3.2 Rusting as a Redox Reaction

Corrosion of Metals
■ Corrosion of Metals

	►	Corrosion of metal is a redox reaction in which oxidation and reduction process occurs simultaneously.
	►	When a metal is exposed to air, it will be oxidized by oxygen.
	►	Metal atoms lose electrons to form ions in oxide form. ○ M → Mn+ + ne (oxidation → loss of electrons)
	►	The properties of ions formed vary from the original metal atoms.
	►	The metal is corroded.

■ This video contains information on the corrosion of metals

■ Electrochemical series and corrosion of metals

	►	The higher the position of the metal in the electrochemical series, the more electropositive metals. This metal is more likely to give up electrons to form metal ion, which is more easily corroded.
	►	Example: ○ Aluminium is more electropositive than zinc. Thus aluminium corrodes more easily.

Rusting of iron
■ Rusting of iron

	►	Rusting is the corrosion of iron.
	►	Rusting is a redox reaction in which the oxidation and reduction occur simultaneously.
	►	Requires water and oxygen.
	►	The animation below shows the stages involved in the rusting of iron.

■ This video contains information on rusting of iron.

■ This video contains information on rusting of iron.

Corrosion of metals and electrochemical series
■ Electrochemical corrosion

	►	Corrosion of a metal associated with the flow of electric current in an electrolyte
	►	Electrochemical corrosion is a redox reaction in which the oxidation and reduction occur simultaneously.
	►	When two metals with different electropositivity come into contact in the presence of an electrolyte, the more electropositive is corroded. On the other hand, the less electropositive is protected from corrosion.

■ Electrochemical corrosion and rusting of iron

	►	When iron is in contact with a more electropositive metal, rusting is protected.
	►	When iron is in contact with a less electropositive metal, rusting is accelerated.

■ This video contains information on the electrochemical corrosion.

Effect of other metals on iron rusting
■ Iron in contact with a more electropositive metal

	►	Iron does not corrode if it is in contact with a more electropositive metal such as magnesium, aluminium or zinc.
	►	Example: Galvanization of iron with a layer of zinc ○ Zinc is more electropositive than iron. ○ Zinc act as the negative terminal and releases electrons to form zinc ion, Zn2+ ◉ Zn → Zn2+ + 2e− (oxidation → reducing agent: zinc metal) ○ Electrons released flow to the iron and prevent the rusting of iron. Zinc is corroded. ○ Electron flow to iron, gained by oxygen and water to form hydroxide ions, OH− at the cathode. ◉ O2 + 2H2O + 4e− → OH− (Reduction) → Oxidising agent: oxygen
	►	Example: Galvanization of iron with a layer of zinc ○ Tin is less electropositive than iron. ○ Iron act as the negative terminal and releases electrons to form iron ion, Fe2+. ◉ Fe → Fe2+ + 2e− (oxidation → reducing agent: iron metal) ○ Electrons released flow to the tin and cause the rusting of iron occurs rapidly. ○ Electron flow to tin, gained by oxygen and water to form hydroxide ions, OH− at the cathode. ◉ O2 + 2H2O + 4e− → OH− (Reduction) → Oxidising agent: oxygen

■ This video contains information on the corrosion of metals

Laboratory Activity 3.2.1: Effect of iron nails when in contact with other metals

Preventing the rusting of iron
■ Methods of preventing rust

	►	Surface protection ○ Greasing and oiling will protect the moving parts of an engine and non-moving articles like tools. ○ Painting, plastic coating, or varnishing will protect any non-moving objects that are unlikely to be scratched. ○ Metal plating with Cr, Ag, Au, Cu, and Sn will give an attractive, corrosion resistant, metallic finish.
	►	Electrochemical protection ○ Zinc metal is generally used for protecting iron and the process is called galvanization. ○ When a zinc coating is scratched the iron is still protected since zinc is above iron in the reactivity series, and the electrons will flow from the zinc to iron.
	►	Alloying ○ By mixing iron with certain metals, alloys which are resistant to corrosion can be produced. These alloys are called stainless steels.
	►	Sacrificial protection ○ If electrons are supplied to iron, it will not rust. ○ By attaching a metal higher in the reactivity series to the iron, the metal will corrode preferentially. ○ Underground pipelines are protected in this way. Bags of magnesium scrap are attached at intervals along the pipeline.

■ This video contains information on preventing the rusting of iron.

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