Gelcasting (GC) process, usually used for ceramic moulding, is adapted for producing spongy or porous metal osteosynthesis components destined to bone void filling. The main objective of the interconnected porosity is to improve the osteoconductive of metal matrix by ingrowth of bone. Further, porosity reduces metal density and Young module, which causes bone resorption, leading to implant failure, phenomenon known as stress shielding. The employed GC is based on the formulation of AISI 316L stainless steel powder suspension in an aqueous solution of organic polymers. This suspension is cast into porous ceramic shells, like those used in lost wax technique, wherein the polymer crosslinking is induced by heating. The shells, containing the resulting hydrogel–metal composite, are subjected to thermal cycle in order to dry, burn the organic phase, sinter the metal particles at 1200 °C, and cool down to room temperature under dry hydrogen permanent flow. The susceptibility to corrosion of 50-60 % porous pieces was analyzed. The results indicated that the lower relation between the open porosity and the total porosity, the lower the corrosion rate.International Congress of Science and Technology of Metallurgy and Materials, SAM – CONAMET 201

Elsner, Cecilia Inés

Garrido, Liliana B.

Gregorutti, Ricardo Walter

Ozols, Andrés

Procedia Materials Science

Gelcasting (GC) process, usually used for ceramic moulding, is adapted for producing spongy or porous metal osteosynthesis components destined to bone void filling. The main objective of the interconnected porosity is to improve the osteoconductive of metal matrix by ingrowth of bone. Further, porosity reduces metal density and Young module, which causes bone resorption, leading to implant failure, phenomenon known as stress shielding. The employed GC is based on the formulation of AISI 316L stainless steel powder suspension in an aqueous solution of organic polymers. This suspension is cast into porous ceramic shells, like those used in lost wax technique, wherein the polymer crosslinking is induced by heating. The shells, containing the resulting hydrogel–metal composite, are subjected to thermal cycle in order to dry, burn the organic phase, sinter the metal particles at 1200 °C, and cool down to room temperature under dry hydrogen permanent flow. The susceptibility to corrosion of 50-60 % porous pieces was analyzed. The results indicated that the lower relation between the open porosity and the total porosity, the lower the corrosion rate.Centro de Investigación y Desarrollo en Tecnología de Pinturas (CIDEPINT

Garrido, Liliana Beatriz

El Servicio de Difusión de la Creación Intelectual

 Procedia Materials Science  9 ( 2015 )  279 – 284 
Available online at www.sciencedirect.com
2211-8128 © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license 
(http://creativecommons.org/licenses/by-nc-nd/4.0/).
Peer-review under responsibility of the Scientifi c Committee of SAM–CONAMET 2014 
doi: 10.1016/j.mspro.2015.04.035 
ScienceDirect
International Congress of Science and Technology of Metallurgy and Materials, SAM – 
CONAMET 2014 
Corrosion in 316L porous prostheses obtained by gelcasting 
Ricardo W. Gregoruttia,*, Cecilia I. Elsnerb, Liliana Garridoc, Andrés Ozolsd 
aLab. de Entrenamiento Multidisciplinario para la Investigación Tecnológica (LEMIT, CIC), Av. 52 e/121 y 122, B1900AYB La Plata, Argentina  
bCentro de Inv. y Des. en Tecn. de Pinturas (CIDEPINT, CONICET), Av. 52 e/121 y 122 - Fac. de Ing. (UNLP), B1900AYB La Plata, Argentina 
cCentro de Tecnología de recursos Minerales y Cerámica Cno. Centenario y 506, (B1897ZCA), Gonnet, Argentina 
dInst. de Ing. Biomédica e Inst. de Tecn. y Cs. de la Ing., Fac. de Ing., Univ. de Bs. As., Av. Paseo Colón 850, C1063ACV CABA, Argentina 
Abstract 
Gelcasting (GC) process, usually used for ceramic moulding, is adapted for producing spongy or porous metal osteosynthesis 
components destined to bone void filling.  The main objective of the interconnected porosity is to improve the osteoconductive of 
metal matrix by ingrowth of bone. Further, porosity reduces metal density and Young module, which causes bone resorption, 
leading to implant failure, phenomenon known as stress shielding. The employed GC is based on the formulation of AISI 316L 
stainless steel powder suspension in an aqueous solution of organic polymers. This suspension is cast into porous ceramic shells, 
like those used in lost wax technique, wherein the polymer crosslinking is induced by heating. The shells, containing the resulting 
hydrogel–metal composite, are subjected to thermal cycle in order to dry, burn the organic phase, sinter the metal particles at 
1200 °C, and cool down to room temperature under dry hydrogen permanent flow. The susceptibility to corrosion of 50-60 % 
porous pieces was analyzed. The results indicated that the lower relation between the open porosity and the total porosity, the 
lower the corrosion rate.  
© 2015 The Authors. Published by Elsevier Ltd. 
Peer-review under responsibility of the Scientific Committee of SAM– CONAMET 2014.Keywords: Corrosion; porous prosthesis; 
gelcasting.  
Keywords: Corrosion; porous prostheses; gelcasting 
 
 
 
 
 
* Corresponding author. Tel. +(54)-221-483-1141; fax: +(54)-221-425-0471.  
E-mail address: metalurgia@lemit.gov.ar 
 
 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license 
(http://creativecommon .org/l censes/by-nc-nd/4.0/).
P er-review under responsibility of the Scientifi c Committee of SAM–CONAMET 2014 
280   Ricardo W. Gregorutti et al. /  Procedia Materials Science  9 ( 2015 )  279 – 284 
1. Introduction 
Stainless steel (ASTM F138, F139 or F745, AISI 316L grade steel), Co-Cr-Mo (ASTM F75, F562 or F799) and 
Ti-6Al-4V (ASTM F620) are used for load bearing applications in partial or total joint replacement, due to their 
satisfactory mechanical properties, corrosion resistance and biocompatibility. The latter property is related to 
biological response of human body, particularly referred to the ability of bone cell proliferation (osteoblasts) onto 
metallic surface, with minimal or any fibrous tissue (formed by fibroblasts), ensuring the implant mechanical 
fixation to bone. The metal surface topography may improve the fixation, by increasing roughness and open 
porosity. The degree of integration to bone seems to reach a maximum, when interconnected porosity is in 50 % to 
80 % (in volume) range, and composed by 100 to 500 ?m pores. These features favor angiogenesis (the growth of 
blood vessels), and thus, the proliferation and colonization of osteoblasts within the pores. At the same time, 
porosity promotes the reduction of density and elastic modulus of mentioned alloys (between 110 and 210 GPa), 
tending to a bone-like magnitude (? 30 GPa). This allows a better transference of the mechanical load on the bone, 
avoiding the accumulation of stress onto the implant, phenomenon known as stress shielding. This process can occur 
when the initial implant fixation is deficient, leading to the relative displacement of metal-bone interface (in teens of 
microns), thus, triggering the osteopenia process, with the consequent loss or bone resorption, tissue inflammation 
and/or infections. This second phenomenon, known as aseptic loosening, leads to bone and metal debris generation 
promoted by wear. The final implications are the loss of the bone-implant interface, the implant failure and the need 
for revision surgery. The need to favor angiogenesis, osteointegration and avoid stress shielding, has driven the 
development of porous or cancellous structure implants, based on Ti, Ta or Ni-Ti alloys, when mechanical 
performance is the leading requirement. These are applied to fracture fixation, providing a scaffold for bone tissue 
regeneration in orthopedic and maxillofacial surgeries [Medlin et al. (2003), Ryan et al. (2006), Li et al. (2008), and 
Chen et al. (2009)]. Controlled porosity can be obtained by means of gelcasting (GC) technique, widely employed 
for molding ceramic components. Gelcasting is based on the formulation of slurry constituted by ceramic or metallic 
particle dispersion in water or organic solution of monomers, polymers or proteins. The slurry is cast into polymeric, 
metal or ceramic molds, wherein the crosslinking of polymers or proteins is induced by heating. The product is a 
composite of a hydrogel with metal particles homogeneously distributed, and with sufficient toughness to withstand 
further handling and sintering processes. This technique has been used for manufacturing Ti and stainless steel 
implants [Li et al. (2008a), Li et al. (2008b), Ozols et al. (2008), Ozols et al. (2009a), Ozols et al. (2009)b]. 
However, the corrosion resistance of these porous alloys may differ from that of full density, when they are 
implanted in physiological medium. Therefore, these materials must be evaluated to ensure their safety for long-
term clinical applications. 
This paper refers to the corrosion resistance of porous AISI 316L implant obtained by gelcasting. This work is a 
continuation of previous studies, which have shown the feasibility of producing implants of various geometries for 
bone fixation, and assessing their osteointegration capability by in vivo tests [Ozols et al. (2009a)].  
 
2. Experimental procedure 
Gelcasting suspension was formulated with 75 % (in wt.) of AISI 316L particles (smaller than 45 ?m) dispersion 
in 20 % (in wt.) and egg albumin devolved in warm distilled water. The slurry was cast in water permeable ceramic 
shells, similar to those used for investment casting process. These shells were wrapped in PVC films and heated in 
thermal water bath at 90 °C for 1 hour, in order to promote the protein gelation. The dried pieces were  heated at 600 
ºC/h rate, up to the sintering of 1200 ºC, where remained for 2 hours, and cooled down to room temperature under 
constant flow of dry hydrogen flow into a AISI 316L gas heated muffle.  
Total porosity (P) was calculated from the relative density (?rel), determined by the relationship between bulk 
density (?ap), measured by immersion water method, and the AISI 316 L theoretical densities of (7.96 g/cm3), 
according to the following relationship: 
 
281 Ricardo W. Gregorutti et al. /  Procedia Materials Science  9 ( 2015 )  279 – 284 
100
1 Prel ???                                                                                                                                                             (1) 
 
Open porosity (P0), defined as the ratio between open pore volume and specimen volume, was determined as 
follows: 
 
0
WG GP
V
??                                                                                                                                                             (2) 
 
Being G the dry weight of the specimen, GW its water saturated weight, and V its apparent volume, including pore 
volume. 
Corrosion resistance was evaluated by means of electrochemical polarization tests in the potential range between 
-0.20 and 0.45 V/SCE, respect to the open circuit potential of the system, where SCE refers to saturated calomel 
electrode potential. The corrosion cell was assembled with the test material, as the working electrode (1 cm2 of 
area), SCE as reference and a Pt electrode as auxiliary. Tests were performed using aqueous solution of 0.9 % (in 
wt.) of NaCl, with pH between 7.0 and 7.4, kept at 37 ºC, to simulate the human body environment. The solution 
was deaerated by N2 bubbling during the tests. The samples were kept in a stabilization period of 1 hour at open 
circuit condition (Eoc). Later, the potential scans were performed at 0.167 mV/s in the anodic direction. 
3. Results and discussion 
3.1.      Porosity measurements 
Table 1 shows the density and porosity of three different samples extracted from the femoral stem obtained by 
gelcasting. 
Table 1. Analyzed AISI 316L sample porosity. 
Sample ?ap (g/cm3) ?rel (g/cm3) P  (%) P0 (%) P0 /P 
M1 3.07 0.38 62.0 60.4 0.95 
M2 3.46 0.44 56.5 55.4 0.98 
M3 2.96 0.37 62.8 50.9 0.81 
 
M1 and M2 samples show a Po/P ratio close to 1.00, evidencing that most of the porosity is interconnected, with 
greater exposed surface area than in isolated porosity situation. In the case of M3 sample, its total porosity was 
similar to that of M1 sample, while the open porosity was significantly lower.  
Fig.1 illustrates the analyzed sample pore irregular geometry and size dispersion. 
 
 
 
Fig. 1. Porosity: (a) sample M1; (b) sample M2; (c) sample M3. 
 
282   Ricardo W. Gregorutti et al. /  Procedia Materials Science  9 ( 2015 )  279 – 284 
M1 and M2 samples, whose Po/P ratios were similar and close to 1.00, exhibited greater uniformity of pore size 
distribution, while sample M3 showed higher dispersion. 
 
3.2. Corrosion behavior 
Corrosion behaviour of porous samples was compared with that of dense AISI 316L, the results are shown in Fig. 
2. 
The analysis of the curves shows that dense AISI 316L exhibited a lower corrosion rate and a wider passive 
region (about 450 mV/SCE) than porous samples. The higher corrosion rate in these samples, at least by one order 
of magnitude, could be ascribed not only to the increased electrochemical activity of the material, but also to the 
higher exposed specific area, resulting from porosity. This was not taken into account in the calculation of the 
current density, determined as a function of the geometric area of the sample. Another feature is the different 
behaviour observed in anodic potential region. Sample M1 showed a large area (? 400 mV/SCE) at which the 
electrochemical activity of the material is lower, while sample M3 is active throughout the defined anodic sweep. 
Sample M2 shows an intermediate behaviour respect to M1 and M2 samples. The reasons for these behaviours have 
not yet been elucidated. 
Table 2 reports the electrochemical parameters obtained from the tests. 
From the results exposed in Tables 1 and 2, the variation of the corrosion rate with ?rel, ?ap and Po/P can be 
determined, as shown in Fig. 3.  
 
 
 
 
Fig. 2. Curves obtained from the electrochemical tests. 
 
 
283 Ricardo W. Gregorutti et al. /  Procedia Materials Science  9 ( 2015 )  279 – 284 
Table 2. Results of electrochemical tests. 
Sample Ecorr  (V/SCE) jcorr  (A/cm2) 
M1 -0.460 1.3×10-5 
M2 -0.267 3.0×10-5 
M3 -0.143 3.7×10-6 
Dense AISI 316L  -0.190 1.0.×0-7 
 
The lowest corrosion rate was observed for sample M3, which reported smaller ?rel, ?ap and P0 /P values, and 
bigger total porosity. While corrosion rate decreased with those parameters, open porosity has the greatest influence, 
because of the lower exposed specific area. In general, jcorr variation is directly correlated with variation of these 
parameters, as also shown in Fig. 3.  
 
 
Fig. 3. Variation of corrosion rate respect to ?rel, ?ap and P0/P. 
284   Ricardo W. Gregorutti et al. /  Procedia Materials Science  9 ( 2015 )  279 – 284 
4. Conclusion 
The results from the electrochemical tests indicated that corrosion rate is higher in porous AISI 316L respect to 
the dense material. Regarding the influence of the type of porosity, it could be established that there is a significant 
effect of open porosity and the total porosity on the corrosion rate. The smaller open porosity implies lower 
corrosion rate. This behaviour can be related to the lower exposed surface area produced when open porosity 
decreases.  
These results highlight the need to find an optimal degree of porosity that allows reconciling corrosion resistance 
with stress shielding on the implant and its biocompatibility. 
 
Acknowledgements 
The authors thank to Comisión de Investigaciones Científicas de la Provincia de Buenos Aires (CICPBA), 
Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), Facultad de Ingeniería UNLP y Facultad 
de Ingeniería UBA for the financial support to carry out the present research paper. 
 
References 
Chen Y., Feng J., B., Zhu Y. P., Weng J., Wang J. X., Lu X.. 2009. Fabrication of porous titanium implants with biomechanical compatibility, 
Materials Letters 63 2659-2661. 
Li Y., Guo Z. M., Hao J. J.. Ren S. B., 2008a. Gelcasting of porous titanium implant.  Powder Metallurgy 51, 231- 236 
Li Y., Z. Guo, J. Hao, S. Ren, 2008b. Porosity and mechanical properties of porous titanium fabricated by gelcasting Rare Metals 27, 282-.286.  
Medlin D. J., Charlebois S., Swarts D., Shetty R., Poggie R. A, 2003. Proceedings of the ASM Materials & Processes for Medical Devices 
Conference. California, USA, 394-398.   
Ozols A., Pagnola M., Faig J., Ferrari E., Tchaghayam M., Cevallos C., Rozemberg S. 2009a Production of Prosthetic Devices of AISI 316 L 
Steel by Gelcasting, Actas SAM/CONAMET 2009, Ed. J. Ovejero Garcia, Buenos Aires, Vol. III, 2035-2040.  
Ozols A., Barreiro M. M., Ferrari E., Tchaghayam N., R. Gregorutti, J. Sarutti. 2009b. Investment Gelcasting of AISI 316l Steel for Prosthetic 
Moulding. Actas SAM/CONAMET 2009. Ed. J. Ovejero Garcia, Buenos Aires, Vol. III, 2041-2046. 
Ozols A., Kokubu G., Grana D. R., Rozenberg S., Barreiro M. 2008. Powder Metallurgy Moulding of 316 L SS by Gelcasting and Its 
Biocompatibility. Congreso Binacional Argentino Chileno SAM/CONAMET 2008 – MEMAT 2008, Santiago de Chile, Chile, trabajo 83 1-8. 
Ryan G., Pandit A. Apatsidis D. P. 2006.  Fabrication methods of porous metals for use in orthopaedic applications. Biomaterials 27 2651-2670.  
 
 
 
 
 


English

Corrosion in 316L Porous Prostheses Obtained by Gelcasting

Centro de Servicios en Gestión de Información

 Procedia Materials Science  9 ( 2015 )  279 – 284 Available online at www.sciencedirect.com2211-8128 © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).Peer-review under responsibility of the Scientifi c Committee of SAM–CONAMET 2014 doi: 10.1016/j.mspro.2015.04.035 ScienceDirectInternational Congress of Science and Technology of Metallurgy and Materials, SAM – CONAMET 2014 Corrosion in 316L porous prostheses obtained by gelcasting Ricardo W. Gregoruttia,*, Cecilia I. Elsnerb, Liliana Garridoc, Andrés Ozolsd aLab. de Entrenamiento Multidisciplinario para la Investigación Tecnológica (LEMIT, CIC), Av. 52 e/121 y 122, B1900AYB La Plata, Argentina  bCentro de Inv. y Des. en Tecn. de Pinturas (CIDEPINT, CONICET), Av. 52 e/121 y 122 - Fac. de Ing. (UNLP), B1900AYB La Plata, Argentina cCentro de Tecnología de recursos Minerales y Cerámica Cno. Centenario y 506, (B1897ZCA), Gonnet, Argentina dInst. de Ing. Biomédica e Inst. de Tecn. y Cs. de la Ing., Fac. de Ing., Univ. de Bs. As., Av. Paseo Colón 850, C1063ACV CABA, Argentina Abstract Gelcasting (GC) process, usually used for ceramic moulding, is adapted for producing spongy or porous metal osteosynthesis components destined to bone void filling.  The main objective of the interconnected porosity is to improve the osteoconductive of metal matrix by ingrowth of bone. Further, porosity reduces metal density and Young module, which causes bone resorption, leading to implant failure, phenomenon known as stress shielding. The employed GC is based on the formulation of AISI 316L stainless steel powder suspension in an aqueous solution of organic polymers. This suspension is cast into porous ceramic shells, like those used in lost wax technique, wherein the polymer crosslinking is induced by heating. The shells, containing the resulting hydrogel–metal composite, are subjected to thermal cycle in order to dry, burn the organic phase, sinter the metal particles at 1200 °C, and cool down to room temperature under dry hydrogen permanent flow. The susceptibility to corrosion of 50-60 % porous pieces was analyzed. The results indicated that the lower relation between the open porosity and the total porosity, the lower the corrosion rate.  © 2015 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the Scientific Committee of SAM– CONAMET 2014.Keywords: Corrosion; porous prosthesis; gelcasting.  Keywords: Corrosion; porous prostheses; gelcasting      * Corresponding author. Tel. +(54)-221-483-1141; fax: +(54)-221-425-0471.  E-mail address: metalurgia@lemit.gov.ar   2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommon .org/l censes/by-nc-nd/4.0/).P er-review under responsibility of the Scientifi c Committee of SAM–CONAMET 2014 280   Ricardo W. Gregorutti et al. /  Procedia Materials Science  9 ( 2015 )  279 – 284 1. Introduction Stainless steel (ASTM F138, F139 or F745, AISI 316L grade steel), Co-Cr-Mo (ASTM F75, F562 or F799) and Ti-6Al-4V (ASTM F620) are used for load bearing applications in partial or total joint replacement, due to their satisfactory mechanical properties, corrosion resistance and biocompatibility. The latter property is related to biological response of human body, particularly referred to the ability of bone cell proliferation (osteoblasts) onto metallic surface, with minimal or any fibrous tissue (formed by fibroblasts), ensuring the implant mechanical fixation to bone. The metal surface topography may improve the fixation, by increasing roughness and open porosity. The degree of integration to bone seems to reach a maximum, when interconnected porosity is in 50 % to 80 % (in volume) range, and composed by 100 to 500 ?m pores. These features favor angiogenesis (the growth of blood vessels), and thus, the proliferation and colonization of osteoblasts within the pores. At the same time, porosity promotes the reduction of density and elastic modulus of mentioned alloys (between 110 and 210 GPa), tending to a bone-like magnitude (? 30 GPa). This allows a better transference of the mechanical load on the bone, avoiding the accumulation of stress onto the implant, phenomenon known as stress shielding. This process can occur when the initial implant fixation is deficient, leading to the relative displacement of metal-bone interface (in teens of microns), thus, triggering the osteopenia process, with the consequent loss or bone resorption, tissue inflammation and/or infections. This second phenomenon, known as aseptic loosening, leads to bone and metal debris generation promoted by wear. The final implications are the loss of the bone-implant interface, the implant failure and the need for revision surgery. The need to favor angiogenesis, osteointegration and avoid stress shielding, has driven the development of porous or cancellous structure implants, based on Ti, Ta or Ni-Ti alloys, when mechanical performance is the leading requirement. These are applied to fracture fixation, providing a scaffold for bone tissue regeneration in orthopedic and maxillofacial surgeries [Medlin et al. (2003), Ryan et al. (2006), Li et al. (2008), and Chen et al. (2009)]. Controlled porosity can be obtained by means of gelcasting (GC) technique, widely employed for molding ceramic components. Gelcasting is based on the formulation of slurry constituted by ceramic or metallic particle dispersion in water or organic solution of monomers, polymers or proteins. The slurry is cast into polymeric, metal or ceramic molds, wherein the crosslinking of polymers or proteins is induced by heating. The product is a composite of a hydrogel with metal particles homogeneously distributed, and with sufficient toughness to withstand further handling and sintering processes. This technique has been used for manufacturing Ti and stainless steel implants [Li et al. (2008a), Li et al. (2008b), Ozols et al. (2008), Ozols et al. (2009a), Ozols et al. (2009)b]. However, the corrosion resistance of these porous alloys may differ from that of full density, when they are implanted in physiological medium. Therefore, these materials must be evaluated to ensure their safety for long-term clinical applications. This paper refers to the corrosion resistance of porous AISI 316L implant obtained by gelcasting. This work is a continuation of previous studies, which have shown the feasibility of producing implants of various geometries for bone fixation, and assessing their osteointegration capability by in vivo tests [Ozols et al. (2009a)].   2. Experimental procedure Gelcasting suspension was formulated with 75 % (in wt.) of AISI 316L particles (smaller than 45 ?m) dispersion in 20 % (in wt.) and egg albumin devolved in warm distilled water. The slurry was cast in water permeable ceramic shells, similar to those used for investment casting process. These shells were wrapped in PVC films and heated in thermal water bath at 90 °C for 1 hour, in order to promote the protein gelation. The dried pieces were  heated at 600 ºC/h rate, up to the sintering of 1200 ºC, where remained for 2 hours, and cooled down to room temperature under constant flow of dry hydrogen flow into a AISI 316L gas heated muffle.  Total porosity (P) was calculated from the relative density (?rel), determined by the relationship between bulk density (?ap), measured by immersion water method, and the AISI 316 L theoretical densities of (7.96 g/cm3), according to the following relationship:  281 Ricardo W. Gregorutti et al. /  Procedia Materials Science  9 ( 2015 )  279 – 284 1001 Prel ???                                                                                                                                                             (1)  Open porosity (P0), defined as the ratio between open pore volume and specimen volume, was determined as follows:  0WG GPV??                                                                                                                                                             (2)  Being G the dry weight of the specimen, GW its water saturated weight, and V its apparent volume, including pore volume. Corrosion resistance was evaluated by means of electrochemical polarization tests in the potential range between -0.20 and 0.45 V/SCE, respect to the open circuit potential of the system, where SCE refers to saturated calomel electrode potential. The corrosion cell was assembled with the test material, as the working electrode (1 cm2 of area), SCE as reference and a Pt electrode as auxiliary. Tests were performed using aqueous solution of 0.9 % (in wt.) of NaCl, with pH between 7.0 and 7.4, kept at 37 ºC, to simulate the human body environment. The solution was deaerated by N2 bubbling during the tests. The samples were kept in a stabilization period of 1 hour at open circuit condition (Eoc). Later, the potential scans were performed at 0.167 mV/s in the anodic direction. 3. Results and discussion 3.1.      Porosity measurements Table 1 shows the density and porosity of three different samples extracted from the femoral stem obtained by gelcasting. Table 1. Analyzed AISI 316L sample porosity. Sample ?ap (g/cm3) ?rel (g/cm3) P  (%) P0 (%) P0 /P M1 3.07 0.38 62.0 60.4 0.95 M2 3.46 0.44 56.5 55.4 0.98 M3 2.96 0.37 62.8 50.9 0.81  M1 and M2 samples show a Po/P ratio close to 1.00, evidencing that most of the porosity is interconnected, with greater exposed surface area than in isolated porosity situation. In the case of M3 sample, its total porosity was similar to that of M1 sample, while the open porosity was significantly lower.  Fig.1 illustrates the analyzed sample pore irregular geometry and size dispersion.    Fig. 1. Porosity: (a) sample M1; (b) sample M2; (c) sample M3.  282   Ricardo W. Gregorutti et al. /  Procedia Materials Science  9 ( 2015 )  279 – 284 M1 and M2 samples, whose Po/P ratios were similar and close to 1.00, exhibited greater uniformity of pore size distribution, while sample M3 showed higher dispersion.  3.2. Corrosion behavior Corrosion behaviour of porous samples was compared with that of dense AISI 316L, the results are shown in Fig. 2. The analysis of the curves shows that dense AISI 316L exhibited a lower corrosion rate and a wider passive region (about 450 mV/SCE) than porous samples. The higher corrosion rate in these samples, at least by one order of magnitude, could be ascribed not only to the increased electrochemical activity of the material, but also to the higher exposed specific area, resulting from porosity. This was not taken into account in the calculation of the current density, determined as a function of the geometric area of the sample. Another feature is the different behaviour observed in anodic potential region. Sample M1 showed a large area (? 400 mV/SCE) at which the electrochemical activity of the material is lower, while sample M3 is active throughout the defined anodic sweep. Sample M2 shows an intermediate behaviour respect to M1 and M2 samples. The reasons for these behaviours have not yet been elucidated. Table 2 reports the electrochemical parameters obtained from the tests. From the results exposed in Tables 1 and 2, the variation of the corrosion rate with ?rel, ?ap and Po/P can be determined, as shown in Fig. 3.      Fig. 2. Curves obtained from the electrochemical tests.   283 Ricardo W. Gregorutti et al. /  Procedia Materials Science  9 ( 2015 )  279 – 284 Table 2. Results of electrochemical tests. Sample Ecorr  (V/SCE) jcorr  (A/cm2) M1 -0.460 1.3×10-5 M2 -0.267 3.0×10-5 M3 -0.143 3.7×10-6 Dense AISI 316L  -0.190 1.0.×0-7  The lowest corrosion rate was observed for sample M3, which reported smaller ?rel, ?ap and P0 /P values, and bigger total porosity. While corrosion rate decreased with those parameters, open porosity has the greatest influence, because of the lower exposed specific area. In general, jcorr variation is directly correlated with variation of these parameters, as also shown in Fig. 3.    Fig. 3. Variation of corrosion rate respect to ?rel, ?ap and P0/P. 284   Ricardo W. Gregorutti et al. /  Procedia Materials Science  9 ( 2015 )  279 – 284 4. Conclusion The results from the electrochemical tests indicated that corrosion rate is higher in porous AISI 316L respect to the dense material. Regarding the influence of the type of porosity, it could be established that there is a significant effect of open porosity and the total porosity on the corrosion rate. The smaller open porosity implies lower corrosion rate. This behaviour can be related to the lower exposed surface area produced when open porosity decreases.  These results highlight the need to find an optimal degree of porosity that allows reconciling corrosion resistance with stress shielding on the implant and its biocompatibility.  Acknowledgements The authors thank to Comisión de Investigaciones Científicas de la Provincia de Buenos Aires (CICPBA), Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), Facultad de Ingeniería UNLP y Facultad de Ingeniería UBA for the financial support to carry out the present research paper.  References Chen Y., Feng J., B., Zhu Y. P., Weng J., Wang J. X., Lu X.. 2009. Fabrication of porous titanium implants with biomechanical compatibility, Materials Letters 63 2659-2661. Li Y., Guo Z. M., Hao J. J.. Ren S. B., 2008a. Gelcasting of porous titanium implant.  Powder Metallurgy 51, 231- 236 Li Y., Z. Guo, J. Hao, S. Ren, 2008b. Porosity and mechanical properties of porous titanium fabricated by gelcasting Rare Metals 27, 282-.286.  Medlin D. J., Charlebois S., Swarts D., Shetty R., Poggie R. A, 2003. Proceedings of the ASM Materials & Processes for Medical Devices Conference. California, USA, 394-398.   Ozols A., Pagnola M., Faig J., Ferrari E., Tchaghayam M., Cevallos C., Rozemberg S. 2009a Production of Prosthetic Devices of AISI 316 L Steel by Gelcasting, Actas SAM/CONAMET 2009, Ed. J. Ovejero Garcia, Buenos Aires, Vol. III, 2035-2040.  Ozols A., Barreiro M. M., Ferrari E., Tchaghayam N., R. Gregorutti, J. Sarutti. 2009b. Investment Gelcasting of AISI 316l Steel for Prosthetic Moulding. Actas SAM/CONAMET 2009. Ed. J. Ovejero Garcia, Buenos Aires, Vol. III, 2041-2046. Ozols A., Kokubu G., Grana D. R., Rozenberg S., Barreiro M. 2008. Powder Metallurgy Moulding of 316 L SS by Gelcasting and Its Biocompatibility. Congreso Binacional Argentino Chileno SAM/CONAMET 2008 – MEMAT 2008, Santiago de Chile, Chile, trabajo 83 1-8. Ryan G., Pandit A. Apatsidis D. P. 2006.  Fabrication methods of porous metals for use in orthopaedic applications. Biomaterials 27 2651-2670.       

Corrosion in 316L porous prostheses obtained by gelcasting

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Corrosion in 316L porous prostheses obtained by gelcasting

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