22 INŻYNIERIA MATERIAŁOWA MATERIALS ENGINEERING ROK XXXIX

Corrosion resistance of sintered AISI 316L–hydroxyapatite biomaterials in Ringer’s solution

Aneta Szewczyk–NykielCracow University of Technology, Institute of Material Engineering, Cracow; *aneta.szewczyk-nykiel@mech.pk.edu.pl

AISI 316L–hydroxyapatite biomaterials were produced by the conventional powder metallurgy technology. In the case of materials such as these, proper and long-term functioning in the aggressive environment of body fluids is very important. Therefore, the purpose of this study was to determine the effect of hydroxyapatite content and sintering temperature on the properties including sintered density, open porosity, and in particular corrosion resistance of AISI 316L–hydroxyapatite biomaterials in Ringer’s solution. The measurement of sintered density and open porosity of studied materials was carried out by the water-displacement method. The corrosion behaviour was studied by open circuit potential measurement and potentiodynamic polarization method. It was stated that the properties of studied biomaterials are dependent on chemical composition of powders mixture and sintering temperature. The results showed that higher sintering temperature ensured to obtain lower values of corrosion current density and corrosion rate, and higher value of polarization resistance. The addition of 5 wt % hydroxyapatite provided to a significant improvement of corrosion resistance in Ringer’s solution in comparison to AISI 316L steel, while a slight decrease in corrosion resistance was observed for AISI 316L–10 wt % hydroxyapatite biomaterials. Passivation ability and better corrosion resistance indicate that sintered at 1240°C AISI 316L–5 wt % hydrohyapatite biomaterials is more appropriate for long-term functioning implants than AISI 316L steel. This biomaterial possessed good densification and the best corrosion resistance among all studied materials, as evidenced by the lowest corrosion current density and corrosion rate combined with the highest polarization resistance.

Key words: biomaterials, AISI 316L, hydroxyapatite, corrosion resistance, powder metallurgy.

Inżynieria Materiałowa 1 (221) (2018) 22÷28DOI 10.15199/28.2018.1.4

MATERIALS ENGINEERING

1. INTRODUCTION

The lengthening of human lifetime, diseases of old age, increase in the number of accidents increase the demand in implants and significant progress in the medicine and bioengineering of materi-als results in an enlargement of the biomaterials application area in medicine [1].

Many years of clinical experiences have shown that biomaterials must meet quite high and varied requirements including biocompat-ibility, but also excellent mechanical properties, corrosion resist-ance, wear resistance and suitable technological properties [1÷17]. Generally, surgical and also dental implants are made of metals, especially austenitic stainless steels and titanium alloys [1÷29]. Austenitic stainless steels were the earliest adapted for implanta-tion in the human body. They exhibit high mechanical properties, relatively good corrosion resistance and at the same time a low pro-duction cost and availability. However, austenitic stainless steels are particularly susceptible to destruction due to the rather high ten-dency to naturally self-passivate (e.g. in comparison to titanium al-loys) and a strong susceptibility to electrochemical corrosion in the environment of body fluids [1, 5, 10, 11, 14÷18, 23, 30]. Moreover, the products of corrosion get to the surrounding tissues and may cause the occurrence of allergic reactions, inflammations or metal-losis [3].

Ceramic biomaterials have been arousing the special interest among scientists for a long time. It can be stated that hydroxyapatite (HA) is one of the most investigated bioceramics [2, 31÷38]. Be-sides mineral origin hydroxyapatite occurring for example in igne-ous rocks, metamorphic limestone or phosphate sedimentary rocks, there is also a natural hydroxyapatite (mainly in the bones and teeth of vertebrates) and a synthetic hydroxyapatite. The synthetic hy-droxyapatite does not exhibit complete biocompatibility with the human bone and teeth and also it is relatively expensive material. That is why during past years there have been many attempts to ob-tain the natural hydroxyapatite. The natural hydroxyapatite has the greatest biocompatibility with the tissues of the human body and it

is much cheaper in comparison to synthetic HA. Currently, natural hydroxyapatite is prepared from different natural materials, which are the waste eg. in animal husbandry. The examples of such natural materials are: bones of animal (eg. cattle, goats, pigs, sheep), egg-shells, and coral skeletons or even human teeth [31, 33÷38].

It is known that hydroxyapatite exhibits excellent biocom-pactibility and osteoconductive properties due to similarity of its chemical and mineralogical composition as well as crystallographic structure to the mineral constituents of human bones and teeth [2]. However, hydroxyapatite has low mechanical properties, such as toughness and yield strength. In particular, it should be pointed out low fracture toughness. The value of KIC coefficient of hydroxyapa-tite ceramic is in the range of 1.1÷1.2 MN∙m–1.5 (in comparison to KIC = 2÷12 MN∙m–1.5 for bone) [23]. The low mechanical proper-ties (brittleness and low strength) of ceramic hydroxyapatite im-pedes its medical applications as long term load-bearing implants [14÷18, 23]. Despite mentioned above weaknesses, hydroxyapatite has been widely used in medicine, especially in dentistry, maxil-lofacial surgery, orthopedics, otolaryngology and plastic surgery [2, 18, 23, 31÷38]. There are several clinically used forms of HA, i.e. powders (or other particulate forms), solid or porous ceramic blocks and as coatings for metallic prostheses in order to improve their biological properties [2]. Hydroxyapatite can be also an effective reinforcing agent for a metal matrix composite. The combination of high strength and toughness of metals along with very good bio-compatibility and corrosion resistance in an environment of body fluids of hydroxyapatite can results in a bioactive material with im-proved corrosion resistance, but also tribological properties [3÷5, 9÷30, 36÷38].

Based on the literature review [3, 4, 9÷25], it can be stated that the proportions of 316L and hydroxyapatite, the manufacture pro-cess influence the properties of composites and enable the design of materials characteristics desirable for use in medicine.

It was found [24] that in the case of 316L–(20, 30%) HA compos-ites HP–HT method led to obtaining a lower corrosion resistance in Ringer’s solution in comparison to conventional powder metallurgy

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techniques what has been confirmed by the higher value of polari-zation resistance and corrosion potential and lower corrosion cur-rent density. Regardless of the method of producing 316L–HA com-posites had greater corrosion resistance than 316L steel. The highest corrosion resistance (Rpol = 57.4 kΩ∙cm2, icorr = 0.0005 mA∙cm–2) was obtained for 316–20% hydroxyapatite composite [11].

The corrosion behaviour of sintered 316L SS–(5, 20 and 50 wt %) HA biocomposites in Ringer’s solution was investigated using electrochemical techniques. These materials were fabricated by mechanical alloying technique, isostatic pressing (200 MPa) and sintering (1200°C, 2 h, vacuum). It was stated that these materials exhibit high corrosion resistant in Ringer’s solution with corrosion current density in the order of 10–6 A∙cm–2. The corrosion resistance decreases with increasing HA content [9].

Also, nickel-free austenitic stainless steel–(0, 5, 10 and 15 wt %) hydroxyapatite biocomposites (produced by mechanical alloying and nitrogen absorption treatment) are passive in Ringer’s solution [7]. The addition of HA caused considerable decrease in the corro-sion current density and the shift of the corrosion potential to a more negative value. It was pointed out that corrosion resistance of these composites increased with increasing HA content up to 10%. But further increase of HA content led to decrease of corrosion resist-ance. The calculated values of polarization resistance were 14 385 and 28 143 Ω∙cm2 for biocomposites with 5% and 10% of HA, re-spectively. For comparison, the value of Rpol for nickel-free austen-itic stainless steel was 3747 Ω∙cm2.

The previous studies [14÷17] related to the influence of hy-droxyapatite in the range of 0÷15 wt % on sintering behaviour, microstructure and selected properties of sintered AISI 316L–HA biomaterials. The aim of this study was to determine the effect of hydroxyapatite content and sintering temperature on the properties, in particular corrosion resistance of sintered AISI 316L–HA bio-composites in Ringer’s solution.

2. MATERIALS AND METHODS

In this study the following materials were used: – water atomized powder of AISI 316L austenitic stainless steel

(produced by Höganäs) with particles size <150 µm. The chemi-cal composition of AISI 316L is shown in Table 1;

– natural origin hydroxyapatite powder obtained by extracting the cortical part of the long bone of the pig. Procedure of preparation of the natural hydroxyapatite involves the following steps: cook-ing bone in distilled water, the mechanical removal of tissues and parts of the spongy bone residue, leaching of organic matter by 4 molar sodium hydroxide solution, rinsing in distilled water until a constant pH, drying at 120°C to constant weight, mill-ing [14÷17, 31, 33, 35, 36]. The results of quantitative chemical analysis shown that hydroxyapatite powder contained: 17.45% P and 39.51% Ca. The value of Ca/P ratio was 1.75.Described above powders of AISI 316L steel and hydroxyapatite

were used to prepare the following mixtures: – AISI 316L–5 wt % HA, – AISI 316L–10 wt % HA.

In addition, pure powder of AISI 316L steel was used to com-pare results.

In the present study cylindrical samples with a diameter of 20 mm and a height of 5 mm were used. The preparation procedure of these samples included: preparation of powders mixtures, press-ing and sintering. The mixing process was conducted in Turbula mixer for 120 minutes. The uniaxial pressing in a rigid matrix was performed at a pressure of 600 MPa. The sintering process took place in Nabertherm P330 (a laboratory tube furnace) in an atmos-phere of dry (dew point below –60°C) and high purity (99.9992%) hydrogen. Sintering was performed at two temperatures: 1180°C and 1240°C. The time of isothermal sintering was 60 minutes. The heating rate to the sintering temperature as well as cooling rate to room temperature was 10°C/min.

The measurement of sintered density and open porosity was car-ried out by the water-displacement method according to the require-ments of PN-EN ISO 2738: 2001.

Corrosion studies was performed using Atlas EU & IA 0531 po-tentiostat with installed Atlascorr05 software. The corrosion resist-ance of investigated materials was evaluated in Ringer’s solution at a temperature of 37±1°C. The qualitative and quantitative chemical composition of Ringer’s solution is presented in Table 2.

A standard three-electrode system consisting of a working elec-trode, a reference electrode and a counter electrode was used for the electrochemical study. The reference electrode was a saturated calomel electrode (SCE). As the counter electrode was used a plati-num electrode. The working electrode was the cylindrical sample of sintered AISI 316L–hydroxyapatite composites as well as AISI 316L steel. The exposed area of samples was 1.33 cm2. The prepa-ration of samples surface included the following steps, grinding up to a 2000 grit, cleaning in ethyl alcohol in an ultrasonic cleaner and compressed-air drying.

Open circuit potential (OCP) measurement and potentiody-namic polarization method were performed to evaluate corrosion behaviour of investigated materials. The open circuit potential was recorded versus time. A period of immersion in Ringer’s solution was 1 hour. When the potential reached a stable value, the poten-tiodynamic polarization measurement was started by increasing the potential. The potential was increased from –0.8 V up to 0.5 V. The scan rate was 1 mV∙s–1. The potentiodynamic polarization corro-sion test allows to determine parameters such as corrosion poten-tial (Ecorr),corrosion current density (icorr), polarization resistance (Rpol), breakdown potential (Eb) and passive current density (ipass). The polarization resistance was evaluated using linear polarization method (called Stern method) and Tafel extrapolate method (called Stern-Geary method). Based on the ASTM G 102-89:2004, corro-sion rate in terms of penetration rate (CR) and mass loss rate (MR) were calculated.

Metallographic cross-sections were prepared. The microstruc-tural study of the biomaterials was done with scanning electron mi-croscopy. An EDS analysis was also performed. It allows identifica-tion of elements constituting the studied material and points out the differences in chemical composition of observed microstructural constituents. A spectrum of EDS presents a dependence of the num-ber of counts as a function of the radiation energy.

3. RESULTS AND DISCUSSION

In the case of sintered materials which are produced using conven-tional powder metallurgy technology the corrosion resistance de-pends on, among other things, porosity. It is known the porosity depends on lots factors like pressing pressure, sintering conditions (mostly temperature and time), as well as size and shape of powder particles. The influence of porosity is associated with increase the

Table 1. Chemical composition of AISI 316L steel, wt % Tabela 1. Skład chemiczny stali AISI 316L, % mas.

Cr Ni Mo Si Mn C Fe

16.8 12.3 2.2 0.8 0.12 0.02 balance

Table 2. Composition of Ringer’s solutionTabela 2. Skład roztworu Ringera

Substance NaCl KCl CaCl2·H2O

Content, g∙dm–3 8.6 0.3 0.33

Ions Na+ K+ Ca2+ Cl–

Concentration, mmol∙l–1 147.2 4.0 2.25 155.7

24 INŻYNIERIA MATERIAŁOWA MATERIALS ENGINEERING ROK XXXIX

active surface exposed to corrosion. It is estimated that real surface is about two orders of magnitude higher than the apparent surface. Interconnected porosity greatly favours the formation of corrosion cells and then it promotes the onset and development of pits. Fur-thermore, it should be mentioned about a lack of passivation within the pores of a sintered materials. Therefore, it can be stated that corrosion resistance of sintered materials can be significantly im-proved through increasing density.

Figure 1 presents the values of sintered density of the investi-gated materials depending on the sintering temperature and amount of hydroxyapatite introduced into AISI 316L steel. The values of sintered density determined in accordance with the rule of mixtures (square markers) in comparison with the measured density values of studied AISI 316L–HA composites with different weight per-cent of hydroxyapatite are also presented in Figure 1. The rule of mixtures is commonly known because of its usefulness to predict various properties of composites. It allows to determine theoretical values of properties such as density, elastic modulus, ultimate ten-sile strength, thermal conductivity, and electrical conductivity. That is why in this study the rule of mixtures was used to evaluate the sintered density of studied composites.

Figure 2 shows the values of open porosity of AISI 316L steel and AISI 316L–hydroxyapatite composites depending on the sinter-ing temperature.

Analysis of presented results (Fig. 1 and 2) lead to the conclu-sion that both the sintering temperature and chemical composition of the powders mixture affects the physical properties of the stud-ied materials. The increase in sintering temperature ranging from 1180°C to 1240°C results in an increase in sintered density and si-multaneously decrease in open porosity of all investigated materi-als, wherein the lowest differences can be observed in the case of sintered austenitic steel. As expected the sintered density decreases with increasing hydroxyapatite content. This dependence may be noted for both sintering temperatures. It is in accordance with the rule of mixtures. Namely, the introduction of reinforcement hav-ing lower density to a matrix material with higher density leads to decrease density of composite. However, the values of the theoreti-cal density are higher than the measured values and the difference between them increases with increasing hydroxyapatite content. Only in the case of AISI 316L–5 wt % HA composite (sintered at 1240°C), the values of theoretical and experimental densities are almost the same. As seen in Figure 2, the open porosity increases with increasing hydroxyapatite content. Nevertheless, the sintering process at a higher temperature results in the same values of open porosity of AISI 316L steel and AISI 316L–5 wt % HA composite.

Figure 3 presents the variation of open circuit potential with time for sintered AISI 316L steel and AISI 316L–HA composites. Gener-ally, the open circuit potential determines the thermodynamically

Fig. 1. Effect of sintering temperature on the sintered density of AISI 316L stainless steel and AISI 316L–HA compositesRys. 1. Wpływ temperatury spiekania na gęstość stali AISI 316L i kom-pozytów AISI 316L–HA

Fig. 3. Variation of open circuit potential with time for sintered AISI 316L stainless steel and AISI 316L–HA composites in Ringer’s solutionRys. 3. Zmiana OCP w czasie dla spiekanej stali AISI 316L i kompozytów AISI 316L–HA w roztworze Ringera

Fig. 2. Effect of sintering temperature on the open porosity of sintered AISI 316L stainless steel and AISI 316L–HA compositesRys. 2. Wpływ temperatury spiekania na porowatość otwartą spiekanej stali AISI 316L i kompozytów AISI 316L–HA

NR 1/2018 INŻYNIERIA MATERIAŁOWA MATERIALS ENGINEERING 25

tendency of a material to electrochemical corrosion reactions with a corrosive medium.

As seen in Figure 3, in the initial period the OCPs of AISI 316L steel and AISI 316L–HA composites increase gradually and then reaches stable values before 1 hour immersion in Ringer’s solution. The values of initial and final OCPs for all materials are equal the lowest and the highest potentials values respectively. It can be ob-served that the higher sintering temperature, the higher OCP value. The shift of the OCP in the positive values direction suggesting the formation of passive layer on the sample surface. Stable potential value indicates thermodynamic stability of the formed layer and its resistance to chemical degradation in Ringer solution. Such behav-iour increases the corrosion resistance of material. Therefore, ma-terials sintered at 1240°C possess higher corrosion resistance. This improvement is supported by a noble shift of open circuit potential.

It was observed for materials sintered at lower temperature that the OCPs values (measured after 1 h immersion in Ringer’s solu-tion) shifted in the negative direction with increasing hydroxyapa-tite content. It was found the same dependence for austenitic stain-less steel–HA composites in Ringer’s solution [23] what means that hydroxyapatite addition turned the composites less noble. It can be clearly seen that the same tendency was not found for studied mate-rials sintered at 1240°C because AISI 316L–5 wt % HA composite possessed higher OCP value compared to AISI 316L steel.

The polarization curves of AISI 316L stainless steel and AISI 316L–HA composites in Ringer’s solution are presented in Figure 4. The shape of presented polarization curves (cathodic and anodic branches) is similar for all investigated materials. Moreover, in the anodic branch the current density is lower than that in cathodic branch. It was found that for AISI 316L–5 wt % HA composite sintered at 1240°C, cathodic and anodic current densities indicate the lowest values in the definite measuring range. The cathodic branches are related to hydrogen evolution. As regards the anod-ic branch, it is observed a „plateau” corresponding to the passive region. A strongly increase in anodic current density follows after reaching breakthrough potential. This is due to the formation and development of pits. Shift of breakdown potential in the direction of higher values and the broader range of passivation results in greater corrosion resistance in Ringer’s solution for AISI 316L–5 wt % HA composite sintered at 1240°C.

Corrosion current density, corrosion potential, polarization re-sistance, breakdown potential and passive current corrosion were determined and the values of these parameters are shown in Table 3 for all studied materials. Table 4 presents calculated values of cor-rosion rate in the form of CR and MR parameters.

Based on analysis of data contained in Tables 3 and 4, it can be concluded that the values of corrosion parameters such as: the corrosion potential, the corrosion current density, polarization re-sistance and corrosion rates depend on material composition as well as the sintering temperature. For all studied materials, higher temperature of sintering process ensured to obtain lower values of corrosion current density and corrosion rate. The lowest values of icorr, CR and MR parameters were obtained in the case of AISI 316L–5 wt % HA composite, while AISI 316L–10 wt % HA com-posite exhibited the highest values of these parameters. A similar dependency can also be observed in the case of polarization resist-ance. It means, the higher the sintering temperature, the higher the value of Rpol. Moreover, AISI 316L–5 wt % HA composite reached the highest value of polarization resistance, while the lowest Rpol value was found for AISI 316L–10 wt % HA. AISI 316L–5 wt % HA composite also possessed the highest Ecorr value and the higher sintering temperature shifted the corrosion potential in the direction of positive values. Likewise, the breakdown potential shifted in the positive direction with increasing sintering temperature. It indicates a lower susceptibility to pitting. The most positive value of Eb was found for AISI 316L–5 wt % HA composite.

Based on the results of potentiodynamic polarization method it can be stated that sintered at 1240°C temperature AISI 316L–5 wt % HA composite reached the highest corrosion resistance in Ringer’s solution. It was confirmed by the highest value of polarization

Fig. 4. Potentiodynamic polarization curves of sintered AISI 316L stainless steel and AISI 316L–HA composites in Ringer’s solutionRys. 4. Potencjodynamiczne krzywe polaryzacji stali AISI 316L i kompo-zytów AISI 316L–HA w roztworze Ringera

Table 3. The values of corrosion potential (Ecorr), corrosion current density (icorr), polarization resistance (Rpol), breakdown potential (Eb) and passive current corrosion (ipass) for sintered AISI 316L stainless steel and AISI 316L–HA compositesTabela 3. Wartości potencjału korozyjnego (Ecorr), gęstości prądu korozji (icorr), oporu polaryzacji (Rpol), potencjału przebicia (Eb) i gęstości prądu pasywacji (ipass) dla stali AISI 316L i kompozytów AISI 316L–HA

Sintering temperature

°C

Ecorr V

icorr A∙cm–2

Rpol1)

W∙cm2Rpol

2)

W∙cm2Eb V

ipass A∙cm–2

AISI 316L

1180 –0.380 3.18∙10–5 1640 2423 0.002 1.54∙10–4

1240 –0.441 1.67∙10–5 2873 4364 0.048 7.60∙10–5

AISI 316L–5 wt % HA

1180 –0.348 1.97∙10–6 23310 26649 0.127 1.42∙10–5

1240 -0.217 9.71∙10–7 56818 69681 0.296 1.29∙10–5

AISI 316L–10 wt % HA

1180 –0.565 7.48∙10–5 646 897 0.077 7.69∙10–5

1240 –0.531 6.59∙10–5 686 907 0.107 8.06∙10–5

1) Stern method 2) Stern-Geary method

Table 4. The values of CR and MR parameters characterizing the cor-rosion rate in Ringer’s solution of the AISI 316L steel and AISI 316L–HA compositesTabela 4. Wartości parametrów CR i MR charakteryzujących szybkość ko-rozji w roztworze Ringer’a stali AISI 316L i kompozytów AISI 316L–HA

Material Sintering temp. °C

CR mm∙y–1

MR g∙cm²∙(µAm²d)–1

AISI 316L1180 0.40 0.71

1240 0.20 0.37

AISI 316L–5 wt % HA1180 0.02 0.04

1240 0.01 0.02

AISI 316L–10 wt % HA1180 0.96 1.43

1240 0.80 1.26

26 INŻYNIERIA MATERIAŁOWA MATERIALS ENGINEERING ROK XXXIX

resistance, and the lowest values of corrosion current density as well as corrosion rate (CR and MR parameters). The comparable values of corrosion parameters (for example icorr = 6.7∙10–7 A∙cm–

2, Eb = 0.214 V vs SCE, Ecorr = –0.229 V vs SCE) were found for 316L–5 wt % HA composite which was produced (using the hy-droxyapatite prepared by wet precipitation method) by mechanical alloying technique, isostatically pressing and sintering at 1200°C for 2 h [9]. A decrease in corrosion resistance was observed for AISI

Fig. 5. Microstructure of sintered AISI 316L–10 wt % HA biomaterial (sintering temperature of 1240°C); SEMRys. 5. Mikrostruktura spiekanego biomateriału AISI 316L–10% mas. HA (temperatura spiekania 1240°C); SEM

Energy [keV]

Energy [keV]

Energy [keV]

Energy [keV] Fig. 6. The EDS spectrums of sintered AISI 316L - 10 wt. % HA biomaterial (sintering temperature of 1240°C) obtained as the result of microanalysis performed at the following points (points are marked in Fig. 5): A) 1, B) 2, C) 3, D) 5. Rys. 6. Widma EDS spiekanego biomateriału AISI 316L - 10 % wag. HA (temperatura spiekania 1240°C) otrzymane w wyniku mikroanalizy przeprowadzonej w następujących punktach (punkty są zaznaczone na rys. 5): A) 1, B) 2, C) 3, D) 5.

Fig. 6. The EDS spectra of sintered AISI 316L–10 wt % HA biomate-rial (sintering temperature of 1240°C) obtained as the result of mi-croanalysis performed at the following points (points are marked in Figure 5): a) 1, b) 2, c) 3, d) 5 Rys. 6. Widma EDS spiekanego biomateriału AISI 316L–10% mas. HA (temperatura spiekania 1240°C) otrzymane w wyniku mikroanalizy prze-prowadzonej w następujących punktach (punkty są zaznaczone na ry-sunku 5): a) 1, b) 2, c) 3, d) 5

316L–10 wt % HA composite, regardless of sintering temperature. It was reflected in the lowest value of polarization resistance and the highest values of corrosion rate and corrosion current density.

The composition of powders mixture as well as the sintering temperature affects the sintered density and open porosity, and thus the corrosion resistance of studied materials.

Because the microstructure evaluation of studied materials was presented elsewhere [14÷17], an exemplary microstructure of sur-face is shown in Figure 5.

The results of EDS microanalysis are presented in Figures 6a÷d. As seen, the spectrum (Fig. 6c) reveals the presence of elements such as iron, chromium and nickel. They are the main elements of AISI 316L stainless steel chemical composition. While the spec-trum in Figure 6d shows peak indicating the presence of phospho-rus. Obviously, peaks pointing out the presence of iron, chromium and nickel are also here. It means that the phosphorus diffused from hydroxyapatite into the austenitic matrix (during sintering process). It is known that hydroxyapatite is a calcium phosphate. Based on the results of EDS analysis (presented in Figures 6a and b), it can be stated that the elements such as oxygen and calcium are present in points 1 and 2, but the peaks from phosphorus are very small. The spectrums reveals also peaks from chromium. This is associated with decomposition of hydroxyapatite and some reaction between AISI 316L steel and HA during sintering [16, 20].

4. CONCLUSION

AISI 316L–hydroxyapatite biomaterials were produced by con-ventional powder metallurgy technology comprising the following steps: preparing the mixture of AISI 316L steel and natural origin hydroxyapatite powders, pressing in the rigid matrix and then sin-tering.

The influence of hydroxyapatite content and sintering tempera-ture on properties such as density, porosity and corrosion resistance was evaluated. The corrosion behaviour of these materials in Ring-er’s solution was studied by open circuit potential measurement and potentiodynamic polarization method.

It can be concluded that higher temperature of sintering pro-cess results in the better densification and greater corrosion resist-ance for all studied materials. Among all studied materials, AISI 316L–10 wt % HA composite possessed the lowest sintered density (and relative density) and simultaneously the highest open porosity. While, almost the same values of open porosity as well as rela-tive density were found for AISI 316L steel and AISI 316L–5 wt % HA composite. All investigated materials showed the passive be-haviour. Compared to the sintered AISI 316L stainless steel, AISI 316L–5 wt % HA composite biomaterial (with similar open poros-ity) exhibited a significant improvement of corrosion resistance in Ringer’s solution, while a slight decrease in corrosion resistance was observed for AISI 316L 10 wt % HA composite. Such depend-ency was found for each of the sintering temperature. Thus, the highest corrosion resistance in Ringer’s solution possessed AISI 316L–5% HA biomaterial sintered at 1240°C temperature. This was confirmed by following corrosion parameters: the greatest values of polarization resistance, corrosion potential and breakdown po-tential, and the lowest values of corrosion current density and cor-rosion rate. Whereas, AISI 316L–10 wt % HA composite had the worst corrosion resistance because of the highest corrosion current density and corrosion rate combined with the lowest polarization re-sistance.

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28 INŻYNIERIA MATERIAŁOWA MATERIALS ENGINEERING ROK XXXIX

Odporność na korozję spiekanych biomateriałów AISI 316L–hydroksyapatyt w roztworze Ringera

Aneta Szewczyk–NykielPolitechnika Krakowska, Instytut Inżynierii Materiałowej, Kraków; *aneta.szewczyk-nykiel@mech.pk.edu.pl

Inżynieria Materiałowa 1 (221) (2018) 22÷28DOI 10.15199/28.2018.1.4

MATERIALS ENGINEERING

Słowa kluczowe: biomateriały, AISI 316L, hydroksyapatyt, odporność na korozję, metalurgia proszków.

1. CEL PRACY

W przypadku biomateriałów właściwe i długotrwałe funkcjono-wanie w agresywnym środowisku płynów ustrojowych jest bardzo ważne. Austenityczne stale nierdzewne przy dobrych właściwo-ściach mechanicznych wykazują jednak znaczną podatność na ko-rozję elektrochemiczną, natomiast hydroksyapatyt charakteryzuje się bardzo dobrą biozgodnością i odpornością na korozję. Wprowa-dzenie dodatku hydroksyapatytu może wpłynąć na poprawę odpor-ności korozyjnej, biozgodności, ale także właściwości tribologicz-nych stali AISI 316L. W pracy dokonano oceny odporności na ko-rozję w roztworze Ringera spiekanych biomateriałów AISI 316L–hydroksyapatyt. Ponadto określono gęstość i porowatość otwartą otrzymanych materiałów. Na podstawie przeprowadzonych badań potencjodynamicznych i wyznaczonych parametrów korozyjnych określono wpływ zawartości hydroksyapatytu i temperatury spie-kania na odporność na korozję badanych materiałów.

2. MATERIAŁ I METODYKA BADAŃ

Do badań zastosowano rozpylany wodą proszek austenitycznej stali nierdzewnej gatunku AISI 316L (skład chemiczny podano w tabe-li 1) oraz proszek hydroksyapatytu pochodzenia naturalnego, wpro-wadzając go do mieszanek w ilości 5 i 10% mas. Przygotowanie próbek do badań obejmowało: mieszanie proszków stali AISI 316L i hydroksyapatytu (120 min), prasowanie (600 MPa) i spiekanie (1180°C i 1240°C, 60 min, wodór). Przeprowadzono badania: gę-stości i porowatości otwartej spieków (PN-EN ISO 2738:2001), odporności na korozję w roztworze Ringera (skład roztworu poda-no w tabeli 2) za pomocą zestawu ATLAS EU&IA 0531. Badania odporności na korozję wykonano metodą bezprądową (rejestracja zmian potencjału w układzie otwartym) i metodą potencjodynamicz-ną. Określono wartości m.in. potencjałów korozyjnych i gęstości prądów korozji metodą prostych Tafela, oporu polaryzacji metodą Sterna i Sterna-Geary'ego oraz szybkości korozji. Przeprowadzono badania mikrostruktury za pomocą SEM i EDS.

3. WYNIKI I ICH DYSKUSJA

W przypadku materiałów spiekanych odporność na korozję zależy między innymi od porowatości. Wraz ze wzrostem temperatury spie-kania w zakresie od 1180°C do 1240°C następuje zwiększenie gę-stości po spiekaniu (rys. 1) i jednocześnie zmniejsza się porowatość otwarta (rys. 2) wszystkich badanych materiałów, przy czym w przy-padku stali AISI 316L można zaobserwować najmniejsze różnice. Zgodnie z regułą mieszanin gęstość biomateriałów AISI 316L–HA zmniejsza się wraz ze wzrostem zawartości hydroksyapatytu. Na rysunku 3 przedstawiono przebiegi zmian potencjału w układzie otwartym w funkcji czasu badania. Dla wszystkich badanych ma-teriałów można zaobserwować początkowo przesunięcie potencja-łu OCP w kierunku wartości dodatnich, co sugeruje tworzenie się

warstw pasywnych na powierzchni próbki, a następnie jego stabi-lizację, przy czym im wyższa temperatura spiekania, tym większe wartości potencjału OCP. Największa wartość potencjału została za-rejestrowana dla spiekanego w 1240°C biomateriału AISI 316L–5% HA. Potencjodynamiczne krzywe polaryzacji wszystkich badanych materiałów zostały zamieszczone na rysunku 4. W tabeli 3 zamiesz-czono wyznaczone wartości potencjału korozyjnego (Ecorr), gęstości prądów korozji (icorr), oporu polaryzacji (Rpol), potencjału przebicia (Eb) i gęstości prądu pasywacji (ipass), natomiast w tabeli 4 obliczone wartości szybkości korozji (parametr CR i MR). Wraz ze wzrostem temperatury spiekania potencjał korozyjny ulega przesunięciu w kie-runku wartości bardziej dodatnich, gęstość prądu korozji wykazuje tendencję spadkową, a wartość oporu polaryzacji wyraźnie wzrasta, czyli zwiększenie temperatury spiekania do 1240°C poprawia odpor-ność na korozję w roztworze Ringera.

Na rysunku 5 zamieszczono obraz mikrostruktury biomateria-łu AISI 316L–10% HA, a na rysunku 6 wyniki przeprowadzonej mikroanalizy składu chemicznego w postaci widm EDS. Główny-mi pierwiastkami w punkcie 3 są Fe, Cr, Ni, śladowo pojawia się również P, co oznacza, że fosfor dyfunduje z hydroksyapatytu do austenitycznej osnowy. W punktach 1 i 2 występują O, Ca i Cr, brak jest natomiast fosforu. Jest to związane z mającą miejsce podczas spiekania dehydratacją hydroksyapatytu i przemianami w układzie AISI 316L–P.

4. PODSUMOWANIE

Wyniki przeprowadzonych badań elektrochemicznych wskazują, że temperatura procesu spiekania oraz ilość wprowadzonego dodatku hy-droksyapatytu wywierają wpływ na odporność na korozję.

Im wyższa temperatura procesu spiekania, tym wyraźniejsze przesunięcie potencjału obwodu otwartego w kierunku wartości dodatnich, większe wartości potencjału korozyjnego oraz oporu polaryzacji, a mniejsze gęstości prądu korozyjnego i pasywacji oraz szybkość korozji. Analiza otrzymanych wyników pozwala na stwierdzenie, że wyższa temperatura spiekania przyczyniła się do uzyskania większej odporności na korozję przy jednoczesnym wzroście stopnia zagęszczenia wszystkich badanych materiałów. W przypadku spiekanego w 1240°C biomateriału AISI 316L–5% mas. HA uzyskano bardzo zbliżone wartości otwartej porowatości i gęstości względnej do stali AISI 316L i wykazuje on większą od-porność na korozję w roztworze Ringera, o czym świadczą wartości parametrów korozyjnych.

Największą odporność na korozję w roztworze Ringera wyka-zał spiekany w temperaturze 1240°C biomateriał AISI 316L–5% mas. HA. Potwierdziły to następujące parametry korozji: najwięk-sze wartości oporu polaryzacji, potencjału korozyjnego i potencja-łu przebicia oraz najmniejsze wartości gęstości prądu korozyjnego i szybkości korozji. Natomiast największa gęstość prądu korozyjne-go i szybkość korozji w połączeniu z najmniejszym oporem polary-zacji uzyskał biomateriał AISI 316L–10% mas. HA.