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International Journal of Corrosion
Volume 2012 (2012), Article ID 146486, 9 pages
Corrosion Behavior of Fe40Al Alloy with Additions of Ti, Ag, and Cr in Molten KCl +
1U.A.E.M.-C.I.I.C.A.P. Av. Universidad 1001, Col. Chamilpa, 62500-Cuernavaca, Mor., Mexico
2I.I.E., Gerencia de Procesos Termico, Av. Reforma 120, Temixco, Mor., Mexico
3U.A.E.M.-Facultad de Biología, Av. Universidad 1001, Col. Chamilpa, 62500-Cuernavaca, Mor., Mexico
4Instituto de Investigaciones Metalúrgicas, Universidad Michoacana de San Nicolás de Hidalgo. Edificio “U”, Ciudad Universitaria, 58066—Morelia, Michoacán, Mexico
Received 10 October 2011; Accepted 27 January 2012
Academic Editor: W. Ke
Copyright © 2012 R. Ademar et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
The effect of 2.5 at.% Cr, Ti, and Ag on the corrosion behavior of Fe40Al intermetallic alloy in KCl-ZnCl2 (1 : 1 M) at 670°C has been evaluated by using electrochemical techniques. Techniques included potentiodynamic polarization curves, linear polarization resistance (LPR), and electrochemical impedance spectroscopy (EIS) measurements. Results have shown that additions of both Cr and Ti were beneficial to the alloy, since they decreased its corrosion rate, whereas additions of Ag was detrimental, since its additions increased the corrosion rate, although the alloy was passivated by adding Ag or Cr. The best corrosion performance was obtained with the addition of Cr, whereas the highest corrosion rate was obtained by adding Ag. This is explained in terms of the stability of the corrosion products formed film.
Sodium and potassium impurities present in the form of chloride or sulfates are very corrosive constituents under certain combustion conditions such as waste incinerators and biomass-fired boilers [1, 2]. Early failure of the thermal components frequently occurs due to the complex reactions between the metallic materials and the hostile combustion environment. Incineration has become a viable technology for disposing of various types of wastes, including municipal, hospital, chemical, and hazardous. Problems with process equipment resulting from fireside corrosion have been frequently encountered in incinerators. The major problem is the complex nature of the feed (waste) as well as corrosive impurities which form low-melting point compounds with heavy and alkali metal chloride which prevents the formation of protective oxide scales and then causes an accelerated degradation of metallic elements . In particular, under reducing conditions such as those typical of the operation of waste gasification plants or even under localized reducing conditions, which frequently arise in the case of incorrect operation of waste incineration systems, it is difficult to form protective oxide scales such as Cr2O3, SiO2, and Al2O3 on the surface of structural materials. Thus, the corrosion attack can be further enhanced under reducing atmospheres in the presence of salt deposits .
The effect of individual KCl, NaCl, and their mixtures with heavy metal chlorides or sulfates on the corrosion behavior of a series of alloy systems has been studied in detail so far [3–9]. It is generally realized that Cr is not as effective element for corrosion resistance of Fe-base and Ni-based alloys due to chloride attack. In contrast, alumina- (Al2O3) forming alloys exhibit promising candidate materials considering the better high temperature resistance of alumina over chromia and their properties such as lower cost, low density, high strength, and good wear resistance [10–13].
In order to obtain more information on the corrosion mechanisms taking place, electrochemical techniques are more appropriated than gravimetric methods. Since a molten salt acts as an electrolyte, electrochemical techniques such as potentiodynamic polarization curves and linear polarization resistance (LPR) measurements can be used just like Ducati et al.  showed in one of the earliest works on this topic. Zeng et al. used electrochemical impedance spectroscopy (EIS) studies for the corrosion of two-phase Cu-15Al alloy in the eutectic (Li, K)2CO3 at 650°C  and for Ni3Al alloy in (Li, Na, K)2SO4 at 650°C . They found that the complex plane of the electrochemical impedance suggested that the fast corrosion rate of these alloys was due to the formation of a nonprotective oxide, and that the corrosion process was controlled by the diffusion of oxidants. Shirvani et al.  used polarization curves to study the corrosion performance of the slurry Si-modified aluminide coating on the nickel base superalloy In-738LC exposed to Na2SO4—20 wt. % NaCl melt at 750°C. Zhu et al.  used Tafel extrapolation, linear polarization resistance, chronopotentiometry, and EIS to determine corrosion rates of iron and iron-based alloys in molten carbonate melts for both cathode and anode in molten carbonate fuel cell (MCFC) environments. Cuevas-Arteaga and coworkers [19–21] have successfully used electrochemical techniques to study the corrosion of materials used in sulfate+vanadate environments and obtained information about both the type and mechanisms of corrosion on these environments. This work deals with an electrochemical study of the effect of adding 2.5 at. % Cr, Ti, and Ag to a Fe40Al intermetallic alloy corroded in KCl-ZnCl2 (1 : 1 M) at 670°C.
2. Experimental Procedure
Cast ingots of binary Fe-40 at. % Al and ternary Fe40Al-X (X = 2.5 at. % Cr, Ti or Ag) alloys were fabricated using a high-frequency vacuum induction furnace. Iron, Aluminum, Chromium of high purity (99.9%) were placed in an alumina crucible placed inside a graphite crucible in order to be induction melted under vacuum. The molten Fe-40Al, Fe40Al-X alloys were poured into a rectangular copper mould. The produced ingots were cut by a diamond wheel cutter in small rectangular parallelepiped pieces of surface area ranging from 1.5 to 2.5 cm2. Electrochemical techniques included potentiodynamic polarization curves, linear polarization resistance (LPR), and electrochemical impedance spectroscopy (EIS) measurements. Details of the experimental setup for the electrochemical cell used in this work are given elsewhere . The body of the cell was a 15 mL ceramic crucible. The most important elements were a reference and auxiliary electrodes, made of a 0.5 mm diameter platinum (Pt) wire inside a mullite tube and filled with commercial refractory, ceramic cement as shown in Figure 1. The amount of salt in each run was 0.5 g for an exposed area of 1.0 cm2 under static conditions, without stirring the molten salt. The electrical contact was made by welding an 80 wt. % Cr—20 Ni, Nicrome wire to the specimen. The stability of the platinum reference electrode was measured by measuring its free corrosion potential versus another identical platinum reference electrode and the results are shown in Figure 2. We can see that during the first ten minutes the potential value shifted towards more active values for up to 55 mV more positive, but after this, the potential value dropped and reached a more or less stable value after approximately one hour or so. Polarization curves were obtained by polarizing the specimens from −500 to +1000 mVPt with respect to the free corrosion potential value, , at a scanning rate of 1.0 mV/s. Corrosion current density values, , were calculated by using the Tafel extrapolation method, which is valid for the sweep rates used here according to  and taking an extrapolation zone of ± 250 mV around the value once it was stable. LPR measurements were carried out by polarizing the specimen from −10 to +10 mV with respect to , at a scanning rate of 1 mV/s every 20 minutes during 24 hours. EIS measurements were done in the frequency interval of 0.05 to 30000 Hz at the value by using a PC4-300 Gamry potentiostat. The amplitude of the input sine-wave voltage was 10 mV as recommended by previous works done on this topic [15, 16]. Working salt consisted of 500 mg/cm2 of an eutectic mixture of KCl-ZnCl2, 1 : 1 M, analytical grade for each test. The testing temperature was 670°C in static air condition. Prior the tests, the surfaces of specimens were prepared by the standard technique of grinding with SiC from 240 to 600 grit emery paper, washed with water, and degreased with acetone. Some specimens taken after the LPR measurements were mounted in bakelite and polished to analyze the corroded surface in a scanning electron microscope (SEM) with an accelerating voltage of 10 keV. Microchemical analysis was carried out with energy dispersive of X-ray analyzer (EDX) attached to the SEM.
3. Results and Discussion
Polarization curves for Fe40Al alloyed with 2.5% Cr, Ti, or Ag are shown in Figure 3. This figure shows that unalloyed Fe40Al intermetallic alloy shows an active behavior only, with an value close to −330 mV and a corrosion rate, expressed in terms of the value, close to 0.3 mA/cm2. The anodic current density increases as the potential is made more anodic, but around 100 mV a kind of anodic limiting current density is reached, maybe because of the formation of a layer of corrosion products. With the addition of both Ti and Ag, the is made more active, reaching values of 156 and −98 mV, respectively. Similarly, the corrosion current density value increases in all cases, especially with the addition of 2.5Ag. Thus, the addition of Ag increases the corrosion current density value and shifted the value towards the active region, which shows that this element is detrimental for the base alloy. On the other hand, a passive region was found with Cr and Ag at a passivating potential value, , of −562 and 87 mV, respectively, reaching a much higher passive current density, , with Cr (around 35 mA/cm2) than that reached with Ti (close to 5 mA/cm2). Table 1 summarizes all these parameters, which shows the beneficial effect of both Cr and Ti.
The change in the linear polarization resistance value with time, , for the different alloys in the KCl + ZnCl2 mixture are shown in Figure 4. It can be seen that by alloying with either Ti or Cr the value increases for up to 3 orders of magnitude in the case of Cr, and thus, a decrease in the corrosion rate. This can be due to the formation of an Al2O3 layer, probably reinforced with Cr2O3. In both cases, the value shows a decrease, but after that it increases once again, indicating a disruption of the external protective corrosion products layer, but a rapid re-establishment of it. Unlike this, the addition of 2.5Ag brings a decrease in the value, and thus, an increase in the corrosion rate. The value for unalloyed Fe40Al and Fe40Al + 2.5Ag intermetallic alloys shows a trend to increase as time increases, indicating a decrease in the corrosion rate and an increase in the protectiveness of the external layer, although this effect is marginal, not sufficient to reach the values shown by the alloys containing either Cr or Ti. This means that the oxides formed in presence of Ag are more unstable as compared to those formed in presence of either Cr or Ti, leading to an increase in the corrosion rate for the alloy containing Ag.
Nyquist diagrams for unalloyed Fe40Al exposed to the KCl + ZnCl2 mixture is shown in Figure 5, where it can be seen that for times shorter than 5 hours approximately, data describe a single capacitive-like semicircle, with its center in the real axis and its diameter increasing with time indicating that the corrosion process is under control by charge transfer from the metal to the electrolyte through the double electrochemical layer. For longer times than 5 hours or so, however, in addition to the high-frequency capacitive semicircle there is what seems an uncompleted low-frequency semicircle. The diameter of the low-frequency semicircle is much bigger than that for the high-frequency semicircle and it has a trend to decrease as time elapses. The emergence of a large capacitive loop at the low-frequency part may indicate the formation of a protective scale on the alloy surface.
For the alloy with addition of 2.5Cr, Figure 6, the data describes, at all exposure times, a semicircle at high and intermediate frequency values followed by a second semicircle at low-frequency values, with its diameter increasing with time. This time, the high-frequency semicircle diameter is lower than that for unalloyed Fe40Al base alloy, Figure 5. A similar behavior is observed for alloys with the addition of either Ag or Ti (Figures 7 and 8). However, the low-frequency semicircle diameter this time shows an erratic behavior, since sometimes it increases with time, and some other times it decreases. This explains why the addition of 2.5Ag increases the corrosion rate of the base alloy, since Nyquist diagrams show the formation of unstable oxides due to the disruption of the external layer formed by the corrosion products or to the dissolution of any protective oxide layer formed on top of the alloy, which is dissolved by the molten salt, Figures 3 and 4.
In order to have a better picture of this behavior, some micrographs of corroded specimens were examined in the scanning electronic microscope. Figure 9 shows a micrograph of corroded Fe40Al base alloy together with the corresponding X-ray mappings of Al, Cl, and O. The presence of O and Al on the external layer, although the Al-layer was not continuous, leads us to the assumption that there was some Al2O3 layer formed on top of the alloy. In addition to this, there is Cl present, which indicates that the molten salt has penetrated and dissolved the external, protective Al2O3 layer, leading to a continuous degradation of the alloy. Similar features were present the alloy with addition of Ag and Ti, Figures 10 and 11, respectively, where it can be seen the presence of an Al-containing oxide on the external surface, dissolved by chloride ions, although the oxide present in the Ag-containing alloy seems to be more porous than that present in the Ti-containing alloy. It can be seen that in the Ti-containing alloy the formed oxide is external and the presence of any Ti-containing oxide, perhaps TiO2 is in very negligible. On the other hand, on the Ag-containing alloy, it can be seen that Ag reacted more with O than Ti, the main corrosion product seems to be silver oxides with, such as AgO and Ag2O and, maybe, some silver chloride, AgCl2; maybe this is the reason why this alloy is less corrosion resistant than the one containing Ti.
EIS results can be represented by equivalent circuit shown on Figure 12, where is the solution or electrolyte resistance, CPEdl the double-layer capacitance, the charge transfer resistance, CPthe capacitance of any formed film on the surface, its resistance. The impedance of a CPE is described by the expression : where is the admittance, is √-1 and is and the used frequency. As an example of this, Tables 2 and 3 give the parameters used to simulate the EIS data for Fe40Al with 2.5Ag and 2.5Ti, respectively. It can be seen that, generally speaking, the values for both alloys are smaller than the corrosion products resistance, , indicating that the corrosion resistance of both alloys is given by this film. However, the value for the Ti-containing alloy shows an erratic behavior as time elapses, that is, the film thickness decreases and increases, indicating that the film is constantly detached from the alloy surface. In addition to this, the values of and deviate from 1, indicating a strong dispersion effect which is common in molten salt corrosion . Moreover, their values at the later stage were smaller than those at the initial stage, which may be related to the partial loss of the molten salt due to evaporation or to the roughness of the surface oxide layer.
A study of the effect of 2.5 at. % Cr, Ag, and Ti on the corrosion behavior of Fe40Al in molten KCl-ZnCl2 (1 : 1 M) at 670°C has been carried out by using electrochemical techniques. Polarization curves have shown that by adding either Cr or Ag the alloy was passivated, although the passive current density was higher with the later. Addition of either Cr or Ti were beneficial, since the lowest corrosion rate was obtained by their addition, whereas the highest corrosion rate, even lower than that obtained with the unalloyed Fe40Al base alloy, was obtained by adding 2.5Ag, being the addition of this element detrimental for the alloy. EIS data showed that the corrosion rate was controlled by the formation of an external corrosion products external film, which was dissolved by the Cl-containing molten salt. The stability of this external film provides the corrosion resistance of the alloy.
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