Chapter 5: Corrosion

Question 1

why do metals corrode?

Answer 1

metals want to have a more stable state

Question 2

types of attack mechanism

Answer 2

chemical attack
electro-chemical attack

Question 3

what is direct chemical attack?

Answer 3

aka oxidation

current or electrons travel through the metal for only a short distance

Question 4

what is electro-chemical attack?

Answer 4

there is anodic and cathodic area in presence of electrolyte

current or electrons flow through more distances than oxidation

Question 5

what is an electrolyte?

Answer 5

liquid containing ions or charged particles

Question 6

difference between anodic and cathodic area

Answer 6

anodic area is more reactive (area where it corrodes) than cathodic area

Question 7

why do oxidation occur in metals?

Answer 7

some metal form oxides to protect the metal (Cr, Al)

Question 8

requirements for electrolytic corrosion to occur

Answer 8

1) presence of electrolyte in contact with metal

2) potential difference between anode and cathode

3) conducting path for electrons

Question 9

which side is corroded and protected? and which is -ve and +ve?

Answer 9

anode will be corroded (-ve)

cathode will be protected (+ve)

Question 10

definition of anode and cathode

Answer 10

anode: oxidation occurs (give out electrons)

cathode: reduction occurs (uses up electrons)

Question 11

anodic reaction formula

Answer 11

M –> M⁺⁺ + 2e^-

Question 12

cathodic reaction formula

Answer 12

H2O + 2e + 1/2 O2 —> 2(OH)^–

Question 13

resulting reaction formula

Answer 13

M²⁺ + 2(OH)^– —> M(OH)₂

Question 14

how is rusting of steel controlled by?

Answer 14

• moisture
• dust (hygroscopic)
• pollutants (SO₂, turns in to H2SO4 to attack the steel)

Question 15

difference between oxidation and electrolytic corrosion

Answer 15

oxidation: dry corrosion
- occur at higher temperatures
- short distances

electrolytic corrosion: wet corrosion
- occur in the presence of water/moisture
- longer distances

Question 16

what is uniform corrosion? and what causes it?

Answer 16

most common form, least damaging

caused by direct chemical attack (acids, oils)

occurs uniformly over the entire surface

Question 17

what is localised corrosion?

Answer 17

all other forms of corrosion (except direct chemical attack)

occurs at specific locations

spreads quickly, makes the steel brittle

Question 18

when does galvanic corrosion occur, and what are the requirements?

Answer 18

occurs when 2 dissimilar metals are in contact

requires the presence of:
electrolyte
conductive path

Question 19

what is the emf series?

Answer 19

ability of metal to resist corrosion (higher better, more corrosion resistance)

gold (more cathodic)
platinum
silver
copper
HYDROGEN
Tin
Nickel
Cadmium
Iron
Zinc
Aluminium
Magnesium (more anodic)

Question 20

what is galvanic series?

Answer 20

ranking of metals relative to their electrochemical reactivity in sea water

Question 21

rate of galvanic corrosion depends on?

Answer 21

difference in pd in the emf series, further away the 2 metals are, greather the pd, faster the corrosion will occur

area ratio:
large anode area (uniform corrosion, slower rate)
small anode area (localised corrosion, faster rate)

Question 22

where is crevice corrosion found?

Answer 22

with stagnant solution, occurs on gaps

Question 23

how is crevice corrosion formed?

Answer 23

crevices form in inaccessible corners by bad design which reduces oxygen within the area (becomes anodic)

Question 24

how does crevice corrosion work?

Answer 24

concentration cell form due to difference in oxygen concentration in electrolyte

more oxygen - cathodic
less oxygen - anodic

Question 25

how can crevice corrosion be prevented?

Answer 25

- welding instead of rivet or bolts - using non-absorbing gaskets - remove accumulated deposit - design to avoid stagnant areas

Question 26

what is pitting corrosion?

Answer 26

localised attack produces small holes or pits in metal difficult to detect (pits covered by corrosion products) very destructive

Question 27

causes of pitting corrosion

Answer 27

started by a break in oxide film water factors: acidity low oxygen concentration high concentration of Cl

Question 28

how does stress corrosion occur?

Answer 28

effect is accelerated when operating with high stress

Question 29

causes of stress corrosion

Answer 29

pd occurs when stress levels are different area with higher stress become anodic often in cold worked materials

Question 30

how to prevent stress corrosion?

Answer 30

- removing stress through design (round corners) or through annealing - remove/neutralise the environment

Question 31

what is intergranular corrosion?

Answer 31

localised attack on or beside grain boundaries

Question 32

causes of intergranular corrosion

Answer 32

caused by precipitates (Chromium Carbide) that are concentrated at grain boundaries, making it anodic

Question 33

how to prevent intergranular corrosion?

Answer 33

- high temp. HT to redissolve chromium carbide particles - rapid cooling in CCR - lower carbon content to < 0.03% - add stabilisers (Nb, Ti)

Question 34

what is microbiological corrosion?

Answer 34

corrosion caused by biological organisms (fungi, bacteria)

Question 35

how does microbiological corrosion work?

Answer 35

organisms grow on metallic surface area beneath organism (anodic)

Question 36

causes of microbiological corrosion

Answer 36

- chemical attack on metals by the by-products of microbes - microbial attack on organic materials

Question 37

corrosion control methods

Answer 37

1) correct choice of material 2) design to minimise effect of corroding agents 3) cathodic protection 4) use of protective coatings 5) environmental control

Question 38

types fo alloy for corrosion control and its effect

Answer 38

Cu: resistance to acid and salt solutions Cr, Al, Si: forming passive oxide film Ti & Nb: resistance to intergranular corrosion Mo: resistance to crevice and pitting

Question 39

what does a good design mean to prevent corrosion?

Answer 39

to prevent any liquids from staying and making contact with the part (allow it to spill off)

Question 40

what is cathodic protection?

Answer 40

supplying the material with electrons and forcing it to be a cathode

Question 41

what is sacrificial anode and how does it work?

Answer 41

sacrificial anode corrodes instead of the material attached to it - sacrificial anode supplies electrons to the material - prevent anode reaction at material - Zn/Mg - will be consumed and must be replaced

Question 42

3 main type of coating for corrosion?

Answer 42

metallic organic inorganic

Question 43

types of metallic coating

Answer 43

1) electroplating 2) coating in liquid state 3) cladding

Question 44

types of inorganic coating

Answer 44

1) ceramic 2) glass

Question 45

types of organic coating

Answer 45

1) paint 2) varnish 3) lacquer

Question 46

environmental control methods to prevent corrosion

Answer 46

1) lowering temperature (corrosion rate increases with temperature) 2) decrease liquid velocity (fast flowing fluid can cause erosion corrosion) 3) remove oxygen from fluids (corrosion rate increases with oxygen concentration) 4) reduce ion concentration 5) adding inhibitors to electrolytes (slow down anodic or cathodic reaction)

Question 47

how does adding inhibitors to electrolytes slow down corrosion?

Answer 47

- forming thin film / bulky precipitate to coat and protect it - combine with corrosion product to form passive layer