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exchange currents
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Book Chapter
Series: ASM Technical Books
Publisher: ASM International
Published: 01 January 2000
DOI: 10.31399/asm.tb.cub.t66910049
EISBN: 978-1-62708-250-1
... for hydrogen evolution Fig. 32 Exchange-current densities for hydrogen evolution of lead, iron, copper, and platinum Fig. 7 Simplified potential-pH diagrams for iron at 25 °C (75 °F) showing (a) areas of immunity (no corrosion), passivity, and corrosion, and (b) reaction/corrosion products...
Abstract
This chapter discusses the principles of corrosion of metals in aqueous environments. The thermodynamics of aqueous corrosion is the subject of the first half of this chapter, which addresses concepts such as corrosion reactions and free-energy change, the relationship between free energy and electrochemical potential, the effect of ionic concentration on electrode potential, and the corrosion behavior of a metal based on its potential-pH diagram. The corrosion (potential-pH) behavior of iron, gold, copper, zinc, aluminum, and titanium are described. Understanding the kinetics of corrosion and the factors that control the rates of corrosion reactions requires examination of the concepts of polarization behavior and identification of the various forms of polarization in an electrochemical cell. These concepts, addressed in the remaining of this chapter, include anodic and cathodic reactions, the mixed-potential theory, and the exchange currents.
Book Chapter
Series: ASM Technical Books
Publisher: ASM International
Published: 01 July 2000
DOI: 10.31399/asm.tb.fec.t65940087
EISBN: 978-1-62708-302-7
... reaction on iron showing the effect of pH. Curve for platinum shows influence of a metal with much higher exchange current density on the position of the hydrogen reduction curve. Source: Ref 5 Fig. 3.17 Theoretical and experimental polarization curves for reduction of oxygen (O 2 + 4H + + 4e...
Abstract
This chapter lays the groundwork for understanding electrode kinetics associated with corrosion. It presents a simple but useful theory relating kinetics to the polarization behavior of half-cell reactions. The theory is based on the observation that electrode potentials vary as a function of current density or charge transfer in a given area. The chapter explains how to measure and plot electrode potentials and currents and how to interpret the resulting polarization curves. It also discusses the effects of concentration gradients, explaining how they cause diffusion and, in some cases, produce changes in electrode potential.
Book Chapter
Series: ASM Technical Books
Publisher: ASM International
Published: 01 December 2008
DOI: 10.31399/asm.tb.ssde.t52310019
EISBN: 978-1-62708-286-0
... , a exp ( ( 1 − α ) n F η a R T ) where the j o,i terms are the exchange current densities and represent the equally large cathode and anode currents at equilibrium (zero overpotential) at the electrodes. The exchange current densities are a measure...
Abstract
Corrosion involves chemical reactions in equilibrium that that are understood through principles of thermodynamics. In practice, the rate at which corrosion reactions occur is the most important consideration. This chapter deals with corrosion kinetics, which allows engineers to to understand rates of corrosion. The discussion covers two kinetic processes, namely electrode reactions at the cathode and anode and conduction of ions in the electrolyte. The chapter also provides information on passivation and its effect on polarization diagrams.
Book Chapter
Series: ASM Technical Books
Publisher: ASM International
Published: 01 July 2000
DOI: 10.31399/asm.tb.fec.t65940183
EISBN: 978-1-62708-302-7
... = , may be determined on inert surfaces such as platinum. The extent to which these curves are applicable when the reactions occur on active metal surfaces must be questioned. The exchange current densities will almost always be lower on the active metals and will differ depending on whether the surface...
Abstract
This chapter discusses the complex polarization characteristics of active-passive metals and addresses related problems in interpreting their corrosion behavior. It begins by presenting several experimentally derived polarization curves for iron, comparing and contrasting them with the iron-water Pourbaix diagram. It then explains how anodic polarization is extremely sensitive to the environment and, as a result, a reasonably complete curve for a given metal-environment system usually can only be inferred. It goes on to describe how such curves are constructed, demonstrating the procedures for a wide range of alloys and environments. The examples also show how factors such as alloy concentration, crystal lattice orientation, temperature, and dissolved oxygen affect corrosion behavior.
Series: ASM Technical Books
Publisher: ASM International
Published: 01 January 2000
DOI: 10.31399/asm.tb.cub.9781627082501
EISBN: 978-1-62708-250-1
Book Chapter
Series: ASM Technical Books
Publisher: ASM International
Published: 01 July 2000
DOI: 10.31399/asm.tb.fec.t65940127
EISBN: 978-1-62708-302-7
..., measurement of the distribution of corrosion. The kinetics of single electrode reactions are discussed in Chapter 3 in which it is demonstrated that the kinetics are governed by the exchange current density and Tafel slope in the region of charge-transfer polarization. In addition, diffusion processes may...
Abstract
This chapter develops a corrosion model that accounts for solution potentials and the effects of coupling between cathodic and anodic reactions. It begins by examining potential differences at various points (in the solution) along a path from the anode to the cathode area. It then presents a simple model of a galvanically coupled electrode, in which the metal is represented as an array of anode and cathode reaction surfaces. The chapter goes on to develop the related theory of mixed electrodes, showing how it can be used to predict corrosion rates based on measured potentials and current densities, polarization characteristics, and physical variables such as anode-to-cathode area ratios and fluid velocity. It also discusses the effect of corrosion inhibitors, galvanic coupling, and external currents, making extensive use of polarization curves.
Book Chapter
Series: ASM Technical Books
Publisher: ASM International
Published: 01 July 2000
DOI: 10.31399/asm.tb.fec.t65940233
EISBN: 978-1-62708-302-7
... is the exchange current for the half-cell reaction. In Chapter 4 , simple corrosion processes (controlled only by charge transfer at the metal interface) are analyzed as coupled half-cell reactions between the oxidation reaction of a metal and the reduction reaction of the species causing corrosion (e.g., H...
Abstract
This chapter discusses the principles and procedures of electrochemical measurements used to investigate corrosion behaviors. It begins by presenting a diagram of a basic potentiostatic circuit, which consists of a working electrode and an auxiliary or counter electrode suspended in an electrolytic solution. It describes how corrosion potentials and current densities are measured and explains how to deal with various sources of error. It also explains how electrochemical impedance measurements are used and describes the underlying theory and procedures in some detail.
Series: ASM Technical Books
Publisher: ASM International
Published: 01 December 2015
DOI: 10.31399/asm.tb.cpi2.t55030082
EISBN: 978-1-62708-282-2
... on the surface; and J 0 is the exchange current density. For the silver-gold system, these variables take the values: Ω = 1.685 × 10 –29 m 3 , q = 1.602 × 10 –19 C, n = 1, a = 2.880 × 10 –10 m, k B = 1.381 × 10 –23 J/K, N s = 1.395 × 10 19 /m 2 for the [111] surface, D s = 2 × 10 –13 m 2 /s...
Abstract
This chapter discusses the effects of metallurgical variables on dealloying corrosion. It begins by describing the processes involved in dealloying of metal alloys in aqueous environments. This is followed by a discussion on the morphology of porous dealloyed structures below and above the critical potential. Some features experimentally observed for dealloying systems are then considered. The chapter concludes by briefly reviewing the proposed mechanisms for the formation of porous metals, namely ionization-redeposition mechanism, surface diffusion mechanism, volume diffusion mechanism, and percolation model of selective dissolution.
Series: ASM Technical Books
Publisher: ASM International
Published: 01 December 2015
DOI: 10.31399/asm.tb.cpi2.9781627082822
EISBN: 978-1-62708-282-2
Book Chapter
Series: ASM Technical Books
Publisher: ASM International
Published: 01 December 2015
DOI: 10.31399/asm.tb.cpi2.t55030180
EISBN: 978-1-62708-282-2
... vessels, but in heat exchangers, it is necessary to extend the electrode around the surface to be protected. Multiple cathodes can be used in parallel to distribute the current and decrease circuit resistance. Reference Electrodes Reference electrodes must be used in anodic protection systems...
Abstract
Anodic protection is used on a smaller scale than other corrosion control techniques due to the fundamental electrochemistry involved. This chapter provides a brief history of the technique, discusses anodic protection use, and compares anodic and cathodic protection. The background and theory of anodic protection are summarized. In addition to briefly describing the various items used for each component of an anodic protection system, the chapter presents design concerns as well as applications of the system. Factors concerning the economic justification of anodic protection are also described.
Book Chapter
Series: ASM Technical Books
Publisher: ASM International
Published: 01 June 1988
DOI: 10.31399/asm.tb.eihdca.t65220077
EISBN: 978-1-62708-341-6
...-handling concepts are described in Chapter 10 . Equipment Cooling Systems All induction heating systems require coolants (primarily water), because current-carrying components dissipate waste heat through I 2 R losses. Obviously, the greater the resistance of the current path and/or the current...
Abstract
This chapter describes two types of auxiliary equipment required in most induction heating installations: cooling systems and device timers. Water- and vapor-based systems used for cooling the power supply and the induction coil are described. The chapter concludes with a brief discussion of timers, with emphasis on open-loop timing systems.
Book Chapter
Series: ASM Technical Books
Publisher: ASM International
Published: 01 August 2015
DOI: 10.31399/asm.tb.piht2.t55050293
EISBN: 978-1-62708-311-9
... Line water to heat exchanger is too hot Monitor line water temperature. Heat exchanger is fouled Clean heat exchanger on PM program Temperature controls not working Repair. Have emergency bypass available Temperature control valve not working Repair. Have emergency bypass available...
Book Chapter
Series: ASM Technical Books
Publisher: ASM International
Published: 01 December 2015
DOI: 10.31399/asm.tb.cpi2.t55030019
EISBN: 978-1-62708-282-2
... corrosion resistant metal decreases, and the surface becomes cathodic. The driving force for corrosion or galvanic current flow is the potential developed between the dissimilar metals. The extent of corrosion resulting from galvanic coupling is affected by the following factors: The potential...
Abstract
This chapter provides a brief account of galvanic corrosion, which occurs when a metal or alloy is electrically coupled to another metal or conducting nonmetal in the same electrolyte. It begins by describing the galvanic series of metals and alloys useful for predicting galvanic relationships, followed by a brief section on polarization of metals or alloys. The effects of area, distance, and geometric shapes on galvanic-corrosion behavior are then discussed. Various alloys susceptible to galvanic corrosion are briefly reviewed. The chapter also discusses various modes of attack that lead to galvanic corrosion, along with methods for predicting and controlling galvanic corrosion.
Book Chapter
Series: ASM Technical Books
Publisher: ASM International
Published: 01 December 2003
DOI: 10.31399/asm.tb.pnfn.t65900089
EISBN: 978-1-62708-350-8
...Abstract Abstract Ion nitriding equipment can be categorized into two groups: cold-wall continuous direct current (dc) equipment and hot-wall pulsed dc equipment. This chapter focuses on these two categories along with other important considerations for ion (plasma) nitriding equipment...
Abstract
Ion nitriding equipment can be categorized into two groups: cold-wall continuous direct current (dc) equipment and hot-wall pulsed dc equipment. This chapter focuses on these two categories along with other important considerations for ion (plasma) nitriding equipment and processing. Other important considerations discussed include the hollow cathode effect, sputter cleaning, furnace loading, pressure/voltage relationships, workpiece masking, and furnace configuration options. The chapter describes five methods of cooling parts from the process temperature to an acceptable exposure temperature after plasma nitriding. The chapter also presents some of the advantages of the pulsed plasma process.
Book Chapter
Series: ASM Technical Books
Publisher: ASM International
Published: 01 August 2013
DOI: 10.31399/asm.tb.ems.t53730051
EISBN: 978-1-62708-283-9
.... This causes small regions, called domains, in which all of the spins are aligned. This lowering of energy by the alignment of spins is called the exchange energy. There is a tendency of a material to form complete magnetic circuits, lowering the magnetostatic energy. The magnetostatic energy is less...
Abstract
This chapter is a review of magnetic materials and how they behave. It begins by discussing the significance of ferromagnetism and comparing the Curie temperature of several ferromagnetic elements. It then discusses the concept of magnetic domains and illustrates how flux paths, and magnetostatic energy, vary based on the size of the domain. It also discusses the process of magnetization and compares and contrasts hard and soft magnetic materials.
Book Chapter
Series: ASM Technical Books
Publisher: ASM International
Published: 01 August 2015
DOI: 10.31399/asm.tb.piht2.t55050025
EISBN: 978-1-62708-311-9
... physical size and improve efficiency. There are two types of direct current power supplies: rectification (ac/dc) and conversion of direct current (dc/dc). Internal circuits convert the dc current to high-frequency outputs. The two main types of circuits in the high-frequency output are parallel...
Abstract
This chapter discusses the basic components in an induction heat treating system. It describes the design and operating characteristics of power supplies, load-matching transformers, tuning capacitors, power regulators, controllers, process monitors, and diagnostic systems. It also provides information on fixtures and work-handling devices, quench systems, and load matching and tuning procedures.
Book Chapter
Series: ASM Technical Books
Publisher: ASM International
Published: 01 January 2000
DOI: 10.31399/asm.tb.cub.t66910407
EISBN: 978-1-62708-250-1
.... The location of the cathode is not a critical factor in simple geometries, such as storage vessels, but in heat exchangers, it is necessary to extend the electrode around the surface to be protected. Multiple cathodes can be used in parallel to distribute the current and to decrease the circuit resistance...
Abstract
This article describes in detail the process of corrosion control by cathodic and anodic protection. The discussion covers the basic concept of cathodic and anodic protection systems, their types and equipment used, and the advantages, limitations, and applications of these protection systems. The types of cathodic protection systems include sacrificial cathodic protection and impressed-current cathodic protection systems. Some of the technical problems associated with cathodic protection include the effects of stray currents on the corrosion of adjacent metal structures, the effects of the chemical reactions occurring at the surface of the protected structure, and the effects of cathodic protection on coatings.
Book Chapter
Series: ASM Technical Books
Publisher: ASM International
Published: 01 August 2015
DOI: 10.31399/asm.tb.piht2.t55050335
EISBN: 978-1-62708-311-9
... there is considerable vaporization and where the smell of the quenchant becomes obnoxious. The quench cooling system must be sized correctly. A source of line water of the required flow and maximum inlet temperature is needed for cooling in the quench system heat exchanger. The quenchant must have the capacity...
Series: ASM Technical Books
Publisher: ASM International
Published: 01 July 2000
DOI: 10.31399/asm.tb.fec.9781627083027
EISBN: 978-1-62708-302-7
Series: ASM Technical Books
Publisher: ASM International
Published: 01 July 2009
DOI: 10.31399/asm.tb.bcp.t52230107
EISBN: 978-1-62708-298-3
.... Diamond R.M. , 1955 . Factors Influencing Ion-Exchange Equilibria in Concentrated Solutions: Behavior of Alkaline Earth and Alkali Ions , J. Am. Chem. Soc. , Vol 77 , p 2978 10.1021/ja01616a013 Everest D.A. , 1964 . The Chemistry of Beryllium , Elsevier , New York, NY , p 38...