ELECTROCHEMICAL KINETICS OF CORROSION Read Re adin ing g Mate Materi rial al:: Chap Chapte terr 3 in “Principles and Prevention of Corrosion”, Denny Jones, Prentice-Hall, 1996.
Dr. Dr. Rama Ramaza zan n Kahr Kahram aman an Chemical Engineering Department King King Fahd Univer Universit sity y of Petrole Petroleum um & Mineral Minerals s Dhahran, Saudi Arabia 1
Faraday’s Law
Charge is related to mass of material reacted in an electrochemical reaction: M
Mn+ + ne-
One mol of metal
n mo mols ls of elect electron rons s To produce one mol of metal ion and Reacts 2
Faraday’s Law (Cont.)
One mole of metal (MW g) contains Avogadro’s number (6 1023) of metal atoms
Hence each mole of metal will produce n times that many number of electrons
Charge on the electron is 1.6
Charge of one mol of electrons (6 1023 electrons) will then be 96500 C
Hence one mole of metal will produce a charge of n 96500 C
96500 C/equivalent is known as Faraday’s constant (also in units of J/V equivalent)
Conversions: 1 A (ampere) = 1 C/s, 1 J = 1 C V
10-19 C (coulomb)
3
Faraday’s Law (Cont.) Q I t M = m= n F n F where
Q = charge (coulomb, C) I = current (amperes, A) (1 A =1 C/s) F = Faraday' s constant (96500 C/equivalent ⋅ mol) n = number of equivalents (mols of electrons) transferred per mol of metal m = mass of metal oxidized (g) M = molecular (atomic) weight of metal (g/mole)
corrosion rate , where
nF = i r = m = I t n F t A t A
A = surface area i = current density, I/A 4
How Fast will Corrosion Occur?
Corrosion kinetics – Concerned with the rates of corrosion reactions
Mixed potential theory: –The corrosion potential will be that potential at which the sum of all anodic (positive) and cathodic (negative) currents on the electrode is zero (i.e. ⏐anodic current⏐=⏐cathodic current⏐) (mixed equilibrium) (not an electrochemical equilibrium)
Polarization – The change in the potential of an electrode as current flows to or from it 5
Corrosion of Zinc in Acid
When zinc is placed in acid the metal will start to dissolve (Zn Zn2+ + 2e-) and hydrogen will start to be liberated (2H+ + 2eH2) according to the potential of the metal
Zn
Zn+2 c
a
e -
H+ H+
H2
6
Corrosion of Zinc in Acid l a i t n e t o P l a c i mEcorr e h c o r t c e l E
Zn
Zn2+ + 2eAt the Corrosion Potential, Ecorr, we have a stable mixed equilibrium
2H+ + 2e-
icorr
H2
Rate Current of Reaction density
Then the corrosion rate may be expressed as the corrosion current density, icorr
As the reaction involves transfer of charge, the rate of reaction may be expressed as a current per unit area, or current density
7
Corrosion of Zinc in Acid (Cont.) l a i t n e t o P l a c i m e h c o r t c e l E
Zn
Zn2+ + 2e-
If the potential is above the Corrosion Potential, then it will fall due to production of electrons
If the potential is below the Corrosion Potential, then it will rise, due to consumption of electrons
2H+ + 2e-
H2
Rate Current of Reaction density 8
Types of Polarization
Activation Polarization
Concentration Polarization
Resistance (IR-drop) Polarization
9
Activation Polarization
Result of a slow step in an electrode reaction at the anode or cathode – For example, when hydrogen evolves at the cathode, the reaction might be considered as: Step 1: Step 2: Step 3:
H+ + e- Hads Hads + Hads H2 Sufficient molecules of H2 combine and nucleate a hydrogen bubble.
Any one of these steps can control the rate of reaction and cause activation polarization.
10
Activation Polarization (Cont.) Example: Hydrogen reduction reaction under activation control.
[“Corrosion Engineering”, Mars Fontana, McGraw-Hill, 1986] 11
Tafel’s Law for Activation Polarization
E = E o + b log where
E = E o = b = η=
i io
or
η
= b log
i io
potential at current i potential at current io (ec for cathodic rxn and ea for anodic rxn) Tafel slope ((+) for anodic rxn, (-) for cathodic rxn) polarization (E -E o at current i )
12
Tafel’s Law at i corr Anodic Polarization
E corr = ea
+ ba log
icorr io
or
η a at icorr
= ba log
icorr io
Cathodic Polarization
E corr = ec + bc log
icorr io
or
η c at icorr
= bc log
icorr io
13
E-log i and Evans Diagrams
Plot E against log |i |, then activation polarization gives a straight line
l a i t n e t o P e d o r t c e l E
Tafel slope expressed as mV per decade of current
mV log |i 2 | - log |i 1 | log |current|
14
E-log i and Evans Diagrams (Cont.) E o and i o for the cathodic reaction
l a i t n e t o P e d o r t c e l E
Anodic reaction, Tafel slope is positive
Mixed equilibrium occurs when sum of all currents is zero, i.e. at E corr and i corr for the corrosion reaction
log |current| E o and i o for the anodic reaction
Cathodic reaction, Tafel slope is negative
15
Exchange Current Density, i o When a piece of metal is sitting in a solution at its equilibrium potential, this does not mean that the rates of the metal dissolution and reprecipitation reactions are zero. Instead it implies that the rates of the two reactions are equal. When the metal dissolution and reprecipitation reactions are in equilibrium, we refer to the (equal and opposite) rates (in terms of current density) of each of the two reactions as the exchange current density, i o . 16
Polarization
17
Effect of Degree of Polarization
As degree of polarization gets higher (i.e. ⏐slope⏐ increases) i corr (corrosion rate) decreases. 18
Example (Zinc in Acid)
19
Example (Cont.)
Note that the reverse reactions, zinc deposition (Zn2++2e-Zn) and hydrogen oxidation (H22H++2e-), do not occur since the corrosion potential lies between 0.0 and -0.762 volt. Zinc deposition (Zn2++2e-Zn) can only occur at potentials more negative than -0.762 V, and hydrogen oxidation (H22H++2e-) only occurs at potentials more positive than 0.0 V.
20
Concentration Polarization
Additional polarization (i.e. slowing down of a reaction) caused by depletion / drop in concentration of a reactant at the electrode surface or an excess of the unwanted species at the electrode surface (diffusion controlled polarization)
Concentration polarization is low / insignificant until a limiting current density, i L, is approached (i.e. it becomes effective at high rates approaching i L)
Limiting current density, i L, is the measure of a maximum reaction rate that cannot be exceeded because of a limited diffusion rate of a reactant 21
Concentration Polarization (Cont.)
[“Principles and Prevention of Corrosion”, Denny Jones, Prentice-Hall, 1996] 22
Concentration Polarization (Cont.) Example: Concentration polarization during hydrogen reduction.
[“Corrosion Engineering”, Mars Fontana, McGraw-Hill, 1986]
23
Concentration Polarization (Cont.) Effect of Solution Conditions on Concentration Polarization
[“Principles and Prevention of Corrosion”, Denny Jones, Prentice-Hall, 1996] 24
Concentration Polarization (Cont.)
η conc
= E − E o =
2.303 RT nF
⎛ i ⎞ log⎜⎜1 − ⎟⎟ ⎝ i L ⎠
Concentration polarization is usually negligible (insignificant) on anodes and usually ignored.
25
Combined Polarization Total Cathodic Polarization η c
= η act ,c + η conc ,c = bc log
ic io
+
2.303 RT nF
⎛ ic ⎞ log⎜⎜1 − ⎟⎟ ⎝ i L ⎠
Anodic Polarization η a
= η act ,a = ba log
ia io
26
Combined Polarization (Cont.)
[“Principles and Prevention of Corrosion”, Denny Jones, Prentice-Hall, 1996] 27
Combined Polarization (Cont.)
Oxygen reduction is often affected by concentration polarization
l a i t n e t o P e d o r t c e l E
Rate of cathodic oxygen reduction without concentration polarization
Rate of cathodic oxygen reduction with concentration polarization Limiting current density - rate of reaction limited by availability of oxygen at the metal surface
log |current density| 28
Combined Polarization (Cont.)
[“Principles and Prevention of Corrosion”, Denny Jones, Prentice-Hall, 1996] 29
Resistance Polarization
If there is a resistance between the anode and the cathode in a cell, then the current flowing through that resistance will cause a potential drop given by Ohm’s Law: V = IR
This is important for paint films and for high resistance solutions
30
Resistance Polarization
l a i t n e t o P e d o r t c e l E
Resistance Polarization causes potential of anode and cathode to differ due to potential drop across solution, hence corrosion current is reduced
log |current density| 31
Sample Polarization Curves
Iron in hydrochloric acid l a i t n e t o P e d o r t c e l E
Anodic iron dissolution Cathodic hydrogen evolution
log |current density| 32
Sample Polarization Curves (Cont.)
Iron in aerated neutral NaCl solution
l a i t n e t o P e d o r t c e l E
Anodic iron dissolution Cathodic oxygen reduction Cathodic hydrogen evolution
log |current density| 33
Sample Polarization Curves (Cont.)
Iron in sulphuric acid (passivity to be studied later)
l a i t n e t o P e d o r t c e l E
Oxygen evolution on passive film (or transpassive corrosion as metal is oxidised to a higher oxidation state) Anodic iron dissolution (with active-passive transition) Cathodic hydrogen evolution
log |current density| 34
Effect of Oxidizer E corr ↑, i corr ↑, rate of hydrogen evolution
↓
[“Principles and Prevention of Corrosion”, Denny Jones, Prentice-Hall, 1996] 35
Effect of Exchange Current Density
[“Principles and Prevention of Corrosion”, Denny Jones, Prentice-Hall, 1996]
The deriving force (difference between the half-cell reaction potentials is much larger for zinc than iron. However, the corrosion rate of zinc, i corr,Zn, is lower than that of iron, i corr,Fe, because of the low exchange current density for hydrogen reduction on zinc compared to iron and the comparatively low exchange current density for zinc dissolution. 36
Effect of Dissolved Ion Concentration Drawing appropriate polarization diagrams, determine the effect of “increasing the concentration of dissolved H+” on “E corr ” and “i corr ” of a metal M corroding to dissolved M+ in a deaerated acid solution under activation control with all other parameters constant.
37
Example Problem
38
Home Work Problems
Prbs. 1, 6, 7 and 8 of Ch.3 in “Principles and Prevention of Corrosion”, Denny Jones, Prentice-Hall, 1996.
39
