Effects of TiO2 Addition on the Electrochemical Performance of the Overlay Welded High-manganese Steel Cladding

Series of overlay flux-cored wires with different content of TiO2 were fabricated, and different claddings of high-manganese steel are welded by flux cored arc welding (FCAW). The influences of TiO2 addition on the electrochemical behaviour of the FCAW claddings over a high-manganese steel were studied. The results demonstrate that all the LPR values of the overlay welded claddings are higher than that of the base metal. However, the LPR values did not increase with the addition of TiO2. The EIS simulation results indicate that the addition of titanium dioxide improves the corrosion resistance of the high-manganese steel cladding, and the trends of the Rp values is highly consistent with the change of LRP value. At the same time, the grain refinement was achieved by the TiO2 addition. In conclusion, the proper content of the titanium dioxide in high-manganese steel cladding contributes to its high LRP and Rp values, in this paper, the suitable content of the TiO2 addition is 20g (0.2wt. %).


Introduction
It is a well-known fact that overlay welding is an excellent technique to improve the surface properties of a substrate.There are many remanufacturing technologies used to accomplish the overlay weld, such as flux cored arc welding, plasma arc welding, electroslag welding and Laser welding [1,2].For example, a cladding weld of high-manganese steel overlaid on the surface of a carbon steel can strengthen the hardness, wear resistance and crack resistance, which is widely used in many fields such as machinery, metallurgy and mining equipment.However, because of the harsh working condition, those high-manganese steel claddings were easy to damage caused by corrosion, fatigue, and stress corrosion cracking.
In order to improve the corrosion resistance, many elements have been added into the base metal of high-manganese austenitic steels.Such as Al, Cr, Cu, and Mo, which improve the tendency to form passive films on the steel surface [3][4][5].However, there are few researches focused on the effects of TiO 2 addition, especially for the electrochemical performance of the overlay welded high-manganese steel cladding.Many researchers have reported that the addition of titanium dioxide can affect the characteristics of the high-manganese steel cladding.This paper aimed to investigate the influence of TiO 2 addition on electrochemical behavior of the high-manganese steel.
In this study, we fabricate series of overlay flux-cored wire with different content of TiO 2 , and made different claddings of high-manganese steel using FCAW.The linear polarization for different deposited metals on the Q235A steel were tested, aim to investigate the effect of the TiO 2 addition on the corrosion properties of the overlay weled high-manganese steel claddings.

Experimental
Titanium dioxide powder of -200μm mesh (99.7% purity level) was added to the base flux-cored wire in the range from 10g (0.1wt.%) to 40g (0.4wt.%) %, the major chemical composition of the weld metal of base overlay flux-cored wire was listed in Table 1.The Q235A steel according to Chinese standard GB/T11253-2007 was used as the experimental base metal with the size of 200 mm × 100 mm × 10 mm , and the chemical composition was listed in Table 2.The different claddings were obtained by FCAW with the following welding parameters as shown in table 3. Using the EWM (Phoenix 521 forceArc puls DW) as the welding machine and use the mixture of 80% Ar+20%CO 2 as protecting gas.The plate was cladded up to a thickness of 15 mm.The prepared claddings were cut to different pieces for microstructure observation and electrochemical measurement by linear cutting.The specimens for metallographic examination were mechanically polished using standard metallographic procedure and etched with a 4 vol% nital solution.
After cladding the welded samples were cut to prepare the electrochemical test with the size of 10 mm × 10 mm × 0.5 mm.The LP and EIS testes were carried out by an Autolab PGSTAT302N electrochemical workstation.And a three-electrode system was used to test the EIS corrosion properties.Using the cladded sample as the working electrode, the Pt plate as the auxiliary electrode, the reference electrode was Ag/AgCl (3 mol/L KCl), and use the 3.5 wt.% NaCl water solution as the test electrolyte.The working electrode was soaked in the electrolyte for 30min before electrochemical test, and set the open circuit potential (OCP) as the EIS mode over a frequency range of 10 4 ~10 -2 Hz.While the EIS test was finished, all samples were immediately charactered by SEM analysis (JEOL, JXA-8100).

Microstructure characterization
The micrograph of cladding metals made by the addition of TiO 2 and without addition of TiO 2 are presented in Fig. 1.The microstructures of the entire clad layer revealed austenite structure.It is observed that the claddings fabricated by the addition of 20g TiO 2 (0.2 wt.%) and 30g TiO 2 (0.3 wt.%) resulted in fine equiaxed grains compared to the others.When increasing the TiO 2 addition to 40g (0.4 wt.%), the fine equiaxed grains disappear, and the grain size also become larger, which might be contributed by the generate of fine particles such as TiC, TiN [6].The EIS measurements were conducted for cladded samples in a 3.5% NaCl solution in order to evaluate the corrosion protection performance of the overlay welded high-manganese steel.Figures 3 show the results for Nyquist, Bode, and Bode-phase plots.It was possible fit well with all the impedance spectra using the equivalent circuit model shown in Fig. 4.
The Nyquist plots of all the clad layer show similar semicircles, represented as the capacitive arc.The diameter of the semicircle of the overlay welded claddings with TiO 2 addition was bigger than that of the base metal without TiO 2 addition.The 1# and 2# sample show similar and also the biggest semicircle diameter which reflected the best passivation behavior.Moreover, the imperfect semicircle diameter for the clad layer of 1# and 2# is higher than that of the 3# and 4#, which agrees well with the LPR values.
As seen in Fig. 3b, the Bode plots show higher electrical resistance value at the low-frequency range, which represent the stronger corrosion resistance and consistent with the result of the above Nyquist plots.One time-constant was observed on the bode plots of all samples (only one maximum phase lag at the middle frequency range).As can be seen from the Bode-phase curves, except the 3# all the clad layer exhibit certain constant plots at the middle frequency range [7].Also, the phase angles values remained far below to 80° except the 3#, which indicated the formation and growth of the corrosion pits at middle frequency [8,9].And the maximum phase angles values of 3# was over 80°, which revealed the formation of a passive film.In other words, it means a pure or ideal capacitive response [10].
Figure 4 shows equivalent circuit best describing the electrochemical processes [11].In this equivalent model, Rs is the solution resistance; Rp stands for the resistance of the sample in the solution, and CPE1 counts for the constant phase element corresponding to the capacitance of the corrosion pits.As the most important characteristic of all the EIS value, the polarization resistance Rp represent the resistant ability to against the corrosion [12,13].Table 5 shows all the values of equivalent circuit elements of the cladding samples in 3.5% NaCl solution.The highest value of Rp was reached to 554.6 Ω•cm2, which indicated the 2# with 20g TiO 2 (0.2 wt.%) possesses best anticorrosion ability of the entire clad layer.The simulation results demonstrate that the addition of titanium dioxide improves the corrosion resistance of overlay welded high-manganese steel cladding.

Corrosion morphologies analysis
Fig. 5 shows SEM images of the corrosion morphologies of the high-manganese steel cladding with different TiO 2 addition after the electrochemical tests in a 3.5% NaCl solution.It is apparent that, the rust films formed at all the test samples.The rust films became increasingly dense and uniform as the increase of the TiO 2 addition.Fig. 4a shows the morphology of the rust presented in a loose porous structure, and obvious cracking phenomena can be easily seen on the clad layer surface.It is obvious that a dense passive film without crack can be observed on the surface of the cladded sample with 20g TiO 2 (0.2 wt.%) addition, which prevents the corrosive penetration trend of the experimental solutions.Meanwhile, this phenomenon well explained and consistent with the close LRP value and EIS measurements result.It is found that there are more corrosive crack appears on the surfaces, as increase the addition amount of the TiO 2 .It will accelerate the corrosive solution penetration trend of the experimental solutions [14].

Conclusion
The results demonstrate that all the LPR values of the overlay welded claddings are higher than that of the base metal.However, the LPR values do not increase with the addition of TiO 2 .To simulate the measured impedance EIS data, the equivalent circuit was used, the simulation results indicate that the addition of TiO 2 improves the corrosion resistance of the high-manganese steel cladding and the trends of the Rp values is highly consistent with the change of LRP value.At the same time, the grain refinement is achieved by the TiO 2 addition.In conclusion, the proper content of the TiO 2 in highmanganese steel cladding contributes to its high resistance values and good corrosive surface morphology, as in this paper, the suitable content of the TiO 2 addition is 20g (0.2wt.%).As resulted in the increase of the polarization resistance to present a similar electrochemical behavior as the highmanganese steel cladding.

Table 1 .
Chemical composition of the weld metal of base overlay flux-cored wire %

Table 3 .
Welding parameters of the welding process

Table 4 .
The linear polarization resistances of the weled metal at different TiO2 Adddition