288 1068-1302/16/0506-0288 2016 Springer Science+Business Media New York 

Powder Metallurgy and Metal Ceramics, Vol. 55, Nos. 5-6, September, 2016 (Russian Original Vol. 55, Nos. 5-6, May-June, 2016)

EFFECT OF Ni3Al ADDITION AND HEATING MODE ON THE ELECTROCHEMICAL RESPONSE ON AUSTENITIC AND FERRITIC STAINLESS STEELS

A. Raja Annamalai,1,2,4 A. Upadhyaya,2 and D. Agrawal3

UDC 669.14;621.762.5;531..754.2

The effect of intermetallic Ni3Al addition and heating mode on the electrochemical behavior of

sintered austenitic (316L) and ferritic (434L) stainless steels is studied. The green compacts were sintered by conventional and microwave methods in solid state at 1350°C in H2 for a period of

60 min. The sintered samples were then subjected to several characterization techniques for evaluating their density and hardness, and finally their electrochemical response was evaluated through potentiodynamic polarization scan (1 mV/sec) in 0.1 N H2SO4. Upon addition of nickel

aluminide, there was an increase in the densification of the composites, which was found more pronounced in case of 434L stainless steel with nickel-aluminide dispersoid composites. A positive response was also observed on the corrosion resistance of the composites as compared with the bare stainless steel compacts.

Keywords: stainless steel, sintering, intermetallic, density, hardness.

INTRODUCTION

Powder metallurgy (P/M) processing of stainless steels offers advantage of net-shaping, high material utilization, and ability to tailor microstructures and modify compositions using dispersoid additions. The parts of complex geometry can be produced to close tolerances in an economical manner through P/M route. Despite these advantages, the P/M stainless steels suffers relatively poor mechanical and corrosion properties attributed to their inherent porosity when compared to their wrought and cast counterparts and which have been reported by various researchers in different environments [1, 2]. To enhance the mechanical and tribological properties of the P/M stainless steels, hard dispersoids such as Al2O3 and SiC have been reinforced. Powder metallurgy processing allows the

microstructural modification with respect to the reinforcement size, shape, and placement [3]. Several researchers have reported the beneficial mechanical and wear behavior of these MMCs (Metal

Matrix Composites) [4–7]. However, the major drawback of these composites is their poor corrosion behavior compared to straight stainless steels. The poor interaction between the matrix and reinforcements promotes the onset of the corrosion attack. Shankar et al. [8] reported that Y2O3 addition marginally increases densification in

1Department of Manufacturing Engineering, School of Mechanical Engineering, VIT University, Vellore, Tamil Nadu 632014, India. 2Department of Materials Science and Engineering, Indian Institute of Technology Kanpur, UP, 208016, India. 3Materials Research Institute, The Pennsylvania State University, University Park, PA 16802, USA.

4To whom correspondence should be addressed; e-mail: raja.anna@gmail.com.

Published in Poroshkovaya Metallurgiya, Vol. 55, Nos. 5–6 (509), pp. 49–58, 2016. Original article submitted January 12, 2015.

DOI 10.1007/s11106-016-9804-1

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316L compacts in solid state and supersolidus sintering condition. This finding was attributed to the interaction of Cr2O3 with the Y2O3 dipersoids. Subsequent analysis revealed no significant degradation of the corrosion resistance

in 316L–Y2O3 composites as compared to straight 316L compacts. Jain et al. [7] have also observed similar

interaction for YAG additions to 316L and 434L stainless steels. An improve ment in the wear and corrosion resistance has been observed as reinforcing with intermetallics produced in situ through reactive sintering [9]. As compared to the oxide and carbide reinforcement the aluminide based intermetallics have been shown to have better interaction with the stainless steel matrix. It is envisaged that such an interaction results in enhanced corrosion resistance [10]. Furthermore, the interaction can aid in smoother compositional variation, which can assist in reducing localized (e.g., intergranular) corrosion.

The present work investigates the effect of intermetallic (Ni3Al) addition and heating mode on the

electrochemical behavior of sintered austenitic (316L) and ferritic (434L) stainless steels. The compacts were sintered in solid-state (1350C) conditions. The sintered samples were characterized for their corrosion resistance by potentiodynamic polarization scan in 0.1 N H2SO4.

EXPERIMENTAL PROCEDURES

The stainless steels (316L and 434L) and nickel-aluminide (Ni3Al) powders were used for the present

study. The powders were supplied by Ametek Speciality Metal Products (USA) and Xform Inc. (USA). The chemical composition of the stainless steel powders is summarized in Table 1. The aluminide powders were found to have a purity more than 99.7%; their characteristics are summarized in Table 2. The composites investigated in the present work are 316L and 434L stainless steels with different proportions (4, 8, and 12 vol.%) of aluminides (Ni3Al).

The powders were mixed in the required proportions in a turbula mixer (model: T2C, supplier: Bachofen, Basel, Switzerland) for 20 min. Cylindrical pellets (16 mm diameter and 6 mm average thickness) were compacted from the mixed compositions at 600 MPa using a 50 ton hydraulic press (model: CTM-50; supplier: FIE, Ichalkaranji, India) with a floating die, using zinc stearate as a die wall lubricant. The green densities of the compacts were around 80% of theoreti-cal density. The green compacts were sintered in a MoSi2 heated horizontal

tubular sintering furnace (model: OKAY 70T-7; supplier: Bysakh, Kolkata, India) at a heating rate of 5°C/min. The as-pressed compacts were consolidated isothermally at 1350°C, which correspond to solid-state sintering. Sintering was carried out in hydrogen atmosphere (dew point: –35°C).

TABLE 1. Composition of the As-Received Powders (in wt.%) Used in the Present Investigation

Powder C Si Mn Ni Cr Mo S P Fe

316L 0.025 0.93 0.21 12.97 16.5 2.48 0.008 0.01 Bal. 434L 0.023 0.71 0.2 – 17 1.0 0.02 0.02 Bal.

TABLE 2. Characteristics of the As-Received Powders with Spherical Shape of the Particles

Property Powder

316L 434L Ni3Al

Cumulative powder size, m: D10 10.3 8.5 36 D50 45.9 35.3 50 D90 85.1 75.1 68

Apparent density, g/cm3 2.7 2.6 3.9 Flow rate, sec/50 g 28 28 15 Theoretical density, g/cm3 7.98 7.65 7.5

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The sintered densities were determined from dimensional and weight measurements. Microwave sintering of the green compacts was carried out using a multimode cavity 2.45 GHz, 6 kW commercial microwave furnace (model: RC/20SE; supplier: Amana Radarange). The sintered compacts were prepared for metallography and chemically etched in Marvels reagent. The microstructural observations were carried out through an optical microscope (model: Lunn Major; supplier: Leitz, Germany) as well as the scanning electron microscope (model: JSM-840A; supplier: JEOL, Japan).

For electrochemical evaluation, the cylindrical samples were polished to mirror finish and ultrasonically cleaned in acetone. The potentiodynamic scans were conducted using a flat cell, supplied by Accutrol Inc., USA, at room temperature. A standard three-electrode (reference, counter, and working) technique was used for the measurement. The reference electrode used for the present experiment was a calomel electrode (SCE) saturated with saturated KCl and the sintered alloy was chosen as the working electrode. A platinum mesh acted as a counter electrode. The area of the sintered sample covered to the solution was 1 cm2. The electrochemical experiments were performed using Gamry Instruments, Inc., potentiostat, (model: PC4) on a CMS100 Framework. Prior to the polarization test, each sample was stabilized for about 3600 sec in the solution to get a stable open circuit potential (OCP). The potentiodynamic polarization tests were conducted at-least three times at a constant scan rate of 1 mV/sec in freely aerated 0.1 N H2SO4. The potentiodynamic scan of a sample was given in graphical form

(voltage versus current density). The critical parameters like corrosion potential (Ecorr), corrosion current (Icorr), and

corrosion rate were evaluated from the polarization curves. The passivation current density (Ip), critical current

density (Icrit), primary passivation potential (Epp), and the trans-passive potential were also determined from the

potentiodynamic polarization scan.

RESULTS AND DISCUSSIONS

The variation of sintered density with Ni3Al addition for 316L and 434L stainless steels consolidated using

conventional and microwave furnace is pre-sented in Table 3. The effect of Ni3Al addition on densification

improved the density of stainless steel compacts. Microwave sintering of stainless steel grades 316L and 434L results in higher sintered density. In

comparison to the density of conventional processed compacts, the enhancement in densification during microwave sintering is more pronounced for 434L–Ni3Al composites. During conventional sintering, the dispersoids, which are

in general refractory in nature, remain segregated at the metal–metal interparticle regions and inhibit compact densification [11]. These observations suggest that Ni3Al particles in the composite independently couple with

microwaves. The interaction of microwaves with this reinforcement is perhaps more effective than even stainless steel powders. This results in formation of localized hot spots within the compacts during sintering. This not only activates densification but also concomitantly results in microstructural coarsening. In summary, it is possible to

TABLE 3. Effect of Ni3Al Addition and Heating Mode on the Sintered Density of Austenitic (316L)

and Ferritic (434L) Stainless Steel Compacts Sintered at 1350C

Ni 3

Al c

onte

nt,

vol.%

316L 434L

Conventional Microwave Conventional Microwave

% Theoretical

density

Average grain size,

µm

% Theoretical

density

Average grain size,

µm

% Theoretical

density

Average grain size, µm

% Theoretical

density

Average grain size,

µm

0 81.6 (*) 83.0 (*) 77.2 (*) 88.5 32 4 77.4 40 84.1 34 83.2 26 86.0 24 8 79.7 36 82.4 30 83.8 22 86.0 20

12 82.2 24 80.8 21 83.0 18 85.5 16

(*) Grain size could not be determined.

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Fig. 1. Optical micrographs of 316L–Ni3Al composites with varying amounts of nickel aluminide addition (0, 4, 8, 12 vol.%) and sintered in conventional and microwave furnaces

engineer local thermal instability and induce anisothermal effects in microwave-sintered ferrous systems by appropriately tailoring the composition. It is envisaged that more in-depth investigation of this effect can result in engineering a novel microstructure through alloying iron with reinforcements having different coupling behavior with microwaves. Peelameduet et al. [12] and Roy and co-workers [13] have demonstrated that in multiphase systems, the phases heat up at different rates. This was shown to result in local anisothermal heating effect. Through model experiments in systems, such as Y2O3–Fe3O4, BaCO3–Fe3O4, and NiO–Al2O3, Peelamedu et al. [12]

demonstrated that the hotter species diffuse rapidly into the relatively colder ones and correlated it with the very rapid diffusion rates observed in microwave processed compacts. To test this hypothesis and to utilize its effect on the enhancement of densification and corrosion resistance in ferrous alloys, stainless steel was selected as a candidate system.

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Fig. 2. Optical micrographs of 434L–Ni3Al composites with varying amounts of nickel aluminide

addition (0, 4, 8, 12 vol.%) and sintered in conventional and microwave furnace

Figures 1 and 2 show the microstructures of 316L–Ni3Al and 434L–Ni3Al composites consolidated in

conventional as well as microwave furnace. In the both grades, aluminide dispersoids are uniformly distributed. Both austenitic and ferritic stainless steel with dispersed Ni3Al show relatively coarse grains in microwave-sintered

condition (Figs. 1b–1d and 2b–2d). The extent of microstructural coarsening seems to enhance with increasing aluminide content. In case of conventional sintering, aluminide addition seems to have an opposite influence on the microstructural coarsening and seems to act as grain growth inhibitors. For conventionally sintered 316L–Ni3Al and

434L–Ni3Al composites, increasing dispersoid addition results in smaller grain size. Figure 3 shows the effect of

Ni3Al addition and sintering mode on the bulk hardness of 316L and 434L compacts. The addition of aluminide

increases the hardness of stainless compacts. In general, the hardness increases with increasing Ni3Al addition. For

each composition, as compared to conventional sintering, microwave sintering results in either equivalent or moderately higher hardness.

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a b

Fig. 3. Effect of heating mode and varying Ni3Al content addition on the bulk Vickers hardness of

316L and 434L stainless steels

Figures 4a and 4b present the potentiodynamic polarization curves for 316L–Ni3Al composites sintered in

conventional and microwave sintering furnaces, respectively. The corresponding polarization curves for Ni3Al

composites are shown in Fig. 5. The trends in anodic polarization for conventionally sintered 316L–Ni3Al

composites reveal a passive behavior in 0.1 N H2SO4 up to an applied potential of 1000 mV. Beyond that, the

increase in the current density increases progressive breakdown of the passive layer on the surface. A similar trend is exhibited by microwave sintered 316L–Ni3Al composites as well (Fig. 4b). A comparison of Figures 4a and 4b

reveals that current density in the anodic region for microwave sintered compacts is lower than its conventionally sintered counterpart. This is reflected in the relatively lower Icorr and corrosion rates in microwave sintered 316L–

Ni3Al compacts (Table 4). Unlike the 316L–Ni3Al compo-sites, the 434–Ni3Al compacts—processed through

conventional as well microwave sintering routes—exhibit active passive transition (Figs. 5a and 5b). However, unlike the austenitic steels, microwave sintering does not result in a marked improvement in the corrosion rate of 434L–Ni3Al composites (Table 4).

Velasco et al. [10] have reported that intermetallic additions, such as Cr2Al and TiAl, improve the

corrosion response of austenitic stainless steels. In sintered multiphase systems, such as composites, the presence of features having different electrochemical potentials results in the formation of microcells. Sites of high energy (e.g., grain boundaries) tend to be anodic, whereas reinforcements tend to be of relatively lower energy and hence are cathodic [12]. Thus, both topological as well as compositional features on the exposed surface influence the corrosion results.

a b

Fig. 4. Comparison of the potentiodynamic polarization curves in 0.1N H2SO4 of 316L–Ni3Al

composites with varying aluminide addition: the compacts were consolidated using conventional (a) and microwave (b) furnace

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TABLE 4. Effect of Heating Mode and Aluminide Content on the Passivity Parameters of 316L and 434L Compacts Obtained from the Potentiodynamic Polarization Investigation

The corrosion resistance of any material strongly depends on its microstructural features. From the

standpoint of corrosion, the addition of the chemically noninert dispersoids to stainless steels has two opposing influences. While Ni3Al addition in optimal level increases the densification, the microstructure coarsening too is

restricted. Hence, the structure is more refined. This would contribute to corrosion through formation of local galvanic couples betweenstainless steel and aluminides. This can be termed as microstructural effect. On the other hand, the activation of densification will reduce the pores that are active sites and thereby result in enhancement in corrosion resistance. In case of microwave sintering, the anisothermal heating effect results in coarser micro- structures in stainless steel–Ni3Al composites, which further contributes to corrosion resistance. This benign role of

Ni3Al on the corrosion response is further confirmed for ferritic stainless steel. However, the high corrosion rates

made the comparison between individual compositions rather difficult. In summary, the trend in the corrosion resistance of sintered stainless steels and their composites can be summarized as.

a b

Fig. 5. Potentiodynamic polarization curves of conventionally (a) and microwave (b) sintered 434L–Ni3Al composites with varying aluminide addition

Composition Sintering mode Icorr, A/cm2 Ecorr, mV Corrosion rate, mpy

316L compacts

316L Conventional 0.65 –404 63 Microwave 0.71 –338 43

316L–4Ni3Al Conventional 1.17 –421 55 Microwave 0.04 –210 2

316L–8Ni3Al Conventional 1.02 –403 68 Microwave 0.08 –278 47

316L–12Ni3Al Conventional 1.50 –344 80 Microwave 0.03 –220 2

434L compacts

434L Conventional 86.7 –462 4016 Microwave 10.9 –341 591

434L–4Ni3Al Conventional 12.1 –531 669 Microwave 21.0 –476 1142

434L–8Ni3Al Conventional 22.3 –447 1181 Microwave 15.0 –543 787

434L–12Ni3Al Conventional 1.60 –394 87 Microwave 9.50 –473 512

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The effect of heating mode and Ni3Al addition on the corrosion resistance (increasing trend) of austenitic

and ferritic stainless steel can be summarized as follows. Conventional sintering:

316L–12Ni3Al 316L–8Ni3Al straight 316L 316L–4Ni3Al,

straight 434L 434L–8Ni3Al 434L–4Ni3Al 434L–12Ni3Al;

Microwave sintering:

316L–8Ni3Al straight 316L 316L–4Ni3Al 316L–12Ni3Al,

434L–4Ni3Al 434L–8Ni3Al straight 434L 434L–12Ni3Al

(the compositions that resulted in significant reduction in the corrosion rate have been underlined). In case of microwave sintering, the positive effect of aluminide addition on the corrosion response can be

attributed to the enhanced microstructural coarsening (Figs. 1 and 2). The sintered microstructures of aluminide-reinforced austenitic and ferritic stainless steels suggest that Ni3Al retains its identity and appears as distinct phases.

The Ni3Al boundaries appear as a dark rim. The phase formation in the interfacial region was critically evaluated

through EDS analysis by Upadhyaya and Balaji [14, 15]. The interface between the aluminide particles and stainless steel that appear in the sintered structure was confirmed to predominantly containing Al2O3. Similar

observation has been reported by Abenojar and co-workers [16] in 316L–3 vol.% Cr2Al stainless steels sintered at

1230°C. Extensive interaction between the intermetallic reinforcements (TiAl, Cr2Al, and TiCr2) with the stainless

steel matrix has been reported by Abenojar and co-workers [16]. The reduction in corrosion rate with Al2O3

addition to the stainless steel was reported by Mukherjee and Upadhyaya [17]. They attributed this to the interaction between Cr2O3 and Al2O3 that results in solid solution formation [18]. Thus, it can be concluded that the

improvement in corrosion properties of nickel-aluminide reinforced stainless steels is due to the interaction of Cr2O3

with the Al2O3 that forms in situ at the Ni3Al–316L/434L interface during sintering.

CONCLUSIONS

The microwave sintering enhances the densification in both austenitic 316L and ferritic 434L stainless steels–aluminide composites. This improvement can be attributed to the faster heating rate in microwave, which enhances the sintering kinetics.

A change in grain morphology and respective grain size and pore closure is observed in case of microwave sintered compacts. The addition of aluminides to both 316L and 434L stainless steels increases the bulk hardness. This effect is attributed to the higher hardness of the dispersoid particles. In the case of microwave sintered composites, Icrit decreases for Ni3Al addition. This could be due to the effect of Ni present in the Ni3Al, which is

known to decrease the Icrit in the stainless steel systems. The slight increase in Ecorr for the microwave sintered

compacts indicates its noble behavior as compared to the conventional sintering compacts.

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