Nace CP 1, Course Manual

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Basic Chemistry and Basic Corrosion Theory 2:4
CP 1 – Cathodic Protection Tester Course Manual
©
NACE International, 2000
02/01/05
above 7. Since the pH scale is logarithmic, for each unit of pH the
environment becomes ten times more acid or alkaline. A medium with a pH
of 6, for example, is ten times more acid than one having a pH of 7.
Neutral pH = 7
Acid pH < 7
Alkaline pH > 7
Acid
Neutral
Alkaline
0 14
ph = –log [H
+
]
Neutral pH = 7
Acid pH < 7
Alkaline pH > 7
Acid
Neutral
Alkaline Acid
Neutral
Alkaline
0 14
ph = –log [H
+
]
Figure 2.3 Illustration of Acid and Alkaline pH
An understanding of pH is important in corrosion and cathodic protection
work. For many metals, the rate of corrosion increases appreciably below a
pH of about 4. Between 4 and 8 corrosion rate is fairly independent of pH.
Above 8, the environment becomes passive and corrosion rates tend to
decrease. This is shown in Figure 2.4, which is typical of the behavior of
steel.
The corrosion rate of aluminum and lead, on the other hand, tends to
increase in environments above a pH of about 8. This is because the
protective oxide film on the surface of these metals is dissolved in most
strong acids and alkalis and the metals corrode. Metals that corrode under
low and high pH levels are termed
amphoteric
metals. Figure 2.4 illustrates
this phenomenon.