AFH 32-1290(I)/MIL-HDBK-1136/1 1 February 1999 48
-0.700 volts DC), annotate those exact locations; add 100 mV to those
readings; or compare future instant off readings to those specific readings at
those specific locations. If a significant percentage of the pipeline is over -
0.700 Volts DC, consider using the -0.85 instant off criterion for the entire
pipeline.
3.3.6.3.2. Determine the instant off reading required to meet the 100 mV
shift criterion for the rest of the structure (example, if the most negative
reading for the rest of the structure was -0.603 Volts DC, the 100 mV instant
off would have to be -0.703 Volts DC).
3.3.6.3.3. Apply that instant off criteria to the rest of the structure.
3.3.6.4. For protected structures:
3.3.6.4.1. If native survey data is available, analyze data and use procedures
as above for unprotected isolated or non-isolated structures, as appropriate.
3.3.6.4.2. If native survey data is not available, is incomplete or otherwise
not useful, consider using the other criteria.
3.3.6.4.3. To use the 100 mV shift criterion after CP has been applied, the
structure must be depolarized, to obtain an approximation of the native
potential, and use procedures as above for unprotected isolated or non-
isolated structures, as appropriate. After current interruption, considerable
time may be required before the potential returns to an approximation of the
native potential value:
60 to 90 days for typical well-coated buried structures.
2 to 30 days for buried structures with poor or no coating.
90 to 150 days for water tank interiors with good coatings and little or
no water circulation.
3.3.6.4.4. Certain soil conditions may slow or speed depolarization,
generally, the higher the corrosion rate, the quicker the depolarization occurs.
Some factors, such as high levels of oxygen, movement or agitation of the
electrolyte (flowing water) will speed depolarization. Some factors, such as