Chapter 10: Corrosion

Question 1

Corrosion

Answer 1

Destructive attack of a metal by electrochemical interaction or chemical reaction with its environment.

Question 2

Corrosion conditions (2)

Answer 2

The presence of an oxidizing agent (O2, CO2, H2O) and a reducing agent (metal being corroded). The reaction must be thermodynamically favorable.

Question 3

Oxidization

Answer 3

Loss of electrons at the anode (LEO)

Question 4

Reduction

Answer 4

Gain of electron at the cathode (GER)

Question 5

Oxidizing agent

Answer 5

A chemical compound that readily transfers oxygen atoms that gains electrons in a redox chemical reaction. It is reduced.

Question 6

Reducing agent

Answer 6

A chemical compound that readily donates electrons in a redox chemical reaction. It is oxidized.

Question 7

Rust on steel

Question 8

Components of an electrochemical (galvanic) cell (4)

Answer 8

Anode, cathode, electrical connection, electrolyte

Question 9

Anode

Answer 9

Gives up electrons and produces ions (cations). Is the reducing agent that undergoes oxidation.

Question 10

Cathode

Answer 10

Accepts electrons and consumes ions (cations) Is the oxidizing agent that undergoes reduction. Is rich in anions (at surface)

Question 11

Electrical connection

Answer 11

Connects and allows electrons to flow from the anode to the cathode.

Question 12

Electrolyte

Answer 12

Conducts current from the cathode to the anode. Must be in contact with anode and cathode.

Question 13

Standard conditions

Answer 13

25°C,1.0M concentration of ions in the electrolyte solution, and 1atm.

Question 14

Electrochemical potential equation

Answer 14

ΔVo = Vcathode - Vanode

Question 15

Spontaneous reaction voltage sign

Answer 15

Positive voltage

Question 16

Non-spontaneous reaction

Answer 16

Negative voltage

Question 17

Electrochemical potential

Answer 17

Helps decide where the anode is (where corrosion occurs)

Question 18

Effect of concentration on potential

Answer 18

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

Question 19

Faraday’s law

Answer 19

The total weight of metal oxidized at the anode.

Question 20

Faraday’s Law formula

Answer 20

w = ItM/nF
I = corrosion current (A)
t = time (s)
M = molar mass (g/mol)
n = valence electrons
F = Faraday constant

Question 21

Corrosion penetration rate

Answer 21

CPR = KW/pAt (mm/yr)
K = constant
W = weight loss
p = density
A = exposed area
t = time

Question 22

Corrosion rate

Answer 22

r = i/nF (mol/m^2s)
i = current density (A/m^2)
n = electrons associated with ionization
F = Faraday constant

Question 23

Types of corrosion (5)

Answer 23

Uniform attack, galvanic corrosion, crevice corrosion, pitting corrosion, intergranular corrosion

Question 24

Uniform attack

Answer 24

Corrosion attack is of uniform intensity throughout the part (EX: silverware tarnishing).

Question 25

Galvanic Corrosion

Answer 25

When two dissimilar metals are in contact with an electrolyte. Less noble metal corrodes (EX: plumbing does not connect stat to tubing).

Question 26

Crevice Corrosion

Answer 26

Corrosion, which occurs because of differences in concentration of a solution is usually because of stagnant flow conditions in a crevice.

Question 27

Pitting corrosion

Answer 27

Cause similar to crevice corrosion. Pits form on surface of material

Question 28

Intergranular corrosion

Answer 28

Corrosion occurs in preferential mode in the structure of the material. Caused by processing, impurities or welding

Question 29

Stress Cell

Answer 29

Stressed area has a higher free energy and becomes the anode. Tensile stress accelerates corrosion, whereas compressive stress slows corrosion.

Question 30

Concentration cell

Answer 30

A dissolved species involved in either the anode or cathode reaction varies in concentration at different locations in the electrolyte.

Question 31

Corrosion control methods (6)

Answer 31

1. Coatings 2. Inhibitors 3. Reduce dissolved oxygen 4. Galvanic protection 5. Cathodic protection 6. Reducing temperature

Question 32

Coatings and Inhibitors

Answer 32

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.

Question 33

Reducing dissolved oxygen

Answer 33

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.

Question 34

Galvanic protection

Answer 34

Where a more anodic metal is hooked up to the metal to be protected and dipped in the electrolyte. Commonly used in metal ships.

Question 35

Cathodic protection

Answer 35

A small DC voltage is used to impress electrons into the steel, rendering the metal cathodic. This is used in addition to protective coatings.