Chapter 10: Corrosion Flashcards
Corrosion
Destructive attack of a metal by electrochemical interaction or chemical reaction with its environment.
Corrosion conditions (2)
The presence of an oxidizing agent (O2, CO2, H2O) and a reducing agent (metal being corroded). The reaction must be thermodynamically favorable.
Oxidization
Loss of electrons at the anode (LEO)
Reduction
Gain of electron at the cathode (GER)
Oxidizing agent
A chemical compound that readily transfers oxygen atoms that gains electrons in a redox chemical reaction. It is reduced.
Reducing agent
A chemical compound that readily donates electrons in a redox chemical reaction. It is oxidized.
Rust on steel
Components of an electrochemical (galvanic) cell (4)
Anode, cathode, electrical connection, electrolyte
Anode
Gives up electrons and produces ions (cations). Is the reducing agent that undergoes oxidation.
Cathode
Accepts electrons and consumes ions (cations) Is the oxidizing agent that undergoes reduction. Is rich in anions (at surface)
Electrical connection
Connects and allows electrons to flow from the anode to the cathode.
Electrolyte
Conducts current from the cathode to the anode. Must be in contact with anode and cathode.
Standard conditions
25°C,1.0M concentration of ions in the electrolyte solution, and 1atm.
Electrochemical potential equation
ΔVo = Vcathode - Vanode
Spontaneous reaction voltage sign
Positive voltage
Non-spontaneous reaction
Negative voltage
Electrochemical potential
Helps decide where the anode is (where corrosion occurs)
Effect of concentration on potential
The higher the concentration of the cation species, the more cathodic (+) the potential.
V = Vo - 0.0592/n*log(1/Cion)
If C < 1, V is more negative
Faraday’s law
The total weight of metal oxidized at the anode.
Faraday’s Law formula
w = ItM/nF
I = corrosion current (A)
t = time (s)
M = molar mass (g/mol)
n = valence electrons
F = Faraday constant
Corrosion penetration rate
CPR = KW/pAt (mm/yr)
K = constant
W = weight loss
p = density
A = exposed area
t = time
Corrosion rate
r = i/nF (mol/m^2s)
i = current density (A/m^2)
n = electrons associated with ionization
F = Faraday constant
Types of corrosion (5)
Uniform attack, galvanic corrosion, crevice corrosion, pitting corrosion, intergranular corrosion
Uniform attack
Corrosion attack is of uniform intensity throughout the part (EX: silverware tarnishing).
Galvanic Corrosion
When two dissimilar metals are in contact with an electrolyte. Less noble metal corrodes (EX: plumbing does not connect stat to tubing).
Crevice Corrosion
Corrosion, which occurs because of differences in concentration of a solution is usually because of stagnant flow conditions in a crevice.
Pitting corrosion
Cause similar to crevice corrosion. Pits form on surface of material
Intergranular corrosion
Corrosion occurs in preferential mode in the structure of the material. Caused by processing, impurities or welding
Stress Cell
Stressed area has a higher free energy and becomes the anode. Tensile stress accelerates corrosion, whereas compressive stress slows corrosion.
Concentration cell
A dissolved species involved in either the anode or cathode reaction varies in concentration at different locations in the electrolyte.
Corrosion control methods (6)
- Coatings
- Inhibitors
- Reduce dissolved oxygen
- Galvanic protection
- Cathodic protection
- Reducing temperature
Coatings and Inhibitors
Coating the surface to isolate it from the electrolyte and/or make adherent ions that coat the surface and isolate it from the electrochemical reaction. These methods must be replenished periodically.
Reducing dissolved oxygen
Inhibiting corrosion by adding hydrazine to water or treating the water using a hydrogen protection cell that replaces the oxygen in the water with hydrogen.
Galvanic protection
Where a more anodic metal is hooked up to the metal to be protected and dipped in the electrolyte. Commonly used in metal ships.
Cathodic protection
A small DC voltage is used to impress electrons into the steel, rendering the metal cathodic. This is used in addition to protective coatings.
