Page 428 - Corrosion Engineering Principles and Practice

396 C h a p t e r 1 0 C o r r o s i o n i n S o i l s a n d M i c r o b i o l o g i c a l l y I n f l u e n c e d C o r r o s i o n 397

Cell potentials also change; some increase, while others decrease.
Some areas which were initially anodic become cathodic. The reverse
also may take place, although not as often. Generally, as time passes,
the total anodic area on a line gets smaller, although total activity
does not decrease at the same rate. The result is that the rate of attack
at the worst locations tends to increase. Finally, among all of the
various cells along the entire line, the most active anode loses enough
metal by corrosion to penetrate the pipe wall completely.
When an impressed current cathodic protection (ICCP) system is
in full operation there is a high possibility for oxygen to be produced
at the anode, and in nearly all cells, hydrogen is formed at the cathode.
If chloride ions are present, chlorine gas may be formed at the anode.
This generation of gas, either oxygen or chlorine, at the anode is not
nearly as likely to occur in a natural corroding cell as it is when an
ICCP system is used, particularly when inert anodes are used.

Hydrogen
The formation of hydrogen at the cathode, however, can occur during
both normal corrosion and under sacrificial CP or ICCP. Initially, it
may be formed as nascent hydrogen, that is, single hydrogen atoms
are produced. These single atoms may then combine with oxygen to
form water, or with some other ion in the environment to form
another compound. Nascent hydrogen may also dissolve in the metal
(cathode area), or combine to form ordinary gaseous hydrogen.
Atomic hydrogen formed at the cathode may diffuse into the steel
and recombine at some depth in the metal to form molecular gaseous
hydrogen that unlike atomic hydrogen cannot migrate any further
through the metal crystal structure. Consequently, an accumulation
of hydrogen gas within the metal may build up to generate enough
pressure to blister or split the solid steel. Hydrogen can also react
with the metal causing hydrogen induced cracking. Chapter 6
provides more details on this particular type of corrosion damage.
When gaseous hydrogen is formed on the metallic surface, it
normally escapes readily in the environment without leaving a trace.
After all, hydrogen is the lightest gas known. However, if the production
of the gas happens to be under a partially defective coating, the formation
of this hydrogen gas may contribute to disbondment and acceleration of
the coating deterioration as explained in more details in Chap. 14.
Electroendosmosis
Another side effect of the passage of electrical current through a
porous medium like soil is electroendosmosis, during which water
is “carried along” with the current. Normally this effect is significant
only with higher current densities than natural corrosion cells
would produce. Like hydrogen formation, it is more likely to be
associated with the use of ICCP in which case water molecules
would be carried to the cathode and away from the anode resulting
in an unwanted increase in anode resistance.