effect of heating mode and y o addition on … · electrochemical response on austenitic and...

Effect of heating mode and Y2O3 addition onelectrochemical response on austenitic andferritic stainless steels

A. Raja Annamalai1, A. Upadhyaya2 and D. K. Agrawal3

The present study compares the effect of Y2O3 addition and heating mode on the electrochemical

response on austenitic (316L) and ferritic (434L) stainless steel during solid state sintering. Up to

12 vol.-%Y2O3 was added to 316L and 434L stainless steel. The compacts were sintered at

1350uC for 60 min. The sintered samples were characterised for their density and hardness. The

effect of heating mode and Y2O3 addition on the microstructural evolution and electrochemical

(corrosion) response for the microwave sintered samples are compared with conventionally

sintered samples. The density of stainless steel samples sintered through the microwave sintering

was higher than that of conventional sintering.

Keywords: Powder metallurgy, Sintering, Hardness, Electrochemical techniques

IntroductionStainless steels produced by powder metallurgy (PM) arewidely used in automotive and structural applications.1

Powder metallurgical processing offers the advantage oflow cost net-shaping with high material utilisation (95%),relatively low temperature processing, and a more refinedand homogeneous microstructure.2 Austenitic stainlesssteels (316L) have been widely used in a variety of productforms for architectural, biomedical, industrial andnuclear applications owing to its excellent corrosion andoxidation resistance, good strength, excellent toughnessand fabrication properties.3 Ferritic stainless steelspossess a good combination of radiation as well ascorrosion resistance. The ferritic stainless steels with fineoxide dispersoids are reported to have comparable hightemperature properties as those of austenitic stainlesssteels.3 Extensive research was carried out on the effect ofY2O3 on the sintering behaviour and corrosion resistanceof 316L stainless steel by Lal and Upadhyaya4 andShankar et al.5,6 Solid state sintering was carried out on316L austenitic powders and the effect of up to 8 vol.-%Y2O3 addition was examined. It was observed that thesintered porosity was more homogeneous for the Y2O3-containing composites than for the straight 316L underidentical sintering conditions. This was attributed to theinteraction of Cr2O3 with the Y2O3 dipersoids. Maximumtensile strength was obtained for the sample containing4 vol.-%Y2O3.7 The hardness of the samples increasedwith increasing percentage of Y2O3. Shankar et al.5,6

investigated the effect of sintering temperature and yttriaaddition on electrochemical behaviour of PM ferritic andaustenitic stainless steel in acidic and chloride environ-ment through polarisation methods. They showed thatthe corrosion behaviour of yttria-dispersed stainless steelis comparable that of the corresponding unreinforcedstainless steels. The beneficial effect of yttria aluminiumgarnet (YAG) addition on the densification and hardnessof the ferritic and austenitic stainless steel is previouslyreported by Jain et al.8 Balaji and Upadhyaya9 andTiwari et al.10 have examined in detail the electrochemicalresponse of the YAG added 316L and 434L stainlesssteels. However, one of the limitations of using highersintering temperatures is microstructural coarsening.Typically, in conventional (electrically heated) furnacecompacts get heated through radiation mode duringsintering.2,11 Consequently, to prevent thermal gradientwithin the compact, a slower heating rate coupled withisothermal holding at intermittent temperatures is pro-vided, which increases the process time, thereby, con-tributing to coarsening. To eliminate this problem, afaster heating rate during sintering is envisaged. However,faster heating rates in conventional furnaces result in athermal gradient within the compacts and led todistortion and inhomogeneous microstructure of thesintered compacts. One of the techniques to achieverelatively homogeneous as well as fast sintering incompacts is through microwave heating.12 Microwavesdirectly interact with the individual particulates withinthe pressed compacts and thereby provide rapid volu-metric heating. This reduces processing time and resultsin energy saving. A range of metallic systems have beenshown to couple with microwaves.13,14 Among ferroussystems, Anklekar et al.15,16 and Raja Annamalaiet al.17,18 have recently demonstrated that steel powdercompacts can be effectively sintered in a microwavefurnace and yield higher mechanical properties as

1School of Mechanical and Building Science, VIT University, Vellore632014, India2Department of Materials Science and Engineering, Indian Institute ofTechnology Kanpur, Kanpur 208016, India3Materials Research Institute, The Pennsylvania State University,University Park, PA 16802, USA

*Corresponding author, email [email protected]

� 2015 Institute of Materials, Minerals and MiningPublished by Maney on behalf of the InstituteReceived 3 November 2013; accepted 22 April 2014DOI 10.1179/1743278214Y.0000000176 Corrosion Engineering, Science and Technology 2015 VOL 50 NO 2 91

compared with conventional sintering. Saitou,19 Pandaet al.20 and Padmavathi et al.21 have demonstrated thatboth austenitic as well as ferritic stainless steel compactscan be consolidated using microwaves. Padmavathiet al.21 have shown that 316L–YAG composites can beprepared by microwave sintering and have a superiormechanical and corrosion response. While much empha-sis has been laid on the mechanical property response ofPM stainless steel, little systematic work has reported ontheir corrosion response. The response of heating rate onYAG added 316L stainless steels have been examined indetail by Panda et al.22,23 However, the electrochemicalresponse was not dealt by them. Chen et al.24 have shownthat number of fine spherical yttrium-rich oxide particlesare not evenly distributed in the steel matrix at 1100uC inrolling conditions; Yu et al.25 found that up to 0?5%yttrium addition to K38 nano-crystalline coatings haveresulted in significant improvement of high temperatureoxidation resistance. However, till date as per theauthor’s knowledge no investigation was observed onmicrowave sintering of Y2O3 addition on stainless steels.In this work, an attempt has been made to study the effectof heating mode on densification response, mechanicaland electrochemical properties of austenitic stainless steel(AISI 316L) and ferritic stainless steel (AISI 434L) usingpotentiodynamic polarisation techniques.

Experimental procedureFor the present investigation, austenitic and ferriticgrades were selected. The nominal compositions of thepowders used in the present investigation are presented inTable 1. For both grades, the powders were prepared bygas atomisation and were in prealloyed form. Thecharacteristics of the as received powders are detailed inTable 2. For both the grades, yttria powder was mixed indifferent proportions (4, 8 and 12 vol.-%) in a turbulamixer (model: T2C, supplier: Bachoffen, Basel,Switzerland) for 30 min. The powders were then uni-axially pressed to cylindrical pellets (16 mm diameter,and 6 mm height) 600 MPa in a hydraulic press (model:CTM-50; FIE, Ichalkaranji, India). To minimise thefriction along the walls of die, zinc stearate was used as adie wall lubricant. Conventional sintering of greencompacts was carried out in a MoSi2 heated horizontaltubular sintering furnace (model: OKAY 70T-7; Bysakh,Kolkata, India). The compacts were heated at a constantheating rate of 5uC min–1 and isothermally held for60 min at 1350uC. Two intermediate isothermal holds at600 and 900uC were provided for delubrication/debindingand reduction of oxides respectively. Sintering wascarried out in hydrogen (dew point: 235uC). Themicrowave sintering experiments were carried out usinga 6 kW, 2?45 GHz, multimode microwave furnace.Temperature measurement was made with an opticalpyrometer (Raytek, Marathon Series), through a smallquartz window at the top of the furnace. The temperatureinside the microwave furnace was monitored using an

infrared pyrometer by considering the emissivity of steel(0?35).26 After sintering, the microwave power wasswitched off and samples were allowed to cool insidethe furnace. Further details of the microwave sinter-ing setup used for the present study is providedelsewhere.14–18,20–23 The sintered density was determinedusing Archimedes method. For microstructural andelectrochemical analysis, the samples were polished tomirror finish and ultrasonically cleaned in acetone. Themicrostructural analyses of the samples were carried outthrough optical microscope (model: DM2500; LeicaImaging System Ltd, Cambridge, UK). Bulk hardnessof the samples was measured using Vickers hardnesstester (Leco V-100-C1 Hardness Tester) at 5 kg load. Inorder to ensure reproducibility, five samples wereexamined. The scanning electron micrographs of thesamples were obtained using back scattered electronimaging mode (Zeiss Evo 50, Carl Zeiss SMT Ltd, UK).The sintered compacts were etched for about 10 s byMarble’s reagent (50 mL HClz25 mL of saturatedaqueous CuSO4 solution). Before corrosion testing,samples were polished with 600 SiC papers for obtaininga smooth surface and the electrochemical behaviour ofthe samples was studied in freely aerated 0?1 N H2SO4

solution (pH 1?31¡0?4) using electrochemical System(model: VersaSTAT 3; Princeton Applied research).Electrochemical tests were carried out in a flat corrosioncell (Accutrol Inc., USA) using a standard three-electrodeconfiguration with the sample as the working electrode,platinum mesh as the counter electrode and Ag/AgCl(saturated with KCl) as reference electrode (z197 mVw.r.t hydrogen). The exposed area of the sample was1 cm2. Before polarisation, the polished samples wereallowed to stabilise for 3600 s in 0?1 N H2SO4 to obtainstable open circuit potential. Electrochemical tests werecarried out in a flat corrosion cell (Accutrol Inc., USA)using a standard three-electrode configuration withplatinum mesh as the counter electrode, Ag/AgCl(saturated with KCl) as reference electrode (z197 mVw.r.t hydrogen electrode) and the sample as the workingelectrode. Potentiodynamic polarisation tests were car-ried out from 2250 mV versus open circuit potential to

Table 1 Composition of austenitic and ferritic grade stainless steels used in present study

Grade* Composition/wt-%

316L Fe–16.5Cr–12.9Ni–2.48Mo–0.93Si–0.21Mn–0.025C–0.008S–0.01P434L Fe–17Cr–1Mo–0.71Si–0.2Mn–0.023C–0.02S–0.02P

*Ametek Specialty Metals Products, USA.

Table 2 Characteristics of powders in as receivedconditions used in present study

Property

Powder

316L 434L

Processing technique Gas atomisation Gas atomisationPowder shape Rounded SphericalCumulative powder size/mD10 10.3 8.35D50 45.9 35.3D90 85.1 75.1Apparent density/g cm–3 2.7 2.6Flowrate, s/50 g 28 28Theoretical density/g cm–3 8.06 7.86

Raja Annamalai et al. Effect of heating mode and Y2O3 addition on austenitic and ferritic stainless steels

92 Corrosion Engineering, Science and Technology 2015 VOL 50 NO 2

z1600 mV versus reference electrode at a scan rate of0?1667 mV s–1. The corrosion potential (Ecorr) andcorrosion current (Icorr), corrosion rate, passivationcurrent density (Ipass), critical current density (Icrit),breakdown potential (Eb) were determined from thepolarisation curves. The corrosion potential (Ecorr) andcorrosion current (Icorr) were determined from theintersection of these two linear plots. The corrosion rate(mmpy) can be determined using 1st-Stern method27 andis expressed as follows

Corrosion rate~0:0033e

rIcorr

where, e is the equivalent weight (g); r is the density ofthe material (mg m–3), and Icorr the corrosion current(mA m–2).

Results and discussion

Densification responseFigure 1a and b shows the effect of heating mode andvarying yttria addition (0, 4, 8, 12 vol.-%) on thesintered density of austenitic and ferritic stainless steel,respectively. For all the stainless–yttria compositions,

microwave sintering results in improved density.Relative to conventional sintering, the effect of densifi-cation is more pronounced in 434L–Y2O3 composites.From the conventional sintering results, it can beinferred that yttria addition does not adversely affectthe sintered density of austenitic stainless steel (Fig. 1a).In contrast, up to 8 vol.-%Y2O3 addition results inimprovement in the sintered density of 434L compacts(Fig. 1b). Another interesting observation from thedensification results (Fig. 1) is that for microwaveprocessed 316L–Y2O3 as well as 434L–Y2O3 composites,the sintered density increases with increasing Y2O3

addition. This underscores good coupling betweenmicrowaves and yttria. It is also interesting to note thatfor 434L–Y2O3 composites, the compact densityattained through microwave sintering increases withincreasing yttria fraction (Fig. 1b). The same trend hasbeen observed by few other researchers.20–23 In case ofconventional sintering, the results from this study(Fig. 1) indicates that up to 8 vol.-%Y2O3 additionenhances the density of stainless steel compacts.Elsewhere, Jain et al.8 have also reported the positiveinfluence of YAG addition on the densificationenchancement in sintered stainless steel compacts. For

2 Effect of heating mode and varying Y2O3 content addition on bulk Vickers hardness of SEM microstructures of a 316L

and b 434L stainless steels

1 Effect of Y2O3 addition and heating mode on sintered density of a 316L–Y2O3 and b 434L–Y2O3 composites


Corrosion Engineering, Science and Technology 2015 VOL 50 NO 2 93

conventionally sintered ferritic stainless steel–yttriacomposites, the positive role of Y2O3 addition on thedensification can be attributed to the chemical interac-tion between Cr2O3 and Y2O3.4,7 Elsewhere, Lal and

Upadhyaya4,7 have reported that the reaction betweenthe Y2O3, Cr2O3 and iron oxide during high temperaturesintering to form spinel of type (YFeCr)3O4 or(YxFeyCr)2O3. It is postulated that the formation of

3 Optical micrographs of 316L–Y2O3 composites with varying amounts of yttria addition (0, 4, 8 and 12 vol.-%) and sin-

tered in conventional and microwave furnace



these reaction product contribute to the protection ofthe surface during anodic polarisation.

HardnessFigure 2a and b graphically shows the effect ofY2O3 addition and heating mode on the bulk hardnessof austenitic and ferritic stainless steel compacts,respectively. As compared to straight stainless steel

grades (316L, 434L), the hardness is higher forsintered stainless steel–Y2O3 composites. However,among the stainless steel–Y2O3 composites, the hard-ness does not seem to be strongly influenced on theamount of Y2O3 addition. For all the compositions,the hardness of microwave sintered compacts is higherthan those measured for conventionally sinteredcompacts.

4 SEM microstructures of a straight austenitic stainless steel and 316L–Y2O3 composites with b 4 vol.-%, c 8 vol.-% and

d 12 vol.-% yttria addition. Compacts were sintered in both conventional as well as microwave furnace



Microstructural results

The effect of Y2O3 addition and heating mode on theoptical as well as SEM microstructures of 316L and434L stainless steels is presented in Figs. 3–6. Allsintered microstructures are characterised by presenceof uniform distribution of residual porosity havingirregular morphology. Most pores are concentrated at

the grain boundaries. For Y2O3 containing composites,the SEM photomicrographs (Figs. 4 and 6) showuniform dispersion of Y2O3 along the stainless steelgrain boundaries. For straight 316L and 434L, themicrostructure of the microwave sintered compact ismore refined (Fig. 3a and 5a). The pores become morerounded and smaller but intragranular when the samesteel is processed in microwave sintering.28 The

5 Optical micrographs of 434L–Y2O3 composites with varying amounts of yttria addition (0, 4, 8 and 12 vol.-%) and sin-

tered in conventional and microwave furnace



comparison of the microstructures of sintered 434L–Y2O3 composites reveals some interesting findings. Forconventionally sintered compacts, the grain size reducesprogressively with increasing yttria reinforcement. Thistrend is typical of that observed in sintered particulatemetal matrix composites. In case of 316L–Y2O3

composites, the sintering mode does not seem tosignificantly influence the microstructure (Fig. 3b–d).In contrast, as compared to conventional sintering, the

grains appear to be coarser in 434L–Y2O3 compositesconsolidated through microwave sintering (Fig. 5b–d).Besides, formation of intermediate reaction product, theenhanced densification in the yttria reinforced stainlesssteel can also be correlated to the grain growthinhibiting effect of the added dispersoids, which inducesmore grain boundary diffusion. This is manifestedprominently in the conventionally sintered 434L–Y2O3

composites (Fig. 3).

6 SEM microstructures of a straight ferritic stainless steel and 434L–Y2O3 composites with b 4 vol.-%, c 8 vol.-% and d

12 vol.-% yttria addition. Compacts were sintered in both conventional as well as microwave furnace at 1350uC



Electrochemical study

To investigate the effect of Y2O3 addition and heatingmode on the corrosion behaviour, electrochemical testswere conducted on the sintered compacts in 0?1 NH2SO4. Figure 7a and b presents the potentiodynamicpolarisation curves for 316L–Y2O3 composites sintered inconventional and microwave sintering furnace, respec-tively. The corresponding polarisation curves for 434L–Y2O3 composites consolidated using both the heatingmode are shown in Fig. 8a and b. The zero current

potential value for 316L and 316L–Y2O3 composites isobserved to be around –300 mV, which is in the activepassive transition regime – typical for stainless steel29–31 –is shown for all sintered compacts. All 316–Y2O3

compositions exhibit passivation response during polar-isation. Tables 3 and 4 present a summary of thecorrosion parameters determined from the polarisationcurves. This is reflected in the invariance in the currentdensity with increasing applied voltage up to 1000 mV inthe anodic region (Fig. 7a and b). Beyond that, thecurrent density sharply increases with increasing potential

7 Comparison of potentiodynamic polarisation curves in 0?1 N H2SO4 of 316L–Y2O3 composites with varying yttria addi-

tion (0, 4, 8 and 12 vol.-%). Compacts were consolidated using a conventional and b microwave furnace

8 Potentiodynamic polarisation curves a conventionally sintered and b microwave sintered 434L–Y2O3 composites with

varying yttria addition (0, 4, 8, 12 vol.-%)

Table 3 Effect of heating mode and yttria content on passivity parameters of 316L compacts obtained frompotentiodynamic polarisation investigation

Composition Sintering mode Icorr/mA cm–2 Ecorr/mV Corrosion rate/mpy

316L CON 0.65 2404 63MWS 0.71 2338 43

316L–4Y2O3 CON 1.40 2361 75MWS 0.86 2339 47

316L–8Y2O3 CON 1.84 2325 98MWS 0.36 2338 20

316L–12Y2O3 CON 6.01 2339 327MWS 0.84 2338 43



Table 4 Effect of heating mode and yttria content on passivity parameters of 434L compacts obtained frompotentiodynamic polarisation investigation

Composition Sintering mode Icorr/mA cm–2 Ecorr/mV Corrosion rate/mpy

434L CON 86.7 2462 4016MWS 10.9 2341 591

434L–4Y2O3 CON 33.4 2336 551MWS 10.2 2339 1969

434L–8Y2O3 CON 33.4 2335 3622MWS 10.6 2342 2047

434L–12Y2O3 CON 86.8 2462 5157MWS 11.2 2340 4173

9 Microstructures of conventionally sintered (left) and microwave sintered (right) a straight 316L stainless steel and b–d

316L–Y2O3 composites after subjecting it to corrosion in 0?1 N H2SO4



which reflects a breakdown in the surface passive layer. Incontrast to 316–Y2O3, the 434–Y2O3 compacts – parti-cularily, the conventionally sintered ones – exhibit arelatively large critical current density (icritical) in theanodic region. Thereafter it undergoes passivation. Theonset of passivation occurs at a rather early stage in caseof microwave sintering. The active–passive transition inthe 434–Y2O3 composites is reflected as a sudden decreasein the current density during anodic polarisation (Fig. 8).A remarkable observation for both microwave sintered

straight 434L as well as 434–Y2O3 composite is that lowcurrent density in the passivation region (ipassive

<1024 A cm–2), which is even lower than any of the316–Y2O3 composition investigated in the present study.The passive current density (ipassive) in case for all themicrowave sintered 434–Y2O3 compositions was lowerthan the corresponding corrosion current density (icorr)values (Fig. 8b). From the data presented in Tables 3 and4, it is evident that the corrosion rates are in generalreduced in for microwave sintered compacts. In case of

10 Corroded microstructures of conventionally sintered (left) and microwave sintered (right) a straight 434L stainless

steel and 434L–Y2O3 composites with b 4 vol.-%, c 8 vol.-% and d 12 vol.-% yttria addition



conventionally sintered 316L stainless steel, Y2O3 addi-tion results in a slight increase in the corrosion rate. Incomparison the corrosion response of microwave sintered316L remains unaffected by the Y2O3 content. Thecorrosion rates of 434–Y2O3 compacts are too high tomake a relative assessment of the influence of varyingyttria addition. However, the high corrosion rates madethe comparison between individual compositions ratherdifficult. In summary, the trend in the corrosionresistance of sintered stainless steel and their compositescan be summarised as

Conventional sintering

316L{12Y2O3?316L{8Y2O3?316L{4Y2O3

?straight 316L

434L{12Y2O3?straight 434L?434L{8Y2O3

?434L{4Y2O3

Microwave sintering

434L{12Y2O3?434L{8Y2O3?434L{4Y2O3

?straight 434L

316L{4Y2O3?straight 316L&316L{12Y2O3

?316L{8Y2O3

Figures 9 and 10 show the microstructures of thecorroded 316L–Y2O3 and 434L–Y2O3 composites respec-tively. The composites prepared using austenitic gradestainless steel show a uniform, general corrosion(Fig. 9a–d). In contrast, the corroded surface of the434L–Y2O3 composites show localised corrosion at theinterfacial region. The compositions and corrosionresistance enhancement over an order of magnitude overothers have been underlined. The trends in the results aresimilar to those reported by Shankar et al.5,6 However,their studies were conducted at 1250 and 1400uC inconventional sintering. The former temperature resultedin inadequate densification whereas in case of latter thesintering was through supersolidus mode, whichdegraded the film formation at the surface. Similar kindof improvement in corrosion behaviour was reported byTiwari et al.10 and Jain et al.8 stainless steels reinforcedwith YAG. The compositions and corrosion resistanceenhancement over an order of magnitude over othershave been underlined. The trends in the results are similarto those reported by Shankar et al.5,6 However, theirstudies were conducted at 1250 and 1400uC. The formerresulted in inadequate densification whereas in case oflatter the sintering was through supersolidus mode, whichdegraded the film formation at the surface.5,6

ConclusionsMicrowave sintering results in higher density in bothaustenitic and ferritic stainless steel and stainless steel –Y2O3 composites. Addition of small amount (4 vol.-%)of Y2O3 to both the ferritic and austenitic stainless steelincreases the densification of the composites. This canbe attributed to the increased diffusion rate as a resultof interaction between the stainless steel matrix andthe Y2O3 particles. However, at higher Y2O3 content

(12 wt-%) this improvement in densification is absentbecause of agglomeration of yttria particles, whichlowers the diffusion rate. Microwave sintering of boththe stainless steel compacts results in microstructuralcoarsening and intragranular pores with roundedmorphology. Corrosion resistance of microwave pro-cessed stainless steels and stainless steel–Y2O3 compo-sites in 0?1 N H2SO4 is higher as compared to that ofconventional sintered compacts in case of microwavesintered compacts.

Acknowledgements

The authors thank Ametek Specialty Metals Products,USA for supplying the powder used for this investiga-tion. The financial support from the Indo-US Scienceand Technology Forum (IUSSTF), New Delhi for thisstudy is gratefully acknowledged.

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