Transcript CORROSION 101 - Western Regional Gas Conference

WESTERN REGION GAS CONFERENCE AUGUST 21, 2012
CORROSION 101
BASIC CORROSION
MADE
CLEAR AS MUD
PRESENTED BY John Brodar P.E. of the Salt River Project
METALLIC PATH
FUEL
OXYGEN
ANODE
IGNITION SOURCE
FIRE TRIANGLE
CATHODE
ELECTROLYTE
CORROSION RECTANGLE
METALLIC PATH
FUEL
OXYGEN
ANODE
IGNITION SOURCE
FIRE TRIANGLE
CATHODE
ELECTROLYTE
CORROSION RECTANGLE
Just as Fire requires all three conditions (Fuel, Oxygen and an Ignition Source) to burn,
several conditions must be present for Corrosion to occur.
Corrosion requires an anode, a cathode, an electrolyte and a metallic path connecting the
anode and cathode. If any one of these conditions is not present or prevented, corrosion
will not occur. Corrosion is electrochemical in nature: the electrolyte and metallic path
are necessary for current to flow. If there is no current flow there is no corrosion.
METALLIC PATH
ANODE
CATHODE
ELECTROLYTE
CORROSION RECTANGLE
METALLIC PATH
ANODE
CATHODE
ELECTROLYTE
CORROSION RECTANGLE
ACME
CAME
MECA
ECAM … REMOVE ANYONE AND THERE IS NO CORROSION.
METALLIC PATH
ANODE
CATHODE
ELECTROLYTE
CORROSION RECTANGLE
REMOVE THE ANODE
REMOVE THE CATHODE
REMOVE THE METALLIC PATH
REMOVE THE ELECTROLYTE AND YOU
STOP CORROSION.
METALLIC PATH
ANODE
CATHODE
ELECTROLYTE
CORROSION RECTANGLE
REMOVE THE ANODE
REMOVE THE CATHODE
REMOVE THE METALLIC PATH
REMOVE THE ELECTROLYTE AND YOU
STOP CORROSION.
METALLIC PATH
ANODE
CATHODE
ELECTROLYTE
CORROSION RECTANGLE
REMOVE THE ANODE
REMOVE THE CATHODE
REMOVE THE METALLIC PATH
REMOVE THE ELECTROLYTE AND YOU
STOP CORROSION.
METALLIC PATH
ANODE
CATHODE
ELECTROLYTE
CORROSION RECTANGLE
REMOVE THE ANODE
REMOVE THE CATHODE
REMOVE THE METALLIC PATH
REMOVE THE ELECTROLYTE AND YOU
STOP CORROSION.
METALLIC PATH
ANODE
CATHODE
ELECTROLYTE
CORROSION RECTANGLE
REMOVE THE ANODE
REMOVE THE CATHODE
REMOVE THE METALLIC PATH
REMOVE THE ELECTROLYTE AND YOU
STOP CORROSION.
WHAT MAKES SOMETHING AN
ANODE?
WHAT MAKES SOMETHING AN
ANODE?
WHAT MAKES SOMETHING A
CATHODE?
WHAT MAKES SOMETHING AN
ANODE?
WHAT MAKES SOMETHING A
CATHODE?
DIFFERENCES!
WHAT MAKES SOMETHING AN
ANODE?
WHAT MAKES SOMETHING A
CATHODE?
DIFFERENCES!
WHAT MAKES SOMETHING AN
ANODE?
WHAT MAKES SOMETHING A
CATHODE?
DIFFERENCES!
WHAT MAKES SOMETHING AN
ANODE?
WHAT MAKES SOMETHING A
CATHODE?
DIFFERENCES!
Illustration of Ohm’s Law
_
+
E = 1 volt
I
I
1 volt
1000 ohms
R = 1000 ohms
 . 001 amps or 1 milliamps
Illustration of Ohm’s Law
_
+
E = 1 volt
I
I
1 volt
1000 ohms
R = 1000 ohms
 . 001 amps or 1 milliamps
Illustration of Ohm’s Law
_
+
E = 1 volt
I
R = 1000 ohms
Illustration of Ohm’s Law
_
+
I
The “I” is
conventional
current.
Illustration of Ohm’s Law
_
+
I
The “I” is
conventional current.
Conventional
current always
leaves the
positive side of
the battery.
Illustration of Ohm’s Law
_
+
I
The “I” is
conventional current.
Conventional current
always leaves the
positive side of the
battery.
In Cathodic Protection the direction of
conventional current is incredibly
important!
METALLIC PATH
ANODE
CATHODE
ELECTROLYTE
CORROSION RECTANGLE
Electrochemical Circuits
Metallic Path
e-
A
A
+ ions

-
ions
C
Electrolytic Path
Conventional Current Flow
Components of a Corrosion Cell
C o rro sio n C ell
• Anode (oxidation reaction)
M eta llic P a th
e-
– corrosion
• Cathode (reduction reaction)
– no corrosion
• Electrolyte (cations and anions)
• External path (usually metallic)
+ io n s
A
- io n s
E lec tro ly tic P ath
C
Electron and Ion Flow
e-
e-
e-
e-
ee-
ee-
Direction of Electron Flow
e- ee- - ee
e- e
e-
ELECTROLYTE
+
+
CATHODE
+
e
ee
e- - ee
e-
+
+
ANODE
Electron and Ion Flow
e-
e-
e-
e-
e-
ee-
e-
Direction of Electron Flow
e-
e- ee- - ee
e- e
ELECTROLYTE
+
+
CATHODE
+
e
ee
e- - ee
e-
+
+
ANODE
Direction of Conventional Current Flow
Direction of Conventional Current Flow
e-
e-
e-
e-
e-
ee-
e-
Direction of Electron Flow
e-
e- ee- - ee
e- e
ELECTROLYTE
+
+
CATHODE
+
e
ee
e- - ee
e-
+
+
ANODE
Direction of Conventional Current Flow
IN THE ELECTROLYTE, AS
CONVENTIONAL CURRENT
LEAVES THE ANODE
IT TAKES IRON IONS
INTO SOLUTION:
CORROSION OCCURS
Anodic Process (half reaction)
ELECTROLYTE
e-
- e
e
Fe++
e- -e- ee +
- e
Fe++ Fe
e
+
++
+
Fe
Fe
+
eANODE
e
e e
Fe++
Fe++ e- ee
e
+
e
++ Fe
Fe
+
AS CONVENTIONAL CURRENT LEAVES THE ANODE IN
THE ELECTROLYTE CORROSION OCCURS
e-
e-
e-
e-
e-
ee-
eDirection of Conventional Current Flow
e- ee- - ee
e- e
e-
ELECTROLYTE
+
+
CATHODE
+
e
ee
e- - ee
e-
+
+
ANODE
Illustration of Ohm’s Law
_
+
I
The “I” is
conventional current.
Conventional current
always leaves the
positive side of the
battery.
In Cathodic Protection the direction of
conventional current is incredibly
important!
Voltmeter Circuit Connection
E
+
_
VOLTS
I
RA
_
RB
RC
Parallel Connection
+
Voltage Sign
20 mV
+
_
Voltage measurement is positive
Potential Measurement Between Two
Reference Electrodes
+ Reading
Reference
Electrode
+
_
Current
Voltmeter with
+ Reading
Reference
Electrode
Sign of Voltage for Dissimilar Metals
.600 V
+
Noble
_
Voltage
measurement
is positive
Active
Sign of Voltage for Dissimilar Metals
.600 V
+
_
Voltage
measurement
is positive
ANODE
NEGATIVE OXIDATION
RUST
LOSE ELECTRONS
LOSE POSITIVE IONS
GAIN NEGATIVE IONS
CATHODE
POSITIVE +
REDUCTION
DOES NOT RUST
GAINS ELECTRONS
GAINS POSITIVE IONS
REPELS NEGATIVE
IONS
Noble
Active
Electrochemical Circuits
Metallic Path
e-
A
A
+ ions

-
ions
C
Electrolytic Path
Conventional Current Flow
P ip e-to -S o il P o ten tial M easu rem en t
Voltmeter Connections
V oltm eter
.900 v
+
_
M eter display is a
postive reading.
R ecord a negative
P /S P otential.
V oltm eter
-.900 v
+
_
R eference
C ell
M eter display is a
negative reading.
R ecord a negative
P /S P otential.
R eference
C ell
E lectrolyte
E lectrolyte
P ipe
P ipe
WHAT ARE THE FOUR MOST
COMMONLY USED METALS
UNDERGROUND?
WHAT ARE THE FOUR MOST
COMMONLY USED METALS
UNDERGROUND?
STEEL (IRON)
WHAT ARE THE FOUR MOST
COMMONLY USED METALS
UNDERGROUND?
STEEL (IRON)
COPPER
WHAT ARE THE FOUR MOST
COMMONLY USED METALS
UNDERGROUND?
STEEL (IRON)
COPPER
GALVANIZED STEEL (ZINC)
WHAT ARE THE FOUR MOST
COMMONLY USED METALS
UNDERGROUND?
STEEL (IRON)
COPPER
GALVANIZED STEEL (ZINC)
MAGNESIUM
WHAT ARE THE FOUR MOST
COMMONLY USED METALS
UNDERGROUND?
STEEL (IRON)
COPPER
GALVANIZED STEEL (ZINC)
MAGNESIUM
WHAT ARE THE FOUR MOST
COMMONLY USED METALS
UNDERGROUND?
WHICH IS AN ANODE?
WHAT ARE THE FOUR MOST
COMMONLY USED METALS
UNDERGROUND?
WHICH IS AN ANODE?
WHICH IS A CATHODE?
WHAT ARE THE FOUR MOST
COMMONLY USED METALS
UNDERGROUND?
WHICH IS AN ANODE?
WHICH IS A CATHODE?
ALL OF THEM CAN BE
EITHER!
DID YOU KNOW THAT EACH
OF THESE METALS HAS A
DIFFERENT NATURAL
VOLTAGE OR POTENTIAL?
STEEL (IRON)
COPPER
GALVANIZED STEEL (ZINC)
MAGNESIUM
COMPARE OTHER METALS TO
STEEL
INTRODUCE THE REFERENCE CELL
TYPICAL POTENTIALS RELATIVE TO
CSE
Reference Electrodes (Half Cells)
Portable Reference Electrodes
Copper-Copper Sulfate Reference Electrode
Removal
Cap
Connection
for Test
Lead
Copper Rod
Clear
Window
Porous
Plug
Saturated Copper
Sulfate Solution
Undissolved Copper
Sulfate Crystals
CORROSION IS AN ELECTROCHEMICAL PHENOMENON.
IN WATER IMMERSION SERVICE
IT IS RELATIVELY EASY, UNDER
SOME CONDITIONS, TO WORK
WITH THE CHEMICAL PORTION
OF THIS PHENOMENON.
ITS CALLED WATER TREATMENT
AND IS USED IN MANY DIFFERENT
INDUSTRIES.
CORROSION IS AN ELECTRO-CHEMICAL PHENOMENON. IN
WATER IMMERSION SERVICE IT IS RELATIVELY EASY, UNDER
SOME CONDITIONS, TO WORK WITH THE CHEMICAL PORTION
OF THIS PHENOMENON.
UNDERGROUND IT IS VERY DIFFICULT
TO WORK WITH THE CHEMICAL
PORTION. THAT’S WHY IT IS SO
IMPORTANT TO UNDERSTAND AND BE
ABLE TO WORK WITH THE ELECTRICAL
PORTION.
LET’S LOOK AT IRON
WHEN AN IRON ATOM CORRODES
SEVERAL THINGS HAPPEN AT THE
SAME TIME
WHEN AN IRON ATOM CORRODES
SEVERAL THINGS HAPPEN AT THE
SAME TIME
THE IRON ATOM GIVES OFF TWO
ELECTRONS AND BECOMES POSITIVE
WHEN AN IRON ATOM CORRODES
SEVERAL THINGS HAPPEN AT THE
SAME TIME
THE IRON ATOM GIVES OFF TWO ELECTRONS AND
BECOMES POSITIVE
THE IRON IS NO LONGER CALLED AN
ATOM IT IS NOW AN ION WITH A
PLUS TWO VALIANCE.
WHEN AN IRON ATOM CORRODES
SEVERAL THINGS HAPPEN AT THE
SAME TIME
THE IRON ATOM GIVES OFF TWO ELECTRONS AND
BECOMES POSITIVE
THE IRON IS NO LONGER CALLED AN ATOM IT IS NOW AN
ION WITH A PLUS TWO VALIANCE.
THE IRON ION NO LONGER STICKS TO
THE OTHER IRON ATOMS, IT GOES
INTO SOLUTION.
WHEN AN IRON ATOM CORRODES
SEVERAL THINGS HAPPEN AT THE
SAME TIME
THE IRON ATOM GIVES OFF TWO ELECTRONS AND BECOMES
POSITIVE
THE IRON IS NO LONGER CALLED AN ATOM IT IS NOW AN ION
WITH A PLUS TWO VALIANCE.
THE IRON ION NO LONGER STICKS TO THE OTHER IRON ATOMS,
IT GOES INTO SOLUTION.
THE IRON ATOM CORRODES AND THE
CORROSION PRODUCT IS AN IRON ION.
CORROSION 101
FREE SAMPLES:
LET’S MAKE RUST
PRESENTED BY John Brodar P.E. of the Salt River Project
WHEN AN IRON ATOM CORRODES
SEVERAL THINGS HAPPEN AT THE
SAME TIME
THE IRON ATOM GIVES OFF TWO ELECTRONS AND BECOMES
POSITIVE
THE IRON IS NO LONGER CALLED AN ATOM IT IS NOW AN ION
WITH A PLUS TWO VALIANCE.
THE IRON ION NO LONGER STICKS TO THE OTHER IRON ATOMS,
IT GOES INTO SOLUTION.
THE IRON ATOM CORRODES AND THE
CORROSION PRODUCT IS AN IRON ION.
++ ++ ++ ++ ++
++ ++ ++ ++ ++ ++
++ ++ +++ ++ ++ ++
++ ++ ++ ++ ++ ++ ++
++ ++ ++ ++ ++ ++ ++
WHEN WILL THIS END?
++ ++ ++ ++ ++ ++
++ ++ +++ ++ ++ ++
++ ++ ++ ++ ++ ++ ++
++ ++ ++ ++ ++ ++ ++
WHAT CAN WE DO?
++ ++ ++ ++ ++ ++
++ ++ +++ ++ ++ ++
++ ++ ++ ++ ++ ++ ++
++ ++ ++ ++ ++ ++ ++
METALLIC PATH
ANODE
CATHODE
ELECTROLYTE
CORROSION RECTANGLE
YOU’RE RIGHT. THE FIRST LINE OF DEFENSE AGAINST CORROSION IS COATINGS.
THEY ARE RELATIVELY CHEAP AND AMAZINGLY EFFECTIVE.
EXCEPT..
COATINGS ARE EFFECTIVE EXCEPT
AT HOLIDAYS (COATING DEFECTS AT THE TIME OF APPLICATION).
OR AT DAMAGED AREAS.
DAMAGE MAY OCCUR DURING
MANUFACTURE, TRANSPORTATION,
INSTALLATION OR IN SERVICE.
HOW BAD CAN THE
CORROSION AT A
DAMAGED AREA OF
THE COATING BE?
FARADAY’S LAW
FOR STEEL FARADAY’S LAW SAYS
THAT ONE AMPERE OF CURRENT
FLOWING OFF OF STEEL FOR ONE
YEAR WILL CAUSE THE CORROSION
OF 20 POUNDS OF STEEL.
FARADAY’S LAW IS VERY MUCH A
MATHEMATICAL RELATIONSHIP.
½ AMP FOR ONE YEAR WILL CONSUME 10
POUNDS OF STEEL
½ AMP FOR TWO YEARS WILL CONSUME 20
POUNDS OF STEEL
2 AMPS FOR ½ YEAR WILL CONSUME 20
POUNDS OF STEEL
CURRENT FLOWING OFF OF YOUR
PIPELINE WILL CONSUME STEEL.
HOW MUCH DOES A ½” DIAMETER HOLE IN A ¼”
WALL PIPE WEIGH?
NOT MUCH!
JUST 0.0558 LBS.
HOW MUCH CURRENT DOES IT TAKE
TO MAKE THAT ½” HOLE?
1 YEAR @ 0.0028 AMPS
2 YEARS @ 0.0014 AMPS OR 1.4 MILLIAMPS
5 YEARS @ 0.6 ma
10 YEARS @ 0.28 ma that’s little more than ¼ ma
Why coatings?
Why coatings?
BECAUSE COATINGS
ARE THE CHEAPEST
THING WE CAN DO
TO STOP CORROSION.
Why cathodic protection?
Why cathodic protection?
Since coatings are not
perfect we have to do
something to protect
the holidays and
damaged areas. AND
Why cathodic protection?
CATHODIC PROTECTION
IS THE EASIEST THING
TO DO TO A PIPELINE
AFTER IT IS INSTALLED.
DEMO PROTECTED PIPE
Microscopic View of a Corrosion Cell
Cathode
Anode
Microscopic Corrosion
Cell on the Surface of a
Pipeline
Cathodic Protection on a Structure (Macroscopic
view)
Metallic
Connection
Cathode
Anode
Cathodic Protection
Current Applied
Electrolyte
Cathodic Protection Anode
Polarization of a Structure
Native
Potentials
-.5
-.6
-.65
-.6
-.7
-.58
-.5
-.6
-.65
-.6
-.7
-.58
-.5
-.6
Corrosion
Mitigated
84 of 40
NATURALLY OCCURING CATHODE. MORE
POSITIVE
NATURALLY OCCURING ANODE. MORE
NEGATIVE
APPLY (PARTIAL) CATHODIC
PROTECTION!!
Prepare to duck.
Polarization of a Structure
Native
Potentials
Corrosion
Mitigated
86 of 40
-.6
-.65
-.6
-.7
-.58
-.58 -.6
-.65
-.6
-.7
-.58
-.5
APPLY (PARTIAL) CATHODIC PROTECTION!!
APPLY MORE CATHODIC PROTECTION
Polarization of a Structure
Native
Potentials
Corrosion
Mitigated
88 of 40
-.6
-.65
-.6
-.7
-.58
-.58 -.6
-.65
-.6
-.7
-.58
-.6
-.65
-.6
-.7
-.6
-.5
-.6
APPLY (PARTIAL) CATHODIC PROTECTION!!
APPLY MORE CATHODIC PROTECTION
APPLY EVEN MORE
CATHODIC PROTECTION
Polarization of a Structure
Native
Potentials
-.6
-.65
-.6
-.7
-.58
-.58 -.6
-.65
-.6
-.7
-.58
-.6
-.65
-.6
-.7
-.6
-.65
-.65
-.7
-.65
-.5
-.6
-.65 -.65
Corrosion
Mitigated
90 of 40
APPLY (PARTIAL) CATHODIC PROTECTION!!
APPLY MORE CATHODIC PROTECTION
APPLY SUFFICIENT CATHODIC
PROTECTION
Polarization of a Structure
Native
Potentials
-.6
-.65
-.6
-.7
-.58
-.58 -.6
-.65
-.6
-.7
-.58
-.6
-.65
-.6
-.7
-.6
-.65
-.65
-.7
-.65
-.7
-.7
-.7
-.7
-.5
-.6
-.65 -.65
Corrosion
Mitigated
92 of 40
-.7
-.7
Cathodic Protection on a Structure (Macroscopic
view)
Metallic
Connection
Cathode
Anode
Cathodic Protection
Current Applied
Electrolyte
Cathodic Protection Anode
Polarization of a Structure
Native
Potentials
Corrosion
Mitigated
-.5
-.6
-.65
-.6
-.7
-.58
-.58
-.6
-.65
-.6
-.7
-.58
-.6
-.6
-.65
-.6
-.7
-.6
-.65
-.65
-.65
-.65
-.7
-.65
-.7
-.7
-.7
-.7
-.7
-.7
9 of 40
Cathodic Protection
Cathodic protection is the cathodic polarization of all noble potential areas (cathodes) to the most active potential on the metal surface. Cathodic
protection is achieved by making the structure the cathode of a direct current circuit. The flow of current in this circuit is adjusted to assure that
the polarized potential is at least as active as the most active anode site on the structure. NACE CP 1
When the potential of all cathode sites reach the open circuit potential of the most active anode site, corrosion on the structure is eliminated.
NACE CP2 Slides
Cathodic protection is the polarization of the most cathodic areas on a structure to a potential equal to or more negative than the most anodic
potential on the structure. When all areas are polarized to a potential equal to or more negative than -850 mv relative to a copper copper sulfate
reference electrode, all corrosion has been halted.
DEMO TEST REELS
Close Interval Potential Survey
Reading
+
Voltmeter
_
Cu/Cu SO4
Ref. Cell
Electrolyte
Pipe
CIS Potential Profile
DEMO TWO WIRES
Shunts
Current Shunts
• Measure voltage drop across a known
resistance.
• Current is calculated using Ohm’s Law.
Shunt Measurement
E
+
RA
I
RB
C u rren t S h u n t w ith
kn ow n resistan ce valu e
is in series w ith th e
circu it
I calcu lated =
_
RC
V o ltm eter is
co n n ected in
p aralle l acro s s
th e cu rren t
sh u n t
V m easu red
R sh u n t
V O LT S
_
+
Current Shunt Calculations #1
Given:
Shunt = .01 ohms
Voltage across shunt = 50 mV
Calculate Current:
1. Convert units of voltage, 50mV = .05 v
2. Calculate current using Ohm’s Law,
I = .05 v /.01 ohms = 5 amps
Current Shunt Calculations #2
Given:
Shunt = 15 amps 50 millivolts
Voltage across shunt = 28 mV
Calculate Current:
I = 28 mV x 15 amps = 8.4 amps
50 mV
Direction of Current Flow
E
+
_
Up Scale
Deflection
VOLTS
_
RA
I
RB
Current Flow is
from Left to Right
RC
+
Typical Current Measurements
There are several current measurements
commonly made in cathodic protection surveys:
• Current output of a galvanic anode system
• Rectifier current output
• Test current for determining current
requirement of a structure
• Current on a structure (this is a voltage
measurement, and current is calculated)
• Current across a bond
Current Along a Pipeline
2-Wire Line Current Test
P ip e size a n d
w a ll th ickn e ss o r
w e ig h t p e r fo o t
m u st b e kn o w n
W est
0.17
mV
N
W ire s m u st b e
co lo r co d e d
P ip e lin e
P ip e S p a n in F e e t
+
_
E ast
Example of 2-Wire Current Line Calculations
•
•
•
•
•
Pipe span = 200 feet
Pipe is 30-inch weighing 118.7 pounds/ foot
Voltage drop across span = 0.17 millivolts
Determined resistance of span = 4.88 x 10-4 ohms
Calculated current flow = 348 milliamps from west to
east
4-Wire Line Current Test
0.17 mV
VOLTS
+
_
+
Power
Source
AMPS
Current
Interrupter
_
+
Pipeline
Pipe Span for Measuring Current
_
Wires
must
be
color
coded
CLEARLLY THE 4 WIRE LINE
CURRENT TEST IS MORE COMPLEX.
YOU ONLY HAVE TO DO IT ONCE
FOR ANY PARTICULAR PIPE
SEGMENT TO DETERMINE THE
RESISTANCE.
IT IS A MULTI STEP PROCESS.
AFTER YOU BECOME AN OHM’S
LAWYER AND CAN WORK WITH
E=IR
I=E/R
R=E/I
YOU WILL UNDERSTAND THAT IF
YOU PASS A KNOWN CURRENT
AND MEASURE A VOLTAGE YOU
CAN USE OHM’S LAW TO
CALCULATE RESISTANCE.
ONCE YOU KNOW THE RESISTANCE
FOR A SECTION OF PIPE, YOU CAN
NOW MEASURE A VOLTAGE DROP
AND, AGAIN USING OHM’S LAW,
CALCULAE THE ACTUAL CURRENT
FLOWING IN THE PIPE.
4-Wire Line Current Test
0.17 mV
VOLTS
+
_
+
Power
Source
AMPS
Current
Interrupter
_
+
Pipeline
Pipe Span for Measuring Current
_
Wires
must
be
color
coded
Example of 4-Wire Current Line
Calculations
• Test current = 10 amps
• Potential shift due to test current (ON= 5.08millivolts
and OFF = 0.17 millivolts) = 4.91 millivolts
• Calibration factor (10/4.91) = 2.04 amps/millivolts
• Voltage drop across span = 0.17 millivolts
• Calculated current flow (2.04 x 0.17) = 347 milliamps
from east to west
WHY ARE ANODES
SOMETIMES NEGATIVE
AND SOMETIMES
POSITIVE?????
A
Galvanic Anode Cathodic Protection
C a th o d ic P ro System
te c tio n S y s te m
CURRENT
STR
UR
T
C
U
E
CURRENT
ANODE
ST
TU
RUC
RE
CURRENT
P ow er
S ou rce
-
+
CURRENT
CURRENT
Impressed Current Cathodic
Im p re s s Protection
e d C u rre n t S y s te m
ANODE
APPLY (PARTIAL) CATHODIC PROTECTION!!
APPLY MORE CATHODIC PROTECTION
APPLY SUFFICIENT CATHODIC
PROTECTION
OVER PROTECT A SEGMENT OF
PIPELINE
Electrical Shielding due to Shorted Casing
Vent Pipe
Pavement
Casing
Pipe Lying on Casing due to
Lack of Insulating Spacers
C a th o d ic P ro te c tio n T u to ria l T w o
 N A C E In te rn a tio n a l, 2 0 0 1
End Seal
4-Wire Line Current Test
0.17 mV
VOLTS
+
_
+
Power
Source
AMPS
Current
Interrupter
_
+
Pipeline
Pipe Span for Measuring Current
_
Wires
must
be
color
coded
Criteria for Cathodic Protection
• Cathodic protection is a polarization phenomenon.
• Cathodic protection is achieved when the open circuit
potential of the cathodes are polarized to the open
circuit potential of the anodes.
• Practical application makes use of
structure-to-electrolyte potentials.
NACE Standards for Underground or Submerged
Iron and Steel
• SP0169 Control of External Corrosion on
Underground or Submerged Metallic Piping
Systems
• SP0285 Corrosion Control of Underground
Storage Tank Systems by Cathodic Protection
Criteria for Underground or Submerged Iron or
Steel Structures
• –0.850 volt potential--Negative (cathodic) potential of at
least 850 mV with the cathodic protection applied
• –0.850 volt polarized potential--Negative polarized potential
of at least 850 mV
• 100 millivolts polarization--Minimum of 100 mV of cathodic
polarization
Voltage (IR) Drops Across a Measuring Circuit
Resistances
Measuring Lead (+)
Contact Lead (+)/Ref. Cell
Reference Cell
Contact Reference Cell
to Electrolyte
Electrolyte
Polarization
Structure
Contact Test Lead/Structure
Test Lead
Contact Test/Measuring Lead
Measuring Lead (-)
Internal Meter
C ath od ic P ro tec tion T u torial T h re e
 N A C E In te rn atio n al, 2 0 0 1
Voltmeter
.900 v
+
_
Reference
Cell
Electrolyte
Polarization
Film
Measurement
& C.P. current
across
electrolyte
Structure
IR Drops Across Electrolyte
• Reference electrode placement
• Current interruption
Pipe-to-Soil Potentials
-
Potential
-850 mV On
IR
-850 mV Instant Off
or IR Corrected
100 mV
Polarization
Depolarization
+
Depolarized Potential
t=0
Time
Santan Raw Water Tank Potentials 8/2/79
0.85
0.81
0.81
Potential to CuCuSo4 (Negative Numbers)
0.8
0.78
0.77
0.76
0.75
0.75
0.74
0.74
0.7
0.69
0.68
0.675
0.67
0.65
0.6
0
10
20
30
40
50
Time in Minutes
60
70
80
90
Santan Raw Water Tank Potentials 8/2/79
0.85
Steady State (48 Amps)
Rectifier Turned Off
Potential to CuCuSo4 (Negative Numbers)
0.8
0.75
0.7
0.65
0.6
0
10
20
30
40
50
Time in Minutes
60
70
80
90
Santan Raw Water Tank Potentials 8/2/79
0.85
Steady State
Rectifier Turned Off
Potential to CuCuSo4 (Negative Numbers)
0.8
0.75
Instant Off Potential
0.7
0.65
0.6
0
10
20
30
40
50
Time in Minutes
60
70
80
90
Santan Raw Water Tank Potentials 8/2/79
0.85
Steady State
I
R
Potential to CuCuSo4 (Negative Numbers)
0.8
D
R
O
P
0.75
Instant Off
0.7
0.65
0.6
0
10
20
30
40
50
Time in Minutes
60
70
80
90
Santan Raw Water Tank Potentials 8/2/79
0.85
Steady State
I
R
Potential to CuCuSo4 (Negative Numbers)
0.8
D
R
O
P
0.75
Instant Off
0.7
Depolarization
Occurs Over Time
0.65
0.6
0
10
20
30
40
50
Time in Minutes
60
70
80
90
Santan Raw Water Tank Potentials 8/2/79
0.85
Steady State
I
R
Potential to CuCuSo4 (Negative Numbers)
0.8
D
R
O
P
0.75
Instant Off
0.7
Depolarization Occurs Over Time
0.65
Rectifier Turned On
(Adjusted to 36 Amps)
0.6
0
10
20
30
40
50
Time in Minutes
60
70
80
90
Santan Raw Water Tank Potentials 8/2/79
0.85
Steady State
I
R
Potential to CuCuSo4 (Negative Numbers)
0.8
D
R
O
P
0.75
Instant Off
I
R
D
R
O
P
0.7
Depolarization Occurs Over Time
0.65
Rectifier Turned On
0.6
0
10
20
30
40
50
Time in Minutes
60
70
80
90
Santan Raw Water Tank Potentials 8/2/79
0.85
Steady State
I
R
Potential to CuCuSo4 (Negative Numbers)
0.8
D
R
O
P
Instant On Potential
0.75
Instant Off
I
R
D
R
O
P
0.7
Depolarization Occurs Over Time
0.65
Rectifier Turned On
0.6
0
10
20
30
40
50
Time in Minutes
60
70
80
90
Santan Raw Water Tank Potentials 8/2/79
0.85
Steady State
I
R
Polarization Increases Over Time.
Potential to CuCuSo4 (Negative Numbers)
0.8
D
R
O
P
Instant On Potential
0.75
Instant Off
I
R
D
R
O
P
0.7
Depolarization Occurs Over Time
0.65
Rectifier Turned On
0.6
0
10
20
30
40
50
Time in Minutes
60
70
80
90
Santan Raw Water Tank Potentials 8/2/79
0.85
THIS POTENTIAL INCREASE IS CATHODIC PROTECTION !
Steady State
I
R
Potential to CuCuSo4 (Negative Numbers)
0.8
D
R
O
P
Instant On Potential
0.75
Instant Off
I
R
D
R
O
P
0.7
Depolarization Occurs Over Time
0.65
Rectifier Turned On
0.6
0
10
20
30
40
50
Time in Minutes
60
70
80
90
Santan Raw Water Tank On Potentials
45
40
35
Height of Tank In Feet
30
25
20
15
10
5
0
0.80
0.90
1.00
1.10
1.20
1.30
1.40
1.50
1.60
1.70
Potential to CuSO4 (Negative Numbers)
11 V 33.5 Amps 10/19/79 Water Flowing 1800 GPM
11 V 36 A 8/20/79 No Flow
1.80
1.90