3: Corrosion

Question 1

corrosion of metals

Answer 1

the destructive and unintentional attack on a metal based on oxidative electrochemical processes involving liquid electrolytes

Question 2

inverse of corrosion

Answer 2

electroplating

Question 3

electroplating vs corrosion

Answer 3

electroplating: metal ions to metal
corrosion: oxidation

Question 4

electrochemical reaction

Answer 4

a chemical rxn involving electron charges. it involves pairs of rxn which are called RedOx rxn.

in our case, it involves solids, electrons, and electrolytes (soln containing mobile ions).

can happen in two separate location, connected by a conductor.o

Question 5

oxidation happens where?

Answer 5

anode

Question 6

reduction happens where?

Answer 6

cathode

Question 7

describe oxidation

Answer 7

• at anode
• an element, often metal, loses electrons (creating free electrons) in an oxidation rxn.
  thus, they become more positive.
• an oxidation rxn can also be the Gian of oxygen or loss of H.
• material is removed from the anode (corrosion)

Question 8

give example of oxidation rxn (Al, w 3e-)

Answer 8

Al -> Al^(3+) + 3e-

Question 9

describe reduction

Answer 9

• at cathode
• an element or molecule gains electrons (capturing free electron) in reduction rxn
  thus, they become more negative
• red rxn can also be loss of oxygen or gain of H
• material is added to cathode (electroplating)

Question 10

give example of reduction rxn (Al, w 3e)

Answer 10

Al^(3+) + 3e- -> Al

Question 11

def electrochemical rxn

Answer 11

electrochemical rxns involve the flow of electrons that “mediates” two sides of a reactions allowing it to occur.

Electrical contact is essential.

it is driven by thermodynamics

Question 12

name two *well-controlled different ways you can have electrochemical rxn (the set-up)

Answer 12

• same metal with a battery driving force
• different metals which produce a potential

Question 13

name a less controlled way an electrochemical rxn can occur

Answer 13

corrosion in pipes, corrosion of zinc in acid soln

Question 14

what is the reference cell, what metal do we use ?
why do we need it ?

Answer 14

• the standard hydrogen half cell
• need: in order to predict the potential of any galvanic couple, the potential of each standard half-cell is measured against a reference half cell: the standard hydrogen half cell

Question 15

def standard half cell

Answer 15

pure metal electrode immersed in a 1M soln of its ions at 25C

Question 16

EMF series: all rxn are represented as what type of rxn?

Answer 16

reduction

Question 17

the potentials for emf are valid in what conditions ? What can we use if these conditions aren’t met ?

Answer 17

emf: 1M soln at 25C, zero current

the Nernst equation provides the cell potential and different temperatures and ion concentrations. it still requires zero current.

Question 18

galvanic series is for what conditions

Answer 18

in sea water

Question 19

same electrode material, two concentrations, which corrodes (high or low) ?

Answer 19

lower concentration will corrode to produce more electrons to even out the concentration of electrons

Question 20

what parts of metals corrode fastest? if a metal has a tear how does this change corrosion ?

Answer 20

corners have lower electron concentration, thus, they corrode faster

at bottom of tear, less electrons, corrode faster

Question 21

if a metal plate is in a soln of its ions, not connected to anything, what happens ?

Answer 21

technically nothing on macro scale

on micro scale, constant equilibrium between ox and red rxns.

r,red = r,oxid = io/nF
io = exchange current density which is an internal current corresponding to the equilibrium rate of the red and ox rxn

Question 22

def polarization

Answer 22

change in electrode potential caused by some effect

Question 23

when an electrochemical cell is doing work and current is flowing through it, what happens to the potential of the electrodes ? why ?

Answer 23

the potential of each electrode change bc of polarization effects

Question 24

def overvoltage

Answer 24

the difference between closed circuit voltage (Vw) and the open circuit voltage (Vemf)

n = Vw-Wemf

Question 25

activation polarization

Answer 25

electrochemical reactions are surface reactions which include a series of steps, one of which is rate limiting. activation is related to the concept of passing the energy barrier for the slowest step. no overvoltage = no net rxn overvoltage = driving force for rxn

Question 26

def concentration polarization. when is it an issue ? what does it affect ?

Answer 26

exists when the reaction rate is limited by diffusion in solution. if the reaction rate is low or the concentration of species is high, diffusion is not rate limiting. However, when the rate is high or concentration is low, three may be a depletion zone next to electrodes. affects only the cathode.

Question 27

study corrosion rates from polarized data

Question 28

def passivity

Answer 28

some metals form a thin oxide film which serves as a barrier to corrosion (mainly chromium, iron, nickel, titanium, alloys) this occurs at anode only

Question 29

def passive region

Answer 29

oxide layer forms on surface, preventing passage of current

Question 30

def transpassive region

Answer 30

the potential is high enough to break the oxide layer

Question 31

name forms of corrosion

Answer 31

1. uniform 2. galvanic 3. crevice 4. pitting 5. intergranular 6. selective leaching 7. erosion-corrosion 8. stress corrosion

Question 32

def uniform attack

Answer 32

- form of corrosion relatively homogeneous corrosion over the whole surface

Question 33

def galvanic corrosion. avoided yb what ?

Answer 33

occurs when two metals or alloys having different composition are electrically coupled while being exposed to an electrolyte

Question 34

how can u avoid galvanic corrosion

Answer 34

- choosing metals that are close in galvanic series (low emf potential) - avoid unfavourable anode-cathode area ratios (have a large anode) - electrical insulation - connect to a their, more anodic, more metal

Question 35

def crevice corrosion

Answer 35

corrosion that occurs because of the concentration differences in ions: occurs in regions of low concentration

Question 36

def pitting corrosion

Answer 36

concentration difference driven like crevice corrosion, however the corrosion pit forms as a deep well, driven by gravity

Question 37

def intergranular corrosion

Answer 37

occurs at grain boundaries often when metals are heated between 500C and 1000C often associated with welding

Question 38

def selective leaching

Answer 38

happens when one element of an alloy is selectively removed by a corrosion process. ex: dezincification of brass leaving porous copper with poor mechanical properties

Question 39

def erosion-corrosion corrosion

Answer 39

occurs from the combined action of chemical attach and mechanical abrasion from fluid motion. especially harmful to alloys that normally passivate (removes protective layer)

Question 40

define stress corrosion

Answer 40

some metals which normally resist a certain corrosive environment may corrode when stress is applied in addition to the corrosive environment. small cracks form and propagate in a direction perpendicular to the stress. the stress does not need to be externally applied.

Question 41

def hydrogen embrittlement

Answer 41

not exactly corrosion but a form of degradation. some metal alloys, specifically steel, experience significant reduction in ductility and tensile strength when atomic hydrogen penetrate the structure of the material. it is typically described as a failure mode. may be induced by cathodic protection.

Question 42

how to prevent corrosion

Answer 42

- select material that doesnt corrode in application environment - make environment less corrosivve - use chemical corrosion inhibitors in the environment which chemically bing the surface and prevent corrosion - use of protective coating - make parts of interest a cathode

Question 43

cathodic prevention

Answer 43

create electrical connection to a more anodic metal which will corrode preferentially (Zn, Mg). the metal to be oxidized is called a sacrificial anode, can also be force by applying an external potential (rectifier)

Question 44

surface oxidation

Answer 44

oxide-reduction reactions are also possible in solid-gas environment (without liquid electrolyte). the oxidation of a metal by atmospheric oxygen is called dry corrosion to obtain good conformal oxide coating, volume of oxide should be the same as volume of metal it is replacing

Question 45

dry corrosion

Answer 45

oxidation of a metal by atmospheric oxygen

Question 46

pilling-bedworth ratio evaluates what

Answer 46

to obtain conformal oxide coating, the volume of the oxide should be the same as the volume of the metal it is replacing

Question 47

surface oxidation: PB ratio is: A. <1 B. =1 C. >1

Answer 47

A = oxide takes less vol than metal B = oxide takes same volume C = oxide take greater volume than metal

Question 48

kinetic of surface oxidation: well adhering films, growth is limited by.... growth kinetics is what shape/relationship

Answer 48

for well adhering films: oxide growth limited by ionic diffusion, which is described by Fick's law. The growth kinetics for these oxides is parabolic

Question 49

for surface oxidation, for films that are porous or flake off, oxidation rate = ?, growth is ? PB is ?

Answer 49

oxidation rate constant growth is linear PB is <1 or >2

Question 50

for surface oxidation, for very thin oxides (<100nm), growing close to room temp, what growth kinetics are observed

Answer 50

logarithmic