corrosion cathodic protection · corrosion four parts needed for a corrosion cell to exist 1. anode...
TRANSCRIPT
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Corrosion&
Cathodic Protection
Presented byMarty Iozzo
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Cost of CorrosionNACE International & U.S. Federal Highway Administration ‐ 2002
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Cost of Corrosion
$276,000,000,000!Each Year!!
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…..So What is Corrosion?
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…..So What is Corrosion?
2Fe + O₂ + 2H₂O → 2Fe⁺⁺ + 4OH‐
What?????
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‐ OR ‐
• An Electro‐Chemical Reaction of a Metal With Its Environment
• The Tendency of a Metal to Return to Its Origin
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CorrosionFour parts needed for a corrosion cell to exist
1. Anode – Where corrosion occurs
2. Cathode – Protected from corrosion
3. Electrolyte ‐ Soil or water (any conductiveenvironment) adjacent to – and containingboth the anode and the cathode
4. Metallic Path ‐ Physically connects theanode to the cathode
*Remove any one part, and the corrosion cell cannot exist
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Galvanic Series
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Typical Corrosion Cell
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Corrosion Cell ‐ Battery
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Galvanic Series
Metal Higher (more negative) on the scale is
the Anode
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Corrosion Cell ‐ Galvanic Anode
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Galvanic Series
Metal Higher is Anode
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New Pipe/Old Pipe
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Galvanic Series
Metal Higher is Anode
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Dissimilar Metals
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Galvanic Series
Metal Higher is Anode
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Dissimilar Metals
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Galvanic Series
Metal Higher is Anode
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Bright Metal
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Galvanic Series
Metal Higher is Anode
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Dissimilar Soils
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Dissimilar Soils
Pipeline in Clay is Anodic
Adjacent Pipeline in Sand is Cathodic
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Differential Oxygen
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Differential Oxygen
Pipeline Under a Roadway
Pipeline Under a Railroad
Pipeline Under a Water Crossing
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Stress Corrosion
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Stress Corrosion
Stress Concentration on Bolts
Bolts in Tension
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CP Interference Corrosion
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AC Induction
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Corrosion Prevention &
Corrosion Control(Cathodic Protection)
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How Cathodic Protection Works
As previously mentioned, corrosion occurs where DC current discharges from the structure to the electrolyte at the anode
The objective is to allow the entire structure to be cathodic
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How Cathodic Protection Works
As the potential of the cathode sites polarizetowards the potential of the anode sites, corrosion is reduced.
When the potential of all cathode sites reach the open circuit potential of the most active anode site, corrosion is eliminated.
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Polarization Reduces the ΔV Along the Structure
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CorrosionFour parts needed for a corrosion cell to exist
1. Anode – Where corrosion occurs
2. Cathode – Protected from corrosion
3. Electrolyte ‐ Soil or water (any conductiveenvironment) adjacent to – and containingboth the anode and the cathode
4. Metallic Path ‐ Physically connects theanode to the cathode
*Remove any one part, and the corrosion cell cannot exist
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Cathodic ProtectionFour parts needed for a CP cell to exist
1. Anode – Where corrosion occurs
2. Cathode – Protected from corrosion
3. Electrolyte ‐ Soil or water (any conductiveenvironment) adjacent to – and containingboth the anode and the cathode
4. Metallic Path ‐ Physically connects theanode to the cathode
*Remove any one part, and Cathodic Protection cannot exist
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CP Cell ‐ Battery
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Cathodic Protection
• Galvanic Anode
• Impressed Current
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Galvanic Anode
• Requires No External Power
• Smaller Diameter Pipe
• Coated Structure
• Isolated Structure
• Lesser Current Requirements
• Lesser Concern For Interference
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Coatings
• Fusion Bonded Epoxy (FBE)• Two Part Liquid Epoxy• Polyethylene & Polypropylene• Coal Tar Enamel• Wax• Mastic• Shrink Sleeve• Tape (Hot & Cold Applied)
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Galvanic Anode
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Galvanic Series
Metal Higher is Anode
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Galvanic CP Design
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Impressed Current
• Requires External Power• Lots of Current Needed• Poorly Coated or Bare• Electrical Isolation Not Possible• Larger Diameter Pipe• Buried Tanks & Tank Bottoms• Long Lines• More Chance For Interference
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Impressed Current
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Impressed Current
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Impressed Current
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Interference Bond
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When Is Cathodic Protection Achieved?
When we can compare our measured cathodic protection potentials against, and satisfy a recognized Standard Recommended Practice ‐ while making considerations for ‘IR Drop / Error’
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+
0 .0 00
C LE AN W AT ER
H IG H IN PU T IM PE DA NC E DIGITA L VO LT METER1 0 M OR G RE ATER
C OP PE R/CO PP ER S UL FA TE R EF ERENCE CELLC LE AN , FU LL Y CH AR GE D & CA LIBRATED
T ES T WIRE S WITH A LL IG AT OR CLIPSS EL EC TION O F SH OR T & LO NG W IT H NO S PLICES
C LE AN W ATERT O SA TU RA TE T HE T ES T LOCATION
M IS C. H AN D TO OLST O MA KE M IN OR R EP AIRS O N-SITE
C .P. TES T EQ UIPM ENT
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The ‘Weakest Link’
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IR Drop / Error ‐ Defined
IR Drop is primarily caused by CP current flowing through some resistance
IR Drop is higher when current is large
IR Drop is higher when Resistance is large
IR Drop is higher on well‐coated structures when the distance to the nearest coating holiday is greater (longer DC path)
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Structure‐to‐Soil Potential Measurement
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‐0.85V Current Applied Criterion w/IR Drop Considered
The reference cell is placed as close as possible to the structure under test (near structure)A structure‐to‐soil potential is read and recordedThe reference cell is placed at ‘remote earth’ from the structureA structure ‐to‐soil potential is read and recordedSubtract the difference in potential readings from the ‘near structure’ potential to obtain ‘IR Drop Free’ potentialThis potential must be at least ‐0.85V to meet criterion
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‐0.85V Polarized Criterion(No DC Current Flow)
Interrupt ALL sources of DC current flowAll influencing rectifiersBonds to foreign structuresSacrificial anodes my not be practical
Interruption must be done quickly and simultaneouslyThe reference cell is placed as close as possible to the structure under testRead and record the ‘OFF Cycle’ potential (Instant Off)The ‘OFF Cycle’ potential must be at least ‐0.85V to meet criteria
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100mV Polarization (Decay) Criterion
Interrupt ALL sources of DC current flowAll influencing rectifiersBonds to foreign structuresSacrificial anodes my not be practical
Interruption must be done quickly and simultaneouslyRecord ‘ON Cycle’ potentialRecord ‘OFF Cycle’ potentialTurn off all sources of DC current flowAllow the Structure to ‘Depolarize’There must be at least 100mV potential decay from the ‘OFF Cycle’ potential to the ‘Depolarized’ potential to meet criteria
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100mV Polarization (Formation) Criterion
Remove ALL sources of DC current flow and allow the structure to completely depolarizeRecord the depolarized baselineEnergize the structure and record the ON potentialInterrupt ALL sources of DC current and record the OFF potential (Instant Off)Allow the structure to polarizeThere must be at least 100mV of potential formation from the depolarized baseline to the OFF potential to meet criteria
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Common C.P. Measurement Errors
Faulty Test EquipmentAll test equipment should be in proper working condition. The voltmeter should be calibrated or “known” to be accurate. Alltest leads and jumper wires should be checked for continuity before each use.
Reference Cell ConditionThe reference cell should be clean, fully charged and calibrated.
Poor Structure ConnectionMake sure contact is being made with the structure under test.
Reference Cell PlacementThe reference cell should be placed as near as possible (without touching) the structure under test. The reference cell should be positioned over native soil only. Never attempt to measure through concrete, asphalt, etc.
Soil ConditionSaturate the soil around the test location with clean water if dry conditions are encountered. Avoid Contaminated soil.
IR (Voltage) Drop (Error)See all conditions listed above.
Outside Air TemperatureThe reference cell is stable and calibrated at an ambient temperature of 70 degrees F. The reference cell will have a potentialdifference of 0.5mV per 1 degree F from ambient temperature.
Inclement WeatherNever conduct potential measurements during severe weather conditions. Also, saturated/conductive equipment and personnel will lead to erroneous potential readings.
Experience
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So……. To Summarize
Corrosion is the degradation of steel due to a reaction with its environment
Cathodic Protection is achieved when the cathodic sites of a structure are polarized in the direction to the potential of the most anodic sites on the same structure
Cathodic Protection can be ‘proved’ by following recommended practices to meet criteria