Corrosion

Introduction

Prince Edward Island soil is famous for its red color. Remember “Bud the Spud from the
Bright Red Mud”? A well-known chemical reaction is actually responsible for this
characteristic color. The actual process is known as corrosion. The spontaneous destructive
oxidation of metals is called corrosion. Corrosion occurs whenever a metal surface is
destroyed by being converted to a metal compound. The reaction is actually more
complicated than most people think.

Elements of the Earth

Roughly 92 chemical elements are known to exist in earth’s crust. Most of these elements
have combined with one or more other elements to form compounds known as minerals.
There are numerous possible combinations of elements and up to 2000 minerals have been
discovered. These minerals exist in mixtures which form the rocks of the earth. Relatively
few of these elements and minerals are of real importance in soils. Approximately 98% of
the earth is composed of only eight chemical elements, most of which is oxygen (O) or
Silicon (Si). The most abundant minerals in soils are light in color. If all soils were
composed of crushed minerals that had undergone little chemical change they would be light
gray. It is obvious that not all soils are gray. The brown, red, and yellow colors of soils are
caused by chemical changes in the elements that make up these minerals. Iron(Fe) is the
element responsible for the chemical changes that occur in Prince Edward Island soil.

Reactions that cause Rusting

To understand what happens in a field or backyard, the events of corrosion must first be
explained. Iron rusts only when there is water and oxygen present. Rust is a complicated
material that contains various types of hydrated iron (III) oxide, Fe O CxH O. Iron begins 2 3 2
to rust at places on its surface where there is an impurity, or where the iron lattice has
imperfections. At these points some of the iron atoms produce iron (II) ions in the solution:

Fe (s) --> Fe(2+) (aq) + 2e-

Here the iron has undergone oxidation. Oxidation is the loss of electrons by ions. As the
iron(II) ions move away they meet hydroxide ions and produce iron (II) hydroxide:

Fe(2+) (aq) + 2OH(-) (aq) --> Fe(OH)2 (s)

Dissolved oxygen will then oxidize the iron (II) hydroxide producing the substance called
rust:

Fe(OH)2 (s) + dissolved oxygen ---> rust (Fe2 O3)

The electrons liberated from the process are taken up by hydrogen ions in the water
producing gas. This is a reduction reaction:

2H(+) (aq) + 2e(-) --> H2 (g)

For a drop of water on an iron surface, rusting will occur near the edges of the drop. This
is because there is more oxygen dissolved from the air near the edges of the drop.

Iron

Iron is the fourth most common element in soil, comprising 5% of the earth’s crust. The iron
in soil is usually found in the soluble cation form (Fe ). This reduced form is more common 2+
because of the lower levels of oxygen in soil. This ion can be readily absorbed by plants.
When high levels of oxygen are present in the air surrounding soil particles, oxidation occurs
and the Fe form of iron prevails. This form of iron is insoluble and therefore not available 3+
to plants. Usually in acid soil sufficient Fe exists in the soil to meet the needs of plants. 2+
However, iron deficiencies are common in alkaline soil. The greater concentration of
hydroxyl causes the oxidation of iron.

Why the Red Soil?

Iron oxides are responsible for the red soil on Prince Edward Island. It is possible to trace
the reactions of iron from the time it is released from rock. Iron olivine is a good example
of a rock which contains iron. This iron can be released due to environmental conditions.
Weathering of iron olivine leads to hydrolysis yielding iron oxide and silicic acid:

Fe2 SiO4 + 2HOH --> 2FeO + H4SiO4

Both of these products are somewhat soluble and can be lost be leaching. However, in the
presence of free oxygen, and when moisture and temperature conditions are favorable for
chemical activity, the iron in the soil minerals is oxidized and hydrated into red and yellow
compounds. The iron oxide (FeO) is oxidized to only slightly soluble iron oxides such as
Fe2O3 or its hydrated counterpart Fe2O3 --> xH2O (the x indicates that the quantity of associated water can vary). This is oxidation reaction:

4FeO + O2 --> 2Fe2O3

Because of the extremely low solubility of these iron oxides, very little of the iron is lost.
This results in a characteristic red color of the soil where the reaction occurs.

Protection from Rusting

There are a few basic methods for protecting metals from corrosion:
One is to slow down the process. Slowing down the corrosion process is done with
protective coatings such as paint or tar. These help to keep out oxygen, water, and
electrolyte salts. The presence of small salt crystals in the air is the major reason why metal
corrodes more rapidly at seacoasts.
Cathodic protection from corrosion occurs when a metal to be protected is coupled with a
metal more easily oxidized than itself. Metal fences, sheets, and nails made of iron can be
protected by galvanizing them. These materials are coated with zinc and said to be
galvanized. The galvanized metal will not corrode until after the zinc coating does because
zinc corrodes more readily than iron. Instead of Fe ions going into solution, Zn ions are 2+ 2+
lost from the zinc. The iron remains unaffected. Tin is also very good at protecting iron and
steel. This is especially evident with tin cans. It is more difficult to plate steel with a thin
layer of zinc than tin. Also, tin is less reactive than zinc and is less likely to dissolve in the
liquids stored in cans. However, tin is not as effective in protection and it will rust if it is
holed.
In anodic protection, the metal to be protected is briefly made positive to form a stable oxide
film on its surface. The stable oxide film then protects the underlying metal from corrosion.
Stainless steels form a protective film of nickel/chromium oxides since they have a high
content of these metals