Scripta Materialia 57 (2007) 651–654 www.elsevier.com/locate/scriptamat
Corrosion behavior of microwave-sintered austenitic stainless steel composites C. Padmavathi,a A. Upadhyayaa,* and D. Agrawalb a
Department of Materials and Metallurgical Engineering, Indian Institute of Technology, Kanpur, UP, India b Materials Research Institute, The Pennsylvania State University, University Park, PA 16802, USA Received 3 April 2007; revised 26 May 2007; accepted 1 June 2007 Available online 5 July 2007
This study compares the electrochemical response of austenitic stainless steel (316L) and yttrium aluminum garnet (YAG)-reinforced 316L composites microwave-sintered in solid-state (1200 °C) and supersolidus (1400 °C) condition. Compared with conventional sintering (through radiative heating), microwave sintering results in better corrosion resistance. This has been correlated with higher densiﬁcation and microstructural attributes in the latter. The 316L–5YAG composites consolidated in microwave furnace at 1400 °C yields the highest corrosion resistance. Ó 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: 316L stainless steel; 316L–YAG composites; Microwave sintering; Densiﬁcation; Corrosion
Powder metallurgical (P/M) stainless steels are increasingly being used for automotive and structural applications. Compared with the conventional cast and wrought route, P/M stainless steel can be processed at a lower temperature, can be near-net shaped, yields a greater material utilization (>95%), and has a more reﬁned and homogeneous microstructure. Despite these advantages, P/M stainless steel has relatively poor corrosion resistance due to the presence of residual porosity. There is considerable scope for improving the properties of P/M components through novel sintering techniques and/or by alloying additives. Stainless steel powders are typically fabricated through atomization, which results in a single-phase prealloyed structure. Such prealloyed powders are amenable to consolidation at relatively higher temperatures through supersolidus liquid phase sintering (SLPS) . Supersolidus sintering involves heating between the solidus and liquidus temperatures to form the liquid phase. The presence of the liquid phase generally enhances densiﬁcation by increasing the diﬀusion kinetics. In addition, capillary stress-induced pore ﬁlling also contributes to densiﬁcation. One of the limitations SLPS is microstructural coarsening . Typically, in a conventional (electrically * Corresponding author. Tel.: +91 512 2597672; fax: +91 512 2597505; e-mail: [email protected]
heated) furnace, compacts get radiatively heated during sintering. Consequently, to prevent a thermal gradient within the compact, a slower heating rate coupled with an isothermal hold at intermittent temperatures is used, which increases the process time and thereby contributes to coarsening. To eliminate this problem, a faster heating rate during sintering is envisaged. However, a fast heating rates in a conventional furnace result in a thermal gradient within the compacts and cause compact distortion and an inhomogeneous microstructure. One technique to achieve relatively homogeneous as well as fast sintering in compacts is through microwave heating . Microwaves directly interact with the individual particulates within the pressed compacts and thereby provide rapid volumetric heating. This reduces processing time and results in energy saving. In addition, the uniform heating minimizes problems such as localized microstructural coarsening, thereby, yielding better properties . Recently, it has been shown that metals could also be coupled by microwaves provided they are in powder form . Subsequently, a range of metallic systems have been shown to couple with microwaves [5– 9]. Among ferrous systems, Porada and Park  and Anklekar et al.  have recently demonstrated that steel powder compacts can be eﬀectively sintered in a microwave furnace and yield higher mechanical properties as compared with conventional sintering. Elsewhere, Saitou  and Panda et al.  have demonstrated that
1359-6462/$ - see front matter Ó 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2007.06.007
C. Padmavathi et al. / Scripta Materialia 57 (2007) 651–654
both austenitic as well as ferritic stainless steel compacts can be consolidated using microwaves. Panda et al.  have shown that stainless steel particulate compacts can be heated at very high rates in microwaves (45 °C min1) and result in about 90% reduction in the overall process time compared with conventional sintering. Besides faster heating, grain coarsening inhibition during sintering has also been achieved through addition of second-phase dispersoids [14–16]. In P/M stainless steels, unlike additives such as Al2O3 and SiC, the addition of such rare-earth-oxide-based dispersoids as Y2O3 and yttrium aluminum garnet (YAG) in optimal amounts has shown to marginally improve both densiﬁcation and mechanical properties [17–19]. The secondphase ceramic addition in the metallic system is expected to result in better microwave-compact coupling  and has been investigated in several systems [14–16]. Recently, Panda et al.  have shown that 316L– YAG composites can be prepared by microwave sintering and have a superior mechanical and tribological response. While much emphasis has been laid on the mechanical property response of P/M stainless steel, little systematic work has reported on their corrosion response. In particular, to the best of the authors’ knowledge, there has as yet been no evaluation of the electrochemical response of microwave-sintered compacts. This study examines the eﬀect of sintering condition (solid-state (1200 °C) and supersolidus sintering (1400 °C)) and YAG addition (5 and 10 wt.%) on the electrochemical response of 316L austenitic stainless steel consolidated both by conventional as well as microwave sintering. The as-received (prealloyed) stainless steel (grade: 316L; composition (wt.%): Fe–16.5Cr–12.97Ni–2.48Mo– 0.93Si–0.21Mn–0.025C–0.008S) and YAG powders (>97% purity) used in the present study were supplied by Ametek Specialty Metals Products, USA and Treibacher, Austria, respectively. The as-received 316L powders were mixed with 5 and 10 wt.% of YAG in a turbula mixer for 30 min. Cylindrical compacts (16 mm diameter and 6 mm height) were uniaxially compacted at 600 MPa using a hydraulic press. The as-pressed (green) densities of the compacts varied between 80% and 82% theoretical density. The compacts were sintered in MoSi2-heated horizontal tubular furnace at constant heating rate of 5 °C min1 at 1200 and 1400 °C. Microwave sintering was carried out using a multimode, 2.45 GHz, 6 kW batch process microwave system. For both, sintering was carried out for 60 min in hydrogen atmosphere. The temperature inside the microwave furnace was monitored using an infrared pyrometer by considering the emissivity of steel (0.35). The details of the experimental setup are provided elsewhere [6,13,21]. The sintered densities were determined from the dimensional and weight measurements. The microstructural analysis was carried out using scanning electron microscopy (SEM) imaging (JSM-840A, JEOL, Japan). The electrochemical experiments were performed using a potentiostat (PC4, Gamry Instruments, Inc., Warminster, PA) on a CMS100 Framework. The surfaces of the samples were polished with 600 grit SiC papers before corrosion testing to produce a smooth surface ﬁnish. This was followed by ultrasonic cleaning
with acetone to obtain a cleaner surface. Prior to polarization, samples were allowed to stabilize for about 3600 s to obtain a stable open circuit potential (OCP). Polarization measurements were carried out in a corrosion cell containing 150 ml of solution using a standard three-electrode conﬁguration: a saturated calomel electrode (SCE) was used as a reference electrode with saturated KCl; platinum mesh was used as a counter electrode; and sintered samples were the working electrode. The pH of the electrolyte was measured using a digital pH meter (model: MK IV, supplier: Systronics, India) and was found to be 1.31. The exposed area of the sample was 1 cm2. The polarization was carried out from 800 to 1800 mV (SCE) at a scan rate of 1 mV s1 in freely aerated 0.1 N H2SO4 solution to construct the Tafel plots (logarithmic variation as a function of voltage). The corrosion potential Ecorr and corrosion current Icorr were determined from the intersection of these two linear plots . Accordingly, the corrosion rate was calculated using the following equation : e ð1Þ Corrosion rateðmmpyÞ ¼ 0:0033 I corr q where e is the equivalent weight and q is the theoretical density determined using inverse rule of mixture. From the potentiodynamic polarization curves, the values of passivation current density (Ipass) were determined. Sintered densities values of pure 316L and 316L– YAG stainless steel composites sintered in both conventional and microwave furnaces at 1200 and 1400 °C are summarized in Table 1. It is interesting to note that microwave sintering results in higher densiﬁcation in both pure 316L and 316L–YAG composites. Furthermore, as compared with solid-state sintering, compacts supersolidus sintered at 1400 °C undergo higher densiﬁcation. This can be attributed to the enhanced densiﬁcation kinetics at higher temperature that is promoted by the formation of liquid (at grain boundaries), which, in turn, assist densiﬁcation through capillary stress-induced grain rearrangement . Our results on density of microwave sintered compacts are similar to those reported by Saitou  and Panda et al. , and can be attributed to restricted microstructural coarsening at the faster heating rate. A reﬁned microstructure implies that the pores are preferentially intergranular and easier to eliminate by grain boundary diﬀusion . Accordingly, the addition of the optimal amount YAG has a similar eﬀect. It is interesting to note that up to 5 wt.% YAG addition does not result in any signiﬁcant degradation in the densiﬁcation of 316L compacts. In fact, microwave-sintered 316L–5YAG at 1400 °C exhibits the highest density (89.3%). In the 316L–YAG composites, microwave couples with both 316L and YAG powders. As suggested by Peelamedu et al. , a two-phase mixture provides diﬀerential microwave absorption and causes localized temperature gradients. They further hypothesized that such condition results in an isothermal heating condition and leads to enhancement in diffusion. It is envisaged that our system behaves in a similar way. Further increase in YAG content to 10 wt.% results in deterioration of densiﬁcation both in microwave and conventional sintering. This is caused
C. Padmavathi et al. / Scripta Materialia 57 (2007) 651–654
Table 1. Eﬀect of sintering condition and YAG content on the densiﬁcation and electrochemical response of 316L stainless steel sintered at 1200 and 1400 °C Sample
Sintering temperature (°C)
SDa (% theo.)
Icorr (lA cm2)
IPass (lA cm2)
Corrosion rate (mm year1)
MWS – microwave sintering; CON – conventional sintering. a SD – sintered density.
by increase tendency for agglomeration at higher YAG contents . The eﬀect of YAG content, sintering temperature and sintering techniques on OCP stabilization curves of microwave and conventional sintered samples at 1200 and 1400 °C are shown in Figure 1a and b. At 1200 °C, microwave-sintered samples exhibit passive behavior due to the progressive formation of a passive ﬁlm on their surface, while conventionally sintered samples show active behavior. The microwave-sintered samples are passive at higher potentials, while conventional samples do not exhibit any passive behavior due to negative corrosion potential values. At 1400 °C, microwavesintered compacts exhibit a similar trend in stabilization. Figure 2a and b shows the potentiodynamic polarization curves for pure and YAG-reinforced 316L stainless steel sintered in microwave and conventional furnaces at 1200 and 1400 °C, respectively. An active–passive transition – typical for stainless steel [24,25] – is shown for all sintered compacts, except the ones that contain YAG and are conventionally sintered at 1200 °C. For these samples, the current measured is too high (10 mA cm2) to be called passive current. Similar behavior was also reported by Fedrizzi et al.  and more recently by Balaji et al.  on austenitic stainless steel sintered in a solid-state condition. From both Figures 1 and 2 and Table 1, it is evident that the Ecorr values become more noble with decreasing anodic current density for
Figure 1. Eﬀect of sintering condition and YAG content on the open circuit potential curves for 316L compacts sintered at 1200 and 1400 °C.
Figure 2. Eﬀect of sintering condition on the potentiodynamic polarization curves for 316L compacts with varying YAG additions sintered at (a) 1200 and (b) 1400 °C.
all microwave-sintered compacts as compared with their conventionally sintered counterparts. A closer examination of the corrosion rates clearly indicates that supersolidus sintering results in a marked improvement in the corrosion resistance. Furthermore, for both solidstate as well as supersolidus sintering conditions, the corrosion rate in microwave-sintered samples is one to two orders of magnitude lower that those observed in conventional sintering. This can be attributed to enhanced densiﬁcation of compacts in response to sintering in microwaves and at higher (supersolidus) temperatures. A high densiﬁcation entails closure of interconnected porosity. Consequently, the access of corrosive medium into pores is greatly reduced, which reduces the corrosion rate. Furthermore, from Figure 2, it is evident that supersolidus-sintered samples exhibit greater passivation behavior. Moreover, for a particular sintering condition, SLPS results in greater microstructural coarsening, which, in turn, reduces the galvanic coupling at the 316L–316L and 316L–YAG interfaces, thereby increasing the corrosion resistance. Similar improvement in the corrosion rate of 316L with YAG and aluminide addition has been recently reported Balaji et al. [27,28]. It is interesting to note the inﬂuence of YAG addition on the corrosion resistance. The greatest passivation behavior (and consequently the slowest corrosion rate) with the smallest anodic current density was
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Figure 3. SEM micrographs of (a) 316L and (b) 316L–5YAG sintered in conventional and microwave furnaces at 1400 °C.
observed for 316L–5YAG compacts microwave-sintered at 1400 °C. This is directly attributed to the fact that microwave-sintered 316L–5YAG compacts show a maximum sintered density at 1400 °C. This can be further correlated with the microstructural diﬀerences between pure 316L and 316L–5YAG composites sintered at 1400 °C in conventional and microwave furnaces (Fig. 3). Note that, compared with conventional sintering, YAG is more homogeneously distributed in microwave-sintered 316L–5YAG compacts. Furthermore, it is also evident that YAG addition and microwave sintering result in a decrease in the sintered porosity. Both these factors result in the enhancement of corrosion resistance. In summary, compared with conventional sintering, microwave sintering results in higher densiﬁcation and a relatively reﬁned microstructure in both pure 316L and 316L–YAG composites. For both sintering modes, compacts sintered in a supersolidus condition (1400 °C) attain greater densiﬁcation. Among all compositions, 316L–5YAG composites microwave-sintered at 1400 °C attain the highest density and are the least corrosion prone. In general, the microwave-sintered samples result in nobler Ecorr, greater passivation behavior and a high corrosion resistance. The implication of this study is that one can envisage a higher density, and a correspondingly more corrosion-resistant, stainless steel by optimally reinforcing it with YAG and consolidating it by supersolidus sintering in a microwave furnace.  R.M. German, Sintering Theory and Practice, WileyInterscience Publications, New York, 1996.
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