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Corrosion is a topic which most of the engineers find difficult. I have tried to simplify the topic & have prepared a presentation for easy understanding covering almost all types of corrosion.

Corrosion is a topic which most of the engineers find difficult. I have tried to simplify the topic & have prepared a presentation for easy understanding covering almost all types of corrosion.

Published in Engineering , Business , Technology
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  • 1. CORROSION MECHANISM, TYPES & PREVENTION
  • 2. HOW DOES IT HAPPEN? IRON ORE → STEEL → RUST • REACTIONS: o Fe → Fe++ +2e- ANODE o 2H+ +½O2 → H2O - 2e- CATHODE o Fe + ½O2 + H2O → Fe(OH) 2 o IRON + WATER WITH OXYGEN → FERROUS HYDROXIDE o Fe(OH) 2 + ½ H2O + ¼O2 → Fe(OH) 3 o IRON + WATER WITH OXYGEN → FERRIC HYDROXIDE o SS (Cr: >11%) - O2 COMBINES WITH CHROMIUM & IRON TO FORM A HIGHLY ADHERENT & PROTECTIVE OXIDE FILM.
  • 3. CONCENTRATION CELL CORROSION • NONUNIFORMITY OF THE AQUEOUS ENVIRONMENTS AT A SURFACE IS CALLED CONCENTRATION CELL CORROSION. • CORROSION OCCURS WHEN THE ENVIRONMENT NEAR THE METAL SURFACE DIFFERS FROM REGION TO REGION. THEY ARE CALLED ANODES & CATHODES WITH RESPECT TO EACH OTHER. • THESE REGIONS DIFFER IN ELECTROCHEMICAL POTENTIAL (ENERGY STORED IN THE FORM OF CHEMICAL & ELECTRICAL POTENTIAL ENERGY) • ANODIC AREAS LOSE METAL. • SHIELDED AREAS ARE PARTICULARLY SUSCEPTIBLE TO ATTACK.
  • 4. PROCESS • STEPS INVOLVED IN THE PROCESS OF CORROSION: o IONS ARE INVOVLED & THEY NEED MEDIUM TO MOVE (USUALLY WATER) o OXYGEN, WHICH IS GENERALLY PRESENT IN WATER IS INVOLVED o THE METAL GIVES UP ELECTRONS TO START THE PROCESS o A NEW MATERIAL IS FORMED, WHICH MAY REACT AGAIN OR COULD PROTECT THE BASE METAL o DRIVING FORCE IS REQUIRED • INTERFERENCE WITH ANY OF THE ABOVE MAY INCREASE OR DECREASE THE RATE OF CORROSION.
  • 5. HOW DOES IT LOOK?
  • 6. TYPES OF CORROSION • UNIFORM CORROSION • LOCALIZED CORROSION o CAVITATION DAMAGE – PITTING o CREVICE • UNDERDEPOSIT CORROSION • GALVANIC CORROSION • DEALLOYING CORROSION • INTERGRANULAR CORROSION • VELOCITY RELATED CORROSION • CRACKING • HIGH TEMPERATURE CORROSION • MICROBIAL CORROSION • TUBERCULATION
  • 7. UNIFORM CORROSION
  • 8. UNIFORM CORROSION • ANODIC REACTION – OXIDATION: o M → M+ + e- • CATHODIC REACTION – REDUCTION: o pH <7: • 2H+ + 2e → H2 REDUCTION OF HYDROGEN IONS o pH>7: • O2 + 2H2O + 4e → 4OH- REDUCTION OF OXYGEN • UNIFORM DISTRIBUTION OF CATHODIC REACTANTS OVER THE ENTIRE EXPOSED METAL SURFACE MAKE IT UNIFORM & THERE IS NO PREFERENTIAL SITE.
  • 9. HOW DOES IT LOOK?
  • 10. HOW DO I STOP THIS? • UNIFORM CORROSION MAY BE REDUCED OR ELIMINATED BY FOLLOWING: o APPROPRIATE CHEMICAL TREATMENT OF WATER ( WITH CORROSION INHIBITORS, DISPERSANTS & FILMERS) o COATING METAL SURFACES WITH WATER IMPERMEABLE BARRIERS (SUCH AS PAINT, EPOXIES, GREASE & OIL) o SUBSTITUTING MORE RESISTING MATERIALS SUCH AS STAINLESS STEEL & COPPER ALLOYS FOR LESS RESISTENT ALLOYS SUCH AS CARBON STEELS. o DEAERATION (MECHANICAL, THERMAL, CHEMICAL & COMBINATION OF THESE). o CATHODIC PROTECTION (SACRIFICIAL ANODES) o PREVENTING SURFACES FROM CONTACTING WATER.
  • 11. FACTS • MOST COMMONLY OBSERVED • EASY TO MEASURE, PREDICT & DESIGN AGAINST THIS TYPE OF CORROSION DAMAGE • MEASUREMENT – COUPONS, NDT ETC. • CAUTION – UNEXPECTED RAPID UNIFORM CORROSION FAILURES: o CONCENTRATION OF AGGRESSIVE ANIONS o VARIABLE WATER CHEMISTRY o INCREASED FLOW RATE o CHEMICAL CHANGE IN ENVIRONMENT
  • 12. LOCALIZED CORROSION: CAVITATIONDAMAGE
  • 13. LOCALIZED CORROSION: CAVITATION DAMAGE • INSTANTANEOUS FORMATION & COLLAPSE OF VAPOR BUBBLES IN A LIQUID SUBJECT TO RAPID, INTENSE LOCALIZED PRESSURE CHANGES. • CAVITATION DAMAGE REFERS TO THE DETERIORATION OF A MATERIAL RESULTING FROM ITS EXPOSURE TO A CAVITATING FLUID.
  • 14. HOW DOES IT HAPPEN?
  • 15. FACTS • CAVITATION DAMAGE RESULTS FROM HYDRODYNAMIC FORCES CREATED BY COLLAPSING VAPOR BUBBLES. • IT GENERATES MICROSCOPIC TORPEDO OF WATER AT VELOCITIES FROM 100 TO 500 m/s. • ENERGY IS ABSORBED BY SURROUNDING FLUID. BUT IF THIS OCCURS NEAR THE SURFACE, DAMAGE IS CAUSED TO THE METAL OXIDE. • WHEN THE METAL IS AFFECTED, CONTINUOUS IMPACTS CAUSE RUPTURE OF METAL.
  • 16. ALLUMINIUM FOIL EXPOSED TO A CAVITATING FLUID FOR 5 SECONDS
  • 17. ALLUMINIUM FOIL EXPOSED TO A CAVITATING FLUID FOR 10 SECONDS
  • 18. ALLUMINIUM FOIL EXPOSED TO A CAVITATING FLUID FOR 20 SECONDS
  • 19. LOCATIONS • WHEREVER SUBSTATNTIAL PRESSURE CHANGES ARE ENCOUNTERED. • SHARP DISCONTINUITIES, SUDDEN ALTERATION OF FLOW DIRECTION, CROSS SECTIONAL AREAS OF FLOW PASSAGES ARE CHANGED. • EXAMPLES: o PUMP IMPELLERS o VALVES o DISCHARGE SIDE OF REGULATING VALVE o TUBE ENDS IN HEAT EXCHANGERS o CYLINDER LINERS IN DIESEL ENGINES
  • 20. WHERE ELSE?
  • 21. HOW DO I STOP THIS? • ELIMINATION: o CHANGE OF MATERIALS: • COVERING OF WEAR RESISTANT & HARD FACING ALLOYS SUCH AS STELLITE. CAN BE INCORPORATED IN SUSCEPTIBLE ZONES. o USE OF COATINGS: • FOR LOW CAVITATION INTENSITIES, COVERING OF RUBBER OR SOME PLASTIC IS USEFUL. o ALTERATION OF ENVIRONMENT: • FOR LOW CAVITATION INTESITIES, APPROPRIATE INHIBITORS CAN BE USEFUL. o ALTERATION OF OPERATING PROCEDURES: • MAINTAINING NPSH, REDUCING FLOW VELOCITY THROUGH A HEAT EXCHANGER • INJECTING AIR INTO CAVITATING SYSTEM, IF NOTHING WORKS o REDESIGN OF EQUIPMENT:
  • 22. CREVICE CORROSION
  • 23. CREVICE CORROSION • PRECONDITIONS: o CREVICE MUST BE FILLED WITH WATER. o SURFACES ADJACENT TO THE CREVICE MUST ALSO CONTACT WATER. • STARTING OF CREVICE CORROSION: o INITIALLY CORROSION IN OXYGENATED WATER OF NEAR NEUTRAL pH OCCURES BY FOLLOWING REACTIONS: o M → M+n + ne- ANODE o O2 + 2H2O + 4e- → 4OH- CATHODE
  • 24. HOW DOES IT START? • MANY REACTION MAY OCCURE NEAR CREVICE, BUT MAIN REACTIONS ARE THOSE WHICH ARE SHOWN IN PREVIOUS SLIDE. • EVENTUALLY OXYGEN BECOMES DEPLETED IN THE CREVICE. • OXYGEN DIFFUSION INTO THE CREVICE IS TOO SLOW TO REPPLACE THE OXYGEN AS FAST AS IT CONSUMED IN CORROSION. • AREA COMPARISON – CREVICE MOUTH & INTERIOR
  • 25. ITS HAPPENING! • OXYGEN CONCENTRATION IS CONSTANT BY WATER FLOW OUTSIDE THE CREVICE. • FORMATION OF DIFFERENTIAL OXYGEN CONCENTRATION CELL. • OXYGENATED WATER ALLOWS CATHODIC REACTION & IT BECOMES CATHODIC & NO METAL DISSOLVES OUTSIDE THE CREVICE. • INSIDE THE CREVICE ANODIC REACTION CONTINUES. • METAL IONS REACT WITH WATER & FORM HYDROXIDES.
  • 26. STILL HAPPENING!! • THE METAL ION CONCENTRATION INCREASES IN THE CREVICE, RESULTING INTO FORMATION OF NET POSITIVE CHARGE IN THE CREVICE ELECTROLYTE. • THIS ATTRACTS THE NEGATIVELY CHARGED IONS DISSOLVED IN THE WATER. (CHLORIDE, SULFATE & OTHER ANIONS) • HYDROLYSIS PRODUCES ACIDS IN THE CREVICE, ACCELERATING THE ATTACK. • pH CAN BECOME AS LOW AS 2. M+Cl- + H2O → MOH ↓ + H+Cl- M2 +SO4 - + 2H2O → 2MOH ↓ + H2 +SO4 -
  • 27. HAS IT FINISHED? • CREVICE ENVIRONMENT BECOMES MORE & MORE ACIDIC. • AREAS IMMEDIATELY ADJACENT TO THE CREVICE RECEIVE MORE & MORE ELECTRONS FROM INSIDE THE CRVICE. • OH IONS ARE FORMED OUTSIDE, LOCALLY INCREASING pH & DECREASING THE ATTACK THERE. • ACCELERATING CORROSION IS REFERRED TO AS AUTOCATALYTIC.
  • 28. LOCATIONS • CREVICE CORROSION OCCURS BETWEEN TWO SURFACES IN CLOSE PROXIMITY, LIKE CRACK COMPONENT LOCATION HEAT EXCHANGERS SHELL & TUBE -ROLLED ENDS AT TUBE SHEET -OPEN WELDS AT TUBE SHEET -BENEATH DEPOSITS -WATER BOX GASKETS -BOLT HOLES, NUTS & WASHER -BAFFLE OPENING PLATE & FRAME -BENEATH GASKETS -PLATE CONTACT POINTS -BENEATH DEPOSITS COOLING TOWERS -THREADED PIPE JOINTS -PARTIALLY EXFOLIATED COATINGS -BETWEEN BUSHING & SHAFTS ON PUMPS
  • 29. HOW DOES IT LOOK?
  • 30. HOW DO I STOP THIS? • FORMS OF PREVENTION: o ELIMINATE THE CREVICE o REMOVE ALL MOISTURE o SEAL THE CREVICE • SYSTEM SPECIFIC EFFECTIVE TECHNIQUE: o DO NOT USE RIVETED JOINTS o EMPLOY SOUND WELDING PRACTICE. POROSITY SHOULD BE MINIMIZED. o ALLOW FOR DRAINAGE OF WATER. o PAINT, GREASE, SOLDER OR SEAL OTHERWISE THE KNOWN CREVICES BEFORE EXPOSURE TO WATER. o AVOID USING HYDROCHLORIC ACID TO CLEAN STAINLESS SYSTEMS IF ANY ALTERNATIVE IS POSSIBLE. o WELD THE TUBE ENDS INTO TUBE SHEETS o JUDICIOUS USE OF CHEMICAL INHIBITORS & CATHODIC PROTECTION. o MAKE SURE ALL GASKETS ARE IN GOOD REPAIR & BOLTS ARE PROPERLY TIGHTENED.
  • 31. UNDERDEPOSIT CORROSION
  • 32. UNDERDEPOSIT CORROSION • COOLING WATER SYSTEM DEPOSITS ARE UBIQUITOUS. • DEPOSITS CAN BE GENERATED INTERNALLY AS PRECIPITATES, LAID DOWN AS TRANSPORTED CORROSION PRODUCTS OR BROUGHT INTO THE SYSTEM FROM EXTERNAL SOURCES. • DEPOSITS CAUSE DIRECT & INDIRECT CORROSION: o DIRECT: DEPOSITS CONTAIN CORROSIVE SUBSTANCES o INDIRECT: SHIELDING OF SURFACES BELOW DEPOSITS PRODUCES INDIRECT ATTACK; CORROSION OCCURS AS A CONSEQUENCES OF SURFACE SHIELDING PROVIDED BY DEPOSIT. • THESE ATTACKS MAY INVOLVE CONCENTRATION CELL CORROSION, TENDANCY IS MORE IN INDIRECT ATTACK.
  • 33. HOW DOES IT HAPPEN? • CONCENTRATION CELL CORROSION • CORROSION BENEATH DEPOSITS CONSUME OXYGEN. • THE DEPOSIT RETARDS OXYGEN DIFFUSION TO REGIONS NEAR THE CORRODING SURFACE FORMING OXYGEN CELL.
  • 34. HOW DOES IT HAPPEN? • SEGREGATION OF AGGRESSIVE ANIONS BENEATH DEPOSITS – CONCENTRATIONS OF SULFATES & CHLORIDES ARE DELETERIOUS. • DIFFERENTIAL AERATION & CONCENTRATION OF AGGRESSIVE IONS BENEATH DEPOSITS - PRODUCE SEVERE LOCALIZED DAMAGE ON STAINLESS STEEL & OTHER METALS SUCH AS ALLUMINIUM, TITANIUM ETC. • DIFFERENTIAL AERATION ALONE – NOT SUFFICIENT TO INITIATE ATTACK ON STAINLESS STEEL.
  • 35. HOW DOES IT LOOK?
  • 36. HOW DOES IT LOOK?
  • 37. HOW DOES IT LOOK?
  • 38. LOCATIONS • ATTACK ALWAYS OCCURS BENEATH A DEPOSIT. • CAN BE FOUND IN VIRTUALLY ANY COOLING WATER SYSTEM AT ANY LOCATION. • SYSTEMS CONTAINING LARGE AMOUNTS OF SAND, GREASE, OIL, BIOMASS, PRECIPITATES, TRANSPORTED CORROSION PRODUCTS & OTHER DETRITUS ARE MORE SUSCEPTIBLE. • BIOLOGICAL ACCULATIONS SUCH AS SLIME LAYERS ARE HARMFUL. • EQUIPMENT IN WHICH WATER FLOW IS SLOW OR INTERMITTENT IS SUBJECT TO DEPOSITION & ASSOCIATED CORROSION.
  • 39. LOCATIONS • NARROW ORIFICES, SCREENS, LONG HORIZONTAL PIPE RUNS, SUMPS OR AT REGIONS OF CONSTRICTED FLOW. • COMPONENTS IN WHICH WATER TEMPERATURE CHAGNGES ABRUPTLY WITH DISTANCE, LIKE HEAT EXCHANGERS, TEND TO ACCUMULATE PRECIPITATES. • SYSTEMS IN WHICH pH EXCURSIONS ARE FREQUENT MAY ACCUMULATE DEPOSITS DUE TO PRECIPITATION PROCESS.
  • 40. FACTS • WATER PERMEABLE DEPOSITS ARE MOST HARMFUL. • DEPOSITS CONTAINING CARBONATE CAN BE PROTECTIVE. • CARBONATES BUFFER THE ACIDITY CAUSED BY THE SEGREGATION OF POTENTIALLY ACIDIC ANIONS IN & BENEATH DEPOSITS. • EFFECTIVENESS OF ALMOST ALL COMMONLY USED CORROSION INHIBITORS INCREASE AS SURFACE CLEANLINESS IMPROVES.
  • 41. MILD STEEL COUPON IN ROLLING MILL COOLING TANK
  • 42. HOW DO I STOP THIS? • DEPOSIT REMOVAL: o REGULAR MECHANICAL CLEANING – WATER BLASTING, AIR RUMBLING & CHEMICAL CLEANING. • DESIGN CHANGES: o INCREASE THE FLOW. DEAD LAGS, STAGNANT AREAS & OTHER LOW-FLOW REGIONS TO BE ELIMINATED. FLOW VELOCITY SHOULD BE >1 M/S. • WATER TREATMENT: o REMOVING SUSPENDED SOLIDS, DECREASING CYCLES OF CONCENTRATION AND CLARIFICATION. o BIODISPERSANTS AND BIOCIDES – BIOFOULED SYSTEMS o JUDICIOUS USE OF CHEMICAL CORROSION INHIBITORS • CATHODIC PROTECTION: o EFFECTIVENESS DEPENDS UPON SURFACE CLEANLINESS
  • 43. GALVANIC CORROSION
  • 44. GALVANIC CORROSION • AN ELECTROCHEMICAL INTERACTION OF TWO OR MORE MATERIALS (1 & 2) HAVING A SUFFICIENTLY DISTINCT GALVANIC POTENTIAL DIFFERENCE. • AN ELECTROLYTE (3) COMMON TO BOTH MATERIALS, THROUGH WHICH AN IONIC CURRENT PASSES. • AN ELECTRICALLY CONDUCTIVE PATHWAY (4) PHYSICALLY LINKING THE TWO MATERIALS.
  • 45. LOCATION • LOCATION SPECIFIC: OCCURS AT BIMETALLIC COUPLE • METAL SPECIFIC: CORROSION AFFECTS THE METAL THAT HAS LESS RESISTANCE. o COOLING TUBES BUNDLE & BAFFLE SHEETS o TRANSPORT OF METAL PARTICLES FORMED BY EROSION CORROSION TO ANOTHER SITE OF DIFFERENT METAL o WHEREVER 2 DISSIMILAR METALS COME INTO CONTACT WITH FAVOURABLE CONDITIONS
  • 46. CRITICAL FACTORS • GALVANIC POTENTIAL: • CONDUCTIVITY OF FLUIDS: o GALVANIC CORROSION IS REDUCED AROUND A BEND IN A TUBE BECAUSE OF INCREASED RESISTANCE TO CURRENT FLOW. • AREA EFFECT: (AREA OF EXPOSED NOBLE METAL) o CORROSION RATE OF ACTIVE METAL = ----------------------------------------------- (AREA OF EXPOSED ACTIVE METAL) o FAVORABLE: LARGE ANODE AND SMALL CATHODE o NOBLE MEMBER SHOULD BE COATED ALWAYS. • FLUID VELOCITY: o MORE THE VELOCITY LESSER THE POTENTIAL OF METALS IN GIVEN ENVIRONMENT.
  • 47. HOW DO I STOP THIS? • PREVENTIVE TECHNIQUES: o AVOID COUPLING MATERIALS HAVING WIDELY DISSIMILAR GALVANIC POTENTIALS. o IF UNAVOIDABLE, USE THE PRINCIPLE OF AREA RATIO o COMPLETELY INSULATE THE MATERIALS FROM ONE ANOTHER AT ALL JUNCTIONS EXPOSED TO A COMMON FLUID. o IF GALVANICALLY INCOMPATIBLE MATERIALS ARE TO BE USED, DESIGN THE ACTIVE MATERIAL COMPONENT SO TTHAT EASY REPLACEMENT IS POSSIBLE, OR ALLOW FOR ANTICIPATED CORROSION BY APPROPRIATELY INCREASING ITS THICKNESS.
  • 48. HOW DO I STOP THIS? • CORRECTIVE TECHNIQUES: o COMPLETELY INSULATE THE MATERIALS FROM ONE ANOTHER AT ALL JUNCTIONS. NONCONDUCTIVE WASHERS, INSERTS, SLEEVES & COATINGS. o ALTER THE CHEMISTRY OF COMMON FLUID TO RENDER IT LESS CONDUCTIVE OR CORROSIVE. o COAT BOTH THE METALS OR THE NOBLE METAL. DO NOT COAT JUST THE ACTIVE METAL. o CATHODIC PROTECTION TECHNIQUE
  • 49. DEALLOYING CORROSION
  • 50. DEALLOYING CORROSION • DEALLOYING OCCURS WHEN ONE OR MORE ALLOY COMPONENTS ARE PREFERENTIALLY REMOVED FROM THE METAL. • REFERED TO AS SELECTIVE LEACHING OR PARTING. • LEACHING OF ZINC FROM BRASS – DEZINCIFICATIONNG • LEACHING OF NICKEL FROM ALLOY (CUPRONICKEL, MONEL) – DENICKELIFICATION • CORRODED AREAS WEAK & POROUS, CAUSING FRACTURE & WEEPING LEAKS.
  • 51. HOW DOES IT HAPPEN? • THEORY 1: o ALLOY DISOLVES WITH A PREFERENTIAL REDEPOSITION OF BASE METAL. • THEORY 2: o SELECTIVE LEACHING OF ZINC/NICKEL, LEAVING COPPER BEHIND. • BOTH MECHANISM MAY OPERATE, DEPENDING UPON THE SPECIFIC ENVIRONMENT.
  • 52. LOCATION • ATTACK OCCURS ONLY IN METALS CONTAINING TWO OR MORE ALLOYING ELEMENTS. • COPPER ALLOYS: BRASSES, CUPRONICKELS & BRONZES – SUSCEPTIBLE IN COOLING WATER ENVIRONMENT. • EXPOSURE TO HIGH TEMPERATURES, ACIDS, SULFIDES OR OTHER VERY AGGRESSIVE ENVIRONMENTS. • SHELL & TUBER HEAT EXCHANGERS & CONDENSERS • COPPER ALLOYS USED IN PUMPS AS BUSHINGS, BEARINGS, IMPELLERS & GASKETS.
  • 53. HOW DOES IT LOOK?
  • 54. HOW DO I STOP THIS? • MATERIAL SUBSTITUTION: o SUBSTITUTION OF MORE RESISTANT MATERIAL. o ARSENIC, ANTIMONY & PHOSPHORUS ADDITION (UPTO 0.1%) • SURFACE CLEANLINESS: o CLEANER THE SURFACE, LESSER THE DEALLOYING o HIGH FLOW, PREVENT SETTLING OF PARTICLES & BIOGROWTH. • CHEMICAL TREATMENT: o CHEMICAL CORROSION INHIBITION. o FILMERS SUCH AS TOLYTRIAZOLE REDUCE CORROSION OF YELLOW METAL • BIOLOGICAL CONTROL: o ANY DEPOSIT CAN INCREASE DEALLOYING o FORMATION OF SLIME LAYERS& IN TURN BIOGROWTH SHOULD BE AVOIDED.
  • 55. TUBERCULATION
  • 56. TUBERCULATION • LUMPS OF CORROSION PRODUCT & DEPOSIT THAT FORM ON THE LOCALIZED REGIONS. • IN OXYGENATED WATER OF NEAR NEUTRAL pH: o HYDROUS FERRIC OXIDE [Fe(OH)3] FORMS o THE LAYER SHIELDS THE UNDERLYING METAL SURFACE FROM OXYGENATED WATER, OXYGEN CONCENTRATION CELL IS FORMED. o HYDROUS FERROUS OXIDE [Fe(OH)2] IS ALSO PRESENT BENEATH THIS SHIELD. o A BLACK MAGNETIC HYDROUS FERROUS FERRITE LAYER FORM BETWEEN FERRIC & FERROUS OXIDES.
  • 57. HOW IS IT?
  • 58. HOW IS IT? • OUTER CRUST: o IT IS COMPOSED OF FERRIC HYDROXIDE, CARBONATES, SILICATES & OTHER PRECIPITATES o FERROUS ION & FERROUS HYDROXIDE GENERATED WITHIN THE TUBERCLE DIFFUSE OUTWARD THROUGH FISSURES, WHERE THEY ENCOUNTER DISSOLVED OXYGEN & FORM FERRIC HYDROXIDE. • INNER SHELL: o THE SHELL IS BLACK IN COLOR & SEPARATES THE HIGH DISSOLVED OXGEN CONCENTRATION REGION OUTSIDE FROM THE LOW DISSOLVED OXYGEN CONCENTRATION REGION INSIDE. o HAS HIGH ELETRICAL CONDUCTIVITY. ELECTRONS GENERATED ARE TRANSFERRED TO THIS REGION & ACT AS CATHOD. o pH INCREASES LOCALLY CAUSING THE CARBONATES TO DEPOSIT ON THE SHELL.
  • 59. HOW IS IT? • CORE: o IT CONSISTS OF FERROUS HYDROXIDE. o HYDROXYL IONS, CARBONATE, CHLORIDES & SULFATE GET ATTRACTED BECAUSE OF +VE CHARGE. • CAVITY: o CAVITY MAY BECOME ACIDIC INTERNALLY. • FLOOR: o LOCALIZED CORRODED REGION PRESENT BENEATH THE TUBERCULE.
  • 60. LOCATIONS • NON STAINLES STELLS & SOME CAST IRONS. • SURFACE MUST CONTACT OXYGENATED WATER DURING GROWTH & MUST REMAIN WET FOR EXTENDED PERIODS. o HEAT EXCHANGERS o STORAGE TANKS o COOLING TOWER COMPONENTS o PUMP COMPONENTS
  • 61. HOW DOES IT LOOK?
  • 62. HOW DO I STOP THIS? • CHEMICAL TREATMENT: o METHODS EMPLOYING CHEMICAL INHIBITORS & DISPERSANTS o USE DISPERSANTS IN SYSTEMS CONTAINING SAND, OIL, GREASE, BIOLOGICAL MATERIAL. o IT INCREASES THE EFFECTIVENESS OF CHEMICAL INHIBITION & ALSO PREVENTS NUCLEATION OF OXYGEN CONCENTRATION CELLS BENEATH FOULANTS. • ALTERING SYSTEM OPERATION: o FLOW o CATHODIC PROTECTION USING SACRIFICIAL ANODES OR APPLIED CURRENT • MATERIAL SUBSTITUTION: o MORE RESISTANT MATERIAL (STAINLESS STELL, BRASSES, CUPRONICKELS ETC) o PROTECTIVE COATINGS.