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Corrosion Characteristics of Medical-Grade AISI Type 316L Stainless Steel Surface After Electropolishing in a Magnetic Field
Originally Published in Corrosion, Volume 64, Number 8, Pages 660-665, August 2008
Ryszard Rokicki, Tadeusz Hryniewicz, K. Rokosz
(This is not a full version. Some photos, tables, and reference material may be absent)
Stainless steel materials are widely used for multiple applications because of their good mechanical properties and very good corrosion resistance in a number of environments. They are also used as biomaterials to manufacture conventional and novel medical devices. The stability of the surface oxide layer is one of the most important features of a biomaterial. Electrochemical polishing (EP) is the most extensively used surface technology for stainless steel. We have improved this surface technology by introducing a magnetic field. With a new process called magnetoelectropolishing (MEP) we can improve metal surface properties by making the stainless steel more resistant to halides encountered in a variety of body fluids, such as blood, saliva, urine, etc. Corrosion research results are presented on the behavior of the most commonly used material-medical-grade AISI Type 316L (UNS S31603) stainless steel-applied for human body implants, stents, and devices. Three basic environments have been adopted for the studies: pure distilled water, the Ringer's body fluid, and 3% sodium chloride (NaCl) aqueous solution (the most aggressive environment). The study results cover open-circuit potential and polarization curve characteristics of austenitic stainless steels (Types 304 [UNS S30400], 304L [UNS S30403], 316 IUNS S31600], and 316L), and a ferritic stainless steel (Type 430 [UNS S43000]). The comparison of the corrosion behavior of the stainless steels' surface after two electropolishing processes carried out in the absence (EP) and presence of the magnetic field (MEP) is reported. The experiments were carried out at room temperature (25°C). It has been found that the proposed MEP process shifts the corrosion potentials into the direction of greater corrosion resistance.
Stainless steel materials are widely used for multiple applications due to their good mechanical properties and very good corrosion resistance in a number of environments. They are also used as biomaterials to manufacture conventional and novel medical devices. The stability of surface oxide layer is one of the most important features of a biomaterial used for that kind of applications. The electrochemical polishing (EP) is the most extensively used surface technology for stainless steel materials known for years. We have developed this surface technology by introducing a magnetic field for that treatment. With the new process called the magnetoelectropolishing (MEP) we can improve metal surface properties by making the stainless steel more resistant for halides encountered in a variety of body fluids, such as blood, saliva, urine, etc. In the paper, the corrosion research results are presented concerning the behaviour of the most commonly used material that is medical grade AISI 316L stainless steel, applied for the human body implants, stents, and devices. Three basic environments have been adopted for the studies, changing from pure distilled water, through the Ringer's body fluid, and 3% NaCl aqueous solution, known as the most aggressive environment. The study results cover open circuit potential, and polarization curves characteristics of austenitic stainless steels (304, 304L, 316, 316L), and a ferritic stainless steel (430). The comparison of the corrosion behaviour of the stainless steels surface after two electropolishing processes carried out: (a) in the absence (EP), and (b) in the presence of the magnetic field is reported. It has been found that the proposed magnetoelectropolishing (MEP) process shifts the corrosion potentials into the direction of more corrosion resistant materials without changing its bulk composition.
Specialty stainless steel alloy 316L and its medical grade is used extensively in pharmaceutical, semiconductors and body implants due to its superior corrosion resistance, smoothness, biocompatibility and cleanability after electropolishing treatment. The remarkable improvement in corrosion resistance of electropolished surfaces of austenitic stainless steels are caused by several interconnected events occurring during the process.
The first of these is the removal of the Beilby layer that consists of inclusions of martensitic phase, foreign material, preexisting oxides, etc, created by forming, machining and mechanical polishing. The second is to create a new corrosion resistant layer that is enriched in chromium oxide due to the anomalous co-dissolution of austenitic steels. The third is to improve the surface smoothness by dissolving the surface peaks preferentially to the surface depressions. The fourth event is the eqipotentializing of grain boundaries on metallic materials.
Different surface treatments are commonly performed on medical implant materials to promote corrosion resistance and biocompatibility. For many years now, electropolishing (EP) used to be recognized as one of the available surface treatments to smooth the surface and to perform the surface passivation on biomaterials. For a given material, the oxide properties are a function of the EP parameters such as applied current density, voltage, temperature, and the composition and concentration of the chemicals used. A stable oxide layer on the passivated metal surface will promote its corrosion resistance and biocompatibility of the implant material in physiological conditions. This passivity could be enhanced by modifying the thickness, morphology, or chemical composition of the surface oxide layer by different treatments. The use of externally applied magnetic field to the EP process provides the treated surface with some new properties and better characteristics, concerning microroughness, hydrophilicity, corrosion resistance, and oxide film morphology. The higher level of metal finishing seems to be very interesting for multiple applications, such as e.g. medical stents and implant devices. The addition of the external magnetic field also significantly minimizes microtopography by lowering microroughness and minimizing actual surface area in micro and nano scales of the various metallic materials.
The process of electropolishing becomes even more complex if the magnetic field is introduced to the system. With the new electropolishing system, an externally applied magnetic force may enhance, or retard, the dissolution process. The electropolishing process is maintained under oxygen evolution to achieve an electropolished surface of the workpiece exhibiting reduced microroughness, better surface wetting and increased surface energy, reduced and more uniform corrosion resistance, minimization of external surface soiling and improved cleanability in shorter time periods. In the paper, the enhanced corrosion resistance of 316L stainless steel is proposed with a new advanced treatment method.
Experimental Procedure 2.1. Material for samples Volume 10, Preprint 45 submitted 3 August 2007. For the experiment, the medical grade AISI 316L stainless steel (Table 1) was used for the investigation. Three sets of AISI 316L stainless steel samples cut of the same sheet-metal of dimensions 30x40x2 mm have been used for the study of corrosion behaviour. The first 3 samples were treated by a standard electropolishing (EP), the second set of 3 samples were electropolished with the same electrolyte composition in the presence of a magnetic field (MEP)  ], and the third set of 3 samples were polished with an abrasive grit paper up to No. 800 (MP - abrasive polishing). Chemical composition of AISI 316L SS vm used for the experiments
2.2. Samples preparation Mechanical abrasive polishing MP of samples was used as a reference. For both electrochemical polishing processes, standard electropolishing EP and magnetoelectropolishing MEP, the same type of a proprietary electrolyte was applied, being a mixture of sulphuric and orthophosphoric acids. During magnetoelectropolishing MEP the bath was unstirred and temperature was kept within 66-68 °C. Prior to corrosion studies the samples were thoroughly degreased in acetone. 2.3. Corrosion measurements Corrosion studies after mechanical abrasive polishing (MP), and two different modes of electropolishing, in the absence (EP), and in the presence of a magnetic field (MEP), were carried out in three solutions: (1) pure distilled water, (2) a typical Ringer's physiological fluid, and (3) in an aqueous 3% NaCl solution, all at the same room temperature of 25 °C. The electrochemical system used for the corrosion measurements consisted of the potentiostat ATLAS 98 with the software IMP98, current platinum electrode Ept-101, and the saturated calomel electrode EK-101P used as a reference. Corrosion investigations were performed to obtain: (1) open circuit potential OCP (2) electrochemical impedance spectroscopy EIS, and (3) potentiodynamic polarisation curves. The corrosion measurements data were obtained in the range of potentials from -1000 mV to 1500 mV recorded every 5 mV with the rate of 1 mV/s. Electrochemical impedance spectroscopy results were obtained each time after holding the samples at open circuit potential for 60 minutes. Afterwards the polarisation curves were investigated, and having them the corrosion rates were calculated. ? 2007 University of Manchester and the authors.
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