MIL-HDBK-1004/10
4.3.2
Chemical Analysis of the Environment
4.3.2.1
pH. pH of either soil or water can affect the corrosivity of the
environment and the current required for cathodic protection. pH can be
measured using several methods. Chemical methods of measuring pH involve
either the use of pH measuring electrodes or indicators whose colors are
dependent on pH. A pH meter measures the difference in potential between a pH
insensitive reference electrode and an electrode whose potential is sensitive
to pH. Figure 11 shows a typical pH meter suitable for field use. Colored
indicators are normally used in the form of pH papers. The paper is wetted
with the solution being measured and the resulting color is compared with
color standards to determine the pH.
Soil pH can also be measured using the antimony electrode method.
The potential of the metal antimony is sensitive to pH. The potential
difference between a pH insensitive electrode and a pH sensitive antimony
electrode can be used to measure pH. This method is particularly applicable
to the measurement of the pH of soils since it can be performed directly on
the surface of the soil. To measure the pH of a soil using an antimony
electrode, the antimony electrode and a copper/copper sulfate electrode are
placed on the surface of the soil and the potential difference between the two
electrodes is measured using an electronic voltmeter. In order to eliminate
the possible effects of stray soil currents, the position of the electrodes is
reversed and the potential is again measured. There should be little or no
difference in the readings if there are no stray currents in the area. If the
readings differ by more than 10 mV the two readings should be averaged to
cancel the effect of the stray currents. The potential readings are converted
to pH values using the graph in Figure 12.
Usually, pH at the surface of the ground is vastly
different than
the pH at pipeline depth. Therefore, pH taken at the surface
of the ground
has no value in determining the conditions at pipeline depth.
All pH
measurements should be taken in the ditch at pipeline or tank
depth, or soil
from borings at proposed pipe or tank depth could be analyzed
for pH.
Coating Conductance. Coatings can both reduce corrosion of
4.3.3
structures and reduce cathodic protection current requirements. In the design
of cathodic protection systems, the condition of protective coatings is an
important factor. For buried structures the condition of the coating can be
determined by electrical testing. The effective electrical resistance of the
coating is determined by these tests. Instead of reporting the measurements
as coating resistance they are converted to conductivity per unit area, or
conductance. Coating conductance is measured by two methods: the short line
method, and the long line method.
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