SCH4C              Lab #17 - The Corrosion of Iron

Introduction:    The corrosion of iron costs industry and consumers billions of dollars annually.  What are some of the factors that contribute to corrosion?  What can be done to control corrosion?
    Corrosion is a term applied to the process in which  metals are converted to their oxides or other compounds.  This causes a gradual and progressive deterioration of the iron. Rusting occurs mainly when iron slowly combines with the oxygen from moist air.   The overall reaction is represented by the equation

                                                4 Fe   +  3 O2  ----->   2 Fe2O3   iron(III) oxide; ferric oxide

     The rusting or corrosion of the metal can be broken down into a series of distinct stages.

a)    In the first stage Fe(s) is converted to Fe2+(aq) and the solution becomes basic.

                            2 Fe(s)  +  O2(g)  +  2 H2O(l)  ------>  2 Fe+2(aq)  +  4 OH-1(aq)

b)    In the second stage the Fe2+(aq)  ions undergo further reaction with oxygen from the air.

                           4 Fe2+(aq)  + O2(g)  +  H2O(l)  ------>  4 Fe3+(aq)  +   4 OH-1(aq)

c)    As the Fe3+(aq) ions are formed they immediately react with the OH-1(aq)  ions to produce insoluble Fe(OH)3(s)

                        Fe3+(aq)  +    3 OH-1(aq)  -------->  Fe(OH)3(s)

d)    Finally, on standing, the Fe(OH)3 eventually dehydrates to produce Fe2O3.

                           2 Fe(OH)3(s)  ---->   Fe2O3(s)   +   3 H2O(g)
 

Notice that both the O2(g) and H2O(l) are needed to cause the rusting of the iron. Rusting does not take place very quickly in regions where humidity is low.  However, dissolved salts will greatly accelerate the corrosion process, which accounts for the rapid rusting of cars in parts of Canada where salt is used to melt ice and snow in the winter.
    In Part A of this experiment you will observe the reaction of an iron nail in acidic, basic, and neutral solutions.  Distilled water is used as a control.  In Part B of the experiment you will be looking for some specific ions that are produced in the corrosion of iron.   The reaction of potassium ferricyanide with Fe+2 ions gives an intense blue colour.  Phenolphthalein reacts with OH-1 ions to give a pink colour.

Problem?  What are some of the factors involved in corrosion?

Apparatus:    250 mL beaker,  ring stand,  iron ring,  wire gauze, Bunsen burner, test tubes, petri dish

Materials:   9 iron nails, steel wool, red and blue litmus paper, copper wire, zinc strip, powdered agar, distilled water, phenolphthalein solution, 0.1 M solutions of sodium hydroxide, potassium hydroxide, sodium chloride, hydrochloric acid, potassium nitrate, potassium ferricyanide and iron(II) sulphate.

Procedure:   CAUTION:  Avoid allowing the solutions to come in contact with your skin.  Wash your hands thoroughly after the experiment.

Part A:   Reaction of Iron with Various Reagents

1.    Polish 7 nails with steel wool and place one in each of 7 test tubes.

2.    Cover each nail with one of the following 6 reagents:  sodium hydroxide, potassium hydroxide, sodium chloride, hydrochloric acid, potassium nitrate, and sulphuric acid.  Cover the last nail with distilled water.

3.    Use red and blue litmus paper to determine if the solution in each of the 7 test tubes is acidic, basic or neutral.

4.    Allow the nails to stand overnight in the solutions.  (Go to Part B in the meantime)

5.    The next day, observe and record any changes that  have taken place.  To each solution add two drops of potassium ferricyanide solution.  Observe any changes.

6.    Add one drop of the potassium ferricyanide solution to about 1 mL of iron(II) sulfate solution.  The potassium ferricyanide solution reacts with Fe2+ ions to form an intense blue precipitate.  Compare this result with that obtained when the potassium ferricyanide solute was added to the various solutions covering the nails.

Part B:  Reactions Involving Two Metals
1.    Prepare about 100 mL of agar mixture.  To do this, heat about 100 mL of distilled water to a gentle boil in a beaker.  Remove the burner and stir in 1 gram of powdered agar.  Resume gentle heating until the agar is dispersed throughout the water.

2.    Add 5 drops of 0.1 mol/L potassium ferricyanide solution and 3 drops of phenolphthalein solution to the agar mixture.  Stir thoroughly.

3.    Polish two nails with the steel wool.  Place one nail on one side of a petri dish.  Bend the second nail sharply with a pair of pliers and place it one the other side of the petri dish.

4.    Obtain a 25 cm length of bare copper wire.  Tightly wrap approximately 10 cm of this wire around the shank of a polished iron nail.  Leave 5 cm and tightly wrap the remaining 10 cm portion around a polished zinc strip.  It is essential for all contacts to be firm.  Place this system into a second shallow petri dish.

5.    Pour enough lukewarm agar carefully over the nails in the petri dishes to completely cover the nails.

6.    Leave the dishes overnight.  Record any observations.

Questions:
1.    a) List the reagents in Part A that do not contribute to the corrosion of the nail.
       b)  List the reagents that did contribute to corrosion.

2.    What pattern is evident? Account for the pattern.

3.    When iron metal (Fe) corrodes,  what ions are formed?

4.    In Part B, what did you observe at the head, at the pointed end, and at the sharp bend of the nails that is different from the rest  of the nail?  What two colours are present?  What ions are indicated by each colour?  How can you account for this in terms of the way the nails are manufactured?

5.    Based on the results of this experiment, what are the factors that contribute to the corrosion of iron?

6.    What methods are used to reduce the corrosion of metals?