Corrosion of Iron
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?
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
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
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
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.
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.
Which parts of the straight nail (the control) oxidized most readily?
What evidence supports this? Suggest a possible reason for the
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?
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.
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#
Petri dishes with covers, 2
Sandpaper or steel wool
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
#Extra materials needed to test each working hypothesis. Consult with your teacher.
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.
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?
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.
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.