Title: Introduction to the iron corrosion.
Objective: To learn how rusting occur and how to prevent rusting.
2567167854710Corrosion of metal, a redox reaction which a metal is oxidized naturally with the releases of electrons to form metal ions. For instance, iron (Fe) releases electrons to form iron (II) ion (Fe2+), the iron said to have corroded, Fe Fe2+ + 2e- . The metals which situated at a higher position in electrochemical series are electropositive. The more electropositive metals will corrode easily. Aluminium (Al), zinc (Zn), lead (Pb), nickel (Ni) and chromium (Cr) will corrode rapidly in the air and caused the formation of thin layer of oxide on its surface which can prevent the metals from further corrosion. Iron (Fe) and magnesium (Fe) form oxide coating. This oxide coating is easily peeled off so it cannot protect the metals from corrosion.
The corrosion of metals is a redox reaction as it involved oxidation and reduction simultaneously. Oxidation is a chemical reaction in which a substance loses of electrons; whereas reduction is a chemical reaction in which a substance gains of electrons.
2989690881269The corrosion of iron also known as rusting of iron. The rusting of iron is a redox reaction and it will occur when the iron come in contact with water and oxygen. This is the equation that shows the rusting of iron: iron + water + oxygen hydrated iron (III) oxide
left447200The centre of the water droplet act as anode (negative terminal). The iron (Fe) loses electrons and is oxidized to iron (II) ions (Fe2+). The Fe2+ ions will dissolve in water and then corroded.
3156392163168Anode (oxidation): Fe Fe2+ + 2e-
Next, the released of electrons by the Fe flow to the edge of the water droplet through the iron. The concentration of dissolved oxygen at the edge of water droplet is higher than the centre of the water droplet. At the edge of water droplet which will act as cathode (negative terminal). The dissolved oxygen and water will received the electrons released by the Fe, then reduced to hydroxide ions (OH-).
2558885184785Cathode (reduction): O2 + 2H2O + 4e- 4OH- Then, the Fe2+ ions and OH- ions combine to form green solid which is known as iron (III) oxide, Fe(OH)2.
1754974167005The equation: Fe2+ + 2OH- Fe(OH)2 .After that, the Fe(OH)2 is oxidised to a brown solid which is known as hydrated iron (III) oxide, Fe2O3 . xH2O (rust) by the oxygen in the air. The equation of the reaction:
174122624511000 4 Fe(OH)2 + 2 H2O + O2 Oxidation 4 Fe (OH)3
8507901875192 Fe (OH)3 Fe2O3 . xH2O
Moreover, there are ways to prevent rusting. The metals can be electroplated by tin, zinc, nickel and so on which act as protection for the metals. For example, the cans of food is covered with a thin layer of tin which provide a protective oxide coating to the cans. Next, alloying can change the metal to become more resistant to corrosion, have a shinier and attractive appearance and so on. For instance, alloying the iron with chromium and nickel will produce stainless steel which is used as surgical instruments, cutlery and so on. The stainless steel contains 18% of chromium and 8% of nickel provide a protective oxide coating. Alloy is a mixture of two or more elements with certain fixed composition in which the major component is alloy. The example of alloy are pewter, brass, steel, and so on. Finally, oil and grease is used as protective coating for movable machine parts. For example, applying oil and greases on the bicycle chains can prevent it from rusting because of the oil can prevent the oxygen and water vapour in the air in contact with the bicycle chain.
Observation and Result:
left34544000 Before Experiment:
Cup 1: Iron nail in 5ml of tap water and without cover.
Cup 2: Iron nail in 5ml of saturated salt water and without cover.
Cup 3: Iron nail in 10ml of 0.02M hydrochloric acid, HCL and with cover.
Cup 4: Iron nail in 10ml of dilute ethanol and with cover.
After Experiment (Observation):
left939800Cup 1: The tip of iron nail was rusted, some brownish rust on the wall of the cup, a little yellowish on the bottom of the cup and the tap water was evaporated completely.
left43008900Cup 2: The head and tip of the iron nail was rusted, many brownish precipitate formed in the solution and many salts are formed.
Cup 3: The whole iron nail was rusted and the solution become yellowish.
Cup 4: The tip of the iron nail was rusted a little.
left10096500Cup A: Magnesium ribbon in 10ml of copper sulphate (CuSO4), with cover and many bubbles was formed..
Cup B: Magnesium ribbon in 10ml of sodium chloride (NaCl), with cover and a little bubbles was formed on its surface.
Cup C: Magnesium ribbon in 10ml of potassium chloride (KCl), with cover and a little bubbles was formed on its surface.
Cup D: Magnesium ribbon in 10ml of zinc chloride (ZnCl2), with cover and many bubbles was formed.
After Experiment (Observation):
left361600Cup A: The magnesium ribbon dissolved incompletely, grey precipitate and white precipitate were formed
2978154635500Cup B: The black dot was on the surface of the magnesium ribbon, bottom part of the magnesium ribbon was corroded and a little white precipitate was formed in the solution.
Cup C: The whole magnesium ribbon became black.
Cup D: The magnesium dissolved completely, many grey and white precipitate were formed.
At the beginning of the experiment of magnesium ribbon, the most bubbles formation on the magnesium ribbon was the magnesium ribbon in copper sulphate as the displacement reaction take place. The magnesium is more reactivity than copper, thus it react with water to form hydrogen gas. The bubbles formed on magnesium ribbon is hydrogen gas. After one week, the tap water in cup 1 was evaporated completely due to the open container as liquid in an open container will evaporate until no liquid is left. The saturated salt water in cup 2 was evaporated slower as compared to tap water in cup 1 although both were without cover as the salt completely dissolved in the water and caused the vapour pressure in the water become lower. The lower the vapour pressure, the lower the ability of water molecules to escape into the air. There are many cubes of salt crystallise from the saturated salt water. The hydrochloric acid turned into yellowish due to the rust. The magnesium ribbon in cup D dissolved completely and the solution was cloudy as compared with the solution before the experiment.
-16851243278809-1739044258665945062371196560793757429500The iron nail in HCl was the most rusted; whereas the iron nail in C2H6O was the least rusted. HCL is a strong acid, the pH is 3, caused the rusting of Fe in the absence of air as the continuous evolution at cathode. The lower the pH, the higher the rate of rusting. The equation of Fe react with HCl: Fe + 2HCL FeCl2 +H2. The increasing order of rusting of iron nail in different solution are in C2H6O, H2O, NaCl, HCl. Next, the Mg in ZnCl2 was dissolved completely and grey precipitate is Zn and white precipitate is MgCl2 were formed as Mg is more electropositive than Zn, Mg displace Zn from ZnCl2 solution. The equation of Mg react with ZnCl2: Mg+ZnCl2 MgCl2+Zn. The Mg in KCl did not dissolve because Mg is less electropositive than K, thus Mg cannot displace K from KCl solution. The equation of Mg react with KCl: Mg+2KCl MgCl2+2K.
-2621970872794-2065959530253The increasing order of reaction of magnesium in different solution: KCl, NaCl, CuSO4, ZnCl2. The equation of Mg react with NaCl: Mg+2NaCl+2H2O MgCl2+2NaOH+H2. The equation of Mg react with CuSO4: 2Mg + 2CuSO4 2MgSO4 + Cu2.
There are factor affect the rate of corrosion. The rate of corrosion can be increased by the concentration of oxygen. Although in an oxygen-deficient environment, the metals will be rust slowly. The higher the concentration of oxygen, the higher the rate of corrosion. The temperature affect the rate of corrosion. The rate of corrosion at normal temperature will be faster than low temperature. The higher the temperature, the higher the rate of corrosion as at higher temperature, the higher the kinetic energy between the particles. The surface of the metals affect the rate of corrosion. The larger the surface of the metals, the more the oxygen and water in contact with the metals, the higher the rate of corrosion. The rate of corrosion will be higher in the medium of acidic compared to neutral and alkaline. The lower the pH, the higher the rate of corrosion. The rate of corrosion in the medium of pH more than 10 is very slow as the protective coating of hydrous iron oxides is formed. Furthermore, the precaution step for iron nail and magnesium ribbon before the experiment are both iron nail and magnesium ribbon should be clean to prevent the impurities which will affect the result of the experiment. Then, make sure all the cup have clean and dry properly. Don’t touch the rusted iron nail directly with wound on bare hand because it will cause tetanus, a serious bacterial infection. Finally, gloves and lab coat should be wear during the experiment.
The iron nail in acid solution will be rusted rapidly as compared with tap water, saturated salt water and ethanol. The magnesium ribbon will be completely dissolve if it come into contact with metals which is less electropositive than it in electrochemical series.
When an iron nail is placed in zinc sulphate solution, the iron will rust.
2541325189257Anode (Oxidation): Half-equation: Fe Fe2++ 2e- .
3479524167640Cathode (Reduction): Half-equation: O2 + 2H2O + 4e- 4OH- .
2368164181969Overall equations: 2Fe + O2 + 2H2O 2Fe2+ +4OH
Mastering Chemical Help. Electrochemical Theory of Rusting. Available on:
http://entrancechemistry.blogspot.com/2010/10/corrosion-corrosion-is-process-in-which.html Accessed 6 September 2018
Thought.Co. How Rust and Corrosion Work. Available on:
https://www.thoughtco.com/how-rust-works-608461 Accessed 6 September 2018