This work deals with the relation between microstructure and electrochemical behavior of four iron-based FeCoC ternary alloys. First, the arc-melted studied alloys were characterized using differential thermal analyses and scanning electron microscopy. The established solidification sequences of these alloys show the presence of two primary crystallization phases (
Cobalt is one of the first transition series of elements. It lays between Fe and Ni and close to Cu in the periodic table. In nature, it shows a strong spatial association with these metals. Cobalt is a critical metal and it has many strategic and irreplaceable industrial uses (supperalloys, magnets, corrosion- and wear-resistant alloys, high-speed steels, cemented carbides, diamond tool, etc.) [
This work is an academic study. It deals with the relation between the microstructure and electrochemical behavior of four iron-based FeCoC ternary alloys.
The solidification behavior of these alloys was studied in an earlier work [
The studied alloys were arc melted in an argon gas atmosphere from pure elements (iron at 99.98 pct and cobalt at 99.5 pct from Aldrich Chemical Co.) and graphite. The solid-liquid and the solid-solid transformation temperatures were followed by a DTA-Netzsch 404S differential thermal analysis (cooling rate of 10 K/min) under argon atmosphere. The observation of the phases was performed using an optical microscope (ZEISSICM405) and a scanning electron microscope (SEM-JEOL).
The electrochemical tests were conducted using a VoltaLAB PGZ301 potentiostat. The corrosive medium consisted of neutral aqueous solution containing 10−3 M NaHCO3 and 10−3 M Na2SO4. The polarisation curves are plotted in potentiodynamic mode. Potential was scanned from −0.8 V/SCE to +1 V/SCE in the direction of the increasing potentials at a scanning rate of 1 mV/s. Before each polarisation, the working electrodes were immersed in the test solution for 45 min. The electrochemical experiments were carried out at 25°C with agitation in presence of oxygen.
In an earlier study [
Compositions, transformation temperatures, and solidification sequences of FeCoC studied alloys. (*Temperature not detected by our differential thermal analysis apparatus limited to temperature lower than 1550°C).
Alloy | Compositions (wt. %) | Temperatures/(°C) | Solidification sequences | ||
Fe | Co | C | |||
Co2 | 90.96 | 4.84 | 4.20 | * | L |
1163 | L | ||||
1150 | L | ||||
753 | Pearlite | ||||
Co3 | 89.37 | 6.50 | 4.13 | * | L |
1170 | L | ||||
1153 | L | ||||
763 | Pearlite | ||||
Co6 | 90.90 | 8.45 | 0.65 | 1496 | L |
1416 | L + | ||||
830 | |||||
756 | Pearlite | ||||
Co8 | 89.52 | 10.00 | 0.48 | 1477 | L |
1463 | L + | ||||
812 | |||||
772 | Pearlite |
Liquidus surface projection of the Fe-Co-C system in the iron-rich corner (metastable system) [
Potentiodynamic polarisation curves of the studied alloys in nondeaerated solution containing 10−3 M NaHCO3 and 10−3 M Na2SO4 at 25°C are presented in Figure
Electrochemical parameters of FeCoC ternary alloys corrosion (immersed in 10−3 M NaHCO3 + 10−3 M Na2SO4, at 25°C).
Alloy | |||||
---|---|---|---|---|---|
Co2 | −395 | 16.8 | 1.7 | 169 | −179 |
Co3 | −390 | 18.2 | 1.3 | 99 | −178 |
Co6 | −347 | 1.8 | 9.3 | 111 | −82 |
Co8 | −337 | 1.7 | 9.8 | 110 | −88 |
Potentiodynamic polarisation curves of Co2, Co3, Co6, and Co8 alloys in nondeaerated solution NaHCO3 10−3 M + Na2SO4 10−3 M, at 25°C.
We gathered in Table
Variation of
Alloy | Co8 | Co6 | Co2 | Co3 |
---|---|---|---|---|
1.7 | 1.8 | 16.8 | 18.2 | |
Fe/C | 186.5 | 184.5 | 21.66 | 21.64 |
Co6 and Co8 steels have a better corrosion resistance than Co3 and Co2 cast iron. This would be allotted to more important carbon content in cast iron.
The Co8 alloy corrosion current density is slightly lower than that of Co6. For these two alloys, the effect of carbon and cobalt content does not appear. However, the microstructures of these alloys (Figures
Optical micrograph (×200) showing the matrix (1) and pearlite (2).
Co8 optical micrograph (×200) showing the matrix (1) and pearlite (2).
In fact, it was reported that pearlitic structures corrode faster than spheroidized materials and steels containing fine pearlite corrode rapidly than those with coarse pearlite. In addition, the degree of dispersion of the carbide is quantitatively characterized by the total amount of interfacial contact between the ferrite and cementite phases [
In addition, Co2 alloy is more resistant than Co3 alloy in the experimental conditions of this study. The examination of the microstructures of these two samples (Figures
Co2 electron micrograph showing graphite (1) and
Co3 electron micrograph showing graphite (1) and
This work follows the study concerning the solidification behavior of iron-based FeCoC ternary alloys. The electrochemical behavior of some of these alloys is reported to solidification observed microstructures.
The results show the presence of two primary crystallization phases (
The interpretation of the electrochemical results in relation with the observed microstructures leads to conclude that Co6 and Co8 steels have better corrosion resistant than Co2 and Co3 cast iron because of the more important carbon content in cast iron. Moreover, the corrosion current density increases with the decrease of in the Fe/C ratio. In addition, it was noted that the corrosion current density increases when the morphology is finer.