In recent years, magnesium alloys have inspired a great interest as eco-friendly structural materials for automobiles and aircrafts due to their properties like high strength to weight ratio, low cost of production, ease of machinability, high damping capacity, castability, weldability and recyclability [1,2]. Apart from these well-known applications, magnesium alloys find their utility as orthopedic biomaterials because of their non-toxicity, lighter density than implant materials, greater fracture toughness compared to hydroxyapatite, similar elastic modulus and compressive yield strength values as that of natural bone and good biocompatibility.
Lightweight and excellent damping characteristics of magnesium alloys make them a popular material choice in sport equipments. Magnesium alloys have great heat transfer property, ability to shield electromagnetic interference and radio frequency interference, etc.
Emerging Materials Research
But the poor corrosion resistance is their limitation in complete utilization. Magnesium alloys are mainly classified into two major alloying systems. The second group of alloys containing various elements such as rare earths, zinc, thorium and silver, but not aluminum, have improved elevated temperature properties compared to the alloys which contains aluminum as major alloying element .
Aluminum increases the corrosion resistance property of the alloy by altering the composition of the hydroxide film formed on the surface.
Corrosion of Magnesium Alloys - 1st Edition
Zinc increases the tolerance limit of iron, copper and nickel. Similarly, manganese improves the corrosion resistance property of the alloy, as well as it reduces the effect of impurities when their tolerance limits were exceeded. The influence of 14 different elements on the corrosion behavior of magnesium alloy in salt water was studied by Hanawalt et al. Considerable amounts of research have been carried out for improving the corrosion resistance property of the magnesium alloy by adding various elements and by different techniques [7—12]..
The reports are available in the literature on the corrosion behavior of magnesium alloys in various aqueous media containing different concentration of sodium chloride, sodium borate buffer, sodium sulfate, acidic, neutral and basic buffer, sodium bicarbonate, simulated biological fluid, etc. But a very few investigations are available in aqueous organic medium like ethylene glycol, which finds significance in automotive industries. Corrosion of engine components by the coolant is a major issue in automotive industries. Song and St.
John have reported the studies on the corrosion behavior of pure magnesium in ethylene glycol solution. Slavcheva and Schmitt have investigated the corrosion behavior of AZ91 magnesium alloy in 50 wt. Huang et al. Slavcheva et al. Cylindrical test coupon was mounted in epoxy resin to get a constant exposed area of 0. This exposed area was ground by using emery papers of different grade — After grinding with emery papers, the surface was polished on a polishing wheel using legated alumina as abrasive to get a mirror finish.
Then the specimen was washed with double distilled water and degreased with acetone. The specimen was dried properly before immersing in the corrosive medium.. Composition of the Mg—Al—Zn—Mn alloy.. Conventional three electrodes compartment Pyrex glass cell with platinum as counter electrode, saturated calomel electrode as reference and Mg—Al—Zn—Mn alloy as working electrode were used.
The polarization studies were carried out immediately after the EIS studies on the same exposed electrode surface without any additional surface treatment.. The cyclic polarization measurements were carried out after the attainment of steady state open circuit potential for 15 min. Impedance measurements were carried out at the open circuit potential by the application of a periodic small amplitude of 10 mV over a wide range of frequencies from kHz to 0. Nyquist plots were used to analyze the impedance data..
In all the above electrochemical measurements, at least three similar results were considered and average values have been reported.. The corresponding EDX spectra were subsequently recorded.
A1 was used to record the optical image after etching the specimen in acetic-picral. Etching reagent was prepared by using 5 mL of acetic acid, 6 g of picric acid, 10 mL of water and mL of ethanol. The alloy surface was etched to reveal the microstructure of the Mg—Al—Zn—Mn alloy ..
Table of Contents
Similar curves were obtained at other temperatures also. From Fig. The anodic polarization curves, representing the anodic dissolution of the magnesium alloy show inflection points characterized by two different slopes at potentials more positive than corrosion potential. This results from some sort of kinetic barrier effect, most probably by the deposition of corrosion product surface film followed by its dissolution at increased anodic overvoltage [27,28].
The cathodic polarization curves are characterized with distinctly linear Tafel regions, and they represent the hydrogen evaluation reactions through the reduction of water . The overall shapes of the polarization curves remain the same in the presence of different concentrations of chloride ions, which indicate no change in the corrosion mechanism as the concentration of chloride ions changes. The catholic polarization curves were used to measure the electrochemical parameters by extrapolating the linear Tafel regions of the curves to the OCP, as the anodic curves do not possess distinct linear Tafel regions.
The potentiodynamic polarization parameters, such as corrosion current density i corr , corrosion potential E corr and cathodic slope b c are tabulated in Table The corrosion rate v corr was calculated using the following Eq. From Table 2 it is evident that the rate of corrosion increases with the increase in chloride ion concentration.
This is attributed to the dissolution of the partially formed Mg OH 2 film in the presence of chloride ions as MgCl 2 , which is having more solubility than Mg OH 2 film . Several studies have established that chloride is an effective corrosive for magnesium and its alloys [2,13,19,17].. The corrosion of magnesium alloys in aqueous medium proceeds through an electrochemical reaction between magnesium and water to produce magnesium hydroxide and hydrogen as per the following Eq.
The difference in potential has been attributed to the formation of Mg OH 2 film on the metal surface .
Effect of solution treatment on stress corrosion cracking behavior of an as-forged Mg-Zn-Y-Zr alloy
The anodic dissolution of magnesium and its alloys involves two oxidation processes as represented in Eqs. As the monovalent magnesium ion is unstable, undergoes oxidation to divalent magnesium ion through a series of reactions involving unstable intermediates like magnesium hydride as shown in Eqs. They are also found to have inter metallic inclusions of MnAl 2 . According to the results published in the literature, magnesium alloys exhibit higher corrosion resistance than pure magnesium . Small additions of Mn have been reported to increase the corrosion resistance of magnesium alloys and reduce the effects of metallic impurities [44,45]..
The cyclic polarization curves are generally used to study the pitting corrosion in corrosive environment. The forward scan represents the polarization behavior of the non-corroded areas, while the reverse scan is associated with the corroded areas . The corrosion potential on the reverse scan is more negative than the forward scan, which indicates that corroded area still acts as anode for further galvanic corrosion and protects the non-corroded area as cathode, resulting in accelerated corrosion in the corroded area, causing the formation of pits . The formation of visible pits on the surface of the alloy is as shown in Fig.
Optical photos of Mg—Al—Zn—Mn alloy surface a before cyclic polarization and b after cyclic polarization.. Electrochemical impedance spectroscopy is a non-destructive technique which provides minimal perturbative signal and can be used to study the response of corroding electrodes to small amplitude alternating potential signals of largely varying frequencies .
Similar plots were obtained at other temperatures also. The Nyquist plots consist of two capacitive loops at higher and medium frequencies and the beginning of an inductive loop at lower frequency region. The high frequency capacitive loop is attributed to the charge transfer of corrosion process and oxide film effects.
The medium frequency capacitive loop represents mass transport as a consequence of diffusion or electrolyte ingress through corrosion product. Song and Xu have suggested that the medium frequency capacitive loop represents the combination of pseudo resistance and capacitance of the film formation and dissolution process . The equivalent circuit models derived by simulating the electrochemical behavior of the alloy-medium interface help in the best understanding of the impedance results.
cockraheadpojust.tk The circuit fitment for the obtained results was done by ZSimpWin software of version 3. The impedance data points neglecting the low frequency inductive loop, can be analyzed using an equivalent electrical circuit EEC as shown in Fig. The simulation of impedance data points is presented in Fig. The high frequency response can be simulated by a series of two parallel resistances — constant phase element R-CPE networks: the charge transfer resistance R ct in parallel with the double layer CPE Q dl and the resistance of the surface film R f in parallel with the film CPE Q f.
The resistance R dif and CPE Q dif associated with diffusion are assigned to the middle frequency response . The constant phase element Q dl is substituted for the ideal capacitive element to give a more accurate fit, as only by the introduction of constant phase element the lack of homogeneity and even porosity of the electrode surface can be accounted ..
Equivalent electrical circuit used for the simulation of experimental impedance data points.. The impedance of the constant phase is given by the following Eq. The capacitance and CPE are related by the following Eq. The collective resistance associated with the high frequency loop R hf is inversely related to the corrosion rate [54,55]. The values of R hf for the corrosion of the alloy are listed in Table 2.
It is evident from Table 2 that the corrosion rate increases with the increase in the concentration of chloride ions as indicated by the reduction in the R hf values.. The capacitance of the double layer is related to the thickness of the double layer by the Helmholtz model Eq. Similar plots were obtained in the presence of other concentrations of chloride ions also. In Fig. Both these facts indicate the increase in the rate of corrosion as the temperature is increased.
The same trend is reflected in the results listed in Table 2 , as the values of i corr and?? The shapes of the polarization curves and Nyquist plots remain unaltered with the change in temperature, indicating that the temperature alters only the rate of alloy corrosion but not the mechanism of corrosion.. Arrhenius equation Eq.
Related Corrosion Behaviuor Of Stressed Magnesium Alloys
Copyright 2019 - All Right Reserved