In this work the corrosion resistance of a high content nickel alloy, Inconel 600, was investigated in mixed NaCl-KCl salts at 700, 800, and 900°C for 100 hours in static air. Investigation was carried out using electrochemical techniques such as polarization curves, rest potential measurements, linear polarization resistance, and electrochemical impedance spectroscopy. Corroded specimens were analyzed by scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS). Electrochemical measurements showed an increased degradation rate of Inconel 600 with increasing test temperature. SEM and EDS analysis show that the damage experienced by Inconel 600 is greater than that determined by electrochemical measurements. This damage was identified as internal corrosion due to the reaction of Cl2 with the alloying elements (Cr and Fe); however, at 900°C the internal damage was minor and it was associated with the nickel content in the alloy.
Problems with process equipment resulting from fireside corrosion have been frequently encountered in waste incinerators and biomass-fired boilers. 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 chlorides which prevent the formation of protective oxide scales and then cause an accelerated degradation of metallic elements [
Chemical composition (% by weight) of the working electrodes (Inconel 600) was 8.26% Fe, 17.20% Cr, 0.49% Si, and balance Ni. Corrosion tests were carried out using specimens with dimensions of 10 × 5 × 3 mm. The specimens were grinded with emery paper until grade 600 and washed with water and finally with acetone. For electrical connection, specimens were spot-welded to a Ni20Cr wire. Ceramic tubes were used for isolating the electrical wire from the molten salt; the gap between the ceramic tube and electrical connection wire was filled with refractory cement. Specimen size, surface preparation, and corrosive mixtures were the same for all electrochemical tests. The NaCl-KCl corrosion mixture was prepared with analytical grade reagents. Dried chloride salts were first weighted in a 1 : 1 mole ratio and then subjected to a mechanical milling in an agate mortar to obtain well-mixed reagents. This mixture has a eutectic temperature of 659°C [
Electrochemical measurements carried out were polarization curves (
After testing, corroded specimens were mounted in thermosetting resin and polished. The specimen cross-section was analyzed by SEM to investigate the morphology and distribution of reaction products. X-ray mapping and microprobe analysis were carried out using an X-ray energy dispersive (EDX) analyzer associated with a Zeiss DSM960 scanning electron microscope.
Figure
Electrochemical parameters of potentiodynamic polarization tests.
Temperature |
|
|
|
|
---|---|---|---|---|
700°C | −992 | 0.115 | 394 | 211 |
800°C | −1139 | 0.500 | 265 | 323 |
900°C | −1103 | 0.730 | 330 | 439 |
Polarization plots of Inconel 600 in NaCl-KCl at the different temperatures.
Figure
Progress of
Progress of
Current density (
The
Experimental impedance spectra, for Inconel 600 in NaCl-KCl at different exposure times, are shown in Figure
Progress of Nyquist plots of EIS response of Inconel 600 in NaCl-KCl at different temperatures: (a) 700°C, (b) 800°C, and (c) 900°C.
It is known that the magnitude of the impedance module and maximum phase angle is associated with a more capacitive response of the protective oxide. In all cases, the plateau zone at the low frequency region was not defined. This shows that, at all test temperatures, the alloy was not able to develop a stable passive layer and hence it had undergone a continuous corrosion process. This behavior indicates that the corrosion process was under charge transfer from the metal to the electrolyte through the double electrochemical layer.
Although electrochemical methods are extremely useful in studying corrosion processes, they alone do not provide enough information to elucidate the mechanism of the system under study. Therefore, the use of complementary techniques like scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) has been suggested in order to clarify both the morphology of the attack and the chemical composition and distribution of elements present. Combination of these methods provides the information to understand reactions occurring on the surface [
Figure
SEM image and EDS analysis of a cross-section of Inconel 600 after corrosion test at 700°C in NaCl-KCl mixture.
SEM image and EDS analysis of a cross-section of Inconel 600 after corrosion test at 800°C in NaCl-KCl mixture.
SEM image and EDS analysis of a cross-section of Inconel 600 after corrosion test at 900°C in NaCl-KCl mixture.
Despite the fact that has been reported that Ni-based alloys have excellent performance in chlorides rich environments [
The appearance shown by the SEM micrographs indicates that the magnitude of the attack is greater than that assumed by the electrochemical measurements. This can occur when there are gaseous species that can diffuse to regions like the spongy zone where molten salts cannot do it easily. Then it is possible that in addition to reactions between molten salt and the protective oxides, there were reactions with gaseous species, such as Cl2, and the alloying elements, as follows [
The corrosion results described above imply clearly that a high Ni content is very effective in improving the corrosion resistance of Inconel 600 in NaCl-KCl melt while Cr plays a detrimental role under the same conditions. This is explained because the breakdown of the protective oxide can occur by dissolution into the molten salts and the degradation rate can be fast if the oxide has a high solubility.
Considering that Cl2 and O2 are the main species of the NaCl-KCl-air system that contribute to the degradation of materials in waste incinerators environments, the construction of phase stability diagrams of these species with the main elements of the alloy is a useful tool for understanding the corrosion behavior at high temperature in this environment. Figure
Chemical reactions and equilibrium constants for the Fe-Cr-Ni-Cl-O system.
Number | Reaction |
|
||
---|---|---|---|---|
700°C | 800°C | 900°C | ||
1 | Cr(s) + Cl2 = |
15.2418 | 13.2619 | 11.7406 |
2 | Fe(s) + Cl2 = FeCl2(l) | 11.8886 | 10.4408 | 9.2427 |
3 | Ni(s) + Cl2 = |
8.5735 | 7.0918 | 5.8697 |
4 | 2Fe(s) + O2 = 2FeO(s) | 22.3934 | 19.7013 | 17.4531 |
5 | 2Ni(s) + O2 = 2NiO(s) | 16.3501 | 13.9913 | 12.0392 |
6 | 6FeO(s) + O2 = 2 |
18.7011 | 15.8875 | 13.5643 |
7 | 3FeCl2(l) + 2O2 = |
7.2749 | 6.1735 | 5.2338 |
8 | 4 |
10.9159 | 8.4684 | 6.4435 |
9 | 2 |
−33.7821 | −30.4114 | −27.1388 |
10 | 2 |
−13.3384 | −11.0541 | −9.1263 |
11 | 4Cr(s) + 3O2 = 2 |
94.7492 | 83.4590 | 74.1012 |
12 | 2FeO(s) + 2Cl2 = 2FeCl2(L) + O2 | 1.3838 | 1.1802 | 1.0322 |
13 | 2NiO(s) + 2Cl2 = 2 |
0.7969 | 0.1923 | −0.2999 |
14 | 2 |
5.0434 | 3.6087 | 2.1961 |
15 | 2FeCl2(L) + Cl2 = 2FeCl3(L) | 0.9457 | 0.5304 | 0.1967 |
16 | 2 |
−23.6952 | −23.1939 | −22.7465 |
17 | 2 |
−11.4470 | −9.9933 | −8.7329 |
Thermodynamic stability diagrams for Fe-Cr-Ni-Cl-O at (a) 700°C, (b) 800°C, and (c) 900°C.
Regarding the metallic elements that constitute the Inconel 600, it is observed that from a thermodynamic perspective Ni is the most stable element. Therefore, Ni will remain immune in O2 and Cl2 environments where the Fe and Cr would be corroding continuously. Therefore, in environments found in garbage incinerators in the temperature of 700–900°C, the Ni-rich alloys will show better performance compared with those richer in Fe or Cr. This analysis is consistent with reported studies where they indicate that Ni or its alloys show better performance in chlorides-rich environments compared with Fe and Cr or their alloys [
The corrosion resistance of Inconel 600 in NaCl-KCl at 700°C, 800°C, and 900°C was studied by electrochemical techniques an SEM and EDS analysis. Corrosive attack was documented by SEM and EDX analysis. Polarization plots showed that corrosion rate (
The authors declare that there is no conflict of interests regarding the publication of this paper.
Financial support from Consejo Nacional de Ciencia y Tecnología (CONACYT, México) (project CVU 270660) and Ph.D. scholarship given to G. Salinas-Solano (Registration no. 219258) are gratefully acknowledged.