1. Engineering Chemistry CHM 406
Corrosion
Engineering Chemistry
CHM 406

2. What is corrosion
A reaction between a material and its environment which results in a deterioration of the material and its properties.
Usually applies to metals, such as iron (Fe).
For metals, this is an electrochemical process – oxidation.
Metal is converted into oxides and hydroxides, in the presence of O2 and H2O.
Carbonates and other salts can also sometimes form, of CO2 / HCO3- or other anions are present.

3. Consequences
Every year, corrosion costs the world’s economy tens of billions of dollars.
Corrosion is a spontaneous process – cannot be completely stopped, only slowed down and minimised.

4. Factors promoting corrosion
Moisture
O2, other oxidising agents.
Acidity (low pH)
The presence of “aggressive” anions, such as Cl- pr SO42-.
Corrosion can take place in the absence of moisture, in dry atmosphere, but only at high temperatures (“dry corrosion.”)
Wet corrosion can take place in air, immersed in water, or in the soil.

5. Rusting of iron

6. Electrochemistry of rusting
Certain parts of the iron surface act as anodes.
Dents, imperfections, areas of stress.
Areas covered in water droplets, in which the cations can dissolve.
Anodic half-reaction:
Fe (s) → Fe2+ (aq) e-
The electrons travel through the metals to regions acting as cathodes, in contact with higher levels of O2 (e.g., edge of water droplets).

7. Cathodic half reactions
One of several possible half reactions can occur.
Acidic conditions (pH = 1-2, i.e., very low):
2 H e- → H2(g)
Slightly acidic conditions (pH = 3-6)
O2(g) + 4 H+ + 4 e- → 2 H2O
Neutral or basic conditions (pH = 7-9)
O2(g) + 2 H2O + 4 e- → 4 OH-

8. Further reactions
The Fe2+ ions migrate through the water to the cathode. They can be further oxidised in the presence of O2.
Fe2+ → Fe e-
The Fe3+ further reacts as follows.
Fe OH- → Fe(OH)3 (s)
Fe(OH)3 (s) → FeO(OH) (s) + H2O
2 Fe(OH)3 (s) → Fe2O3 (s) H2O
Also formed are FeO, Fe3O4 (Fe2O3.FeO), and hydrated oxides.

9. Inhibition of corrosion
Passivation: protective coating of metal oxide.
Use of corrosion inhibitors.
Inert protective coatings: paints and varnishes. These act as a barrier between the metal and O2.
Sacrificial anodes.
Coatings of other metals, e.g., galvanised iron.

10. Passivation
This is the formation of a tight film of a non- conducting metal oxide on the surface of the metal.
This prevents O2 from approaching the metal surface and/or electrons from the metal being transferred to O2.
Al and its alloys are protected from corrosion by such a layer of Al2O3; will not corrode despite being so high (active) on the electrochemical series.

11. Stainless steel
Steel is an alloy (solid solution of metals) of Fe with C and selected other metals.
Stainless steel contains Cr.
Like Al, Cr forms a film of oxide (Cr2O3) on the surface that passivates stainless steel and protects it from corrosion.
Electroplating a layer of Cr on to steel can also provide protection against corrosion.

12. Corrosion inhibitors
These slow down either the anode reaction (dissolution of the metal into metal ions), or the cathode reaction (reduction of O2), or both.
The most widely used inhibitors are
Chromate (K2CrO4): oxidises and passivates the surface of the metal; in the case of Fe, Cr2O3 is deposited.
Phosphate (Na3PO4): Deposits a protective film of Fe3(PO4)2 or FePO4 on the steel surface.

13. Sacrificial anodes
If the metal, e.g., Fe, is electrically connected to a piece of another metal which is more reactive in the electrochemical series, e.g., Zn or Mg, the latter functions as the anode, tending to oxidise; the Fe then becomes a negatively charged cathode, and is prevented from oxidising until the Zn or Mg is used up.
The piece of Zn or Mg is called a sacrificial anode, since it is consumed while preventing the Fe from rusting.
If the Fe is connected to a less reactive metal, e.g., Cu or Sn, it becomes an anode, and is more reactive towards corrosion. Especially a problem with Al.

14. Galvanising and metal coatings
Coating a metal such as Fe with another metal such as Zn (e.g., galvanised iron) or Cr (chrome plating) provides two types of protection.
Barrier to prevent contact with O2, including passivation.
In the event of the coating being scratched, sacrificial anode protection.
Achieved by coating with molten metal (galvanising with Zn) or electroplating.