Week 11: Corrosion Flashcards
What is corrosion?
Destruction of a metal or alloy because of CHEMICAL or ELECTROCHEMICAL REACTIONS with SURROUNDING ENVIRONMENT or MEDIUM
Why does corrosion matter?
It degrades a material and can change its properties leading to failure
What is a requirement for corrosion to occur?
Ions that combine with the electron have to be electrically connected
Why does corrosion occur?
Because the electrolyte is in contact with the anode and cathode
Anode
Gives up electrons to the circuit and corrodes (oxidation)
Cathode
Receives electrons from the circuit (reduction)
Path
Anode and cathode need to be electrically connected
Electrolyte
The medium where metallic ions leave anode and move to cathode to accept electrons
Must be in contact with anode and cathode
What are the 2 types of iron oxides that rust is composed of?
Ferrous and ferric oxide
Reduction
OIL RIG
Occurs at the cathode
(Reduction is gain)
Oxidation
OIL RIG
Occurs at the anode
(Oxidation is loss)
Galvanic series
Ranks reactivity of metals/alloys in seawater
How does corrosion occur?
An electrochemical process where the electrons from one chemical species transfer to another
What are the forms of corrosion?
UA - UA
G - gives
C - courteous
P - people
I - in
SL - SL
E - extra
S - space
HE - here
- Uniform attack
- Galvanic corrosion
- Crevice corrosion
- Pitting corrosion
- Intergranular corrosion
- Selective leaching
- Erosion-corrosion
- Stress corrosion
- Hydrogen embrittlement
Uniform Attack
Oxidation & reduction reactions occur uniformly over surfaces.
Galvanic Corrosion
Occurs when two metals or alloys having different compositions (inert or active) are in electrical contact while exposed to an electrolyte.
This leads the anode to give electrons out to the cathode and corrodes the anodic side where the cathodic remains unharmed
Localised Corrosion
Crevice or pitting corrosion
Crevice Corrosion
Narrow and confined spaces causing high in ion concentrations which accelerate oxidation
Intergranular Corrosion
Occurs in specific alloys along grain boundaries in particular environments
Precipitates eat away at the chromium and make the material less resistant to corrosion
Pitting Corrosion
Downward propagation of small pits and holes, microns in size
Selective Leaching
Preferred corrosion of one element of an alloy.
This occurs in brass where zinc is corroded away to leave only the copper, impairing the properties of the alloy
Erosion Corrosion
Combined chemical attack and mechanical wear
Stress Corrosion
Corrosion at crack tips when a tensile stress is present
Hydrogen Embrittlement
Loss of strength due and metal becoming brittle due to Hydrogen absorbed through a metal surface
How can you prevent/protect against corrosion?
MS - might sell
C - candy
I - in
T - there
- Material selection
- Coating
- Inhibitor
- Thermodynamics method
Passivate
Metals that form thin oxide layer that slows corrosion
Coating
Paints, plastics/rubbers/glass/ceramics, metals
Inhibitor
Substances which decrease corrosiveness when added in low concentrations to the environment
What does an inhibitor depend upon?
Alloy and corrosive environment
Anode Inhibitor
Slows down the anodic half-reaction of corrosion by the metal surface become passive
What is a material called if it’s easy to give out electrons?
Active
Cathode Inhibitor
Slows down the rate of reduction reaction at a cathode
Cathodic Protection
By using a sacrificial anode or external voltage
Swelling
Liquid or solute diffuses into and is absorbed within the polymer
Dissolution
A continuation of swelling when a polymer is completely soluble
Scission
Rupture of molecular chain bonds in polymers
Why is rupture in polymers bad?
Reduces the molecular weight and lowers mechanical strength and corrosion resistance
How can bond rupture occur?
Radiation, heat or chemical reactions
What are examples of weathering?
- UV rays starts oxidation
- Water can reduce hardness and stiffness if absorbed into polymers
What is a material called if it’s difficult to give out electrons?
Inert
What do swelling and dissolution both lead to?
A polymer with a lower Tg and E, but with more viscous behaviour
How to prevent swelling and dissolution?
- Increase molecular weight
- Increase degree of crystallinity
- Decrease temperature
Why does chromium in stainless steels make them corrosion resistant as compared to carbon steel?
The chromium forms an oxide layer to prevent corrosion.
In carbon steels, rust forms instead of a protective coating,
Briefly explain why cold-worked metals are more susceptible to corrosion than noncold-worked
metals.
CW is more susceptible to corrosion because of increase in dislocation density.
More dislocation density means there’ll be a high energy state there so more corrosion can occur
What possible mechanism accounts for an inhibitor’s effectiveness?
The formation of a very thin and protective coating on the corroding surface.
Which metal is typically used in the sacrificial cathodic protection of steel?
Zinc
