Manganese Constituent in Stainless Steels Oxidized in Oxygen Containing Water Vapor at 800 o C: High Temperature Oxidation and Volatilization

. Many industrial operations, such as solid oxide fuel cells (SOFCs) or combustion processes, have water vapor environments. Stainless steel was applied as part of these applications at high temperatures. The present work investigates high and low manganese content stainless steel that oxidized in an oxygen-containing 5% water vapor at 800 o C. SEM and EDX were performed to analyze oxide formation and ICP-OES was used to detect the volatile species that evaporated from steel surfaces. The results show that manganese in steel increases spallation and reduces Cr volatilization rates in high Mn stainless steel. The average Cr volatilization of high Mn samples is 0.09 × 10 -11 g cm -2 s -1 , which is about 14 times lower than the ones (1.25 × 10 -11 g cm -2 s -1 ), while Fe and Mn volatilization rates increase by 10 and 62 times, respectively.


Introduction
Stainless steel has been extensively used in various industrial applications.This is due to their excellent oxidation resistance and good mechanical properties.Chromium oxide (Cr2O3) can form on stainless steel which has a high chromium content (> 10.5-12.0wt.%) [1].It provides a dense, continuous, adherent steel substrate and a relatively slow growth rate.The prevention of Cr2O3 helps to increase high-temperature corrosion resistance and reduce the maintenance process due to material loss.Stainless steel is substantially more corrosive in the atmosphere containing water vapor than dry air [2,3], and this often arises in many applications.It has been reported that the amount of water vapor important to the hot-rolling process is around 19.5% [4] and can reach up to 40.0% in the combustion process [5].Including solid oxide fuel cells (SOFCs), which are potential technologies for producing sustainable energy [6].The water vapor can induce Cr evaporation from the chromium oxide in the form of CrO2(OH)2(g) at lower temperatures of 1500 o C, as follows: (1) [7].
As a result of chromia evaporation and inadequate Cr supplied from the steel, the protective Cr-rich oxide transforms into a non-protective Fe-rich oxide, and breakaway oxidation ensues [8].Besides chromium in stainless steel, manganese is a common constituent element in alloys and affects oxidation resistance.When high content is added to alloys, it can create an austenite phase, which is regarded as a nickel substitute for the production of austenitic stainless steel grades [9,10].It also is seven times cheaper than nickel at an equivalent weight [11,12].Indeed, various studies have been conducted on the effect of low-level manganese additions on the oxidation resistance of (Fe, Ni, or Co)-Cr alloys since 1970.The scales created were proportional to the bulk chromium content [13][14][15].They observed that adding up to 5% manganese to alloys containing around 20% chromium resulted in the creation of MnCr2O4.
For SOFCs with metallic interconnects, H. Falk-Windisch et.al., [16] studied Sanergy HT (0.25 wt.%Mn) and Crofer 22 H (0.40 wt.%Mn) stainless steels of chromium content ~23 wt.% in air-3% H2O at 650, 750 and 850 o C.They found that the presence of manganese in steels at high temperatures with low content contributes to the formation of a duplex-layered oxide scale with an outer layer of (Mn, Cr)3O4 whereas Cr2O3 is underneath.Moreover, the mass gain of Crofer 22 H stainless steel was higher than the others but reduced in chromium volatilization.This is due to the effect of Mn on the production of a continuous top layer of (Cr,Mn)3O4, which decreases Cr activity at the metal substrate compared to Cr2O3.Despite the fact that prior research [13][14][15][16]  the volatilized phase that causes mass loss from steel substrates.To better understand, the effect of manganese on chromium volatilization and oxidation resistance was comparable.In the present work, stainless steel with high manganese (Fe-15.3wt.%Cr-8.9wt.%Mn) and low content as AISI 430 stainless steel (Fe-16.4wt.%Cr-0.8wt.%Mn) were studied.The samples were oxidized in an oxygen atmosphere containing water vapor at 800 o C and then collected chromium, manganese, and iron from volatile species.The oxidation mechanism of the alloy is also discussed through the dependence of manganese content on the alloy.

Experimental procedures 2.1 Material
An AISI 430 stainless steel (low Mn) and special grade (high Mn) were cut into rectangular shapes (15 x 15 x 1 mm 3 ) and its chemical composition was measured using optical emission spectroscopy (Thermo Electron Corporation, ARL 3460) and reported in Table 1.The sample was ground on silicon abrasive papers of 180 up to 1000 grits and cleaned with deionized water and ethanol in an ultrasonic bath, respectively.It was dried by an air dryer.

Table 1. Chemical composition of the studied high and low
Mn stainless steel in wt.%.

Experimental setup
To begin, suspend each specimen one at a time inside a controlled-atmosphere furnace in a vertical direction, as the hottest part of the furnace is in the center.The desired furnace temperature was set at 800 o C for the test and the oxidation times were set at 1, 24, 48, 72, and 96 h.The velocity of argon gas was 1.0 cm s -1 .It flowed directly through the specimen in the vertical furnace while waiting for the temperature inside the furnace to rise to 800 o C. When the temperature reaches 800 o C, the oxygen gas is switched from argon with a velocity flow rate at 1.0 cm s −1 into a water flask.A 5% H2O content was controlled by maintaining the water temperature at 31 o C. The temperature was calculated according to the Clausius-Clapeyron equation using the enthalpy of water vaporization, 40,893 J mol −1 [17].The humidified oxygen flowed through the sample surfaces and condensed at the bottom column of the furnace.This condensate was known as a concentration solution.The tube and condenser were cleaned with 0.1 M HCl, and the cleaning solution was added to the concentration solution, which was then used to analyze the number of elements from vaporization using inductively coupled plasma optical emission spectroscopy (ICP-OES).Before and after each test, the samples were weighed using a precision balance.

Characterization
Scanning electron microscopy (SEM) was performed to provide the surface image of the sample, and SEM equipped with energy-dispersive X-ray spectroscopy (EDS) was used to examine elements.The X-ray diffraction (XRD) technique was utilized at room temperature to characterize the phases present on oxidized stainless steels, and the incident Cu-Kα, α = 1.5406Å was utilized.XRD patterns obtained from the experiment were matched with the standard patterns by X'Pert HighScore program which was reported as an ICDD number.

Results and discussion
The results of the testing were reported in four sections.Firstly, oxidation was tested of both low and high Mn stainless steel oxidized in O2-5%H2O for 1 h up to 96 h at 800 o C. Second, the volatilization rates of the samples were measured in O2-5%H2O at 800 o C for 96 h.SEM-EDS results of the oxidized sample and the XRD were exhibited in the last section.

Oxidation test
During the experimental workpieces, measured the weights of the specimens before and after putting the specimen in the furnace with a five-digit weighting machine.It was converted to mass gain which weight change divided by surface area of the specimen.Figure 1 shows mass gain in mg.cm -2 as a function of time for both stainless steel samples during the oxidation test in O2-5%H2O at 800 o C. We found that the mass gain of the high Mn stainless steel sample has values lower than zero and begins at 4 h.It means that spallation behaviors of the thermal oxide growth occur on the surface of the sample.In contrast, the mass gain of low Mn steel rises with time.The result provides that the Mn content in metal substrates has an impact on the formation of thermal oxide.

Volatilization rates
For the volatile species phase of the thermal oxide growth on stainless,  2 C 2023 entrapped at the end of the furnace tube in a condenser bottle.The quantitative determination of volatile metal was evaluated by the ICP-OES method.This analytic method can enable the elements in the 0.1 to 50 ppb [18].The volatilization rate was presented in terms of the mass of volatile metals per surface area and time.It was found that Cr volatilization rates in high Mn stainless steel are 14 times lower than in low Mn stainless steel, while Fe and Mn volatilization rates increase by 10 and 62 times.The dominant volatilization species phases may be hydroxide compounds of Cr Mn and Fe.There were CrO2(OH)2, Mn(OH)2 and Fe(OH)2, respectively [19].Table 2. Cr, Mn and Fe-species volatilization rates of both low and high Mn stainless steel oxidized in O2-5%H2O at 800 o C for 96 h.

SEM and EDS
Cross-sectional SEM images and EDS mapping results of the oxidized low Mn stainless steel after the test in O2-5%H2O at 800 o C for 96 h is revealed as shown in Figure 2 at 170x magnitude image.It can see that the thermally grown oxides on the stainless-steel substrate have two types.There is a large nodule of oxide and a thin continuous oxide layer.For the EDS mapping, the outer part of nodule oxide consists of Fe and O which is the main element distribution in outward growth nodule oxide.Moreover, Mn, Fe, and O were detected covering the outer part of nodule oxide.The inner part of the nodule oxide has Cr, Fe, and O.It was indicated that this oxide is an inward growth to metal substrates.Nearly the nodule oxide, a thin continuous oxide layer is found.Mn, Cr, and O were obtained in this layer.For high Mn stainless steel 96 h shown in Figure 3, it has a thicker oxide layer composed of Cr, Mn, Fe and O which surface of the oxide is covered by a thin oxide layer of Mn, Fe and O but most of the outer oxide layer consists of Cr and O.

XRD phase identification
To identify the oxide phases formed after the test, the Xray diffraction technique was applied to the samples.Figure 4 shows XRD pattern of the oxidized low Mn stainless steel in O2-5%H2O at 800 o C for 96 h.The peaks of MnFe2O4 (ICDD 00-010-0319), Cr2O3 (ICDD 00-002-1362), (Cr, Fe)2O3 (ICDD 00-002-1357) and Fe2O3 (ICDD 00-013-0534) were detected on the surface sample.For the oxidized high Mn stainless steel for 1 h at the same oxidation atmosphere and temperature test, peaks MnCr2O4 (ICDD 01-075-1614) and Fe0.902O (ICDD 01-086-2316).When increasing the oxidation time to 96 h.The sample has peaks of MnCr2O4 (ICDD 01-075-1614), Mn2O3 (ICD 00-001-1061), Cr2O3 (ICDD 00-002-1362) and Fe3O4 (ICDD 01-074-0748).Figure 5 shows the behavior of oxide growth on low Mn stainless steel.The nodule oxide was formed at an angle on the sample.This nodule occurs because of the direction of the gas flow, the angle was turbulent.Nevertheless, the thin oxide layer beside the nodule was covered by Cr2O3 and MnCr2O4.It is close to the SEM-EDS mapping and XRD results.That is why Cr is highly volatile shown in Table 2.According to Asteman et.al. [8] found that the nodule oxide formed (Breakaway oxidation) on 304L stainless steel at 500-800 o C in O2-40%H2O atmospheres as a result of the flow rate.It was obtained from (Cr,Fe)2O3 and Fe2O3 oxide formation.In this case, the results were similar.
From the results of high Mn stainless steel, it can be inferred that the thermal oxide growth was shown in Figure 6.At the initial time, MnCr2O4 covering FeO was fabricated by Cr and Mn reacting with O.After that MnCr2O4 and FeO interdiffusion to create the Mn-Cr-Fe oxide.In the long term, Mn and Fe diffuse through the Mn-Cr-Fe oxide to form (Mn,Fe)2O3 or (Mn,Fe)3O4 on the top surface.This phase contributes to the volatile phases of Mn and Fe.It related to volatilization rate results, which are measured.When the sample was cooled down from high temperatures to room temperature.Spallation can occur.Moreover, the results were closed to A.L. Marasco and D.J. Young [20].They found that the high Mn element in stainless steel oxidized at 900 o C at an oxygen partial pressure of 26.7 kPa, which makes bixbyite-(Mn, Fe)2O3-and α -Fe2O3 oxides and increases the high oxidation rate, then spallation occurs.

Conclusion
In the present work, stainless steel with high manganese content (Fe-15.32wt.% Cr-8.94 wt.% Mn) and low manganese content as AISI 430 stainless steel (Fe-16.41wt.% Cr-0.79 wt.% Mn) were oxidized in O2-5%H2O atmospheres at 800 o C for 1 h up to 96 h.The summary was described as follows.
1) Manganese in stainless steel induces spallation of oxide after oxidization in O2-5%H2O at 800 ๐ C as a result of nodule oxide formation.
2) Cr volatilization rates in high Mn stainless steel are lower than the other by a factor of 14, while Fe and Mn volatilization rates increase by 10 and 62 times, respectively.It may be possible that Mn-Fe oxide formed at the outer layer.
As a recommendation, the selection of stainless steel for industrial Mn concentration should be suitable to reduce Cr loss and avoid spallation of the oxide scale.

Fig. 1 .
Fig. 1.Mass gain as a function of time of the sample during the oxidation test in O2 5%H2O at 800 o C.

Fig. 2 .
Fig. 2. Cross-sectional micrographs and EDS mapping results of low Mn stainless steel oxidized after 96 h (170x magnitude image).

Fig. 3 .
Fig. 3. Cross-sectional micrographs and EDS mapping results of high Mn stainless steel oxidized after 96 h (1,000x magnitude image).

Fig. 4 .
Fig. 4. XRD patterns of high Mn stainless steel oxidized in O2-5%H2O at 800 o C for 96 h (a), 1 h (b) and low Mn stainless steel for 96 h (c)

Fig. 5 .
Fig. 5. Schematic sketch behavior of oxide growth on low Mn stainless steel.

Fig. 6 .
Fig. 6.Schematic sketch behavior of oxide growth on high Mn stainless steel.

Table 2
shows Cr, Mn and Fespecies volatilization rates of both low and high Mn stainless steel oxidized in O2-5%H2O for 96 h at 800 o C.During the oxidation test, the metal volatile phase was