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CHEE 380 - Materials Science > 3: Corrosion > Flashcards

3: Corrosion Flashcards

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1
Q

corrosion of metals

A

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

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2
Q

inverse of corrosion

A

electroplating

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3
Q

electroplating vs corrosion

A

electroplating: metal ions to metal
corrosion: oxidation

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4
Q

electrochemical reaction

A

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

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5
Q

oxidation happens where?

A

anode

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6
Q

reduction happens where?

A

cathode

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7
Q

describe oxidation

A
  • 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)
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8
Q

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

A

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

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9
Q

describe reduction

A
  • 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)
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10
Q

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

A

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

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11
Q

def electrochemical rxn

A

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

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12
Q

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

A
  • same metal with a battery driving force
  • different metals which produce a potential
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13
Q

name a less controlled way an electrochemical rxn can occur

A

corrosion in pipes, corrosion of zinc in acid soln

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14
Q

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

A
  • 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
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15
Q

def standard half cell

A

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

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16
Q

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

A

reduction

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17
Q

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

A

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.

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18
Q

galvanic series is for what conditions

A

in sea water

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19
Q

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

A

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

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20
Q

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

A

corners have lower electron concentration, thus, they corrode faster

at bottom of tear, less electrons, corrode faster

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21
Q

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

A

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

22
Q

def polarization

A

change in electrode potential caused by some effect

23
Q

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

A

the potential of each electrode change bc of polarization effects

24
Q

def overvoltage

A

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

n = Vw-Wemf

25
Q

activation polarization

A

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

26
Q

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

A

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.

27
Q

study corrosion rates from polarized data

A
28
Q

def passivity

A

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

29
Q

def passive region

A

oxide layer forms on surface, preventing passage of current

30
Q

def transpassive region

A

the potential is high enough to break the oxide layer

31
Q

name forms of corrosion

A
  1. uniform
  2. galvanic
  3. crevice
  4. pitting
  5. intergranular
  6. selective leaching
  7. erosion-corrosion
  8. stress corrosion
32
Q

def uniform attack

A
  • form of corrosion

relatively homogeneous corrosion over the whole surface

33
Q

def galvanic corrosion. avoided yb what ?

A

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

34
Q

how can u avoid galvanic corrosion

A
  • 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
35
Q

def crevice corrosion

A

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

36
Q

def pitting corrosion

A

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

37
Q

def intergranular corrosion

A

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

often associated with welding

38
Q

def selective leaching

A

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

39
Q

def erosion-corrosion corrosion

A

occurs from the combined action of chemical attach and mechanical abrasion from fluid motion.

especially harmful to alloys that normally passivate (removes protective layer)

40
Q

define stress corrosion

A

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.

41
Q

def hydrogen embrittlement

A

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.

42
Q

how to prevent corrosion

A
  • 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
43
Q

cathodic prevention

A

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)

44
Q

surface oxidation

A

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

45
Q

dry corrosion

A

oxidation of a metal by atmospheric oxygen

46
Q

pilling-bedworth ratio evaluates what

A

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

47
Q

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

A

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

48
Q

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

A

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

49
Q

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

A

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

50
Q

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

A

logarithmic

CHEE 380 - Materials Science (6 decks)

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