45 Corrosion

Background

  Each year, the corrosion of metals does untold damage to cars, homes, and factories. Corrosion is a complex reduction-oxidation (redox) reaction in which metals are changed to their oxides or other compunds. In a corrosion reaction, electrons flow from the anode to the cathode. The anode and the cathode may be two different parts of the metal being corroded, or the cathode may be a different object that is in electrical contact with the metal being corroded.

In this experiment you will study a variety of factors involved in the corrosion of iron.

 Goals

Equipment

Materials
 

9 uncoated iron nails, Fe	litmus paper or Hydrion paper
thin zinc strip, Zn	phenolphthalein solution
copper wire, Cu	0.1 M iron (II) sulfate, FeSO4
steel wool	ferricyanide,
0.1 M potassium	K3Fe(CN)6
agar, powdered	distilled water

 
 

Solution Set 1
0.1 M sodium hydroxide NaOH	0.1 M sodium chloride, NaCl
0.1 M sodium dichromate, Na2Cr2O7	0.1 M hydrochloric acid, HCl
Solution Set 2
0.1 M potassium hydroxide KOH	0.1 M potassium nitrate, KNO3
0.1 M sodium carbonate, Na2CO3	0.1 M nitric acid, HNO3
Soution Set 3
0.1 M sodium phosphate, Na3PO4	0.1 M sodium thiocyanate, NaSCN
0.1 M sodium oxalate, H2SO4	0.1 M sulfuric acid, Na2C2O4

Safety

Procedure

NOTE (May 11, 1999): The following procedural changes have been made to the standard procedure: Day 1 (Part A): Place all 4 nails in one petri dish. Make sure they are not touching. Day 2 (Part A): use 10 drops of iron (II) sulfate.

Day 1 Experiments

Part A. Reaction of Iron with Aqueous Solutions

Copy Table 45.1 into your labratory notebook. As you perform the experiment, record your observations in this table.

 1. Clean five iron nails with steel wool. Place each nail in a separate small test tube.

2. Your techer will assign you one of the three sets of chemicals listed in the Materials section. Fill each of four test tubes with different soultion from the set, until each nail is just covered. Fill the fifth tube with distilled water. Put the tubes in a test-tube rack.

3. Use litmus or Hydrion paper to determine whether each solution is acidic, basic, or neutral. Record the results in Table 45.1.

4. Allow the test tubes to stand overnight. You will study them tomorrow, when you begin step 10.

 Part B. Effects of Stress; Protection by Other Metals

 5. Heat 100 mL of distilled water to boiling in a 250-mL beaker. Remove the gas burner. CAUTION: Hot agar causes severe burns to the skin. Add, while stirring, 1g of powdered agar. Replace the burner; heat and stir the mixture until the agar forms a suspension. Be careful not to burn the agar. Stop heating and turn off the gas burner.

6. Add 5 drops of 0.1 M potassium ferricyanide and 3 drops of phenolphthalein to the agar suspension. Stir to mix thoroughly. Allow the agar to cool, but not set, while you proceed to the next step.

7. Clean four iron nails with steel wool. Place one nail in a petri dish. Use pliars to bend a second nail into a right angle. Place the bent nail beside, but no touching, the straight nail as shown in Figure 45.1. Tightly wrap a 10-cm piece of copper wire around a third nail. Wrap the fourth nail tighly with a thin strip of zinc metal. Place these two metal-wrapped nails in a second petri dish. Be sure that the nails do no touch.

8. Pour the warm agar suspension into the petri dishes. The nails and attatched pieces of metal should be covered by agar to a depth of at least 2 mm.

9. View the dishes against a white background and make observations at the end of the class period. Cover the dishes and keep them undisturbed overnight. You will observe them again tomorrow.

Day 2 Observations

 Part A

10. Observe the test tubes that have stood overnight against a white background. Record any evidence of reaction in Table 45.1.

11. Test for the presence of ferrous ions, Fe2+. In a separate small test tube, add on drop of 0.1 M potassium ferricyanade to 1 mL of 0.1 M iron (II) sulfate. Record your observations.

12. Now, test each of the five test tubes containing the nails present for the presence of ferrous ions by adding 1 or 2 drops of 0.1 M potassium ferricyanide. Record your observations. The presence of ferrous ions in the test tubes is evidence that corrosion has occured.

 Part B

13. Observe the dishes against a white background. Record your observations by sketching the dishes. Show the location and color of any reaction products.

14. Follow your teacherís instructions for proper disposal of the materials.

 Data Record

 Table 45.1 Observations
 

Test Solution	pH	Initial Observations	Observations-12 hours	Test
Set I
NaOH
Na2Cr2O7
NaCl
HCl
Set II
KOH
Na2CO3
KNO3
HNO3
Set III
Na3PO4
Na2C2O4
NaSCN
H2SO4
Controls
H20
F3SO4

 
Data Analysis
 
1. Obtain class data for the two sets of chemicals that you did not use in Part A. Enter this data in Table 45.1
2. List the chemicals used in Part A for which there was no evidence of corrosion.
3. List the chemicals used in Part A for which there was evidence of corrosion.
4. Did either copper or zinc appear to protect the iron nail against corrosion? Explain.
5. Explain how the colors that developed in the petri dishes identify the anode and cathode for each reaction.
6. What were the usual sites where corrosion took place for the nails embedded in the agar.
 
Conclusion
 
1. Examine the data for Part A. Are there any ions that seem to inhibit corrosion or to promote it? Try to explain these effects.
2. Explain the results obtained in distilled water.
3. Consider your answer to question 6 in Data Analysis. What effect does bending seem to have on the tendency or iron to corrode?
4. Explain the effects caused by wrapping the nails with zinc or copper. Discuss the relative ease of oxidation or iron, zinc, and copper in your answer.
5. Is it correct to say that corrosion did not take place in the nail wrapped with zinc? Explain.
 
Extensions
 
1. Design an experiment to test other ways of preventing the corrosion of iron. Methods might include the use of various types of coatings, by adjusting pH, or by using so-called sacrificial metals.
2. Do some library research on corrosion effects that are due to air pollution. Design an experiment to test some of the effects that you learn about.