MIL-HDBK-1004/10
When protective currents are totally interrupted, corrosion will
usually return to a normal rate after a short period of time.
Consequences of Overprotection. In addition to the chemical
3.7
corrosion damage that can occur on aluminum and lead structures if limiting
potentials are exceeded in the negative direction, excessive negative
potentials can also damage other metals. In addition to being wasteful of
anode material or electrical power, excess potentials can cause disbondment of
protective coatings and can cause hydrogen embrittlement of certain types of
steels, especially high strength steels.
Coating Disbondment. Excess cathodic protection potentials can
3.7.1
result in the generation of hydrogen gas. When the cathodic protection
potential reaches the polarized potential of -1.12 V (instant off), with
respect to a copper/copper sulfate reference electrode, the generation of
hydrogen gas will occur. When hydrogen gas is generated it is often trapped
between the coating and the surface and causes blisters and disbonding of the
coating.
Electrolyte can subsequently fill the gap between the coating and
the metal and, as the coating is an electrical insulator, sufficient current
for effective cathodic protection cannot reach the affected area and corrosion
will occur. Coating disbondment is a particular problem in water tanks. In
soil environments when high quality coatings are used, disbondment is seldom
encountered at potentials less negative than -1.6 V (current on) or -1.12
polarized potential (instant off).
Hydrogen Embrittlement. The hydrogen produced when cathodic
3.7.2
protection currents are excessive can also result in the reduction of the
ductility of steel. This is particularly true for high strength steels (in
excess of 130,000 pounds per square inch (psi) yield strength).
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