Pitting Corrosion behaviour of Austenitic Stainless-Steel Coated on Ti6Al4V Alloy in Chloride Solutions


 This study aims to investigate the influence of adding a coating layer of austenitic stainless steel type 316L on Ti6Al4V alloy on corrosion behaviour. Samples of 316L, Ti6Al4V, and 316L on Ti6Al4V were prepared by hot-press sintering of their powders. The potentiodynamic polarization technique was used to characterize the corrosion behaviour of the samples in 0.9 and 3.5 wt. % NaCl concentrations. The corrosion potential (Ecorr.), current density (icorr) and corrosion rate (CR) of the sintered samples were compared in this study. The results showed that 316L samples had the best corrosion resistance, although micropits were observed on the surface, while Ti6Al4V samples had the lowest. This corrosion behaviour of sintered 316L samples can be interrelated to the existence of a passive layer on stainless steel alloys that can be attacked by chloride ions and causing localized corrosion. In general, the CR values of Ti6Al4V samples coated by 316L were between the 316L and Ti6Al4V samples. The CR values of the samples, in 0.9 wt. % NaCl, did not show significant changes with increasing time, as the CR for 316L values were around 0.003 mm/year, while for Ti6Al4V the CR values changed noticeably from 0.018 mm/year of 0 hr, to 0.015 mm/year for 24 hours. However, the changes were less than that of Ti6Al4V. For 3.5 wt. % NaCl solution, although the same order of CR remained, i.e., the CR values of coated Ti6Al4V samples were between 316L (lowest) and Ti6Al4V (highest), the overall CR values for the samples were higher than 0.9 wt. % NaCl.


INTRODUCTION
Titanium alloys are widely used in different applications such as automotive, defence, aerospace, and oil and gas industries. This wide usage is attributed to their high fatigue strength, low density, and high corrosion resistance [1][2][3][4]. In the last two decades, among titanium alloys, type Ti6Al4V (grade 5) in particular, has widely been used in the biomedical field. The main two reasons which have attracted the researchers and manufacturers to use this type of Ti alloy are their biocompatibility with human tissue and having a modulus of elasticity that is close to that of bones. Thus, the alloy has been used for orthopedics, bone implant, hip prosthesis, dental implants, surgical screws [5][6][7][8].
Although Ti6Al4V has become popular, however, the low wear resistance makes its use limited [9,10]. Many researchers have attempted to solve this problem by applying various surface treatments and coating processes to increase surface wear resistance [11][12][13][14]. Gao et al. [11] showed that a biofunctional magnesium coating, deposited on porous Ti6Al4V scaffold, can improve adhesion, extracellular matrix (ECM) mineralization compared with bare Ti6Al4V. Jemat et al [12] investigated the effect of the adequate roughness role of Ti alloys in providing a surface for bone-implant contact and cell proliferation. The results showed a significant role of surface roughness despite the mechanical properties of the alloys. Hou et al. [13] deposited nano-TiC particles on Ti6Al4V by vacuum hot-press sintering (HPS) and hot extrusion. To improve the mechanical properties of Ti6Al4V, they found that the TiC coating improves tensile strength. Luan J.H., et al. [14] investigated the effects of boron (B) and yttrium (Y) on the mechanical properties and oxidation behaviour of cast Ti-6Al-4V alloys. The results indicated that boron is beneficial for improving the ductility by not only grain-size refinement but also grain-boundary enhancement. At the same time, yttrium is effective in increasing the oxidation resistance. Together with their high strength, these improved properties have made a microalloyed cast Ti-6Al-4V alloys competitive for practical engineering applications.
On the other hand, austenitic stainless steel type 316L alloy provides better wear resistance than Ti6Al4V [15][16][17]. Brończyk et al. [15] worked on the tribo-corrosion behaviour of Ti6Al4V, and AISI 316L simulated in normal and inflammatory conditions. The results showed that 316L has a lower tribo-corrosion wear rate than the Ti6Al4V alloy. Therefore they recommended using 316L stainless steel alloy as an implant. In the context of joint and bone replacement which mainly depends on the stability of the mechanical fixation, Mahesh M. Sonekaret et al. [16] made a comparative study on the tribological behaviour between 316L and Ti6Al4V using a wear test under different loads. The results showed better friction behaviour of AISI 316L steel. The author, Yamanoğlu et al. [17], studied the wear behaviour of coated 316L on Ti6Al4V powder, using a uniaxial hot press. The results showed that coating of 316L on Ti alloy reduced the wear rate of the powder alloy. In terms of the biocompatibility tests of coating austenitic stainless-steel alloy on Ti alloys, recent investigations [18,19] show that there is a relationship between pitting corrosion resistance and microbiologically induced corrosion (MIC), based on the similarities of the localized corrosion of MIC in nature, which is the phenomena of pitting and crevice corrosion. The previous paper of the author [17] indicates that the coated Ti6Al4V with 316L powder alloy, were tested in a bacterial environment of E. coli and S. aureus bacteria, and the results showed good antibacterial properties, no bacteria observed on the surface of 316L.
Ibragim et al. [20] used a potentiodynamic polarization technique complemented with XRD and SEM measurements to study the corrosion behaviour of some austenitic stainless steels in 2.5 M NaCl. The electrochemical parameters Ecorr, jcorr and Epit were used to compare the corrosion resistance of the tested samples. The results showed that an increase in either NaCl concentration or solution temperature decreases the corrosion resistance. While the increase in Mo or Cr content in the alloys of the stainless steels improves their corrosion resistance [21]. Fossati et al. [22] showed that the nitriding process on austenitic stainlesssteel alloys does not support pitting corrosion resistance due to the microcavities produce during nitriding coating. Yang and Luo [23] investigated the effect of hydrogen -which was introduced catholically to the samples through applying a constant current density -and the applied stress on austenitic stainless steel in chloride solution. The results showed that austenitic stainless-steel coated on Ti6Al4V alloy in chloride solutions hydrogen significantly decreases the critical chloride concentration for the breakdown of passive films, which indicates that hydrogen promotes the breakdown of passive films. Also, with an increase in the tensile stress, the critical chloride concentration decreased.
Although the above researches showed different attempts to treat the surface of Ti alloys with different coating techniques as biomaterial applications, at the same time some approaches indicate the existence of relationships between biocompatibility and corrosion behaviour. For example, there are as yet unconfirmed approaches indicating an interconnection between microbiologically induced corrosion (MIC) and pitting corrosion behaviour. Therefore, to fill such gaps, this research aims to study the electrochemical polarization behaviour of coated Ti6Al4Valloy by 316L using powder metallurgy.

MATERIALS AND EXPERIMENTAL WORKS
The preliminary materials used in this study were powders of hydride-dehydride HDH Ti6Al4V powders (Table 1) and water atomized stainless steel powders type AISI 316L ( Table 2). The chemical composition of AISI 316L stainless steel (UNS S31603) is outlined in Table 3    A hot-press sintering technique was used in this study to prepare the samples. Stainless steel powders type AISI 316L was placed (coated), with 250 μm thickness, on titanium alloy powders, type Ti6Al4V in a graphite die and, in-situ, sintered by uniaxial hot pressing, besides, each alloy, Ti and 316L, was prepared individually. A schematic illustration of the hot press process is shown in Figure 2. The sintering process was carried out at two different temperatures, 950 and 1050 o C, vacuumed at 10 -4 mbar, for 30 minutes, under 50 MPa pressure.

Fig. 2. Schematic of in-situ coating process by hot press
The samples' electrochemical characterization and corrosion behaviour were studied using the potentiodynamic polarization as a function of time, in a chloride solution with two concentrations of 0.9 and 3.5 wt. % of NaCl at 25 o C, which is similar to the human body environment [18][19][20][21]. The electrochemical tests were performed using a model Bio-Logic SP-150 potentiostat measuring instrument. The experimental set-up was included an electrochemical cell with three electrodes: a platinum counter electrode, a saturated calomel electrode SCE/Hg/Hg2Cl2as a reference electrode and preparation working electrodes, which were used in the prepared samples. The working electrodes were prepared for the electric contact made by AP wire glue. Where the copper wire was glued to the sample, the surface was masked with lacquer, and only the working part of the sample remained uncoated. The exposed surface area varied from 2 cm 2 to 6 cm 2 .
Before starting the electrochemical tests, to remove dust and dirt, the samples were placed in an ultrasound device, cleaned using acetone and distilled water, and dried. Then, the samples were introduced in two different aerated electrolytic solutions, freshly prepared from analytical grade chemicals of NaCl and distilled water, immersed at different times 0, 0.5, 1, 3, 5 and 24 h.
The evaluation of the corrosion potential as a function of the immersion time was measured at a scan rate of 1.5 mV/s. The potentiodynamic polarization curves were recorded by a potential sweep from -250 to + 250 mV. Figure 3 shows the potentiodynamic polarization curves as a function of time, at different concentrations of aerated two NaCl concentrations and pH values, at 25 o C. austenitic stainless-steel coated on Ti6Al4V alloy in chloride solutions   Table 4 shows that the current density, icorr., of 316L in 0.9%was decreased from 0 to 5h then increased at 24h. This result can be explained by the high degree of surface corrosion at the beginning of contact with the solution. Table 5 shows that the icorr. values of 3.5% solution decreased from 0 to 1h, increased at 3h, and then continued to decrease until 24h. Meanwhile, a significant passivation rate is observed in the 3.5 wt. % NaCl solution during the periods of 30 min, 1h and 24 h. This can be attributed to the formation of the oxide protective layer [20,21], then reduced the times, 3 and 5h due to breakage of the passive film, as a result of the deterioration and dissolution [22][23][24][25] due to localized attack by adsorption of ion Cl - [26][27][28]. austenitic stainless-steel coated on Ti6Al4V alloy in chloride solutions Table 5 shows that the electrochemical behaviour of the 316L samples was stable with increasing the time of immersion. However, an increase was observed for the CR in the 3.5% solution at times 30 and 5 hours, CR = 0.008 mm/year, which was also remained insignificant until 24 hours, when reached CR = 0.004 mm /year. These results are similar to what other researchers have found [20,21]. Table 4 shows the CR value of 316Lin 0.9 % solution was lower than that of 3.5 % solution, as the resistance to corrosion decreases with the increase of concentration of NaCl [20].

Ti6Al4V Titanium alloy
The anodic parts of both curves showed the presence of a pseudo-passivation stage with the presence of fluctuations due to the propagation of the Clion, which corresponds to the breakdown of passivation at the potential of 0 V for immersed samples in a 3.5% solution and -0.3 V for immersed samples in 0.9 %. Figures 3c and d show extrapolation of certain Ti6Al4V electrochemical parameters. Tables 4 and 5 show that the corrosion potential (Ecorr.) of Ti6Al4V in both solutions increased with increasing immersion time. This can be attributed to the passivation layer formation [29,30]. Moreover, the values of icorr in all samples were increased with increasing immersion time in both NaCl solutions. This reflects the occurrence of dissolution and release of metal ions in the long term in the clinical failure of orthopedic or dental implants [29,30]. Tables 4 and 5 show that the CR values of Ti6Al4V slightly decreased with increasing the immersion time in both NaCl solutions. The CR of Ti6Al4V in 0.9 % solution was lower than those of 3.5% solution.

Coating 316L on Ti6Al4V
Based on the data in Table 4 and Figure 3, the icorr. of the coated 316L samples in 0.9% of NaCl solution were increased from 0 to 1h, then decreased from 3 to 24h, while the Ecorr. was decreased with time while become more anodic due to the continuous dissolution. Table 5 shows that for coated 316L samples, the Ecorr. values in 3.5% NaCl solution decreased from 0 to 1 h, increased at 3 h and continued to decrease to 24 h, while the icorr values increased with increasing immersion time due to dissolution of the passive film. Overall, Tables 4 and 5 show that the 316L samples had the lowest value of the CR followed by 316L coated with Ti6Al4V, then Ti6Al4Valloy. The very low values of the CR of 316L can be attributed to the passive layer, which acts as a barrier to protect the surface. Figures 4a and b show the surface morphology of 316L and Ti6Al4V samples respectively, which were immersed in 3.5 % solution for 24 h. Figure 4a shows the existence of micropores on 316L sample, also -due to the process of elaboration by HPS -micropits are shown. This is an indication of the localized corrosion attack by Clions, cause micropits on the surface. The qualitative analysis over a specific area of the surface, see Figure 4, indicates a significant amount of oxygen of 18 (at. %t) which confirms the oxidation of the surface of 316L in contact with the aggressive solution of 3.5 wt. %. In terms of Ti6Al4V, the existence of micropores was observed. However, there was no clear indication of corrosion types, e.g., micropits, on the surface after an immersion time of 24 hours. section of the coated sample,  Figure 5 shows the bonded coating thickness of stainless steel on Ti6Al4V alloy, as in average equals 250 m, which shows relatively high homogeneity of the stainless steel layer. This homogeneousness is an important property for the coated material because the interlayer between bonded materials is strongly affects the final properties of the materials. Thick coating materials and techniques are necessary for tribological and load bearing applications. It should be noted that corrosive behaviour is one of the most important factors affecting the biocompatibility of metal implants because metal ions released during corrosion can cause various adverse effects, both in the tissues surrounding the implant, and allergic reactions. The corrosion rate influences the reaction of tissues, that is, the number of metal ions released [30]. Therefore, although the generally relatively corrosion resistance of the titanium alloy Ti6Al4V observed in our study, which is in good agreement with the results of Vora H.D. et al. [29], implantation may lead to an increase in the amount of titanium released in the tissue due to the influence of proteins, for example, albumin [31]. It should also be noted that, unlike 316L stainless steel, titanium alloy Ti6Al4V can easily osseointegrate during implantation, that is, promote bone formation in direct contact with a metal surface [32]. An essential role in the success of osseointegration is played by the oxide layer on the alloy surface, while thicker layers promote rapid osseointegration for titanium alloy Ti6Al4V [32,33]. The oxide austenitic stainless-steel coated on Ti6Al4V alloy in chloride solutions layer also has the effect of passivating the metal, so that corrosion is inhibited and the release of titanium ions is minimized [12,30,33]. Therefore, for both materials of this study, stainless steel and titanium alloy, the formation of coatings on their surfaces may increase their biocompatibility. The study of their corrosion resistance plays an important role in determining the requirements for the materials and microstructure of such coatings.

CONCLUSIONS
Corrosion behaviour, through applying potentiodynamic tests, were investigated on stainless steel powders, were in situ coated on Ti6Al4V alloy, in two NaCl concentrations. The main conclusions are: -The 316L samples showed higher corrosion resistance than coated and uncoated Ti6Al4V.
However, micropits were observed on the surface. -The 316L samples did not show a significant change with increased time in 0.9% NaCl, but the change was noticeable in 3.5% NaCl. -The corrosion resistance of the coated Ti6Al4V by 316L has a lower corrosion resistance than 316L alloy. -The Ti6Al4V samples showed the lowest corrosion resistance; however, a passivation region was observed. -Increasing the concentration of NaCl caused an increase in the corrosion rate for all the samples.

ACKNOWLEDGMENTS
This is part of the Further Funded VC2020 project funded by De Montfort University -DMU, in the United Kingdom, to establish a research collaboration with Kocaeli University in Turkey. The research was supported by the Scientific Research Unit at Kocaeli University, also by Semilab Semiconductor Physics Laboratory Co. Ltd., in Hungary, University A/Mira Bejaia in Algeria and D. Serikbayev East Kazakhstan Technical University in Kazakhstan. The authors are grateful to DMU to support this research to build this research cooperation.