This document discusses various types of corrosion, including uniform corrosion, localized corrosion like pitting and crevice corrosion, galvanic corrosion, and underdeposit corrosion. It explains the mechanisms of each type of corrosion through chemical reactions and diagrams. Key points made include that crevice corrosion occurs in confined spaces where oxygen is depleted, galvanic corrosion results from dissimilar metals in contact, and underdeposit corrosion takes place beneath insulating surface deposits where oxygen and ion concentrations can vary greatly. The document provides examples of where each type typically occurs and methods for preventing or reducing corrosion in industrial systems.

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.