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Investigatory Project on Metal Coupling






Rust is formed when iron article remains in contact with oxygen and water containing dissolved carbon dioxide for a few days. As a result of this, the iron article gets corroded and destroyed. Rust is hydrated ferric oxide, Fe2O3 .xH2O. Rust easily peel off exposing fresh surface for further action. A number of factors affect corrosion. Coupling with a metal (impurity) in the electrochemical series is a deciding factor. Because of the impurity present in iron, a galvanic cell sets up between the two metals - one metal becomes the cathode and the other anode. In other words corrosion of metal is a slow electrochemical process. The metal which becomes anode goes in solution and thus corrodes. Suppose copper is present as impurity in iron and iron is placed in an atmosphere of moist air. Then corrosion will take place according to the following mechanism.


 Fe  Fe2+ +2e- (anode, oxidation)


Copper, which is less electropositive element than iron will behave as cathode. The electrons from the above anode reaction move towards the cathode and form hydroxyl ions as follows:


2H2O + 2e-  H2 + 2OH- (cathode, reduction)


Hydroxyl ions so formed will combine with iron ions and form ferrous hydroxide.


Fe2+ + 2OH-  Fe (OH)2


Ferrous hydroxide is further oxidised to ferric hydroxide, by the oxygen of the atmosphere and rust or hydrated ferric oxide results.


If a more electropositive metal (Zn, Mg. Al, etc.) than iron is present as impurity, then that particular metal will become anode and passes preferentially into the solution. Thus the reaction, Fe  Fe2+ +2e- does not occur and hence rusting does not take place.


Apparatus required:


Two petridishes with covers, beaker (250 ml), glass rod, iron nails, sand paper, gelatin, copper wire, zinc strip, aluminium wire and magnesium ribbon.


Chemicals required:


Powdered agar-agar (2.5g), water, 0.1 M potassium ferricyanide solution, 0.1% phenolphthalein and carbon tetrachloride




Clean five iron nails by rubbing with sand paper. Then wash the nails with carbon tetrachloride till bright shining surfaces are obtained. Dry the nails in an oven at 100°C. Then wind a clean copper wire around one nail, a clean zinc strip around the second, a clean aluminium wire on the third and a clean magnesium ribbon on the fourth. Place one nail (without coupling any other metal) and four other nails coupled with different metals in two petridishes as shown in figure in such a way that they do not touch each other.

Heat 5 g of agar-agar in 100 ml of water in a breaker till whole of gelatin dissolves in water and a clear solution is obtained. Add a small crystal of potassium ferricyanide and two drops of 0.1% phenolphthalein solution to it. Stir the contents with a glass rod and pour it carefully into the two petridishes to cover the nails to a depth of about 2 mm. Cover the dishes with the lids and leave them undisturbed for a day. Formation of two patches around a nail indicates rusting of it. Observe the colour and its intensity on the patches formed around each nail. Blue coloured patch is formed by the interaction Fe2+ ions with potassium ferricyanide which indicates anodic region while pink patch formed from the interaction of hydroxyl ions, produced in the corrosion process, with phenolphthalein indicates cathode region. Intensity of the colour indicates the extent of rusting.





Metal couple

Colour of patches

Rusting occurred or not


Iron nail without coupling






















Rusting of iron increases on coupling with less electropositive metals like copper and decreases on coupling with more electropositive metals like zinc, aluminium and magnesium.


 [Preparation of agar-agar solution: Heat about 2.5 g of well powdered agar-agar in about 150 ml of distilled water in a beaker and heat it to boiling (agar-agar is a gelatin like material obtained from certain seaweeds). Stir the solution well till the solid agar-agar is dispersed completely in the solution. Use the solution while hot].


Viva Questions on Effect of Metal Coupling on Rusting of Iron


1. Is rusting a chemical or physical process?

Ans: Rusting is a chemical process.


2. What compound is formed on rusting of iron?

Ans: Hydrated ferric oxide, Fe2O3.xH2O is formed during rusting.


3. What is the general name given to the rusting of iron?

Ans: Corrosion.


4. How does coupling of iron with zinc prevents rusting?

Ans: Since zinc is more reactive than iron, passes preferentially into the solution as Zn2+ ions leaving iron unaffected.


5. Give names of two metals which can prevent rusting of iron on coupling with it?

Ans: Zinc and magnesium.


6. How does metal coupling affect rusting of iron?

Ans: Rusting of iron increases on coupling with less electro positive metals like copper and decreases on coupling with more reactive metals like magnesium and zinc.


7. Under ground iron pipes are prevented by coupling with an active metal. Account for it.

Ans: Zinc is used to prevent underground iron pipes. Zinc is more active than iron and undergoes oxidation. Corrosion of iron is prevented. The process is called cathodic protection.

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