Evaluation of Coated Stainless Steels for Bipolar Plates in Fuel Cells Type PEM

Evaluation of Coated Stainless Steels for Bipolar Plates in PEM Type Fuel Cells

M. Rendón-Belmontea, J.T. Pérez-Quirozb, J. Porcayo-Calderónc, G. Orozcoa.

aCentro de Investigación y Desarrollo Tecnológico en Electroquímica S.C. Parque Sanfandila S/N, Pedro Escobedo, C.P. 76703, Querétaro, México.

bInstituto Mexicano del Transporte, Sanfandila, Pedro Escobedo, C.P. 76700 Querétaro, México.

cInstituto de Investigaciones Eléctricas, Calle Reforma 113, Col. Palmira, C.P. 62490, Cuernavaca, Morelos.

Abstract

An electrochemical study of Ni20Cr, NiCrAlY and Cr3C2(NiCr)alloys used in coating form made with the Tafel extrapolation and Resistance techniques for polarization. The icorr values obtained for Ni20Cr and NiCrAlY coatings, the material dissolution, and the low resistance to corrosion eliminates the possibility that these coating could be candidates for construction bipolar plates for PEM type fuel cells; however with the Cr3C2(NiCr) coating, icorr

values below 0.016 mA/cm2 were obtained. Materials must fulfill this criterion to be considered as candidates in applications such as a bipolar plate.

Introduction

Stainless Steels are considered strong candidates for use as bipolar plates because they are easily produced, a wide variety of different types of stainless steel are available, they have a high mechanical resistance and they are economical compared to other materials (such as gold). A disadvantage of stainless steel is that in the acidic environment of a fuel cell, it is unstable and when in direct contact with the acid of the membrane/electrolyte, stainless steel experiences certain amount of corrosion. This corrosion could contaminate the catalyst and as a consequence, could diminish the cell’s efficacy (1,2,3,4). As a result, there is a need to look for more resistant materials that can withstand these conditions. One option is to use noble metals such as stainless steel and to coat these metal with alloys of chrome, nickel, nitrates and carbides, it is well-known that a way to increase a materials’ resistance to corrosion is with the application of a coating with a higher chemical stability than the substrate (1,5).

Experimental Development

Two types of stainless steel have been used in this work, types AISI S4300 and S4400. Their chemical composition is shown in Table 1. Three coatings consisting of Ni20Cr, NiCrAlY and Cr3C2(NiCr) alloys in powder form were used. Their chemical composition was determined by spectroscopy of atomic absorption and is shown in Table 2.

ECS Transactions, 15 (1) 61-70 (2008)10.1149/1.3046620 © The Electrochemical Society

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Resaltado

TABLE I. Chemical composition of selected steels (weight %) SS Designation C Cr Mn Si Ti Al Nb Fe439 S4300 0.03 17-19 1.00 1.00 0.32 0.15 - Bal.441 S4400 0.03 17.5-

18.51.00 1.00 0.10-0.60 - 0.39 Bal.

TABLE II. Chemical composition of coatings evaluated (weight %) Coatings Ni Cr Al Y Cr3C2 NiCrNi20Cr 80 20 - - - -

NiCrAlY60.9 28.2 9.9 1 - -

Cr3C2(NiCr)

- - - - 75 25

These coatings were applied on steel sheets with a surface area of 10x10 cm. The S4300 stainless steel was coated with the Ni20Cr alloy and the S4400 stainless steel was coated with the NiCrAlY alloy. These two coatings were applied with the thermal spray using the powder flame technique. The Cr3C2(NiCr) alloy was deposited on S4400 by means of the HVOF (High Velocity Oxygen-Fuel) system and with a Sulzer-Metco Model DJ2700 using a flame generated by the combustion of an oxygen-propane mixture. Before the coatings were applied, the steel sheets’ surface was prepared with a ceramic abrasive burst according to the NACE No. 1/SSPC-SP 5 (6) standard and cleaned with acetone. The electrolyte used was a solution made with H2SO4 0.5 M + 2 ppm F¯ atambient temperature.A mercuric sulfate electrode (Hg2SO4) was used as a reference electrode and a graphite bar electrode (Fig. 1) was used as a reference counter-electrode. For the coatings’ electrochemical behavior two techniques were utilized: Resistance to polarization (Rp) and Tafel’s Extrapolation (Potentio-dynamic curves) with a sweeping speed of 0.15 mV/s according to ASTM G5 and G59 (7,8). In both cases a Gamry potentiostat was used. The morphological aspects of the coatings were analyzed through SEM (sweeping electronic microscopy) and their composition through the EDS before and after the electrochemical preparations.

Figure 1. Electrochemical Cell used for the evaluation of coatings in H2SO4 0.5 + 2 ppm F¯

ECS Transactions, 15 (1) 61-70 (2008)


Results and discussion

Graphic 1 shows the behavior of Ni20Cr coating applied to type S4300 stainless steel.It is observed that at the beginning of the electrochemical evaluation the icorr obtained is 1.7 mA/cm2 and tends to diminish during the first 8 days (192 hours) of exposure to 0.2mA/cm2. However after 8 days of exposure the icorr value increases up to a 2.0 mA/cm2

value, and after 15 days (360 hours), the evaluation was terminated because of coating dissolution that provoked a color change of the electrolyte (from transparent to green). This result is attributed to the coating porosity and heterogeneity resulting from the low velocity of the impact (24-36 m/s) of the particles which is a characteristic of this technique. However the alloy may have a better performance with a different application technique such as the high velocity of fuel oxygen (HVOF) technique, wich has a higher impact velocity to obtain better adherence and to minimize porosity.

Graph 1. Behavior of Type S4300 stainless steel with a Ni20Cr coating in an H2SO4 0.5M + 2 ppm F¯ medium.

The Graph 2 shows the behavior of NiCrAlY coating applied on S4400. It is observed that at the beginning of the electrochemical evaluation the icorr value obtained is 1.35 mA/cm2 with a trend to diminish 13 days after (312 hours) of exposure to a value of 0.1 mA/cm2. This current stayed relatively stable (0,1-0,05 mA/cm2) for only 2 days (48 hours), after which an increase of the corrosion velocity occurred that provoked the dissolution of the coating after (480 hours).



Graph 2. Behavior of S4400 stainless steel with a NiCrAlY coating in an H2SO4 0.5 M + 2 ppm F¯ medium.

During the electrochemical evaluations of both coatings the icorr values obtained were 0.1 mA/cm2 and 2 mA/cm2, compared with levels such as 0.016 mA/cm2 reported in the literature for the materials studied (3,9,10), the icorr values of these coatings were higher and therefore didn´t fulfill the desired criterion. However, it is important to recall that the coatings have been already evaluated in their condition as deposited; this means that an additional finish was not generated. This difference indicates that the real area of reaction of the coatings is much higher than the measured area, due to the morphology of the coating surface, as it is shown later in this paper.

Figure 2a shows the superficial aspect of Ni20Cr coating, once it had been deposited on the S4300 stainless steel and before the electrochemical evaluation. This figure shows the morphological condition which is typical of a coating obtained by the thermal projection process were some flat and/or agglomerated particles were observed as a consequence of the projection of the coating against the substrate. During the projection the particles can be in a melted or semi-melted state and react with the atmosphere during their trajectory, experiencing a certain degree of oxidation. Also, with impact they generate heterogeneities and porosity in the coating (4). Figure 2b shows the aspect of Ni20Cr coating deposited on Type S43000 stainless steel after the electrochemical evaluation, can be observed the presence of corrosive products as small crystals. EDS analysis determined that the sheet composition contained a higher amount of Nickel before the evaluation than after it. This demonstrates that nickel was the main element to suffer dissolution during the evaluations along with chrome in a minor quantity; see Table 3. In Figure 3 the thickness of the coating obtained can be appreciated.



2a) 2b)

Figure 2a) Micrography at 200X of the Ni20Cr coating, deposited on Type S4300 stainless steel before the electrochemical evaluation. Figure 2b) Micrography at 200X of the Ni20Cr coating, deposited on Type S4300 stainless steel after the electrochemical evaluation.

Figure 3. Micrography at 100X of the plaque profile, to determine the thickness of the Ni20Cr coating deposited on Type 4300 stainless steel.

Figure 4a shows the superficial aspect of NiCrAlY coating deposited on Type S44000 stainless steel before the electrochemical evaluation. The same characteristics are observed in the Ni20Cr coating in this figure; this is normal because both formulations were deposited with the same process. Figure 4b shows the superficial aspect of NiCrAlY coating deposited on Type S44000 stainless steel after the electrochemical evaluation. The surface shows that the coating suffered dissolution when subjected to an acid medium. A partial dissolution of the coating was observed in some areas which resulted in the direct contact of the electrolyte with the base metal (S4400). According to the EDS analysis, nickel suffered the main dissolution. It was also observed that the Chrome quantity in the coating is lower before the evaluation than after wards. This is due to the dissolution of chrome contained in the substrate (see Table 3).



The fact that this coating had a better resistance to the corrosive attack of the electrolyte than the Ni20Cr coating is very noticeable. In the case of the Ni20Cr coating, the dissolution was practically total while in the case of the NiCrAlY, the dissolution was only observed in some areas. In Figure 5 the thickness of the coating can be observed.

a)

b) Figure 4a) Micrography at 200X of the NiCrAlY coating deposited on Type S4400 stainless steel before its electrochemical evaluation. 4b) Micrography at 200X and 1000X of the NiCrAlY coating deposited on the Type S4400 stainless steel after its electrochemical evaluation.

Figure 5. Micrography at 100X of the plaque profile, in order to determine the thickness of the NiCrAlY coating deposited on Type S4400 stainless steel.



TABLE III. Results of coatings evaluated (weight %)

Element ss 44000 with

coating before the

electrochemical evaluation

ss 44000 with coating after the electrochemical

evaluation

ss 43000 with coating before the

electrochemical evaluation

ss 43000 with coating after the electrochemical

evaluation

Al 20.50 20.11 - -

Cr 10.56 12.39 12.80 10.38

Ni 9.25 5.39 16.38 12.94

Y 0.45 0.35 - -

Figure 6 shows the aspect of the surface finish of Cr3C2(NiCr) coating deposited on Type S4400 stainless steel evaluated in its “as-deposited” condition. The characteristics are typical for a coating deposited with the HVOF process with a wrinkled surface that corresponds to the particle’s size used for deposition (~37 m) (11). This aspect is an important variable that needs to be taken into account since the real surface area of the coating is much larger than the area used to calculate the corrosion velocity.

Figure 6. Micrography at 200X of the superficial aspect of the Cr3C2(NiCr) deposited on Type S4400 stainless steel.

Figure 7 shows the typical aspect of a coating deposited through the HVOF process. Due to the process characteristics coatings deposited with this process have a high density and a minor porosity of about 1% (11). The projection velocity of the particles is high at 500 m/s and because of the high kinetic energy as the particles impact the surface, coverings with high density and low porosity are generated. The apparent porosity observed in this figure is due to the detachment of particles of chrome carbonate during the metallographic preparation of the sample. The coatings adherence to the substrate is mainly due to the inlay of particles deformed when they are impacted against the rough surface of the substrate.



Figure 7. Micrography at 500X of the aspect on a transversal section of the Cr3C2 (NiCr)coating deposited on Type S4400 stainless steel.

Figure 8 shows the aspect of the interface coating-surface where it can be observed that the first particles that impacted the substrate surface were completely deformed and they penetrated the surface imperfections. This guarantees the adequate adherence strength.

Figure 8. Micrography at 500X of the aspect of the interface Cr3C2(NiCr)¯ substrate.

Graph 3 shows the electrochemical behavior of Cr3C2(NiCr). The icorr oscillates between 0.00168 and 0.00295 mA, which are values that are comparable with the criterion reported in literature and the material satisfies the icorr =0.016 mA/cm2 criterion (3,9). Therefore this coating is considered as candidate for development of bipolar plate.



Graph 3. Behavior of Type S4400 stainless steel with coating Cr3C2(NiCr) in an H2SO4

0.5 M + 2 ppm F¯ medium.

However it is important to mention that in this work the corrosion resistance of the material is the main problem that is being explored. It is also important to select a material as candidate for bipolar plate, construction that meets other important criteria such as a good electric conductivity and a high thermal conductivity (12,13,14).

Conclusions

According to the results obtained from the electrochemical evaluation, fusing the Tafel Extrapolation Techniques and polarization resistance, it is concluded that the Ni20Cr coatings deposited on Type S4300 stainless steel and the NiCrAlY coatings deposited on Type S4400 stainless steel and sprayed by a powder flame do not have a sufficient resistance to corrosion to be considered as candidates for use in PEM type fuel cells. However the Cr3C2(NiCr) coatings showed good results during the electrochemical evaluation. The icorr values were lower than 0,016 mA/cm2 which is the criteria to consider a material as candidate to be used as a bipolar plate.

Acknowledgements

I thank CONACYT for the scholarship for the postgraduate studies and the development of this research. To the Instituto Mexicano del Transporte for allowing us to realize a part of the experimental Project for this research in their facilities. To IIE for their support to perform the sample analysis. To CIDETEQ for allowing us to be part of it and to my adviser J.T. Perez-Quiroz for his support.



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Evaluation of Coated Stainless Steels for Bipolar Plates in Fuel Cells Type PEM