Materials End-of-Life Flashcards
What is corrosion?
The destruction/deterioration of a materials as a result of reaction with its environment.
What do we usually refer to as corrosion?
The reaction of a metallic species with water or oxygen (oxidation).
Why do metals oxidise when exposed to water/oxygen?
Most metals are less stable than their oxides, hence they have a thermodynamic tendency to oxidise.
What is the rate of corrosion controlled by?
The kinetics of the anodic and cathodic reactions.
Can the anode and cathode always be separated?
No. In the case of iron in sulphuric acid both reactions happen side-by-side on adjacent parts of the surface.
What is the usual cathodic reduction of oxygen in a neutral aerated environment?
O2 + 2H2O +4e -> 4OH-
Under what conditions are Fe2+ ions stable?
Only under acidic conditions.
In alkali conditions, the ions react to form iron(II) hydroxide or oxides.
In neutral conditions hydrolyses to form H+ ions and Fe(OH)+ or Fe(OH)2
For many metals, what happens to the environment around the anode?
It becomes acidic.
Similarly, the environment around the anode often becomes alkaline.
Can we measure electrode potentials on an absolute scale?
No. It is a scale relative to the standard hydrogen electrode. It doesn’t matter on the electrode and often the standard calomel electrode is used (SCE)
What is happening when an electrode establishes an equilibrium against a reference electrode?
The cathodic and anodic reactions are taking place at the same rate. The electrons from one are being consumed by the other.
What is the reversible potential of an electrode?
The potential of an electrode when it is in equilibrium with a reference electrode.
What happens if the potential is shifted to be more positive than the reversible potential of an electrode?
The anodic reaction starts to predominate.
What happens if the potential is shifted to be more negative than the reversible potential of an electrode?
The cathodic reaction dominates.
What is the basic principle of cathodic protection?
A supply of electrons from a current lowers the potential below the reversible value thus a metals in solution stays as a metal.
Derive the Nernst equation
Check
In corrosion, we are dealing with a ‘mixed electrode’. What does this mean for the equilibrium potential?
It will be an intermediate value between the two riverside potentials of the half reactions.
How to test to make a polarisation curve?
Connect a second electrode to form a cell such that a potential difference can be applied between the two electrodes and a current is forced to flow. A polarisation curve can be plotted as we investigate how the rate of anodic and cathodic reactions are affected by the potential.
Derive the Tafel equation.
Check
Write the Tafel equations in terms of overpotential and logs.
Check
What is the point in a Tafel plot where the anodic and cathodic lines cross?
The reversible potential
What is an Evans diagram?
A plot of potential against current where polarisation curves for the cathodic and anodic reactions of species can be plotted.
What is the current when two lines cross on an Evans diagram?
The corrosion current.
On Evans diagrams, why do cathodic curves often sweep downwards at increased current?
There is oxygen depletion at the surface if the metal which causes a potential shift (concentration polarisation). The current at which this happens is the limiting current density and it is caused by the cathodic current being limited by the rate of oxygen diffusion.
What is cathodic control?
A system in which the rate of metal attack is determined by the rate of the cathodic reaction.
Give an example of cathodic control.
Steel in reinforced concrete in the sea. The rate of corrosion is determined by oxygen corrosion through the concrete (which is slow).
What is the big assumption when plotting an Evans diagram?
That there is an equipotential across the surface (may not be satisfied for a large structure or where medium around metal has high electrical resistivity).
How does pure zinc behave in acid?
It dissolves slowly in acid but quickly in alkaline conditions. This is due to a low reversible potential so lower driving force for the cathodic reaction.
Which metals are poor catalysts for the reduction of hydrogen?
Hg, Pb and Zn.
Why does impure zinc containing iron dissolve quickly in acid?
The iron in the metal surface provides a good surface for cathodic reaction.
Does lead have a fast or slow cathodic reaction in acid?
Slow therefore the rate of corrosion is slow.
What happens to corrosion of lead when it is in an acid such as sulfuric acid?
The lead forms a very insoluble lead salt that protects the metal beneath (passivation).
What happens to magnesium in HF?
It forms an insoluble fluoride.
What do base and nobel metal mean in the context of corrosion?
Base refers to the metal with the more negative corrosion potential in that medium and nobel the more positive.
When can galvanic corrosion happen?
When two metals of differing potential are in electrical contact.
Explain how galvanic corrosion happens between zinc (base) and iron (nobel).
When in electrical contact, the two metals cannot be at differing potentials so the zinc shifts upwards and the iron downwards to make a compromise potential.
At this new potential the anodic current on Zn increases (faster attack) and that of iron decreases. Conversely, the cathodic current of Zn is reduced and of Fe increased.
Although currents no longer balance on the metals, there is still a current balance in the whole system as electrons flow from the anode (Zn) to the cathode (Fe).
How can galvanic corrosion be used to protect a metal? Use the example of Zn and Fe.
The anodic current of Zn increases and that in Fe decreases when the two metals are in electrical contact.
If the relative surface of Zn is large enough compared to that of Fe, then attack on Fe ceases completely. This is cathodic protection by a sacrificial anode.
How can galvanic corrosion be avoided?
By choosing combinations of metals with a small difference in corrosion potential for a given environment.
Can reversible potentials be used to make good decisions in materials selection?
No. Corrosion potentials should be used or a galvanic series as each environment is different. For example Sn is more nobel than Fe so Sn coatings provide no protection for steel yet they are used inside food cans as in this environment Sn is more active than Fe so the Fe doesn’t corrode. The polarity has reversed and Sn is sacrificial.
Why are the corrosion potentials of Al, Ti and SS odd?
They tend to take more positive potentials than their chemistry would lead us to predict as they form a protective oxide film.
What is passivation?
The forming of an oxide film that acts as a barrier to corrosion.
How can non-metallic elements such as carbon, stimulate attack on metals, eg in steel?
They can lead to local corrosion, for example, in grey cast irons, the graphite provides cathodic sites for attack on the matrix.
How can variation in composition leading to local corrosion be useful?
In etching to reveal microstructure.
Describe why weld decay is an issue in SS.
Welding causes Cr to be depleted from close to the gbs. The bands of Cr depletion along the gbs act as anodes coupled to the Cr rich grains that act as cathodes. The gbs are rapidly removed and the alloy falls apart. Can be avoided by using low C steel or using Nb or Ti to form stabilised SS (they bind with C).
What is differential aeration?
When there is a varying exposure to oxygen caused by crevices, leaky seals etc.
What normally happens to corrosion if there is a depletion of oxygen?
The corrosion current reduces due to cathodic control.
What happens in a crevice to corrosion if there is a depletion of oxygen?
The anodic reactions inside the crevice can be supported by cathodic reactions outside the crevice.
Since the corrosion potential of the deaerated region is lower than that of an aerated region, a galvanic cell is established with current passing out of the crevice.
This results in pH falling inside the crevice and rises outside due to the hydrolysis of cations, then aggressive anions build up in the crevice driven by the electric field.
How thick is a typical passive film?
4-10nm
Describe the shape of an anodic polarisation curve for a metal that passivates.
There is an active peak at a low potential where the metal dissolves as ions. This is followed by a sudden fall in current at a critical potential which indicates the onset of passivity. The current remains roughly constant at increasing potential, due to the film thickening as potential increases but the electric field in the oxide remaining unchanged.
How can we compare how easy it is for a metal to passivate using polarisation curves?
The further to the left (lower the current) and lower the potential that the active peak occurs at, the easier it is to passivate that metal.
Derive the Pourbaix diagram for Al.
Check
Is it true that if a solid oxide is the most stable product on the Pourbaix diagram that the passivity will occur?
No!
It is true to argue though that passivity is not possible if no solid product can form though.
What condition is needed for spontaneous passivation
The cathodic reaction must provide sufficient current to surmount the active peak.
What is transpassivity?
If the potential is too high, the passive film breaks down due to oxidation of the species that form the oxide ( in SS this is Cr(III) to Cr(VI) which is soluble) thus the protection is lost.
What is reductive dissolution?
When the passive oxide species is reduced to a soluble species caused by the anodic oxidation of the underlying metal. This is seen in iron when it forms an Fe(III) passive oxide in air and it is reduced in acidic conditions to soluble Fe(II) ions.
What causes pitting?
Aggressive ions such as Cl- which bring about local failures in the protective oxide which leads to pits forming.
How does corrosion occur within a pit?
The metal within the pit acts as an anode and the surrounding passive metal is the cathode.
There is a high rate of attack inside the pit and once the pit forms, the high rate causes high concentrations of dissolved metal inside the pit.
The ions are then hydrolysed by water resulting in acidity within the pit.
Anions accumulate inside the pit due to the anode current, thus the pit is acidic and has a high Cl- conc which won’t allow a passive film to reform.
How to reduce pitting rate.
Have a high pH environment outside the pit.
Use inhibitors that impede pitting.
What happens on a polarisation curve when pitting occurs?
At a certain critical potential the current increases rapidly (this is known as the pitting potential). On a backwards scan, the repassivation potential is lower as pitting continues to occur until below a new repassivation potential.
How can inhibitive species affect critical Cl- levels for pitting?
In concrete, the inhibitive species is OH- ions that increase the pH of the pore water, thus increasing the concentration of Cl- required.
Under what humidities is atmospheric corrosion possible?
Above about 65% corrosion becomes appreciable as liquid can condense on the metal.
What are the most dangerous species for corrosion in marine and industrial environments?
Marine: Cl-
Industrial: Sulfur dioxyde which oxidises to sulfuric acid and sulfate ions that catalyse corrosive attack.
High temperature oxidation is an issue in what environments?
Refineries, boilers, nuclear power stations, gas turbines and vehicle engines.
Polymer degradation occurs under the influence of what factors?
Heat Light Microbes Chemicals Residual stresses
When can polymer degradation be beneficial?
Lower molecular weight for recycling
Pro-oxidant additives that can break chains down after use for easier recycling.
Bio-resorbable materials eg. surgical sutures.
What kinds of degradation can happen to polymers in their service lives?
Photo-induced degradation
Thermal degradation
Chemical degradation
Biodegradation
Why does photo-induced degradation occur in polymers?
The energy of sunlight radiation that reaches the surface of the Earth is energetic enough to break bonds in hydrocarbons.
What happens during photo-induced degradation of polymers?
Light is absorbed at the surface and there is homolytic fission of bonds to form free radicals. The free radicals then rapidly react with present oxygen.
What two types of degradation can be induced thermally within polymers?
Depolymerisation
Substituent reactions
What happens during thermal depolymerisation degradation?
The polymer chain backbone breaks, but the products are similar to the parent material with distinguishable monomer units.
What happens in degradation of polymers by substituent reactions?
The side groups attached to the backbone of the polymer react away, changing the repeat unit, but leaving the chain structure intact.
What happens in chemical degradation of polymers?
Usually a chemical reaction with surrounding oxygen.
What is the most common type of polymer degradation?
Oxidation of hydrocarbons. Breaking of double bonds within the polymer structure.
What is the rate of polymer degradation by the oxidation of hydrocarbons?
It is auto accelerating in that it starts slowly and gradually accelerates.
What types of chemical degradation can take place other than the oxidation of hydrocarbons?
Solvolysis, ozonolysis, chlorine induced cracking and galvanic action.
How does biodegradation of polymers occur?
Mirco-organisms secrete enzymes that promote the breakdown into lower molecular weight molecules. The enzymes act on the polymer provided it can be broken down.
What methods are there of protecting polymers from degradation with extra layers?
Scavenger theory
Protective film theory
Re-linking theory
Self-healing theory
Outline how scavenger theory can protect polymers from degradation?
A protective layer reacts with the environment instead of the polymer.
Outline how protective film theory can protect polymers from degradation?
Once the protective layer reacts with the polymer, a film is produced on the polymer surface that is inert and cannot be breached
Outline how re-linking theory can protect polymers from degradation?
The protective layer causes broken double bonds to be reformed.
Outline how self-healing theory can protect polymers from degradation?
The protective layer reacts with the degraded polymer chains to form low-molecular weight material which forms an inert film on the surface.
How can stabilisers prevent polymer degradation?
They prevent the reactions that cause degradation from happening. For example, hindered amine light stabilisers (HALS) scavenge free radicals from photo-oxidation. UV-absorbers stabilise by absorbing UV and converting to light. Antioxidants stabilise by terminating chain reactions due to absorption of UV, eg C-black in tyres.
What is one of the biggest issues with recycling polymers?
Plastics can be very different in type making separation for recycling difficult.
What are the advantages and disadvantages of reducing the types of plastics that we use?
Increase in volume of specific types would make it more economical to recycle, however it would be met by resistance from industry and governments not willing to enforce.
Describe floatation sorting of plastics for recycling.
Plastics sorted depending on whether or not they float.
Describe tirboelectric sorting of plastics for recycling.
Plastics are charged by friction and then sorted depending on if they are positively or negatively charged (only works if plastics are completely dry).
What is the most common way of treating recycled plastics in plants once sorted?
They are ground and extruded into small pellets (no change in chemistry) then melted and pressurised into a mould to resolidify into the desired shape.
How can plastics be chemically recycled once sorted?
Polymers can be broken down into monomers and later repolymerised to high-quality products. Alternatively can be broken into oligomers (shorter chains) so that original characteristics of the polymer are preserved.
What is activation polarisation?
Where one of the steps in the half-cell reactions controls the flow of charge.
What is concentration polarisation?
When electrochemical reaction is controlled by diffusion of electrolyte.
What is uniform corrosion?
When corrosive attack occurs evenly across the whole surface.
What us localised corrosion?
selective removal of material at specific areas in contact with a corrosive environment.
What initiates crevice corrosion?
Depletion of inhibitor
Depletion of oxygen
Shift to acidic conditions
Build up of aggressive ion species
What methods are there of corrosion prevention?
Materials Selection Alteration of environment Design Cathodic and anodic protection Coatings and paints
What environment is SS typically used in?
nitric acid
What environment is Ni/Ni alloys typically used in?
caustic solutions
What environment is Monel typically used in?
HF
What environment are hastelloys typically used in?
Hot HCl
What environment is lead typically used in?
Dilute sulfuric acid
What environment is Al typically used in?
Nonstaining atmospheric exposure
What environment is Sn typically used in?
Distilled water
What environment is Ti typically used in?
Hot oxidising solutions
What environment is tantalum typically used in?
Anything that requires ultimate resistance
What environment is steel typically used in?
Conc sulfuric acid
How can environments be altered to reduce corrosion?
Lowering temp (exception is boiling water is usually less corrosive) Decreasing velocity (too fast causes erosion of passive layer) Removing oxygen or oxidisers Changing conc (sometimes reducing in the case of Cl- but increasing conc of sulfuric and phosphoric acid causes them to become more inert)
What is a corrosion inhibitor?
A substance that when added in small cones to the environment decreases the corrosion rate (considered a retarding catalyst.
List the different types of corrosion inhibitors.
Adsorption-type Hydrogen evolution poisons Scavengers Oxidisers Vapour-phase Cathodic Anodic
How does an adoption type inhibitor work?
Compound that adsorbs onto the metal surface and suppresses metal dissolution and reduction reactions.
How does a hydrogen-evolution poison work as an inhibitor?
It is a substance that retards the hydrogen evolution reaction.
How does a scavenger work as an inhibitor?
It is a substance that removes corrosive reagents from solution.
How does an oxidiser work as an inhibitor?
It inhibits the corrosion of metals with active-passive transitions.
How does a vapour-phase inhibitor work?
It is a substance with a high vapour pressure that is place close to the metal and protects is by adhering to it by sublimation or condensation.
How does a cathodic inhibitor work?
Metal ions added to form a precipitate on the surface of a metal under alkaline conditions that blocks the diffusion of oxygen and causes the corrosion rate to fall.
What are the two types of anodic inhibitors and what are the differences between them?
Passivating and non-passivating.
Both cause passivation, but passivating inhibitors are oxidising agents that can act without oxygen present (eg chromate)
How can we design objects to be more corrosion resistant?
Mechanical strength requirements need to be considered with an allowance for corrosion. Generally increasing wall thickness by 2 times that of which is expected for the desired lifetime.
General rules for designing against corrosion:
Weld rather than rivet
Design to avoid stagnant areas
Design to allow components that will fail to be eaisly replaced
Avoid electrical contact between dissimilar metals
What is cathodic protection?
Where the cathode is coupled with a sacrificial anode or an external current is applied to the cathode to suppress the anodic current. If electrons are supplied to the cathode then the anodic reaction will be suppressed.
When is anodic protection used?
When cathodic protection cannot in strongly alkaline or acidic environments.
How does anodic protection work?
It is based on the idea that anodic surfaces can be polarised, then form a protective oxide layer. A potentiostat is used to keep the metal at constant potential wrt a reference electrode.
What kinds of coatings can be used to limit corrosion?
Metallic and inorganic
Organic (paints).
What is sacrificial coating?
Prevents corrosion by applying a thin layer of metal to the surface that has a lower electrode potential.
What are the most common methods of metallic coating?
Electrodeposition/electroplating Flame spraying or metallising Cladding Hot dipping Vapour coating Diffusion coating Chemical conversion Phosphatising Chromatising Galvanising Anodising
How does electroplating work?
The part to be coated is immersed in solution of the metal to be plated. A direct current passed between the part and another electrode.
How does flame spraying work?
A metal wire (or powder) is fed through a melting flame so that the metal is blown onto the surface that is to be protected. These coatings are normally porous and the surface must be roughened to obtain a good mechanical bond.
How does cladding protect a metal from corrosion?
A surface layer of sheet metal is attached to another metal by rolling the two sheets together.
How is hot dipping done to protect metals from corrosion?
Hot dip coatings applied by immersing the metals in molten metal bath of low melting point metal.
Galvanising is the most common example.
Thicker coating than electroplates
Often heat treated to improve alloy bond.
How does vapour deposition work?
Coating metal is vaporised in high vacuum chamber. The vapour deposits on the surface to be coated. Can be PVD or CVD.
How does diffusion coating work?
Heat treatment to cause alloy formation by diffusion of one metal into the other.
Parts to be coated can be packed in solid materials or exposed to gaseous environments.
How does chemical conversion protect a metal from corrosion?
It works by producing a coating by corroding the metal surface to form an adherent and protective corrosion product. They provide limited protection but are good bases for painting.
What is phosphatising?
When steel is immersed in phosphoric acid, causes hydrogen to be evolved, raising the pH and precipitating iron phosphate. Commercial phosphatic baths contain Zn and Mn cations that give more protective films.
What is chromatising?
The process of treating Al alloys and Zn by anodic attack on the metal surface with chromate solutions, forming a protective oxide layer.
How does anodising work?
Electrolytic passivation process to increase the thickness of the natural oxide layer on the surface of metal parts. Usually used on Al alloys and are sealed by immersion in boiling water.
What kinds of coatings does organic coatings refer to?
Paints, varnishes, lacquers etc.
Organic coatings are developed together with what industry?
The petroleum industry.
How do organic coatings slow corrosion by being barrier coatings?
They exhibit resistance inhibition and act as ionic filters that minimise current transfer between the anodic and cathodic sites. They can also deprive oxygen from the metal, slowing the cathodic reaction.
How can paints provide cathodic protection?
If a pigment of a more anodic metal is added to the primer, the paint can protect against current discharge to the environment.
How can inhibitive primers be used in paints?
Inhibitive species can be added to the primers to control corrosion by modifying the interfacial environment so that passivation of the substrate metal can be achieved and is maintained.
What are paints made from?
Binders that form the film that hardens.
Pigments that give colour or act as barriers or anticorrosives.
Solvent that reduces the viscosity for application.
How is a surface prepared for painting?
Cleaned and blasted to remove scales and provides good surface profile for primers.
What layers are applied in painting a surface?
Primers
Intermediate coats or undercoats
Finishes or topcoats
Problems with paints
Environmental degradation
Radiation damage
Microbial effects
Physical damage
Why is adhesion of paints important and how can water affect paints?
Adhesion is important to stop problems such as blistering.
Water lowers adhesion and when water accumulates beneath a paint, corrosion cells can develop. Paints are permeable.
How to avoid paint failure.
Correct surface prep Selection of suitable paint system Correct application Correct drying, curing and overcoating times Protection against water.
What factors affect the accessibility of remaining resources?
Environmental damage of mining
Politics/war
Economics of mining
What is important in terms of time to do with renewable resources?
The materials need to be able to renew in the same timescale as they are consumed, else they aren’t really renewable.
What is embodied energy?
(Ashby) The energy of the fossil fuel consumed to produce 1kg of material.
or
(Building and Construction) The energy consumed during the extraction, processing of raw material, manufacture materials and components plus energy used during construction and demolition.
What units of measure can be used for embodied energy?
Energy/mass
Energy/volume
What are the issues with landfill?
Toxins Leachate Greenhouse gases Land use Expensive Look ugly
Draw a typical lifecycle of a material.
Check lecture 1 pg 13
6 reasons for end of life.
End of technical life End of functional life End of legal life End of economic life End of physical life End of desirability
What things does recycling require?
Cost effective collection, sorting and recycling tech Developed markets Logistics QC recycle materials Eduction of industry and consumers Clear labelling Infrastructure for collection, sorting and reprocessing Government policy
What is a lifecycle analysis (LCA)?
An analysis of the impact of a product of its lifetime (one cycle). It doesn’t typically consider the impact at the end of its life or the limitations of input resources.
What is a lifecycle indicator (LCI)?
It is a single parameter quantified in an LCA.
How does an LCI deal with global warming?
It quantifies the effect of all the gases in a single parameter based on the effectiveness of each element and is then expressed in terms of the ‘equivalent’ of one of the chemicals.
What are sustainable materials by definition?
Materials used for consumer of industrial use that must be produced in the required volumes without depleting non-renewable resources or disrupting the establishes steady state equilibrium of the environment and key natural resource systems.
Is it easy to make sustainable materials?
No. Nearly always some input of energy in processing or transport etc.
What steps are involved with the recycling of metals?
Collection Sorting Shredding Melting Purification Solidification Transportation
Why can’t most steel that reaches market be recycled?
Demand is so high that there is not enough scrap to meet this demand. As such ‘new’ steel contains on ≈40% recycled.
How can non-ferrous metals be separated for recycling?
By using eddy current separators (rotating magnet under the conveyor) to separate metallic and non-metallic materials.
What is dross in Al production?
The formation of solid oxides in the blast furnace.
What is the difference between white and black dross?
White is mostly from primary Al production and has a higher Al purity.
Black is mostly from recycling process and has a lower Al content. Also considered toxic and hazardous waste which is banned from landfill under EU law.
What is fluxing and why is it needed in Al production?
Fluxing is the addition of salt to break the skin of dross that forms on the surface in Al production to allow the Al out. Typically a mix of KCl and NaCl.
Is the disposal of black dross economically viable?
No. But it is required by legislation
Can Al alloys be recycled and have the same use as its original?
No. There is some sorting due to source, eg aircraft (2xxx and 7xxx alloys). Often recycled Al alloys have a high Si content so are alright for casting but less food for forging and sheets. Fe and Is present challenges along with Mg, Ni and V.
How sustainable are ceramics?
Depends.
Glass recycling is well established.
Other ceramics are mostly crushed for drainage, base of roads, composite for aggregates otherwise landfill.
What are the challenges in polymer recycling?
Polymer cross-contamination
Additives
Non-polymer impurities
Degradation of properties
How could thermosets become recyclable in the future?
With degradable cross-links that respond to particular triggers
Are biodegradable polymers necessarily sustainable?
No. Many biodegradable polymers can all be made from petrochemical resources
In what order are biodegradable polymers designed to breakdown in?
Industrial composter, home composter, litter, marine environment.
