corrosion and its inhibition in sa213-t22 tig weldments used in power plants under neutral and...

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Materials Science and Engineering A 432 (2006) 47–51

Corrosion and its inhibition in SA213-T22 TIG weldments used inpower plants under neutral and alkaline environments

S. Natarajan ∗, S.P. Kumaresh BabuDepartment of Metallurgical & Materials Engineering, National Institute of Technology, Tiruchirappalli 620 015, Tamilnadu, India

Received 18 February 2006; received in revised form 21 April 2006; accepted 12 June 2006

bstract

Failures of industrial boilers have been reported to be mainly due to water side corrosion and scaling. Boiler corrosion is due to highly alkaliner acidic conditions of the boiler water. The present work has been aimed at investigating the corrosion behaviour of T22 (2.25 Cr–1 Mo) boilerteel TIG weldments in neutral and alkaline media. The parent metal and weld root regions are chosen as regions of exposure for the study made atoom temperature and at 100 ◦C. Electrochemical polarization technique such as Tafel line extrapolation (Tafel) was used to measure the corrosion
urrent. Corrosion inhibition using thiourea at 100 ppm concentration was studied in these experiments. Micro-structural observation and surfaceharacterization using SEM and XRD studies have been made on samples exposed at 100 ◦C. The results show that parent metal experiences higherorrosion rate than weld metal and thiourea as inhibitor is found unsuitable in NaOH medium.
2006 Elsevier B.V. All rights reserved.

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eywords: Boiler corrosion; T22 TIG weldments; Neutral and alkaline media;

. Introduction

Fossil boilers experience corrosion problems in both water-ide and fireside of the tubing as well as elsewhere in the plants.ailures of industrial boilers have been reported mainly due toaterside corrosion and scaling. Corrosion causes difficulty in

wo respects. The first is the deterioration of the material itselfnd second is the deposition of the corrosion products in higheat release areas of the boiler. Corrosion beneath certain typesf boiler deposits can so weaken the metal that tube failure mayccur. In steam condensate systems, replacement of lines andquipment due to corrosion can be costly.

On load boiler corrosion results from the local formation ofconcentrated alkaline or acidic solutions at the metal surface.wo types of accelerated on load corrosion can be caused by theccumulation of boiler water impurities. The first type causticouging is caused by boiler water treatment additives and seconds caused by local concentration of chloride impurities which are
cidic or become acidic when become heated.
In all arc welding processes [1–4], the intense heat sourceroduced by the arc and the associated local heating and cool-

∗ Corresponding author. Tel.: +91 431 2500133; fax: +91 431 2500133.E-mail address: drs [email protected] (S. Natarajan).

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921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved.oi:10.1016/j.msea.2006.06.051

rea Inhibitor; SEM and XRD

ng results in a number of consequences in material corrosionehaviour and several metallurgical phase changes occur in dif-erent zones of a weldment. Because the occurrence of corrosions due to electrochemical potential gradient developed in thedjacent site of a weld metal, it is proposed to study the effectsf welding on the corrosion behaviour. The fossil fuel fired boil-rs [5–7] and power generating equipment experience corrosionroblems in such components as steam generators, waterwallsurrounding the furnace and in front and rear portions of theuperheater and reheater. These components are often madef carbon and low alloy steel. The water used for steam rais-ng in any boiler installation often contain gaseous impuritiesnd dissolved solids. These can cause scaling and corrosionn the boiler plant. Apart from these, some of the inorganicalts hydrolyze to produce acidity causing corrosion of boilerubes.

This paper describes an experimental work carried out at RTnd 100 ◦C to evaluate and compare corrosion and its inhibitionn SA213 Gr.T22 (2.25 Cr–1 Mo) steel weldments prepared byIG welding process in ammonium nitrate and sodium hydrox-

de media. Organic compounds such as hexamine and thiourea
re normally used as inhibitors for the prevention of corrosion.n the present study thiourea is employed as inhibitor. SEMnd XRD analyses have also been made on samples exposed at00 ◦C.
mailto:[email protected]

dx.doi.org/10.1016/j.msea.2006.06.051

4 ls Science and Engineering A 432 (2006) 47–51

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Table 2Welding process parameters for TIG welding

Parameters Values

Current 80–90 AVoltage 18 VElectrode diameter 3.15 mmGas flow rate 20 L/minShielding gas Argon

Table 3Corrosion current in mA/cm2 at RT for T22/TIG/NH4NO3/NaOH/thiourea

Concentrationof medium

PM WM

NaOH NH4NO3 NaOH NH4NO3

0.01 MWOI 1.35 9.80 0.68 8.20WI 11.0 1.50 1.10 0.98PIE (IAC) (84.6) (IAC) (88.0)



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8 S. Natarajan, S.P. Kumaresh Babu / Materia

. Experimental work

.1. Preparation of weldments

T22 Cr–Mo steel in tube form was welded by TIG processnd it was stress relived at 730 ◦C. The studies were made usingmall coupons measuring 90 mm × 10 mm, cut from the plateomprising parent metal and weld metal regions.

.2. Corrosive media and Inhibitor

NH4NO3 and NaOH solutions of 0.01, 0.1, 0.5 M concen-rations were prepared using analar grade chemicals. The pHf the solutions were found to be: 5.6, 6.0, 6.7, respectively forH4NO3 and 12.0, 13.0, 13.7, respectively for NaOH medium.hiourea at 100 ppm was used as the inhibitor throughout thetudy. Each test coupon was surface polished using conventionalethods (degreasing, polishing with emery papers of various

rades, etc.). Except the desired area of regions of exposure,he other regions were masked using Teflon. Studies were madeoth at room temperature and at 100 ◦C.

.3. Electrochemical polarization studies

All studies were made using Model-362 scanning poten-iostat (Princeton Applied Research Corp., USA). The studiesere made at room temperature and at 100 ◦C with and with-ut inhibitor mentioned above. The following technique wasdopted to find Icorr (corrosion current) values.

.4. Tafel line extrapolation

Application of potentials between ±50 and 250 mV is takenor Tafel slope. A graphical plot of E versus log i was made in allhe experiment and tangents were drawn which on extrapolationo Ecorr intersected at a point that represented on the X-axis, thecorr value.

.5. SEM analysis

Studies on the surface morphologies of T22 Cr–Mo steel sam-les for WM region were carried out for the 0.5 M NH4NO3 andaOH media containing thiourea as inhibitor at 100 ppm level

xposed at 100 ◦C.

.6. XRD studies

A computer controlled wide angle X-ray diffractometer sys-em JEOL (Japanese make) model; JDX 8030 using Cu K�

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able 1hemical composition in wt.% and mechanical properties in MPa for SA213 Gr.T22

Element

C Mn Si P S

ase metal 0.16 0.60 0.10 0.040 maximum 0.040iller wire 0.105 1.41 0.47 0.017 0.01

ote—PIE: percentage inhibitor efficiency; the term within the parentheses: PIEor TIG process; WOI: without inhibitor; IAC: inhibitor accelerates corrosion;

I: with inhibitor.

adiation and λ = 1.5418 A (Ni filter) with a scanning range–65◦ 2θ was used to investigate the weld region of the speci-en.

. Results and discussion

The chemical composition, mechanical properties and weld-ng conditions for the SA 213 T22 grade Cr–Mo steel are givenn Tables 1 and 2.

.1. Selection of electrochemical technique

Among the many corrosion monitoring techniques available,afel method is selected due to the reason that it provides greaterlectrochemical perturbation to the test electrode. The values of
corr in Tafel electrochemical technique for room temperaturend 100 ◦C are given in Tables 3 and 4. The PM shows higherorrosion rate than WM region both at room temperature and00 ◦C.
steel

Mo Cr T.S Y.S

maximum 1.00 2.25 415.0 207.00.01 maximum 0.01 maximum 480.0 400.0

S. Natarajan, S.P. Kumaresh Babu / Materials Sci

Table 4Corrosion current in mA/cm2 at 100 ◦C for T22/TIG/NH4NO3/NaOH/thiourea

Concentrationof medium

PM WM

NaOH NH4NO3 NaOH NH4NO3



0.5 MWOI 0.92 1.40 0.50 0.72WI 1.30 5.05 0.76 3.00PIE (IAC) (IAC) (IAC) (IAC)

Note—PIE: percentage inhibitor efficiency; the term within the parentheses: PIEfor TIG process; WOI: without inhibitor; IAC: inhibitor accelerates corrosion;WI: with inhibitor.

Table 5Tafel slope values at RT for T22/TIG/NH4NO3/NaOH/thiourea

Medium Molarconcentration (M)

ba, bc (mV/decade)

PM WM

WOI WI WOI WI

NaOH 0.01 60, 112 90, 120 70, 110 81, 1240.1 86, 113 90, 110 82, 121 86, 1190.5 90, 129 86, 122 89, 111 86, 122

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fluid during the operation of any plant. In NH4NO3 medium,thiourea shows 88.0% efficiency at room temperature and 34.6%efficiency at high temperature (100 ◦C) for WM region whereasfor PM region it shows 84.6% efficiency at room temperature

H4NO3 0.01 90, 110 64, 112 88, 114 70, 1190.1 86, 120 69, 118 64, 115 65, 1200.5 85, 116 79, 120 87, 127 69, 122

Tafel slope values namely ba and bc have been furnished inables 5 and 6, respectively for RT and 100 ◦C test conditions.

.2. Effect of alkali concentration

With increase in alkali concentration, increase in Icorr values not much prominent. The Icorr value for both PM and WM
ncreases up to a particular concentration (0.1 M) and then startso decrease to lower values. Natarajan et al. [8] have observed theimilar trend in pressure weldments. This results from formationf protective oxide film over the surface. NaOH can concentrate
able 6afel slope values at 100 ◦C for T22/TIG/NH4NO3/NaOH/thiourea

edium Molarconcentration (M)

ba, bc (mV/decade)

PM WM

WOI WI WOI WI

aOH 0.01 87, 120 86, 122 81, 120 78, 1190.1 77, 111 79, 122 77, 113 83, 1220.5 81, 120 82, 122 89, 127 86, 125

H4NO3 0.01 90, 115 72, 118 79, 121 78, 1190.1 89, 117 69, 116 83, 115 88, 1240.5 88, 116 71, 115 79, 116 81, 119 F

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ence and Engineering A 432 (2006) 47–51 49

o high pH levels leading to caustic corrosion because the mag-etite film is unstable and soluble at pH values above 12. Thisappens at both parent metal and weld metal regions becausef micro-level galvanic cells setup due to differences in surfacerofiles. At high pH levels, the protective oxide layer becomesoluble which leads to enhanced corrosion rate. In the case ofxposure to lower pH corrosive environments namely acids,atarajan et al. [9–11] have found enhanced corrosion rate in

ll regions.

.3. Influence of welding process and micro-structure

Compared to parent metal, the weld metal region shows lowerorrosion rate which indicates that TIG is beneficial to Cr–Moow alloy steel. The low heat input of TIG process results in lessilution of WM and minimum loss of alloying elements even-ually leading to low corrosion rate in the WM region. Also thelloy steel under consideration will not undergo any metallur-ical phase transformation at 100 ◦C and so no micro-structuralariations are to be expected between room temperature and00 ◦C. The PM microstructure consists of polygonal ferrite andearlite. The WM shows a combination of widmanstatten typeerrite, pearlite and bainite. The presence of Cr and Mo in thelloy enhances hardenability and promotes bainite formationven on relatively slow cooling. During the bainitic formation,oarsening of carbide particle occurs and the number of poten-ial galvanic cells per unit area of the exposed surface decreasesnd hence the corrosion rate decreases. The microstructures ofM and WM are shown in Figs. 1 and 2.

.4. Role of inhibitor

It is the weld root region which always carries the corrosive

ig. 1. Microstructure consists of coarse polygonal ferrite and pearlite T22 (2.25r–1 Mo)/TIG/parent metal.

50 S. Natarajan, S.P. Kumaresh Babu / Materials Science and Engineering A 432 (2006) 47–51

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Fig. 3. SEM photo: T22/TIG/WM/0.5 M NaOH/100 ppm thiourea at 100 ◦C.Features: steam chimneys with small globules.

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ig. 2. Microstructure consists of widmanstatten ferrite and bainite T22 (2.25r–1 Mo)/TIG/weld metal.

nd 89.9% at high temperature (100 ◦C) independent of the con-entrations of the medium used.

In NaOH medium the presence of thiourea has not broughtny beneficial effect at all. Instead it has shown the negativeffect leading to acceleration of corrosion. However, at this junc-ure, this should not be taken to mean that thiourea becomesesponsible for acceleration but striking observation is that dueo high pH levels of NaOH, the protective layer of the steelecomes soluble leading to rapid corrosion. In a nut-shell, it cane inferred that the high pH of the NaOH has outweighed thexpected beneficial role of thiourea. Thus, thiourea acceleratesorrosion in NaOH medium both at room temperature and at00 ◦C. A comparison of PIE for different media at room tem-erature and at 100 ◦C is shown in Table 7.

.5. SEM analysis

The WM region exposed at 100 ◦C in 0.5 M NH4NO3 andaOH containing 100 ppm thiourea as inhibitor was subjected toEM examination and the morphological features are provided

n Figs. 3 and 4. In this connection it is worthwhile to quoteelevant literature [12–16] available on surface characteristicsf oxide film formation on steels used in fossil fired boilersertaining to case studies. Similar observation has been madey Natarajan and Kumaresh Babu [17–20] in the investigation
arried out on laboratory scale employing 1Cr–0.5Mo powerlant low alloy steel. Magnetite film (Fe3O4) formation [21] ishe dominant effect of corrosion. The fissuring of the oxide films very much prone to occur under two situations: (a) excessive
ps

able 7omparison of PIE at RT and 100 ◦C for T22/TIG/NH4NO3/NaOH/thiourea

oncentration of medium (M) PM

NaOH NH4NO3

RT 100 ◦C RT

.01 IAC IAC 84.6

.1 IAC IAC 61.1

.5 IAC IAC 35.7

ig. 4. SEM photo: T22/TIG/WM/0.5 M NH4NO3/100 ppm thiourea at 100 ◦C.eatures: honeycomb type structure with microfissures.

rowth of film; (b) existence of high pH levels of the alkali used.he authors of this technical paper have also noticed similarssuring of the film in this present investigation as depicted inEM photographs.

.6. XRD studies

The XRD pattern obtained on WM for both media showseaks which correspond to formation of iron oxide. This can beeen from Figs. 5 and 6.

WM

NaOH NH4NO3

100 ◦C RT 100 ◦C RT 100 ◦C

76.4 IAC IAC 88 34.689.5 IAC IAC 56.33 33.6IAC IAC IAC 66.6 IAC

S. Natarajan, S.P. Kumaresh Babu / Materials Sc

Fig. 5. XRD pattern of WM/T22/TIG/0.5 M NaOH/100 ppm thiourea at 100 ◦C.

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ig. 6. XRD pattern of WM/T22/TIG/0.5 M NH4NO3/100 ppm thiourea at00 ◦C.

. Conclusions

The T22 grade parent metal experiences by and large twoto three times higher corrosion rate when compared to weldmetal region at all concentrations in both the media at bothtemperatures.
In general, NH4NO3 medium shows higher corrosion ratethan NaOH at room temperature and at 100 ◦C.Thiourea as inhibitor is found unsuitable in NaOH mediumboth at room temperature and at 100 ◦C whereas it is found
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ience and Engineering A 432 (2006) 47–51 51

beneficial in NH4NO3 medium with maximum inhibitor effi-ciency of around 90%.SEM examination reveals even distribution of a globularadherent corrosion product with few steam chimneys andmicro-fissures.The XRD shows few peaks of appreciable sharpness indicat-ing formation of iron oxide (Fe3O4).

eferences

[1] R.L.O. Brien, Welding Handbook, vol. 2: Welding Processes, 8th ed.,AWS, Miami, FL, USA, 1995.

[2] T.G. Gooch, Process Industry Corrosion: Theory and Practice, NACEPublication, 1982, pp. 739–755.

[3] M. Henthorne, Corrosion 30 (1974) 39.[4] G.E. Linnert, Welding Metallurgy, Carbon and Alloy Steels, vol. 1, 4th

ed., AWS, Miami, FL, USA, 1994.[5] G.S. Theus, P.L. Daniel, Proceedings of the Eighth International Brown

Boveri Symposium, Baden, Switzerland, 1983, pp. 185–188.[6] NACE, Corrosion Testing of Chemical Cleaning Solvents, vol. 21: Mate-

rials Performance, 3M, NACE Publication, 1982, p. 48.[7] A.N. Rothwell, J. Corros. Prev. Contr. 39 (1992) 113–118.[8] S. Natarajan, F. Ginoprakash, R. Vaideeswaran, J. Steel Relat. Mater. 2

(2004) 159.[9] S. Natarajan, V. Sivan, Bull. Electrochem. 6 (5) (1990) 501–504.10] P.J. Antony, S. Natarajan, R. Vaideeswaran, J. Corros. Prev. Contr. 50

(2003) 181–188.11] S. Natarajan, V. Sivan, Int. J. Join. Mater. 15 (2003) 17–23.12] W.M.M. Huijbregts, Proceedings of the Fifth ICMC, Tokyo, 1972, p.

857.13] P.H. Effertz, Proceedings of the Fifth ICMC, Tokyo, Japan, 1972, p.

920.14] G.M.W. Mann, Br. Corros. J. 12 (1977) 6–13.15] P. Mayer, A.V. Manolesu, Proceedings of the Seventh ICMC, Ontaria,

Canada, 1980, pp. 1480–1487.16] L. Kayafas, J. Corros. (1990) 443.17] S. Natarajan, V. Sivan, J. Corros. Prev. Contr. 50 (2003) 7–19.18] S.P. Kumaresh Babu, S. Natarajan, J. Steel Relat. Mater., in press.
19] S. Natarajan, Australas. Weld. J. Second Quart. 50 (2005) 33–39.20] S.P. Kumaresh Babu, S. Natarajan, Int. J. Join. Mater. 18 (1) (2006)
19–26.21] Metals Hand Book, vol. 13: Corrosion, 9th ed., ASM International, Ohio,

USA, 2001, pp. 985–995.

corrosion and its inhibition in sa213-t22 tig weldments used in power plants under neutral and...

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