CORROSION Flashcards
In a corrosion scenario in which there is only one metal present, where will corrosion typically occur?
In the corrosion of a single metal, the anode site will form either…
- Where the lattice is disrupted (e.g. metal has been scratched, bent, welded, cut, etc) as electrons are more readily lost (and if a metal loses electrons its being oxidised or corrode
- At the site of lowest oxygen concentration (e.g. underground, beneath the paint, at the tail end of the screw) as a consequence of DIFFERENTIAL AERATION: when there are regions of differing oxygenation.
What is corrosion?
Corrosion is the destructive oxidation of metals caused by oxidants (usually O2 and water) in the surrounding chemical environment.
What are the two types of corrosion?
The two types of corrosion are distinguished by whether there is water present:
Wet corrosion:
- Electrochemical cell in which products form rust
Oxidation of iron half equation
Reduction of water half equation
Then products bond to form rust Fe(OH)2(s) or Fe(OH)3(s).
Dry corrosion:
- Very slow reaction
- 4Fe(s) + 3O2(g) -> 2Fe2O3(s)
What is the formula for red rust?
Fe2O3xH2O(s)
What are the properties of rust?
Rust is an ionic lattice, so it’s…
- Weak
- Brittle
- Absorbs water
- Porous
- Greater volume than original metal
Colour depends on degree of dehydration.
Compare this to metal which is…
- Strong
- Ductile
- Malleable
- Conductive
Where does rust form?
Rust forms when the products of the oxidation of iron (Fe^2+ ions) and the reduction of water (OH^- ions) meet in between reduction and oxidation sites and form a precipitate (Fe(OH)2(s) which will oxidise further to Fe2(OH)3(s).
Because Fe2+ ions are more mobile rust tends to appear closer to the anode.
In answering a question about the rust formed you would ALWAYS add the red/ox half eqs.
Where will corrosion occur if there are two metals in contact?
If two metals are in contact the stronger reducer will be preferentially oxidised and so be the anode. The weaker will be the cathode (where WATER and oxygen are reduced).
What increases rate of corrosion?
- Two metals being in contact (e- transfer more efficient through metallic lattice)
- Strength of reducers/oxidisers: reactivity of metals (stronger reducers are oxidised more readily aka they oxidise faster.
- O2 and H2O availability (both MUST be present)
- Metallic lattice being disrupted (e- more readily lost)
- More surface area exposed
- Increased temperature (see C6)
- Increased availability of salt (which increases efficiency of electrical conductivity through a solution and acts as an effective salt bridge.
How can corrosion be prevented?
- Surface protection
- Alloys
- Electrochemcial protection (sacrificial anodes)
- Impressed current
What is the key principle of electrochemical protection?
Electrochemical protection is a form of corrosion prevention in which a reducer with a higher oxidation potential (readiness to lose e-) than the metal to be protected is added to the system in some way, be it through painting a zinc layer or through burying a block of the stronger reducer underground adjacent to the metal to be protected and connecting them by a wire.
This stronger reducer will then be preferentially oxidised: give equation, so the weaker reducer (the protected metal) will form the cathode site for the reduction of water and oxygen give equation and the target metal will thus remain intact.
How does impressed current work as a corrosion prevention method?
By installing an impressed current to a system we supply electrons to the metal to be protected, and given this excess of elecrons they are gained in the reduction of water and oxygen give equation thus making it a cathode as the metal can’t be further reduced, while electrons are usually lost at an inert electrode connected by a wire. Thus, metal remains intact.
When corrosion occurs across two areas of differing oxygen concentrations (e.g. at the base of a screw or underground vs aboveground or underwater vs above water) what do we say this is a consequence of?
DIFFERENTIAL AERATION!!!
How does surface protection prevent corrosion?
Surface protection prevents corrosion by blocking contact of reactants (Fe(s), O2(g) and H2O(l) and preventing electron transfer and ion migration, hence preventing a redox reaction from occurring.
What are some types of surface protection?
- Paint
- Noble coating (a weaker metal than protected metal, purely serves as covering).
- Grease (e.g. for moving parts)
- Tin cans: actually steel cans covered by a layer of tin
- Galvanising: Zn(s) covering
Why do metal things corrode more quickly on the coast?
Increasing availability of salts, O2 and water, may be warmer.
- Saltwater in corrosion is an electrolyte and effective salt bridge: allows ion migration, completes the circuit.
