Nace CP 1, Руководство по эксплуатации

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RP0169-2002
14 NACE International
6.2.2.3.3 The use of excessive polarized
potentials
on externally coated pipelines
should be avoided to minimize cathodic
disbondment of the coating.
6.2.2.3.4 Polarized potentials that result in
excessive generation of hydrogen should be
avoided on all metals, particularly higher-
strength steel, certain grades of stainless
steel,
titanium, aluminum alloys, and
prestressed concrete pipe.
6.2.3 Aluminum Piping
6.2.3.1 The following criterion shall apply: a
minimum of 100 mV of cathodic polarization
between the structure surface and a stable
reference electrode contacting the electrolyte. The
formation or decay of this polarization can be used
in this criterion.
6.2.3.2 PRECAUTIONARY NOTES
6.2.3.2.1 Excessive Voltages: Notwith-
standing the minimum criterion in Paragraph
6.2.3.1, if aluminum is cathodically protected
at voltages more negative than -1,200 mV
measured between the pipe surface and a
saturated copper/copper sulfate reference
electrode
contacting the electrolyte and
compensation is made for the voltage drops
other than those across the pipe-electrolyte
boundary, it may suffer corrosion as the result
of the buildup of alkali on the metal surface.
A polarized potential more negative than -
1,200 mV should not be used unless previous
test results indicate that no appreciable
corrosion
will occur in the particular
environment.
6.2.3.2.2 Alkaline Conditions: Aluminum
may suffer from corrosion under high-pH
conditions
and application of cathodic
protection tends to increase the pH at the
metal
surface. Therefore, careful
investigation or testing should be done before
applying cathodic protection to stop pitting
attack on aluminum in environments with a
natural pH in excess of 8.0.
6.2.4 Copper Piping
6.2.4.1 The following criterion shall apply: a
minimum of 100 mV of cathodic polarization
between the structure surface and a stable
reference electrode contacting the electrolyte. The
formation or decay of this polarization can be used
in this criterion.
6.2.5 Dissimilar Metal Piping
6.2.5.1 A negative voltage between all pipe
surfaces
and a stable reference electrode
contacting the electrolyte equal to that required for
the protection of the most anodic metal should be
maintained.
6.2.5.2 PRECAUTIONARY NOTE
6.2.5.2.1 Amphoteric materials that could be
damaged
by high alkalinity created by
cathodic protection should be electrically
isolated and separately protected.
6.3 Other Considerations
6.3.1 Methods for determining voltage drop(s) shall be
selected
and applied using sound engineering
practices. Once determined, the voltage drop(s) may
be used for correcting future measurements at the
same location, providing conditions such as pipe and
cathodic protection system operating conditions, soil
characteristics, and external coating quality remain
similar. (Note: Placing the reference electrode next to
the pipe surface may not be at the pipe-electrolyte
interface. A reference electrode placed at an externally
coated pipe surface may not significantly reduce soil
voltage drop in the measurement if the nearest coating
holiday
is remote from the reference electrode
location.)
6.3.2 When it is impractical or considered
unnecessary to disconnect all current sources to
correct for voltage drop(s) in the structure-to-electrolyte
potential measurements, sound engineering practices
should be used to ensure that adequate cathodic
protection has been achieved.
6.3.3 When feasible and practicable, in-line inspection
of pipelines may be helpful in determining the presence
or absence of pitting corrosion damage. Absence of
external corrosion damage or the halting of its growth
may indicate adequate external corrosion control. The
in-line inspection technique, however, may not be
capable of detecting all types of external corrosion
damage, has limitations in its accuracy, and may report
as anomalies items that are not external corrosion. For
example, longitudinal seam corrosion and general
corrosion may not be readily detected by in-line
inspection. Also, possible thickness variations, dents,
gouges, and external ferrous objects may be detected
as corrosion. The appropriate use of in-line inspection
must be carefully considered.
6.3.4 Situations involving stray currents and stray
electrical gradients that require special analysis may
exist.
For additional information, see Section 9,
“Control of Interference Currents.”
6.4 Alternative Reference Electrodes
6.4.1 Other standard reference electrodes may be
substituted for the saturated copper/copper sulfate