CORROSION
MECHANISM, TYPES & PREVENTION
HOW DOES IT HAPPEN?
IRON ORE → STEEL → RUST
• REACTIONS:
o Fe → Fe++ +2e- ANODE
o 2H+ +½O2 → H2O - 2e- CATHODE
o Fe + ½O2 ...
CONCENTRATION
CELL CORROSION
• NONUNIFORMITY OF THE AQUEOUS ENVIRONMENTS AT A SURFACE IS CALLED
CONCENTRATION CELL CORROSI...
PROCESS
• STEPS INVOLVED IN THE PROCESS OF CORROSION:
o IONS ARE INVOVLED & THEY NEED MEDIUM TO MOVE (USUALLY WATER)
o OXY...
HOW DOES IT LOOK?
TYPES OF CORROSION
• UNIFORM CORROSION
• LOCALIZED CORROSION
o CAVITATION DAMAGE – PITTING
o CREVICE
• UNDERDEPOSIT CORROS...
UNIFORM CORROSION
UNIFORM CORROSION
• ANODIC REACTION – OXIDATION:
o M → M+ + e-
• CATHODIC REACTION – REDUCTION:
o pH <7:
• 2H+ + 2e → H2 R...
HOW DOES IT LOOK?
HOW DO I STOP THIS?
• UNIFORM CORROSION MAY BE REDUCED OR
ELIMINATED BY FOLLOWING:
o APPROPRIATE CHEMICAL TREATMENT OF WAT...
FACTS
• MOST COMMONLY OBSERVED
• EASY TO MEASURE, PREDICT & DESIGN AGAINST THIS
TYPE OF CORROSION DAMAGE
• MEASUREMENT – C...
LOCALIZED CORROSION:
CAVITATIONDAMAGE
LOCALIZED CORROSION:
CAVITATION DAMAGE
• INSTANTANEOUS FORMATION & COLLAPSE OF
VAPOR BUBBLES IN A LIQUID SUBJECT TO RAPID,...
HOW DOES IT HAPPEN?
FACTS
• CAVITATION DAMAGE RESULTS FROM
HYDRODYNAMIC FORCES CREATED BY COLLAPSING
VAPOR BUBBLES.
• IT GENERATES MICROSCOPIC...
ALLUMINIUM FOIL EXPOSED TO A
CAVITATING FLUID FOR 5 SECONDS
ALLUMINIUM FOIL EXPOSED TO A
CAVITATING FLUID FOR 10 SECONDS
ALLUMINIUM FOIL EXPOSED TO A
CAVITATING FLUID FOR 20 SECONDS
LOCATIONS
• WHEREVER SUBSTATNTIAL PRESSURE CHANGES ARE
ENCOUNTERED.
• SHARP DISCONTINUITIES, SUDDEN ALTERATION OF
FLOW DIR...
WHERE ELSE?
HOW DO I STOP THIS?
• ELIMINATION:
o CHANGE OF MATERIALS:
• COVERING OF WEAR RESISTANT & HARD FACING ALLOYS SUCH AS
STELLI...
CREVICE CORROSION
CREVICE CORROSION
• PRECONDITIONS:
o CREVICE MUST BE FILLED WITH WATER.
o SURFACES ADJACENT TO THE CREVICE MUST ALSO CONTA...
HOW DOES IT START?
• MANY REACTION MAY
OCCURE NEAR CREVICE, BUT
MAIN REACTIONS ARE THOSE
WHICH ARE SHOWN IN
PREVIOUS SLIDE...
ITS HAPPENING!
• OXYGEN CONCENTRATION IS
CONSTANT BY WATER FLOW
OUTSIDE THE CREVICE.
• FORMATION OF DIFFERENTIAL
OXYGEN CO...
STILL HAPPENING!!
• THE METAL ION
CONCENTRATION INCREASES IN
THE CREVICE, RESULTING INTO
FORMATION OF NET POSITIVE
CHARGE ...
HAS IT FINISHED?
• CREVICE ENVIRONMENT
BECOMES MORE & MORE
ACIDIC.
• AREAS IMMEDIATELY
ADJACENT TO THE CREVICE
RECEIVE MOR...
LOCATIONS
• CREVICE CORROSION OCCURS BETWEEN TWO
SURFACES IN CLOSE PROXIMITY, LIKE CRACK
COMPONENT LOCATION
HEAT EXCHANGER...
HOW DOES IT LOOK?
HOW DO I STOP THIS?
• FORMS OF PREVENTION:
o ELIMINATE THE CREVICE
o REMOVE ALL MOISTURE
o SEAL THE CREVICE
• SYSTEM SPECI...
UNDERDEPOSIT
CORROSION
UNDERDEPOSIT
CORROSION
• COOLING WATER SYSTEM DEPOSITS ARE UBIQUITOUS.
• DEPOSITS CAN BE GENERATED INTERNALLY AS
PRECIPITA...
HOW DOES IT HAPPEN?
• CONCENTRATION CELL
CORROSION
• CORROSION BENEATH
DEPOSITS CONSUME
OXYGEN.
• THE DEPOSIT RETARDS
OXYG...
HOW DOES IT HAPPEN?
• SEGREGATION OF AGGRESSIVE ANIONS BENEATH
DEPOSITS – CONCENTRATIONS OF SULFATES &
CHLORIDES ARE DELET...
HOW DOES IT LOOK?
HOW DOES IT LOOK?
HOW DOES IT LOOK?
LOCATIONS
• ATTACK ALWAYS OCCURS BENEATH A DEPOSIT.
• CAN BE FOUND IN VIRTUALLY ANY COOLING WATER
SYSTEM AT ANY LOCATION.
...
LOCATIONS
• NARROW ORIFICES, SCREENS, LONG HORIZONTAL
PIPE RUNS, SUMPS OR AT REGIONS OF CONSTRICTED
FLOW.
• COMPONENTS IN ...
FACTS
• WATER PERMEABLE DEPOSITS ARE MOST HARMFUL.
• DEPOSITS CONTAINING CARBONATE CAN BE
PROTECTIVE.
• CARBONATES BUFFER ...
MILD STEEL COUPON IN ROLLING
MILL COOLING TANK
HOW DO I STOP THIS?
• DEPOSIT REMOVAL:
o REGULAR MECHANICAL CLEANING – WATER BLASTING, AIR RUMBLING &
CHEMICAL CLEANING.
•...
GALVANIC CORROSION
GALVANIC CORROSION
• AN ELECTROCHEMICAL
INTERACTION OF TWO OR
MORE MATERIALS (1 & 2)
HAVING A SUFFICIENTLY
DISTINCT GALVAN...
LOCATION
• LOCATION SPECIFIC: OCCURS AT BIMETALLIC
COUPLE
• METAL SPECIFIC: CORROSION AFFECTS THE METAL
THAT HAS LESS RESI...
CRITICAL FACTORS
• GALVANIC POTENTIAL:
• CONDUCTIVITY OF FLUIDS:
o GALVANIC CORROSION IS REDUCED AROUND A BEND IN A TUBE B...
HOW DO I STOP THIS?
• PREVENTIVE TECHNIQUES:
o AVOID COUPLING MATERIALS HAVING WIDELY DISSIMILAR GALVANIC
POTENTIALS.
o IF...
HOW DO I STOP THIS?
• CORRECTIVE TECHNIQUES:
o COMPLETELY INSULATE THE MATERIALS FROM ONE ANOTHER AT ALL
JUNCTIONS. NONCON...
DEALLOYING
CORROSION
DEALLOYING
CORROSION
• DEALLOYING OCCURS WHEN ONE OR MORE ALLOY
COMPONENTS ARE PREFERENTIALLY REMOVED
FROM THE METAL.
• RE...
HOW DOES IT HAPPEN?
• THEORY 1:
o ALLOY DISOLVES WITH A PREFERENTIAL REDEPOSITION OF BASE METAL.
• THEORY 2:
o SELECTIVE L...
LOCATION
• ATTACK OCCURS ONLY IN METALS CONTAINING
TWO OR MORE ALLOYING ELEMENTS.
• COPPER ALLOYS: BRASSES, CUPRONICKELS &...
HOW DOES IT LOOK?
HOW DO I STOP THIS?
• MATERIAL SUBSTITUTION:
o SUBSTITUTION OF MORE RESISTANT MATERIAL.
o ARSENIC, ANTIMONY & PHOSPHORUS A...
TUBERCULATION
TUBERCULATION
• LUMPS OF CORROSION PRODUCT & DEPOSIT THAT
FORM ON THE LOCALIZED REGIONS.
• IN OXYGENATED WATER OF NEAR NEU...
HOW IS IT?
HOW IS IT?
• OUTER CRUST:
o IT IS COMPOSED OF FERRIC HYDROXIDE, CARBONATES, SILICATES & OTHER
PRECIPITATES
o FERROUS ION &...
HOW IS IT?
• CORE:
o IT CONSISTS OF FERROUS HYDROXIDE.
o HYDROXYL IONS, CARBONATE, CHLORIDES & SULFATE GET ATTRACTED
BECAU...
LOCATIONS
• NON STAINLES STELLS & SOME CAST IRONS.
• SURFACE MUST CONTACT OXYGENATED WATER
DURING GROWTH & MUST REMAIN WET...
HOW DOES IT LOOK?
HOW DO I STOP THIS?
• CHEMICAL TREATMENT:
o METHODS EMPLOYING CHEMICAL INHIBITORS & DISPERSANTS
o USE DISPERSANTS IN SYSTE...
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Corrosion Guide

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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.

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Corrosion Guide

  1. 1. CORROSION MECHANISM, TYPES & PREVENTION
  2. 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. 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. 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. 5. HOW DOES IT LOOK?
  6. 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. 7. UNIFORM CORROSION
  8. 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. 9. HOW DOES IT LOOK?
  10. 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. 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. 12. LOCALIZED CORROSION: CAVITATIONDAMAGE
  13. 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. 14. HOW DOES IT HAPPEN?
  15. 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. 16. ALLUMINIUM FOIL EXPOSED TO A CAVITATING FLUID FOR 5 SECONDS
  17. 17. ALLUMINIUM FOIL EXPOSED TO A CAVITATING FLUID FOR 10 SECONDS
  18. 18. ALLUMINIUM FOIL EXPOSED TO A CAVITATING FLUID FOR 20 SECONDS
  19. 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. 20. WHERE ELSE?
  21. 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. 22. CREVICE CORROSION
  23. 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. 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. 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. 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. 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. 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. 29. HOW DOES IT LOOK?
  30. 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. 31. UNDERDEPOSIT CORROSION
  32. 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. 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. 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. 35. HOW DOES IT LOOK?
  36. 36. HOW DOES IT LOOK?
  37. 37. HOW DOES IT LOOK?
  38. 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. 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. 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. 41. MILD STEEL COUPON IN ROLLING MILL COOLING TANK
  42. 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. 43. GALVANIC CORROSION
  44. 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. 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. 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. 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. 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. 49. DEALLOYING CORROSION
  50. 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. 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. 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. 53. HOW DOES IT LOOK?
  54. 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. 55. TUBERCULATION
  56. 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. 57. HOW IS IT?
  58. 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. 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. 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. 61. HOW DOES IT LOOK?
  62. 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.
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