Corrosion behavior of AISI 316 L borided and non-borided steels immersed in a simulated body fluid solution
Introduction
AISI 316 L stainless steel has been successfully applied in medicine due to its biocompatibility and relatively low cost [1]. Different surface engineering techniques can be performed to increase the corrosion resistance of the base metal, in particular when exposed to electrolytes simulating a body fluid [1], [2], [3], [4], [5], [6], [7], [8], which lead to diverse applications as biomaterial.
The powder-pack boriding is a surface thermochemical treatment, which improves the hardness, wear and corrosion resistance of diverse metallic materials. Further, because of its low cost and ease of processing, compared with other thermochemical treatments [9], [10], [11], [12], [13], [14], [15], [16], [17], boriding has been successfully applied to a wider sort of steels.
Boriding can enhance the corrosion resistance of ferrous materials if it leads to the formation of boride coatings on the surface component. Boride coatings passivate in non-oxiding dilute acids and alkaline media [15], [16]. For industrial applications, it is important to consider that the corrosion resistance of boride-coated steel components greatly depends on the amount of microcracking in the coating and the specific coating-solution conditions [18], [19], [20], [21].
Likewise, boride coatings corrode by either direct chemical or physical mechanisms. In the case of chemical attack of boride coatings, the composition of the environment may cause the coating to either become soluble or to be changed into soluble corrosion (chemical species) products. Thus, the importance to understand and to know the chemical species formed on the surface of boride coatings is related to the direct information of the oxidation/reduction behavior, and therefore, the corrosion and/or metal release mechanisms.
Particularly, a study of the corrosion resistance on AISI 316 L borided steel exposed to a simulated body fluid solution (SBFS) during 1 and 168 h was developed by Kayali et al. [2], using the potentiodynamic polarization technique. For AISI 316 L steel, the corrosion resistance was higher compared to the borided steel. This effect was attributed to the porosity of borided coatings and to the development of soluble complex salts over the surface of borided steel. Nevertheless, in the study of Kayali et al. [2], the chemical species formed during the corrosion tests on the material were not reported.
Moreover, during the work proposed in [2], the wear resistance of AISI 316 L borided and non-borided steels were estimated using the ball-on-disk system in dry and wet (SBFS) mediums. A decrease in the wear rates both in dry and wet mediums was observed in AISI 316 L borided steel when increasing boriding temperature. In the dry medium, the wear rates of borided steels were 30 times lower than that of AISI 316 L steel, while in the SBFS medium were 3 times lower.
In comparison with the work proposed in [2], in this study, the corrosion resistance of AISI 316 L borided and non-borided was estimated during 10 days of immersion in the Hanks' solution using the Electrochemical Impedance Spectroscopy (EIS) technique. The days of immersion in the Hanks' solution were selected to verify the behavior of the corrosion resistance of both non-borided and borided steels, the evolution and type of the chemical species developed on the surface, and corrosion mechanisms formed by the electrolyte-surface system by the aid of SEM, EDS and XPS techniques. In this case, similar days of immersion in a SBFS were proposed by different authors [2], [3], [4], [5], [6], [7], [8] to estimate the corrosion resistance of coatings formed on the surface of AISI 316 L steel. The corrosion results were analyzed by means of Nyquist and Bode plots in order to fit a suitable electrical equivalent circuit to estimate the electrochemical parameters for both, borided and non-borided samples immersed in the SBFS medium. Finally, from all the experimental evidence, electrochemical reactions are proposed to explain the behavior of corrosion resistance of the borided steel after 10 days in the Hanks' solution.
Section snippets
The powder-pack boriding process
Commercial samples of medical grade AISI 316 L steel, with 25.4 mm OD and 6.35 mm long were used in this study. The chemical composition of the steel is given in Table 1.
Prior to boriding, the specimens were ground sequentially using 100–2000 grit SiC papers then ultrasonically cleaned in isopropyl alcohol. The samples were packed in a stainless steel container (AISI 304) in contact with commercial Ekabor 2 powder mixture. During the boriding process, the container was transferred to a resistance
Microstructure of the boride coating
Examination on the cross section of AISI 316 borided steel revealed the presence of a relatively thick FeB/Fe2B coating as shown in Fig. 1. The SEM image showed a slightly darker gray coating (FeB), forming an outermost borided zone, which gives way to a second undercoating lighter in tone (Fe2B), where most of the existing pores seem to have concentrated. The whole compound coating was separated from the underlying alloy substrate probably due to a large cooling contraction. It is relevant to
Conclusions
Corrosion resistance of AISI 316 L borided and non-borided steels after 10 days in Hanks' solution was estimated by EIS. According to the different analyses on the borided steel after 10 days of immersion in the Hanks' solution, the corrosion resistance was affected, basically, by the presence and accumulation of both B2S3(s) and FePO4(s) electrochemical species in crystal cluster forms, which drastically changed the morphology and chemical composition over the boride coating. Furthermore, the
Acknowledgments
This work was supported by research grant 224248 of the National Council of Science and Technology, and research grant 20150039 of the National Polytechnic Institute in Mexico. The authors wish to thank the Nano Science and Micro and Nano Technologies Center of the National Polytechnic Institute for their cooperation. M.P-P. and M.R-R. would like to thank SNI for the distinction of their membership and the stipend received.
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