Corrosion Flashcards
What is an electrochemical reaction
A reaction which involves charge transfer (electro) and formation of a new compound from chemical elements
What is required for charge transfer to occur and what functions to fill this requirement
Charge transfers requires a charge carrier
+ve → ions
-ve → electrons
(T/F) metal ions do not act as a charge carrier for positive charge
F
(T/F) In an electrochemical cell, something losses change and something gains charge
T
Parts of an electro chemical cell
1) electrodes → anode, cathode
2) electrolyte: solution where electrical conduction is carried out by ions
3) electrical contact→ wire
Anode____ electrons, cathode ___ electrons
Losses, gains
Anode = ___. Cathode=____
Oxidization, reduction
What can be an electrolyte
Any thing that allows the flow of ions
What can be an electrical contact
Any thing that allows electrons to flow (current)
How much of an electrochemical cell is submerged
Every thing but the wire/electrical contact
Which way do electrons flow and why
From anode to cathode
Cathode has higher electron affinity
What does current meassure
Charge
What does voltage measure
Electrical potential difference
(T/f) since there is a current, there is a voltage
T
Spontaneous def
The direction a reaction proceeds in naturally
What is required to proceed with an unspontaneous reaction
Induced charge
As electrons more from ___ to ___ through ___ , ions move from ___ to ___ through ___.
Anode
Cathode
Electrical contact
Anode
Cathode
Electrolyte
What is a half-cell reaction
A reaction which describes a element of intent in relation to a standard cell, allows us to compare elements of interest
How to tell anode vs cathode on an EMF table
More positive = cathode = wants electrons
What form is the EMF series in
Reduction form, typically based on a standard hydrogen electrode
What is a standard electrode
An electrode that is used as a reference to compare reactions or elements against
Describe SHE
Standard hydrogen electrode
Electrolyte of I mol/L [ ] of metal ions
298 k
Saturated with 1 atm of hydrogen gas
Platinum electrode
E not = 0.0 v
(T/f) liquid is compressible
F
Does potential change if conditions change from standard conditions? How do we account for this?
Yes, nerst’s eg
Examples of standard electrodes
Standard hydrogen electrode
Silver-silver chloride ( agci)
Calomel +0.244 V
Ag Cl standard electrode vs SHE
+ 0.222 V
What does the degree symbol mean?
Standard conditions
What are standard conditions?
298.15 K→ 25 C
I mol/ L cathode ions near cathode and same for anode
1 atm pressure
Voltage units
V= j/c
E meaning and relation to spontaneous
E = electrical potential
Positive = spontaneous
Delta g?
Gibs free energy, energy change in a system
>0 = cathode = absorbs energy
<0= anode = release energy
What is n
N = # of electrons transferred
F
Fairiday’s constant
Total energy change per mol of elections
What does the sign of delta G depend on
E
Corrosion def
Gradual deterioration of materials over time due to chemical or electrochemical reactions with surrounding environment leads to dissolution or formation of non-metallic scale or film
Results in deterioration of properties optical, physicals anything that effects function)
Occurs in metals and ceramics
Why is corrosion bad?
Leads to premature failure
Degradation of mechanical properties
Decrease of material integrity and thus structural strength
Costs money to fix things when they break
Ex of corrosion
Rust (iron)
Patina ( copper)
Oxidization
Chemical attacks
Anodizing
Can corrosion ever be stopped?
No, there will always be current, it can only be greatly slowed down
High level, how to slow down corrosion
Proper design, material selection and processing
Nickname for corrosion
Environmental attack
Degradation def
Corrosion in polymers due to internation with it’s environment
Requirements for corrosion (an electrochemical cell) to work
1) materials with different desire for elections (E not cell not equal to 0)
2) path for electron transfer
3) path for ion transfer
Concentration of solid species
Always 1 since they do not change
Can nearst eq be used for half-cell reactions
Yes be careful using it in reduction form
What are the two main ways corrosion can be categorized
By uniformity
By mechanism
Categories of uniformity of corrosion
Uniform corrosion → slow (slow ro corrosion), easy to spot (large area)
Localized corrosion → fast (fast ro corrosion), difficult to spot (small area)
Mechanisms of corrosion
Electro chemical cell
Galvanic Cell
Multiphase / impurity cell
Intergranular corrosion
Grain-grain boundary cell
* Stress-driven corrosion
Concentration cell
Crevice corrosion
Pitting corrosion
* microbial corrosion
What is galvanic corrosion
Two metals or alloys with different E in electrical contact with each other in a corrosive electrolyte
Is essentially an electrochemical cell
What is the most common electrolyte
Water
(T/f) dropping water on a part of a cell would not start concentration cell corrosion
F
Why does measure electrical potential between two electrodes sometimes not and up to the theoretical value
Water tends to be very active as an electrode and can react to beach an electrode, replacing ore of the existing electrodes
What ions can water be split into
H+ and OH-
What happens when water reacts instead of the intended electrodes
Instead of plating the other electrode, the anode its will react with H+ or OH- to create a gas
What determines which water ion interferes with a reaction
The presence of oxygen
Oxygen-free = H+
Aerated = OH-
What is the reaction quotient for half-cell reactions
1/ [ ion]
Are high ph substances corrosive? What about basic?
Ph = power of hydrogen = # of hydrogen ions
High ph = more hydrogen ions = more corrosive since more hydrogen electrode to react
Same premise for basic on an OH- scale
What is a multi-phasic or impurity cell
A electrochemical cell is created between different phases of an alloy
This creates an area of fast localized corrosion
Common in alloys since more alloys rely on multiplphases
Localized corrosion takes over and the alloy is no longer protected
*electrochemical potential difference created due phases with different potentials
What is sanitization
Multi-phasic cell corrosion in stainless steel
Usually occurs due to poor heat treatment
Anode = ferrite (2+) or austensite
Cathode = cemented (OH-)
What is grain-grain boundary cell corrosion
- potential difference created by pot diff that exists between grains and GB
It is very small but is enough to cause corrosion
Ex: etching
In a grain-grain boundary cell, what are the electrodes
G.B = cathode
Grain = anode
What is intergrainular corrosion
Preferential attack on GB due to potential difference created by multiphasic cells and grain-grain boundary cells
Applies to all metals
(T/F) Intergranular Corrosion is the localized from of grain-grain boundary corrosion
F, opposite (at GB)
What is a concentration cell
A cell created due to a difference in concentration of something which creates a E
occurs naturally
Often accompanied by stagnation
What is a concentration cell
A cell which causes corrosion due to low or high concentrations of something
The driving force for the redox reaction is the concentration difference of an ion.
Occurs naturally
(T/F) E diff determine corrosion rate
F
What is crevice corrosion
Concentration cells created by the trapping of ions in small crevices
Crevices must be large enough for electrolyte to get in
Stagnation occurs
What is stagnation
When it is difficult for ions to get out creating a rapid cycle of corrosion occur due to the little movement
(T/F) Temperature difference can lead to diff in E
T
(T/F) Stress changes E not
T
What is pitting corrosion
A type of concentration cell corrosion
A extremely localized corrosion due to scratched and composition variations
Once started, it acts as crevice corrosion
How do we know which chemical form (for example different charges) reacts as the electrode?
Compare the overall energy change of the systems
Higher E = more likley
Ensure comparison is made under same conditions
What are e-ph diagrams
Plots the likley hood of each form of a element or compounds based on likley hood by ph and E
Basically a plot of Nernst eq for each reaction based on what’s more favourable for the overall reaction
Only favourable reactions are shown, less energetic forms don’t react and thus don’t show
What is passivation
Formation of a protective jacket or film which limits corrosion
Does passivation stop corrosion
No, it limits it by slowing it down
Corrosion current is not 0
Does passivation always create a protective film
No, the film may be protective and stick to the surface it may be too small or too big and not protect it
If the film is too small, protection is not achieved
If the film is too big, the film flakes
What is the Peanut butter ratio
Pilling Bedworth Ratio (P-BR)
Measure of the susceptibility of a metal to oxidization and corrosion
Compares vol change when a metal is oxidized and it’s normal volume
Determines how likley a metal is to form a protective film (passivation)
How to interpret the PBR
PBR < 1 - porous oxide, corrosion may continue
1 <= PBR <= 2 - protective jacket
PBR > 2 0 chance of flaking or exploliate
Steps to check for Passivation
1) Will it form a compound based on E- is it spontaneous?
2) check PBR
Does the E change in a solution that is not water
Yes, substances may become more anodic or catholic depending on the solution used as an electrolyte
Likley hood of corrosion changes
What is the relationship between PBR and E of a cell
Describes how the driving force of a raction
What is the driving force
Describes if a reaction is favourable to go forwards
Does not describe how fast or how long it will take, just if the reaction will occur
What is Thermodynamics
How likley a RX is to go forwards
Not time related
Measure by V
Basically the driving force
What are the kinetic aspects of corrosion
The rate of reaction
How long and how fast
What is the corrosion current
Number of atoms leaving the anode per time
What is the current density and units
Current normalized per unit area
A/m2
C/S
Assumptions made to calculate rate of corrosion
Assume flats surface with uniform distribution of electrons on surface- uniform corrosion
What does CPR mean and assume
Change in thickness per time
Assumes unifrom corrosion over entire areas- same assumption as corrosion rate
What is an acceptable corrosion limit
0.5 mm/yr
CPR for very low corrosion
0.0254 mm/yr
CPR for low corrosion
0.25 mm/yr
CPR for fairly low corrosion
0.5
CPR for high corrosion
1.27
E units
V
Delta G units
J
F units
C/S
M unit
G/mol
P units (density)
G/vol
Current density units
A/m^2
Corrosion rate r unit
Mol/m^2s
CPR units
Mm/yr
What is stress driven corrosion
E is created due to residual stress
The stress changes local potential, creating a pot diff
Requires corrosion and stress to work
High stress area is anode
Low stress is cathode
(T/F) Elastic and plastic strain change E
T
What is microbial corrosion
Corrosion caused by excretions of bacteria as they form corrosive products
Products lead to concentration cells
Leads to localized corrosion by nucleating a pit
What type of bacteria cause microbial corrosion
Iron-related bacteria
Acid-producing bacteria
Sulfate-reducing bacteria
How to prevent corrosion
Remove the needed components:
E
Ion pathways
Pathways for e
How can dissimilar electro-potential be prevented
Proper material selection- minimizes basic galvanize cells
Proper heat treatment - minimized stress driven cells and multi-phasic cells
Proper design and manufacturing - minimizes geometries with crevices
Remove electrical contract by insulating diff materials
Use a smaller area of exposed dissimilar materials - slows down CPR
Avoid rivets - prone to crevice corrosion - same material tho
Avoid solder or braze - prone to galvanic cells as they are diff material from original - may cause stress driven corrosion
Why does a lemon battery stop working
When lemon juice is used up, no more H plus supply so no more [] diff and thus no more E diff
Welding vs Rivet vs Brase/Solder
Welding is the best if done properly
Avoid rivets - prone to crevice corrosion - same material tho
Avoid solder or braze - prone to galvanic cells as they are diff material from original - may cause stress driven corrosion
How can electrolyte contact be limited
Anodizing
Coatings : tight sealing paint, oil, grease
Use inhibitors : WD:40
Prevent water pooling
Seal up gaps and crevices
What is anodizing
forcibly increase thickness of passive film by using power supply to force higher rate
What is Glavanization
A process which can be done to steel
Slows down the rate of corrosion
Sacrificial anode but for steel
When in a corrosive environment, Zn is forced to be the sacrificial anode and steel becomes the cathode
Zn reacts again to for ZnO and protect again
Requirement for anodizing
Metal with a PBR of between 1 and 2 must be passivated
What is sacrificial anode protection
A metal is coated onto another metal with the idea that in a corrosive environment, the coating will become a sacrificial anode to act as a layer of protection for the cathode
The anode reacts, the ions get taken by the air and thus the coating weakens until its gone
What is a requirement for galvanization and sacrificial anode for it to work well
Large anode to cathode ratio
Slows down the RX further
What is catholic protection
Involves either using a sacrificial node or an applied potential (inert cathode)
Passive vs active catholic protection
Passive = natural - sacrificial anode
Active = need constant applied power supply to drive current - applied potential
What is inert cathode
Also called applied potential
Active form of catholic protection
Ex : electronic rust inhibitor module for cars
Essential undoing the reactions which produce rust or other forms of corrosion
What is swelling
Absorption of liquid (solvent) or a solute (polymer)
How does swelling effect polymers
Makes it softer, more ductile
Lowers Tg
Grows in size
chain does not break chains
How does chain length effect properties of molecules
Increase chain length = increase elastic modulus and yield strength
How does swelling effect the properties of a polymer
No change, since chains aren’t broken, no change in properties other than volume
What is dissolution
Occurs after swelling
Chains are disentangled
May break chains
Thus may lower elastic modulus and yield strength
When is swelling and dissolution likley to happen to polymers
When the solvent and polymer have high similarity in chemical simularity
What is scission
Severance of molecular chain bonds
Can be due to
Radiation (x-ray, gamma, UV) - ionization of bonding e - ionizing energy is the energy that can break up chains
Chemical reaction
Thermal effect- due to elevated temperature- when this occurs depends on the melting temp of the polymer
What is weathering
Degradation of polymers in outdoor conditions
Ex: oxidation, UV radiation, moisture
What are the steps to degreation
Swelling, dissolution, scission
Why do we want materials to be durable but not too durable
We want it to be strong and last but we need to be able to dispose of it at the end of its life cycle
What is embodied energy
Energy needed to create 1 kg of stock material
What is LCA
Life cycle analysis
Accesses the environmental impact throughout the life cycle of the product
What LCI
Life cycle inventory
A list of natural material used and their emissions
What is the material life cycle
In a endless circle
Product manufactured — product used — end of life —- material produced
Natural products can be added to the material production stage
Raw material refining - manufacturing - transport - use- end of life (disposal, recycling)
What counts as emissions in a material life cycle LCI
Co2
NO2
SOX
Particulates
Toxic waste
Low grade heat
Biodegradable
Once in the landfill, material can quickly return to the earth in its original state through interaction with its environment in a mearurable amount of time
Recyclable
Materials that can be reprocessed at the end of life and returned to the life cycle as in a different product
What are the types of recycling
Open loop recylcing
Close loop recylcing
Co-firing
What is open loop recycling
Recycled product re-enters back into the same system
What is close loop recycling
Recycled product re-enters into a pool and distributed across multiple systems
What is co-firing
Incineration to produce energy for other uses
Only some (very little) of the embodied energy is recuperated
Re-cycling processes from more environmentally friendly to least
Closed loop
Open loop
Co-firing
(T/F) Most metals are not biodegradable
F
Most are biodegradable since it can be corroded by various chemicals found in nature 0 thought this takes a long time and may create things that are toxic for wildlife
(T/F) Metals are recyclable
T
RE-melting and hence re-casting and or reforming metal into new parts or products is an option but still needs energy
Why is recycling of metals still a concern for sustainablity
Needs energy
(T/F) With recycling, all energy is recovered
F
Only a portion of embodied energy can be recovered
Why is recycling metals possibly not advantageous
Metals and alloys have a very specific composition
Anything less or extra is an impurity which may effect the properties of the metal and thus make it considered poor quality
Makes recycling difficult
What is idle at
An app that creates a simplified LCA
Describes eco cost of producing a product
Shows us how environmentally unfriendly something is
What is eco-cost
Money needed to prevent the environmental burden of producing a product, material or process
Why is light weighting beneficial and disadvantagoues
Using lighter (Less dense) materials requires less energy to transport and use
But may be lower is strength/ toughness so it may require strengthening
T/F galvanic corrosion is a type of electrochemical cell
T
