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Lasers in Dental Science

, Volume 2, Issue 3, pp 137–146 | Cite as

Corrosion behavior of erbium chromium-doped yattrium-scandium-gallium-garnet (Er,Cr:YSGG 2780 nm) laser-treated titanium alloy used for dental applications at different pH conditions (in vitro study)

  • Dalia A. Abd El daym
  • Mostafa E. Gheith
  • Nadia A. Abbas
  • Laila A. Rashed
  • Zeinab A. Abd El Aziz
Original Article
  • 231 Downloads

Abstract

Titanium alloys are most often used in the design of dental implants. Although these biomaterials have good mechanical and biological properties, their corrosion resistance is still critical for the overall success of the treatment procedure. Implant failure is more likely to occur in inflammatory diseases related to low pH levels. Er,Cr:YSGG laser is most often used in implant dentistry.

The aim of the this study was to assess the effect of Er,Cr:YSGG laser surface treatment on the corrosion behavior of titanium alloy (Ti–6Al–4V) at different pH.

Materials and methods

A total of 40 discs were used. Twenty discs were irradiated with Er,Cr:YSGG laser, which was operating in a normal room atmosphere and temperature at power 2 W. The corrosion behavior was investigated in simulated body fluid at pH of 7.40 and 5.20. At each pH, corrosion behavior was studied for up to 864 h at 192, 360, 696, and 864 h intervals using potentiodynamic polarization test and electrochemical impedance spectroscopy. The laser-treated and -untreated discs were examined with scanning electron microscope before and after the electrochemical tests.

Result

Laser treatment significantly improves the corrosion resistance compared to the untreated group. Immersion time and pH also significantly affect the corrosion behavior. The acidity yielded more aggressive changes on the untreated titanium alloy.

Conclusion

Er,Cr:YSGG laser could improve the corrosion resistance of Ti–6Al–4V.

Keywords

Corrosion behavior Laser treatment Polarization test Ti–6Al–4V 

Introduction

Titanium and its alloys are suitable for biomedical materials due their superior qualities, such as low specific gravity, high corrosion resistance, low elasticity modulus, and good biocompatibility [1]. The high corrosion resistance of titanium and its alloys is partly due to a protective titanium dioxide passive film spontaneously formed on the titanium surface [2].

Titanium oxide is a stable and dense layer, which acts as a protective barrier to continued metallic oxidation. In the event of damage, titanium oxide has the ability to spontaneously reform under normal physiological conditions. However, events such as abnormal cyclic loads, implant micromotion, acidic environments, and their conjoint effects, can result in permanent breakdown of the oxide film, which may consequently lead to exposure of the bulk metal to an electrolyte. Active dissolution of metal ions can occur upon exposure of the bulk metal [3]. Titanium dental implants are generally surface-modified to reduce corrosion, improve osseointegration, and increase the biocompatibility. To achieve this, surface treatments, such as surface machining, sandblasting, acid etching, electro polishing, anodic oxidation, plasma-spraying, and biocompatible/biodegradable coatings are used to improve the quality and quantity of the bone-implant interface of titanium-based implants. Laser processing is now being used in implant applications [4].

Erbium lasers are solid state lasers categorized in the mid-infrared range of the electromagnetic spectrum, with light emitted as invisible, nonionizing thermal radiation. They are free-running pulsed lasers. Wavelength is a major factor in the absorption of the laser light by biologic tissue. The largest absorption peak for water is just below 3000 nm, which is at the erbium wavelength. Erbium is also well absorbed by hydroxyapatite. Pulsed MIR lasers have high tissue absorption so that they are effective ablating lasers [5, 6]. The role of lasers in dental implantology was explored by Romanos et al., and they found that soft-tissue lasers could be of benefit for improvements in hemostasis, as an adjunct to soft-tissue peri-implant re contouring, and for improving wound healing. Hard tissue lasers (Er,Cr:YSGG and Er:YAG wavelengths) were valuable for laser-assisted osteotomies, and for the improvement in early osseointegration after fixture placement. They also mentioned that lasers could be used for the treatment of peri-implantitis [7]. Hard tissue lasers allow precise bone sectioning and ablation with minimal thermal effects upon the adjacent tissues [8, 9, 10]. The Er,Cr:YSGG laser exhibits almost no absorption of laser irradiation in titanium, and thus prevent excessive energy transformation in the form of heat development. It can be used safely on implant surfaces with adequate water spray without increase in temperature [11]. Most metal corrosion occurs via electrochemical reactions at the interface between the metal and electrolyte solution. Because corrosion occurs via electrochemical reactions, electrochemical techniques are ideal for the study of the corrosion processes using a metal sample with a surface area of a few square centimeters to model the metal in a corroding system [12]. Corrosion normally occurs at a rate determined by equilibrium between opposing electrochemical reactions. The first is the anodic reaction, in which a metal is oxidized, moving electrons into the metal. The other is the cathodic reaction, in which a solution species (often O2 or H+) is reduced, removing electrons from the metal. When these two reactions are in equilibrium, the flow of electrons from each reaction is balanced, and no net electron flow (electrical current) occurs [13]. Monitoring the relationship between electrochemical potential and current generated between electrically charged electrodes is called polarization resistance. It allows the calculation of the corrosion rate to determine the quantitative assessment of corrosion and to measure the susceptibility to localized corrosion for corrosion resistant materials [14]. The polarization resistance is the ratio of the applied potential and the resulting current. The rate of corrosion is directly proportional to corrosion current while inversely related to polarization resistance (www.Corrosionpedia.com). Impedance measurements are useful and informative method of corrosion assessment. It can be used to follow actively corroding systems [15]. The aim of the present study was to present a better understanding of the effect of a Er,Cr:YSGG (2780 nm) laser surface treatment on the corrosion behavior of titanium alloy (Ti–6Al–4V) at different pH conditions and at different time intervals.

The hypothesis was that laser treatment could improve the corrosion resistance of Ti–6Al–4V.

Materials and method

Materials

A 6-mm diameter cylindrical rod of conventional biomedical titanium alloy Ti–6Al–4V (Bredent, Germany—GmbH) was used. Forty titanium alloy samples were used in the present study. The samples were divided into two groups: laser-treated and -untreated groups (20 samples each). Then, 20 samples of laser treated and untreated (ten samples each) were immersed in Hank’s solution at pH 5.2. Twenty samples of laser treated and untreated (ten samples each) were immersed in Hank’s solution at pH 7.4.

Preparation of the work samples

The rod was cut into 40 discs with a thickness of 2 mm using a silicon carbide cut-off wheel at 3800 rpm, under continuous flowing coolant.

Preparation of the mold

Forty Teflon molds of 10 mm diameter and 10 mm thickness were prepared for mounting the discs. A hole of 6 mm diameter and 2 mm depth was drilled from the upper side of the mold to allow the disc to be secured in it exposing 0.28 cm2 of the disc’s surface. Another hole of 3 mm in diameter and 8 mm depth was drilled and screwed in the opposite side of the mold to allow for placement of a 3-mm diameter copper rod. The exposed surface of the samples was finished with different grades of silicon carbide grit papers up to 2400-grit in a single direction to achieve regular and similar morphology for all samples. Final polishing was carried out with alumina paste. All discs were washed with distilled water, dried, and then sterilized in alcohol before the experiment.

Laser treatment

Laser surface treatment of 20 discs was carried out using the Er,Cr:YSGG laser (Waterlase MD, Biolase Technology, USA), which operated in a normal room atmosphere and temperature. Parameters used in the study were summarized in Table 1. The delivery system consists of a fiber optic tube terminating in a zirconia tip 600 μm in diameter. A water cooling system of 40% water and 60% air was used. The titanium samples were irradiated and hand guided at constant distance of 0.5–1 mm with the laser system. Each disc was irradiated in parallel movements moving the laser beam continuously and not staying too long in one spot. The disc was scanned once to standardize the treatment time. The angle created by the laser beam and the disc surface was approximately 90°.
Table 1

Laser parameters used in the study

Laser type

Wave length (nm)

Power (W)

Energy (m J)

Spot size (mm)

Fluence (J/cm2)

Frequency (Hz)

Power density (W/cm2)

Total time (s)

Pulse duration μ sec

Dose (J/cm2)

Er,Cr:YSGG

2780

2

100

0.6

35.4

20

707

20

50

14,140

Corrosion testing procedure

Potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) were performed using a IviumStat potentiostat (Ivium technologies; Eindhoven, Netherlands) scanning unit controlled by a personal computer and a software which connected to three-electrode cell assembly consisting of titanium alloy disc as the working electrode and platinum wire as the counter electrode. The reference electrode that all potentials are referred to is Hg/Hg2Cl2/Cl saturated calomel electrode (SCE) of Eo = 240 mV versus normal hydrogen electrode (NHE). To prepare the working electrodes, the samples were joined at one end to insulated copper rods, which were screwed in the Teflon molds to be in contact with the titanium alloy discs.

In order to simulate the physiological conditions of human body, Hank’s solution was used for in vitro corrosion studies, and its chemical composition was given in Table 2. It is prepared with double-distilled water and analytical grade reagents. The prepared Hanks solution was poured into a clean cell. The electrodes were immersed in Hank’s solution at pH of 7.40 and 5.20. A fresh solution was used for each experiment. Measurements were performed at various intervals of 192, 360, 696, and 864 h at room temperature.
Table 2

The composition of Hank’s solution

Reagent

Composition g/l

NaCl

8.00

KCl

0.40

NaHCO3

0.35

CaCl2

0.14

MgCl2 6H2O

0.10

Na2HPO42H2O

0.06

KH2PO4

0.06

MgSO4.7H2O

0.06

C6H12O6

1.00

An open circuit potential (OCP) was measured in aerated conditions in each sample. After the stabilization of OCP, the potentiodynamic scans were started at 250 mV below OCP, at a rate of 0.5 mV s−1. The results were analyzed in terms of geometric surface area. The metal corrosion behavior was studied by measuring the potential between the specimen (working electrode), and the reference electrode and plotting the E-log I (voltage–current) diagram. Electrochemical impedance measurements for the different samples of untreated and treated titanium alloys were carried out in Hank’s solution at pH (7.40 and 5.20) with the applied frequency ranges from 35 kHz to 100 mHz. The impedance behavior of the specimens was expressed in Nyquist plots of Z” as a function of Z’. The experiments were repeated for all samples to ensure the reproducibility of the electrochemical curves.

Surface analysis

The discs were examined under scanning electron microscope using SEM Model Quanta 250 FEG (Field Emission Gun, FEI Company, the Netherlands). The surface characteristics of the machined- and laser-treated titanium discs before and after the electrochemical assessment at the different pH of Hank’s solutions (pH 5.2 and pH 7.4) were recorded.

Statistical analysis

The descriptive statistics for the corrosion rate for the different test groups are shown in Table 3. The results of multifactorial ANOVA for the contribution of different factors as laser treatment (treated/untreated sample), pH values (pH 5.2/ pH 7.4), and the immersion time (192, 360, 696, and 864 h) to the corrosion rate are shown in Table 4.
Table 3

Descriptive statistics of the corrosion rate (mm/y) for the different test groups

Laser treatment

pH

Immersion time (hours)

Mean

SD

SE

CI

Range

Max

Min

Untreated sample

pH 5.2

192

0.01

0.00

0.00

0.00

0.01

0.01

0.00

360

0.01

0.00

0.00

0.00

0.01

0.01

0.01

696

0.06

0.01

0.01

0.02

0.03

0.08

0.05

864

0.09

0.02

0.01

0.04

0.05

0.11

0.06

pH 7.4

192

0.01

0.00

0.00

0.00

0.00

0.01

0.00

360

0.01

0.00

0.00

0.00

0.00

0.01

0.00

696

0.04

0.01

0.01

0.02

0.03

0.06

0.03

864

0.05

0.02

0.01

0.03

0.03

0.06

0.03

Treated sample

pH 5.2

192

0.07

0.00

0.00

0.00

0.00

0.01

0.01

360

0.01

0.00

0.00

0.00

0.00

0.01

0.01

696

0.01

0.00

0.00

0.00

0.01

0.01

0.01

864

0.01

0.00

0.00

0.00

0.00

0.01

0.01

pH 7.4

192

0.00

0.00

0.00

0.00

0.00

0.00

0.00

360

0.01

0.00

0.00

0.00

0.00

0.01

0.01

696

0.01

0.00

0.00

0.00

0.00

0.01

0.01

864

0.02

0.00

0.00

0.00

0.01

0.02

0.01

Table 4

Multifactorial analysis of variance for corrosion rate

Source

Sum of squares

Df

Mean square

F ratioa

P value

Main effects

 A: Laser treatment

0.01

1

0.01

131.80

P ≤ 0.00*

 B: pH

0.00

1

0.00

14.14

0.00*

 C: Immersion time

0.01

3

0.01

65.46

P ≤ 0.00*

Interactions

 AB

0.00

1

0.00

14.07

0.00*

 AC

0.01

3

0.00

40.08

P ≤ 0.00*

 BC

0.00

3

0.00

2.94

0.04*

 ABC

0.00

3

0.00

7.52

0.00*

 Residual

0.00

48

0.00

  

 Total (corrected)

0.04

63

   

*Significant difference at P ≤ 0.05 (factor have a statistically significant effect on corrosion rate at the 95.0% confidence level)

The ANOVA table decomposes the variability of corrosion rate into contributions due to various factors. Since Type III sums of squares have been used, the contribution of each factor is measured having removed the effects of all other factors

aAll F-ratios are based on the residual mean square error.

All the factors in the study had a significant contribution in the change of the corrosion rate with the highest change attributed to laser treatment, followed by immersion time and finally pH with F ratio: 131.80, 65.46, and 14.14 respectively.

Result

Potentiodynamic polarization curve (Tafel curves)

The metal corrosion behavior was studied by measuring the potential between the specimen (working electrode) and the reference electrode by plotting the E-log I (voltage–current) curves. Figure 1a–d shows Tafel lines and Table 5 summarizes electrochemical corrosion parameters derived from these Tafel lines for untreated and laser-treated Ti–6Al–4V alloys for these intervals 192, 360, 696, and 864 h at room temperature. Table 5 illustrates the polarization resistance (Rp), corrosion potential (Ecorr), corrosion current densities (Icorr), and corrosion rates in millimeters per year for all the investigated samples. From Table 5, a clear shift is observed of the corrosion potential (Ecorr) value from more negative direction in case of untreated titanium alloy toward more positive direction in case of the laser-treated alloy indicating the noble behavior.
Fig. 1

Tafel potentiodynamic polarization curves of laser-treated and -untreated Ti–6Al–4V at two pH (7.4 and 5.2) a at 192 h, b at 360 h, c at 696 h, d at 864 h

Table 5

The mean corrosion parameters for Tafel analysis of laser-treated and -untreated Ti–6Al–4V at various time intervals

Laser treatment

pH

Immersion time (hours)

Ecorr (V) + OCP

Icorr (A/cm2)

Rp (Ohm)

ba (V/dec)

bc (V/dec)

Untreated sample

pH 5.2

192

− 0.24

8.19E-07

117,250

0.09

0.18

360

− 0.20

1.26E-06

72,447.5

0.10

0.15

696

− 0.19

6.69E-06

14,667.5

0.11

0.16

864

− 0.19

1E-05

10,439

0.11

0.17

pH 7.4

192

− 0.23

6.69–07

124,775

0.11

0.10

360

− 0.17

8.14E-07

113,150

0.11

0.14

696

− 0.20

4.88E-6

16,132.5

0.10

0.11

864

− 0.22

5.25E-6

20,837.5

0.13

0.64

Treated sample

pH 5.2

192

− 0.20

7.77E-7

59,470

0.06

0.06

360

− 0.20

1.03E-6

45,985

0.07

0.06

696

− 0.18

1.15E-6

49,750

0.08

0.07

864

− 0.17

1.24E-6

129,000

0.14

0.42

pH 7.4

192

− 0.22

9.48E-8

972,675

0.12

0.12

360

− 0.16

9.2E-7

57,462.5

0.09

0.06

696

− 0.16

1.31E-6

27,065

0.06

0.04

864

− 0.16

1.87E-6

20,915

0.06

0.04

Where Ecorr is the corrosion potential, Icorr is the corrosion current density, Rp is the polarization resistance, ba is the slope of anode branch, and bc is the slope of cathode branch

Electrochemical impedance spectroscopy measurements (EIS)

The results of EIS data are displayed as typical Nyquist (Zre versus Zimg) for time intervals of 192, 360, 696, and 864 h at room temperature, and they are shown in Fig. 2a–d. Moreover, the electrochemical corrosion parameters derived from EIS curves for untreated and treated Ti–6Al–4V alloy for various intervals are summarized in Table 6.
Fig. 2

Nyquist plots of laser-treated and -untreated Ti–6Al–4V at two pH (7.4 and 5.2) a at 192 h, b at 360 h, c at 696 h, d at 864 h and inserted inside the figure the equivalent electrical circuit used to fit the impedance spectra

Table 6

Values of fitting parameters of laser-treated and -untreated Ti–6Al–4V at different pH for various time intervals at room temperature

Laser treatment

pH

Immersion time (hours)

R s

R p

R b

C p

C b

Untreated sample

pH 5.2

192

4.13E + 02

3.53E + 03

1.15E + 04

1.01E-04

9.30E-04

360

1.20E + 02

1.77E + 03

1.08E + 04

6.03E-05

7.74E-05

696

4.88E + 02

4.35E + 02

3.49E + 03

2.11E-04

5.15E-04

864

1.95E + 02

5.49E + 02

3.03E + 02

4.67E-04

3.58E-04

pH 7.4

192

8.24E + 02

4.80E + 04

2.83E + 04

2.19E-04

2.07E-05

360

5.71E + 02

2.62E + 03

2.15E + 04

3.12E-04

2.67E-05

696

8.04E + 02

6.18E + 03

2.65E + 03

1.32E-03

4.69E-05

864

5.43E + 02

6.53E + 03

2.65E + 03

1.14E-06

7.16E-04

Treated sample

pH 5.2

192

7.67E + 02

1.24E + 04

3.02E + 04

2.30E-05

1.63E-04

360

1.81E + 04

9.32E + 04

8.74E + 04

4.57E-06

2.86E-06

696

1.64E + 03

7.24E + 03

6.09E + 02

1.91E-06

1.91E-06

864

7.57E + 03

6.40E + 04

6.06E + 04

1.08E-05

7.96E-06

pH 7.4

192

2.73E + 04

7.26E + 04

7.26E + 04

1.28E-04

1.28E-04

360

5.94E + 02

1.01E + 04

2.51E + 04

2.13E-05

2.40E-04

696

1.49E + 03

1.76E + 04

1.01E + 04

1.26E-04

8.26E-05

864

2.49E + 02

8.59E + 03

4.10E + 04

1.73E-04

2.30E-06

Where Rs is the solution resistance, Rp is the porous layer resistance, Rb is the barrier layer resistance, Cp is porous layer capacitance, and Cb is the barrier layer capacitance

The equivalent electrical circuit used to fit the impedance spectra inserted inside the Fig. 2 in which Rs, Rp, and Rb represent the solution, porous layer (corrosion product), and barrier layer resistance, respectively. Cp and Cb are the capacitances of the porous layer and the barrier layer, respectively.

Surface analysis

Figure 3a–b shows SEM images for untreated and laser-treated discs respectively before electrochemical tests. Laser-treated discs images showed sealing of the scratches due to machining and polishing. No voids, inclusions, pits, or micro- fractures were observed in laser-treated discs. Figure 4a–d shows SEM images for laser-treated and -untreated discs after electrochemical tests. Pitting was clearly shown for both treated and untreated discs at both acidic and neutral media. Pits tend to appear more aggressive in acidic solution. However, smoother pitting was produced on the laser-treated discs.
Fig. 3

Electron microscopic image before electrochemical test

Fig. 4

Electron microscopic image after electrochemical test

Discussion

Er,Cr:YSGG (2780 nm) is a hard tissue laser used currently in dentistry. During irradiation, parameters such as output power, energy, dose, and duration should be considered. The power setting chosen was 2 W, 20 Hz over a period of 20 s, and the energy dose 100 mJ/pulse to allow adequate dose to be delivered to the titanium alloy surface without undesirable results [16].

In the present study, the corrosion rate increases by time for both laser-treated and -untreated Ti–6Al–4V alloys immersed in acidic and neutral Hanks solution. This could be due to that the oxide structure changes over time upon immersion in bio-electrolyte solutions. The high concentration of chloride ions in these fluids is highly aggressive for biomaterials [17]. In acidic chloride solutions (at pH of 5.2), the concentration of H+ ions will increase and enhance the rate of dissolution there by reducing the oxide thickness. Also, aggressive ions like Cl ions diffuse and migrate through the oxide layer, and then adsorb on the oxide surface which is responsible for passive layer breakdown due to formation of titanium chloride [18]. This explains the significant increase of corrosion rate by decreasing the pH for both laser-treated and -untreated Ti–6Al–4V alloys. Laser treatment significantly decreased the corrosion rate. This could be due to the laser-treated alloy releasing electrons easier, thus reaching equilibrium faster and preventing electron flow (electrical current) so more noble metal was achieved. This was proved in the present study by Tafel analysis which showed significant decrease of corrosion current and increase of polarization resistance of laser-treated alloy compared to untreated alloy. The potentiodynamic polarization curves of laser-treated and -untreated Ti-6Al-4 V alloys immersed in acidic and neutral Hanks solution for different time intervals were broadly similar except for the current density variation with increase in potential. For laser-treated samples, the current density (Icorr) decreased compared to untreated samples at different time intervals. The corrosion potential (Ecorr) increased with laser treatment compared to untreated samples at different time intervals, which means that it acts as an electron donor to electrolyte indicating a better corrosion resistance [15]. Both laser-treated and -untreated samples showed rapid increase in current density at high potentials, which mean localized corrosion characteristics in the form of pitting.

The impedance behavior of laser-treated and -untreated Ti–6Al–4V alloys immersed in acidic and neutral Hanks solution for different time intervals was expressed by Nyquist plots. For the interpretation of the electrochemical behavior of a system from EIS spectra, an appropriate electric circuit model of the electrochemical reactions occurring on the electrodes is necessary. Many circuit models were tried, to obtain the closest fit with least chi-square value. From Fig. 2, the results suggest that the two-time constants present in the Nyquist plot for all investigated samples, indicated the occurrence of two processes during corrosion. The EEC model with two RC couples was based on two contributions, R1C1 and R2C2, related to high and low frequency time constants, respectively [19, 20]. According to the studied system, these time constants get different physical meaning. In this given equivalent circuit, R is attributed to the electrolyte resistance between the reference electrode and the surface of the working electrode (Rs). This is the first time the constant (at high frequency) is attributed to the inner barrier layer (corrosion product) compounds which can be partially protective; R1 and C1 were associated to the inner layer impedance represented by resistive (Rb) and capacitive (Cb) elements. While, the second time (at low frequency) is linked with the double-layer capacitance (Cdl) at the electrolyte/outer surface interface and the charge transfer resistance of the outer porous surface (Rct), respectively. The total polarization resistance (Rp) is the sum of Rb, Rct. The barrier layer is compact, having a high resistance, whereas, the outer layer is porous.

The nature of the alloy-solution interface did not change with immersion time. At all immersion times, the system fits into the same circuit model. The barrier resistance was high at 192 h of immersion and then slowly decreased until 864 h of immersion for untreated samples at both pH (5.2 and 7.4). The initial increase in the barrier layer resistance could be due to the growth of the oxide layer in the solution, and the slow decrease afterwards was due to attack by chloride ions from Hanks solution. Inner films formed at pH 7.4 showed higher values of barrier layer resistance compared to the values of barrier layer resistance obtained at pH 5.2, which indicates that the barrier resistance is influenced by pH [19]. The general formation of oxide layer was explained by Stern double-layer theory, which proposes that electrical double layer is formed whenever a metal is exposed to an aqueous environment. This double layer consists of an inner barrier layer and outer diffusion layer, which were confirmed by an equivalent circuit employed in EIS modeling [21]. In the untreated samples, the outer porous layer resistance values were higher at pH 7.4 condition than at pH 5.2 condition. The increase in the resistance values suggests that there was thickening of the passive layer, so by decreasing pH condition the resistance values decrease and the passive layer thickness decrease, consequently, an oxide layer formed in acidic condition containing chloride ions shows less corrosion resistance than the oxide layer formed in neutral media [22].

For laser-treated samples at both pH (5.2 and 7.4), the barrier resistance showed a nonspecific trend with time and pH, which may be due to change in the characteristics of barrier layer due to laser treatment. The values of barrier layer resistance in laser-treated samples for each pH condition were higher than the values of barrier layer resistance in untreated samples at the same pH condition, which is associated to high impedance and is responsible for high corrosion resistance of laser-treated samples [20]. The porous layer resistance in laser-treated samples did not follow specific pattern. This could be due to the incorporation of ions into the pores from the solution. The porous outer oxide layer can accommodate the adsorbed ions in the oxide film matrix and increase the biocompatibility of the implant material. This means that the protection provided by the passive layer was attributed to the barrier layer, while the ability to osseointegrate could be due to the presence of the porous layer [19]. The values of porous layer resistance in the laser-treated samples for each pH condition were higher than the values of the porous layer resistance in untreated samples at the same pH condition. The increase in the resistance values suggests the increase of thickening of the outer passive layer on the laser-treated samples, which facilitates oseointegration and improves biocompatibility.

In the present study, SEM revealed no undesirable changes on the surface of machined titanium alloy after irradiation with an Er,Cr:YSGG laser at 2 W. No voids, inclusions, pits, or micro fractures were observed in the laser-treated discs. It showed sealing of scratches due to machining and polishing. An SEM study on the effects of laser irradiation on machined and anodized titanium discs reported that machined and anodized Ti disc surfaces were affected by irradiation with an Er,Cr:YSGG laser at powers over 3 W [23]. Another study showed that, Er,Cr:YSGG did not cause significant changes on a Ti plasma spray coated implant even at 6 W [24].

SEM images after polarization tests showed localized corrosion in the form of pitting for both treated and untreated discs at acidic and neutral media. Pits tend to appear more aggressive in acidic solution. A smoother pitting was produced on the laser-treated discs. Acidic conditions exhibit a high reactivity with chloride ions, and the concentration of chloride ions has a significant influence on the pitting intensity [25]. The SEM results confirm the result of Tafel analysis and impedance measurements.

Another study reported that pitting potentials were reduced in laser-melted samples compared to the untreated samples. They attributed the poor corrosion resistance of untreated Ti–6Al–4V to the local enrichment of aluminum (α stabilizer) and the formation of a galvanic cell between the two different phases α and β. Partitioning of aluminum α-phase during electrochemical corrosion is the primary mechanism of degradation of the aluminum containing Ti-alloys, whereas, the laser-treated Ti-6Al-4V lacks this partitioning effect [26]. Biswas et al. reported better corrosion resistance in laser-melted surfaces compared to laser-nitrided surfaces. The reduced pitting corrosion provided by surface melting was possibly because of partial suppression of β phase formation in the microstructure and the change in morphology from granular to acicular [27]. Zaveri studied treated Ti–6Al–4V alloy with a pulsed-wave Nd:YAG laser under various conditions to obtain a surface oxide layer for improved corrosion resistance. Corrosion resistance studies were carried out in three different simulated bio-fluids (SBFs), namely, NaCl solution, Hank’s solution, and Cigada solution. Tafel analysis showed that the laser-treated Ti–6Al–4V was more corrosion resistant than bare specimens in any of the above SBFs [28].

Conclusion

Implant failure is more likely to occur in inflammatory diseases related to low pH levels. The corrosion behavior of laser-treated and -untreated titanium alloys under the different pH conditions at 192, 360, 696, and 864 h intervals was studied. The result shows that all the factors in the study had a significant contribution on the corrosion behavior of titanium alloy with the highest change attributed to laser treatment, followed by immersion time, and finally pH. We can conclude that the Er,Cr:YSGG laser could improve the corrosion resistance of Ti–6Al–4V in the acidic and neutral medium.

Notes

Conflict of interest

The author declares that there is no conflict of interest.

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Copyright information

© Springer International Publishing AG, part of Springer Nature 2018

Authors and Affiliations

  • Dalia A. Abd El daym
    • 1
  • Mostafa E. Gheith
    • 2
  • Nadia A. Abbas
    • 3
  • Laila A. Rashed
    • 4
  • Zeinab A. Abd El Aziz
    • 5
  1. 1.Dental Research Center, Ministry of Health and PopulationCairoEgypt
  2. 2.Department of laser applications in dental surgeries, National Institute of Laser Enhanced ScienceCairo UniversityCairoEgypt
  3. 3.Department of Prosthodontic, Faculty of Oral and Dental MedicineCairo UniversityCairoEgypt
  4. 4.Department of Medical Biochemistry and Molecular Biology Faculty of MedicineCairo UniversityCairoEgypt
  5. 5.Central Metallurgical Research and Development Institute (CMRDI)CairoEgypt

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