Corrosion Tutorial

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CORROSION TUTORIAL

Dated: June 14, 2004 Hota GangaRao Eung H Cho Sucharitha Bachanna Srinivas Aluri Robert Creese

Constructed Facilities Center

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FOREWORD

Corrosion is the result of chemical reaction between a metal and its environment. It is the tendency of the refined metal to return to its mineral state. Corrosion engineers study the corrosion mechanisms to determine the causes of corrosion and the methods to minimize the resulting damage. Also, corrosion engineers design and apply various methods to prevent corrosion by practical and economical means. The importance of corrosion is not only related with economy, but also with safety issues. The loss of material due to corrosion results in the failure of the machines, structures, bridges, etc. resulting in damages worth billions of dollars. The annual direct cost of corrosion and of protection against corrosion in the United States for Department of Transportation alone is estimated to be around 276 billion in the year 2001. Direct costs mean the costs of replacing corroded structures and machinery. Indirect costs resulting from actual or possible corrosion are more difficult to evaluate and maybe more than $276 billion. Some of the major industries affected by corrosion are 1) Defense, 2) Nuclear power plants, 3) Aircraft, 4) Pipeline, 5) Storage Tanks, 6) Highways and bridges, 7) Water systems, 8) Gas distribution, 9) Transportation, 10) Petroleum, 11) Oil and natural gas, 12) Chemical plants, etc.

In this tutorial, some of the basics issues dealing with corrosion are explained. The essential elements of electrochemistry that are needed to understand the basics of corrosion reactions are presented in Chapters 1 and 2. Corrosion has been classified in many different ways. One way is to classify by the forms in which corrosion occurs. The forms of corrosion are discussed in Chapter 3. Corrosion of composites is typically called Aging. Composites have different mechanical properties compared to most metals. Hence aging of composites is discussed in Chapter 4. Use of composites is rapidly becoming prevalent in many applications. Some of the important applications are discussed in Chapter 5. Although corrosion cannot be prevented, its rate can be retarded using many different methods. Some of the important methods are discussed in Chapter 6. Chapter 7 consists of brief information on typical metals and composites that are used extensively in the industry. Chapter 8 deals with advanced electrochemical kinetics of the corrosion reactions.

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7.6 COMPOSITES TERMINOLOGY 204 7.7 REAGENTS 212 7.7.1 SULFURIC ACID 212 7.7.2 HYDROCHLORIC ACID 212 7.7.3 NITRIC ACID 213 7.7.4 HYDROFLUORIC ACID 213 7.7.5 PHOSPHORIC ACID 214 8 CORROSION KINETICS 215 8.1 POLARIZATION 215 8.1.1 ACTIVATION POLARIZATION 215 8.1.2 CONCENTRATION POLARIZATION 220

8.2 MIXED POTENTIAL THEORY 222

8.3 EXPERIMENTAL POLARIZATION CURVES 224

LIST OF FIGURES

Figure 1-1: Electrochemical Cell _________________________________________________ 2 Figure 2-1: Schematic Active-Passive Behavior of the Anodic Polarization of a Metal _______ 9 Figure 2-2: Schematic Representation of Corrosion of Stainless Steel with Two Oxidation

Reagents ___________________________________________________________________ 10 Figure 2-3: Comparison of Galvanostatic and Potentiostatic Anodic Polarization Curves_ __ 11 Figure 2-4: Cathodic and Anodic Polarization Curves________________________________ 12

Figure 3-1: Uniform Corrosion 17

Figure 3-2: Example of uniform corrosion damage on a rocket assisted artillery projectile 18

Figure 3-3: Atmospheric corrosion of galvanized anti crash railing due to marine aerosol condensation on wooden post 19

Figure 3-4: Galvanic Corrosion of Brass Coupled With Black Iron 20

Figure 3-5: Mechanism of Galvanic Corrosion of a Two Metal Couple 21

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Figure 3-7: Corrosion rate determination for a two electrode process system 23

Figure 3-8: Corrosion rate determination for a three electrode system 24

Figure 3-9. The introduction of a less noble metal will decrease the corrosion rate of the more noble metal. 25

Figure 3-10: Effect of cathode to anode ratio in galvanic corrosion 28

Figure 3-11: Occurrence of Stray current corrosion in pipelines. 33

Figure 3-12: Ionic current flow onto the pipeline 34

Figure 3-13: Current flow onto pipeline at coating discontinuities 35

Figure 3-14: External stray current sources. 36

Figure 3-15: Corroded surface of carbon steel in its natural condition 37

Figure 3-16: Pitting in Aluminum 39

Figure 3-17: Propagation of Pitting Corrosion 41

Figure 3-18: Crevice Corrosion 43

Figure 3-19: Mechanism of Crevice Corrosion 44

Figure 3-20: crevice corrosion in rivets 45

Figure 3-21: A crevice formed into an open atmosphere 46

Figure 3-22: Example of Pack Rust 46

Figure 3-23: Mechanism of Filiform Corrosion 47

Figure 3-24: “worm like” filiform corrosion tunnels. 48

Figure 3-25: Filiform Corrosion Causing Bleed Through a Welded Tank 49

Figure 3-26: Stress Corrosion Cracking Showing Branched Cracks in Aluminum Plates 50

Figure 3-27: Schematic of Active-Passive Behavior of the Anodic Polarization of a Metal 51

Figure3-28: Liquid Metal Embrittlement 53

Figure 3-29: Solid Metal Induced Embrittlement of a cadmium plated B7 bolt 54

Figure 3-30: Brittle crack in a cadmium plated B7 bolt from solid metal induced embrittlement ____________________________________________________________________________55 Figure 3-31 Impingement corrosion in a bent tube 59

Figure 3-32: Exfoliation of Aluminium 62

Figure 3-33:Exfoliation of aircraft component 63

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Figure 4-1: Fibers as reinforcement a) Random Fibers________________________________70 Figure 4-1: Fibers as reinforcement b) Continuous Fibers (Long)______________________ _70 Figure 4-2: The combined effect on modulus of the addition of fibers to the resin matrix_____ 73

Figure 4-3: Typical Sorption Curve (Vijay et al., 2001)______________________________ 83 Figure 4-4: The Sorption Curves for Epoxy, Vinyl ester, and Isopolyester Resin When Exposed to

the 3 Different Solutions (Chin et al., 1999)_________________________________________84 Figure 4-5: Fickian Diffusion Curves for Epoxy in (a) Water, (b) Salt Solution, and (c) Concrete Pore Solution at 22 °C (Chin et al., 1999)__________________________________________ 86 Figure 4-6: Thermal Expansion Measured by Stokes (Stokes, 1990)_____________________ 88 Figure 4-7: Moisture-Induced Thermal Expansion vs. Temperature (Stokes, 1990) _________ 88 Figure 4-8: (A) Moisture Absorption/Swelling Response of Carbon Phenolic Specimen as a function on the humidity of conditioning environment. (Stokes, 1990) ___________________ 90 Figure 4-8: (B) Moisture Absorption/Swelling Response of Carbon Phenolic Specimen in Across Ply Direction (Stokes, 1990)_____________________________________________________91 Figure 4-8: (C) Moisture Absorption/Swelling Response of Carbon Phenolic Specimen in Ply Direction (Stokes, 1990)________________________________________________________92 Figure 4-8: (D) Moisture Absorption/Swelling Response of Carbon Phenolic Specimen in Wrap Direction (Stokes, 1990)________________________________________________________93 Figure 4-9: Difference Between (a) Diamine and (b) Aniline Hardener __________________ 99 Figure 4-10: Bonds Between Glass Fiber and Coupling Agent. _______________________ 102 Figure 4-11: Typical Creep Behavior of Plastics (GangaRao, 2001) ___________________ 105 Figure 4-12: Maxwell's Model__________________________________________________ 108 Figure 4-13: Kelvin Model_____________________________________________________110 Figure 4-14: Four Element Model _______________________________________________111 Figure 4-15: Behavior of creep and stress relaxation in four element model ______________112 Figure 4-16: Behavior of creep when subjected to a series of stresses ___________________113 Figure 4-17: Schematic Representation of the Effects of Time, Temperature, and Moisture on Creep Compliance (Liao,1998) ________________________________________________ 116 Figure 4-18: Moisture Absorption Behavior_______________________________________ 118 Figure 4-19: Effect of Physical Aging on Creep_____________________________________120

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Figure 4-20: Fatigue Damage Mechanism in Unidirectional Composites Under Loading Parallel to Fibers: (a) Fiber Breakage, Interfacial Debonding; (b) Matrix Cracking; (c) Interfacial Shear Failure [Talreja, 1987]________________________________________________________125 Figure 4-21: Two-Stage Strength Degradation Model for Fatigue Reliability of Composites [Talreja, 1987] _____________________________________________________________ 127 Figure 4-22: Comparison of S-N Curve for Three Different Unidirectional Composite Materials [Curtis and Dorey, 1986] _____________________________________________________ 128 Figure 4-23: Comparison of S-N curve for Four Different Materials with Different Carbon Fibers in Same Epoxy Resin [Curtis and Dorey, 1986] ______________________________ 129 Figure 4-24: Fatigue Life Diagram of Unidirectional Composites Under (a) Loading Parallel to Fibers, (b) Off-Axis Loading (Dotted line correspond to on-axis loading) [ Talreja, 1987] __ 130 Figure 4-25: Normalized S-N Curves for (0/±45) CFRP Laminates with Varying Percentage of 0o Fibers [Curtis and Dorey, 1986] _____________________________________________ 131 Figure 5-1: F-22 Raptor Aircraft -Tactical Fighter Aircraft __________________________ 157 Figure 5-2: RAH-66 Comanche ________________________________________________ 157 Figure 5-3: a.GAU –19A b. F18C/D ____________________________________________ 158 Figure 5-4: Reactive Armor and XM-301 Gun ____________________________________ 158 Figure 5-5: Objective Crew Served Weapon ______________________________________ 159 Figure 5-6: Delta II _________________________________________________________ 160 Figure 5-7: Missiles from the Hydra 70 Family____________________________________ 160 Figure 5-8: Destroyers _______________________________________________________ 161 Figure 5-9 Goalkeeper: In-Ship Defense System ___________________________________ 161 Figure 5-10: Composite Fire Truck Panels _______________________________________ 162 Figure 5-11: All Composite Bridge, Laurel Lick, CFC-WVU _________________________ 162 Figure 5-12: Energy Plant Towers ______________________________________________ 163 Figure 5-13: Third Rail Protection in Monorail System _____________________________ 163 Figure 5-14: MRI Units ______________________________________________________ 164 Figure 5-15: Composite Computer Chip _________________________________________ 164 Figure 5-16: Waste Water Plant________________________________________________ 165 Figure 5-17: Telecommunication Towers_________________________________________ 166

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Figure 5-19: Sheet Piling and Fender Applications_________________________________ 167 Figure 6-1: Steel Tank Protected by Sacrificial Anode System ________________________ 169 Figure 6-2: Mechanism of Anodic Protection System _______________________________ 170 Figure 6-3: Steel Tank Protected by Impressed Current System _______________________ 171 Figure 6-4: Mechanism of Impressed Current Systems Explained Using Anodic Polarization Curves ____________________________________________________________________ 171 Figure 6-5: Mechanism of Electroplating ________________________________________ 174 Figure 6-6: Effects of Applied Anodic Current on the Behavior of Active-Passive Alloy ____ 181 Figure 7-1: Particulate Reinforcement___________________________________________ 193 Figure 7-2: Whisker Reinforcement _____________________________________________ 194 Figure 7-3: Continuous Fiber Reinforcement _____________________________________ 194 Figure 7-4: Braided Fabrics___________________________________________________ 195 Figure 7-5: Hybrid Fabrics ___________________________________________________ 195 Figure 7-6: Knitted or Stitched Fabrics __________________________________________ 196 Figure 7-7: Woven Fabric ____________________________________________________ 196 Figure 7-8: Hand Lay Up _____________________________________________________ 198 Figure 7-9: Tube Rolling _____________________________________________________ 199 Figure 7-10: Resin Transfer Molding Machine (CFC-WVU) _________________________ 200 Figure 7-11: VARTM –Tabletop Model of VARTM and Schematic Process of Manufacture _ 200 Figure 7-12: Injection Molding Machine (CFC-WVU) ______________________________ 201 Figure 7-13: Compression Molding Machine (CFC-WVU)___________________________ 202 Figure 7-14: Schematic Representation of Pultrusion Process (Bedford Plastics)____________ 202

Figure 7-15: Basic Extruder___________________________________________________ 203 Figure 7-16 a: Winding Machine Showing Carriage and Mandrel b: Filament Winding ____ 204 Figure 8-1: Activation Polarization _____________________________________________ 216 Figure 8-2: Butler-Volmer Equation and Tafel Equation ____________________________ 220 Figure 8-3: Combined Polarization _____________________________________________ 222 Figure 8-4: Behavior of Metal M in Acid Solution__________________________________ 223 Figure 8-5: Behavior of Coupled Metals in Acid Solutions ___________________________ 224 Figure 8-6: Showing Cathodic and Anodic Polarization Curves_______________________ 226

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LIST OF TABLES

Table 1-1: E0 Values for Metals __________________________________________________ 4 Table 1-2: E0 Values for Common Oxidation Reagents ________________________________ 5 Table 3-1: Galvanic Series of Metals/Alloys in Seawater _____________________________ 32 Table 4-1: Diffusion Coefficients of Epoxy, Vinyl Ester, and Isopolyester Resins___________ 85 Table 4-2: Variation of Equilibrium Moisture Content with Degree of Cure ______________ 98 Table 4-3: Effect of Hardener on Equilibrium Moisture Content ______________________ 100 Table 4-4:Calculated Values of Gth _____________________________________________ 103 Table 4-5: Knockdown Factors (Vijay,1998) ______________________________________ 146

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1 FUNDAMENTALS OF CORROSION

In the design and fabrication of machines and structures, the choice of the material depends on many factors, including its corrosion behavior. Although corrosion resistance is an important factor, the final choice frequently depends on many other factors like economics, availability, etc. In the case of applications that require aesthetic appeal, appearance is the most important consideration.

Corrosion resistance or chemical resistance of a material depends primarily on thermodynamics, electro-chemistry, and metallurgy among others. Thermodynamics can determine whether or not the material is prone to corrosion. Electro-chemistry along with environmental factors, design, etc., can predict the approximate rate of corrosion. Metallurgical factors influence the corrosion resistance of the material. Physical chemistry and its various disciplines are very useful for studying the mechanisms of corrosion reactions, the surface conditions of metals, and other basic properties. This chapter introduces the fundamentals of the electro-chemistry needed to understand the basic corrosion mechanism. The electrochemical cell, standard electrochemical potential, Nernst equation, and free energy and electrode potential are briefly discussed.

1.1 Electrochemical Cell

Corrosion is an electrochemical reaction between the metal and its environment. In order to determine how corrosion occurs, we must understand the formation of a corrosion cell. The corrosion cell consists of an anode, a cathode, an electrolyte, and an anionic membrane.

Figure 1.1 shows an electrochemical cell, which consists of a steel anode, a platinum cathode, sulfuric acid catholyte and an anionic membrane (allows only negatively charged ions to pass through the membrane).

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Figure 1-1: Electrochemical Cell

At the anode, iron is oxidized and dissolved into the electrolyte.

Fe = Fe2+ + 2e- …(1.1) Equation (1.1) is an anodic reaction and it is also called half-cell reaction. This reaction cannot occur alone; it needs a partner cathodic reaction. The cathodic reaction takes place on the surface of the platinum electrode according to:

2H+ + 2e- = H2 (g = ‘gas’) …(1.2)

This cathodic reaction is also called half-cell reaction. We can see that cathodic reaction is a reduction of H+ to H2 (g).

The anionic membrane allows only the anions (negatively charged ions) of the sulfate ions to pass through. This transfer of anions is needed to maintain the electrical neutrality of the solutions at both anodic and cathodic compartment. For example, the dissolution of Fe introduces Fe++ ions into the anodic compartment. So, one mole of Fe++ _{introduced needs one mole of SO}

42- transferred from the cathodic compartment to

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1.2 Standard Electrochemical Potential

Standard electrochemical potential is defined as the potential under standard conditions, i.e., 25 °C and 1 atmosphere pressure and when the reactants of the reaction have unit activity.

The list of E0_{values for various metals is provided in Table 1.1. Table 1.2 shows}

the E0_{values for common oxidation reagents. The E}0_{values for both tables are given}

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Table 1-1: E0 Values for Metals

Reaction Standard Potential E0 (volts vs. SHE) Au3+ + 3e- = Au +1.498 Noble Ag+ + e- = Ag +0.799 Hg22+ + 2e- = 2Hg +0.799 Cu2+_{+ 2e}-_{= Cu} _+0.342 Pb2+ + 2e- = Pb -0.126 Sn2+ + 2e- = Sn -0.138 Ni2+ + 2e- = Ni -0.250 Co2+ + 2e- = Co -0.277 Cd2+ + 2e- = Cd -0.403 Fe2+ + 2e- = Fe -0.447 Cr3+_{+ 3e}-_{= Cr} _-0.744 Zn2+ + 2e- = Zn -0.762 Al3+ + 3e- = Al -1.662 Mg2+ + 2e- = Mg -2.372 Na+ + e- =Na -2.71 K+ + e- = K -2.931 Active

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Table 1-2: E0 Values for Common Oxidation Reagents

Reactions Standard Potential E0_{(volts vs. SHE)}

Cl2 + 2e- = 2Cl- +1.358 O2 + 4H+ + 4e- = 2H2O (pH 0) +1.229 NO3- +4H+ + 3e- = NO + 2H2O +0.957 O2 + 2H20 + 4e- = 4OH- +0.82 Fe3+ + 3e- = Fe +0.771 O2 + 2H2O + 4e- = 4OH- (pH 14) +0.401 Sn4+_{+ 2e}-_{= Sn}2+_+0.15 2H+_{+ 2e}-_{= H} 2 0.000 2H2O + 2e- = H2 + 2OH- (pH 7) -0.413 2H2O + 2e- = H2 + 2OH- (pH 14) -0.828

The E0 values can be calculated from ΔG0 (GFE = Gibbs Free Energy) value of the reaction. For example, the reverse reaction of reaction (1.1) is:

Fe2+ + 2e- = Fe …(1.3) The GFE change for reaction (1.3) is 20.30 kcal/mole. But,

ΔG0 = - nFE0 …(1.4) where F is the Faraday constant and is 23.06 kcal/equivalent-volt and n is the number of electrons or equivalents/mole. Then, the E0 of reaction (1.3) will be -0.44 eV.

1.3 Nernst Equation

Nernst equation is used to calculate E values from the E0values. The E values are under the conditions that deviate from the standard condition as defined in section 1.2. The Nernst equation can be derived from:

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ΔG=ΔG0_{+RTln(K) …(1.5)}

where R is the gas constant (=1.987cal/mole-K), T is the absolute temperature, K is the equilibrium constant, and F is the Faraday constant (=23060 cal/equivalent-volt). Substitution of equation (1.4) and the similar term for ∆G into Equation (1.5) yields:

E=E0–(RT/nF)*ln(K) …(1.6) Substitution of R, F, and T (=2980K) into Equation (1.6) and converting ln (log to the base e) to log (log to the base 10) yields:

) log( 23060 * 303 . 2 * 298 * 987 . 1 0 _K n E E = − ) log( 059 . 0 0 _K n E E = − …(1.7) Equation (1.7) is called the Nernst Equation.

1.4 Free Energy and Electrode Potential

The overall electrochemical reaction is a combination reaction of cathodic and anodic reactions. If the number of electrons is not identical in both half-cell reactions, mathematical arrangement should be made to equate the number of electrons, so that the overall electrochemical reaction results in no electrons.

Since potential is an intensive (having same potential value for every subdivision of a system) property, the potentials of the half-cell reactions cannot be added in order to determine the free energy of the overall reaction. The free energy of the overall reaction can be determined as follows: First E value of the half-cell reactions should be calculated using the Nernst equation. Then, these values should be converted to free energy, ΔG. In this procedure, n for each half-cell reaction should be determined to give no electrons in the overall reaction. Then the free energies of the half-cell reaction can be added to obtain the free energy of the overall reaction.

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For example, the overall reaction for Figure 1.1 can be written as: Fe + 2H+_{= Fe}++_{+ H}

2 …(1.8)

The cathodic reaction is Equation (1.2) and the anodic reaction is Equation (1.1). To calculate the free energy of reaction (1.8) when [Fe++] = 10-2 molar, H+ = 10-1 molar and P_H₂ = 1 atmosphere

The E value for cathodic reaction is:

2 2 0.059 0 log 0.059 2 [ ] H P E H+ = − = − …(1.9)

Then the ΔG = -2F(-0.059) = 2.72 kcal/mole. Similarly the E value for reaction (1.3) is:

0.059 1 0.44 log 0.499 2 [ ] E Fe++ = − − = − …(1.10)

This E value is the potential for the cathodic reaction. Since we need the potential of the anodic reaction, its potential should be 0.499 eV. Then the free energy is:

ΔG = -2F(0.499) = -23.01 kcal/mole.

Now the free energy of the overall reaction (1.8) is the sum of those for the half-cell reactions, which is -20.29 kcal.

The sign of this free energy indicates that the overall reaction is spontaneous. If ΔG is negative, the reaction is spontaneous because the energy level at the final state is lower than that of initial state. Also the reverse is true. When ΔG is zero, the reaction is at equilibrium.

1.5 Potential Measurement of Half-Cell Reaction

The reference electrode is needed to measure the potential of the half-cell reaction. Since the potential of reference electrode is known and we measure the

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potential difference between the half-cell reaction and the reference electrode, we can measure the potential of the half-cell reaction. The measurement should be conducted in a circuit with a high impedance value. Otherwise, a significant current flows through the circuit and then the potential changes to give a wrong value of the half-cell reaction.

There are several types of reference electrodes. The standard electrode is prepared with platinum wire immersed in a chamber, which contains 1 molar H+ ion, and pure hydrogen gas is bubbled through the solution. The potential of this reference electrode is given as:

2H+ + 2e- = H2 E0 = 0 eV …(1.11)

Thus, the standard hydrogen electrode is not practical because it involves flammable hydrogen gas. Thus, other reference electrodes are used. One of them is calomel electrode and it has the following half-cell reaction:

Hg2Cl2 + 2e- = 2Hg + 2Cl- E0 = 0.241 eV …(1.12)

The potential at the saturation of chloride ion is 0.241 eV.

The next reference electrode is copper-copper sulfate electrode and it has the following half-cell reaction:

CuSO4 + 2e- = Cu + SO4- E0 = 0.318 eV …(1.13)

The potential at the saturation of copper ion is 0.318 eV.

The next reference electrode is silver-silver chloride electrode and it has the following half-cell reaction:

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2 PASSIVITY AND ELECTROCHEMICAL CORROSION

MEASUREMENTS

2.1 Passivity

When an oxidizer such as ferric ion is added to a solution in which a metal (e.g., iron, nickel, chromium, titanium) is immersed, a typical polarization curve as shown in Figure 2.1 is produced.

Figure 2-1: Schematic Active-Passive Behavior of the Anodic Polarization of a Metal

In the active zone, the curve follows Tafel behavior. However, when the voltage increases (by adding more oxidizer), the current starts decreasing until it reaches ipass

(passive current). This passive current is retained until the potential reaches Etp active Log i Epp (passivation potential) Etp transpassive ipass (passive current) passive Ecorr (corrosion potential) E Oxygen evolution icc (critical current)

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(transpassive potential). Above Etp, the current starts increasing again like in the active

zone.

The passive zone has the lowest current due to the formation of metal hydroxide/oxide film. This film is usually impermeable for the corrosion reagents such as oxygen and water thus reducing the metal corrosion drastically. At Etp the film

becomes unstable making the film break down, and the current starts increasing above its potential.

The passivity is one of the important aspects in corrosion. Passive films are intentionally produced to control the corrosion rates. For example, in anodic protection, applying a potential to form a passive film protects acid storage tanks. Titanium alloys are frequently used in an aggressive environment to induce passive film. Since titanium is an active metal, it induces passive film easily. Stainless steels are generally immune to atmospheric corrosion where oxygen is involved but they are not immune to acid environment. The mechanism is illustrated in Figure 2.2. It can be seen from Figure 2.2 that the higher potentials with oxygen can induce passivity while the lower potential with acid induces higher corrosion rate.

Log i E

2H2O+O2+4e

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There are two methods to generate anodic polarization curves. Figure 2.3 compares both methods. The dotted line is produced by galvanostat where potential is measured at each increment of current. Thus, the passivity cannot be revealed by this method. However, since the potentiostat measures current at each increment of potential, the passive zone will be revealed. This potentiostat should be used to measure the passivity.

Figure 2-3: Comparison of Galvanostatic and Potentiostatic Anodic Polarization Curves

The methods available for measurement of corrosion by electrochemical polarization are as follows:

1. Tafel extrapolation 2. Polarization resistance

Polarization measurements are found to be useful in engineering and research applications due to its inherent advantages such as accuracy, speed, continuous monitoring and non-destructive measurements.

Log i

E Galvanostat

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2.2 Tafel Extrapolation

The Tafel extrapolation technique uses the data obtained from the cathodic polarization measurements. Consider a metal M in de-aerated acid solution. In Figure 2.4, the solid lines represent the experimentally measured cathodic polarization and anodic polarization data. As the current density increases, the region is found to be linear compared to less current density values. This linear region is referred to as Tafel region. To determine the corrosion rate from this measurement, the Tafel region is extrapolated to obtain the corrosion potential. The accuracy of this technique is better than the conventional techniques. The disadvantage of this system is that it cannot be applied for systems having more than one reduction reactions.

Figure 2-4: Cathodic and Anodic Polarization Curves Log i Experimental Tafel equation 2H++2e-=H2(g) Fe=Fe2++2e -Ecorr E icorr

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2.3 Linear Polarization Resistance Method

Linear Polarization Resistance Method (LPR) is the only corrosion monitoring method that allows corrosion rates to be measured directly in real time. The response time and data quality makes this technique superior to other methods. This method is limited to electrolytically conducting liquids.

2.3.1 Derivation of the Polarization Resistance

It is experimentally observed that iapp is almost linearly related to applied potential

within a few millivolts of polarization from Ecorr. Stern and Geary simplified the kinetic

expression to provide an approximation to the charge transfer controlled reaction kinetics given by Equation (2.1) for the case of small overvoltage with respect to Ecorr.

2.3( ) 2.3( ) [exp( corr ) exp( corr )]

app corr a c E E E E i I β β − − = − …(2.1)

where βa = anodic Tafel slope

βc = cathodic Tafel slope

Icorr / icorr = corrosion rate/corrosion current density

Ecorr = corrosion potential

Equation (2.1) can be approximated mathematically by taking its series expansion (e.g., ex = 1 + x + x2/2! + x3/3! +….) and by neglecting higher terms when ΔE/β < 0.1. This simplified relationship has the following form

corr a c p (E - E ) 0 app E * R = [ ] = i 2.3i_corr( _a _c) β β β β → ₊ ...(2.2) Rearranging * 2.3 ( ) a c corr p a c p B i R R β β β β = = + …(2.3) Rp is the polarization resistance

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βa = anodic Tafel slope

βc = cathodic Tafel slope

B is the proportionality constant

The units of Rp are ohms as obtained from E-Iapp data when the applied current is

not normalized by electrode area (such data must be multiplied by electrode area to yield Rp (Ω-cm2)). If the electrode area is doubled, the measured Rp value in ohms is

halved, and this intrinsic polarization resistance value remains the same. This gives the result that corrosion rate per unit area is independent of electrode area. However, the working electrode area must be known to calculate the corrosion rate.

The B factor is dominated by the smaller of the two anodic and cathodic Tafel slopes (βa, βc), if unequal. Therefore, cathodic mass transport control results in B =

βa/2.3 and similarly anodic mass transport control results in B = βc/2.3. Knowledge of

Rp, βa, and βc enables direct determination of the corrosion rate at any instant in time

using equation (2.3). iapp is approximately linear with potential within ± 5 mV to 10 mV

of Ecorr . Consequently the method is called linear polarization method (LPR). The

extent of approximately linear E – iapp region can vary considerably among corroding

systems. It can be infered that corrosion rate is inversely proportional to the Rp.

2.3.2 Principle of Measurement

Before making any measurements, residual potential difference between the reference electrode (R) and the test electrode (T) should be nullified. After which, current will flow from the auxiliary electrode (A) onto the test electrode. The flow of current between the auxiliary electrode and test electrode will increase until the test electrode potential is shifted 10 mV with respect to the reference electrode. The current (ΔI) required to sustain the 10 mV potential shifts is proportional to the corrosion rate of the test electrode.

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2.3.3 Advantages of Polarization Resistance Measurements

• Rapid Measurement.

• Accuracy of the measurement of even smaller corrosion values.

• Can be used to monitor corrosion continuously which cannot be inspected visually or by other methods.

2.3.4 Errors and Limitations in the Use of Polarization Resistance

Measurements

• The Stern-Geary relationship is only mathematically valid only when ΔE is equal to zero

• Curvature of polarization curves about the corrosion potential, Ecorr can cause

errors in the measurement of polarization resistance when linearization techniques are involved.

• This equation is valid only for activation controlled process

• This method is not applicable under the special non-Tafel conditions corresponding to passivity or cathodic diffusion limiting current densities.

• The corrosion rate icorr must be much larger than any other exchange currents of

half-cell reactions, or the latter rates will dominate the polarization resistance measurement.

• Resistances from the presence of films on the electrodes and the electrolyte resistance between the working and reference electrode in high resistivity media can produce an underestimation of corrosion rates due to IR (current times resistance) losses on ΔE and must be compensated to obtain accurate measurements.

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2.4 Other Methods to Determine Polarization Resistance

2.4.1 Electrochemical Impedance Spectroscopy

Electrochemical impedance spectroscopy is a well-established laboratory technique used to determine the electrical impedance of the metal-electrolyte interface at various AC frequencies. Impedance measurements combine the effects of DC resistance with capacitance and inductance. In order to make impedance measurements, it is necessary to have a corrosion cell of known geometry, a reference electrode, and instrumentation capable of measuring and recording electrical response of the test corrosion cell over a wide rate of AC excitation frequencies. Again, with the evolution of more rugged computers, some investigation of this method is now being made in the field. AC impedance is capable of characterizing the corrosion interface more comprehensively and with good quality equipment specifications of achieving measurements in lower conductivity solution or high resistivity coatings. AC impedance measurements can be used to predict corrosion rates and characterize systems under study and are commonly used for performance studies of chemical inhibitors and protective coatings to evaluate the resistance of alloys to specific environments etc.

2.4.2 Electrochemical Noise

Electrochemical noise is a monitoring technique, which directly measures naturally occurring electrochemical potential and current disturbances due to ongoing corrosion activity. It has the same media conductivity limitations and requirement for a known electrode area as other electrochemical techniques. It is generally less quantitative than linear polarization resistance for corrosion rate calculations; it is useful in detecting transient effects in marginally conductive situations. Laboratory and field interpretations are still under development. One advantage is that this technique can provide a large quantity of information, which can be useful in determining what is actually happening in real time with corrosion activity in piping or equipment being

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especially, when there is a contact but acceptable level of corrosion activity in the system or another source of potential electrical disturbance in the system.

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3 FORMS OF CORROSION

The various forms of corrosion that are prevalent in metals are discussed in terms of their characteristics, mechanisms, and preventive measures in this chapter.

3.1 Uniform Corrosion

Uniform (or general) corrosion is a form of corrosion where there is uniform reduction of thickness over the surface of a corroding material. This is the most important form of corrosion on the basis of tonnage wasted. Uniform corrosion can be easily measured and predicted. This leads disastrous failures relatively rare. The breakdown of protective coating systems on structures often leads to this form of corrosion.

Figure 3-1: Uniform Corrosion.

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Figure 3-2: Example of uniform corrosion damage on a rocket assisted artillery projectile (Reference: http://www.corrosion-doctors.org/Forms/projectile.htm)

By selecting suitable materials and protective coatings, uniform corrosion can be controlled. Cathodic protection and corrosion inhibitors are other preventive methods.

“It is relatively easy to monitor uniform corrosion; generally the simplest methods suffice (coupons, ER, NDT techniques for thickness measurements). Much data on uniform corrosion has been published that can be used for design purposes and estimating a corrosion allowance".

In most practical cases, corrosive environments tend to differ from "textbook" cases (even small differences can be very significant). Furthermore, actual uniform corrosion rates tend to vary with time; this variability is not accounted for by single "textbook values". Corrosion monitoring is therefore advisable.

Unexpected rapid uniform corrosion failures can occur if the material's surface changes from the passive (low corrosion rate) to the active (high corrosion rate) state. The resultant increase in uniform corrosion rate is typically several orders of magnitude. This undesirable transition can occur if the passive surface film is disrupted by mechanical effects, flow rate changes, a chemical change in the environment etc. Real-time corrosion monitoring systems can detect such transitions.” (This excerpt is taken from www.corrosion-club.com)

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3.1.1 Atmospheric corrosion

Atmospheric corrosion is defined as the corrosion or degradation of the material exposed to the air and its pollutants. This has been identified as one of the oldest forms of corrosion and has been reported to account for more failures in terms of cost and tonnage than any other single environment. The rate at which the corrosion takes place and the severity is primarily dependant upon the properties of the surface formed electrolyte, which in turn depends upon the factors such as relative humidity, climatic conditions, temperature, atmospheric pollutants etc.

3.1.1.1 Mechanism of Atmospheric Corrosion

Atmospheric corrosion is an electrochemical process, which takes place in the presence of an electrolyte. Depending upon the climatic conditions, relative humidity and temperature, a certain humidity level is reached which tends to form a thin electrolyte on the metallic surfaces. The following reactions take place.

Anodic reaction 2M Æ 2M2+ + 4e-

Cathodic reaction O2 + 2H2O + 4e- Æ 4OH-

Figure 3-3: Atmospheric corrosion of galvanized anti crash railing due to marine aerosol condensation on wooden post

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It is an established principle that if a change occurs under which a system is in equilibrium, the system will tend to adjust itself so as to nullify the effect of that change. Therefore, in the presence of an electrolyte, atmospheric corrosion proceeds by balancing cathodic and anodic reaction. The anodic reaction involves the dissolution of the metal in the electrolyte, while the cathodic reaction involves reduction. Oxygen from the atmosphere is readily supplied to the electrolyte in thin film conditions.

3.1.1.2 Prevention of Atmospheric Corrosion

There are two approaches to prevent Atmospheric corrosion. The first is a temporary one which is used during storage and which consists of lowering of atmospheric humidity by using a desiccant, heating devices, or by treating the surface with vapor phase or surface inhibitors. Permanent solution is by applying organic, inorganic and metallic coatings effectively.

3.1.2 Galvanic Corrosion

When two dissimilar metals are coupled and immersed in an electrolyte solution, a galvanic cell is formed. In a galvanic cell, the more active metal is corroded while the noble metal is protected. Corrosion of active metal is increased and of noble metal is decreased upon galvanic coupling. Figure 3.4 shows galvanic corrosion between the pipe made of black iron and a brass valve.

Figure 3-4: Galvanic Corrosion of Brass Coupled With Black Iron (Reference: http://www.eci-ndt.com/gallery_a.htm)

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When two dissimilar metals M and N, with N being more active, are connected and immersed in an acid solution, the corrosion mechanism may be depicted in a graphical representation as shown in Figure 3.5.

Figure 3-5: Mechanism of Galvanic Corrosion of a Two Metal Couple

The more noble metal M undergoes the cathodic reaction while the active metal N undergoes the anodic reaction. The electrons produced by the dissolution of N are consumed by the cathodic reaction that takes place on the noble metal surface. Thus, corrosion of the noble metal retards while that of the active metal accelerates because of the galvanic effect generated on the metal surface.

The magnitude of the galvanic current flow is controlled by the potential difference and the total resistance to current flow as shown in the Figure 3-6, which can be broken down further into the following resistance components.

Resistance to current flow of the electrolyte.

Resistance to current flow in the conducting materials and in the connection between them.

Resistance associated with polarization behavior of the anode and cathode.

The Figure 3-6 illustrates a cell showing the corrosion process in its simplest form. This cell includes the following essential components: a metal cathode, metal

M N _N2+

2H+ H2

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-contact with the anode and the cathode. They are so arranged to form a closed electric path.

Figure 3-6: Galvanic cell showing corrosion process in its simplest form (Reference: http://www.edstrom.com/Resources.cfm?doc_id=131)

The cathode is positively charged and anode is negatively charged. The difference in charge provides potential voltage which is the driving force for the current to flow in the cell.

Hydrogen gas is produced at the cathode and no destruction will occur while the anode gives off its ions in the form of rust, this is where the corrosion occurs. The rate of which depends upon the relative sizes of the anode and cathode and also the potential difference between cathode and anode.

If, for instance, the anode is very small compared to the cathode, the rate of corrosion would be very rapid. The opposite would be true if there was a very large anode compared to the cathode.

When contact with a dissimilar metal is made, however, the self-corrosion rates will change: corrosion of the anode will accelerate; corrosion of the cathode will decelerate or even stop.

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This is shown in Figures 3-7 to 3-9. Figure 3-7 shows the corrosion rate for a single metal in solution.

Cathodic Reaction 1 Anodic reaction 2 icorr Ecorr E (V)

log Current Density μA/cm2

Figure 3-7: Corrosion rate determination for a two electrode process system

(Reference: http://www.egr.uri.edu/che/course/CHE534w/chapter3EnivronmentalCorrosion.htm)

Figures 3-8 and 3-9 shows the rate determination when a third electrode process is added at a potential between the first two electrode reactions. The rule that must be applied is that the ‘total oxidation rate must be equal to the total reduction rate.’

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Cathodic Reaction 1 Anodic Reaction 2 icorr 1+2 Ecorr 1+2 E (V)

log Current Density μA/cm2 Cathode Reaction 3

Anode Reaction 3

Total Cathode 1+3

Figure 3-8: Corrosion rate determination for a three electrode system.

(Reference: http://www.egr.uri.edu/che/course/CHE534w/chapter3EnivronmentalCorrosion.htm)

From Figure 3-8, it is seen that the corrosion rate for electrode 2 has increased from icorr to icorr 1+2 as it is the only anodic reaction. Two cases are shown in Figures 3-8

and 3-9; when the corrosion potential for three electrodes is above the two electrode potential and when the three electrode corrosion potential is below the two electrode potentials respectively. In Figure 3-8 the resulting corrosion potential is more negative than the third electrode reverse potential, thus contributing to the cathodic reaction and protecting the third electode from corrosion. The second electrode dissolution rate increased significantly by the introduction of the third electrode processes.

In Figure 3-9, the resulting corrosion potential from the three electrodes is more negative than the double electrode potential. In this case both the second and third electrodes are corroding, but the third electrode corrodes at a lower rate than the second electrode.

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Cathodic Reaction 1 Anodic Reaction 2 icorr 1+2 Ecorr 1+2 E (V)

log Current Density μA/cm2 Cathode Reaction 3

Anode Reaction 3

Total Anode 1+3

Figure 3-9. The introduction of a less noble metal will decrease the corrosion rate of the more noble metal. (Reference: http://www.egr.uri.edu/che/course/CHE534w/chapter3EnivronmentalCorrosion.htm)

Both these Figures 3-8 and 3-9 show that introducing a more anodic metal will decrease the corrosion rate in a more noble metal. This is the process behind galvanic corrosion. It can also be used for protection by galvanizing.

The seawater Galvanic Series, shown in table 3-1 below can be used to predict which metal will become the anode and how rapidly it will corrode. The metals below are arranged according to their tendency to corrode galvanically. Metals with negative voltage charges (anodic–least noble) are listed first, followed by metals with positive charges (cathodic–more noble).

3.1.2.1 CORRODED END (ANODIC OR LEAST NOBLE)

• MAGNESIUM

• MAGNESIUM ALLOYS • ZINC

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• CADMIUM

• ALUMINUM 2117, 2017, 2024

• MILD STEEL (1018), WROUGHT IRON

• CAST IRON, LOW ALLOY HIGH STRENGTH STEEL • CHROME IRON (ACTIVE)

• STAINLESS STEEL, 430 SERIES (ACTIVE)

• 302, 303, 304, 321, 347, 410,416, STAINLESS STEEL (ACTIVE) • NI - RESIST

• 316, 317, STAINLESS STEEL (ACTIVE) • CARPENTER 20 CB-3 STAINLESS (ACTIVE) • ALUMINUM BRONZE (CA 687)

• HASTELLOY C (ACTIVE) INCONEL 625 (ACTIVE) TITANIUM (ACTIVE) • LEAD - TIN SOLDERS

• LEAD • TIN • INCONEL 600 (ACTIVE) • NICKEL (ACTIVE) • 60 NI-15 CR (ACTIVE) • 80 NI-20 CR (ACTIVE) • HASTELLOY B (ACTIVE) • BRASSES • COPPER (CA102)

• MANGANESE BRONZE (CA 675), TIN BRONZE (CA903, 905) • SILICON BRONZE

• NICKEL SILVER

• COPPER - NICKEL ALLOY 90-10 • COPPER - NICKEL ALLOY 80-20 • 430 STAINLESS STEEL

• NICKEL, ALUMINUM, BRONZE (CA 630, 632) • MONEL 400, K500

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• SILVER SOLDER • NICKEL (PASSIVE) • 60 NI- 15 CR (PASSIVE) • INCONEL 600 (PASSIVE) • 80 NI- 20 CR (PASSIVE) • CHROME IRON (PASSIVE)

• 302, 303, 304, 321, 347, STAINLESS STEEL (PASSIVE) • 316, 317, STAINLESS STEEL (PASSIVE)

• CARPENTER 20 CB-3 STAINLESS (PASSIVE), INCOLOY 825 • NICKEL - MOLYBDEUM - CHROMIUM - IRON ALLOY (PASSIVE) • SILVER

• TITANIUM (PASS.) HASTELLOY C & C276 (PASSIVE), INCONEL 625(PASS.) • GRAPHITE

• ZIRCONIUM • GOLD • PLATINUM

3.1.2.2 PROTECTED END (CATHODIC OR MOST NOBLE)

From the list 3.1.2.1, it is true that each metal has a different electrical potential when immersed in the same electrolyte (an electrically conductive fluid such as sea water). As a result, if two dissimilar metals are placed in the same electrolyte, their different electrical potentials will produce a voltage that can be measured on the two pieces of metal. According to the potential difference of these two metals, the current flows from higher voltage metal to the lower one. This action raises the voltage of the voltage metal above its natural potential. To establish the equilibrium, the lower-voltage metal discharges a current in to the electrolyte. The current passes through the electrolyte back to the higher-voltage metal and completes the electrical circuit between the two pieces. The current flowing through the electrolyte is generated by an electrochemical reaction that steadily consumes the lower-voltage metal a process

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In couple A, the aluminum rivet is comparatively small, and the C/A ratio is large. In couple B, the situation is reversed: the stainless steel rivet is small, and the C/A ratio is also small. Corrosion of the aluminum rivet in couple A will be severe. However, corrosion of the large aluminum plate in couple B will be much less, even though the potential difference is the same in each case.

The two major factors affecting the severity of galvanic corrosion are (1) the voltage difference between the two metals on the Galvanic Series, and (2) the size of the exposed area of cathodic metal relative to that of the anodic metal. Corrosion of the anodic metal is both more rapid and more damaging as the voltage difference increases and as the cathode area increases relative to the anode area.

Figure 3-10: Effect of cathode to anode ratio in galvanic corrosion (Reference: http://www.ocean.udel.edu/seagrant/publications/corrosion.html)

The effect of the second factor, the cathode-to anode area ratio, C/A, is illustrated in Figure 3-10 for a rivet in a plate. In both couples A and B, aluminum is the anode, and stainless steel is the cathode. In couple A, the aluminum rivet is comparatively small, and the C/A ratio is large. In couple B, the situation is reversed: the stainless steel rivet is small, and the C/A ratio is also small. Corrosion of the aluminum rivet in couple A will be severe. However, corrosion of the large aluminum plate in couple B will be much less, even though the potential difference is the same in each case.

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3.1.2.3 Factors Affecting Galvanic Corrosion

Area Effect

The current flowing between anode and cathode will be the same independent of the surface area of each electrode. For the anodic and cathodic reactions, it is the current rather than current density which is equal. Since the corrosion occurs at anode, the rate of corrosion depends on the current density in the anode. To minimize the galvanic corrosion, the anode area should be very large compared to the cathode area. Inorder to protect the system from galvanic corrosion, the cathode of the system should be painted. If the paint is damaged, then small cathode to large anode area ratio is formed which results in minimizing corrosion rates. Conversely, if the anode is painted, then the damage to the paint causes large cathode to small anode ratio resulting in large corrosion rates in the anode and thus penetrating into the metal. The anode to cathode area effect is an important characteristic. It is important in several other forms of corrosion including pitting corrosion, crevice corrosion, stress corrosion cracking and corrosion fatigue.

Distance Effect

Distance effect is another important factor for galvanic corrosion. Galvanic corrosion rates are the largest at the interface between the anode and cathode and decrease with distance away from the contact region. The transportation of the ions becomes more difficult when the distance between anodic and cathodic reaction site increases thus decreasing the corrosion rate. Essentially the resistance of the electrolyte increases with distance. This is an important factor in determination of the form of corrosion. If a galvanic corrosion is suspected, then according to the rule as explained, the rate of corrosion adjacent to the galvanic contact region should be higher. If the corrosion rate is far away from this area then different type of corrosion may be involved. For example if corrosion appears at a constriction some distance from a galvanic contact, then erosion corrosion was the cause and not the galvanic corrosion.

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Distance Apart in the Galvanic Series

Galvanic series is an empirical listing of the corrosion resistance of metals. Its advantage over the Redox series is that it refers to alloys in a real environment. Metal at the top of the system are highly cathodic while metals at the bottom highly anodic. For selection purposes metals close together on the list are desirable as there is little driving force for corrosion to be accelerated. Clearly, it is undesirable to connect metals widely spaced on the galvanic series.

3.1.2.4 Galvanic Series

Galvanic series is a list of metals/alloys according to their corrosion potentials. The list is formed by polarization of two or more half-cell reactions to a common mixed potential, Ecorr on the corroding surface. Corrosion potentials in the galvanic series are

measured in real or simulated service conditions. The galvanic series gives only tendencies for galvanic corrosion, not the rate. In the galvanic series, the potential changes due to changes in electrolyte composition and temperature. For every change in environmental conditions, a new series needs to be established. Table 3.1 lists some of the metals/alloys for the galvanic series in seawater. In Table 3.1, titanium is a noteworthy element. Titanium is a very active metal with E0 value being -1.63 V for the reduction of:

Ti2+ + 2e- = Ti …(3.1) However, it has a very noble corrosion potential. This may be due to the fact that titanium surface is easily covered with passive film with normal potential oxidizing reagents such as oxygen.

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Table 3-1: Galvanic Series of Metals/Alloys in Seawater Noble ⇑ ⇓ Active Platinum Gold Titanium Silver

Hastelloy C (62 Ni, 17 Cr, 15 Mo) 18-8 Stainless steel (passive)

Inconel (passive) (80 Ni, 13 Cr, 7 Fe) Cast Iron Wrought Iron Copper Red Brass Cadmium Manganese Bronze Aluminum 52SH Tin Lead Zinc Magnesium Alloys Magnesium

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3.1.2.5 Prevention of Galvanic Corrosion

• Coupling of two dissimilar metals should be avoided when they are much apart in the galvanic series.

• In the case where such coupling is necessary, break the electrical contact between the two metals by using insulation such as gaskets and rubber. • Do not paint anodic area because if the paint is ruptured in a small area, the

corrosion rate in the area will increase drastically. Instead, paint the cathodic area.

3.1.3 Stray Current Corrosion

Stray current corrosion is the current flowing through unintended paths due to some kind of leakage from extraneous sources. The damage caused to the metal components due to this unwanted current refers to stray current corrosion. Metals like aluminum under soil or water are affected due to this kind of corrosion. This current mostly originates due to bad earthen systems of electrical equipments and eventually leaks through the metal structures or other conductive systems. The common sources of these stray current include electric railway systems, cathode protection systems of nearby equipment or pipelines, DC-driven elevators, etc.

The mechanism involved in stray current corrosion is that an electrolysis cell is formed that forces the metal structure through which it passes to act as an anodic site. Thus local oxidation occurs and the metal is consumed rapidly.

3.1.3.1 Direct Stray Current Corrosion

This type of corrosion occurs due to direct current from sources like rail transit system, DC welding equipment. Again this type of corrosion can be classified as dynamic current corrosion when the current is not steady or flows irregularly and as static current corrosion when the flow is steady. The effects of direct stray current are very severe compared with alternative currents.

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3.1.3.2 Alternating Stray Current Corrosion

This type of corrosion is caused due to alternate currents from sources like overhead AC power lines.

3.1.3.3 Telluric Effects

The disturbances in the earth’s magnetic field called geomagnetic activity may lead to the production of dynamic stray currents. These currents induced naturally due to the geomagnetic activity are called telluric effects.

These may flow onto a buried pipeline varying the magnitude of current flow and the position of current pick-up. The discharge areas will also fluctuate with time.

Figure 3-11: Occurrence of Stray current corrosion in pipelines. (Reference:http://www.corrosion-club.com/stpickup.htm)

Basic Theory

A protective coating is used as the primary form of protection for buried pipelines. Additionally, cathodic protection is designed to provide protection at coating

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protection system is used to reduce the corrosion risk factor. Figure 3-12 schematically illustrate current flow in an impressed current cathodic protection system (the principle is similar for a sacrificial anode system). Under these idealized conditions, current flows through the electrolyte (soil) onto the pipeline, in the form of ionic current.

Figure 3-12: Ionic current flow onto the pipeline. (Reference:http://www.corrosion-club.com/sttheory.htm)

The schematic diagram in Figure 3-13 details the current flow onto the pipeline at coating discontinuities, under the protective influence of the cathodic protection system.

Figure 3-13: Current flow onto pipeline at coating discontinuities. (Reference:http://www.corrosion-club.com/sttheory.htm)

Current flow in the electrolyte that does not originate from the cathodic protection system designed to protect the pipeline is referred to as stray current. Such external stray current sources interfere with the normal operation of the cathodic protection system leading to corrosion problems.

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