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COURSE IN CP INSPECTION METHODS FOR CORROCEAN

COURSE IN CP INSPECTION METHODS FOR CORROCEAN. Part I – Cathodic Protection Part II – CP Inspection Methods. COURSE IN CP INSPECTION METHODS. Part I Cathodic Protection. Offshore Corrosion. C orros i on: Based on the L atin word “corrodere” = to gnaw.

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COURSE IN CP INSPECTION METHODS FOR CORROCEAN

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  1. COURSE IN CP INSPECTION METHODSFOR CORROCEAN • Part I – Cathodic Protection • Part II – CP Inspection Methods

  2. COURSE IN CP INSPECTION METHODS • Part I • Cathodic Protection

  3. Offshore Corrosion Corrosion: Based on the Latin word “corrodere” = to gnaw

  4. Wetcorrosion inan electrolyte containing oxygen

  5. Electrode Potentials

  6. CATHODICPROTECTION • PRINCIPLE: • The material to be protected is supplied with • an externalcathodiccurrent • The electrochemical potentialof the protected material is movedin a negative direction to the immune area • The material is completelyprotected when it reaches the Protection Potential

  7. Pourbaix Diagram

  8. ELEKTROCHEMICAL REACTIONS Corrosionof FE: a) Fe2+ + 2 e- = Fe b) Fe3O4 + 8 H+ + 8 e- = 3 Fe + 4 H2O c) Fe3O4 + 8 H+ + 2 e- = 3 Fe 2+ + 4 H2O d) Fe2O3 + 6 H+ + 2 e- = 2 Fe 2+ + 3 H2O e) O2 + 4 H+ + 4 e- = 2 H2O f) 2 H+ + 2 e- = H2

  9. Applications of the Pourbaix Diagram • Shows what reactionswhichcan occur with different pH and potential • Indication on the composition of the corrosion/oxidation products • Shows the changes of the environment (pH and potential) which are nessesary to avoid corrosion

  10. TECHNICALSOLUTIONS • Sacrificial Anodes • Galvanic couplingto sacrificial anodesmade of Al-alloyorZinc • ImpressedCurrent • Use of source for direct current (DC) andnone corroding anodes

  11. Impressed Cathodic Protection Cathodic Protection

  12. SACRIFICAL ANODE SYSTEMS • Advantages: • Robust system, reduced maintenance • Used on every platform on the Norwegian continental shelf • Disadvantages: • Limited driving voltage (0.25 V) • More anodes necessary for protection • More anodes necessary for securing long operating time • (Not suited for media with low conductivity, e.g. in soil)

  13. IMPRESSED CURRENT • Advantages: • High driving voltage (30 V) • Few anodes – reduced resistance • Disadvantages: • Vulnerablecomponents • Need for regulation/control system • Risk of overprotection of highly charged materials • Coating damages – cathodic accouplement • Need for/recommended protection shield around the anodes • Need for maintenance

  14. Example of Impressed Current Installation

  15. Applications of Impressed Current • Appliedon steel in seawater or soil • Oil Platforms in steel and concrete • Subsea Pipelines • Hull • Quay structures and sheet pile curtains • Concrete bridges placed in seawater • Pipelines buried in soil • Vessels/tanks buried in soil

  16. ELECTROCHEMICAL POTENTIALS • St eel • Corrosion potential ca. -650 mV Ag/AgCl • Protectedat ca. -800 mV Ag/AgCl • Al-anode and Zn-anode • Corrosion potential ca. -1050 mV Ag/AgCl

  17. CATHODICPROTECTION Anodic reactions: Zn = Zn2+ + 2e- Al = Al3+ + 3e- Cathodic reactions: 2 H2O = 4 H+ + 4OH- O2 + 4 H+ + 4 e- = 2 H2O ---------------------------------- O2 + 2 H2O + 4 e- = 4OH- 2 H+ + 2 e- = H2 (g) Anode and cathode reactions are always balanced, i.e congestion of electrons does not exist

  18. CRITERIA FOR CATHODICPROTECTION • Potential Criteria: maximum -800 mV, ref Ag/AgCl • minimum -1100 mV, ref Ag/AgCl • Demand for current: varywith O2 inthe electrolyte • solubility • flow velocity • temperature • construction geometry • geographical site

  19. Calcareous deposit reduces the demand for current: • Calcareous deposits reduce the effective cathodic surface area thereby lowering demand for current. The calcareous deposit is formed when MgOH2and CaCO3 salts precipitate on the cathode (steel surface). • The following changes the composition and quality of the calcareous layer: • current density • temperature • pressure • seawater quality • flow velocity

  20. CATHODIC PROTECTION • The most commonly used sacrifical anode materialsare: • Al-Zn-In • Zn • Mg • Magnesium • relativelyexpensive • low capacity of currentbecause of highselfcorrosion • may cause overprotection • short operating time • Often used where the electrolyte has low conductivity

  21. Zinc: • classical anodematerial • low driving voltage (230 mV) • lowcapacity of current results inhighweight of anodes (780 A/kg) • temperature limits < 40 Co • Often usedon subsea piplinesandconstructionsburied in mud • Aluminium: • has to be alloyed otherwise it is passive • high capacity of current (2500 Ah/kg) • long operating time saves weight • high driving voltage • Al-Zn-In anodesmost commonly used offshore

  22. PRACTICAL CP DESIGN • wherewill the construction be placed? • what kind of environmental parameters should be taken into account (temp.,res.) • areas to protect • operating lifetime • what kind of design standards should be used (DnV, NORSOK, NACE) • what demand for currentis expected • will the construction be protected by coating,if so, what kind of coating • degradation mechanisms for coating (Coating Breakdown) • possible current drainage toe.g.wells, poles, other structure • influence from other structures, pipelines etc.

  23. DEMAND FOR CURRENT INITIALDEMAND FOR CURRENT: Demand for current to polarize the structure down to a safe protection potential ( -800 mV) and build a good calcareous deposit. AVERAGE CURRENT: Demand for current tomaintain a safe protection potentialafter polarization of the structure. Used to calculate necessary anode weight. FINAL DEMAND FOR CURRENT: Demand for current to repolarizethe structure after a possiblebreakdown/damageof the calcareous deposit (after winter storms). It also gives the demand for current at the end of the operating lifetime.

  24. Illustration of design current density Initial current density Current density (mA/m2) Final (peak) current density Average current density Time

  25. REQUIREMENT OF CURRENT FOR PROTECTION • Bare steel inseawater: • 100 - 200 mA/m2 • Bare steel insoil: • 10 - 20 mA/m2 • Reinforced concrete: • 1-3 mA/m2

  26. CATHODICPROTECTIONANDCOATING • Reduces the requirement of current • Lowers anode weight • Easier to achieve good current distributionand consequent protection of the entire structure

  27. CALCULATECATHODICPROTECTION Structure: - calculate area to protect (m2) - calculatecurrentrequirement, I (A) - calculate anode weightrequirement, W (kg) Anode data: - anode material - anode type and dimensions - calculate anode weight, Wa (kg) - calculate anode resistance, Ra (ohm) - calculate driving voltage, DE, (mV) - calculate anode current output, Ia (A)

  28. CP DESIGN Current requirement: I = i * Area Anode weightrequirement:W = I*L*8760 C * U Anode current output: Ia = DE Ra Anode weight: Wa = Voluma*d

  29. REQUIREMENT OF ANODES Calculate necessary number of anodesto meet the current requirement (initial and final current): N1 = I Ia Calculatenecessary number of anodesto meet the anode weight requirement for the total operating lifetime: N2 = W Wa

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