Corrosion

Question 1

Oxidation reaction

Answer 1

• When a metal loses electrons + becomes positive - takes place at anode + can be gain of oxygen/loss of H
• Material removed from anode (corrosion)

Question 2

Reduction reaction

Answer 2

• Element gains electrons + becomes more negative - site of reduction is cathode + can be loss of oxygen/gain of H
• Material added to cathode (electroplating)

Question 3

Standard half cell

Answer 3

Pure metal electrode immersed in 1M solution of its ions at 25 degrees C

Question 4

When are potentials calculated for EMF series valid?

Answer 4

For 1M solution at 25 degrees C + requires 0 current

Question 5

Does Nernst equation require 0 current?

Answer 5

Yuh

Question 6

Polarization

Answer 6

Change in electrode potential caused by some effect - when electrochemical cell doing work + current flowing through it the potential of each electrode changes due to polarization

Question 7

Overvoltage

Answer 7

Difference between closed + open circuit voltage

Question 8

Activation polarization

Answer 8

Potential difference beyond value of equilibrium needed to generate currents depending on energy activation of redox rxn - over voltage is driving force for rxn

Question 9

Concentration polarization

Answer 9

Part of polarization of electrolytic cell resulting from changes in electrolyte concentration due to passage of current through electrode/solution interface - affects cathode only and when diffusion is rate limiting

Question 10

Passivity

Answer 10

When thin oxide film forms as barrier to corrosion - occurs only on anode

Question 11

Passive region

Answer 11

Oxide layer forms on surface which prevents passage of current

Question 12

Transpassive region

Answer 12

Potential is high enough to break oxide layer

Question 13

8 types of corrosion

Answer 13

Uniform, galvanic, crevice, pitting, intergranular, selective leaching, erosion-corrosion, stress corrosion

Question 14

Galvanic corrosion

Answer 14

Occurs when 2 metals w/diff composition are electrically couples while being exposed to electrolyte

Question 15

How to avoid Galvanic corrosion?

Answer 15

Choose metals close in galvanic series, avoid unfavourable anode-cathode area ratios, electrical insulation + connect to a more anodic metal

Question 16

Crevice corrosion

Answer 16

Occurs because of concentration differences in ions - occurs in region of low concentration

Question 17

Pitting

Answer 17

Concentration difference driven like crevice corrosion but corrosion pit forms as deep well driven by gravity

Question 18

Intergranular corrosion

Answer 18

Occurs at grain boundaries often when metals are heated

Question 19

Selective leaching

Answer 19

When 1 element of alloy is selectively removed by corrosion process

Question 20

Erosion corrosion

Answer 20

Occurs from combined action of chemical attack and mechanical abrasion from fluid motion - especially harmful to alloys that normally passivate as it removes protective layer

Question 21

Stress corrosion

Answer 21

When metals that normally resist certain corrosive environment corrode when stress is applied in addition to corrosive environment - small cracks form + propagate in direction perpendicular to stress

Question 22

Hydrogen embrittlement

Answer 22

• Reduction in ductility + tensile strength when atomic H penetrates structure of material - failure mode
• May be induced by cathodic protection

Question 23

General corrosion prevention

Answer 23

• Select material that doesn’t corrode in application environment
• Make environment less corrosive
• Use chemical corrosion inhibitors in environment which chemically bind surface + prevent corrosion
• Make parts of interest a cathode

Question 24

Cathodic protection

Answer 24

Create electrical connection to more anodic metal which will preferentially corrode - can be forced by applying external potential

Question 25

Pilling-Bedworth ratio

Answer 25

Evaluates volume of oxide vs volume of metal it’s replacing

Question 26

P-B ratio < 1

Answer 26

Oxide takes less volume than metal

Question 27

P-B ratio = 1

Answer 27

Oxide takes same volume as metal

Question 28

P-B ratio > 1

Answer 28

Oxide takes greater volume than metal

Question 29

Kinetics of well-adhering films

Answer 29

Oxide growth limited by ionic diffusion described by Fick’s law - growth kinetics for these is parabolic

Question 30

Kinetics of films that are porous/flake off

Question 31

Kinetics of very thin oxides growing close to room temperature

Answer 31

Logarithmic growth kinetics