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Marine cathodic protection.


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Cathodic protection of pipelines, ship hulls, offshore plateforms in marine environment.

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Marine cathodic protection.

  2. 2. Contents:  Introduction  Anode materials in marine applications.  Cathodic protection of marine pipelines  Cathodic protection of offshore structures  Cathodic protection of ship hulls. 23/15/2017
  3. 3. Cathodic protection can be defined as a technique of reducing or eliminating the corrosion of a metal by making it the cathode of an electrochemical cell and passing sufficient current through it to reduce its corrosion rate.  ICCP,  SACP 33/15/2017 Introduction
  4. 4. Anode Materials  The choice of anode material depends on whether active (ICCP) or passive (SACP) systems are under consideration. Sacrificial anode : Should be anodic to steel. Must corrode inexpensive and durable Commercial sacrificial anodes are magnesium, aluminum, or zinc or their alloys like Al-Zn-In alloy etc. 43/15/2017
  5. 5. 3/15/2017 5 IC ANODES:  insoluble and corrosion resistant,  sustain high current density, exchange current density,  lower power consumption  examples: High-silicon caste iron, precious metals, Mixed-metal oxide, polymer anodes.
  6. 6. Cathodic Protection of Marine Pipelines • Protective coatings • supplemental CP. 63/15/2017
  7. 7. Effective coating resistance in ohms for one average square foot Current required in amperes Bare Pipea 500 10,000 14.91 25,000 5.964 50,000 2.982 100,000 1.491 1,000,000 0.1491 5,000,000 0.0298 Perfect coating 0.000058 73/15/2017
  8. 8. Coating & Cathodic Protection Cost Coating Cathodic Protection 0 % 100 % Total Cost 83/15/2017
  9. 9. 2.Supplemental CP  The CP system supplements coatings and is intended to control corrosion at holidays in coatings.  Large pipelines : ICCP at one/ both ends.  Most marine pipelines are protected by SACP.  Zn, or Al sacrificial anodes  Electrical contact by insulated copper cables. 3/15/2017 9
  10. 10. Design Considerations. The average cathodic protection current density required to protect a marine pipeline will depend on:  the type of coating applied, the amount of damage, whether or not burial is specified, and the location of the pipeline.  Large-diameter pipe lines can be protected by installing an ICCP system atone or both ends of the pipeline. 103/15/2017
  11. 11. 113/15/2017 Bracelet-type zinc or aluminum /alloys sacrificial anodes Spacing between anodes o Small dia pipeline(≤14in.) = 150m o Large dia pipeline = 300m.
  12. 12. Design procedure  Calculation of current:  Surface area of pipe segment  Fraction of steel assumed to be bare.  Anodes are then sized to fit the condition:  W/C > IL ; where W is the anode weight (kg), C is the alloy consumption rate in kilogram/amp year (kg/Ayr), I is the anode current output (A), and L is the desired design life in years.  I=E/R and R=0.315r/√A (McCoy’sEquation) ; where r is the electrolyte resistivity (Ω cm), and A is the anode area (cm2), R= anode –electrolyte resistance, E= net driving voltage. 3/15/2017 12
  13. 13. Cathodic Protection of Offshore Structures • Most plateforms are not painted below the waterline, CP system causes change in pH near cathode which causes precipitation of minerals and formation of protective scale. 133/15/2017
  14. 14. 3/15/2017 14 • Problems with new technology:  Offshore structures are now being built in deeper, colder water where mineral deposits are less likely to form. mineral deposit require current densities 750 to 1000 mA/m2 (70 to 93 mA/ft2).  Under-protection as current requirement change with depth (Gulf of Mexico and Santa Barbara Channel).  carbonate scales are harder to deposit in deep, cold waters.
  15. 15. Methods used for protection  SACP systems: simple and rugged, do not depend on external electric power supplies.  the weight of sacrificial anodes can be a serious consideration for deep-water platform.  Hybrid designs: ICCP (primary CP) + SACP. Example Murchison Platform 3/15/2017 15
  16. 16. Cathodic protection of ship hulls  Ships normally have protective coatings as their primary means of corrosion control.  Cathodic protection systems are then sized so that an adequate electric current will be delivered to polarize the structure to the desired level.  Bare steel  Holidays in coatings. 3/15/2017 16
  17. 17. Anode material  Aluminum anodes are available for ship hulls, but they can passivate and become inactive on ships that enter rivers or brackish estuaries. For this reason, zinc anodes are almost universally used in commercial service.  ICCP systems are used on very large ships. 3/15/2017 17
  18. 18. ICCP of ship hulls 3/15/2017 18
  19. 19. 3/15/2017 19 Anode surrounded by di-electric shield Impressed-current cathodic protection systems can produce overprotection in some cases. Organic coatings can disbond because of the formation of hydrogen gas bubbles underneath coatings. Coating disbondment can produce increased surface areas that require more cathodic protection and is controlled by placing dielectric shields between the impressed-current anode and the hull.
  20. 20. Thank you 3/15/2017 20