Nace CP 1, Руководство по эксплуатации

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RP0176-2003
6
NACE International
________________________________________________________________________
Section 4: Criteria for Cathodic Protection
4.1 Introduction
4.1.1 This section lists criteria and related inspection
methods for CP. Compliance with the criteria either
separately or collectively indicates whether the struc-
ture is protected.
4.2 General
4.2.1 The objective of using CP is to control the corro-
sion of metallic surfaces in contact with electrolytes.
4.2.2 The criteria in Paragraph 4.3 have been devel-
oped through laboratory experiment or field experience.
Structure protection need not be limited to these criteria
if it can be demonstrated by other means that control of
corrosion has been achieved.
4.2.3 The selection of a particular criterion for achiev-
ing the objective in Paragraph 4.2.1 depends, in part,
on past experience with similar structures and environ-
ments in which the criterion has been used success-
fully.
4.2.4 No single criterion for evaluating the effective-
ness of CP has proved to be satisfactory for all condi-
tions. A combination of criteria may be needed for a
single structure.
4.3 Criteria
4.3.1 Potential measurements
4.3.1.1 A negative (cathodic) voltage of at least
-0.80 V measured between the structure surface
and a silver/silver chloride (20 ohm-cm seawater)
reference electrode (Ag/AgCl [sw]) contacting the
water is necessary. Normally, this voltage should
be measured with the protective current applied.
This -0.80 V criterion includes the voltage drop ac-
ross the steel/water interface, but does not include
the voltage drop in the water (see Paragraphs
4.5.1.1 and 4.6.1).
4.3.1.2 As an alternative to the criterion in Para-
graph 4.3.1.1, a minimum negative (cathodic) volt-
age shift of 300 mV is produced by the application
of protective current. This voltage shift shall be
measured between the structure surface and a ref-
erence electrode contacting the water; it includes
the voltage drop across the steel/water interface,
but does not include the voltage drop in the water
(see Paragraphs 4.5.1.1 and 4.6.1).
4.3.1.3 The potential at which corrosion is con-
trolled is a function of the temperature and the
environment. In waters exposed to the air and at
typical ambient temperatures, the criteria listed in
Paragraphs 4.3.1.1 and 4.3.1.2 have proved to be
satisfactory. For other circumstances, the poten-
tial to control corrosion can be estimated using the
Nernst equation.
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4.3.2 Visual inspection
4.3.2.1 Any visual inspection method (diver visual
or feel, physical measurement, photography, or
television) should indicate no progression of cor-
rosion beyond limits acceptable for intended struc-
ture life throughout the structure.
4.3.3 Coupons
4.3.3.1 Corrosion type and rate must be within
limits acceptable for intended structure life.
4.4 Alternative Reference Electrodes
4.4.1 Other standard reference electrodes that may be
substituted for the Ag/AgCl (sw) are listed below with
their voltage equivalent to -0.80 V referred to a Ag/AgCl
(sw):
4.4.1.1 Saturated copper/copper sulfate reference
electrode (CSE): -0.85 V (or more negative for pro-
tection). NOTE: This electrode is not stable for
long-term immersion service.
4.4.1.2
High-purity zinc reference electrode:
+0.25 V (or less positive for protection) corre-
sponding to ASTM
(2)
(B 418
7
Type II, high-purity
zinc having the following composition limits: alumi-
num: 0.005% maximum; cadmium: 0.003% maxi-
mum; iron: 0.0014% maximum; zinc: balance.
4.4.1.3 Saturated calomel reference electrode
(SCE [saturated KCl]): -0.78 V (or more negative
for protection).
4.4.1.4 MIL specification
(3)
zinc reference elec-
trode: +0.25 V (or less positive for protection) cor-
responding to MIL-A-18001
8
having the following
composition limits: aluminum: 0.10% to 0.50%;
cadmium: 0.025% to 0.15%; iron: 0.005% maxi-
mum; lead: 0.005% maximum; copper: 0.005%
maximum; silicon: 0.125% maximum.
__________________________________________
(2)
ASTM International, 100 Barr Harbor Drive, West Conshochocken, PA 19428.
(3)
Military Specification, available from Government Printing Office, Washington, DC 20402.