Influence of Nitrogen on the Pitting Corrosion Behavior of 904L Weld Clad-raja

Corrosion Science\ Vol[ 39\ No[ 09\ pp[ 0598Ð0514\ 0887Þ 0887 Elsevier Science Ltd[ All rights reserved[\ Pergamon Printed in Great Britain[

9909Ð827X:87:,Ðsee front matter

PII] S9909Ð827X"86#99063Ð0

INFLUENCE OF NITROGEN ON THE PITTING CORROSIONBEHAVIOR OF 893L WELD CLAD

V[ S[ RAJA\�$ S[ K[ VARSHNEY\$ R[ RAMAN% and S[ D[ KULKARNI%

$ Corrosion Science and Engineering\ Indian Institute of Technology\ Bombay\ Mumabi\ 399 965 India% Department of Metallurgical Engineering and Materials Science\ Indian Institute of Technology\ Bombay\

Mumabi\ 399 965 India

Abstract*The e}ect of nitrogen on the weld cladding of 893L _ller metal towards microstructure and elec!trochemical corrosion was investigated[ The initial addition of nitrogen to the shielding gas was found to decreasethe pitting resistance of the clad as compared to that obtained without nitrogen addition[ Subsequent additions ofnitrogen\ however\ gradually enhanced the pitting resistance[ Microstructural examination showed that secondaryaustenitic phase formed along the interdendritic cell boundaries of the primary austenite\ in the absence of nitrogenaddition to the shielding gas\ o}ered good resistance to the pitting corrosion of the interdendritic cell boundaries[On initial introduction of N\ reduction in the formation of secondary austenite occurred making the clad inferiorto pitting corrosion resistance[ Subsequent additions of N decreased the formation of secondary austenite\enhanced pitting resistance of the weld clad by stabilizing the passivity of the interdendritic cell boundaries[Þ 0887 Elsevier Science Ltd[ All rights reserved[

INTRODUCTION

Weld overlay cladding of carbon steel by corrosion resistant materials such as stainless steelis one of the most widely accepted cost e}ective method to prevent corrosion[ Howeversimilar to welded structure\ cladded structures are also prone to localized corrosion[ Thisis because the solidi_ed structure is characterized by dendrites formation\ microsegregationof alloying elements along interdendritic and interface regions and the formation of del!eterious phases which are not present in wrought counterparts[ Though not much literatureon the localized corrosion behavior of weld clads are available\ stainless steel weldmentsare known to be more susceptible to localized attacks such as pitting0\1 and stress corrosioncracking[2\3 Notably\ it has been shown in earlier studies\ that the di}erence in pittingresistance between the welded and the wrought alloy tends to become wider the higher thematerial is alloyed especially with Mo"1#[ Heat treatment can\ in general\ bring down thechemical heterogeneities and improve the corrosion resistance[ However\ in practice thismay neither be viable nor economical[ Nitrogen addition to the weld clad can be one of themost e}ective methods to improve the corrosion resistance of the welded structure[

The bene_cial e}ects of N towards improving pitting\ crevice and intergranular cor!rosion resistance have been known for some time and has formed the basis amongst otherthings for the commercial development and usage of high N!containing austenitic stainless

�Author to whom correspondence should be addressed[Manuscript received 2 May 0886^ in amended form 10 November 0886

0598

V[ S[ Raja et al[0509

steels[4 Several studies have been directed at understanding the role of N in improving thelocalized corrosion resistance[5Ð7 The recent review of Grabke describes various mechanismsproposed by di}erent authors in this regard[8 Though a few studies have been carried outto show the bene_cial e}ect on N in the weldment "for example ref[ 09# towards pittingcorrosion resistance\ the studies on the role of N introduced along with shielding gas onthe localized corrosion behavior of stainless steel weldments are sparse[00Ð02 It is importantto study the role of N introduced through the shielding gas on solidi_ed structure\ becauseN introduced during welding has two roles to play as observed by Rajashekhar\03 i[e[ "i#crucially it in~uences the solidi_cation mode and hence the room temperature micro!structure of the weldment and "ii# as in the case of N!containing wrought alloys improvesthe localized corrosion resistance[ A detailed systematic study of the above combined e}ectsis very essential to properly take advantage of the bene_cial e}ects of N[

This paper describes the in~uence of N on 893L weld cladding with respect to bothmicrostructural and corrosion resistance[ Though the work involves cladded surface\ theresults can also be extended to understand the e}ect of N on the welded structure[

EXPERIMENTAL DETAILS

Materials and claddin`Table 0 shows the compositions of the mild steel substrate "ASTM A405 grade# and

the _ller metal "893L# used and the overlay clad obtained in the study[ The intentionof choosing low S content _ller wire and the mild steel was to prevent any possible hot!cracking of the weld clad[ The surface of the substrate was cleaned before cladding\ bypickling in 34) HCl solution for 19min[ Pickling removed any adherent oxide layerfrom the base metal surface[ Tungsten inert gas "TIG# welding with Ar as the shieldingwas used to develop a weld clad[ Di}erent levels of N in the weld clad were introducedby varying the volume percentage of nitrogen gas in the Ar shielding gas[ The gases\nitrogen and argon\ were passed through a mixer for proper mixing before they werefed to the torch[ A _xed argon gas ~ow rate of 19 l:m was used in all the experiments[Variation in the N!content of the shielding gas was obtained by adjusting the ~ow rateof N into the mixer[ The total pressure in the mixer was approximately 3[9 atm[ gauge[Vol) of N1 in the shielding gas used were "i# 9[4\ "ii# 1[9\ "iii# 4[9\ and "iv# 09[9 andthe corresponding N! content in wt) of the welds obtained were "i# 9[92\ "ii# 9[94\ "iii#9[08\ and "iv# 9[14[ The morphology of clad layers is shown in Fig[ 0[ This involvedbuilding the _rst two layers by initially placing separated layers and then giving a layerbetween them[ Afterwards top layers were developed by continuous overlapping overthe preceding layers[ Here\ the initial run provided layers that were separated and thesubsequent run was given to _ll!up the gaps which existed in the previous run[ Finally

Table 0[ Chemical composition of the base metal and _ller wire

ALLOY C Mn Si P S Cr Ni Mo Cu

Base 9[1 0[98 9[12 9[907 9[998 * * * *Filler Wire 9[91 0[73 9[25 9[901 9[998 19[90 13[68 3[91 0[64Clad 9[92 0[53 9[20 9[915 9[909 07[79 10[80 2[13 0[43

In~uence of nitrogen on the pitting corrosion behavior of 893L weld clad 0500

Fig[ 0[ Schematic diagram sectional view of the clad illustrates how the clad layers were built] a#First two layers were built by initially placing separated layers and then giving a layer between

them\ b# Top layers were developed by continuous overlapping over the preceding layers[

overlapping layers were obtained without any gap[ This method was adopted tosimultaneously optimize the dilution as well as smoothness of the surface[ Welding wascarried out using a welding current and voltage of 069A and 06[4V respectively withan arc gap of 1mm[ Traveling speed was maintained at 10 cm:min[

Chemical composition\ except nitrogen\ of the clad was determined using ARL!23999Quantometer[ For this purpose\ samples of 3 cm ×2 cm cut from the clad and polishedupto 019 grit emery paper were used[ Nitrogen analysis was done by induction meltingmethod in a Leybold!Heraus NOA 1999 model instrument[ For this purpose drill chipswere taken from the clad layer[

Microstructural and phase analysisThe Fisher make ferritoscope was used for measuring the ferrite content of the clad

layer[ The Ferritoscope was calibrated before the commencement of the measurements[For the determination of delta ferrite\ an average of 24Ð39 readings were taken[ X!ray


di}raction studies were carried out using Phillips PW0719 di}ractometer with CuKa

radiation having a wavelength of 0[4395_[For metallographical examination\ the samples were prepared and polished down

with 9[4mm diamond paste[ Etching was carried out using Beraha|s reagent "9[8Ð0[9 gm[Na1S1O2\ 19ml HCl and 099ml distilled water# as described in a previous study[04

Polarization studiesFor electrochemical corrosion studies of the clad\ the base metal was removed by

machining process[ 0×0 cm samples were cut from the clad layer so obtained[ A copperwire was soldered on to the base metal side of the clad layer for electrical contact whilethe other side was used for carrying out corrosion studies[ The side with Cu contactwas mounted with cold setting resin leaving the other side free to carry out corrosionstudies[ Mounted samples were wet polished down with 0mm size alumina[ The interfacebetween the mount and the sample was covered with a epoxy resin to avoid the possibleformation of crevice[

Potentiodynamic polarization studies were carried out using a PARC EG+G Potent!iostat:galvanostat Model 162 driven by M241 software[ Studies were carried out usinga Pt sheet as counter electrode and saturated calomel electrode as a reference electrodeat a scan rate of 9[22mV:s[ Before carrying out the cyclic polarization test\ the specimenwas conditioned at potential −1[9V for 019 s and kept in the solution for 899 s[ tostabilize at the corrosion potential[

Double loop electrochemical potentiokinetic reactivation test "DL!EPR# tests werecarried out as using the procedures described by Majidi and Streicher[05 A fresh solutionof 9[4M H1SO3¦9[90M KSCN was used for each test[ Samples were allowed to stabilizefor 09min in the solution before commencement of the test[ The samples were polarizedfrom corrosion potential "Ecorr# to 299mV "SCE# with a scan rate of 5V:h[

RESULTS AND DISCUSSION

MicrostructureX!ray di}raction patterns obtained on 893L clad layers for various N content are

shown in Fig[ 1[ The peaks in all these patterns correspond only to austenite[ Thus anyother possible phases viz[ d!ferrite\ carbides or intermetallics could not be revealed byx!ray di}raction patterns[ Comparison of intensities of "000# plane\ whose I:Imax accordingASTM x!ray di}raction _le No[01!625 supposed to be 099\ to that of "199# planeshowed that the intensity of "199# plane increased at the expense of "000# plane with Ncontent in the weld clad[ Since\ x!ray di}raction corresponds to as!cladded sheets\ itcould possibly indicate the existence of preferred orientation in the solidi_ed structurein the presence of N[ In fact Lippold and Savage06 had indicated that ³099× are easygrowth directions for both austenite and ferrite phases in the stainless steel weldments[Interestingly\ while faster cooling rates are supposed to promote more directional growth\along easy growth direction ³099× due to better heat transfer condition\ it is not clearwhy N addition results in more textural growth and no work is found in literature inthis regard and a detailed investigation is beyond the scope of the present work[

The microstructural variation in the weld clad due to N addition are demonstratedin Fig[ 2[ The color metallographs delineate the interdendritic boundaries to be distinctlydi}erent from the interdendritic grains[ In the interpretation of colour metallography


Fig[ 1[ X!ray di}raction patterns of 893L weld clad with various N content in it[ a# 9[92 wt) N\b# 9[94 wt) N\ c# 9[08 wt) N and d# 9[14 wt)[ Notice the gradual variation in the relative

intensities of "000# and "199# planes[


)

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(a)

(b)

Fig[ 2[ Microstructural features of the weld clad shown for various N additions[ The variations inthe microstructure of the equiaxed zone close to the centre of the weld clad are shown[ a# 9[92 wt)

N\ b# 9[94 wt) N\ c# 9[08 wt) N and d# 9[14 wt) N[

phase having the reddish yellow color contrast was attributed to d!ferrite by Berana andShpigler[04 It should however be pointed out that x!ray di}raction patterns Fig[ 0 andferritoscope! employed to measure d!ferrite! showed no detectable d!ferrite[ This is inspite of the fact that quantitative metallography showed this phase to constitute about19 volume percent in the weldment obtained without N in the shielding gas as has beenshown in Fig[ 2"a#[ Therefore\ it is obvious that this phase cannot be attributed to d!ferrite[ This apparent contradiction between the optical metallograph on one hand andthe results of x!ray di}raction pattern and ferritoscope on the other hand can be resolved


)

)CMYK Page 0504

(c)

(d)

Fig[ 2[ Continued[

by examining the pseudo!binary diagram as well as inherent solidi_cation characteristicsof a weld clad and will be shown that these are secondary austenitic regions rich in Cr[Based on metallography\ from the area measurements\ the volume fraction of this phasewas calculated for various N contents of weld clad[ Fig[ 3 illustrates how this phasegradually reduces with more addition of N to the clad[

Based on pseudo!binary diagram of Fe!Cr!Ni ternary system "Fig[ 4#[ and electronprobe microanalysis Lippold and Savage06 had shown that high Ni stainless steelsundergo primary austenite solidi_cation[ In such cases the interdendritic cells\ the lastsolidifying liquid\ may be enriched with Cr[ Though this region is richer in Cr contentthan the primary austenite\ only very high levels of Cr and low levels of Ni can result


Fig[ 3[ Variation of volume fraction of the interdendritic phase with varying N content[

in stable ferrite on cooling[ Otherwise this interdendritic region transforms\ on cooling\to austenite! called secondary austenite! in spite of high levels of Cr in the interdendriticregions[ Accordingly the reddish yellow regions can be considered as austenite phasehaving higher levels of Cr and Ni than the primary austenite[ Since the region hashigher Cr content its response to the etchant is similar to that of ferrite[

The increasing addition of N to the weld clad seems to reduce the interdendritic!phaseunder identical conditions of cladding\ as shown in Fig[ 3[ Notably the clad layers did notexhibit any hot cracking even though the weld clad did not contain detectable d!ferrite[

Electrochemical corrosionPotentiodynamic polarization curves of weld clad containing di}erent amounts of N

additions in 2[4) NaCl\ 0M H1SO3 and 0M HCl solutions are respectively shown inFigs[ 5Ð7[ The electrochemical parameters such as corrosion potential\ Ecorr\ pittingpotential\ Epit\ transpassive potential\ Etp\ critical current density ic\ passive currentdensity\ ip\ have been derived from the above polarization curves for 2[4) NaCl and0M H1SO3 respectively[ The data are summarized in Tables 1 and 2 for NaCl andH1SO3 media\ respectively[ The unstable passivity exhibited by these clads in 0M HCl\preclude any such comparison[ In the case of H1SO3 medium\ the weld clad did notshow any pit when observed under microscope after completion of the anodic polarizationand hence the steep raise in anodic current exhibited by the weld clad after passiverange was attributed to transpassive dissolution[

Examination of Table 1 shows that there has not been any systematic and signi_cantvariation observed with respect to either Ecorr or ip within the weld clad to make anymeaningful interpretation in the case of 2[4) NaCl solution[ However\ with respect toEpit a clear trend seems to emerge[ Thus the initial addition of N up to 9[94 wt)decreases the Epit while a subsequent increase only leads to a steep raise in Epit value[


Fig[ 4[ Schematic pseudo!binary diagram of Fe!Cr!Ni terinary system illustrating the e}ect ofcomposition on the stability of various phases[06

The initial decrease in Epit value due to N addition is rather unexpected trend in thelight of the fact that N addition in austenitic stainless steels had been reported to havea signi_cant bene_cial e}ect towards pitting resistance in the wrought condition[4\7\07

It should\ however\ by emphasized that wrought alloys represent an entirely di}erentstate of the alloy where the alloy is completely homogenous both interms of chemistryand microstructures[ Interestingly\ while there had been several studies on the role of Nin wrought stainless steels no systematic work has been reported in the literature on thee}ect of controlled introduction of N in the shielding gas on stainless steel weldmentcorrosion except of Kamachi Mudali and Co!workers00\01 and Ogawa et al[02 The limitedwork on the weldments of N!bearing stainless steels indicate that the N in the weldmentsuppresses localized corrosion[09\08 All these authors have reported that N additionincreases the Epit value irrespective of the N content of the alloy which is in disagreementwith our results[ These papers do not show how a systematic variation in the N contentof the weldment would a}ect its behavior[ The weldments studied by previous workers00Ð

02 contained higher N contents that actually are at the favourable side of the N e}ects


Fig[ 5[ Typical cyclic polarization curves of weld clad under various N contents in 2[4) NaCl[

Fig[ 6[ Typical cyclic polarization curves of weld clad under various N contents in 0M H1SO3[


Fig[ 7[ Typical cyclic polarization curves of weld clad under various N contents in 0M HCl[

Table 1[ Electrochemical parameters derived from the anodic polarizationcurves in 2[4) NaCl solution for various N content of the weld

Wt[) N Ecorr\mV "SCE# Epit\ mV "SCE# ip\ mA:cm1

9[92 −238 406 9[39[94 −183 280 9[69[08 −255 645 9[59[14 −180 0972 9[5

Table 2[ Electrochemical parameters derived from the anodic polarization curves in H1SO3 solution for variousN content of the weld

Wt[)N Ecorr\ mV"SCE# Etp\ mV"SCE# icrit\mA:cm1 ip\mA:cm1 Passive Range "mV#

9[92 -−154 796 09[1 1[2 8269[94 −149 718 5[7 1[4 8799[08 −115 794 5[6 1[7 8379[14 −100 798 1[3 1[3 883

in our present system[ Thus the detrimental e}ect of N at low levels in the weldmentmust have been missed[ Nevertheless\ what is more interesting is that the results obtainedin our laboratory show consistently the detrimental e}ect of N at low concentrationsover a variety of _ller metals employed[03\19 10 Thus it is believed that the detrimental


Fig[ 8[ Typical double loop EPR curves obtained for various N containing clad are shown[ Notethe variation in if\ ir and the nature of reverse scans[ The ~uctuations in the anodic current during

reverse scan disappears for weld clad having 9[14 wt) N[

e}ect of N at lower levels is not an artifact but it is a trend arising out of N additionin the weldment which is mostly not observed in wrought stainless steels[

To understand\ this apparent anomalous behavior of N\ the possible e}ects on thestability of passive _lm formed in weld clad was examined using the electrochemicalpolarization reactivation "EPR# technique[ Fig[ 8 shows EPR test curves obtained for893L weld clad under varying N addition[ Examination of these curves shows thefollowing points[

"a# The if "peak current density of the anodic curve during the forward scan# ishigher for 9[94 wt N) containing clad than for the 9[92 wt N) containing clad[

"b# Subsequent addition of N decreases if["c# The reverse scan indicates a positive hysteresis "being lower in current density

than the forward scan# indicating that the _lm formed is stable in all thosealloys[

"d# A closer examination of the reverse curve\ however\ reveals that the reactivationin the case of clad having 9[940 wt) N is more than that of other clads ingeneral and 9[92 wt) N in speci_c[

"e# The ~uctuations in the reverse curves is an indication of the inherent instabilityof the _lm at this potential[ Thus the absence of such ~uctuation in the weldclad having 9[14 wt) N is indicative of the fact that the _lm formed is morestable[

While the point a# indicates that N promotes active dissolution of the weldment thepoint b# indicates that N indeed promotes active dissolution of interdendritic boundaries


"similar to intergranular corrosion#\ though to a marginal extent\ when present in smallerlevels[ On the contrary the active dissolution is suppressed when present in a largerextent[ The points b# and e# show that only when N is present in a larger amount itpromotes very good stability of the passive _lm and o}ers resistance to active dissolutionand at moderate levels N is not e}ective[ This becomes obvious from the fact that whenthe reverse anodic curves exhibit many unstable passivity and when N is above 9[14wt) there is complete stability[ Polarization results in HCl seem to be complementaryto the EPR tests[ The trend in active dissolution has exhibited by ic is similar to whatis observed in EPR tests[ Further more it is the 9[14 wt) N weld clad that exhibits astable passive region\ though to a smaller extent\ than the remaining clads[ It should benoted that in HCl medium\ even the wrought stainless steels having N do not exhibitgood passivity[11

The anomalous behavior of the e}ect of N addition seems to happen only when thereare depassivators such as Cl− or KSCN[ In the absence of these passivation\ there is asteady decrease in active dissolution[ Thus in pure sulfuric acid "Fig[ 6 and Table 2#\ thealloys behave di}erently in the sense that ic monotonically decreases with N addition[

A closer examination of microstructures of the weldments shown in Fig[ 2 and thepitted samples shown in Fig[ 09 brings out several salient features relating to the e}ectof N on solidi_cation and its consequent e}ect on pitting tendency[ The color opticalmetallographs of Fig[ 2"a# and 2"b# show that when N content of the weldment is 9[92wt) the secondary austenite\ a chromium rich phase forms a continuous network alongthe primary austenite dendritic boundaries[ When N content is enriched\ the volumefraction of this phase reduces steadily as brought out in subsequent micrographs of Fig[2"bÐd# and the plot of Fig[ 3[ Simultaneously\ favorable conditions prevail with regardsto the primary austenite in two aspects[ Firstly\ its N content increases with moreaddition of N and secondly\ more amount of Cr and Mo will be retained in the primaryaustenitic phase as the volume fraction of secondary austenite decreases[ Under suchcircumstances the interdendritic cell boundaries become prone to attack even as theprimary austenitic dendrites themselves becoming more resistant to attack[

Interestingly when the volume fraction of N exceeds a certain level\ in this case it is9[14 wt) N\ the interdendritic boundaries develop far more superior resistance againstnot only to pit initiation\ but also to pit propagation[ The views are corroborated bythe following observations[

Incidently\ the color micrographs seem to provide additional inferences regarding thetendency of the weld clad at various N levels[ It is known that the interference colorsdeveloped on a phase is related to the thickness of the _lm\ which in turn dependsinversely on the corrosion resistance[04 Thus austenite with lower Cr and Mo which areresponsible for better passivity! form thicker _lm than that of ferrite!having higher Crand Mo[ Therefore\ the blue color due to thicker _lm is indicative of a lesser passivatingtendency\ in sharp contrast to the reddish! yellow color due to thinner _lm indicativeof a better passivating tendency[

Based on the above characteristics of metallography\ it can be seen that theinterdendritic regions become vulnerable to attack ðsee the color change of interdendriticregion in Fig[ 2"a# and "b#Ł when the N addition is just 9[94 wt) N[ The subsequentaddition of N strengthens the grain boundary against attack ðFig[ 2 "cÐd#Ł[ Therefore\the nature of cyclic polarization shown in Fig[ 5 and EPR test Fig[ 8 should re~ect thistendency[ The facts that Eprot is almost the same\ albeit the alloy having 9[92 wt) N


Fig[ 09[ Optical metallographs obtained after subjecting the samples for polarization in 2[4)NaCl solution[ a# 9[92 wt) N\ b# 9[94 wt) N and c# 9[14 wt) N[ Note the change in the mode of

attack[ In a# and c# the interdendritic regions exhibit good resistance to attack[


(a)

(b)

Fig[ 00[ Microstructural features of the weld clad brought out for various N additions[ Thevariations in the microstructure of the columnar zone close to unmixed zone are brought out[ a#9[92 wt) N\ b# 9[94 wt) N\ c# 9[08 wt) N and d# 9[14 wt) N[ Note\ the gradual reduction in

the secondary arms of the primary austenitic dendrites with N addition[

showing a marginally higher value\ barring the clad with 9[14 wt) N and the existenceof unstable passivity in the reverse anodic curve Fig[ 8\ barring the clad with 9[14 wt)N\ vindicates the metallographic observation[ As additional evidence\ optical micrographsof the weld clad obtained after subjecting them to cyclic polarization in 2[4) NaCl arebrought out in Fig[ 09[ These micrographs were obtained by subjecting the samples toanodic polarisation[ The micrograph of the pitted samples\ on comparision with thecolor micrographs of Fig[ 2\ shows that when the weld clad contains 9[92 wt) N\ theattack is preferentially within the primary austenite ðFig[ 09"a#Ł[ When the N content in


(c)

(d)

Fig[ 00[ Continued[

the clad becomes 9[94 wt) the attack seen both along the interface and within theprimary austenitic grains ðFig[ 09"b#Ł[ However\ the presence of 9[14 wt) N clearlymakes the grains more resistance to attack ðFig[ 09"c#Ł[

It is worthwhile to highlight yet another observation made in the study which furtherneeds a detailed investigation[ Comparison of micrographs given in Fig[ 2 ðnote that Fig[2"a# shows a gradual variation of color across the boundaries\ while the same is not seenFig[ 2"b#Ł and Fig[ 00 shows that the interface between the primary and secondary austenitesbecomes sharper when the N is added[ This is indicated by the near absence of secondaryarms of primary austenites in Fig[ 00"bÐd# as against their clear presence in Fig[ 00"a#[ Ithas earlier been reported that increase in cooling rate leads to sharp boundaries[ This was


attributed to the enhancement of solute in the solidifying phase at higher cooling rateleading to a reduction in the accumulation of solute in the liquid phase[06 From thecorrosion view point it becomes quite relevant\ since the corrosion resistance of the interfacewill be poor if passivating solute elements such as Cr and Mo are not allowed to accumulateat the interface due to variations in the solidi_cation process[ In the present work\ N seemsto modify the solute distribution ðsee the gradual color change of primary and secondaryaustenite interface in Fig[ 2"a# and its absence in Fig[ 2"bÐd#Ł and there by make the cladmore susceptible to pitting at least during initial stages[ Indeed\ Okagawa et al[12 havereported that\ the N can suppress the formation of secondary arms of the dendrites as itcan bring out the change in solute partitioning on the solid front[ The fact that no secondaryarms are visible in 9[94wt) N ðFig[ 00"b#Ł containing clad as against the clear existence ofsecondary arms in the weld clad with 9[92wt) N ðFig[ 00"a#Ł agrees with the above[ Thisaspect of solute partitioning and its e}ect on localised corrosion needs a detailed investigationas the interfaces are more prone to attack in welds[

CONCLUSIONS0[ 893L weld clad exhibits primary austenite solidi_cation with a continuous net work of

Cr rich secondary austenite along the interdendritic regions[1[ Nitrogen addition to the weld clad reduces the amount of secondary austenitic phase

and as a consequence the clad becomes more susceptible to pitting at low N contents"lower than 9[08 wt)#[ Further increase in N content stabilized the interdendritic regionsand promotes pitting resistance[

2[ The colormetallographyof theweldments can be used to assess the localized corrosion attack[

REFERENCES0[ Marshall\ P[I[ and Gooch\ T[G[ Corrosion\ 0882\ 38\ 403[1[ Garner\ A[ Met[ Pro`[\ 0874\ 016"4#\ 20[2[ Fang\ Z[ and Wu\ Y[\ Zhu\ R[ Corrosion\ 0883\ 49\ 060[3[ Baselack III\ W[A[\ Savage\ W[F[ and Duquette\ D[J[ Met[Trans[\ 0868\ 09A\ 0318[4[ Bandy\ R[ and Vau Rooyan\ D[ Corrosion\ 0874\ 30\ 117[5[ Reynders\ B[ and Stratmann\ M[ Materials Science Forum 074Ð077\ 0884\ 620[6[ Newman\ R[C[ and Shahrabi\ T[ Corros[ Sci[\ 0876\ 16\ 716[7[ Ahila\ S[\ Reyenders\ B[ and Grabke\ H[J[ Corros[ Sci[\ 0885\ 27\ 0880[8[ Grabke\ H[J[ ISIJ International\ 0885\ 25\ 666[

09[ Agarwal\ D[C[ The new nitrogen forti_ed 5) Mo super austenitic stainless steels] their weldability andcorrosion resistance\ Prof[ of the Materials Weldability Symposium\ eds[ R[A[ Patterson and K[W[ Mahin\Detroit\ 7Ð01\ October 0889\ p[ 096[

00[ Kamachi Mudali\ U[\ Dayal\ R[K[\ Gill\ T[P[S[\ Gnanamoorthy\ J[B[ Corrosion\ 0889\ 35\ 343[01[ Kamachi Mudali\ U[\ Dayal\ R[K[\ Gill\ T[P[S[\ Gnanamoorthy\ J[B[ Werkstoffs and Korrosion\ 0875\ 26\ 526[02[ T[ Ogawa\ S[ Aoki\ T[ Sakamoto and T[ Zaizen\ Weldin` J[\ May\ 0871\ 028!s[03[ Rajasekhar\ K[ Studies on the In~uence of Nitrogen on Weldability of Stainless Steels[ Ph[D[ Thesis\ submitted

to Indian Institute of Technology Bombay\ 0885[04[ Berana E[ and Shpigler\ S[ Color Metallography\ ASM\ Ohio\ 0866\ p[11 + 29[05[ Majidi\ A[P[ and Streicher\ M[A[ Corrosion\ 0873\ 39\ 473[06[ Lippold J[C[ and Savage\ W[F[\ Weldin` J[\ May\ 0879\ 37!s[07[ Reynders\ B[ and Stratmann\ M[ Materials Science Forum 074Ð077\ 0884\ 620[08[ Walkar\ R[A[ and Gooch\ T[G[ Br[ Corros[ J[\ 0880\ 15\ 40[19[ Prabhakaran\ T[\ Localized Corrosion behavior of welded austenitic stainless steels[ M[ Tech Dissertation\

Indian Institute of Technology\ Bombay\ 0885[10[ Varshney\ S[K[\ Development of Corrosion Resistance Weld overlay cladding[ M[ Tech Dissertation\ Indian

Institute of Technology\ Bombay\ 0885[11[ Newman\ R[C[ and Sharabi\ T[ Corros[ Sci[\ 0876\ 16\ 716[12[ Okagawa\ R[K[\ Dixon R[D[ and Olson\ D[L[\ Weldin` J[\ Aug[ 0872\ 194!s[

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Influence of Nitrogen on the Pitting Corrosion Behavior of 904L Weld Clad-raja