Page 48 - Corrosion Engineering Principles and Practice

28 C h a p t e r 2 C o r r o s i o n B a s i c s 29

water can increase its conductivity or change other properties of the
liquid, but does not increase or decrease its acidity. For instance,
+
whether a given amount of H ion is produced in water by introducing
hydrochloric (HCl), sulfuric (H SO ), or any other acid is immaterial.
4
2
The pH of the solution will be the same for the same number of
dissolved hydrogen atoms.
The pH may be measured with a meter or calculated if certain
parameters are established. Water itself dissociates to a small extent
−
+
to produce equal quantities of H and OH ions displayed in the
following equilibrium:
+
H O  H + OH − (2.3)
2

pH, originally defined by Danish biochemist Søren Peter Lauritz
Sørensen in 1909, is a measure of the concentration of hydrogen ions.
The term pH was derived from the manner in which the hydrogen
ion concentration is calculated; it is the negative logarithm of the
hydrogen ion (H ) concentration:
+
pH = − log (a H + (2.4)
)
10

where log is a base-10 logarithm and a is the activity (related to
H+
10
concentration) of hydrogen ions. The “p” in Eq. (2.4) stands for the
German word for power, potenz, so pH is an abbreviation for power
of hydrogen.
A higher pH means there are fewer free hydrogen ions, and that a
change of one pH unit reflects a tenfold change in the concentrations of
the hydrogen ion. For example, there are 10 times as many hydrogen
ions available at pH 7 than at pH 8. The pH scale commonly quoted
ranges from 0 to 14 with a pH of 7 considered to be neutral.
Substances with a pH less that 7 are considered to be acidic, and
substances with a pH equal to or greater than 7 are considered to be basic
or alkaline. Thus, a pH of 2 is very acidic and a pH of 12 very alkaline.
However, it is technically possible to have very acidic solutions with a
pH lower than zero and concentrated caustic solutions with a pH greater
than 14. Such solutions are in fact typical of many ore extracting processes
that require the digestive power of caustics and acids.
Low-pH acid waters accelerate corrosion by supplying hydrogen
ions to the corrosion process. Although even absolutely pure water
contains some free hydrogen ions, dissolved carbon dioxide (CO ) in
2
the water can increase the hydrogen ion concentration. Dissolved
CO may react with water to form carbonic acid (H CO ), as shown in
2
3
2
Eq. (2.5):
[H CO ]
2
×
CO + H O  H CO with K eq = [CO ] 3 = 1 .7 × 10 − 3 at 25 C (2.5)
o
2
3
2
2
2
where K is the reaction equilibrium expressed in equation.
eq