Corrosion of Iron

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 oxidationreduction
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 oxidationreduction reaction takes place via two separate but simultaneous halfreactions
(Equations 1 and 2).
Oxidation halfreaction:
Fe(s) ? Fe2+(aq) + 2eEquation
1
Reduction halfreaction:
O2(g) + 2H2O(l) + 4e?
4OH(
aq) Equation 2
Combining the oxidation and reduction halfreactions
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 reducedthe
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 OHions
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, reddishbrown
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.
PreLab
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, semisolid
gel. Phenolphthalein, an acidbase
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> 810)
due to reaction with OHions.
Potassium ferricyanide,