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LOCALIZED CORROSION STUDY OF 316L BIO-IMPLANT IN SIMULATED BODY FLUIDS.

Byline: H. Khurshid, K.M. Deen - Kashif_mairaj83@yahoo.com, S. L. Ahmad and R. Ahmad

ABSTRACT: This study was to investigate the behavior of corrosion resistance of 316L austenitic stainless steel bio-implant. The localized corrosion behavior of 316L in simulated body fluids (ringer, ringer + 0.1% blood serum, Ringer + 0.4% blood serum and 1% lactic acid solution) was investigated by electrochemical cyclic polarization test method. It was deduced by electrochemical test results that pitting resistance of 316L SS increased with the addition of blood serum in ringer solution. Formation of meta-stable pits was evident from polarization curves with the addition of blood serum. The addition of lactic acid further reduced the stable pitting tendency of 316L SS.

Key words: Corrosion, Stainless steel, Electrochemical, Pitting

INTRODUCTION

Stainless steel high resistance to corrosion derives from a dense film of chromium oxide (passive film) that naturally forms on the iron surface. Generally first requirement of any material serving in biological system is that it should be inert and not causing any undesirable reaction with its surrounding. When stainless steel is placed inside a tissue, the interaction between the implant and tissue determines the degree of biocompatibility. It implies to the ability of the material to perform effectively with an appropriate host response for the desired applications.[1] Common medical devices include hip replacements, prosthetic heart valves, neurological prostheses and drug delivery systems. [2]

Corrosion as an electrochemical process commences on the surface of stainless steel implants and subsequently affects the body response. This is undesirably followed by release of ions such as chromium and nickel in surrounding tissue which can negatively affect the response of host body to biomaterial. The literature has proven that nature concentration of ions Cr+3, Fe+2, Ni+1 inside the body may severely prevent the tissue functionality and can ultimately lead to severe health related consequences , therefore corrosion of material is the first issue to be considered when the material is designed as a bio implant.

It is also necessary to establish an understanding of biological factors effecting corrosion. There are various in vivo and in vitro studies on corrosion of metal implants which employed simulated body fluids. [3]

EXPERIMENTAL WORK

The composition of AISI 316L Austenitic stainless steel was determine by PMI (Positive Material identification) test method. The %age composition is tabulated in table 1. Standard method of metallography was employed to establish a relationship between microstructure and electrochemical behavior.

The specimen of dimension (2" x 2") was wire cut for metallography. Each specimen was ground with silicon carbide grit papers of 120, 320, 400, 500, 600, 1000, 1200 and 1500 grit sizes sequentially. Then electrolytic polishing was performed in 50mL perchloric acid,750 mL ethanol and 140mL H2O and etched electroletically in 10% oxalic acid at 6V for 60 seconds. The specimens were then examined by metallurgical microscope before and after etching to study the microstructural aspects.

Table 1: Result of the PMI of the AISI Base metal

Element###Composition (%)

C (Carbon)###0.025

Cr (chromium)###16.74

Ni (Nickel)###9.87

Mo (molybdenum)###2.28

Fe (Iron)###70.29

Mn (Mangenese)###7.44

Cu (Copper)###0.29

P (phosphorus)###0.065

S (sulphur)###0.062

Corrosion behavior of Austenitic Stainless steel AISI 316L may be influenced by surface preparation. The primary objective of surface preparation for electrochemical analysis was to produce surface similar to commercial surface. Samples of dimensions (2" x2") were mounted and then sequentially ground up to 600 silicon carbide grit paper. The corrosion analysis of 316L was carried out using DC electrochemical cyclic polarization method in four simulated body fluids such as ringer solution (RS), ringer +0.1% blood serum solution (RS1B), ringer + 0.4% blood serum solution (RS4B) and 1% lactic acid (1LA). The specific composition of the simulated test ringer solution is provided in table 2.

Table 2: composition of Ringer's solution

Chemicals###Quantity (g/L)

NaCl###18

CaCl2###0.48

KCl###0.86

NaHCO3###0.4

The open circuit potential was determined in the intial delay of 10 seconds. Acquistion of electrochemical data was obtained afterwards. The input values to the polarization scan are give in table 3.

RESULTS AND DISCUSSION

The microstructural image obtained by standard metallography practice is given in figure 1. Microstructural investigation revealed typical homogenized austenitic structure which consists of equiaxed austenitic grains

Table 3: Parameters for Cyclic Polarization potential.

Input Parameters###Values

Initial potential###-500mV vs. SCE

Apex potential###1000mV vs. SCE

Final potential###-500mV vs. SCE

Forward scan rate###2.5 mV/s

Reverse scan rate###2.5 mV/s

Apex current density###10 mA/cm2

Sample period###1 s

Sample area###1.44 cm2

Density###7.87 g.cm2 (SS316L)

Eq.wt###26.58 (SS 316L)

structure and twin boundaries. The globular intermetallic sigma phase was also observed at few locations within the austenitic grains. Delta ferrite presence in the microstructure may decrease the pitting potential and hence reduces pitting resistance. [4]

Cyclic polarization tests were conducted to investigate localized corrosion susceptibility of 316L SS samples towards simulated body fluids (SBFs). The curve was analyzed in terms of the pitting resistance and pitting protection potential. The polarization scan in SBFs are shown in fgure 2. It was observed that corrosion potential (Ecorr) moved to noble direction from -212.0mV in RS to -71.82mV when serum concentration was increased to 0.4% in RS as tabulated in table 1. The potential in 1LA was +81.35mV which indicated higher corrsion resistance of 316L in lactic acid than in ringer solution. Bursts of anodic current near pitiing potential was observed in RS1B and RS4B. It was evaluated by the addition of blood serum in RS reduces the induction time of satable pit formation and enhances the tendancy of metastable pitting but increased the resistance of stable pit formation with slight increase in pitting current (11.26uA).

The magnitude of pitting current was higher in RS which further decreased by the addition of blood serum as shown in figure 3. This validated that increased in concentartion of nonhalide ions can affect the pitting tendancy and induction time for stable pit formation and number of pits. [5] The addition of lactic acid reduces the tendancy of pitting in RS which is apperent in figure 4. The negative loop of cyclic polarization cleary indicated the fact. The protection tendancy was also inhanced by decrease in diffrential protection

Table 4: Output data of Cyclic Polarization

Solutions###Ecorr(mV)###Eprot(mV)###Epit(mV)###Pitting###Protection

###resistance###tendency

###Epit - Ecorr Eprot - Ecorr

RS###-212.0###423.4###-153.2###58.8###635.4

RS1B###-154.2###269.5###-92.32###61.88###423.7

RS4B###-71.82###369.3###151###222.82###441.12

1LA###+81.35###-###-###-###-

CONCLUSION

Ringer solution showed lower pitting resistance without the addition of blood serum. Increased blood serum concentration in ringer solution increases the pitting resistance and reduces the differential protection potential.

Future developments

Surface treatments and modification would be the next step in this research work to enhance biocompatibility in simulated body fluids.

REFERENCES

1. Yang Leng,Donglu Shi,"Introduction to biomaterials" Tinsinghua University Press, 2006,pg 601-637

2. U Kamachi Mudali, T M Sridhar and Baldev Raj, "Corrosion of bioplants", Sadhana Vol.28, Parts 3 and 4, June/August 2003, Pg 601-637

3. Omanonic S,Roscoe SG."Electrochemical studies of the absorbtion behavior of bovine serum albumin on stainless steel", Langmuir 1999;15(23); pg 8315-21

4. H.S. Khatak, Baldev Raj, "Corrosion of austenitic stainless steels, Mechanisims, Mitigation and Monitoring", Narosa Publishing House, 2002, pg. 80-83

5. K.M. Deen, M.A. Virk, C.I. Haque, R. Ahmad and I.H. Khan," Failure investigation of heat exchanger plates due to pitting corrosion",J. Engineeering Failure Analysis, Volume 17, Issue 4, June 2010, Pages 886-893

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Publication:Science International
Article Type:Report
Geographic Code:9PAKI
Date:Dec 31, 2011
Words:1322
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