Negative Effect of Laser Treatment with Nd:YAG on the Corrosion of Amalgam in Artificial Saliva

This work involves study the corrosion behavior of amalgam in artificial saliva at pH=6 and 73 o C after using Nd: YAG, where this laser used for healing the cancer. This study indicates that laser surface treatment affecting corrosion resistance of amalgam, and the results indicate that the laser treatment don`t give the desired purpose because the nature of amalgam and the presence of mercury in this alloy which may be evaporate and it give toxicity for human and the energy of treatment must be low to avoid evaporation. In this work, laser treatment with two pulses under experimental conditions gave little improvement in corrosion resistance of amalgam compared with one and three pulses through the corrosion parameters of polarization behavior.


Introduction
Hitherto, the possible danger from the internal source of corrosion products, dental amalgam fillings, has been mostly overlooked.Presently, this problem is devoted special attention [1].Though a proof of amalgam biocompatibility has never been presented, amalgam is still by far the most extensively used material for dental restorations [2].One of the possible reasons may be the very interdisciplinary character of the problem.For correct answer, specialist competence in the following fields is required: materials science, corrosion/electrochemistry, toxicology, medicine/diagnostics, physical biology, analytical chemistry.The common type of dental amalgam is an alloy containing typically in weight-%; 05 Hg, 70 Ag, 15 Sn, Cu, Zn.
There are many authors studied the behavior of dental alloys in artificial saliva to measure the resistance of these alloys toward many materials.Rajendran et al. were studied the corrosion behavior of mild steel (MS) , zinc coated mild steel (MS-Zn) and stainless steel 716L (SS) in artificial saliva at pH=6.0 and 73 o C in presence of D-Glucose by utilizing potentiodynamic polarization study and AC impedance spectra.The results of this study indicated that in the presence D-Glucose all three alloys showed better corrosion resistance than in the absence D-Glucose because in presence D-Glucose will form metal (Ni-Cr-Fe)-glucose complex on metal surface, it is compact, close packed and stable [10].In another study, Rajendran et al. studied corrosion behavior of same materials in the presence of spirulina powder (a tiny aquatic plant has been eaten by human since prehistoric times) using the same technique and they showed that the corrosion resistance was SS716L > MS > MS-Zn [16].
The aim of this work is an attempting to improve the corrosion resistance of amalgam in artificial saliva at pH=6 and 73 o C by leaser treatment.

Preparation of Samples and Electrolyte
The used alloy in this study was amalgam;(Ag 06.3wt%, Sn 27.6 wt%, Cu 14.3 wt%) which prepared by amalgamator.And then cold mounted using pyrax polymers to obtain only surface area.
The open side was polished mechanically to a mirror finish, rinsed in distilled water and stored in desiccators.The electrolyte reference used was modified Fusayama artificial saliva [13], which closely resembles natural saliva, with composition of (5.4 g/L KCl, 5.4g/L NaCl, 5.956 g/L CaCl 2 .2H 2 O, 5.69 g/L NaH 2 PO 4 .2H 2 O, 5.550g/L Na 2 S.9H 2 O and 1g/L urea).

Corrosion Test:
Polarization experiments were performed in WINKING M Lab 255 Potentiostat/Galvanostat from Bank-Elektronik with electrochemical standard cell with provision for working electrode (amalgam), auxiliary electrode (Pt electrode), and a Luggin capillary for connection with saturated calomel electrode SCE reference electrode.Electrochemical measurements were performed with a potentiostat by SCI electrochemical software at a scan rate 7 mV.sec - .
The main results obtained were expressed in terms of the corrosion potentials (E corr ) and corrosion current density (i corr ) in addition to measure the Tafel slops by Tafel extrapolation method.

Laser Surface Modification (LSM)
The laser that used in this work is Nd:YAG laser with an output of second harmonic generation of 1564nm, laser energy of 665mJ, 75ns pulse duration, and 5.0mm spot size.One advantages of laser surface modification LSM was the obtainment of smooth surface for treated alloy.

Surface Characterization
The microstructures of specimens of amalgam as shown in Fig.
(1) for untreated and LSM treated specimens.They indicate laser spots in the specimens.
LSM treatment has effect on surface of specimens due to laser melting and rapid cooling, while the bulk of material (which known as saturated solid solution) remained unaffected.

Open Circuit Potential Measurements (OCP)
The electrical potential at the metal electrolyte interface is strongly dependent on the nature and the concentration of the electrolyte, pH and surface conditions.As a result, the electrochemical reactions at this interface vary with time.
The OCP-Time was measured with respect to SCE for 15 minute with scan rate 15mV in aerated electrolyte of artificial saliva at pH=6 and 73+1 o C for amalgam.
Fig. (2) shows OCP-time measurement for untreated and treated amalgam in artificial saliva.OCP-time measurement for untreated amalgam shows gradual rising in potential magnitudes for all duration test to reach -955 mV.The potential increase shows that the alloys become thermodynamically more stable with time.While the open circuit potential (E oc ) for treated amalgam decreases due to gradually breakdown of passive layer and gradually decreased in potential magnitudes because the passive layer don't cover alloy surface completely, and the laser treatment affecting the phases in amalgam especially mercury.But generally the E oc were nobler than for untreated specimen.

Linear Polarization
This test is a simple method for evaluating the corrosion rate, sufficiently sensitive to measure even a low corrosion rate [17], the test was conducted at a scanning rate of 5.0mA/sec.The current densities were calculated with reference to the samples geometrical area.
During anodic or cathodic polarization, or at open circuit, ion transfer reaction of metal ions and oxygen ions will take place.A clear process is the anodic corrosion of a passive metal in the steady state.In such a case, metal ions travel through the oxide film with a constant rate and are transferred in an ion transfer reaction at the interface oxide/electrolyte.The rate of that passive film dissolution depends on the local potential drop at the interface, the pH and the activity of the metal ions at the oxide surface [19].
The polarizations test started at a cathodic potential in relation to the corrosion potential, therefore the surface passive film was at least partially removed due to the highly reducing initial potentials as shown in Fig. (2) which illustrates the linear polarization for untreated three dental alloys in oral environments (artificial saliva) only.
After immersion in the electrolyte for 15 min., a linear polarization test was carried out for the specimens in artificial saliva to determine the values of polarization data.The dissolution of amalgam take place in anodic region (upper section of curve).
While the lower section represents reduction reaction which includes evolution of hydrogen molecules because of the acidity of electrolyte (artificial saliva) as follows: In addition to reduction of oxygen as follows [25]: The corrosion current densities (i corr ) were obtained from the polarization curves by extrapolation of the cathodic branch of the curves to the corrosion potential E corr .The average values of corrosion parameters are shown in Table (1).
These data indicate that laser treatment of the amalgam with one and three pulses shift E corr toward noble direction, while laser treatment with two pulses shifts E corr toward active direction.The corrosion current densities values (i corr ) shift to higher values for treatment with one and three pulses, but the treatment with two pulses shift (i corr ) to lower value.It is known that any factor that enhances the value of (i corr ) results in an enhanced value of the corrosion rate on pure kinetic ground.The rate (C R mpy) of corrosion in a given environment is directly proportional with its corrosion current density (i corr ) in accordance with the relation [21]: ……(7) where C R (mpy): corrosion rate in mil per year, e: equivalent weight of alloy (gm), ρ: density of alloy (gm/cm 7 ) and i corr : corrosion current density (mA.cm -2 ).
The laser treatment leads to increment in the surface hardness which was very little with laser treatment in comparison with untreated specimen.This increment can be due to the fact that laser radiation has caused a smoother surface.Generally, change in surface hardness shows a microstructure modification in metallic bonds.The comparison of linear polarization curves indicate a few important points which are as follows: the corrosion rate for laser treated specimen reduced for treated amalgam only with two pulses.This implies that treated specimen release hydrogen easier and act as an electron donor to electrolyte [22].
Increase of corrosion resistance probably means that most inclusions at the surface have been dissolved in the structure due to melting or alternatively, they are covered by molten material.
Treatment with one and three pulses shows no significant changes in its behavior.An interesting feature is the systematic shift of the pitting potential in the active region with a laser treatment despite the formation of a semipassive film at lower current densities, i.e. corrosion is observed.This indicates the formation of unstable passive film that is subtle to severe pitting with laser treatment.This is observed by NasserAl-Aqeeli in his work about the corrosion behavior of electrodeposited and laser irradiated Ni-Co nanostructured alloy [27].

Cyclic polarization
Cyclic polarization data were recorded by potentiostat with electrochemistry software.The polarization scan was initiated after immersing the specimen for 15 minutes and scanning the potential in the more noble direction at the scan rate of 7mV/sec.When -355mV were reached, the scanning direction was reversed.
Potentiodynamic measurements were carried out in order to determine the initiation and propagation of local corrosion, which is associated with the breakdown of passive protective film.
The breakdown potential (E br ) is the one at which the anodic current increases considerably with applied potential.The potential, at which the hysteresis loop is completed upon reverse polarization scan, is known as the protective potential or repassivation potential.
Breakdown potential is a sign of local corrosion but the measure of pitting susceptibility is the difference between the breakdown potential and the repassivation one.The protection potential represents the potential at the intersection of hysteresis curve with passive domain.Below this potential the propagation of existing localized corrosion will not occur.If the difference between breakdown and the repassivation potential is increasing, the chance in the appearance of pitting is greater and its propagation in depth is more intense.In other words, the hysteresis loop increases as the susceptibility of material to corrosion increases.
From Figure (7), it can be seen that the forward scan is the same of reverse scan, this type of cyclic polarization curve is known to resist localized corrosion [24].It is also observed that the reverse scan curves meet the forward scan curve along the passive range.The potential for the reverse scan curves are

Conclusion
The laser treatment always used to improve the corrosion resistance, but in this work can be conclude that the laser treatment for amalgam don`t give the desired purpose because the nature of amalgam and the presence of mercury in this alloy which may be evaporate and it give toxicity for human and the energy of treatment must be low to avoid evaporation.In this work, laser treatment with two pulses under experimental conditions gave little improvement in corrosion resistance of amalgam compared with one and three pulses through the corrosion parameters of polarization behavior.
Chang et  al. [17]studied electrochemical behavior on microbiology related corrosion of metallic dental materials in the presence of streptococcus mutants.While Latifa Kinani et al. studied effect of eugenol (occurs widely as a component of essential oils and is a major constituent of clove oil) on the titanium corrosion in artificial saliva enriched with eugenol at different concentration by utilizing electrochemical measurement and scanning electron microscopy[14].

Negative Effect of Laser Treatment with Nd:YAG on the Corrosion of Amalgam in Artificial Saliva
……………………………………………………. Lec.Fatima I.Sultan more positive than that for the forward scan in the laser treatment with two pulses.

Table ( 1): Corrosion
parameters of untreated and treated amalgam with leaser in artificial saliva at pH=6 and temperature 73 o C.