CORROSIONMECHANISM, TYPES & PREVENTION

HOW DOES IT HAPPEN?

IRON ORE → STEEL → RUST • REACTIONS:

o Fe → Fe++ +2e- ANODEo 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.

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.

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.

HOW DOES IT LOOK?

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

UNIFORM CORROSION

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.

HOW DOES IT LOOK?

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.

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 CHEMISTRYo INCREASED FLOW RATEo CHEMICAL CHANGE IN ENVIRONMENT

LOCALIZED CORROSION: CAVITATIONDAMAGE

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.

HOW DOES IT HAPPEN?

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.

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 DIRECTION, CROSS SECTIONAL AREAS OF FLOW PASSAGES ARE CHANGED.

• EXAMPLES:o PUMP IMPELLERSo VALVESo DISCHARGE SIDE OF REGULATING VALVEo TUBE ENDS IN HEAT EXCHANGERSo CYLINDER LINERS IN DIESEL ENGINES

WHERE ELSE?

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:

CREVICE CORROSION

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

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

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.

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

-

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.

LOCATIONS• CREVICE CORROSION OCCURS BETWEEN TWO

SURFACES IN CLOSE PROXIMITY, LIKE CRACKCOMPONENT 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

HOW DOES IT LOOK?

HOW DO I STOP THIS?

• FORMS OF PREVENTION:o ELIMINATE THE CREVICEo REMOVE ALL MOISTUREo SEAL THE CREVICE

• SYSTEM SPECIFIC EFFECTIVE TECHNIQUE:o DO NOT USE RIVETED JOINTSo 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.

UNDERDEPOSIT CORROSION

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 SUBSTANCESo 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.

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.

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.

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. • 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.

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.

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.

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.

• 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 SYSTEMSo JUDICIOUS USE OF CHEMICAL CORROSION INHIBITORS

• CATHODIC PROTECTION:o EFFECTIVENESS DEPENDS UPON SURFACE CLEANLINESS

GALVANIC CORROSION

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.

LOCATION• LOCATION SPECIFIC: OCCURS AT BIMETALLIC

COUPLE• METAL SPECIFIC: CORROSION AFFECTS THE

METAL THAT HAS LESS RESISTANCE.o COOLING TUBES BUNDLE & BAFFLE SHEETSo TRANSPORT OF METAL PARTICLES FORMED BY EROSION CORROSION

TO ANOTHER SITE OF DIFFERENT METALo WHEREVER 2 DISSIMILAR METALS COME INTO CONTACT WITH

FAVOURABLE CONDITIONS

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 CATHODEo NOBLE MEMBER SHOULD BE COATED ALWAYS.

• FLUID VELOCITY:o MORE THE VELOCITY LESSER THE POTENTIAL OF METALS IN GIVEN

ENVIRONMENT.

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.

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

DEALLOYING CORROSION

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.

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.

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.

HOW DOES IT LOOK?

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 DEALLOYINGo 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 DEALLOYINGo FORMATION OF SLIME LAYERS& IN TURN BIOGROWTH SHOULD BE

AVOIDED.

TUBERCULATION

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.

HOW IS IT?

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.

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.

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

HOW DOES IT LOOK?

HOW DO I STOP THIS?

• CHEMICAL TREATMENT:o METHODS EMPLOYING CHEMICAL INHIBITORS & DISPERSANTSo 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 FLOWo CATHODIC PROTECTION USING SACRIFICIAL ANODES OR APPLIED

CURRENT

• MATERIAL SUBSTITUTION:o MORE RESISTANT MATERIAL (STAINLESS STELL, BRASSES,

CUPRONICKELS ETC)o PROTECTIVE COATINGS.