3 and 4 as oxidation and reduction electrochemical reactions. Basically, this Chapter is an extension of the principles of electrochemistry, in which partial reactions were introduced as half-cell reactions, and their related kinetics were related to activation and concentration polarization processes. The principles and concepts introduced in this chapter represent a unique and yet, simplified approach for understanding the electrochemical behavior of corrosion (oxidation) and reduction reactions in simple electrochemical systems. 

Reference electrodes used in electrochemical corrosion measurements should fulfil three basic requirements ... 

It has been demonstrated that BE modeling can accurately predict experimental results. BE methods also can be used to evaluate the effect of a single parameter on system performance. In this way basic understanding of electrochemical corrosion and parameter interactions can be obtained. Several parametric studies have, for example, been published on damage levels in the propeller area, seawater conductivity, and paint resistance effects, as well as on the influence of stray current source on system performance [18]. 

Definitions of the anode and the cathode are among basic definitions in electrochemical corrosion. The area of the metal surface that corrodes i.e., where the metal dissolves and goes into solution) is called the anode. The cathode is the area of the metal surface that does not dissolve. In the literature of electrochemistry, reduction and oxidation reactions are defined as when metals lose electrons i.e., oxidation) or gain electrons (reduction) ... 

We will start our microbial corrosion journey by reviewing some basic corrosion - to put it more precisely, electrochemical corrosion, in Chapter 1. In this chpter, some basic facts regarding electrochemical corrosion are reviewed to a limited extent that may be useful to understand the logic behind using methods and techniques such as cathodic protection, coating and use of inhibitors which are explained in Chapter 2 in the section Technical Mitigation of Corrosion . This much can be foimd in almost every book written on corrosion or microbial corro-... 

Nevertheless, there are many instances where electrochemical corrosion mechanisms may play a primary role in affecting the service performance of bonded joints. It should be noted that such mechanisms of attack involve both the presence of (a) anodic sites, where reaction with the metallic substrate occurs and electrons are generated, and (b) cathodic sites, where the electrons are consumed. The major reaction leads to the generation of hydroxyl ions, and the liquid present at these sites will become strongly basic and so possess a relatively high pH. Thus, typically an aqueous (electrolyte) layer needs to be present, since, without such an aqueous film, no electrical current can flow from the anodic to the cathodic sites. These aspects are illustrated, for example, by the schematic electrochemical corrosion mechanism for an organic coating on a steel substrate shown in Fig. 4, which is discussed in detail in Section 2.S.2.2. 

Chapter 1 provides an overview of the current rmderstanding of the problem of corrosion. The chapter also provides a brief introduction to nanomaterials in this context. Chapter 2 discusses corrosion basics with referetrce to nanostmctured materials. Chapter 3 addresses theoretical aspects of grain size reduction on corrosion with a model example and comparison with experimental resirlts of nanocrystalline zirconium and its alloys. Chapter 4 provides a good accoimt of the relevant electrochemical aspects of nanostructured materials. The nature of passive film and its correlation with nanocrystallization are explained. Chapter 5 gives a good description of fabrication of electrodeposited nanostructured materials. 

Cooling System Corrosion Corrosion can be defined as the destmction of a metal by chemical or electrochemical reaction with its environment. In cooling systems, corrosion causes two basic problems. The first and most obvious is the failure of equipment with the resultant cost of replacement and plant downtime. The second is decreased plant efficiency to loss of heat transfer, the result of heat exchanger fouling caused by the accumulation of corrosion products. 

Y"et, corrosion engineering and science is no longer an empirical art dissecting a large corrosion problem into its basic mechanisms allows the use of quite sophisticated electrochemical techniques to accomplish satisfactory results. On that positive side, there is real satisfaction and economic gain in designing a component that can resist punishing seiwice conditions under which other parts fail. In some cases, we cannot completely prevent corrosion, but we can try to avoid obsolescence or the component due to corrosion. 

In oilfield situations we are generally faced with corrosion attacks in aqueous environments. Basically all attacks in aqueous solutions are electrochemical in nature. This means that besides the chemical reaction there will also be a flow of electrons, resulting in a flow of current. The current flows from a higher potential to a lower one. Hence, there are two reactions taking place simultaneously in the system. One reaction occurs as the electrons are discharged from the surface, called the anode. The released electrons are consumed in the other... 

Corrosion is fundamentally a problem associated with metals. Since plastics are electrically insulating they are not subject to this type of damage. Plastics are basically non-corrosive. However, there are those that can be affected when exposed to corrosive environments. It is material deterioration or destruction of materials and properties brought about through electrochemical, chemical,... 

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