Basic Corrosion Basic Corrosion and and Cathodic Protection - - PowerPoint PPT Presentation

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Basic Corrosion Basic Corrosion and and Cathodic Protection - - PowerPoint PPT Presentation

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Basic Corrosion & Cathodic Protection Basic Corrosion Basic Corrosion and and Cathodic Protection Cathodic Protection Jeff Schramuk NACE CP Specialist #7695 www.cpsolutionsinc.net 1 Topics to be Covered Why Should We Be Concerned

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Jeff Schramuk

NACE CP Specialist #7695 www.cpsolutionsinc.net

Basic Corrosion Basic Corrosion and and Cathodic Protection Cathodic Protection

Basic Corrosion & Cathodic Protection

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Topics to be Covered

Why Should We Be Concerned about Corrosion? Definitions and Terminology Forms of Corrosion Pipe Coatings and Cathodic Protection Cathodic Protection using Magnesium Anodes Advantages & Limitations of Galvanic Anode CP Systems Impressed Current Cathodic Protection Measurement and Testing of CP Systems Field Test Equipment Cathodic Protection Criteria.

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Why Should We Be Concerned about Corrosion? Definitions and Terminology Forms of Corrosion Pipe Coatings and Cathodic Protection Cathodic Protection using Magnesium Anodes Advantages & Limitations of Galvanic Anode CP Systems Impressed Current Cathodic Protection Measurement and Testing of CP Systems Field Test Equipment Cathodic Protection Criteria.

Basic Corrosion & Cathodic Protection

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Effects of Infrastructure Corrosion

Life Safety Economics Environmental Regulatory Compliance

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Why Should We Be Concerned about Corrosion? Definitions and Terminology Forms of Corrosion Pipe Coatings and Cathodic Protection Cathodic Protection using Magnesium Anodes Advantages & Limitations of Galvanic Anode CP Systems Impressed Current Cathodic Protection Measurement and Testing of CP Systems Field Test Equipment Cathodic Protection Criteria.

Basic Corrosion & Cathodic Protection

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Corrosion Can be Defined as:

Practical Definition Scientific Definition

The Tendency

  • f a Metal to

Revert to its Native State Electrochemical Degradation of Metal as a Result of a Reaction with its Environment

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Corrosion - A Natural Process

IRON OXIDE REFINING MILLING IRON CORROSION IRON OXIDE

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Four Basic Parts of a Corrosion Cell Anode – A metal electrode in contact with the electrolyte which corrodes Cathode - A metal electrode in contact with the electrolyte which is protected against corrosion Electrolyte – A solution or conducting medium such as soil, water or concrete which contains

  • xygen and dissolved chemicals

Metal Path – An external circuit that connects the anode and the cathode

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Electron Flow vs. Conventional Current Flow of conventional current is from positive (+) to negative (-) Conventional current flow from (+) to (-) will be from the cathode to the anode in the metal path Conventional current flow from (+) to (-) will be from the anode to the cathode in the electrolyte.

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Anodic Area (Metal Loss)

DC Current

Cathodic Area (Protected)

Definitions - Anodes & Cathodes

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Copper at -200mV Steel at -600mV

The Simplified Corrosion Cell

Copper at -200 mV Steel at -600 mV

  • 1. Anode
  • 2. Cathode
  • 3. Electrolyte
  • 4. Metal Path
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Components of a Familiar Corrosion Cell

CARBON ROD (Cathode) ZINC CASE (Anode) NH4 and Cl- Paste (Electrolyte) WIRE (Metallic Path) I I I I I e-

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Material Potential*

Pure Magnesium

  • 1.75

Magnesium Alloy

  • 1.60

Zinc

  • 1.10

Aluminum Alloy

  • 1.00

Mild Steel (New)

  • 0.70

Mild Steel (Old)

  • 0.50

Cast / Ductile Iron

  • 0.50

Stainless Steel

  • 0.50 to + 0.10

Copper, Brass, Bronze

  • 0.20

Gold +0.20 Carbon, Graphite, Coke +0.40

* Potentials With Respect to Saturated Cu-CuSO4 Electrode

Less Active More

Practical Galvanic Series*

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Corrosion Reaction and Ohm’s Law Ohm’s Law States that: I = ∆E/R where: ∆E = Driving Potential (EA minus EC) EA = Anode Potential (measured in volts) EC = Cathode Potential (measured in volts) I = Current Flow (measured in amperes) R = Resistance (measured in ohms)

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Some Common Electrical Quantities

Current Flow: 1 ampere (A) = 1000 milliamps (mA)

Examples: A sacrificial anode’s output is measured in mA A CP rectifier’s output is can be up 100 A

Voltage: 1 volt (V) = 1000 millivolts (mV)

Examples: A magnesium anode’s potential is ~1.6 V (1600 mV) A CP rectifier can have a DC voltage of up to 100 V

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Corrosion Cell - Anodic Reactions

Copper at -200mV Steel at -600mV Cathode Anode I e- Fe++ Fe++ Fe++ OH- OH- OH- I I

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Corrosion Cell - Cathodic Reactions

Copper at -200mV Steel at -600mV Cathode Anode I e- H+ H+ H+ H+ e- e- e- e- I

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Corrosion Cell – Combined Reactions

Copper at -200mV Steel at -600mV Cathode Anode I e- H2 H2 e- H2 H2 Fe2(OH)3 Fe2(OH)3 Fe2(OH)3 I

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Why Should We Be Concerned about Corrosion? Definitions and Terminology Forms of Corrosion Pipe Coatings and Cathodic Protection Cathodic Protection using Magnesium Anodes Advantages & Limitations of Galvanic Anode CP Systems Impressed Current Cathodic Protection Measurement and Testing of CP Systems Field Test Equipment Cathodic Protection Criteria.

Basic Corrosion & Cathodic Protection

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General Corrosion Corrosive environment is uniform around the structure

Anode area is uniformly distributed over the structure Corrosion rate is usually constant over the structure

Environments where uniform attack can occur

Atmospheric, Aqueous, Concrete

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True Uniform Corrosive Attack

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Galvanic Corrosion When two different metals are connected and placed into a corrosive environment. Corrosion current is proportional to the difference in electrochemical energy between the two metals Area Effect

Avoid small anode connected to a large cathode

Distance Effect

Area closest to anode will have the greatest corrosion

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Material Potential*

Pure Magnesium

  • 1.75

Magnesium Alloy

  • 1.60

Zinc

  • 1.10

Aluminum Alloy

  • 1.00

Mild Steel (New)

  • 0.70

Mild Steel (Old)

  • 0.50

Cast / Ductile Iron

  • 0.50

Stainless Steel

  • 0.50 to + 0.10

Copper, Brass, Bronze

  • 0.20

Gold +0.20 Carbon, Graphite, Coke +0.40

* Potentials With Respect to Saturated Cu-CuSO4 Electrode

Less Active More

Practical Galvanic Series*

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Galvanic Corrosion Bimetallic Connection

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Old Pipe (Cathode) New Pipe (Anode)

Old-New Pipe Corrosion Cell

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Steel in Concrete-Soil

Cathodic Zone Anodic Zone Concrete Encasement Pipe in Soil Corrodes Note: Arrows Indicate Direction of DC Current Flow

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Dissimilar Surface Conditions

Pipe (Cathode) Threads Bright Metal (Anode) Scratches (Anode)

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Concentration Cell Corrosion

Due to differences in the environment Differential Soil Aeration – Very common

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Aerated Soil

Differential Soil Aeration

Oxygen diffusing through backfill sustains corrosion to cathodic (top) area of pipe Lack of oxygen at bottom of pipe creates relative corrosion cell to (top) area of pipe Clay soil Clay soil

Anodic Zone Cathodic Zone O2 O2

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Differential Aeration on Cast Iron Pipe

Cathodic Zone Anodic Zone

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Clay (moist low oxygen) Sandy Loam (well drained, high oxygen) Anode Cathode Cathode

Differential Soil Aeration

Factors contributing to an increased corrosive attack are de-icing salts and agricultural fertilizers

Pavement Sandy Loam (well drained, high oxygen)

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Pitting Corrosion Random and highly localized Depth greater than area of attack Most destructive form of corrosion Pit location and growth difficult to predict

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Pitting of Coated Carbon Steel in Soil

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External Pitting: Ductile Iron Water Main

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Selective Leaching Corrosion Selective Leaching

Graphitization (Gray Cast Iron) Dezincification (Brass)

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Dealloying Corrosion (Graphitization)

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Why Should We Be Concerned about Corrosion? Definitions and Terminology Forms of Corrosion Pipe Coatings and Cathodic Protection Cathodic Protection using Magnesium Anodes Advantages & Limitations of Galvanic Anode CP Systems Impressed Current Cathodic Protection Measurement and Testing of CP Systems Field Test Equipment Cathodic Protection Criteria.

Basic Corrosion & Cathodic Protection

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Eliminating the Corrosion Cell

Anode Cathode

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Apply a Bonded Tape Wrapping

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Pitting at a Coating Defect

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Coat the Structure & Electrically Isolate It

What’s Wrong Here?

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Encase the Pipe in a “Corrosion Barrier”

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Why Should We Be Concerned about Corrosion? Definitions and Terminology Forms of Corrosion Pipe Coatings and Cathodic Protection Cathodic Protection using Magnesium Anodes Advantages & Limitations of Galvanic Anode CP Systems Impressed Current Cathodic Protection Measurement and Testing of CP Systems Field Test Equipment Cathodic Protection Criteria.

Basic Corrosion & Cathodic Protection

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Corrosion occurs where current discharges from metal to electrolyte The objective of cathodic protection is to force the entire surface to be cathodic to the environment.

How Cathodic Protection Works

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Current is obtained from a metal of a higher energy level.

Galvanic Anode Cathodic Protection

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Material Potential*

Pure Magnesium

  • 1.75

Magnesium Alloy

  • 1.60

Zinc

  • 1.10

Aluminum Alloy

  • 1.00

Mild Steel (New)

  • 0.70

Mild Steel (Old)

  • 0.50

Cast / Ductile Iron

  • 0.50

Stainless Steel

  • 0.50 to + 0.10

Copper, Brass, Bronze

  • 0.20

Gold +0.20 Carbon, Graphite, Coke +0.40

* Potentials With Respect to Saturated Cu-CuSO4 Electrode

Less Active More

Practical Galvanic Series*

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Copper -200mV Steel -600mV Magnesium -1.7V

  • 1. Anode
  • 2. Cathode
  • 3. Electrolyte
  • 4. Metal Path

Galvanic Corrosion – No C.P. Benefit

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Cathode Cathode Anode

  • 1. Anode
  • 2. Cathode
  • 3. Electrolyte
  • 4. Metal Path

Galvanic Corrosion - Mitigated w/CP

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CP Performance - Can Be Verified

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Sacrificial Anode on a Buried Pipeline

Sacrificial Anode Coating Defect Connection to Pipe Grade

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Coating Defect Connection to Pipe Grade Sacrificial Anode

Sacrificial Anode w/Test Station

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CP Test Station - Terminal Board

structure lead wire anode lead wire insulated terminal board calibrated shunt resistor

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Magnesium Anodes

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Proper distance of anode from pipe

At least 3’ from a coated pipe At least 6’ from bare steel At least 1’ deeper than pipeline Evaluate pipe coating

Install anode carefully – don’t lift by the lead wire Tamp earth firmly around anode package.

Packaged Magnesium Anode Natural Gas PL

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Leave slack in the anode lead wire Wet area thoroughly around anode Make a secure electrical connection to the pipe (e.g. exothermic weld) Repair pipe coating to match original Place test box where it is protected from damage and can be easily located Do not allow any foreign pipeline contacts.

Packaged Magnesium Anode Natural Gas PL (cont.)

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Packaged Magnesium Anode Natural Gas PL (cont.)

*Detail courtesy of Midwest Energy Association *Detail courtesy of Midwest Energy Association

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Why Should We Be Concerned about Corrosion? Definitions and Terminology Forms of Corrosion Pipe Coatings and Cathodic Protection Cathodic Protection using Magnesium Anodes Advantages & Limitations of Galvanic Anode CP Systems Impressed Current Cathodic Protection Measurement and Testing of CP Systems Field Test Equipment Cathodic Protection Criteria.

Basic Corrosion & Cathodic Protection

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No external AC power is required Effective utilization of protective current Simple and inexpensive to install on new underground structures Seldom cause stray DC interference Minimal maintenance requirements.

Galvanic Anode CP Advantages

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Limited driving potential ∆E = (Ea – Ec) Limited current output I = ∆E / Rt Large number of anodes will be required on bare or poorly coated structures Ineffective in high-resistivity soil environments (Rt ).

Galvanic Anode CP Limitations

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Why Should We Be Concerned about Corrosion? Definitions and Terminology Forms of Corrosion Pipe Coatings and Cathodic Protection Cathodic Protection using Magnesium Anodes Advantages & Limitations of Galvanic Anode CP Systems Impressed Current Cathodic Protection Measurement and Testing of CP Systems Field Test Equipment Cathodic Protection Criteria.

Basic Corrosion & Cathodic Protection

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Rectifier Anode Groundbed ( - ) ( + ) Pipeline (Structure)

Surface (Horizontal) Anode System

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Deep Anode (Vertical) Anode System

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Continuous Linear Anode System

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Impressed Current Transformer Rectifier

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Why Should We Be Concerned about Corrosion? Definitions and Terminology Forms of Corrosion Pipe Coatings and Cathodic Protection Cathodic Protection using Magnesium Anodes Advantages & Limitations of Galvanic Anode CP Systems Impressed Current Cathodic Protection Measurement and Testing of CP Systems Field Test Equipment Cathodic Protection Criteria.

Basic Corrosion & Cathodic Protection

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Have you checked your rectifier lately?

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Monitoring Data for a CP Rectifier

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Can you locate your test stations?

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Potential Profile Survey Technique

Reference Cells Test Station Voltmeter-Computer Wire Dispenser & Distance Chainer

Pipeline

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Why Should We Be Concerned about Corrosion? Definitions and Terminology Forms of Corrosion Pipe Coatings and Cathodic Protection Cathodic Protection using Magnesium Anodes Advantages & Limitations of Galvanic Anode CP Systems Impressed Current Cathodic Protection Measurement and Testing of CP Systems Field Test Equipment Cathodic Protection Criteria.

Basic Corrosion & Cathodic Protection

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CP Test Equipment - Multi-Meters

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Multi-Meter Characteristics Basic Functions

Reads AC & DC Volts Reads Ohms (optional diode checker) Reads AC and DC Amps (be careful here!)

Performance Criteria

Field rugged, water/drop resistant High input impedance (min. 20 M-Ω)

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Test Equipment Quality Assurance

Perform pre-test operational checks in accordance with the manufacturer instructions Verify the battery strength (if so equipped) Initiate corrective action for equipment out of specification Have the equipment calibrated each year

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Reference Electrode Basic Components

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Reference Electrode - Maintenance

Periodically verify cell against a known standard Keep porous plug covered when not used Clean and refill the reference cell annually Clean copper rod with a non-metallic abrasive pad Replace w/fresh Cu/CuSO4 solution (½ full at all times) Some Cu/CuSO4 crystals should always remain in suspension Wash hands after using – Cu/CuSO4 solution is hazardous

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P/S Potential Readings

Connect voltmeter to pipe and reference Ensure reference cell plug has good contact with moist soil – not pavement Place reference cell away from anodes Read P/S on DCV scale Record P/S reading using standard forms If polarity is positive, notify corrosion dept.

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Meter Connections

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Why Should We Be Concerned about Corrosion? Definitions and Terminology Forms of Corrosion Pipe Coatings and Cathodic Protection Cathodic Protection using Magnesium Anodes Advantages & Limitations of Galvanic Anode CP Systems Impressed Current Cathodic Protection Measurement and Testing of CP Systems Field Test Equipment Cathodic Protection Criteria.

Basic Corrosion & Cathodic Protection

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Cathodic Protection Criteria

  • 0.85 V (w/IR-drop consideration)
  • 0.85 V Instant-Off

100 mV polarization decay Other criteria determined to be “appropriate” by regulatory authority

DOT Standard – Part 192.463

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NACE International – CP Criteria

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DOT Standard – Part 192.465

Monitoring of Cathodic Protection Potentials tested every 12 months at intervals not exceeding 15 months, or 10% per year to sample entire line every 10 years Rectifiers and critical bonds checked every 2 months at intervals not exceeding 2-1/2 months.

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Do We Have a Good Reading?

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Questions? Questions?

Jeff Schramuk

NACE CP Specialist #7695 www.cpsolutionsinc.net

Basic Corrosion & Cathodic Protection