REFERENCES Kle
1 M.G. Fontana, Corrosion Engineering, Third edition, McGraw-Hill Book Company, New York, 1986 . 2 T.N. Andersen and H. Eyring, Chapter 3 Principles of Electrode Kinetics, in Physical Chemistry An Advanced Treatise, Vol. IXA Electrochemistry, Edited by Henry Eyring, Academic Press Inc., 1970 . 3 D.W. Shoesmith, Kinetics of Aqueous Corrosion, in Corrosion, ASM Handbook, Vol. 13, American Society for Metals International, Ninth edition, 1987 . 4 D.A. Jones, Principles and Prevention of...
Relevant Data
The intend in this section is to provide that reader some experimental data available in the literature. In practice, cathode materials and reference RE electrodes used in anodic protection are given in Tables 9.1 and 9.2, respectively. Table 9.1 Cathode materials for anodic protection 2 Table 9.1 Cathode materials for anodic protection 2 Sulfuric Acid, Kraft Digester Liquid Liquid Fertilizers Nitrate Solutions Figure 9.4 shows temperature dependency current density curves for 316 stainless...
Activation Polarization
In general, the activation polarization is basically an electrochemical phenomenon related to a charge-transfer mechanism, in which a particular reaction step controls the rate of electron flow from a metal surface undergoing oxidation. This is the case in which the rate of electron flow is controlled by the slowest step in the half-cell reactions 1-4 . Despite that eq. 3.22 is a generalized expression, it represents a measure of anodic polarization for corrosion studies and indicates that r 0...
Kinetics Of Passivation
Assume a defect-free single crystal and a mechanism of oxide film growth by vacancy migration. Thus, the rate of film formation for a single crystal, related to Faraday's law, can be approximated as 13 dx dt Rate of film formation cm s ip Passive current density A cm2 z Valence F 96,500 CA I to I A.s mol p Density of metal g cm3 Note that eq. 6.1 mathematically resembles eq. 3.48 , but both have different meanings. In addition, eq. 6.1 is related to several factors shown in Figure 6.12. Figure...
Polarization
This section partially treats the copious literature for supporting the mathematical models for characterizing the kinetics of charge transfer mechanism involved in an electrochemical system. Thus, electrode reactions are assumed to induce deviations from equilibrium due to the passage of an electrical current through an electrochemical cell causing a change in the working electrode WE potential. This electrochemical phenomenon is referred to as polarization. In this process, the deviation from...
Impressed Current Technique
The impressed current technique is a simple and yet, significant form of cathodic protection of underground steel pipelines as shown in Figure 8.5a. A buried pipeline is connected to the positive terminal of a rectifier power supply and the anode to the negative terminal. Both terminals must be well insulated otherwise, current leakage stray-current occurs and the structure may not be protected adequately. The significance of the cathodic protection setups shown in Figure 8.5 is that the...
b ptype oxide
Figure 10.6 Schematic diagrams for oxide semiconductors. Analysis of Figure 10.6 indicates that n-type oxides have metal-excess and oxygen-deficit conditions. Also, interstitial cations M 2 and electrons are free to move within the oxide lattice toward the oxide-gas interface, where the following reactions occur Thus, the nonstoichiometric. MxOy oxide has its own lattice defects such as vacancies. In this process, both M 2 and electrons e diffuse through the oxide lattice until the oxide is...
Electrorefining
Electrorefining ER is an electrochemical technique suitable for refining metals produced by electrowinning or smelter. The final metal product from a smelter and an electrowinning EW processes contains impurities, but further refinement can be accomplished by electrorefining cells, which deliver high purity metal M, such as copper, and a secondary recovery of precious metals may follow. A plant layout for electrorefining is shown in Figure 7.3. The impurities are more noble than the metal being...
Mixedelectrode Potential
Electrochemical corrosion systems can be characterized using the kinetic parameters previously described as Tafel slopes, exchange and limiting current densities. However, the mixed potential theory requires a mixed electrode system. This is shown in Figure 5.1 for the classical pure zinc Zn electrode immersed in hydrochloric C acid solution 1,8-9 , This type of graphical representation of electrode potential and current density is known as Evans Diagram for representing the electrode kinetics...
Mathematics Of Electrowinning
This section is mainly concerned with simple formulas based on Faraday's law of electrolysis and energy consumption for producing high purity metals from electrolytes through an aqueous electrolysis. The mathematics is simple to understand and easy to use in basic engineering calculations. However, complications may arise when diffusion and migration of solute are coupled. Diffusion itself is rather a complex subject that depends on the type of diffusion process to be analyzed. The economics of...
Designing Pressure Vessels
The objective of this section is to provide the reader with some insights of pressure vessels used nowadays in our daily life and subsequently derive expressions for predicting the current and potential developed during cathodic protection of pressure vessels using sacrificial anodes. Among many type of pressure vessel designs the domestic electric water heater schematically depicted in Figure 821 is of interest in this section. There are also gas water heaters available in the market....
Corrosioninduced Spalling
Figure 1.15 shows a spalling-induced corrosion of a steel frame, which was initially protected by an organic coating paint . Notice that spalling is a separation of the surface coating. This particular case is another atmospheric-related corrosion phenomenon. Spalling can also occur on metal oxides and refractory materials due to thermal cycling. Spalling is a unique defect that represents local disruption of the original protective coating. Figure 1.15 illustrates a severe case of spalling...
Ac Polarization
An A.C. polarization is caused by an alternating current A.C. , which in turn, affects the concentration periodically in the vicinity of the cathode electrode surface, but the charged double-layer is assumed to remain undisturbed 31 . Under these A.C. conditions, eq. 3.55 becomes Inserting this expression into eq. 4.23 gives the alternating concentration gradient According to Erdey-Gruz 31 , the periodic concentration is defined as According to Erdey-Gruz 31 , the periodic concentration is...
Limiting Current Density
Assume that the mass transfer is due to diffusion and that the diffusion molar flux Jx is the chemical rate of the mass transfer under steady-state condition. Subsequently, the diffusion process can be described by Fick's first law. Thus, eq. 4.18 becomes where Ax x - xa 6 JnDt since xQ 0 at the electrode surface. Combining eqs. 4.8 and 4.70 yields the cathodic current density for a steady-state condition AC lt 0 Recall that product is the number of coulombs required to convert 1 mole of metal...
Standard Electric Potential
In this section, a procedure to carry out measurements of the electromotive force emf is described. This emf is known as the standard potential E already introduced in Table 2.2. The standard hydrogen electrode SHE is used as the reference electrode in conducting these measurements. In general, a reference electrode one selects to measure E of a metal has to be reversible since classical thermodynamics applies to all reversible processes. The SHE cell diagram for the potential E M measurement...
Polarization Methods
The polarization resistance Rp of a metal electrolyte system and the pitting or breakdown potential Eb can be determined using at least two-electrode system. Subsequently, the rate of metal dissolution or corrosion rate is calculated using a function of the form icorr f , Rp gt ia. The methods are Linear Polarization LP as schematically shown in Figure 3.4 covers both anodic and cathodic portions of the potential E versus current density i curve for determining Rp. Tafel Extrapolation technique...
Polarization Curves
Figure 6.5 shows a stepwise potentiostatic polarization curve for a eutectoid steel in deaerated sulfuric acid solution 11 . This polarization curve is clearly divided into anodic and cathodic regions. The Tafel slopes for these regions and the corrosion current density can be determined very easily. The pertinent experimental data extracted from this figure are included as legend. Notice that the polarization curve covers data near the corrosion potential -0.51 Vsce only. It is clearly...
Electrical Double Layer
When a metal M is immersed in a suitable electrolyte, initially its atoms oxidize at a relatively high rate, and subsequently, the oxidation dissolution process gradually ceases and eventually stops due to a negative charge build-up on the metal surface. Thus, the dynamic equilibrium is attained, provided that no other complex electrochemical reactions take place. In this process, metal atoms are removed from their lattice sites to ionize as cations M I into the electrolyte, forming a...
REFERENCES Dfm
1 D.A. Jones, Principles and Prevention of Corrosion Macmillan Publishing Company, New York, 1992 442. 2 V. Ashworth, Principles of Cathodic Protection, in Corrosion Control, Third edition, edited by L.L. Shreir, R.A. Jarman, and G.T. Burstein, Butterworth-Heinemann, 1994 10 24. 3 H.H. Uhlig and R.W. Revie, Corrosion and Corrosion Control John Wiley, New York, 1985 217-223. 4 S.L. Pohlman, Stray-Current Corrosion in Corrosion Vol 13, Ninth edition, ASM Handbook,ASM International, 1987 87. 5...
PROBLEMSQUESTIONS Pya
8.1 Why cathodic protection is not generally recommended for stress corrosion problems on high-strength ferritic steels 8.2 A steel structure exposed to seawater is to be cathodically protected using the sacrificial anode technique, calculate a the number of Zn anodes N hat will be consumed in a year if the Zn anode capacity is 770 A.h Kg average value as per Table 8.2 and the current is 0.70 amps,.and b the individual weight M of anodes so that NM gt W, where W is the total theoretical weight...
Pourbaix Diagrams
Potential-pH plots are known as Pourbaix diagrams since they were first made by Pourbaix in 1938. A compilation of these diagrams is available in the Atlas of Electrochemical Equilibria in Aqueous Solutions 9 . In practice, Pourbaix diagrams are suitable for studies of corrosion, electrowinning, electroplating, hydrometallurgy, electrolysis, electrical cells, and water treatment since they are electrochemical maps indicating the domain of stability of ions, oxides and hydroxides 9 . This map...
Passive Oxide Film
oxide compound that protects the metal against further oxidation and ranges from 1 nm to 10 nm in thickness 12 , Apparently, thin films in the order of 1-2 nm in thickness are of higher quality than thick films due to lesser atomic defects. A metal that exhibits passivity is thermodynamically unstable within a potential range independent or nearly independent of current or current density. This means that the metal is unstable in the passive state since a slight disturbance may increase the...
Cyclic Polarization Curves
Essentially, it is a mixed potential phenomenon in which the anodic and cathodic polarization processes are carried out using a reverse scan rate. The scan rate is reversed at a predetermined potential, leading to a cathodic polarization in the passive region until both anodic and cathodic curves intersect. The output of this technique is schematically shown in Figure 6.11. Figure 6.11 Schematic cyclic polarization curve showing the protected This cyclic curve shows the protective potential...
Corrosion Rate
During corrosion oxidation process, both anodic and cathodic reaction rates are coupled together on the electrode surface at a specific current density known as icorr. This is an electrochemical phenomenon which dictates that both reactions must occur on different sites on the metal electrolyte interface. For a uniform process under steady state conditions, the current densities at equilibrium are related as ia ic icorr Ecorr. Assume that corrosion is uniform and there is no oxide film...