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 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. 2
Basic Corrosion & Cathodic Protection 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. 3
Effects of Infrastructure Corrosion Regulatory Life Safety Compliance Economics Environmental 4
Basic Corrosion & Cathodic Protection 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. 5
Corrosion Can be Defined as: The Tendency Practical of a Metal to Revert to its Definition Native State Electrochemical Scientific Degradation of Metal as a Result of a Definition Reaction with its Environment 6
Corrosion - A Natural Process IRON OXIDE REFINING MILLING IRON CORROSION IRON OXIDE 7
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 oxygen and dissolved chemicals Metal Path – An external circuit that connects the anode and the cathode 8
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. 9
Definitions - Anodes & Cathodes Cathodic Area DC Current (Protected) Anodic Area (Metal Loss) 10
The Simplified Corrosion Cell 1. Anode Steel at -600mV Copper at -200mV 2. Cathode Copper at -200 mV Steel at -600 mV 3. Electrolyte 4. Metal Path 11
Components of a Familiar Corrosion Cell CARBON ROD (Cathode) ZINC CASE I (Anode) I I NH 4 and Cl - Paste (Electrolyte) I I WIRE e - (Metallic Path) 12
Practical Galvanic Series* Material Potential* Pure Magnesium -1.75 Magnesium Alloy -1.60 Zinc -1.10 Active More 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 Less Copper, Brass, Bronze -0.20 Gold +0.20 Carbon, Graphite, Coke +0.40 * Potentials With Respect to Saturated Cu-CuSO 4 Electrode 13
Corrosion Reaction and Ohm’s Law Ohm’s Law States that: I = ∆ E/R where: ∆ E = Driving Potential (E A minus E C ) E A = Anode Potential (measured in volts) E C = Cathode Potential (measured in volts) I = Current Flow (measured in amperes) R = Resistance (measured in ohms) 14
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 15
Corrosion Cell - Anodic Reactions I I Fe++ Steel at -600mV e- Copper at -200mV OH - Fe++ Cathode OH - Anode I OH - Fe++ 16
Corrosion Cell - Cathodic Reactions I Steel at -600mV Copper at -200mV e- e- H+ Cathode e- H+ Anode I e- H+ e- H+ 17
Corrosion Cell – Combined Reactions I Steel at -600mV e- Copper at -200mV e- H 2 Fe 2 (OH) 3 Cathode H 2 Anode I H 2 Fe 2 (OH) 3 H 2 Fe 2 (OH) 3 18
Basic Corrosion & Cathodic Protection 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. 19
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 20
True Uniform Corrosive Attack 21
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 22
Practical Galvanic Series* Material Potential* Pure Magnesium -1.75 Magnesium Alloy -1.60 Zinc -1.10 Active More 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 Less Copper, Brass, Bronze -0.20 Gold +0.20 Carbon, Graphite, Coke +0.40 * Potentials With Respect to Saturated Cu-CuSO 4 Electrode 23
Galvanic Corrosion Bimetallic Connection 24
Old-New Pipe Corrosion Cell Old Pipe New Pipe (Cathode) (Anode) 25
Steel in Concrete-Soil Concrete Note: Arrows Indicate Direction of DC Current Flow Encasement Pipe in Soil Corrodes Cathodic Anodic Zone Zone 26
Dissimilar Surface Conditions Pipe Scratches (Cathode) (Anode) Threads Bright Metal (Anode) 27
Concentration Cell Corrosion Due to differences in the environment Differential Soil Aeration – Very common 28
Differential Soil Aeration Aerated Soil O 2 O 2 Oxygen diffusing through backfill sustains corrosion to cathodic (top) area of pipe Cathodic Zone Clay soil Clay soil Anodic Zone Lack of oxygen at bottom of pipe creates relative corrosion cell to (top) area of pipe 29
Differential Aeration on Cast Iron Pipe Cathodic Zone Anodic Zone 30
Differential Soil Aeration Pavement Sandy Loam Sandy Loam Clay (moist (well drained, (well drained, low oxygen) high oxygen) high oxygen) Cathode Anode Cathode Factors contributing to an increased corrosive attack are de-icing salts and agricultural fertilizers 31
Pitting Corrosion Random and highly localized Depth greater than area of attack Most destructive form of corrosion Pit location and growth difficult to predict 32
Pitting of Coated Carbon Steel in Soil 33
External Pitting: Ductile Iron Water Main 34
Selective Leaching Corrosion Selective Leaching Graphitization (Gray Cast Iron) Dezincification (Brass) 35
Dealloying Corrosion (Graphitization) 36
Basic Corrosion & Cathodic Protection 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. 37
Eliminating the Corrosion Cell Cathode Anode 38
Apply a Bonded Tape Wrapping 39
Pitting at a Coating Defect 40
Coat the Structure & Electrically Isolate It What’s Wrong Here? 41
Encase the Pipe in a “Corrosion Barrier” 42
Basic Corrosion & Cathodic Protection 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. 43
How Cathodic Protection Works 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. 44
Galvanic Anode Cathodic Protection Current is obtained from a metal of a higher energy level. 45
Practical Galvanic Series* Material Potential* Pure Magnesium -1.75 Magnesium Alloy -1.60 Zinc -1.10 Active More 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 Less Copper, Brass, Bronze -0.20 Gold +0.20 Carbon, Graphite, Coke +0.40 * Potentials With Respect to Saturated Cu-CuSO 4 Electrode 46
Recommend
More recommend
