Corrosion of Iron

Introduction

Corrosion is defined as the chemical or electrochemical degradation of metals due to their reaction with the environment. The corrosion of iron, better known as rusting, is an oxidation-reduction process that destroys iron objects left out in open, moist air. In the United States alone, it is estimated that the cost of corrosion, in equipment maintenance, repair, and replacement, exceeds $300 billion per year. What kinds of chemical treatments, surface coatings, or combinations of metals will prevent the corrosion of iron?
Background
When iron metal is exposed to oxygen and water, a familiar result is observed-rust. The rusting process consists of several steps. In the first step, iron is oxidized to iron(II) ions, Fe2+, and oxygen from the air is reduced to hydroxide ions, OH-. This oxidation-reduction reaction takes place via two separate but simultaneous half-reactions (Equations 1 and 2).

Oxidation half-reaction: Fe(s) → Fe2+(aq) + 2e-          Equation 1

Reduction half-reaction: O2(g) + 2H2O(l) + 4e- → 4OH-(aq)     Equation 2

Combining the oxidation and reduction half-reactions gives the balanced chemical equation for the overall reaction of iron, oxygen, and water (Equation 3). Notice that two iron atoms are oxidized for every oxygen molecule that is reduced-the number of electrons gained by one oxygen molecule is equal to the number of electrons given up by two iron atoms.

2Fe(s) + O2(g) + 2H2O(1) → 2Fe2+(aq) + 4OH-(aq)         Equation 3

Fe2+ and OH- ions may combine to form solid iron(II) hydroxide, Fe(OH)2 (Equation 4). This is almost never observed, however, because iron(II) hydroxide reacts further with oxygen and water to form hydrated iron(III) oxide, Fe2O3•nH2O, the flaky, reddish-brown solid known as rust (Equation 5).

Fe2+(aq) + 2OH-(aq) → Fe(OH)2(s)                 Equation 4

4Fe(OH)2(s) + O2(g) + xH2O(I)→ 2Fe2O3•(x+4)H2O(s) Rust     Equation 5
    
Experiment Overview
The purpose of this activity is to investigate chemical additives, surface coatings, and metal combinations that will reduce or prevent the corrosion of iron. Each group of students will be responsible for developing a hypothesis and designing a "fair test" to determine how and why different conditions affect the corrosion of iron. In order to compare results obtained by different student groups, the corrosion of iron will be studied using a standard test method.

Pre-Lab Activity
The following demonstration illustrates the standard test method that will be used in this experiment and provides evidence for the electrochemical nature of corrosion.

Two iron nails were cleaned and sanded, and one of the nails was bent to a 90° angle. The nails were placed in a Petri dish and covered with warm agar containing two indicators. Upon cooling, the agar formed a stable, semi-solid gel. Phenolphthalein, an acid-base indicator, was added to detect the formation or presence of hydroxide ions. Phenolphthalein is colorless in acidic or neutral solutions but turns bright pink in basic solutions (pH> 8-10) due to reaction with OH- ions. Potassium ferricyanide, K3Fe(CN)6 was added to detect the formation or presence of iron(II) ions. Ferricyanide ions react with Fe2+ ions to form a dark blue mixed iron(II)/iron(III) compound, Fe3[Fe(CN)6]2 commonly known as Prussian blue (Equation 6).
                3Fe2+(aq) + 2Fe(CN)6 3-(aq) → Fe3[Fe(CN)6]2 (s)    Equation 6
                                      Yellow         Prussian blue

1. Observe the nails and the indicator colors in the standard corrosion test. Record all observations in the diagram below.

2. Which parts of the straight nail (the control) oxidized most readily? What evidence supports this? Suggest a possible reason for the observation.

3. Compare the results obtained for the bent nail versus the control. Did bending the nail change where oxidation of the metal was most likely to start or the amount of rust that was observed? Explain.

4. According to the electrochemical model for iron corrosion, the corrosion process takes place via two separate half-reactions. Electrons flow through the metal, like electricity through a wire, from the site where iron is oxidized to the site where oxygen is reduced. Do the indicator color changes support this model for iron corrosion? Explain.

Materials
 
Agar, 1.5 g*
Distilled water, 150 mL*
Iron nails, 4
Phenolphthalein indicator solution, 1 % in
 alcohol, 1 mL
Potassium ferricyanide solution,
K3Fe(CN)6 0.1 M, 1 mL
Chemical additives or test solutions#
Metal wires, strips or ribbons#
Surface coatings or linings#
Beaker, 400-mL
Hot plate
Petri dishes with covers, 2
Sandpaper or steel wool
Spatula
Stirring rod
Weighing paper
 

*Prepare a 1 % suspension of agar in boiling water. Dissolve 1.5 g of agar in 150 mL of boiling water to give enough agar for two corrosion tests (Petri dishes).
#Extra materials needed to test each working hypothesis. Consult with your teacher.

Safety Precautions
Potassium ferricyanide solution is a skin and eye irritant. Contact with concentrated acids may generate a toxic gas; avoid contact with strong acids. Phenolphthalein is an alcoholbased solution-it is a flammable liquid and moderately toxic by ingestion. Keep away from flames and other sources of ignition. Avoid contact of all chemicals with eyes and skin. Wear chemical splash goggles, chemical-resistant gloves, and a chemical-resistant apron. Wash hands thoroughly with soap and water before leaving the lab.

Procedure
Study the mechanism of the corrosion process (see the Background section) and the evidence for this mechanism (see the Pre-Lab Activity).
1. Form a working group with two other students and brainstorm the following questions.
• What chemical additives might reduce or prevent the corrosion of iron?
• What type of surface coatings might inhibit the corrosion of iron?
• What combinations of metals might prevent the corrosion of iron?
• What other types of metal treatment might reduce the corrosion of iron?
2. Choose one general type of metal treatment and develop an "if/then" hypothesis to describe its effect: If a nail is combined or treated with , then the amount of corrosion should , because _
Write the hypothesis on the Data Sheet.
3. Design a (fair test" experiment to test the hypothesis-choose at least 3-4 specific examples of metal treatment that may provide evidence both for and against your hypothesis. What other variables might affect the test? How can these variables be controlled?
4. Write a detailed, step-by-step procedure for your experiment and verify the procedure and the required safety precautions with your instructor. Carry out the experiment and record observations on the Data Sheet.