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Caustic Corrosion

Typical Locations

Water-cooled Tubes:

At flow disruptions Horiz / inclined tube

High Heat flux zone

Flame impingement zone

Probable Root Cause

Concentration of NaOH from

boiler water chemicals

Feed water system corrosiondeposits

Condenser leakage

Temp. increase due to internal

deposits

Corrective Action

Control Boiler WaterChemistry

Reduce corrosion

product ingress Chemical cleaning

Reweld irregular welds

Use T11 type steel orrifled tube

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Caustic Corrosion Gauging

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CAUSTIC GOUGING STRATIFICATION

CAUSTIC GOUGING

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Hydrogen Damage

Typical Locations

Water-cooled Tubes:

At flow disruptions Horiz / inclined tube

High Heat flux zone

Flame impingement zone

Probable Root Cause Concentration of acidic salts and

low pH water chemistry

Condenser leakage and ingressof corrosion products

Feed water system corrosion

deposits

Chemical cleaning contamination

Corrective Action

Control Boiler Water

Chemistry Check corrosion product

ingress

Chemical cleaning Replace affected tubes

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HYDROGEN EMBRITTLEMENT

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FLATTENING TEST FAILURE HYDROGEN EMBRITTLEMENT

GRAIN BOUNDARY CRACKS - HYDROGEN

EMBRITTLEMENT

ROUNDED DISSOLVED GAS PITTING CORROSION

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Internal Pitting (Localised Corrosion)

Typical Locations Anywhere in Eco, SH, RH,

Non-heated portion of WW Eco Inlet

Horizontal Tubes

Probable Root Cause Exposure of tube to waterwith high conc. of oxygen.

High Feed water oxygen at

start-ups and low loads

Condensation in bends at

shut downs

Corrective Action Control Feed water oxygen

levels to

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Pitting Damage

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Stress Corrosion Cracking

Typical Locations

SH, RH-at regions of :

conc. of chlorides,sulphates or hydroxide

stressed in fabrication,

service etc. like bends,

attachment weldsProbable Root Cause

Corrosive conc. From drumcarry-over or attemperatorspray

SS Tube material sensitized

Stresses

Corrective Action

Replacement

Surveillance for carry-over

Heat Treatment of bends

Care during chemicalcleaning

Use of 347H tube

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STRESS CORROSION CRACK IN SS 304H

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STRESS CORROSION CRACK IN COLD BENT TUBE

TRANSGRANULAR STRESS CORROSION CRACKS

INTERGRANULAR STRESS CORROSION CRACKS

CORROSION FATIGUE CRACKS

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Vibration Fatigue

Typical Locations

welded tie type spacers

between vertical WW and

steam cooled screen tubes

Welded or fixed attachments

on horiz.

steam cooled tubes

Probable Root Cause

Gas flow induced vibration

Corrective Action

Install vibration snubbers

between or across tubes

Evaluate against boiler flow

conditions

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Thermal Fatigue

Typical Locations

Water splashed bottom ash

hopper region Soot blower spray region with

condensate

Regions like Ecoinlet and Pry

SH with rapid fluid

temperature changes

Probable Root Cause

Multiple cycles of suddencooling of tube metal by

water quenching

Corrective Action

Tube Replacement

Checking soot blower, ashhopper systems

Control of Feed water

temperature (once thro type)

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Thermal Fatigue

FATIGUE CRACK

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FATIGUE CRACK

FATIGUE CRACK

THERMAL FATIGUE

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Corrosion Fatigue

Typical Locations

Regions with differences in

thermal expansion rates and

directions (cracks originatingexternally)

Corrosion and cyclic stresses

(cracks originating internally

at weld attachments)

Probable Root Cause

Thermal expansion stress

cycles Stress concentration regions

like pits, notches etc.

Dissolved oxygen or acidic

conditions

Corrective Action

Redesign of attachments to

reduce restraints to thermal

expansion Chemical cleaning

Restrictions on cyclic

operation

Heat Treatment and

contouring of welds

Flexible connection

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Corrosion Fatigue

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CORROSION FATIGUE CRACKS

CORROSION FATIGUE CRACKS

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FATIGUE CRACK ATTACHMENT WELD

CORROSION FATIGUE CRACK IN COLD BEND

CORROSION FATIGUE CRACKSCORROSION FATIGUE CRACKS

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Fly Ash Erosion

Typical Locations

Gaps between tube banks and

duct walls.

Gas by-pass channels

Protrusions of rows.

Areas close to large ash

accumulation.

Probable Root Cause

Non-uniform, excessive gas flow

with fly ash particles.

High ash coal with -quartz.

Tube misalignment.

Corrective Action

Changing operating

conditions like reduced load,low excess air etc.

Protections like shields,

baffles etc.

Flow Model study

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Fly Ash Erosion

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Falling Slag Erosion

Typical Locations Lower furnace sloping wall

near bottom opening

Probable Root Cause

Slagging of coal andmassive clinker fall

Coal properties and

boiler design

Corrective Action

Change in fuel Wear bars

Increase tube wall

thickness

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Cont to Tube Fai3