Aluminide Coatings on 304 Stainless Steel

Aluminide Coatings on 304 Stainless Steel Kevin L. Smith, Armen Kutyan, Shaghik A. Abolian, Tom F. Krenek, Stephanie A. Salas, Vilupanur A. Ravi California State Polytechnic University, Pomona 3801, W. Temple Avenue Pomona, CA 91768 USA ABSTRACT Type 304 stainless steel coupons were aluminized at 650, 750 and 850 C for coating times in the 1-25 hour range via halide activated pack cementation. Mass gains per unit area and coating thicknesses were plotted as a function of time. Empirical rate equations and the corresponding parabolic rate constants were obtained for the kinetics of aluminization for these two metrics, i.e., mass change and thickness of coating. The Vickers micro-hardness values of the coated regions ranged between 700-1114 HV. Coating surface phases were identified using X-ray diffraction analysis as Al 5 FeNi, Al 5 Fe, Al 86 Fe 14 and AlFe with one or more absent depending upon coating temperatures and times. Scanning electron microscopy coupled with energy dispersive spectroscopy was utilized to obtain diffusion profiles for aluminum, chromium, nickel and iron. Key words: coatings, pack cementation, aluminizing, stainless steel, high temperature corrosion INTRODUCTION Austenitic stainless steels are becoming increasingly important in high temperature applications largely due to a combination of desirable mechanical properties, ease of fabrication, room temperature corrosion resistance during shutdown or maintenance periods and overall cost effectiveness. 1-4 The optimization of mechanical properties has to be balanced against environmental stability. 5 Depending on the industrial application, various forms of corrosion can occur simultaneously which can include (but is not limited to) sulfidation, carburization, hot corrosion and oxidation. One approach to retaining the mechanical properties of the structural alloy while defending against corrosion is by the application of protective coatings. Diffusion coatings are an effective method of achieving corrosion resistance against the harsh conditions prevalent in high temperature environments. The

deleterious effects of oxidation can be mitigated through the application of an aluminide diffusion coating. 4 During high temperature exposures, the aluminum-rich surface oxidizes to a highly protective aluminum oxide layer that inhibits corrosion by acting as a diffusion barrier. 6 One of the more economical and effective methods for the application of aluminide coatings is by halide activated pack cementation (HAPC). HAPC is a coating process in which a halide vapor is generated inside a pack and ultimately deposits a coating element on the surface of the substrate. 7-8 A typical pack in this process includes a filler, master alloy, activator salt and a substrate. The filler is inert and is simply used to achieve a uniform coating by allowing well distributed halide vapors consisting of the master alloy and activator salt to reach the substrate. The activator salt constituent is used to react with the master alloy to produce the halide vapor that will be deposited, and finally, the master alloy is the constituent that contains the element that will be deposited on the substrate. Aluminide coatings in this study were applied using pure aluminum as the masteralloy. In this study, the HAPC process was used to aluminize the austenitic stainless steel alloy (304 SS). The 300 series of austenitic stainless steels are a widely used class of materials and of these, type 304 is a widely used grade, and was thus selected as a meaningful substrate for this project. 1 EXPERIMENTAL PROCEDURE Stainless steel coupons, approximately 5 mm thick and 12.5 mm in diameter, were ground down to a 600 grit finish. The mass and dimensions of each sample were measured. The samples were then placed inside crucibles that contained packs of aluminum oxide, aluminum and aluminum chloride powders. The experimental protocol was to use one sample per crucible. The crucibles were sealed using a ceramic cement and heated in a furnace under a flowing argon atmosphere to minimize oxidation. Austenitic stainless steel specimens were aluminized at 650, 750 and 850 C for coating times in the 1-25 hour range. The mass and dimensions for the coated samples were measured again and subjected to X-ray diffraction (XRD) analysis and cross-sectional micro-hardness. Coating morphologies were examined using optical and scanning electron microscopy (SEM) coupled with energy dispersive spectroscopy (EDS). Coating Kinetics RESULTS AND DISCUSSION As denoted by the Secondary Electron Images (SEI) in Figures 1 and 2, coating thickness increased for higher coating temperatures (constant coating time) and longer coating times (constant coating temperature). The images illustrate a cross-sectioned sample, where the top, darker portion is the aluminide coating and the bottom lighter portion is the stainless steel substrate.

(a) (b) (c) Figure 1: Secondary Electron Images SEI of 304 SS aluminized for 1 h showing an increase in coating thickness as temperature increases. a) 650 C, b) 750 C and c) 850 C (a) (b) (c) (d) (e) Figure 2: Secondary Electron Images SEI of 304 SS aluminized at 650 C showing an increase in coating thickness for longer coating times. a) 1h, b) 4h, c) 9h, d) 16h and e) 25h Figure 3 is a plot of the average coating thickness as a function of coating time. Each data point is the average coating thickness based on 10 different measurements on 1 3 replicate samples. The error bars represent one standard deviation from the mean. The data points for each coating temperature demonstrate that as the time was increased, the thickness increased and followed a parabolic behavior of the form δ = k p (t 1/2 ) + C in the 1 25 h time range, where δ is coating thickness in μm, k p is the thickness rate constant in μm/hr 1/2 and t is time in hours. 9 Linear trend lines were fitted to each data set in the 1 to 25 hour range for 650, 750 and 850 C shown below:

Thickness (µm) δ = 10.05*t 1/2 + 12.68 @ 650 C (R 2 =0.96) (1) δ = 7.76*t 1/2 + 36.56 @ 750 C (R 2 =0.90) (2) δ = 9.15*t 1/2 + 50.84 @ 850 C (R 2 =0.95) (3) Coating thickness rate constants (k p ) values were obtained at 650, 750 and 850 C from the trend line slopes and were determined to be 10.05 μm/hr 1/2, 7.76 μm/hr 1/2 and 9.15 μm/hr 1/2 respectively. The coefficients of determination (R 2 ) are all reasonably high, with the 750 C data showing a noticeably lower value. 120.0 100.0 650 C 750 C 850 C 80.0 60.0 40.0 20.0 0.0 0 1 2 3 4 5 Square Root of Time ( hr) Figure 3: Thickness of the aluminide coating vs. square root time of the aluminization process. The average mass gain per unit surface area also increased as time increased in a similar manner as thickness, however with a higher degree of uncertainty as shown in Figure 4. Each data point represents an average of 2-5 replicates with the error bar representing one standard deviation from the mean. The samples in the 650 C temperature range exhibited fewer errors as denoted by the smaller error bars. The samples in the 750 C temperature range had larger mass gains than the 650 C range as anticipated, but showed larger scatter in the data. The samples in the 850 C temperature range had larger mass gains than the 650 C range but lower than the 750 C range. In addition, the 850 C trend line had a lower slope than the corresponding ones at 650 and 750 C which was indicative of lower rates of mass gain at this temperature. One possibility as to why the 850 C temperature coatings had lower mass even though they formed thicker coatings may be attributable to a higher amount of porosity. The other possibility is that the density of the phases formed may be lower. The data at 650 C show a good fit to a parabolic rate equation (see equation 4 below) with a rate constant of 1.99 mg/ (cm 2 h 1/2 ), indicating a behavior of the form, ( m/a) = k m (t 1/2 ) + C where m/a is the

Mass Gain (mg/cm 2 ) mass gain per unit area, k m is the rate constant, t is the time and C is a constant. The coefficient of determination (R 2 ) is reasonably high (0.94) for this fit. 20.00 15.00 650 C 750 C 850 C 10.00 5.00 0.00 0 1 2 3 4 5 Time ( hr) Figure 4: Mass gain vs. square root time for 304 SS aluminized at different times and temperatures. Mass gain = 1.99*t 1/2 + 2.73 @ 650 C (R 2 =0.94) (4) It is important to note that the thickness equations corresponding to Figure 3 and the mass gain per surface area equations corresponding to Figure 4 should only be utilized in the 1 to 25 hour range and are not valid in the 0 to 1 hour time range. At t = 0, the coating thickness should be small and will be only due to the time interval during the temperature ramp-up. Moreover the operating mechanisms may be different during this time. X-Ray Diffraction X-ray diffraction (XRD) was used to establish phase identities on the surface of the stainless steel. From these studies, it was established that for long coating times (25h) at 650 C, the surface of the coated alloys consisted mainly of the phases Al 5 Fe 2 and Al 5 FeNi, while the corresponding phases for the same coating time (25h) at 750 C were Al 0.5 Fe 0.5 and Al 86 Fe 14 and at 850 C the phase AlFeNi alone was detected. An example of the evolution of the coating with time can be illustrated with the 750 C example. In this case, the surface composition of the coatings evolved from AlFe and Al 5 Fe 2 (1h) to Al 0.5 Fe 0.5, AlFeNi, and Al 86 Fe 14 for longer coating times (9h and 25h). The corresponding coating evolution at higher temperatures, 850 C, was from the phases Al 86 Fe 14 and AlFeNi (1h) to AlFeNi at longer times (25h). As shown in Figure 5, for coating times of 9 h, the surface of the coated alloy consisted mainly of Al 5 FeNi and Al 5 Fe 2 at 650 C, while the corresponding phases for the 750 C coatings were Al 86 Fe 14 and Al 5 FeNi as shown in Figure 6. At 850 C, however, Al 86 Fe 14 and AlFe were detected (see Figure 7). For longer times and higher temperatures the concentration of aluminum increases. Also noted above, as times and temperatures increased, mass gain per surface area increased denoting a higher ratio of the concentration of aluminum to iron with a higher change in mass per surface area.

Figure 5: X-Ray Diffraction Pattern for 304 SS Aluminized at 650 C for 9h. Figure 6: X-Ray Diffraction Pattern for 304 SS Aluminized at 750 C for 9h. Figure 7: X-Ray Diffraction Pattern for 304 SS Aluminized at 850 C for 9h.

Microhardness (HV) Micro-hardness Figure 8 shows a micro-hardness profile as a function of distance into the 304 SS. The size of the indents increased when transitioning from the coating to the substrate. Figure 9 shows the Vickers micro-hardness values as a function of distance into the 304 SS. The 850 C profile shows a small set of high hardness values close to the surface (HV 700 1114) with a subsequent sudden drop at the coating/substrate (approximately HV 222 378). These values were substantially less than those coated at 650 and 750 C. Figure 8: Vickers micro-hardness profile for 304 SS aluminized at 750 C for 4h. 1200 1000 650 C 750 C 850 C 800 600 400 200 0 0 25 50 75 100 125 150 175 200 Depth (micrometers) Figure 9: Plot of Vickers micro-hardness vs. depth for different coating temperatures.

Scanning Electron Microscopy (SEM) SEM images of the coated cross-sections were obtained for representative samples subjected to various coating temperatures (650, 750 and 850 C) and times (1 to 25 h). Coating thicknesses ranged from 15 to 65μm for the 650 C temperature range, 30 to 70μm for the 750 C temperature range, and 35 to 100μm for the 850 C temperature range. Using Energy Dispersive Spectroscopy (EDS) in conjunction with the SEM, the presence of aluminum at the surface of 304 stainless steel was confirmed as shown in Figure 10. The figure shows a backscattered electron image on the left and the corresponding elemental dot map for aluminum on the right. (a) (b) Figure 10: a) Secondary Electron image of the cross-section of 304 SS aluminized at 850 C for 1 h and b) Al X-ray map illustrating aluminum at the surface The progress of the inward diffusion of aluminum was followed by plotting concentration (at. %) versus distance into the alloy (micrometers). These concentration profiles were over-laid on the corresponding electron image to give a visual representation of the effect of changing chemical composition on the alloy microstructure. Figure 11 shows the variation of the concentrations of Al, Cr, Fe and Ni as a function of the distance into the substrate. In general, the aluminum concentration decreases with depth into the SS304 substrate. The aluminum concentration profile shows noticeable step changes in concentration at approximately 10μm and again at 25μm into the substrate. These sharp drops in aluminum concentration demarcate phase boundaries.

Al Concentration (Atomic %) Figure 11: Concentration vs. Depth for Al, Cr, Fe and Ni. Figure 12 shows that the Al surface concentration remained fairly constant for the 650 and 750 C runs; however, the 850 C samples showed a decrease in surface concentration with respect to time. This decrease is possibly due to activator and/or master alloy depletion in the pack, subsequently leading to lower values for the mass gain at 850 C relative to 750 C as shown earlier in Figure 4. Further experiments are underway to verify this hypothesis. 75.00 70.00 65.00 60.00 55.00 50.00 45.00 40.00 35.00 30.00 650 C 750 C 850 C 0 5 10 15 20 25 time, (h) Figure 12: Plot of Al surface concentration vs. aluminization time for 304 SS aluminized at different times and temperatures.

The increase in the coating layer thickness and composition can be demonstrated by an example (Figure 13), which shows the effect of increasing the process times on an aluminide coating formed at 750 C. Initially the major phase is Al 5 Fe 2 and as time increases the major phase becomes Al 86 Fe 14 an intermetallic phase with an approximately 6 to 1 atomic ratio. Figure 13: Schematic for Aluminized 304 SS at 750 C CONCLUSIONS Aluminum was successfully deposited onto type 304 stainless steel through the halide activated pack cementation process. Rate equations and rate constants were obtained for mass gain/unit area and coating thickness plots at 650, 750 and 850 C in the 1 25 h coating period range. X-ray diffraction identified the surface phases as Al 5 FeNi, Al 5 Fe, Al 86 Fe 14 and AlFe depending upon the coating temperature and time. Coated steels had Vickers microhardness values of 700-1114 HV. ACKNOWLEDGEMENTS The authors would like to acknowledge Edwards Life Sciences for their support in providing access to scanning electron microscopy. In addition, they would also like to thank Mr. Ulus Ekerman, Mr. Alejandro Cuevas, Mr. Samad Firdosy and Mr. Jordan Koch for help and support. Financial support from Ms. Sylvia Hall, the LA Section of NACE International, Western States Corrosion Seminar, Western Area Conference and the NACE International Foundation is gratefully acknowledged. REFERENCES 1. A. John Sedriks, Corrosion of Stainless Steels, Second Edition (John Wiley and Sons, Inc. New York, 1996): p. 13. 2. F.J. Perez, M.P. Hierro, F. Pedraza, C. Gomez and M. C. Carpintero, Aluminizing and Chromizing Bed Treatment by CVD in a Fluidized Bed Reactor on Austenitic Stainless Steels, Surface and Coatings Technology 120-121 (1999): pp. 151-157. 3. V. Ravi, J. Koch, C. Kouttjie and T. K. Nguyen, Engineered Protective Coatings for Enhanced High Temperature Performance of Stainless Steel, Stainless Steel World America 2012 (Houston, October 5-7 th 2012). 4. S. R. Pillai, High Temperature Corrosion of Austenitic Stainless Steels, in Corrosion of Austenitic Stainless Steels Mechanism Mitigation and Monitoring, H.S. Khatak and B. Raj, Ed., (Woodhead Publishing Limited, Cambridge, England, 2002): pp. 265 286

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