Page 309 - Corrosion Engineering Principles and Practice

280 C h a p t e r 8 C o r r o s i o n b y W a t e r 281

Nations Scientific, Education, and Cultural Organization (UNESCO)
committee to determine a more precise relation between chlorinity
and salinity. The definition of 1969 produced by UNESCO is written
in Eq. (8.2):

S (‰) = 1.80655 Cl (‰) (8.2)

The definitions of 1902 and 1969 give identical results at a salinity
of 35‰ and do not differ significantly for most applications. The
definition of salinity was reviewed again when techniques to
determine salinity from measurements of conductivity, temperature,
and pressure were developed. The Practical Salinity Scale defined in
1978 is a complex function related to the ratio (K) of the electrical
conductivity of a seawater sample to that of a potassium chloride
(KCl) solution with a mass fraction in KCl of 0.0324356, at the same
temperature and pressure.

S = 0.0080 – 0.1692K 0.5 + 25.3853 K
+ 14.0941 K 1.5 – 7.0261 K + 2.7081 K 2.5 (8.3)
2
Note that ‰ is no longer used in this definition. In fact, a value of
35‰ would simply correspond to a value of 35 on the Practical Salinity
Scale.

Precipitation of Calcareous Deposits. The natural presence of calcium
and magnesium in seawater has been advantageously used to coat
internal walls of vessels such as ballast tanks with a protective film of
calcareous deposits. These films, once they are formed by the cathodic
polarization of metal surfaces in seawater, greatly reduce the current
density needed to maintain a prescribed cathodic potential. For most
cathodic surfaces in aerated waters, the main reduction reaction is
described by Eq. (8.4):

−
−
O + 2 H O + 4e → 4OH (8.4)
2
2
In cases where the potential is more negative than the reversible
hydrogen electrode potential, the production of hydrogen as described
in Eq. (8.5) becomes possible:
2H O 2e → H + 2OH (8.5)
+
−
−
2
2
In either case, the production of hydroxyl ions results in an
increase in pH for the electrolyte adjacent to the metal surface. This
situation causes the production of a pH profile in the diffuse layer
where the equilibrium reactions can be quite different from the bulk
seawater conditions. Temperature, relative electrolyte velocity and
electrolyte composition will all influence this pH profile. In seawater,