Claims:WE CLAIM:

1. High and low Manganese stainless steel having exceptional properties including higher yield strength, higher tensile strength and higher elongation and having bipolar plate application in automobile and power generating fuel cells characterized in that the said steel is having a composition comprising Mn, Ni, Mo, Cu, N along with Cr- 17 to 20 %, C - 0.06 to 0 .1%, S- up to 0.01%, P- upto0.01%, Si - 0.10 to 0.45% and the balance being Fe with the proviso that when the Mn content is as low as 6 to 8%, then Ni content is 5 to 7 %, Mo is 2 to 3%, Cu is 0.5 to 0.8%, N is 0.2 to 0.4%andwhen the Mn content is as high as 16 to 20%, then Ni content is 1 to 3%, Mo is 0 to 1.5 %, Cu is 1 to 2%, and N is 0.3 to 0.5 %.

2. High and low Manganese stainless steel as claimed in claim 1, which is HIC resistant and has a fully austenitic microstructure with grain size of about 10 to 11 micro metre.

3. High and low Manganese stainless steel as claimed in claims 1 and 2, wherein the steel has the yield strength 350 MPa to 450 MPa, ultimate tensile strength 700 MPa to 780 MPa, elongation 60 to 70 % and hardness of 90 to 97 HRB.

4. High and low Manganese stainless steel as claimed in claims 1 to 3, which is super corrosion resistant and with higher fuel cell efficiency in the range of 110 to 120 mW cm-2.

5. High and low Manganese stainless steel as claimed in claims 1 to 3 with superior film resistance at the anode side in the range of 45 to 80 K ohm cm2 and at the cathode side in the range of 40 to 45 K ohm cm2 in simulated fuel cell environment as determined by electrochemical impedance test.

6. High and low Manganese stainless steel as claimed in claims 1 to 3, has less defect concentration in the passive film for the acceptor concentration at both the anode (H2) and the cathode (O2) side in the range of 8 to 12 X 1020cm-3 and for the donor concentration at the anode side (H2) in the range of 7 to 11 X 1020 cm-3 and at the cathode side (H2) in the range of 6 to 11 X 1020 cm-3 in stimulated fuel cell environment as determined by Mott- Schottky analysis.

7. High and low Manganese stainless steel as claimed in claims 1 to 3, which has shown low interfacial contact resistance (ICR) in the range of 20 to 30 m ohm-2 at load of 140N cm-2.

8. High and low Manganese stainless steel as claimed in claims 1 to 7, which is of lower cost as compared with the conventional stainless steel AISI 316L.

9. A process to manufacture low cost high and low Manganese stainless steel as claimed in claims 1 to 8, comprising the following steps :
i) Reheating of ingots/ slabs and soaking in a reheating furnace to 1150 to 1250 deg. C
ii) Hot rolling the same to 16 mm plate with finish rolling temperature of 1000 to 1100 deg. C ;
iii) Reducing final rolling to 30-35% ;
iv) Annealing the strips after hot rolling being carried out in the range of 1100 to 1200 deg. C ;
v) Water quenching of the annealed steel.

10. A process as claimed in claim 9, wherein the ingots/slabs have the composition comprising Mn, Ni, Mo, Cu, N along with Cr -17 to 20%, C -0.06 to 0.1%, S – up to 0.01%, P – 0.01%, Si - 0.10 to 0.45%, and the balance being Fe with the proviso that when the Mn content is as low as 6 to 8%, then Ni content is 5 to 7%, Mo is 2 to 3%, Cu is 0.5 to 0.8%, N is 0.2 to 0.4%, and when the Mn content is as high as 16 to 20%, then Ni content is 1 to 3%, Mo is 0 to 1.5%, Cu is 1 to 2%, and N is 0.3 to 0.5%.

Dated this 14th day of March, 2018

, Description:LOWCOST HIGH AND LOW MANGANESE AUSTENITIC STAINLESS STEEL FOR BIPOLAR PLATE APPLICATION IN AUTOMOBILE AND POWER GENERATING FUEL CELLS AND A PROCESS FOR THE MANUFACTURE OF SAID STEEL

FIELD OF THE INVENTION

This invention relates to low cost high and low manganese austenitic stainless steel for bipolar plate application in automobile and power generating fuel cells. The invention includes a process for the manufacture of said steel.

BACKGROUND OF THE INVENTION AND PRIOR ART

Fuel cell is a device that converts the chemical energy from a fuel into electricity through a chemical reaction with oxygen or another oxidizing agent. There are many types of fuel cells, but they all consist of an anode (negative side), a cathode (positive side) and an electrolyte that allows charges to move between the two sides of the fuel cell. Electrons are drawn from the anode to the cathode through an external circuit, producing direct current electricity. The combination of anode and cathode is termed a cell, where anodes and cathodes are called bipolar plates. Proton exchange membrane (PEM) is sandwiched between two bipolar plates (between
anode and cathode). On anode side hydrogen gas is passed where H+ ions pass

through PEM and reaches the cathode and react with the O2 gas which is passed on cathode side. Due to this chemical reaction (flow of ions) between the anode and cathode, an electrical potential of about 0.7 volts is created in the cell or between the bipolar plates. In order to meet an application requirement, the cells are “stacked”, or placed in series to increase the voltage.

The general requirements of the bipolar plates are high electrical conductivity, high thermal conductivity, high mechanical strength, impermeability to reactant gases, high resistance to corrosion, and low cost for mass production. The most commonly used bipolar material is graphite because of its chemical stability to withstand the fuel cell environment and also low electrical resistivity, resulting in high electrochemical power output. However, its low mechanical strength and high machining costs drive the search for alternative cost effective material for bipolar plates.

Metallic materials can overcome these drawbacks, but the challenge is achieving two important properties namely corrosion resistance and surface contact (interfacial)

resistance. Bipolar plates made of Al and Ti-based materials show better corrosion characteristics but are in general poor in surface contact resistance. Stainless steels can be used as alternative to graphite, particularly AISI 316L because of its excellent localized corrosion resistance owing to presence of Mo. But recent studies have confirmed that there is loss in fuel cell efficiency due to increase in contact resistance due to compact nature of the passive film formed by Mo in AISI 316L. Another drawback of AISI 316L steel is cost owing to Ni and Mo alloying. In recent times there has been an increase in the usage of low Ni and Mn-containing low cost stainless steels for various applications. Addition of N to Mn-containing also increases the corrosion resistance. Recent studies also have revealed that Mn increases the contact resistance property through altering the defects structures of oxide scales in stainless steel.

The presently available AISI 316L austenitic stainless steel possess excellent localized corrosion resistances with pitting potential >300 mV and excellent mechanical properties (YS of >300 MPa and elongation > 55%). But these properties are achieved by adding costlier alloying elements like nickel (~8 wt.%) and molybdenum ( 2-3 wt% ).Attempts were made in the pastto produce low cost Mn stainless steel with less Ni content with all requisite properties, but the same did not meet with much success as the processes involved complicated and costly routes. Examples of such arts are given here by references, US 9145598, CN 102965584A (powder metallurgy route), CN 105002431A (duplex metallurgy route), CN
102965584A (ladle refining process). The properties both corrosion resistance and mechanical properties were achieved through alloy modification technique (KR
20120050085A, TW 200831685A, Patent Publication no. MX 2012012874A, CN

103154291A Indian Patent No. 223848), lowering nickel content < 1-2% and increasing Mo content to 4-5 % (KR 20120050085A, US 8877121 and US 8858872).

Thus, there exists a genuine need to develop a low cost Mn stainless steel with all desired properties as stated above in the field.

OBJECT OF THE INVENTION

The main object of the present invention is to provide for both high as well as low Manganese stainless steel having higher yield strength, higher tensile strength and higher elongation as compared with the conventionally known 316L stainless steel and having bipolar plate application in automobile and power generating fuel cells.

The other object of the invention is to provide for high and low manganese stainless steel which is HIC resistant and has a fully austenitic microstructure.

Another object of the invention is to provide for high and low manganese stainless steel which is super corrosion resistant and higher fuel cell efficiency than the conventional 316L stainless steel.

Still another object of the invention is to provide for high and low manganese stainless steel which is having superior film resistance at the anode and the cathode side in simulated fuel cell environment.

Another object of the invention is to provide for high and low manganese stainless steel which has less defect concentration in the passive film for the acceptor and donor concentrations at the respective anode (H2) and cathode (O2) side.

Still another object of the invention is to provide for high and low manganese stainless steel which can show low interfacial contact resistance.

Another important object of the invention is to provide for high and low manganese stainless steel which is of lower cost as compared with conventional AISI 316L stainless steel.

Still another object of the invention is to provide for a process to manufacture the above high and low manganese low cost stainless steel.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS :

Table 1: Comparison of chemical composition of Mn stainless steel (MnSS) with commercially available AISI 316L stainless steel in wt.%

Table 2: Room temperature tensile properties and bulk hardness of MnSS in comparison with 316L austenitic stainless steel

Table 3: Typical values deduced from the potentiodynamic plots obtained for the sample Stainless steel tested in fuel cell environment

Table 4: Electrical parameters obtained by fitting the impedance spectra of 316L and

MnSS and calculated oxide thickness with respect to the applied potential

Table 5: The donor ( ND) and acceptor (NA) concentration density of 316 L and MnSS

specimens function of potentials

Table 6: Pilot scale fuel cell testing conditions

Fig. 1: Thermocal thermochemical software analysis showing thermodynamic equilibrium and stability of various phases in MnSS-2 stainless steel as a function of temperature

Fig. 2: Thermocal thermochemical software analysis showing thermodynamic equilibrium and stability of various phases in MnSS-3 stainless steel as a function of temperature

Fig. 3: Thermocal thermochemical software analysis showing thermodynamic equilibrium and stability of various phases in MnSS-4 stainless steel as a function of temperature

Fig. 4(a): Light optical micrographs of (a) 316 L; (b) MnSS – 2 stainless steel showing fully austenitic stainless steel with annealing twins

Fig. 4(b): Light optical micrographs of (c) MnSS-3; (d) MnSS-4stainless steel showing fully austenitic stainless steel with annealing twins

Fig. 5 : Potentiodynamic polarization of 316L SS in 0.5 M H2SO4 solution containing 2 ppm of HF at a scan rate of 1 mVs-1; (a) H2 purged ( simulated anode environment) and (b) O2 purged ( simulated cathode environment )

Fig. 6: Potentiodynamic behaviour of the samples recorded at RT and at 80 deg.C in

0.5 M H2SO4 with 2 ppm HF. The potential was ramped from – 0.6 V to 1.2 V vs SCE
at a scan rate of 1 mVs-1.

Fig. 7: (a) Schematic of potentiodynamic curve where theoretical cathodic curve intersects anodic curve more than once and (b) the resulting cathodic (negative) loop

Fig. 8: Comparison of Potentiodynamic behaviour of the samples MnSS-2 and MnSS-4 with 316L SS recorded at (a) room temperature (b) 80oC in 0.5 M H2SO4 with 2 ppm HF. The potential was ramped from 0.6 V to 1.2 V vs SCE at a Scan rate of 1 mVs-1.

Fig. 9: Potentiodynamic polarization of MnSS -2, MnSS-4 and 316L stainless steels in 0.5 M H2SO4 solution containing 2 ppm of HF at 80oC at a scan rate of 1mVs-1 : (a) H2 purged (simulated anode environment ) and (b) O2purged ( simulated cathode environment)

Fig. 10: Potentiostatic polarization of MnSS-2 and MnSS-4 and 316L stainless steels in simulated fuel cell environment ( 0.5 5 M H2SO4 and 2 ppm HF at 80oC with H2 and O2 purging for 3600 sec (a) anode environment and ( b) cathode environment

Fig. 11: Nyquist plots for 316L and MnSS-2, MnSS-4 stainless steels recorded in (a)

anodic and (b) cathodic in fuel cell environments

Fig. 12: Bode plots for 316L and MnSS-2, MnSS-4 stainless steel recorded in (a)

anodic and (b) cathodic in fuel cell environment

Fig.13: Polarization resistance MnSS and 316L stainless steels from impedance measurements at the film formation potentials of -0.1 V and 0.6 V tested in fuel cell environment

Fig.14: Oxide thicknesses of MnSS and316L stainless steels from impedance measurements at the film formation potentials of -0.1 V and 0.6 V tested in fuel cell environment

Fig. 15: A schematic showing bi-layer structure of passive film formed on stainless steel

Fig. 16: Mott-Schottky plots of MnSS-2, MnSS-4 and SS-316L stainless steels at the

(a) anodic and (b) cathodic environments of fuel cell

Fig. 17: The schematic diagram of interfacial contact resistance test

Fig. 18: Variation of contact resistance of 316L and MnSS-2 stainless steels with respect to applied compaction force

Fig. 19: (a, b) The parallel serpentine flow-field design machined Mn stainless steel, (c) various components used in the fuel cell assembly and (d) the in-house fuel cell setup

Fig. 20: Fuel cell polarization and performance curves for H2-O2 PEFCs for MnSS-2 and 316L SS bipolar plates at 30oC and atmospheric pressure with gaseous H2 (80% RH ) and gaseous O2 (80% RH) fed to the anode and cathode of the PEFCs respectively.

DESCRIPTION OF THE INVENTION

According to the invention there is provided high and low manganese stainless steel having exceptional properties including higher yield strength, higher tensile strength and higher elongation and having bipolar plate application in automobile and power generating fuel cells characterized in that the said steel is having a composition comprising Mn, Ni, Mo, Cu, N along with Cr –17 to 20%, C- 0.06 t 0.1%, S - up to
0.01%, P- up to 0.01%, Si- 0.10 to 0.45%, and the balance being Fe with the proviso that when the Mn content is as low as 6 to 8%, then Ni content is 5 to 7%, Mo is 2 to
3%, Cu is 0.5 to 0.8%, N is 0.2 to 0.4% and when the Mn content is as high as 16 to

20%, then Ni content is 1 to 3%, Mo is 0 to 1.5%, Cu is 1.00 to 2%, and N is 0.3 to

0.5%.

The high and low manganese stainless steel of the invention is of lower cost as compared with the conventional AISI 316L stainless steel. The said cost advantage could be achieved by the unique selection of the alloying elements including their relative proportions. This involves keeping the Ni content low with high localized corrosion resistance and which includes at least the main component as iron, the main alloy components as chromium, manganese, nitrogen, and low nickel (1 to 2 wt
%).

The invention includes a process for the manufacture of the said low cost steel which comprises of the following steps:
i) Reheating of the ingots / slabs of stainless steel of the above composition and soaking in a reheating furnace to 1150 to 1250o C ;

ii) Hot rolling the same to 16 mm plate with finish rolling temperature of 1000 to

1100o C ;

iii) Reducing the final rolling to 30 – 35% ;

iv) Annealing the strips after hot rolling being carried out in the range of 1100 to

1200o C ;

v) Water quenching of the annealed steel.

The alloy design for low-Ni and high Mn stainless steels (MnSS) was evolved through formulation of appropriate chromium and nickel equivalents using the well-known Schaeffler-Delong diagram to achieve austenite (?) in the experimental steels. The Schaeffler-Delong diagram was commonly used for predicting the stability of austenite, ferrite and martensite phases in stainless steels as a function of Cr and Ni- equivalents. The following formulae were used for computing the Cr and Ni- equivalents:

Creq = % Cr + 2 (% Si) + 1.5 (% Mo) + 5 (% V) + 5.5 (% Al) +

1.75 (% Nb) + 1.5 (% Ti) + 0.75 (% W)

Nieq = % Ni + % Co + 30 (% C) + 25 (% N) + 0.5 (% Mn) + 0.3 (% Cu)

The values of Cr- and Ni-equivalents chosen for evolution of MnSS compositions and the designed chemical compositions of the steels have been presented in the Table
1 as shown in the accompanying drawings. For the comparison commercially available 316L SS was chosen of which the chemical composition is also given in the Table 1.

Additionally, for developing an understanding of the phase transformations in designed Mn containing austenitic stainless steels, the thermodynamic equilibrium and stability of various phases in Mn containing austenitic stainless steels were analyzed as a function of temperature using Thermocal thermo-chemical software (version Thermo 3.0) for different temperature ranges (Figs.1–3 of the accompanying drawings):

The following salient observations on phase transformation and stability can be deduced from these thermodynamic plots:
1. The liquidus temperature for the steel is ~1400o C.

2. The primary nucleating phase in liquid steel is ferrite (? or ?) i.e. the primary solidification mode of the steel is ferritic.

3. Austenite phase begins to nucleate at around 1330o C.

4. Secondary phases such as carbides of M23C6 type start nucleating at ~900-950o C

5. Inter-metallic phases such as sigma (?) and chi (?) begin to nucleate in steel at around ~650o C.
6. Nucleation of other inter-metallic compounds and secondary phases such as Alpha prime (??), Laves (?), nitrides (Cr2N) is thermodynamic feasible in the lower temperature range of 600-300o C.

Heats of designed MnSS compositions were melted in an Inductotherm-USA make

100 kg air induction furnace in separate campaigns using low nickel stainless steel SSLNA slab pieces and precisely weighed alloy additions of nitrided Mn, nitrided FeCr, low carbon FeCr & FeMo lumps, Mn metal chips, shredded copper scrap, and nickel briquettes. In all, four laboratory heats were made and target chemistries were effectively achieved in three heats i.e. one AISI 216L and two high Mn stainless steel heats. The molten steel from each heat was cast into 100 mm square cross- sectioned 25 kg ingots. Two ingots were obtained for each heat. The top and bottom end of the ingots were cropped to exclude the pipe and other solidification defects.
The ingots were subsequently reheated and soaked in a furnace at 1250o C for 3

hours for thermal/ compositional homogenization and then hot-rolled in Hillé-UK make experimental rolling mill in 2 rolling campaigns to 17 strips with finish rolling temperatures of 1000 - 1050o C to avoid edge cracking. The 100 mm square cross- sectioned ingots were initially hot-rolled to 50 mm plates using 4-pass draft schedule,
100?95?78?65?50 mm and then further down to 17 mm strips using a 3-pass draft schedule, 50?37?25?17 mm, after reheating the plates at 1250o C for 1 hr. After each rolling campaign, the plates were allowed to air cool. The hot rolled strips were subsequently conferred a solution annealing treatment by soaking them at
1150o C for 2 hours followed by rapid quenching in water for dissolution of deleterious

inter-metallic compounds and secondary phases and to prevent their re-precipitation in the steels.

The newly developed Mn containing stainless steels (Table -1) compositions were met as per the designed chemistry. Topographical micro-structural analysis was carried out for all the steels at hot rolled solution annealed condition and the microstructure were given in Fig.4 of the drawings. All the steels showed a fully austenitic microstructure with average grain size of about 10 to 11µm.

The mechanical properties of the developed MnSS were given in the Table 2 of the drawings. For comparison the mechanical properties of commercial available AISI
316L SS are also given in the table. The developed MnSS steels revealed an exceptional combination of properties: higher yield strength, higher tensile strength and higher elongation compared to that of commercial available AISI 316L SS. The hardness of Mn stainless steels was in the range of 90-96 HRB. The yield strength of Mn containing austenitic stainless steels works out to be about 1.6-1.8 times that of conventional austenitic stainless steel grade, AISI 316L SS. The high strength of these steels can be effectively used to an engineering advantage for reducing wall thickness of piping/ tubing and weight of structures in corrosion-prone offshore oil platforms, oil & gas refineries, chemical processing, paper & pulp and fertilizer industries. Since the Mn stainless steels are composed of only fully austenite in their microstructures with higher grain size, their overall elongations were found to be marginally inferior to conventional austenitic stainless steel AISI 316LSS.

The electrochemical studies in fuel cell environment were carried out with conventional three electrode cell with saturated calomel electrode (SCE) as reference electrode, graphite as counter electrode and polarization experiments were performed using a Gamry 600 potentiostat/galvanostat. The specimens were exposed to 0.5 M H2SO4 containing 2 ppm HF. The temperature of the electrolyte
was at room temperature and some cases the temperature was set to 80±1oC,

simulating the fuel cell operation temperature. This was achieved by connecting the double walled electrochemical cell to a thermostat.

The polarization experiments were carried out by applying a potential ramp from –

0.6 V, cathodic (with respect to reference electrode) to 1.2 V in the anodic direction (with respect to reference electrode) at a scan rate of 1 mVs-1. To study the potentiodynamic behavior of alloys at simulated anode and cathode environment, tests were conducted by maintaining the temperature of the electrolyte at 80°C and by purging either with hydrogen or oxygen, respectively. The shaded region in the figures(Fig. 5) indicates the possible potential range a bipolar plate can experience during the operation of a fuel cell. The passive range of the 316L in the cathodic and anodic environments are observed in the potentials between -0.210 – 0.790 and -
0.180 - 0.780, respectively, while the passive currents density, ip remains similar in both the environments.

Polarization graph of Mn stainless steel (Fig. 6 of the drawings) shows typical active, passive and trans-passive regions for both at the room temperature and at 80° C. The MnSS–4 also showed similar passive characteristics at room temperature, however at 80° C it displayed unstable passivation behaviour till 0 V vs. SCE upon exposure at 80o C. Among all the specimens, MnSS-3 showed poor/unstable passivation behaviour both in room temperatures as well as at 80° C. It is well known
that the unstable passivation behaviour occurs when theoretical cathodic curve intersects theoretical anodic cure more than once as shown in Fig. 7of the drawings
.The typical values deduced from the potentiodynamic plots obtained for all the specimens are shown in Table 3.Based on the potentiodynamic analyses, MnSS–2 and MnSS–4 specimens are promising candidates in terms of Ecorr, passive region and the breakdown potentials. Further, the potentiodynamic behaviour of MnSS–2 and MnSS–4 specimens were compared with the commercial-metallic bipolar material namely, 316L SS as shown in the Fig. 8.The curves were recorded at room temperature and at 80° C. Interestingly, the Ecorr and the break down potentials of MnSS–2 and MnSS–4 steels are higher than that of 316L SS, indicating better corrosion behaviour.

In order to investigate the corrosion characteristics of Mn-SS steels in the anodic and cathodic conditions, the potentiodynamic tests were done in H2-containing and O2- containing environments, respectively. Fig. 9 of the drawings shows the potentiodynamic polarization curves of Mn SS specimens in comparison with 316L SS in the simulated PEMFC anodic and cathodic environments. The usual potential ranges of PEM fuel cell anode and cathode are marked with shadows on the polarization curves.

The typical anodic potential of -0.1 V and the cathode potential of 0.6 V vs SCE were selected as the bench marks to evaluate the corrosion properties of Mn-SS steels. Potentiodynamic polarization curves of MnSS-2 and MnSS-4 steels in the simulated H2 purged anodic environments, exhibits quite similar behavior with that of SS 316L with a stable passive region. However, Mn SS steels exhibits cathodic current loop and additional anodic peak, which is similar to often seen in Mn containing stainless steels (201). This signifies that Mn-SS show are unstable at potential of - 0.1 V, which could dissolve actively if the passive layer is disturbed. On the other hand, Mn- SS steels in the cathodic atmosphere (0.6 V) presented identical passive current densities and breakdown potential as SS 316L. This signifies that all steels behave similar in O2 purged cathodic conditions.

Potentiostatic tests were conducted to examine the corrosion activities of Mn SS steels and 316L SS in the simulated and accelerated PEM fuel cell environments under operating electrode potential conditions. For potentiostatic tests two potentials were chosen, - 0.1 V and 0.6 V simulating the anodic and cathodic potentials that exists in PEM fuel cells, respectively. The surfaces of stainless steels were polarized at the selected potential for an hour at 80º C in H2 purged (anodic environment) O2 purged solution (cathodic current) as shown in the Fig. 10 of the drawings.

In the anodic atmosphere, the MnSS steels shows the negative (cathodic) currents, indicating that these steels are still in the cathodic regions, similar to the behavior which was also recorded in potentiodynamic curves (Fig. 10(a)) i.e., at - 0.1 V both the steels are in cathodic loop. These results indicate that, MnSS steels show unstable passivation in the anodic fuel cell atmosphere. Under identical conditions,
SS 316L steel shows the passive behavior with current density less than 1 µAcm-2,

which is well within the DOE 2020 target value. However, long term exposure study is absolutely necessary to corroborate this behavior.

On the other hand, in the cathodic atmosphere (0.6 V), both the MnSS steels show passive behavior as SS 316L, however higher magnitude of passive current densities (2 µAcm-2 and 4 µAcm-2) than SS 316L (1 µAcm-2). The higher current densities of MnSS steels in comparison with SS 316L indicate that the passive layer on MnSS steels is not as protective as 316L (Fig. 10(b)).In general, the O2 atmosphere favors the formation of oxide film, which inhibits the dissolution rate, however the stability of the oxide film governs the magnitude of the passivation current density. It should be noted that for this test duration, both Mn SS steels (MnSS-2 and MnSS-4) shows higher passive current density than the DOE 2020 target value, whereas 316L
satisfies the norm. A long term exposure study is necessary, where it is normal that passive current density reduces further (less than 1 µAcm-2).

The EIS analysis were carried out after potentiostatically polarizing the specimens at selected potentials (- 0.1 V or 0.6 V) in simulated fuel cell environment. The Nyquist and Bode plots for both SS 316L and MnSS stainless steels tested in anodic and cathodic environments are shown in Fig. 11 and 12 of the drawings.

The thickness of the oxide layer was calculated from impedance data by using a parallel plate expression for capacitors.

C ? ee? A
t

where C is capacitance, ?o is the permittivity of free space (8.85 × 10-14 F cm-1), ? is the dielectric constant of the oxide layer, taken as 15.6 for stainless steel. A is the surface area of the steel (in cm2), and t is the thickness of oxide layer (in cm). From the capacitance of the oxide film, its thickness can be calculated.
The capacitance related to CPE was calculated by,

?Q.R 1
C ? p
Rp

where Rp is polarization resistance (?cm 2), Q represents a constant phase element with units of ?-1 sa cm-2, and a is a numerical exponent.

Electrical parameters obtained by fitting the impedance spectra of 316L and MnSS with the equivalent electrical circuit and the values of calculated passive film thickness are presented in Table. 4. Generally, higher polarization resistance/oxide resistance (Rp) values indicate better corrosion resistance (Fig. 13 of the drawings). From the Table. 4 of the drawings it can be observed that the Rp values of MnSS steels are similar to 316L SS both in the H2 and O2 atmospheres. The MnSS–4 steels shows even higher Rp values than the 316L SS in H2 atmosphere. On the other hand, the thickness of the oxide films for MnSS steels are higher in case of H2 atmosphere, where as it is less in O2 atmospheric conditions as compared to 316L SS (Fig. 14 of the drawings).

Mott-Schottky analysis was carried out to understand the effect of temperature on the electronic properties of oxide. Mott-Schottky plots were obtained by sweeping the potential in cathodic direction from 1 V to -1 V and the capacitance value was recorded for a frequency of 1 kHz with a scan rate of 25 mV per step to avoid electro- reduction of passive film. The bi-layer formation on stainless steel passive films and the p-type behavior was attributed to an inner Cr oxide and the n-type behavior was associated with outer Fe based oxides and hydroxidesas shown in the Fig. 15 of the drawings.

The general behavior of Mott-Schottky plots of alloys both tested in anodic and cathodic environments is shown in Fig. 16 of the drawings and is qualitatively similar. This indicates that, the semiconducting characteristics of the alloys are similar in both conditions. The p-type behavior at Region II (- 0.6 V to -0.3 V) and n-type behavior at

Region III (0.1 V – 0.6 V) indicates two different layers in stainless steel passive film. The bi-layer formation on stainless steel passive films and the p-type behavior was attributed to an inner Cr oxide and the n-type behavior was associated with outer Fe based oxides and hydroxides as shown in the Fig. 16The NA and ND values for both
steels are in the order of 10 20 to 10 21 cm-3

All steels show p-type and n-type behaviour over the range of formation potential. This suggests a bi-layer structure of the surface oxide on all these steels. A p-type corresponds to an inner Cr rich-oxide and n-type corresponds to outer Fe rich-oxide. Surface characterization by x-ray photoelectron spectroscopy (XPS) or Auger electron spectroscopy (AES) is required to confirm the composition of bi-layer structure. From Fig. 16(a),it can be observed that, in the anodic (H2 atmosphere), the
capacitance value of MnSS–2 and MnSS–4 steels is lower (C-2 is higher) than that of

316L SS (region II), indicating presence of less charge carriers. This suggests that the passive film (Cr-oxide & Fe-oxide) formed on Mn SS steels at anodic conditions are slightly better than the 316 L SS, particularly for MnSS–4, as also seen in the impedance studies (Fig. 11 and 12). In the cathodic conditions (Fig. 16 (b)), the capacitance value for p-type oxide is the same for Mn SS steels as well as 316L SS indicating the passive film equally stable. However, the capacitance value or charge carriers are more for n-type (Fe-rich) oxide for Mn SS steels, which is the reason for higher passive current density for these two steels as compared to 316L.

The calculated donor and acceptor concentrations of all the steels are represented in the Table 5. It is generally, the acceptor concentration (NA - inner chromium oxide layer) which decides the stability of the steels. From the table it is observed that the calculated NA values in the anodic (H2 atmosphere) are lower compared to the 316L SS, indicating that MnSS steel contain lesser defects in the inner passive film layer than 316L SS. Hence, MnSS steel possess better stability in the anodic atmosphere than the 316L SS, if the passive film is not damaged. In the cathodic (O2) condition, the NA and ND values of MnSS and 316L are same indicating a similar defect concentration in the both the layers hence similar passive behaviour.

ICR measurements were carried out at room temperature with naturally formed oxide on the surface. For these measurements, the stainless steel specimen was placed between two carbon papers (TGP-120) and sandwiched between two copper plates. The copper plates were connected to the micro Ohm meter as depicted in Fig. 17 of the drawings. The Ohm meter applies the current between 6.7 mA and 67 mA

according to the target resistance. Compaction force was provided using a automatic and digitally controlled hydraulic compressive press. To avoid any surface roughness effect on contact resistance, diamond finished surface was used for all the measurements. The total resistance (twice the contact resistance between carbon paper and copper plate and twice the contact resistance between carbon paper and stainless steel) was measured with respect to the applied pressure and then contact resistance between carbon paper and stainless steel was calculated by calibration.

The Fig. 18 of the drawings shows the calculated interfacial contact resistance (ICR) between MnSS–2 and 316L SS and carbon paper. The ICR of both the steels decreased with increasing compaction force. This is due to the increase in interfacial contact between the 316L SS and the carbon paper when pressed together with increasing pressure, enhancing both electrical and thermal conductivity. DOE 2020 target defines ideal bipolar plate should have the contact resistance less than 10 m?
-2 at a pressure of 140 Ncm-2. At 140 Ncm-2, the ICR of 316L SS and Mn SS-2

stainless steels is 33 and 28 m? -2 respectively, under identical conditions, indicating that both steels exhibited slightly higher ICR values than the DOE target.

Commercial SGL DC 35 gas diffusion layers (GDLs) were used as a substrate for the anode and cathode electrodes in MEA fabrication. For the cathode catalyst layer, commercial Pt/C catalyst was dispersed in Iso propyl alcohol (IPA) and ultra sonicated for 30 min. followed by the addition of 30 wt % Nafion solution. The resultant slurries were further sonicated for 1 h and coated on top of the GDL till to
obtain a Pt loading of 0.50 mg cm-2. For anode catalyst layer, commercial Pt/C was

used with 7 wt % Nafion till to obtain a Pt loading of 0.50 mg cm-2. MEA is obtained by sandwiching the pre-treated Nafion-212 membrane between the cathode and the anode followed by their hot-compaction under a pressure of 20 kg cm-2 at 130 °C for
3 min. MEAs were coupled with teflon gas-sealing gaskets and placed in single-cell test fixtures with parallel serpentine flow-field machined on Mn SS – 2 steels. The galvanostatic polarization data were with gaseous H2 (~ 80% RH) and gaseous O2 (~
80% RH) to the anode and cathode of the PEFCs, respectively. The humidified gases

are passed to the fuel cell with a thermostat through the bubble humidifiers kept at 10

°C higher than the cell temperature. The flow rates of the reactants were fed with the help of controllable rota meters. MEAs are evaluated with an active area of 25 cm2 in PEFCs at cell temperature of 60 ºC. The parallel serpentine flow-field design machined on MnS –2 steel, various components used in the fuel cell assembly and the In-house fuel cell set up are shown in the Fig. 19 of the drawings .Fuel cell

polarization curves of the MEAs with MnSS-2 and 316L SS as bipolar plates were evaluated under experimental conditions as described in Table 6 of the drawings.

Fuel cell polarization curves of the MEAs with MnSS–2 and 316L SS as bipolar plates were evaluated under experimental conditions as described in Table 6and shown in Fig. 20 of the drawings. Since, all the MEAs were made with a similar process; any variation in performance is attributed to the metallic bi-polar plates made with MnSS–2 and 316L SS. Superior behaviour and improved power density for MnSS–2 specimens is perceived than 316L SS. The difference in the performance could be attributed to the different composition of the steels, however the difference in flow filed design of the plates and active area also need to be considered in the present testing. The performance results of the fuel cell tests agree with the corrosion resistance results in the simulated environments. This result reveals that the MnSS-2, having a lower ICR value and a rational corrosion resistance, produces a high current density than the 316L SS.

? The uniqueness of the present invention lies in the formulation of a comprehensive methodology for manufacturing low cost high and low Mn stainless steel.

? Alloy composition:

Low Mn stainless steel composition-1 of C (0.06 to 0.1%), Mn (6 to 8%), S (up to

0.01%), P (up to 0.01%), Si (0.10 to 0.45%), Cr (17 to 20%), Ni (5 to 7%), Mo (2 to

3%), Cu (0.5 to 0.8%) and N (0.2 to 0.4%) and balance being Fe.

High Mn high N stainless steel composition-2 of C (0.06 to 0.1%), Mn (16 to 20%), S(up to 0.01%), P(up to 0.01%), Si (0.10 to 0.45%), Cr (17 to 20%), Ni (1 to 3%), Mo (0 to 1.5%), Cu (1.00 to 2%) and N (0.3 to 0.5%) and balance being Fe.

? Processing: Manufacturing method of claimed high strength HIC resistant steel, comprising following steps:
• The steels ingots/slabs are reheated and soaked in a reheating furnace to
1150 to 1250 oC for 3 hours.

• Hot-rolled to 16 mm plate with finish rolling temperatures of 1000 to

1100oC.

• The final rolling reduction is about 30-35%.

• Annealing the strips after hot rolling was carried out in the range of 1100 to

1200o C and subsequent water quenching is done

? Microstructure: The claimed steel composition has a fully austenitic microstructure with grain size in the range of about 10 to 11 µm

? Mechanical properties:

? Tensile properties

• The claimed steel compositions(both low Mn and high Mn stainless steels) has a yield strength in the range of 350 MPa to 450 MPa, ultimate tensile strength in the range of 700 MPa to 780 MPa and elongation in the range of 60 to 70%
? Hardness

• The claimed steel compositions (both low Mn and high Mn stainless steels) has a hardness in the range of 90-97 HRB

? Corrosion resistance:

? Fuel cell environment at room temperature

• The claimed low Mn stainless steel composition has break down potential in the range of 850 to 950 mV in stimulated fuel cell environment as determined by potentiodynamic polarizationtest.
• The claimed low Mn stainless steel composition has passive layer in the range of -50 to 950 mV in stimulated fuel cell environment as determined by potentiodynamic polarizationtest.
• The claimed high Mn stainless steel composition has break down potential in the range of 870 to 970 mV in stimulated fuel cell environment as determined by potentiodynamic polarizationtest.
• The claimed low Mn stainless steel composition has passive layer in the range of -100 to 950 mV in stimulated fuel cell environment as determined by potentiodynamic polarizationtest.

? Fuel cell environment at 800C

• The claimed low Mn stainless steel composition has break down potential in the range of 850 to 900 mV in stimulated fuel cell environment as determined by potentiodynamic polarizationtest at room temperature.

• The claimed low Mn stainless steel composition has passive layer in the range of 50 to 900 mV in stimulated fuel cell environment as determined by potentiodynamic polarizationtestat room temperature.
• The claimed high Mn stainless steel composition has break down potential in the range of 800 to 900 mV in stimulated fuel cell environment as determined by potentiodynamic polarizationtestat room temperature.
• The claimed low Mn stainless steel composition has passive layer in the range of 0 to 900 mV in stimulated fuel cell environment as determined by potentiodynamic polarizationtestat room temperature.

? Film resistance:

? at anode side (H2)

• The claimed high Mn stainless steel composition has superior film resistance in the range of 45 to 80 KO.cm2 in stimulated fuel cell environment as determined by electrochemical impedance spectroscopy test.

? at cathode side (O2)

• The claimed high Mn stainless steel composition has superior film resistance in the range of 40 to 45 KO.cm2 in stimulated fuel cell environment as determined by electrochemical impedance spectroscopy test.

? Oxide thickness:

? at anode side (H2)

• The claimed high Mn stainless steel composition has less film thickness in the range of 0.15 to 0.25 nm in stimulated fuel cell environment as determined by electrochemical impedance spectroscopy test.

? at cathode side (O2)

• The claimed high Mn stainless steel composition has less film thickness in the range of 0.45 to 0.55 nm in stimulated fuel cell environment as determined by electrochemical impedance spectroscopy test.

? Defect concentration in the passive film

? Acceptor concentration:

? at anode side (H2)

• The claimed high Mn stainless steel composition has less defect concentration in the range of 8 to 12 X1020 cm-3 in stimulated fuel cell environment as determined by Mott-Schottky analysis.

? at cathode side (O2)

• The claimed high Mn stainless steel composition has less defect concentration in the range of 8 to 12 X1020 cm-3 in stimulated fuel cell environment as determined by Mott-Schottky analysis.

? Donor concentration:

? at anode side (H2)

• The claimed high Mn stainless steel composition has less defect concentration in the range of 7 to 11X1020 cm-3 in stimulated fuel cell environment as determined by Mott-Schottky analysis.

? at cathode side (O2)

• The claimed high Mn stainless steel composition has less defect concentration in the range of 6 to 11X1020 cm-3 in stimulated fuel cell environment as determined by Mott-Schottky analysis.

? Interfacial contact resistance(ICR): The claimed Mn stainless steel composition showed low interfacial contact resistance (in the range of 20 to 30 m?-2) than that of commercial available AISI 316L stainless steel (33m?-2) at load of 140 Ncm-2

? Pilot fuel cell testing: The claimed Mn stainless steel composition showed higher fuel cell efficiency (in the range of 110 to 120mW cm-2) than that of commercial available AISI 316L stainless steel (84mW cm-2).

The invention presents a viable prospect of being utilized/ commercialized manufacturing the low cost Mn stainless steel with superior mechanical and corrosion resistance than that of conventionally known AISI 316L stainless steel as a bipolar material for automobile and power generating fuel cells.

The invention as disclosed is well adapted to obtain the objects and advantages mentioned as those inherent therein, while presently preferred embodiments of the invention have been described for the purpose of this disclosure, numerous changes

in the composition of the stainless steel can be made by those skilled in the art, which changes are encompassed within the spirit of this invention as defined by the appended claims.