Corrosion Control and Anti-fouling Systems
Nature and Forms of Corrosion
There is a natural tendency for nearly all metals to react with their environment.
The result of this reaction is the creation of a corrosion product
which is generally a substance of very similar chemical composition to the
original mineral from which the metal was produced.
ATMOSPHERIC CORROSION Protection against atmospheric corrosion
is important during the construction of a ship, both on the building berth
and in the shops. Serious rusting may occur where the relative humidity is
above about 70 per cent; the atmosphere in British shipyards is unfortunately
sufficiently humid to permit atmospheric corrosion throughout most
of the year. But even in humid atmospheres the rate of rusting is determined
mainly by the pollution of the air through smoke and/or sea salts.
CORROSION DUE TO IMMERSION When a ship is in service the
bottom area is completely immersed and the waterline or boot topping
region may be intermittently immersed in sea water. Under normal operating
conditions a great deal of care is required to prevent excessive corrosion
of these portions of the hull. A steel hull in this environment can provide
ideal conditions for the formation of electro-chemical corrosion cells.
ELECTRO-CHEMICAL NATURE OF CORROSION Any metal in tending
to revert to its original mineral state releases energy. At ordinary
temperatures in aqueous solutions the transformation of a metal atom into
a mineral molecule occurs by the metal passing into solution. During this
process the atom loses one or more electrons and becomes an ion, i.e. an
electrically charged atom, with the production of an electric current (the
released energy). This reaction may only occur if an electron acceptor is
present in the aqueous solution. Thus any corrosion reaction is always
accompanied by a flow of electricity from one metallic area to another
through a solution in which the conduction of an electric current occurs by
the passage of ions. Such a solution is referred to as an electrolyte solution;
and because of its high salt content sea water is a good electrolyte solution.