Applied Electrochemistry. Dept. Chem. & Chem. Eng. Lecture 14 Metal Electrochemistry. Dept. Chem. & Chem. Eng. 1. Electrodeposition. 2. Corrosion. 3. Industrial electrolytic process. Outline. Definition.
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Dept. Chem. & Chem. Eng.
Dept. Chem. & Chem. Eng.
Industrial electrolytic process
Corrosion is the deterioration of materials by chemical interaction with their environment. The term corrosion is sometimes also applied to the degradation of plastics, concrete and wood, but generally refers to metals.
--the destructive electrochemical attack of a material.
--Al Capone's ship, Sapona, off the coast of Bimini.
--4 to 5% of the Gross National Product (GNP)*
--this amounts to just over $400 billion/yr**
Photos courtesy L.M. Maestas, Sandia National Labs. Used with permission.
* H.H. Uhlig and W.R. Revie, Corrosion and Corrosion Control: An Introduction to Corrosion Science and Engineering, 3rd ed., John Wiley and Sons, Inc., 1985.
**Economic Report of the President (1998).
The Rusting Mechanism (Peel)
Basic “rusting” or corrosion requirements
1. The metal is oxidized at the anode of an electrolytic cell
2. Some ions are reduced at the cathode
3. There is a potential or voltage difference between the anode and cathode
4. An electrolyte (fluid) must be present
5. The electrical path must be completed
Corrosion of zinc in acid
• Two reactions are necessary:
-- oxidation reaction: Zn → Zn2+ + 2e-
-- reduction reaction: 2H+ + 2e → H2(gas)
Adapted from Fig. 17.1, Callister 6e. (Fig. 17.1 is from M.G. Fontana, Corrosion Engineering, 3rd ed., McGraw-Hill Book Company, 1986.)
• Other reduction reactions:
-- in an acid solution
-- in a neutral or base solution
• Metal with smaller
• Ex: Cd-Ni cell
• EMF series
Data based on Table 17.1, Callister 6e.
Thermodynamic Driving Force
F = Faraday’s constant (total charge on Avogadro’s number of electrons)
n = the number of electrons transferred
E = The electrode potential
The Basic equation which describes ALL corrosion reactions
For the Given Example:
Note: pH = -log10[H+]
EFFECT OF SOLUTION CONCENTRATION
• Ex: Cd-Ni cell with
standard 1M solutions
• Ex: Cd-Ni cell with
n = #e-
• Reduce VNi - VCd by
H 2H+ +2e-
2H+ +2e- H2
POTENTIAL VOLTS (SHE)
Fe Fe+2 +2e-
Fe+2 +2e- Fe
CURRENT DENSITY, A/cm²
Kinetics Describes Rate of Reaction Evan’s Diagram
= Area Inside Crevice (Anodic)
= Area Outside Crevice (Cathodic)
Aa << Ac
M M+ +e-
O2 /OH +
½ O2 + H2O + 2e- 2OH-
Effect of Oxidizer Concentration (e.g., Oxygen) on the Electrochemical Behavior of an Active - Passive Metal
[Fontanna and Greene, Corrosion Engineering, McGraw-Hill, 1967]
Passivity is defined as corrosion resistance due to formation of thin surface films under oxidizing conditions with high anodic polariza tion.
For example Fe is passivated in concentrated nitric acid.
Effect of Temperature and Dissolved O2
FRESH WATER @ 50°F
FRESH WATER @ 90°F
FRESH WATER @ 120°F
VELOCITY = 2.5 FPS
CORROSION RATE (MPY))
DISSOLVED O2 (PPM)
DISSOLVED O2 (Mg/H2O)
Forms of corrosion
• Stress corrosion
Stress & corrosion
at crack tips.
• Uniform Attack
Oxidation & reduction
occur uniformly over
Break down of passivating
layer by erosion (pipe
• Selective Leaching
Preferred corrosion of
(e.g., Zn from brass (Cu-Zn)).
of small pits & holes.
Fig. 17.8, Callister 6e.
(Fig. 17.8 from M.G.
Engineering, 3rd ed.,
often where special
Dissimilar metals are
physically joined. The
more anodic one
17.2) Zn & Mg
• Crevice Between two
pieces of the same metal.
Fig. 17.6, Callister 6e. (Fig. 17.6 is courtesy LaQue Center for Corrosion Technology, Inc.)
Fig. 17.9, Callister 6e.
Types of Aqueous Corrosion Cells
Factors Affecting Corrosion
• Self-protecting metals!
--Metal ions combine with O2
to form a thin, adhering oxide layer that slows corrosion.
• Reduce T (slows kinetics of oxidation and reduction)
• Add inhibitors
--Slow oxidation/reduction reactions by removing reactants
(e.g., remove O2 gas by reacting it w/an inhibitor).
--Slow oxidation reaction by attaching species to
the surface (e.g., paint it!).
• Cathodic (or sacrificial) protection
--Attach a more anodic material to the one to be protected.
Adapted from Figs. 17.13(a), 17.14 Callister 6e. (Fig. 17.13(a) is from M.G. Fontana, Corrosion Engineering, 3rd ed., McGraw-Hill Book Co., 1986.)
Corrosion of Metals
Passivity of Metals
Atmospheric Corrosion of Steels