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.
