Electrochemical Analysis of Some Biodegradable Mg-Ca-Mn Alloys

BOGDAN OPRISAN, DECEBAL VASINCU*, STEFAN LUPESCU*, CORNELIU MUNTEANU, BOGDAN ISTRATE*, DIANA POPESCU, CATALIN PLESEA CONDRATOVICI, ALINA RAMONA DIMOFTE, KAMEL EARAR Gr.T. Popa Medicine University of Iasi, Faculty of General Medicine, 16 University Str., Iasi, Romania University of Medicine and Pharmacy Grigore T. Popa, Faculty of DentalMedicine, 16 Universitatii Str. 700115, Iasi, Romania Gheorghe Asachi University of Iasi, Faculty of Mechanical Engineering Department, 43 DimitrieMangeron Str., 700050, Iasi University „Dunărea de Jos”of Galați, Faculty of Medicine and Pharmacy, 35 Al. I. Cuza Str., Galati, Romania

We also reported that the distribution coefficient (k) of Mn (1.1) means that it does not accumulate on the solid / liquid side, and Mn can be a solute within the primary dendrites without forming compounds containing Mn. The Mn solute in dendrites changes the surface film composition of Mg alloys, which also affects the exchange density of the reaction current of the evolution of the hydrogen cathode [20]. The corrosion behavior of the aqueous solution of Mg alloys is the result of an electrochemical process in which Mg ionizes anodic, with the predominant hydrogen evolution of the cathodic reaction based on hydrogen. Refining the microstructure with the addition of Mn is beneficial for the alloy [21].
In this paper, we systematically investigated the microstructure and, in particular, the corrosion resistance of some alloys in the Mg-0.5Ca-xMn system. This study has been executed with the increase of Mn content in the alloy. Table 1 presents the quality certificate of MgCa and MgMn ingots, sent by the company from where they were purchased, Hunan China Co.

Materials and methods
The alloys Mg-0.5Ca-0.5Mn, Mg-0.5Ca-1Mn, Mg-0.5Ca-1.5Mn, Mg-0.5Ca-2Mn, Mg-0.5Ca-3Mn were made from high purity compounds, the chromium composition is presented in table 1. The melting and alloying operations were carried out in a graphite crucible under a gas mixture (Ar) at 750 C. The melting was maintained for 30 min at 750 C to ensure that all the required alloying elements were dissolved in the molten alloy.Afterwards, the crucibles were left to cool, followed by the removal of the alloys from crucibles and thenrefined into the shape of a bar.
The bars were cut into pills of 12 mm in diameter and the frame of 4 mm thick, sanded with abrasive paper up to 2500SiC and finished with alumina suspension, the samples were left to dry in the open air and chemically attacked to observe the microstructure of the surfaces.

Used alloys:
a) melting of the material b) the resulting ingots Fig. 1

Microstructure
The microstructure of the cast alloys was observed using an optical microscope. The intermetallic secondary phases of the cast alloys were analyzed by X-ray diffraction (XRD) using mono-chromatic Cu-K(alpha) radiation.

Electrochemical testing
For the corrosion test, a Volta Lab 21 potentiostat was used. The corrosion current determined in this way represents the corrosion current that appears at the metal / medium interface when the metal is immersed in the solution and can be measured directly by electrochemical methods. To calculate the corrosion rate of an alloy immersed in a corrosive environment, it is necessary to know the density of the instantaneous current that is determined by the polarized resistance method. A cell containing three electrodes was used: platinum auxiliary electrode, saturated calomel electrode and working electrode. The measurements were made at 20ºC and the electrolyte was naturally aerated, the linear polarization curves were recorded at a scanning speed of the electrode potential of 1mV / s and the cyclical polarization curves were performed at a scanning speed of 10mV / s. This method is used to determine the corrosion current at the corrosion potential of the metal or alloy, from the linear polarization curve obtained for relatively small overvoltages. The corrosion current thus determined is actually the  corrosion current that appears at the metal / corrosive medium interface when the metal is immersed in the solution and cannot be measured directly by electrochemical methods. This is actually an instant corrosion current.

Surface analysis
The cross sections of the samples were also prepared by focusing the focused ion beam (FIB) to analyze their surface films by high resolution transmission electron microscopy (NU AM FOLOSIT ASTA-refaci) (Quanta 3D scanning microscope) and energy dispersion X-ray spectroscopy ( EDS).

Microstructure and XDR analysis
Optical micrographs for Mg-0.5Ca-xMn alloys are shown in Figure 1. It was observed that by increasing the quantity of Mn, the microstructure is refined, and the particles of the second phase that contain Ca tend to distribute in the interdendrite areas. It can be observed that the compound Mg2Ca is formed at the grain boundary of Mg. All alloys showed α-Mg peaks. From the EDS analysis, the presence of other impurities such as Cu, Ni and Fe can be explained by this impurity content of the alloy composition.
XRD analysis shows us three major phases: α-Mg, Mg2Ca and Mg0.975Mn0.025, the previously specified compounds resulting from the peaks of the graph in Figure 3. It is shown that Mg0.975Mn0.025 has the same hexagonal crystallographic structure as pure Mg and Mg2Ca has a monoclinic structure. All alloys showed α-Mg peaks.

Electrochemical testing
In the table 3 are presented the parameters obtained from the electro-corrosion resistance tests of the experimental alloys Mg0.5Cax (x = 0.5; 1; 1.5; 2 and 3 wt.%) Mn, tests performed in SBF solution. Since magnesium alloys eliminate high amounts of gas from the surface of the samples, gas bubbles are still forming. These gas bubbles were removed by using a magnetic stirrer that operated at a relatively slow rate of agitation of the electrolyte solution. At each test, the fresh electrolyte solution was changed. Chemical composition of immersion solution is presented in table 2. In all cases are observed in processing the presence of both types of reception for anodic and cathodic tea while the branches may have values close to the opposite sign. A larger difference is observed only under test conditions with 2% Mn. The anodic branch is much larger than the cathodicand this fact may be a less pronounced passage of a metallic element while looking at it in others. With the exception of the cases with 0.5% and 2% Mn, similar corrosion rates with 0.56 mm per year were obtained. Electro-corrosion currents (jcor) confirm this behavior. The corrosion current is thus determined during the corrosion cure current, the interface of the metal / corrosive medium appears when it is metallic and is immersed in solutions and cannot be measured by the direct electrochemical method. This is actually an instantaneous corrosion current. The resistance of the polarization and other experimental ones are close to this one.
The Tafel variations show the similar behavior of the experimental alloys with a small differentiation of the potential of the MgCaMn alloy. The linear polarization resistance method is probably one of the most widespread applications of electrochemical measurements in the laboratory and is very well applied in the case of uniform (generalized) corrosion.  Figure 5 shows the cyclical curves characteristic of the experimental samples from the MgCaMn system. From the analysis of the cyclical polarization curves (cyclic voltamograms), one of them can obtain information regarding the type of electrochemical process that takes place at the electrode / environment interface (such as: generalized corrosion, localized corrosion, passivation, oxidation reductions of the solution species), evaluation characteristic potentials (corrosion potential, piercing potential, re-passivation potential, protection potential). The general appearance of the corrosion is of generalized corrosion more accentuated for the Mg0.5Ca2Mn(SCRIE FORMULA) and MgCa1Mn samples with a larger loop given by the corrosion intensity. In both cases mentioned, with 1% and 2% Mn respectively, the generalized corrosion is supported by many pitting corrosion zones which evolve very quickly and which, by combining their effects, it transforms in a very short time into generalized corrosion. Surface analysisof corroded samples Figure 6 shows the SEM images after the 5000x magnification corrosion test, there are no traces of corrosion at points and the degradation occurs by oxidizing the surface layer of the alloy and detaching it from the substrate.
It is observed the identification of a smaller percentage of oxygen on the surface of the material compared tothesample Mg0.5Ca0.5Mn (6.6 comparedto 25% -Tab.2), also the quantities of salts are lower or equal to the chlorine-based compounds. Adding 1% Mn increases the electro-corrosion resistance of the MgCa alloy and significantly reducing the amount of oxide on the surface compared to the behavior of the Mg0.5Ca0.5Mn sample, but,the best resistance to electrocorrosionisthe Mg0.5Ca3Mn alloyfollowedby Mg0.5Ca2Mn.

Conclusions
Thisresearchshowsthat with the increase of the Mn content in the alloy, the microstructure is more refined thus showing an increase in the strength of the alloy.Following the results of X-ray diffraction tests, we can observe a series of Mg2Ca compounds having a monoclinic form and Mg and Mg0.975Mn0.025 having a hexagonal shape.On the morphology of the surface we can see that the compound Mg2Ca is formed at the boundary of Mg grains, and Mg0.975Mn0.025 are formed separately. Mg-05Ca-0.5Mn Mg-05Ca-1Mn Mg-05Ca-1.5Mn Mg-05Ca-2Mn Mg-05Ca-3Mn Fromtheanalysis of the results recorded in the case of electro-corrosion resistance experiments it was observed that the highest corrosion rate is the alloy with 0.5% Mn respectively 0.85 mm / year and the smallest alloy with 3% respectively 0.55 mm / year.The Tafel curves show close values of the ba and bc branches which represents a close activity for the two types of reactions, reduction and oxidation or anodic and cathodic, which take place at the contact between an alloy and an aqueous electrolyte environment.The cyclical polarization curves indicate for all samples a generalized surface corrosion more pronounced in the case of samples with 1 and 2% Mn respectively.The experimental alloys showed, in all cases, a combined corrosion between galvanic, intercrystallineandpointcorrosion.
The microstructuralsurfaceanalysis by SEM electron microscopy highlights a generalized surface corrosion that was observed with the formation of compounds on the surface of the materials in the electrolyte solution. Zonal traces of corrosion at points were also observed due to the inhomogeneities or inclusions that appear in the experimental alloys.