WESTERN REGION GAS CONFERENCE AUGUST 21, 2012 CORROSION 101
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Transcript of WESTERN REGION GAS CONFERENCE AUGUST 21, 2012 CORROSION 101
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WESTERN REGION GAS CONFERENCE AUGUST 21, 2012
CORROSION 101
BASIC CORROSIONMADE
CLEAR AS MUDPRESENTED BY John Brodar P.E. of the Salt River Project
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IGNITION SOURCE
FUEL OXYGEN
METALLIC PATH
ELECTROLYTE
ANODE CATHODE
FIRE TRIANGLE CORROSION RECTANGLE
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IGNITION SOURCE
FUEL OXYGEN
METALLIC PATH
ELECTROLYTE
ANODE CATHODE
FIRE TRIANGLE 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.
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METALLIC PATH
ELECTROLYTE
ANODE CATHODE
CORROSION RECTANGLE
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METALLIC PATH
ELECTROLYTE
ANODE CATHODE
CORROSION RECTANGLE
ACMECAMEMECAECAM … REMOVE ANYONE AND THERE IS NO CORROSION.
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METALLIC PATH
ELECTROLYTE
ANODE CATHODE
CORROSION RECTANGLE
REMOVE THE ANODEREMOVE THE CATHODE
REMOVE THE METALLIC PATHREMOVE THE ELECTROLYTE AND YOU
STOP CORROSION.
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METALLIC PATH
ELECTROLYTE
ANODE CATHODE
CORROSION RECTANGLE
REMOVE THE ANODEREMOVE THE CATHODE
REMOVE THE METALLIC PATHREMOVE THE ELECTROLYTE AND YOU
STOP CORROSION.
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METALLIC PATH
ELECTROLYTE
ANODE CATHODE
CORROSION RECTANGLE
REMOVE THE ANODEREMOVE THE CATHODE
REMOVE THE METALLIC PATHREMOVE THE ELECTROLYTE AND YOU
STOP CORROSION.
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METALLIC PATH
ELECTROLYTE
ANODE CATHODE
CORROSION RECTANGLE
REMOVE THE ANODEREMOVE THE CATHODE
REMOVE THE METALLIC PATHREMOVE THE ELECTROLYTE AND YOU
STOP CORROSION.
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METALLIC PATH
ELECTROLYTE
ANODE CATHODE
CORROSION RECTANGLE
REMOVE THE ANODEREMOVE THE CATHODE
REMOVE THE METALLIC PATHREMOVE THE ELECTROLYTE AND YOU
STOP CORROSION.
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WHAT MAKES SOMETHING AN ANODE?
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WHAT MAKES SOMETHING AN ANODE?
WHAT MAKES SOMETHING A CATHODE?
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WHAT MAKES SOMETHING AN ANODE?
WHAT MAKES SOMETHING A CATHODE?
DIFFERENCES!
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WHAT MAKES SOMETHING AN ANODE?
WHAT MAKES SOMETHING A CATHODE?
DIFFERENCES!
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WHAT MAKES SOMETHING AN ANODE?
WHAT MAKES SOMETHING A CATHODE?
DIFFERENCES!
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WHAT MAKES SOMETHING AN ANODE?
WHAT MAKES SOMETHING A CATHODE?
DIFFERENCES!
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Illustration of Ohm’s Law_ +
E = 1 volt
R = 1000 ohmsI
milliampsorampsohms
volt1001.
1000
1I
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Illustration of Ohm’s Law_ +
I
milliampsorampsohms
volt1001.
1000
1I
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Illustration of Ohm’s Law_ +
I
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Illustration of Ohm’s Law_ + The “I” is
conventional current.
I
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Illustration of Ohm’s Law_ + The “I” is
conventional current.
Conventional current always
leaves the positive side of
the battery.
I
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Illustration of Ohm’s Law_ + The “I” is
conventional current.
Conventional current always leaves the positive side of the
battery.
I
In Cathodic Protection the direction of conventional current is incredibly
important!
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METALLIC PATH
ELECTROLYTE
ANODE CATHODE
CORROSION RECTANGLE
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Electrochemical Circuits
C
e-Metallic Path
+ ions
- ions
Electrolytic Path
Conventional Current Flow
A C
Metallic Path
+ ions
ions
Electrolytic Path
Conventional Current Flow
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Components of a Corrosion Cell
• Anode (oxidation reaction)
– corrosion
• Cathode (reduction reaction)
– no corrosion
• Electrolyte (cations and anions)
• External path (usually metallic)
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Electron and Ion Flow
++
+
+
ELECTROLYTE
e-
e-
e-
e-
e-
e-e-
e-
e-
e-
e-e-
e-e-
e-
Direction of Electron Flow
ELECTROLYTEELECTROLYTE
CATHODE ANODE
e-
e-
e-
e-e-
e-
e-
e-
e-e-
e-e-
e-
Direction of Electron Flowe-
e-
e-e-e- e- e-
e-
+
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Electron and Ion Flow
++
+
+
ELECTROLYTE
e-
e-
e-
e-
e-
e-e-
e-
e-
e-
e-e-
e-e-
e-
Direction of Electron Flow
ELECTROLYTEELECTROLYTE
CATHODE ANODE
e-
e-
e-
e-e-
e-
e-
e-
e-e-
e-e-
e-
Direction of Electron Flowe-
e-
e-e-e- e- e-
e-
+
Direction of Conventional Current Flow
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Direction of Conventional Current Flow
++
+
+
ELECTROLYTE
e-
e-
e-
e-
e-
e-e-
e-
e-
e-
e-e-
e-e-
e-
Direction of Electron Flow
ELECTROLYTEELECTROLYTE
CATHODE ANODE
e-
e-
e-
e-e-
e-
e-
e-
e-e-
e-e-
e-
Direction of Electron Flowe-
e-
e-e-e- e- e-
e-
+
Direction of Conventional Current Flow
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IN THE ELECTROLYTE, AS CONVENTIONAL CURRENT
LEAVES THE ANODE
IT TAKES IRON IONS INTO SOLUTION:
CORROSION OCCURS
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Fe++
Fe++
Fe++
Fe++
Fe++
Fe++
Fe++
Fe++
Fe++
e-
e- e-
e-
e-e-
e-
e-
e-
e-
e-
e-
e-e-
e-
e-e-
e-
ANODE
ELECTROLYTE
Anodic Process (half reaction)
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AS CONVENTIONAL CURRENT LEAVES THE ANODE IN THE ELECTROLYTE CORROSION OCCURS
++
+
+
ELECTROLYTE
e-
e-
e-
e-
e-
e-e-
e-
e-
e-
e-e-
e-e-
e-
Direction of Conventional Current Flow
ELECTROLYTEELECTROLYTE
CATHODE ANODE
e-
e-
e-
e-e-
e-
e-
e-
e-e-
e-e-
e-
e-e-
e-e-e- e- e-
e-
+
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Illustration of Ohm’s Law_ + The “I” is
conventional current.
Conventional current always leaves the positive side of the
battery.
I
In Cathodic Protection the direction of conventional current is incredibly
important!
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Voltmeter Circuit Connection
+ _
+_
VOLTS
Parallel Connection
RA RBRC
E
I
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Voltage Sign
Current
+_
Voltage measurement is positive20 mV
20 mV
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Potential Measurement Between Two Reference Electrodes
+ Reading
+ _
ReferenceElectrode
Voltmeter with+ Reading
ReferenceElectrode
Current
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Sign of Voltage for Dissimilar Metals
Noble Active
+_
Voltage measurement is positive
.600 V
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Sign of Voltage for Dissimilar Metals
Noble Active
+_
Voltage measurement is positive
.600 V
ANODENEGATIVE -OXIDATIONRUSTLOSE ELECTRONSLOSE POSITIVE IONSGAIN NEGATIVE IONS
CATHODEPOSITIVE +REDUCTIONDOES NOT RUSTGAINS ELECTRONSGAINS POSITIVE IONSREPELS NEGATIVE IONS
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Electrochemical Circuits
C
e-Metallic Path
+ ions
- ions
Electrolytic Path
Conventional Current Flow
A C
Metallic Path
+ ions
ions
Electrolytic Path
Conventional Current Flow
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Voltmeter ConnectionsPipe-to-Soil Potential Measurement
.900 v
+_
Pipe
Electrolyte
ReferenceCell
Voltmeter Meter display is apostive reading.Record a negativeP/S Potential.
-.900 v
+ _
Pipe
Electrolyte
ReferenceCell
Voltmeter Meter display is anegative reading.Record a negativeP/S Potential.
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WHAT ARE THE FOUR MOST COMMONLY USED METALS
UNDERGROUND?
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WHAT ARE THE FOUR MOST COMMONLY USED METALS
UNDERGROUND?
STEEL (IRON)
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WHAT ARE THE FOUR MOST COMMONLY USED METALS
UNDERGROUND?
STEEL (IRON)
COPPER
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WHAT ARE THE FOUR MOST COMMONLY USED METALS
UNDERGROUND?
STEEL (IRON)COPPER
GALVANIZED STEEL (ZINC)
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WHAT ARE THE FOUR MOST COMMONLY USED METALS
UNDERGROUND?
STEEL (IRON)COPPER
GALVANIZED STEEL (ZINC)
MAGNESIUM
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WHAT ARE THE FOUR MOST COMMONLY USED METALS
UNDERGROUND?
STEEL (IRON)COPPER
GALVANIZED STEEL (ZINC)MAGNESIUM
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WHAT ARE THE FOUR MOST COMMONLY USED METALS
UNDERGROUND?
WHICH IS AN ANODE?
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WHAT ARE THE FOUR MOST COMMONLY USED METALS
UNDERGROUND?
WHICH IS AN ANODE?WHICH IS A CATHODE?
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WHAT ARE THE FOUR MOST COMMONLY USED METALS
UNDERGROUND?
WHICH IS AN ANODE?WHICH IS A CATHODE?
ALL OF THEM CAN BE EITHER!
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DID YOU KNOW THAT EACH OF THESE METALS HAS A
DIFFERENT NATURAL VOLTAGE OR POTENTIAL?
STEEL (IRON)COPPER
GALVANIZED STEEL (ZINC)MAGNESIUM
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COMPARE OTHER METALS TO STEEL
INTRODUCE THE REFERENCE CELLTYPICAL POTENTIALS RELATIVE TO
CSE
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Reference Electrodes (Half Cells)Reference Electrodes (Half Cells)
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Portable Reference Electrodes
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Copper-Copper Sulfate Reference Electrode
Copper Rod
Saturated CopperSulfate Solution
Undissolved CopperSulfate Crystals
PorousPlug
ClearWindow
Removal Cap
Connection for Test Lead
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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.
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ITS CALLED WATER TREATMENT AND IS USED IN MANY DIFFERENT
INDUSTRIES.
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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.
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LET’S LOOK AT IRON
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WHEN AN IRON ATOM CORRODES SEVERAL THINGS HAPPEN AT THE
SAME TIME
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WHEN AN IRON ATOM CORRODES SEVERAL THINGS HAPPEN AT THE
SAME TIME
THE IRON ATOM GIVES OFF TWO ELECTRONS AND BECOMES POSITIVE
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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.
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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.
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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.
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CORROSION 101
FREE SAMPLES:LET’S MAKE RUST
PRESENTED BY John Brodar P.E. of the Salt River Project
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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.
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++ ++ ++ ++ ++++ ++ ++ ++ ++ ++ ++ ++ +++ ++ ++ ++
++ ++ ++ ++ ++ ++ ++++ ++ ++ ++ ++ ++ ++
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WHEN WILL THIS END?++ ++ ++ ++ ++ ++ ++ ++ +++ ++ ++ ++
++ ++ ++ ++ ++ ++ ++++ ++ ++ ++ ++ ++ ++
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WHAT CAN WE DO?++ ++ ++ ++ ++ ++ ++ ++ +++ ++ ++ ++
++ ++ ++ ++ ++ ++ ++++ ++ ++ ++ ++ ++ ++
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METALLIC PATH
ELECTROLYTE
ANODE CATHODE
CORROSION RECTANGLE
YOU’RE RIGHT. THE FIRST LINE OF DEFENSE AGAINST CORROSION IS COATINGS.
THEY ARE RELATIVELY CHEAP AND AMAZINGLY EFFECTIVE.
EXCEPT..
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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.
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HOW BAD CAN THE CORROSION AT A
DAMAGED AREA OF THE COATING BE?
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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.
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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
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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.
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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
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Why coatings?
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Why coatings?
BECAUSE COATINGS ARE THE CHEAPEST THING WE CAN DO
TO STOP CORROSION.
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Why cathodic protection?
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Why cathodic protection?
Since coatings are not perfect we have to do something to protect
the holidays and damaged areas. AND
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Why cathodic protection?
CATHODIC PROTECTION IS THE EASIEST THING TO DO TO A PIPELINE
AFTER IT IS INSTALLED.
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DEMO PROTECTED PIPE
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AnodeCathode
Microscopic Corrosion Cell on the Surface of a Pipeline
Microscopic View of a Corrosion Cell
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AnodeCathode
Metallic Connection
Electrolyte
Cathodic Protection Anode
Cathodic Protection Current Applied
Cathodic Protection on a Structure (Macroscopic
view)
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84 of 40
-.5 -.6 -.65 -.6 -.7 -.58
-.5 -.6 -.65 -.6
-.6
-.5
NativePotentials
CorrosionMitigated
Polarization of a Structure
NATURALLY OCCURING CATHODE. MORE POSITIVE
NATURALLY OCCURING ANODE. MORE NEGATIVE
-.7 -.58
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APPLY (PARTIAL) CATHODIC PROTECTION!!
Prepare to duck.
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86 of 40
-.5 -.6 -.65 -.6 -.7 -.58
-.58 -.6 -.65 -.6 -.7 -.58
NativePotentials
CorrosionMitigated
Polarization of a Structure
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APPLY (PARTIAL) CATHODIC PROTECTION!!
APPLY MORE CATHODIC PROTECTION
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88 of 40
-.5 -.6 -.65 -.6 -.7 -.58
-.58 -.6 -.65 -.6 -.7 -.58
-.6 -.6 -.65 -.6 -.7 -.6
NativePotentials
CorrosionMitigated
Polarization of a Structure
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APPLY (PARTIAL) CATHODIC PROTECTION!!
APPLY MORE CATHODIC PROTECTION
APPLY EVEN MORE CATHODIC PROTECTION
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90 of 40
-.5 -.6 -.65 -.6 -.7 -.58
-.58 -.6 -.65 -.6 -.7 -.58
-.6 -.6 -.65 -.6 -.7 -.6
-.65 -.65 -.65 -.65 -.7 -.65
NativePotentials
CorrosionMitigated
Polarization of a Structure
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APPLY (PARTIAL) CATHODIC PROTECTION!!
APPLY MORE CATHODIC PROTECTION
APPLY SUFFICIENT CATHODIC PROTECTION
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92 of 40
-.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
NativePotentials
CorrosionMitigated
Polarization of a Structure
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AnodeCathode
Metallic Connection
Electrolyte
Cathodic Protection Anode
Cathodic Protection Current Applied
Cathodic Protection on a Structure (Macroscopic
view)
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9 of 40
-.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
NativePotentials
CorrosionMitigated
Polarization of a Structure
Cathodic ProtectionCathodic 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.
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DEMO TEST REELS
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Close Interval Potential Survey Close Interval Potential Survey Reading
+_
Cu/Cu SO4
Ref. Cell
Voltmeter
Pipe
Electrolyte
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CIS Potential ProfileCIS Potential Profile
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DEMO TWO WIRES
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Shunts
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Current Shunts
• Measure voltage drop across a known resistance.
• Current is calculated using Ohm’s Law.
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Shunt Measurement
Vmeasured
Rshunt
Icalculated =+ _
+_
VOLTS
RA
RB
RC
E
I
Current Shunt with known resistance value is in series with the circuit
Voltmeter is connected in parallel across the current shunt
Vmeasured
Rshunt
Icalculated =Vmeasured
Rshunt
Icalculated =+ _
+_
VOLTS
RA
RB
RC
E
I
Current Shunt with known resistance value is in series with the circuit
Voltmeter is connected in parallel across the current shunt
+ _
+_
VOLTS
RA
RB
RC
E
I
Current Shunt with known resistance value is in series with the circuit
Voltmeter is connected in parallel across the current shunt
+ _
+_
VOLTS
RA
RB
RC
E
I
Current Shunt with known resistance value is in series with the circuit
Voltmeter is connected in parallel across the current shunt
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Current Shunt Calculations #1
Given:Shunt = .01 ohmsVoltage across shunt = 50 mV
Calculate Current:1. Convert units of voltage, 50mV = .05 v2. Calculate current using Ohm’s Law,
I = .05 v /.01 ohms = 5 amps
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Current Shunt Calculations #2
I = 28 mV x 15 amps 50 mV
= 8.4 amps
Given:Shunt = 15 amps 50 millivoltsVoltage across shunt = 28 mV
Calculate Current:
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Direction of Current Flow
+ _
+_
VOLTS
RA
RB
RC
E
I
Current Flow is from Left to Right
Up Scale Deflection
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Typical Current MeasurementsTypical 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
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Current Along a Pipeline
+_
+_
Voltage measurement is positive20 mV
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2-Wire Line Current Test
Pipe Span in Feet
Wires must be color coded
0.17 mV
+ _
Pipe size and wall thickness or weight per foot must be known
Pipeline
N
West East
Pipe Span in Feet
Wires must be color coded
0.17 mV
+ _
Pipe size and wall thickness or weight per foot must be known
Pipeline
N
Pipe Span in Feet
Wires must be color coded
0.17 mV
+ _
Pipe size and wall thickness or weight per foot must be known
Pipeline
Pipe Span in Feet
Wires must be color coded
0.17 mV
+ _
Pipe size and wall thickness or weight per foot must be known
Pipeline
N
West East
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Example of 2-Wire Current Line CalculationsExample 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
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4-Wire Line Current Test4-Wire Line Current Test
Pipe Span for Measuring Current
Wires must be color coded
0.17 mV
+ _
Pipeline
PowerSource
+_
CurrentInterrupter +
_
AMPS
VOLTS
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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.
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AFTER YOU BECOME AN OHM’S LAWYER AND CAN WORK WITH
E=IRI=E/RR=E/I
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YOU WILL UNDERSTAND THAT IF YOU PASS A KNOWN CURRENT
AND MEASURE A VOLTAGE YOU CAN USE OHM’S LAW TO CALCULATE RESISTANCE.
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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.
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4-Wire Line Current Test4-Wire Line Current Test
Pipe Span for Measuring Current
Wires must be color coded
0.17 mV
+ _
Pipeline
PowerSource
+_
CurrentInterrupter +
_
AMPS
VOLTS
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Example of 4-Wire Current Line Calculations
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
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WHY ARE ANODES SOMETIMES NEGATIVE
AND SOMETIMES POSITIVE?????
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Galvanic Anode Cathodic Protection SystemA Cathodic Protection System
ANODE
CURRENT
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Impressed Current Cathodic ProtectionImpressed Current System
ANODE
CU
RR
EN
T
PowerSource
+-
CU
RR
EN
T
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APPLY (PARTIAL) CATHODIC PROTECTION!!
APPLY MORE CATHODIC PROTECTION
APPLY SUFFICIENT CATHODIC PROTECTION
OVER PROTECT A SEGMENT OF PIPELINE
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Electrical Shielding due to Shorted Casing
Pipe Lying on Casing due toPipe Lying on Casing due toLack of Insulating SpacersLack of Insulating Spacers
Vent PipeVent Pipe
End SealCasingCasing
PavementPavement
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4-Wire Line Current Test4-Wire Line Current Test
Pipe Span for Measuring Current
Wires must be color coded
0.17 mV
+ _
Pipeline
PowerSource
+_
CurrentInterrupter +
_
AMPS
VOLTS
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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.
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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
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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
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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
Voltage (IR) Drops Across a Measuring CircuitVoltage (IR) Drops Across a Measuring Circuit
Cathodic Protection Tutorial Three NACE International, 2001
.900 v
+ _
PolarizationFilm Structure
Electrolyte
ReferenceCell
Voltmeter
Measurement & C.P. current across electrolyte
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IR Drops Across Electrolyte
• Reference electrode placement• Current interruption
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Time
Pote
nti
al
+
-
t=0
-850 mV On
-850 mV Instant Offor IR Corrected
IR
Depolarization
Depolarized Potential
100 mVPolarization
Pipe-to-Soil PotentialsPipe-to-Soil Potentials
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Santan Raw Water Tank Potentials 8/2/79
0.81 0.81
0.74
0.69
0.680.675
0.67
0.74
0.75
0.76
0.77
0.78
0.6
0.65
0.7
0.75
0.8
0.85
0 10 20 30 40 50 60 70 80 90
Time in Minutes
Po
ten
tial
to
Cu
Cu
So
4 (N
egat
ive
Nu
mb
ers)
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Santan Raw Water Tank Potentials 8/2/79
0.6
0.65
0.7
0.75
0.8
0.85
0 10 20 30 40 50 60 70 80 90
Time in Minutes
Po
ten
tial
to
Cu
Cu
So
4 (N
egat
ive
Nu
mb
ers)
Steady State (48 Amps) Rectifier Turned Off
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Santan Raw Water Tank Potentials 8/2/79
0.6
0.65
0.7
0.75
0.8
0.85
0 10 20 30 40 50 60 70 80 90
Time in Minutes
Po
ten
tial
to
Cu
Cu
So
4 (N
egat
ive
Nu
mb
ers)
Steady State
Instant Off Potential
Rectifier Turned Off
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Santan Raw Water Tank Potentials 8/2/79
0.6
0.65
0.7
0.75
0.8
0.85
0 10 20 30 40 50 60 70 80 90
Time in Minutes
Po
ten
tial
to
Cu
Cu
So
4 (N
egat
ive
Nu
mb
ers)
Steady State
Instant Off
IR
DROP
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Santan Raw Water Tank Potentials 8/2/79
0.6
0.65
0.7
0.75
0.8
0.85
0 10 20 30 40 50 60 70 80 90
Time in Minutes
Po
ten
tial
to
Cu
Cu
So
4 (N
egat
ive
Nu
mb
ers)
Steady State
Instant Off
IR
DROP
Depolarization Occurs Over Time
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Santan Raw Water Tank Potentials 8/2/79
0.6
0.65
0.7
0.75
0.8
0.85
0 10 20 30 40 50 60 70 80 90
Time in Minutes
Po
ten
tial
to
Cu
Cu
So
4 (N
egat
ive
Nu
mb
ers)
Steady State
Instant Off
IR
DROP
Depolarization Occurs Over Time
Rectifier Turned On (Adjusted to 36 Amps)
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Santan Raw Water Tank Potentials 8/2/79
0.6
0.65
0.7
0.75
0.8
0.85
0 10 20 30 40 50 60 70 80 90
Time in Minutes
Po
ten
tial
to
Cu
Cu
So
4 (N
egat
ive
Nu
mb
ers)
Steady State
Instant Off
IR
DROP
Depolarization Occurs Over Time
Rectifier Turned On
IR
DROP
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Santan Raw Water Tank Potentials 8/2/79
0.6
0.65
0.7
0.75
0.8
0.85
0 10 20 30 40 50 60 70 80 90
Time in Minutes
Po
ten
tial
to
Cu
Cu
So
4 (N
egat
ive
Nu
mb
ers)
Steady State
Instant Off
IR
DROP
Depolarization Occurs Over Time
Rectifier Turned On
IR
DROP
Instant On Potential
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Santan Raw Water Tank Potentials 8/2/79
0.6
0.65
0.7
0.75
0.8
0.85
0 10 20 30 40 50 60 70 80 90
Time in Minutes
Po
ten
tial
to
Cu
Cu
So
4 (N
egat
ive
Nu
mb
ers)
Steady State
Instant Off
IR
DROP
Depolarization Occurs Over Time
Rectifier Turned On
IR
DROP
Instant On Potential
Polarization Increases Over Time.
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Santan Raw Water Tank Potentials 8/2/79
0.6
0.65
0.7
0.75
0.8
0.85
0 10 20 30 40 50 60 70 80 90
Time in Minutes
Po
ten
tial
to
Cu
Cu
So
4 (N
egat
ive
Nu
mb
ers)
Steady State
Instant Off
IR
DROP
Depolarization Occurs Over Time
Rectifier Turned On
IR
DROP
Instant On Potential
THIS POTENTIAL INCREASE IS CATHODIC PROTECTION !
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Santan Raw Water Tank On Potentials
0
5
10
15
20
25
30
35
40
45
0.80 0.90 1.00 1.10 1.20 1.30 1.40 1.50 1.60 1.70 1.80 1.90
Potential to CuSO4 (Negative Numbers)
Hei
gh
t o
f T
ank
In F
eet
11 V 33.5 Amps 10/19/79 Water Flowing 1800 GPM 11 V 36 A 8/20/79 No Flow