Effect of Microstructural Characteristics on Corrosion Behaviour of Microwave Sintered Stainless Steel Composites

C.Padmavathi1, Shubhranshu Shekar Panda1, D. Agarwal2, A.Upadhyaya1

1Department of Materials and Metallurgical Engineering Indian Institute of Technology, Kanpur, INDIA

2Materials Research Institute The Pennsylvania State University, University Park, USA

Keywords: Stainless Steel-YAG Composites; Microwave Sintering; Densification; Polarization

Abstract

With the advantages of near net shape and tailor made materials, powder metallurgical (P/M) processing of stainless steel (SS) has become the most prominent fabrication technique, especially in automobile industry. However, the use of SS is being restricted because of its lower sintered density, which deteriorates the corrosion behavior of the final product. In the present work, an attempt has been made to enhance the corrosion response of 316L and 434L (YAG based) stainless steel composites by modifying the microstructure through microwave sintering (MWS) at 1200°C (solid state sintering) and at 1400°C (supersolidus liquid phase sintering). The corrosion resistance of the MWS composites was studied in 0.1 N H2SO4solution. The microstructure – processing – corrosion property correlation has been determined for better understanding of MWS stainless steel composites. The dependence of electrochemical behavior of MWS composites on YAG content, interconnected porosity and processing parameters has been investigated. The results showed substantial improvement in corrosion resistance of MWS composites due to mainly full density attainment and reduced interconnected porosity

Introduction

P/M is a suitable technology to fabricate complex parts with net shaping capabilities, appreciable dimensional precision and high productivity [1]. P/M stainless steel is generally used in special applications, where enhanced properties are required ascompared to the low alloy steel [2]. However the wider application of P/M stainless steel is limited due to their poor mechanical and corrosion properties. Therefore, there is a need to improve the corrosion resistance and mechanical properties of P/M stainless steel by modifying the microstructural characteristics through the addition of second phase dispersiods or by utilizing novel sintering techniques [3] such as microwaves.

The P/M stainless steel consolidated by solid state sintering does not attain the full density and porosity as high as 15% is present [4]. Hence, in the recent years attempts have been made to attain the full densification through the liquid phase sintering methods. The prealloyed stainless steel powders led to the supersolidus liquid phase sintering (SLPS) which involves the heating of the prealloyed powder between the solidus and liquidus temperatures to form a liquid phase [5]. During supersolidus liquid phase sintering the fragmentation of the individual grains and rearrangement of these particles occurs followed by solution re-precipitation resulting in enhanced densification. The superior properties are attained through SLPS route due to the presence of the high diffusivity liquid allowing for rapid densification [4]. Liquid formation can be achieved through addition of the second phase dispersoids. Extensive studies have been carried out on yttria dispersed P/M stainless steel by Lal et al [6], which resulted in homogenous porosity. This was attributed to the interaction of Cr2O3 with Y2O3 dispersoids. The effect of SiC addition to P/M SS on sintering behaviour was investigated by Patankar and Tan [7] resulting in higher sintered density. Al2O3 was added up to 8 vol. % of 434L solid state sintered stainless steel resulting in to higher mechanical properties [8]. Recently Shankar and Upadhyaya reported that an addition of greater than 5% yttria to P/M SS improves the sintered density appreciably [9].

Also it was shown by the Debata and Upadhyaya that the corrosion resistance of the nickel based super alloys improved due to the formation of yttrium aluminate [9]. The literature on the corrosion behaviour of the second phase dispersoids strengthening stainless steel are very limited.

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The limitations of the high temperature sintering resulting in the microstructural coarsening can be overcome by applying faster heating rates and shorter soaking times during sintering. Faster heating rate in the conventional furnace causes thermalgradient leading to compact distortion and inhomogeneous microstructure. Recently, the novel techniques like microwave sintering has been used to achieve the faster heating rates combined with homogenous microstructures [10-11]. It was shown that the microwaves can also couple with metals in powder form rather than as a monolithic [12]. It was also shown that the steel powder compacts with Cu and Ni additives couple with microwave leading to effective sintering and better mechanical properties [13]. Shubhranshu et al have shown that the microwave sintering of the YAG – stainless steel composites have resulted in improved densification and mechanical properties [14].

The literature on the electrochemical behavior of dispersion strengthened P/M SS is very limited. There are no published reports on the corrosion behaviour of the microwave sintered stainless steel composites. The present study was carried out to evaluate the corrosion behaviour of 316L and 434L – YAG dispersed stainless steel composites processed by microwave sintering at 1200°C and 1400°C in comparison with conventional sintered SS. The effect of the YAG content and sintering temperature on the corrosion resistance of the microwave sintered stainless steel has been studied in detail.

Experimental Procedure

Prealloyed 316L and 434L stainless steel (Supplier: Ametek Specialty Metals Products, USA) and YAG powders (Supplier: Treibacher, Austria) were used in this study. The compositions (wt %) of the both as-received stainless steel and YAG powders are given in Table I.

Table I Composition (in wt. %) of as-received powders used in the research work

Powder Cr Mo Si Mn C S P Fe Y Al Ti Ni

316 L 16.5 2.48 0.93 0.21 0.025 0.008 0.01 balance - - - 12.97

434L 17 1.0 0.71 0.2 0.023 0.02 0.02 balance - - - -

YAG - - 0.014 - 0.08 - - 0.014 45.05 22.5 <0.01 -

The powder characteristics of 316L stainless steel, 434L stainless steel and YAG powders are given in Table II.

Table II Characteristics of the powders in as-received conditions used in the present study

Property Powder316L YAG 434L

Processing technique Gas atomization Chemical reduction Gas atomization Powder shape Rounded Rounded Spherical

Cumulative powder size, mD10 10.3 0.5 D50 45.9 1.5 D90 85.1 2.1

8.5 35.3 75.1

Apparent density, g/cm3 2.7 0.70 2.6 Flow rate, s/50g 28 98 28

Theoretical density, g/cm3 8.06 4.50 7.86

The 316 L and 434L -YAG composites were prepared by mixing the different proportions of YAG (5 and 10 wt %) in a tubular mixer (T2C, supplier: Bachoffen, Basel, Switzerland) for 20 min. The cylindrical compacts of 16 mm diameter and 6 mm thickness were uniaxially compacted at 600 MPa from the stainless steel mixed powders using hydraulic press (model: CTM-50, supplier: FIE, Ichalkaranji, India) with floating die. To minimize the friction along the walls of die zinc state was used as lubricant. The green densities were 80 ± 2% of the theoretical density for 316L compacts and 75 ± 3% of the theoretical density for 434L compacts. Conventional sintering of green compacts was carried out in a MoSi2 heated horizontal tubular sintering furnace (model: OKAY 70T-7, supplier: Bysakh, Kolkata, India) at 1200°C (solid state sintering) and 1400°C (supersolidus sintering) at a heating rate of 5°C/min and holding time of 1h in pure hydrogen atmosphere (dew point: -35°C). Microwave sintering of the green compacts was carried out using a multi-mode cavity 2.45 GHz, 6 kW commercial microwave furnace at 1200 and 1400oC in H2 with soaking time of 1h.

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For all samples the temperature measurements were recorded by considering emissivity of steel as 0.35. The sintered density was determined by dimensional measurements. The sinterability of the compact was also determined through densification parameter, which is expressed as:

Densification parameter (D.P) = (s intered density - green density)

(theoretical density - green density) (1)

The microstructural analyses of the samples were carried out using an optical microscope. Marble’s reagent (50ml HCl and 25ml of saturated aqueous solution of copper (II) sulphate) was used as etching reagent for 10s. The corrosion resistance of the conventionally and microwave sintered 316L and 434L stainless steel composites was studied in 0.1N H2SO4 solution.

The electrochemical studies were also carried out for all microwave and conventional sintered compacts using Gamry Instruments, Inc. potentiostat, (model: PC4) on a CMS100 Framework.. The surface of the sintered compacts was slightly polished with 600 grit SiC papers before corrosion testing to obtain a smooth surface finish. Prior to the potentiodynamic polarization test, samples were stabilized for 3600s for attaining the 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 was used as reference electrode with KCl as the counter electrode (supplier: Accutrol Inc., USA). The exposed area of the sample was 1 cm2. The polarization was carried out from –800 mV to 1800 mV (SCE) at a scan rate of 1 mVs-1 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 [9]. Before polarization test, sample was stabilized about 3600s in the solution to acquire a stable open circuit potential (OCP). The current density at that intersection point gives the Icorr. By using 1st-Stern method as given below:

C ICorrCorrosion Rate= (2)

Where C is conversion factor, is the equivalent weight (g); is the density of the material (mgm–3) and Icorr the corrosion current (mA/m2). The tafel slopes were estimated from corresponding anodic and cathodic curves. The critical parameters like corrosion potential (Ecorr), corrosion current (Icorr) and corrosion rate were evaluated from the polarization curves. Besides the tafel extrapolation results, the passivation current density (Ip), critical current density (Icrit), primary passivation potential (Ep) and the trans-passive potential were also determined from the potentiodynamic polarization scan.

Results and Discussions

Densification Behaviour

Figure 1 shows the effect of sintering temperature and YAG addition on the sintered density of 316L SS sintered in microwave and conventional furnaces, respectively. It was observed that pure 316L and 316L – YAG composites sintered in microwave at 1400°C exhibited higher density than the compacts sintered at 1200°C. This has also been validated by densification parameter (D.P) which is an indication of densification response. The densification parameter as shown in Figure 2a and b followed the same trend as sintered density.

The densification enhancement at higher temperature may be attributed due to the increased diffusivity [4]. During the supersolidus sintering at 1400°C formation of liquid occurs at grain boundaries assisting in densification through capillary stress induced grain rearrangement [4]. With an exception of pure 316L sintered stainless steel, all SS compacts showed higher densification response at 1400C. This is believed to be due to the presence of a liquid phase and faster sintering kinetics provided by microwaves.

The 316L microwave sintered compacts showed marginal improvement in sintered density at 1200°C as when compared to conventional, while at 1400°C it resulted in lower density [14]. The presence of two phase system (YAG and SS) provides different degree of microwave absorption causing localized temperature gradients also known as anisothermal situation, and as a result an enhancement in sintering behavior occurs [15]. This enhanced sintering behaviour retards the grain growth providing smaller grain size than in the case of conventionally sintered samples.

The 316L –YAG composites with an optimal amount of 5 wt % of YAG contributes to highest compact densification. While in case of 10 wt % YAG resulted in decreased sintered density due to YAG-YAG interaction and agglomeration.

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Figure 1a and b: Effect of YAG addition on sintered density of 316L stainless steel sintered at 1200°C and 1400°C.

Figure 2a and b: Effect of YAG addition on densification parameter of 316L stainless steel sintered at 1200°C and 1400°C.

Figure 3 shows the variation of sintered density with YAG addition and sintering temperature. It was seen that 434L and 434L-YAG compacts showed higher density at 1400°C as when compared to 1200°C due to melt formation at grain boundaries which favors the densification response by enhancing diffusion kinetics [16]. From Figure 4a and b it is evident that 434L and 434L-YAG conventional sintered at 1200°C do not exhibit significant densification parameter and follow the same trend as the sintered density. While microwave sintered 434L and 434L-YAG compacts exhibited higher density with a marginal difference as when compared to conventional. Microwave sintered compacts show large improvement in density at 1200°C over the conventional ones. It was interesting to note that at 1400°C there was not much variation in density due to removal of almost all pores. This occurred due to the effective coupling between the 434L-YAG composites and microwave sintered compacts and anisothermal situation.

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Figure 3a and b: Effect of YAG addition on sintered density of 434L stainless steel

sintered at 1200°C and 1400°C.

Figure 4a and b: Effect of YAG addition on densification parameter of 434L stainless steel sintered at 1200°C and 1400°C.

Electrochemical Behaviour

Corrosion Potential Curves

In case of powder metallurgical steels, Ecorr is quickly established [17] so initially the corrosion potential of 316L and 434L samples was stabilized in H2SO4 electrolytic solution before conducting potentiodynamic polarization test. The Figure 5a and b shows the stabilization curves with free corrosion potential versus time for 316L and 316L-YAG compacts sintered in microwave and conventional furnaces, respectively. Figure 5a shows that for samples sintered in microwave at 1200°C; free corrosion potential is shifted more rapidly towards the nobler potentials and then stabilized as when compared to conventional sintered ones. This may be attributed due to progressive formation of passive film on sample surface on

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immersion, as an exception microwave sintered 316-10YAG samples exhibits stabilization of corrosion potential from noble to active region due to unstable passiviation of surface film. The microwave sintered 316L and 316L-YAG compacts at 1400°C shows the similar trend in stabilization as at 1200°C and it occurs at nobler potentials as when compared to conventional sintered ones. But 316L-5YAG compacts shows the stabilization at the active potential due to breakdown of the passive film.

Figure 5a and b: Open corrosion potential curves of 316L and 316L-YAG samples sintered at 1200°C and 1400°C.

Figure 6a and b shows the corrosion potential stabilization curves at 1200°C and 1400°C for 434L and 434L-YAG composites samples. It is quite interesting that at 1200°C; pure 434L sintered in microwave stabilized at nobler potential indicating the formation of surface film as when compared to convetional ones. But the 434L-YAG dispersed composites shows entirely different trend; where the stabilization of potential occurs towards active region with marginal differences forboth microwave and conventional sintered samples. This is probably because of preferential attack on more active zones of surface film and extensive dissolution of passive layer. At 1400°C; 434L and 434L-YAG microwave sintered samples stabilized at nobler potential as when compared to conventional ones.

Figure 6a and b: Open corrosion potential curves of 434L and 434L-YAG samples sintered at 1200°C and 1400°C.

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Open corrosion potential test were done to understand the nature and stability of the passive film formed on surface. As the stabilization of the corrosion potential was not consistent, potentiodynamic polarization test were conducted immediately which is well confirmed from the literature [18].

Potentiodynamic Polarisation Curves

Figure 7a and b shows potentiodynamic curves of all 316L compacts; which depicts the influence of YAG content and sintering temperature upon corrosion behaviour at 1200°C and 1400°C sintered in microwave and conventional furnaces. There is a usual active-passive transition for both compacts sintered at 1200°C and 1400°C. It was observed that Ecorr values were nobler for all microwave sintered samples which is from evident from Table III; conventional sintered showed the immediate dissolution in electrolytic solution due to discontinuous passive film on surface. The corrosion resistance of 316L microwave sintered compacts at 1200°C improved as when compared to conventional, which may be attributed due to increased sintered density.

Figure 7a and b: Potentiodynamic polarization curves for 316L and 316L-YAG samples sintered at 1200°C and 1400°C.

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Table III Potentiodynamic polarization results for 316L and 316L –YAG composites sintered at 1200°C and 1400°C

Composition Sintering Techniques

Sintering Temperature

°C

Icorr

µA/cm2IPass

µA/cm2Ecorr

mVCorrosion Rate

mmpy

1200 265 119 -303 2.77Pure- 316L Conventional

1400 40.30 102 -290 0.41

1200 297 565 -333

The 316L compacts sintered in microwave at 1400°C resulted in better corrosion behaviour due to higher sintered density and closure of interconnected pores by supersolidus liquid phase sintering mechanism. Thus the access of corrosive medium into the pores network is reduced. This may be attributed to the fact that melt formation is more prominent during the supersolidus liquid phase sintering [4], which is evident from SEM image showing the homogenous distribution of melt along the grain boundaries as seen in Figure 8a. The conventional sintered compacts at 1400°C (higher temperature) resulted in microstrutural-coarsening more pronounced during supersolidus melt formation as compared to microwave compacts.

The YAG content greatly influences the corrosion resistance too, the greatest passivation with smallest anodic current density was found for the 316L-5YAG microwave sintered samples at 1200°C and 1400°C (highest density). From microstrutural point of view, optimum YAG dispersed composites contains smaller and lesser pores then the straight compacts as shown in Figure 8b. The 316L-5YAG compacts sintered in microwave at 1400°C appeared well sintered and well bonded with matrix, whereas conventional compacts showed reduced corrosion resistance due to open interconnected pores as a consequence electrolyte stagnation occurs in pores leads to formation of hydrogen concentration cells and localized corrosion like crevice [21].

Also during sintering the depletion of chromium from 316L compact surface leads to compound formation or chromium evaporation [22].The higher density compacts can overcome this phenomenon taking place i.e. smaller surface area in which chromium depletion occurs [23].

It was interesting to note that even with higher YAG content upto 10 wt % in 316L compacts sintered in microwave at 1400°C showed better passivation behavior and reduced corrosion rate which is well evident from microstrutural characteristics as shown in Figure 8c. Based on elemental analysis of YAG-added SS Jain et al. [19] have reported that chemical interaction between 316L and YAG might have caused active-passive transition behaviour in 316L-10YAG composites sintered at both temperatures. When compared to solid state sintering, the decreased corrosion rate was observed in supersolidus sintered 316L compacts due to decreased porosity content and also larger grain size which decreases the effective grain boundary sites which act as main corrosion initiation sites [20].

Microwave sintered 316L –YAG composites exhibited higher passivity than conventional at both 1200°C and 1400°C. The passive nature of the surface film was well understood from the ipass parameter determined from the potentiodynamic polarization curve. The passive current density in turn reflects the nature of the passive film as function of potential [20].

3.24 316L-5 YAG Conventional

1400 142 511 -334 1.42

1200 386 768 -319 4.28 316L- 10YAG Conventional

1400 174 197 -336 1.74

1200 1.66 52.64 108.6 0.03594 Pure- 316L Microwave

1400 0.99 58.34 -311 0.1542

1200 0.0078 29.53 95.04 0.0036 316L-5 YAG Microwave

1400 0.0276 22.77 170 0.01189

1200 11.81 35.39 -280.4 0.3619 316L- 10YAG Microwave

1400 0.13 61.27 46.20 0.0563

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Figure 8: Microstructures of 316L and 316L-5YAG compacts sintered in microwave at 1400°C (a) SEM image of pure 316L (b) SEM image of 316L-5YAG (c) optical image of 316-10YAG.

Figure 9a and 9b shows the potentiodynamic curves for 434L and 434L-YAG SS composites sintered at 1200°C and 1400°C by microwave and conventional furnaces, respectively. The 434L samples microwave sintered at 1400°C exhibited nobler Ecorr values with lower current density resulting in enhanced corrosion resistance as given in Table IV. This may be attributed due to melt formation and microstrutural coarsening effects at higher temperatures as seen in Figure 10a. It was interesting tonote that both 434L solid state and supersolidus liquid phase sintered samples although resulted in reduced corrosion rate and nobler Ecorr values with lower current densities; higher sintered densities did not result in any passivation behaviour. These are attributed due to the crevice corrosion mechanism in pores and precipitation of carbides during sintering [24]. This was well confirmed by the OCP curves as discussed before.

The 434L-5YAG compacts microwave sintered at 1200°C showed the better corrosion resistance and passivation as when compared to conventional compacts. While at 1400°C both microwave and conventional sintered composites showed the best passivation behaviour. It was interesting that the YAG content upto 10 wt % did not degrade the corrosion property; instead the superior corrosion resistance was obtained, despite the marginal improvement in sintered density due to the YAG-YAG interaction and agglomeration at 1400°C. This is because of homogenous distribution of melt along the grain boundaries which forms the continuous network acting as the barrier for corrosion as seen in the Figure 10b and c.

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Figure 9a and b: Potentiodynamic polarization curves for 434L and 434L-YAG samples sintered at 1200°C and 1400°C.

Table IV Potentiodynamic polarization results for 434L and 434L –YAG composites sintered at 1200°C and 1400°C

Composition Sintering Techniques

Sintering Temperature °C

Icorr

µA/cm2IPass

µA/cm2Ecorr

mV

Corrosion Rate mmpy

1200 325 80 -485 3.59Pure- 434L

Conventional 1400 327 49 -502 3.23

1200 801 32 -522 8.87 434L-5 YAG

Conventional 1400 513 53 -526 5.10

1200 4570 599 -523 50.6 434L- 10YAG

Conventional 1400 409 69 -522 4.15

1200 4.73 - -500 3.12 Pure- 434L

Microwave 1400 1.98 x 10-5 - -303 0.00855

1200 17.40 9.522 -490 7.5 434L-5 YAG

Microwave 1400 11.40 5.83 -475 0.0086

1200 22.50 56.93 -501 10.75 434L- 10YAG

Microwave 1400 180.6 8.30 -337 5.60

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Figure 10: Microstructures of 434L and 434L-5YAG compacts sintered in microwave at 1400°C (a) SEM image of pure 434L (b) SEM image of 434L-5YAG (c) optical image of 434L-10YAG.

Conclusions

1. The microwave sintering of both 316L and 434L SS samples at supersolidus temperature (1400°C) resulted in enhanced densification response and improved corrosion resistance as when compared to solid state and conventional sintered samples.

2. This improved corrosion behaviour may be due to homogenous distribution of melt formation along the grain boundaries, smaller and lesser porosity.

3. The superior corrosion resistance and higher passivation behaviour of 316Land 434L SS with 5 wt% YAG at 1400°C is mainly due to microstructural coarsening, homogeneous melt distribution along the grain boundaries and removal of almost all pores.

4. YAG addition upto 10 wt% in both 316L and 434L SS sintered in microwave did not deteriorate corrosion property as when compared to conventional.

5. The corrosion behavior of both SS -YAG composites is greatly influenced by optimum amount of YAG content, sintering temperature and sintering techniques (in turn heating rate) in 0.1N H2SO4 solution.

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