Materials for engineering, 3rd Edition - (Malestrom)

Metals and alloys 125
3.4.2 Corrosion of metals and alloys in aqueous
environments
Although we will use examples of metals being immersed in a liquid, the
corrosion processes which occur are also encountered under conditions of
ordinary atmospheric exposure, when the relative humidity is high.
Condensation of moisture upon the metal surface then takes place and
degradation by ‘wet corrosion’ is observed.
It is well known that the mechanism of aqueous corrosion is electrochemical:
a local anode is formed where the metal (M) enters solution:
M → M 2+ + 2e [3.13]
Electrons flow through the metal and are neutralized at a local cathode.
There is a variety of possible reactions for this electron discharge and one of
the most important, involving the absorption of oxygen, can be written as:
O 2 + 2H 2 O + 4e → 4OH – [3.14]
So OH – ions are produced in the liquid at the cathode, which thus becomes
locally alkaline.
Let us consider an iron specimen partially immersed in an electrolyte
(Fig. 3.35). Atmospheric oxygen dissolves in the liquid, so the iron just
beneath the surface of the liquid is surrounded by liquid with the highest
dissolved oxygen content and, therefore, acts as a cathode. The bottom of the
specimen, furthest removed from the surface, is surrounded by liquid with
Iron
Air
Hydroxyl ions at cathode
OH – OH – NaCl solution
Electron flow
e
Ferrous ions at anode
Fe 2+ Fe 2+
3.35 A simple corrosion cell.
Precipitate of rust away from
both anode and cathode