b. Each metal on the chart has a different potential, this means that a metal "UPPERMOST" on the chart
from those lower will make it the "anode."
c. Those metals on the chart "LOWER" than any metal above it on the chart is the "cathode."
NOTE: This means that the material higher up on the chart will be the one to corrode.
4.
Galvanic Corrosion.
a. Galvanic corrosion is the self-generated corrosion activity which results when a pipe is placed in the
soil. Differences in potential develop song the pipe or between different pips.
b. These differences can result from dissimilar metal as a result of differences within the soil.
c. A simple form of the galvanic cell is the flashlight battery. A dissimilar metal such as aluminum pipe
connected to a cast iron valve.
(1) Galvanic corrosion can accrue as there is a difference in electrical potential, this dissimilar metal
makes the aluminum pipe anode, and the cast iron valve the cathode.
(2) This means the aluminum pipe will corrode giving up energy in the form of D-C current, to
protect the cast iron valve.
d. Where "new" (pipe not used in the past), and "old" (pipe currently in use) pipes are connected
(welded), the new pipe will corrode, thereby protecting the old pipe from corrosion. The new pipe will not last as
long due to the fact it is corroding to protect the old pipe for whose corrosion is halted.
5.
Dissimilar Soils.
Different soils and mineral contents effect the rate of corrosion on a underground structure.
a. The type and consistency of the soils are determined prior to actual construction as well as after
construction. A soil survey is conducted to determine the makeup of the soil.
b. The different soils go from "gravel" which gives good drainage, through "silt" which will have a high
concentration of corrosive matter (for example, acids, salts and other impurities).
c. The pH is a measure of the acidity or alkalinity of a solution. When the pH is numerically equal to
"7" it is a neutral solution. For a pH less than "7," the solution is acidic. For a pH greater than "7," the solution is
basic or alkaline. The numerical rating will decrease (down to 0) as the acidity increases (vegetation as it
composts). The numerical rating will increase (up to 14) as the alkalinity increases (mineral salt).
16-3
QM5200
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