Effect of Solid Solution Temperature on Microstructure and Corrosion Resistance of CoNiCrMo Alloy

Abstract

Influence of solid solution temperature on microstructures and corrosion properties of CoNiCrMo alloy were investigated by means of optical microscopy (OM), scanning electron microscope (SEM) and electronic differential system. The corrosion resistance of CoNiCrMo alloy in 0.9%NaCl aqueous solution was also investigated. The results indicated that with the increase of solution treatment temperature, the dendrites of CoNiCrMo alloy gradually disappeared. Thus, the corrosion resistance of the alloy decreased. When the solution temperature is 1200ºC, the dendrites almost disappeared and most carbide dissolved, and begins to spheroidization. The corrosion resistance of CoNiCrMo alloy treated at 1000ºC for 1h is the highest than the one at 1100 ºC and 1200ºC. Its self-corrosion potential is -424mV and self-corrosion current density is 1.02×10^-8A/cm².

Keywords:
Co-based alloys; heat treatment; corrosion resistance; microstructure

1. Introduction

Co-based alloys that have excellent comprehensive properties, such as high temperature fatigue resistance, low thermal expansion coefficient, heat corrosion resistance and wear resistance, good biocompatibility, high mechanical properties and high thermal conductivity, were widely used in the fields of military, chemical industry, energy and Bioengineering ^1-3.

At present, the researches on biomedical metal materials have become a hot spot in the global material industry ^4-6. It is well known that Co-based alloys have been widely used in various medical implants to replace and repair the body bones, damaged joints, dental decay, mutant necrotic tissues and organs^7,8. Moreover, Co-based alloy has stronger corrosion resistance and the similar biocompatibility compared with stainless steel, and the abrasive resistance of Co-based alloys is the best among all biomedical metal materials. Co-based alloys are more suitable for long-term implant under poor load conditions^9,10. Some studies ^10-13 indicated that Co-based alloys are very sensitive to heat treatment process, and different treatment temperature and holding time during the solution and aging will greatly affect the microstructures, mechanical properties and corrosion resistance of this alloy. For example, Laser cladding Co-based-VN alloy-Ti composite coatings was aging treated at 550 ºC, 650 ºC and 750 ºC, respectively. It was found that the intensities of Cr₂₃C₆, TiN and VC diffraction peaks were gradually improved as aging temperature increased.The microhardness and wear resistance of Co-based-VN alloy-Ti composite coatings were firstly reduced and then increased as aging temperature increased, the microhardness was increased by 7.9%, and the wear loss was reduced by 8.4%, for aging treatment at 750 ºC. The microstructure and tensile properties of Co-27Cr-6Mo (mass%) alloys heat-treated at 673-1373 K were studied. When low-temperature heat treatment (LTHT) was applied at 673-873 K, both the ultimate tensile strength and elongation synchronously improved compared with the solution-treated alloy.

According to above references, Co-based alloys have excellent comprehensive properties used in biomedical field. When Ni is added to co-based alloys, this alloy will have high toughness and elasticity. Therefore, the CoNiCrMo alloy will be studied in this paper. However, now, it is lack of no systematic report on the effect of different heat treatment conditions on the microstructures and corrosion properties of CoNiCrMo alloy. Therefore, it is quite necessary to know the relation between heat treatment process and microstructural properties for Co-based alloys.

In this paper, the effect of solid solution temperature on the microstructures and corrosion properties of CoNiCrMo alloy will be deeply studied, and the mechanism of heat treatment process on the corrosion resistance of CoNiCrMo alloy will be clarified.

2. Experimental Techniques

In this experiment, CoNiCrMo alloy was melted in high purity argon atmosphere by medium frequency induction furnace. Then cast to remove the outer surface of the non-uniform composition, and obtain CoNiCrMo alloy ingot of uniform composition. The chemical composition of this alloy is shown in Table 1. And then, a series of samples were cut into Φ 9mm×19mm cylinders by lining cutting machine. Some samples were firstly treated by solid solution at 1000ºC, 1100ºC and 1200ºC, and then these samples were treated by water cooling after holding for 1h. All above samples treated will be polished and polished. The microstructure of the alloy was observed by means of metallographic microscope, and a solution that consisted of HCl : HNO₃=3:1 was used. The surface morphology and element distribution of the samples were observed by JSM-6380LV scanning electron microscopy (SEM) and energy dispersive spectrometer (EDS).

Table 1
The Chemical composition of CoNiCrMo alloy (wt.%).

Co	Ni	Cr	Mo	Fe	Si	C
27.40	30.50	22.97	5.44	11.79	1.75	0.15

Potentiodynamic polarization curves and electrochemical impedance spectroscopy (EIS) of the alloy were measured using Princeton Electrochemical Workstation Three-electrode system (PEWT). The working electrode was the sample to be tested, and the reference electrode was the saturated calomel electrode. Moreover, the auxiliary electrode was the platinum electrode, and the corrosion medium was in 0.9% NaCl solution. The temperature was maintained at 37±0.5ºC. The test area of the sample is 1cm². Before testing, the sample was placed in the 0.9% NaCl solution for 1h to stabilize. The potentiodynamic scanning rate is 0.5mV/s and the scanning range was from -1V to 1V. The frequency range of electrochemical impedance (EIS) measurement is from 10 mHz to 100 kHz, and the amplitude of excitation voltage is 10mV. The spectrum is analyzed by Zsimpwin software.

3. Results and Analysis

3.1 Microstructures

As Co and Ni can form infinite solid solution and Ni is an austenite stabilizing element. Therefore，the CoNiCrMo alloy with higher Ni content should be austenite structure. Fig. 1 shows the metallographic photograph of CoNiCrMo alloy after solution treatment at different temperatures. It can be seen from the graph that with the increase of solution temperature, the dendrites of Co-based alloy gradually disappeared and melted into the matrix. Meanwhile, the interdendritic precipitates also decreased. When the temperature rose to 1200ºC, the dendrites almost disappeared and most carbide dissolved, and begins to spheroidization.

Figure 1
Microstructure of CoNiCrMo alloy at different solution temperatures for 1h (a) 1000ºC; (b) 1100ºC; (c) 1200ºC

Figs. 2-4 are the SEM and EDS results of CoNiCrMo alloy after solution treatment at 1000ºC, 1100ºC and 1200ºC. As can be seen from the graph, the deeper color of the dendrites branches indicates that they are corroded, while the lighter color of the parts between the dendrites branches indicates that they are more resistant to corrosion. EDS analysis indicated that the dendrites branches were mainly composed of Co elements, thus the content of Co elements was significantly higher than that of the inter-branches of dendrites, while the inter-branches of dendrites were composed of corrosion-resistant elements. As shown in Fig. 2, the content of Co at B area is 27.40%, much higher than that at A area (11.99%). With the increase of solution temperature, the corrosion resistant elements between dendrites branches gradually dissolve into the matrix, but the residual content of Cr among dendrites branches is still very high. Further observation by EDS showed that the particulate matter contained higher Cr element, and there were obvious corrosion pits around the particulate matter, which were more evident at 1100ºC and 1200ºC (see Fig. 3 and Fig. 4). Some studies indicated that carbides in Co-based alloys often occurred in the form of M₂₃C₆ (M = Co, Cr, Mo) ^14-16. In this experiment, it was found that the content of Cr in carbides reached to 63.33% (point A in Fig. 4). Corrosion pits mostly occur around carbides, and the local corrosion performance is greatly reduced due to the presence of carbides.

Figure 2
SEM and EDS of CoNiCrMo alloy for 1h at 1000ºC solid solution treatment.

Figure 3
SEM and EDS of CoNiCrMo alloy for 1h at 1100°C solid solution treatment.

Figure 4
SEM and EDS of CoNiCrMo alloy for 1h at 1200°C solid solution treatment.

3.2 Corrosion resistance analysis

In order to compare the corrosion resistance of CoNiCrMo alloy in saline solution under different heat treatment conditions, the dynamic polarization curves and electrochemical impedance experiments were carried out respectively. Normally, the self-corrosive potential (Ecorr) in the polarization curve is a thermodynamic parameter to evaluate the corrosion resistance of alloys. The self-corrosive potential reflects the degree of corrosion difficulty of alloys. With the increase of self-corrosion potential, the corrosion of the alloys is more difficult. Corrosion current and polarization resistance are the dynamic parameters reflecting the corrosion resistance of alloys. Higher corrosion current indicates that once the material is corroded, the corrosion speed will be accelerated. Corrosion current reflects the corrosion degree and corrosion rate of alloys^17,18.

Fig. 5a is the dynamic polarization curve of CoNiCrMo alloy after different solution temperatures. The polarization curves of different treatment processes are approximately the same. Tafel linear fitting analysis of the polarization curves can obtain the relevant electrochemical parameters, as shown in Table 2.

Figure 5
Polarization and EIS curves of CoNiCrMo alloy at different solution temperature.

Table 2
Corrosion resistance parameters of CoNiCrMo alloy.

Temperature	Ecorr /mV	Icorr /(A•cm-2)	Rp/(kΩ•cm-2)	βa/V•dec-1	βc/V•dec-1
1000°C	-424	1.02×10-8	16700	0.926	-0.276
1100°C	-436	3.72×10-8	4900	0.649	-0.256
1200°C	-461	47.9×10-8	474	0.693	-0.298

From the fitting values in Table 2, it can be seen that the corrosion current densities of CoNiCrMo alloys treated by solid solution in saline water are relatively low. Among them, the corrosion current density is the smallest and the corrosion potential is positive after 1000ºC solid solution treatment. It can be seen from Fig. 5a that with the increase of solution treatment temperature the corrosion potential of CoNiCrMo alloy becomes more and more negative, and the corrosion current becomes larger and larger. From the point of view of polarization resistance, the value of polarization resistance decreases sharply with the increase of solution treatment temperature. With the increase of the polarization resistance, the corrosion resistance of the alloy is also improved further. With the increase of solution temperature, the polarization curve shifts to the right and downward gradually. This indicates that the corrosion potential is more negative, the corrosion current density is higher, and the corrosion resistance is worse. Therefore, the comprehensive analysis of self-corrosion current, self-corrosion potential and polarization resistance shows that the corrosion resistance of CoNiCrMo alloy after 1h solid solution treatment at 1000ºC is the best.

The corrosion resistance of CoNiCrMo alloy treated at 1000ºC for 1h is the best. Its self-corrosion potential is -424 mV, self-corrosion current density is 1.02×10^-8A/cm², polarization resistance is 16700 kΩ/cm², anode slope is 0.926 V/dec and cathode slope is -0.276 V/dec. The results are according to above microstructural analysis. With the increase of solution temperature, the poor Cr phenomenon and serious pitting corrosion occur around carbides in CoNiCrMo alloy after solution treatment at different temperatures. In addition, it can be seen that the breakdown potential (E_b) of each sample will occur corrosive pitting rapidly when the applied potential is greater than the breakdown potential (E_b). The breakdown potential of CoNiCrMo alloy after solution treatment at 1000ºC is the highest, 480 mV. This indicates that its resistance to pitting corrosion is the best, while that of CoNiCrMo alloy after solution treatment at 1200ºC is the lowest and its resistance to pitting corrosion is poor, which basically coincides with the analysis of scanning electron microscopy.

Fig. 5b shows the electrochemical impedance spectra of CoNiCrMo alloy under different solution temperature. The X-axis is the real part of impedance and the Y-axis is the imaginary part of impedance. It can be seen that there is only one capacitive impedance arc arc in the impedance test of CoNiCrMo alloy in saline solution different solution temperatures. The radius of capacitive impedance arc reflects the resistance value during charge transfer. The larger the radius of capacitance impedance arc, the larger the electron transfer resistance and the worse the conductivity, and the better the corrosion resistance of the alloys ^19,20. The radius of capacitive impedance arc of the alloy treated at 1000ºC is obviously larger than that of the other two temperatures, which indicates that the corrosion resistance of the CoNiCrMo alloy treated at this temperature is obviously better than that of the other two solid solution treatment temperatures.

4. Conclusions

(1) The dendrites of CoNiCrMo alloy gradually disappeared and melted into the matrix with the increase of solution temperature. Meanwhile, the interdendritic precipitates also decreased. When the solution temperature is 1200ºC, the dendrites almost disappeared and most carbide dissolved, and begins to spheroidization. However, the residual content of Cr among dendrites branches is still high. Corrosion pits mostly occur around carbides, and the local corrosion performance is greatly reduced due to the presence of carbides.

(2) With the increase of solution treatment temperature the corrosion potential of CoNiCrMo alloy becomes more and more negative. The corrosion resistance of CoNiCrMo alloy after 1h solid solution treatment at 1000ºC is the best. Its self-corrosion potential is -424mV and self-corrosion current density is 1.02×10^-8A/cm². The electrochemical impedance spectra (ESI) also indicated that the corrosion resistance of the CoNiCrMo alloy treated at this temperature is obviously better than that of the other two solid solution treatment temperatures.

5. Acknowledgement

This research is financially supported by the National Natural Science Foundation of China (61703200), the Innovation Foundation of Nanjing Institute of Technology (CKJA201404), the Opening Project of Jiangsu Key Laboratory of Advanced Structural Materials and Application Technology (ASMA201610) and the Shandong Provincial Natural Science Foundation, China (Grant No. ZR2016JL017).

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Publication Dates

• Publication in this collection
  24 Oct 2019
• Date of issue
  2019

History

• Received
  24 Jan 2019
• Reviewed
  24 Aug 2019
• Accepted
  27 Aug 2019