Localized corrosion monitoring device for limited conductivity fluids
View Patent ↗A localized corrosion monitoring (LCM) device is modified to obtain polarization resistance (R p ) from electrical resistance of a probe having a strip of metal under investigation (test electrode), a reference electrode and a temperature sensor (e.g. thermocouple). This configuration allows the probe to be used in areas where only a thin film of conductive fluid is available or required to provide potential monitoring. Thus, the applicability of the LCM technique is broadened. All the above devices are expected to be configured in one item of equipment, except for the probe which may be remotely located from the rest of the equipment.
1. A method for measuring localized corrosion comprising:
(a) placing a test electrode of a metal strip under investigation having length L in contact with a conductive fluid;
(b) placing a temperature sensor in proximity to the metal strip;
(c) measuring the resistance across the length L of the test electrode over a time period Δt to give first and second resistance values R 1 and R 2 ;
(d) calculating changes in resistance due to temperature and subtracting them from measurements of R 1 and R 2 ;
(e) determining polarization resistance R p from the relationship:
R
p
=
[
B
a
B
c
M
W
2.3
(
B
a
+
B
c
)
·
Density
·
2
F
·
L
2
ρ
]
Δ
t
(
R
1
R
2
R
2
-
R
1
)
(
Eq
.
1
)
where:
B a and B c are Tafel slopes of anodic and cathodic reactions, respectively, obtained separately or from known values,
MW is the molecular weight of the metal of the test electrode, Density being its density,
ρ is the specific conductance of the metal,
F is Faraday's constant,
L 2 is the square of the length of the test electrode, and
R 1 and R 2 are the resistance measurements over time period Δt;
(f) measuring the potential of the metal strip, V, relative to a reference electrode;
(g) performing these two measurements (c) and (f) independently; and
(h) subsequently calculating the localized corrosion from I=V/Rp, wherein I is current, where the current is data are calculated from measurements by an electrical resistance (ER) technique.
2. The method of claim 1 where the temperature sensor is selected from the group consisting of a thermocouple, a resistance thermometer and combinations thereof.
3. The method of claim 1 where the conductive fluid has a physical thickness sufficient to conduct charge.
4. The method of claim 1 where the localized corrosion is selected from the group of characteristics consisting of: i) occurrence of individual localized corrosion events, ii) the duration of corrosion events, iii) surface area of a pit associated with corrosion event, iv) depth of penetration of a pit associated with corrosion event, v) rate of penetration of a pit associated with corrosion event, vi) volume of metal displaced by corrosion event and vii) a type of localized corrosion event.
5. The method of claim 1 further comprising estimating a rate of penetration of a pit associated with the localized corrosion from a time rate of change of the measured potential and calculated current.
6. A method for measuring localized corrosion comprising:
(a) placing a test electrode of a metal strip under investigation having length L in contact with a conductive fluid;
(b) placing a temperature sensor in proximity to the metal strip, the temperature sensor selected from the group consisting of a thermocouple, a resistance thermometer and combinations thereof;
(c) measuring the resistance across the length L of the test electrode over a time period Δt to give first and second resistance values R 1 and R 2 ;
(d) calculating changes in resistance due to temperature and subtracting them from measurements of R 1 and R 2 ;
(e) determining polarization resistance R p from the relationship:
R
p
=
[
B
a
B
c
M
W
2.3
(
B
a
+
B
c
)
·
Density
·
2
F
·
L
2
ρ
]
Δ
t
(
R
1
R
2
R
2
-
R
1
)
(
Eq
.
1
)
where:
B a and B c are Tafel slopes of the anodic and cathodic reactions, respectively, obtained separately or from known values,
MW is the molecular weight of the metal of the test electrode, Density being its density,
ρ is the specific conductance of the metal,
F is Faraday's constant,
L 2 is the square of the length of the test electrode, and
R 1 and R 2 are the resistance measurements over time period Δt;
(f) measuring the potential of the metal strip, V, relative to a reference electrode;
(g) performing these two measurements (c) and (f) independently; and
(h) subsequently calculating the localized corrosion from I=V/Rp, wherein I is current, where the localized corrosion is selected from the group of characteristics consisting of: i) occurrence of individual localized corrosion events, ii) the duration of corrosion events, iii) surface area of a pit associated with corrosion event, iv) depth of penetration of a pit associated with corrosion event, v) rate of penetration of a pit associated with corrosion event, vi) volume of metal displaced by corrosion event and vii) a type of localized corrosion event,
where the current is calculated from measurements by an electrical resistance (ER) technique.
7. The method of claim 6 where the conductive fluid has a physical thickness sufficient to conduct charge.
8. The method of claim 6 further comprising estimating a rate of penetration of a pit associated with the localized corrosion from a time rate of change of the measured potential and calculated current.