High Temperature Corrosion Failure of a SecondarySuperheater Tube in a Thermal Power Plant Boiler

D. Ghosh*, H. Roy, S. Ray and A. K. Shukla

NOT and Metallurgy DivisionCentral Mechanical Engineering Research Institute, Durgapur-713209, India

(Received April 23, 2009; final form April 30, 2009)

ABSTRACT

Failures of boiler tubes are a foremost cause of theunscheduled outages of the thermal power plant boiler.The failure of the boiler tubes occurs due to variousreasons like creep, fatigue , corrosion and erosion. Thispaper highlights a case study of typical prematurefailure of a platen superheater tube of thermal powerplant boiler. Visual examination, dimensionalmeasurement, chemical analysis, oxide scale thicknessmeasurement, microstructural examination, EnergyDispersive spectroscopy (EDS) were carried to ascertainthe probable cause/causes of failure of inner leg ofplaten super heater tube. From the investigation, it wasfinally concluded that the presence of potassium (K) andsulphur (S) in the deposited ash may be responsible forformation of low melting compound alkali- irontrisulfate. The combination of localized high tube metaltemperature and wall thinning due to high temperaturefireside corrosion led to the premature tube failure.

Keywords: Boiler; High temperature corrosion;Failure; Platen Superheater.

1. INTRODUCTION

Low alloy ferritic steels viz. 2.25 Cr-1 Mo and 1 Cr-0.5 Mo are used extensively for the construction ofwater wall, economizer, superheater and reheater tubepanels/ banks of the boiler /1,2/. Platen superheater tubebanks are located in radiant zone in the furnace and veryoften exposed to high temperature. Different hightemperature damage mechanisms like creep, firesidecorrosion are becoming operative in superheater tubebanks during operation and causes material degradationwhich finally leads to premature failure /3/. Firesidecorrosion of the liquid molten ash deposits at the hightemperature can cause wall thinning and contributing tostress rupture failure. Fire side corrosion problem oftenexists in the superheater tube above temperature 853 K/4/. In pulverized coal fired boilers corrosive ash willform the deposit over outer surface of the tubes. Thepresence of both Na and K and Sin the ash in presenceoxidizing atmosphere will form low melting eutecticalkali tri iron sulfate (Na,K)3 Fe (SO4). The alkali irontrisulphate forms by reaction of alkali sulphates withiron oxide in the'presence of SO3 and have a maximummelting temperature (1:1 mixture of sodium andpotassium salts) of 825 K, whereas melting points ofsimple sulphates are 1157 K for Na2SO4 and 1342 K for

Corresponding Author:Dcbashis Ghosh

NOT & Metallurgy Division,Central Mechanical Engineering Research Institute, Durgapur-713209, IndiaTel.: +91-343-6452083/6510233; Fax: +91-343-2546745/2548204E-mail address: dghosh@cmeri.res.in, dbsl012000@yahoo.co.in

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K2SO4. 15-61 .This paper highlights a case study onprobable cause in a typical premature platen superheatertube failure in a thermal power plant boiler of 210 M Wcapacity. The as received failed tube is shown in Fig 1.The material specification, design and operatingparameters of the tube as obtained from the plant aregiven as follows.1. Material specification of the tube: SA-213 T22 (2.25

Cr-l Mo steel)2. Working Temperature and pressure of the tube: 798

K&90X104kg/m2 .3. Location of failure: Bottom bend of platen

superheater tube4. Effective running hours: 1,26,543 hours5. Nominal dimension of the tube: 44.5mm (Outside

diameter) X 4.5 mm Thk.

ailed Zone

dherentash deposits

Fig. 1: As received failed tube

2. EXPERIMENTAL PROCEDURE

The following tests were carried out to identify theprobable cause/causes of failure.

Visual examinationDimensional measurementChemical analysisOxide scale thickness measurementSulfur printMetallographie examinationEDS X-ray microanalysis

2.1 Visual examination and dimensionalmeasurement

The visual inspection revealed a small puncturealong the fireside of the failure. Longitudinal scale

cracking and thick adherent ash deposit was foundaround the failure zone (Fig 2). A thick deposit on thefireside was observed in the outer surface of the tubeslightly away from the puncture zone (Fig 3). The outerdiameter (OD) and wall thickness were measured byusing Vernier caliper and ultrasonic wall thicknessgauge (Type: DM-3, Krautkramer, Germany). Theminimum thickness found near the region of failure is1.6 mm .whereas outside diameter is 50.0 mm. Thedimensional measurement revealed significant reductionin wall thickness and increase in diameter in the form ofbulging around the failure zone.

Longitudinal scalecracking

Failure location

Fig. 2: Close view of the failure showing longitudinalscale cracking

Fig. 3: Thick ash deposits on outer surface of the tube(Adjacent to the failure zone)

2.2 Chemical analysis

The adherent ash deposits are removed from thefailed tube and chemically analyzed. The results aregiven in Table 1 (a); the chemical composition confirmsthe specification of SA-213 T22 (2.25 Cr-l Mo steel).

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D Ghosh, H Roy, S. Ray and A. K. Shukla High Temperature Materials and Processes

The Fireside ash deposit is removed from failed tubes in composition. The results of ash analysis are given inthe form of powder and subjected to chemical Table 1 (b).

Table l(a)Chemical composition of the tube material (mass %)

LocationTube material

C0.15

S0.008

P0.029

Si0.22

Μη0.44

Cr2.27

Mo1.43

Table l(b)Chemical composition of the deposited ash (mass %)

LocationAsh

Si20.54

Cr

—

Mo

—

Al2.89

Mg1.57

Ρ2.33

S1.58

K31.77

Ca1.97

Ti3.13

Fe31.99

Zn2.31

2.3 Oxide scale thickness measurement

Small specimens were cut from the failure zone ofthe failed tube, polished up to 1000 grade emery paperfollowed by cloth polishing and then viewed underoptical microscope in unetched conditions (Fig 4). Theaverage oxide scale thickness at inner diameter (I.D)side was found to be 0.18 mm. The average tube metaltemperature calculated from the standard rate laws foroxide scale growth kinetics as suggested by Paterson eto/./7/is858K.

2.4 Sulphur print

Sulphur print of the failed tube samples from theadjacent area of the failure is done by using a piece ofphotographic paper dipped in 2% sulphuric acidsolution and excess acid is wiped off. The prepared tubesamples were pressed on the damp photographic paperfor 3-4 minutes and then removed. The sulphur printshowed presence of sulfur on the outer surface of thetube around the failure zone (Fig. 5).

Fig 5:

Fig. 4: Oxide scale at side of the tube near failure zone

Sulfur print shows the presence of sulfur onthe outer surface of the tube (indicated bybrownish line)

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Vol. 28. Nos. 1-2, 2009 High Temperature Corrosion Failure of Secondary Superheater Tube

2.5 Metallographie examination

The specimen from the failed area is selected formetal lographic examination. The microstructuralfeatures of the failure zone at unetched conditionconfirmed the presence of grain boundary sulfide attacknear the failure zone (Fig. 6). The microstructuralexamination at etched condition also confirmed,partially spheroidised carbides and isolated creepcavities (Fig. 7).

Grain boundarysulfide attack

Fig. 6: Grain boundary sulfide attack near failure zone

%

Fig. 7: Microstructure revealed ferrite, partiallyspheroidised carbides and isolated creepcavities at the grain boundaries.

2.6 EDX microanalysis

The EDS X-ray microanalysis was carried out oninterface between tube OD using EDS (make:Thermonoran USA) attached with scanning electronmicroscope (HITACHI S-3000N). The analysis wascarried out and is shown in (Fig. 8 and Table 2).

Fig. 8: EDS microanalysis in Interface between tubeOD and ash deposits

Table 2Chemical composition in mass percent

ElementMgAlSiΡSKCaTiFe

' Zn

Mass percent1. 57

20.5431.772.331.582.891.973.13

31.912.31

3. RESULTS AND DISCUSSIONS

The failure is associated with small narrowpunctures with longitudinal scale cracking. Thickadherent ash deposits are also covered on the outersurface in and around the failure zone (Fig. 2 and Fig.3). Considerable reduction of wall thickness combinedwith bulging of the tube around the failure zone is alsoobserved. The tube metal temperature estimated fromthe ID side oxide scale thickness (Fig. 4) as suggestedby Paterson et αϊ. ΠI is 858 Κ near the failure zone. Thesulphur print (Fig. 5) of the samples indicated thepresence of sulfur in between base metal and outside

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D Ghosh. H Roy. S. Ray and A. K. Shukla High Temperature Materials and Processes

adherent ash deposit. Microstructural examination atunetched condition revealed grain boundary sulfideattack initiated from outer ash deposits surfaces to theinner side (Fig. 6). The microstructure of the failurezone also revealed isolated creep cavities along withferrite and spheroidised carbides which also supporthigh temperature exposure in the failure zone (Fig. 7).The chemical composition of the ash showed theappreciable amount of potassium (K) and sulftir(S)[Table l(b)]. The estimated tube metal temperature nearthe failure zone is above the temperature of low meltingeutectic of potassium tri iron sulfate, which sticks onouter surface of the tube and accelerates the metalwastage due to fire side ash corrosion. The results ofEDS X-ray microanalysis on the interface of the tubemetal and ash deposit confirm the presence of potassium(K) and sulfur(S) (Fig. 8 and Table 2).

4. CONCLUSIONS

The presence of potassium (K) and sulphur (S) in thedeposited ash may be responsible for formation of lowmelting eutectic alkali-iron trisulfate on the outersurface of the tube. The estimated tube metaltemperature at the failure zone is very favorable for theformation of low melting alkali tri iron sulfate andfurther causes fireside corrosion on the outer surface ofthe tube. The drastic wall thinning from the outersurface due to high temperature corrosion caused thepremature failure of the tube.

ACKNOWLEDGEMENT

The authors would like to thank the director, CentralMechanical Engineering Research Institute, Durgapur,India for his kind permission to publish this paper.

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