Chapter 9 - Electrochemical Cells Flashcards
What is Corrosion?
Corrosion is an electrochemical process (REDOX) in which a metal is oxidised in the presence of agents like water and oxygen gas.
Water is important, Oxygen is the oxidising agent.
When metals corrode to form an oxide that is crumbly and brittle, making the material is no longer suitable for its original use.
Extremely costly problem.
- not sustainable, greenhouse gas emissions.
Which types of cells corrode?
More reactive metals (HIGHER OXIDATION POTENTIAL) like Na, Ca, Mg, and Fe show a greater tendency to corrode than those of lower reactivity (eg. Cu, Ag, Pt and Au).
- Despite this, some quite reactive metals like Al, Cr and Zn show little or no effects of corrosion.
- DUE TO THE NATURAL FORMATION OF PASSIVATING LAYER, A THIN PROTECTIVE OXIDE COATING THAT FORMS UPON EXPOSURE TO AIR
- Al2O3 on Al
- Cr2O3 on Cr
This layer excludes oxygen and water from the underlying metal thus inhibiting its ongoing corrosion.
Even if the passivating layer is scratched or damaged it simply self-repairs on further exposure to water and air.
What is Dry corrosion?
Direct corrosion = Dry corrosion
Direct reaction with oxygen in the air to form a metal oxide
Metals affected by dry corrosion or not.
Sodium - so reactive it must be kept in oil to prevent contact with oxygen.
Aluminium - forms a tough impervious coating of aluminium oxide, which protects the metal underneath from further contact with oxygen.
- the aluminium window frame is lightweight and corrosion-resistant.
- therefore, it is an excellent material in situations where maintenance would be difficult.
Iron - much less reactive than Na or Al; dry corrosion of iron is LOW AT ROOM TEMP.
- when iron does corrode, it forms a coating that flakes off readily, leaving the metal underneath exposed to further corrosion.
What is wet corrosion?
The presence of moisture accelerates the corrosion of iron.
- process called wet corrosion.
It can occur in moist air or by direct immersion of water.
Several factors affect the rate at which wet corrosion occurs.
In general, What accelerates wet corrosion?
- the presence of water
2. impurities such as salt and acidic pollutants that dissolve in the water.
Simply, What is the process of wet corrosion?
- The conducting iron allows for electrons to travel between the anode (anodic site) and the cathode (cathodic site)
- At the anode, oxidation occurs and at the cathode, reduction occurs.
Explain the process of Rust
Step 1: Iron is oxidised to form Fe+2 ions at one region on the iron surface.
- at the same time, at another region on the surface, using the electrons produced by the oxidation process, oxygen is reduced in the presence of water to hydroxide ions.
Step 2; The formation of a precipitate of Iron (II) hydroxide
Step 3: Further oxidation of iron(ii) hydroxide occurs in the presence of oxygen and water to produce iron(iii) hydroxide, a red-brown precipitate.
Step 4: In air, the iron(iii) hydroxide loses water to form hydrated iron (iii) which is known as rust.
- The porous rust easily flakes off the surface of the iron underneath to further oxidation.
Step 5: the wet corrosion of iron is accelerated if the water contains dissolved ions.
- particularly evident in coastal areas where iron objects are exposed to seawater.
- the presence of dissolved ions in water increases its conductivity, which results in an increase in the rate of iron corrosion.
What are the factors affecting the rate of corrosion of iron include:
- Oxygen; this is the OXIDISING AGENT.
- It must be present for corrosion to occur
- higher the concentration of oxygen, the faster the rate of corrosion. - Water; the presence of water GREATLY ENHANCES THE RATE OF CORROSION
- enables dissolving ions to flow efficiently between the CATHODIC and ANODIC regions that form during corrosion. - pH; the lower pH of water (more acidic) the greater the rate of corrosion.
- This happens as the REDUCTION POTENTIAL of OXYGEN INCREASES as the pH DECREASES.
- the reduction potential for oxygen in an aqueous solution increases as the pH decreases.
* **This means oxygen reduction and hence corrosion, is more favourable in an acidic environment. - Less reactive metals;
where irons contacts less reactive metal, it corrodes faster.
- eg. screwing a copper pipe to a steel frame makes the steel frame corrode faster - Electrolytes: provide an efficient salt bridge contact between the anodic and cathodic sites of corrosion.
- their presence increases the rate of corrosion.
eg. rusting in or near marine environments is much faster than away from these salty areas. - More reactive metals; contact with more reactive metal prevents ions from corroding.
- eg. bolting a piece of magnesium to a ship’s steel hull prevents the hull from corroding. - Temperature; as with any reaction, a higher temperature leads to a higher corrosion rate.
List the ways of preventing corrosion of metals
by the use of
- galvanising metals
- surface protection
- use of Cathodic protection
- use of a sacrificial anode
Explain Cathodic protection using a sacrificial anode
- Sacrificial anodes are made of metals more reactive than iron
eg. Zn, Al or Mg - The sacrificial anode is BOLTED to the IRON needing protection and must be in salt bridge contact with iron.
- They only work in VERY DAMP OR WET ENVIRONMENTS such as water storage, tanks, ship hulls or underground pipelines.
- The process works as the MORE REACTIVE METAL of the sacrificial ANODE is OXIDISED in preference to iron.
- As the sacrificial anode SLOWLY OIXDISES IT RELEASES ELECTRONS ONTO THE IRON SURFACE MAKING IT CATHODIC, SO PREVENTING ITS OXIDATION
( only reduction can occur at the cathodic site) - As the sacrificial anode is eventually fully oxidised, it must be replaced for the protection to continue.
Explain the process of Galavnising to reduce corrosion.
- It requires little or no maintenance and its protection lasts for many years.
- It involves coating iron with more reactive metal ZINC, (E for oxidation of Zn= 0.76 V, While E for Fe =0.44V)\
- As with non-metallic and inert metallic coatings this method works by excluding O2 and H2O.
- As Zinc oxidises in preference to iron, due to higher oxidation potential, it will protect the underlying iron even if the zinc is scratched and the iron is exposed to O2 and H2O.
- –This effect is known as cathodic protection. - Also the Zinc surface naturally forms a passivating layer of ZnCOZn(OH)2 which protects the zinc layer from excess oxidation.
Explain cathodic protection using DC current to prevent corrosion.
- Involves connecting iron or steel structure to the negative terminal of a low voltage DC circuit.
- As the structure is supplied with electrons (made cathodic) it cannot oxidise (the only reduction occurs at the cathodic site).
- The positive terminal of the DC power supply is attached to a piece of scrap iron, the anode, which over time becomes oxidised.
- other types of anode, like those made of carbon, high silicon steel or platinum, are longer lasting.
- With platinum anodes, water is oxidised instead of the anode, so these last indefinitely. - For cathodic protection to be effective, the anode must be in salt bridge contact with the iron structure being protected.
- Thus a very damp or wet environment is essential, eg. steel wharf piers, underground pipelines, water tanks and ship hulls,
Explain inert non-metallic coating for the prevention of corrosion.
Apply an inert non-metallic coating TO THE IRON OR STEEL STRUCTURE.
- eg GREASE, PAINT, PLASTIC, OR PORCELAIN
this works by excluding O2 and H2O from iron, thus preventing reduction and so oxidation will not occur either.
This is ONLY EFFECTIVE WHILE THE BARRIER IS INTACT.
- ONCE SCRATCHED, CORROSION WILL COMMENCE.
Explain inert Metallic coatings for the prevention of corrosion.
Inert metallic coatings, eg. Copper and Tin Coatings, Can also be used to control corrosion.
These work by excluding O2 and H2O from the iron surface.
However unlike non-metallic coatings, if the metallic barrier is damaged CORROSION OCCURS FASTER THAN NORMAL.
- this happens because the iron BENEATH IS MORE REACTIVBE THAN THE INERT METAL COATING.
The LESS REACTIVE METALLIC COATING CAUSES THE EXPOSED IRON TO BECOME ANODIC AND OXIDISED MORE QUICKLY THAN IT OTHERWISE WOULD.
