Corrosion

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Corrosion

  1. 1. Corrosion
  2. 2. Type of Corrosion • Uniform • Galvanic • Pitting and Crevice Corrosion • Stress Corrosion Cracking • Erosion • Microbiologically Induced Corrosion
  3. 3. Uniform Corrosion • Can be a good or a bad thing • Corrosion occurs evenly over the surface • Oxide layers can be very tough – Magnetite • Fe3O4
  4. 4. Effect of pH • Magnetite (for example) – Low pH no oxides form – High pH porous Fe(OH)3.xH2O forms – pH 10-12 ideal range for carbon steel • Different metals require different conditions for uniform corrosion • pH is controlled to control uniform corrosion
  5. 5. Galvanic Corrosion • Chemical reactions • Electrons removed from one reactant travel through an external circuit • On material tends to disappear
  6. 6. Galvanic Cell ANODECATHODE -e -e -e (Fe)(Cu) Fe ++ ions Corrosion Product ELECTROLYTE (H2O + NaCl) H2 Bubbles 2e- + 2H+ H2 (bubbles) Fe++ + 2 OH - Fe (OH)2 (corrosion product) Fe Fe++ + 2e- In the Electrolyte: At the Cathode: Reactions At the Anode: H2O OH- + H+
  7. 7. Cathodic Protection • Each metal has a different potential to donate electrons • Anode always corrodes • To protect a metal – Select a metal that more easily donates electrons – Build a cell with metal to be protected as cathode
  8. 8. Factors Affecting Galvanic Corrosion • Oxygen concentration of the electrolyte • Temperature • Conductivity of the electrolyte • Cathode/anode surface area
  9. 9. Pitting and Crevice Corrosion Scale Region where pit will form Metal Electrolyte (H2 O plus dissolved oxygen)
  10. 10. Stress Corrosion Cracking • Three conditions required for most common kind – Metal under tensile stress – Dissolved Oxygen – Chloride Ion • Brittle cracks form at the sites of stress • Failure can be fast • Failure can occur at stress loads far below yield strength
  11. 11. Erosion Corrosion • Flow removes protective layer • New protective layer forms using up metal • Promoted by – High flow – Turbulent conditions – Particulates in fluid • Concern with feeders in HTS
  12. 12. Example of Erosion Corrosion
  13. 13. Microbiologically Induced Corrosion • Bacteria in water – Can be in presence of oxygen or not – Bacteria form a nodule • Similar to pitting corrosion.
  14. 14. General Corrosion Control • Eliminate oxygen • Eliminate chloride ion • Maintain pH levels • Prevent stagnation • Chlorinate
  15. 15. HTS pH Control inner zone outer zone TEMP. ( °C ) 225 240 255 270 285 300 0 2 4 6 8 10 pH 9.3 pH 10.2 pH 9.7 342SOLUBILITYOFFeO(µgFe/kgHO) PNGS BNGS-A Reactor Inlet Header PNGS BNGS-A Reactor Outlet Header
  16. 16. pH Control • Add LiOH for low pH • Ion exchange columns for high pH • Lithium tends to collect in the pressurizer – pH goes up if water goes to main system – Called lithium hide out – Possible pH excursions during cool down
  17. 17. Dissolved Oxygen HTS • Oxygen is corrosive – More so in hot water • Attacks zirconium, iron, their protective oxide coatings • Continuous source through radiolysis of D2O • Controlled by adding excess D2 or H2 • Explosion Hazard
  18. 18. Conductivity HTS • Used for troubleshooting • Not controlled
  19. 19. pH Secondary Side • pH controlled by adding chemicals – Ammonia (from decomposed hydrazine) – Morpholine • Chemicals must be volatile
  20. 20. Dissolved O2 Secondary Side • O2 is corrosive particularly if Cl- is present • Three levels of removal – Air extraction – Deaerator – Hydrazine addition • Environmental hazard – chemicals discharged in boiler blow off
  21. 21. Conductivity Secondary Side • Conductivity of boiler water shows the presence of bad boiler things • Specifically cation conductivity is measured – Measures the conductivity of anions Cl-, SO4 -, HCO3 - – IX is used to replace cations with H+ – Conductivity sensitive to the anions
  22. 22. Moderator Normal • Stainless steel system • Requires neutral pH and low conductivity • Deuterated IX columns remove all impurities • Conductivity monitored • pH is used for troubleshooting
  23. 23. Moderator Over Poisoned • pH 4-6 to prevent gadolinium nitrate precipitation • pH is monitored
  24. 24. Moderator Chemistry Control • Proper use of IX’s • Prevent air in-leakage
  25. 25. Sludge and Scale Boiler Tube HT Fluid Scale Flakes Boiler Water Deposits/Sludge Pile Corrosion occurs here Boiler Tubesheet or Tube Support Plate
  26. 26. For You to Do • Read the section on corrosion pp. 19-37 • Answer questions p. 38

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