5
2
Corrosion Mechanisms
Most commonly used metals are unstable in the atmosphere. These unstable
metals are produced by reducing ores artificially, and therefore they tend to
return to their original state or to similar metallic compounds when exposed
to the atmosphere. Exceptions to this are gold and platinum, which are
already in their metal state.
Corrosion by its simplest definition is the process of a metal returning
to the material’s thermodynamic state. For most materials, this means the
formation of the oxides or sulfides from which they originally started when
they were taken from the earth before being refined into useful engineering materials.
These changes are electrochemical reactions that follow the laws of thermodynamics. Understanding the interactions of materials with their environment now takes on the added dimension of chemistry and electricity.
These concepts help explain why corrosion processes are time and temperature dependent. They also establish that the corrosion reactions, or rates, are
affected by ion and corrodent concentrations, and explain why some reactions are reversible or controllable while others are not.
Corrosion in aqueous solutions is the most common of all corrosion processes. Water, seawater, and various process streams in industry provide an
aqueous medium. Moisture in the atmosphere and water in the soil account
for the aqueous corrosion in these media. In all these cases, water is seldom
present in pure form. Rather, various salts and gases remain dissolved in it,
and their dissociation renders the water somewhat conducting. For all practical purposes, it acts as an electrolyte. The chemical nature of this electrolyte
may be acidic, alkaline, or neutral.
One of the most basic corrosion reactions involves the oxidation of a pure
metal when exposed to a strong acid. A familiar case is that of pure iron
coming in contact with hydrochloric acid. The resulting chemical reaction is
obvious, with the solution beginning to bubble violently. The reaction can be
expressed as follows:
Fe HCl FeCl H
2
+
→
+
2
2 ↑
(2.1)
We can see the result of this reaction by the gradual disappearance of the
iron and the hydrogen bubbles rising rapidly to the surface. On an electrochemical level, there is also an exchange of electrons taking place:
2
Corrosion Mechanisms
Most commonly used metals are unstable in the atmosphere. These unstable
metals are produced by reducing ores artificially, and therefore they tend to
return to their original state or to similar metallic compounds when exposed
to the atmosphere. Exceptions to this are gold and platinum, which are
already in their metal state.
Corrosion by its simplest definition is the process of a metal returning
to the material’s thermodynamic state. For most materials, this means the
formation of the oxides or sulfides from which they originally started when
they were taken from the earth before being refined into useful engineering materials.
These changes are electrochemical reactions that follow the laws of thermodynamics. Understanding the interactions of materials with their environment now takes on the added dimension of chemistry and electricity.
These concepts help explain why corrosion processes are time and temperature dependent. They also establish that the corrosion reactions, or rates, are
affected by ion and corrodent concentrations, and explain why some reactions are reversible or controllable while others are not.
Corrosion in aqueous solutions is the most common of all corrosion processes. Water, seawater, and various process streams in industry provide an
aqueous medium. Moisture in the atmosphere and water in the soil account
for the aqueous corrosion in these media. In all these cases, water is seldom
present in pure form. Rather, various salts and gases remain dissolved in it,
and their dissociation renders the water somewhat conducting. For all practical purposes, it acts as an electrolyte. The chemical nature of this electrolyte
may be acidic, alkaline, or neutral.
One of the most basic corrosion reactions involves the oxidation of a pure
metal when exposed to a strong acid. A familiar case is that of pure iron
coming in contact with hydrochloric acid. The resulting chemical reaction is
obvious, with the solution beginning to bubble violently. The reaction can be
expressed as follows:
Fe HCl FeCl H
2
+
→
+
2
2 ↑
(2.1)
We can see the result of this reaction by the gradual disappearance of the
iron and the hydrogen bubbles rising rapidly to the surface. On an electrochemical level, there is also an exchange of electrons taking place: