Metal Corrosion

The structure of metals The arrangement of the atoms Metals are giant structures of atoms held together by metallic bonds Metallic bonds - atoms surrounded by delocalised electrons ("sea of electrons")

The structure of metals

The structure of metals The electrons can move freely within the metallic bonds each electron becomes detached from its parent atom - the electrons are delocalised The metal is held together by the strong forces of attraction between the positive nuclei and the delocalised electrons

The structure of metals The more electrons you can involve - the stronger the attractions Transition metals tend to have particularly high melting points and boiling points They can involve many delocalised electrons Metallic character increases as we move to the right of the periodic table 

The Corrosion of Metals Most metals react with their surroundings to form oxides and hydroxides E.g. copper forms copper hydroxide iron rusts to form iron oxide Sodium corrodes in air to form a layer of sodium oxide on the metal surface

Metals That Don’t Corrode Only a few metals can resist corrosion: Gold & Platinum (don’t react with oxygen) Stainless steel (iron + carbon + chromium (form stable film against corrosion)

Definitions Corrosion - is the changing of the surface of a metal element into a compound (oxide) Silver + oxygen silver oxide Rusting - is the special name given to the corrosion of iron Iron + oxygen iron (III) oxide

What Happens When Iron Rusts? Rusting involves iron atoms (metal) losing electrons to form ions. It occurs over two steps Fe(s) Fe2+(aq) + 2e- Fe2+(aq) Fe3+(aq) + e- The electrons lost from the iron are accepted by the water and oxygen 2H2O(l) + O2(g) + 4e- 4OH-(aq) oxidation oxidation

Experiment: Conditions For Rusting Dry air Boiled water + oil water salty water Experiment Air water electrolyte Did Rusting Occur? 1 yes no 2 3 Yes (small) 4

Experimental Conclusions Water and oxygen are both required for rusting to take place An electrolyte must also be present and the speed of rusting is increased An electrolyte is an ionic substance dissolved in water and provides free ions to carry a current

Corrosion and the Reactivity series Potassium Sodium Lithium Calcium Magnesium Aluminum Zinc Iron Tin Lead Copper Mercury Silver Gold The uses of metals depends on their position in the reactivity series Why don’t we make nails from potassium or gold? What are the benefits and disadvantage of having silver jewelry? Why are bridges not built from stainless steel? Increasing speed of corrosion

Corrosion and the Reactivity series

Cell Potentials We can calculate the magnitude of electron flow by measuring the voltage e- Anode (-) Cathode (+)

Allows ion flow without mixing solutions Allows ions to pass between solutions but doesn’t allow the solutions to mix

Zn(s) + Cu2+(aq) Zn2+(aq) + Cu(s) Figure 21.5 A voltaic cell based on the zinc-copper reaction Displacing electrons from zinc to copper Over time, the anode will decrease in mass, and the cathode will increase. The Cu2+ solution will also become more clear as the Cu2+ ions are reduced into solid Cu Cu2+(aq) + 2e- Cu(s) Zn(s) Zn2+(aq) + 2e- Zn(s) + Cu2+(aq) Zn2+(aq) + Cu(s)

Summary of Corrosion and reduction potential E cell = E anode – E cathode Anode (-) Cathode (+) MX (s)  M+ (aq) + X- (aq) Oxidation: lose electrons at anode (-) M  M+ + e- Reduction: gain electrons at cathode (+) X + e-  X-

Displacement Reactions Chemical reaction in which a less reactive element is replaced in a compound by a more reactive one For example, the addition of zinc metal to a solution of copper(II) sulphate displaces copper metal: Zn(s) + CuSO4(aq) → ZnSO4(aq) + Cu(s) The copper is taken out of the solution and is deposited as a solid In the electrochemical series an element can be displaced from a compound by any element above it in the series

Corrosion and the Reactivity series The more negative the metal in the series the more reactive it is (its reaction is fast and more exothermic) - it wants to lose electrons to form an oxide Therefore the reverse reaction becomes difficult (oxide -> pure metal) Hard to extract a metal from its ore (stable) The pure metal is also more susceptible to corrosion with oxygen and water

Electronegativity – the ability to gain or lose electrons

Atomic radius

Atomic radius Elements with a large atomic radius are high on the reactivity series (lose electrons easily) The number of protons increases across a period as does the effective nuclear charge Electrons within a shell cannot shield each other from the attraction to protons This causes the atomic radius to shrink (less reactive)

Summary of trends in the Reactivity Series High on activity series (-) Large atomic radius LHS of periodic table (lose electrons) Low on activity series (+) Smaller atomic radius RHS periodic table (gain electrons)

Protection Against Corrosion

What Happens When Metals Corrode? Corrosion is a chemical reaction It involves the metal atoms losing electrons Fe(s) Fe2+ + 2e- Metals corroding are examples of oxidation reactions Cu(s) Cu2+ + 2e-

Methods of Protection There are two main methods of protecting metals from corrosion: Physical protection - placing a barrier to water and oxygen on the surface of the metal Chemical Protection - providing the metal with a source of electrons to prevent oxidation

Physical Protection Plastic coating Oil and grease Paint Tin plating

Questions Plastic Oil and grease Paint Tin-plating What are the advantages and disadvantage of each type of physical protection: Plastic Oil and grease Paint Tin-plating

Physical Protection Electro-plating - coating the iron with a less reactive metal Gold, silver, chromium, and copper are a few of the metals that can be used as coatings The metal item to be plated is used as the negative (-) electrode The item is placed in a solution of the metal coating The metal in solution is reduced from ions to atoms and deposited on the metal item Power pack + - gold electrode item to be coated gold ion solution Au2+(aq) + 2e- Au(s)

Problems with Electroplating When the electroplating is broken an electrochemical cell is set up If iron is higher in the electrochemical series than the coating metal electrons flow away from the iron Rusting is speeded up Electron flow V gold iron electrolyte solution

Physical Protection Galvanizing - coating the iron with zinc The iron or steel is dipped in molten zinc The zinc provides a physical coating on the surface If the zinc coating is damaged sacrificial protection occurs. This also is a form of chemical protection (see later) This is expensive and requires special equipment to achieve

Chemical Protection When a metal corrodes it loses electrons Mg Mg2+ + 2e- Chemical protection supplies the metal with a flow of electrons by two methods: 1. Direct current power supply 2. Metal higher in the reactivity series

Direct Electrical Protection This involves connecting the iron to the negative terminal of a battery or power supply Connecting the negative terminal of a car battery to the car body slows corrosion Ocean liners use direct electrical protection when docked

Sacrificial Protection What happens when you connect a more reactive metal to a less reactive metal in a simple cell? Electron flow The electrons flow from the metal higher up the electrochemical series to the metal lower zinc iron The flow of electrons prevents the iron from rusting V zinc iron electrolyte solution Zn2+ + 2e- Zn

Sacrificial Protection Mg steel pipeline To protect underground pipes from rusting the pipe is connected to scrap magnesium by a wire The magnesium sacrificially corrodes giving electrons to the iron pipe Rusting is slowed down The scrap magnesium needs regularly replaced

Questions What happens to a metal when it corrodes? How can we prevent this loss of electrons from a corroding metal?

Questions Name the two types of corrosion prevention. What two chemicals are required for rusting to occur? What else is required for rusting to occur? Clue: It contains charged particles that allow electrons to be lost more easily from the iron

Questions What metals are used to plate steel and iron? Where do most of these metals sit in the electrochemical series? What terminal on the power pack is the metal to be coated connected?

Questions What metal is used in galvanising? What type of protection does this offer the iron/steel? Give 3 examples of galvanising being used to prevent rusting. Which metal is higher in the electrochemical series - iron or zinc?

Questions When does electro-plating prevent rusting? When does electro-plating cause rusting to occur faster? When rusting occurs what metal is losing electrons? What metal is being protected from corrosion?

Questions What do you use to provide direct-electrical protection to a metal surface? Are there any disadvantages to this method? Give two examples of where this is used

Questions Where must the metal used for sacrificial protection be on the E.C. series? What is the next most suitable metal for sacrificial protection after zinc? Which metal would provide the best protection out of these two?

Testing For Protection Against Rusting Set up the following experiments: Half fill the petri dish with warm agar solution. Add 5 drops of ferroxyl indicator and 3 drops of phenolpthalein indicatior into the dish. Gently stir. Place a nail in each dish as shown below. Ferroxyl indicator + iron nail Ferroxyl indicator + iron nail wrapped in magnesium Ferroxyl indicator + iron nail wrapped in copper

Testing For Protection Against Rusting After leaving the experiment for 20-30 minutes, draw a before and after diagram. What did you observe happening? Ferroxyl indicator + iron nail Ferroxyl indicator + iron nail wrapped in magnesium Ferroxyl indicator + iron nail wrapped in copper

Testing For Protection Against Rusting Set up the following experiments: Half fill the petri dish with warm agar solution. Add 5 drops of ferroxyl indicator and 3 drops of phenolpthalein indicatior into the dish. Gently stir. Place a nail in each dish as shown below. Ferroxyl indicator + iron nail Ferroxyl indicator + iron nail wrapped in magnesium Ferroxyl indicator + iron nail wrapped in copper