High Temperature Corrosion Behavior of HVOF, Fe3Al Coatings

This work evaluates the suitability of iron aluminide coatings for use in high temperature fossil fuel combustion environments, such as boiler applications. The coatings are applied using High Velocity Oxy-Fuel (HVOF) thermal spray techniques. Bulk Fe3Al coatings are known to exhibit excellent oxidation and sulfidation resistance at high temperatures, however, the behavior of HVOF-deposited Fe3Al coatings has not been documented. HVOF thermal spray deposition was used to produce freestanding Fe3Al coatings for high temperature corrosion and oxidation studies. The free standing coatings were then subjected to temperatures up to 1000 o C in various furnace atmospheres during exposure tests exceeding 1000 hours. The furnace atmospheres explored included, dry air, wet air, oxidizing (various ratios of CO2:CO), carburizing (CO2, CO, CH4 mixtures) and a simulated fossil fuel combustion atmosphere (N2-15CO2-5O2-1SO2-20H2O, in volume percent). The mass gain was monitored as a function time from which the parabolic rate constant for oxidation of the coating was determined. Free-standing, HVOF Fe3Al samples exhibited limited corrosion at the sample surface while Inconel Alloy 600 samples experienced greater mass gain with significant oxidation at the sample surface and internal oxidation during oxidation experiments in air at 1000 o C. Fully dense Fe3Al coatings exhibited parabolic rate constants for oxidation comparable to those of bulk Fe3Al reported in literature. However, HVOF Fe3Al coatings with open porosity exhibited breakaway corrosion after relatively short exposure times (a few hundred hours). Also, during exposure to a simulated, fossil fuel combustion atmosphere, the free-standing, HVOF Fe3Al samples exhibited mass gain similar to those observed during oxidation in air. Introduction Increasing the operating temperature of fossil power plants directly increases plant efficiency and reduces the emissions. There has been a steady increase in fossil fuel-fired power plant operating temperatures – 530-560 o C for many current fossil fuel power plants [1], to 600 o C for some advanced plants [2] and up to 760 o C for the proposed advanced, ultra-supercritical power plant [3, 4]. These systems are limited by the high temperature mechanical properties and corrosion resistance of the materials in combustion environments. High operating pressure (>13 MPa) inside super-critical and ultra-supercritical fossil systems necessarily require creep resistance of structural materials. Often times, materials that satisfy the high temperature, structural requirements do not possess the required corrosion resistance. While bulk Fe3Al has been shown to be highly corrosion resistant in simulated fossil fuel combustion atmospheres [5,6], its mechanical properties make is unsuitable in high temperature structural applications [7]. Therefore, one option is to use Fe3Al as a coating on a suitable high temperature, structural alloy. Iron aluminide powder is readily available and can be applied as a relatively thick coating using the High Velocity Oxy-Fuel (HVOF) thermal spray coating technique. This is a versatile deposition technique which is capable of controlling the residual stress in the Fe3Al coating. Powder particles during HVOF coating application are typically semi-solid and moving at many hundreds of meters/second. These semisolid particles can cause significant deformation of the substrate surface upon impact, effectively “peening” the surface. This “peening” stress in combination with quenching stresses and Fe3Al coating/substrate CTE mismatch can be used to control the residual stress in the coating and thereby Table 2 HVOF Thermal Spray Parameters HVOF Thermal Spray Torch JP 5000, 10.2 cm barrel Standoff distance 35.6 cm Chamber pressure P1 620 kPa P2 720 kPa P3 340 kPa Kerosene flow rate 26.5 l/h 16.7 l/h Oxygen flow rate 820 slm 520 slm Equivalence Ratio 1 1 Powder Fe3Al, Lot #0376601 Carrier gas flow rate 5 slm Rotation 5 rpm Table 1 Composition of the Iron Aluminide Powder Supplier: AMETEK Product: FAS-C (-270) Lot #: 037601 Element Fe Al Cr Zr C Wt. % Bal. 15.7 2.4 0.2 0.02 generate coatings with a high resistance to cracking. (The peening stress that develops during HVOF is absent or insignificant in the various other types of coating techniques, e.g. plasma spray, twin wire arc spraying, aluminizing, CVD, etc.., and manipulation of the stress state in coatings made by these methods is limited.) The relative contributions of the quenching, peening and CTE mismatch stresses sum to give a residual stress state in the coating that can range from tensile to neutral to compressive [8,9] and is largely controlled by the combustion chamber pressure, Pc, in the HVOF thermal spray gun. A compressive stress state is most desirable for iron aluminide coatings exhibiting limited ductility, since tensile stresses in the coating would tend to promote cracking. HVOF thermal spray deposition incorporates minor amounts of oxides and/or porosity and it must be determined whether these impurities and defects in HVOF Fe3Al coatings can degrade the excellent oxidation, corrosion and sulfidation resistance typically associated with bulk iron aluminides. This work reports on the degradation behavior of free-standing Fe3Al material deposited by HVOF thermal spray techniques in various furnace atmospheres at high temperature and compares the results to the behavior of bulk Fe3Al reported in literature. Experimental Methods The composition of the powder used for HVOF coating deposition is shown in Table 1 and corresponds to Fe3Al with minor additions of chromium and zirconium for enhanced mechanical properties. The particle size of the powder was -270 US Standard Mesh Size with the majority of the powder (>80%) being -400 US Standard Mesh Size. Free-standing Fe3Al coatings were made with HVOF thermal spray technique by applying a relatively thick coating (~1-1.5 mm) to a plate of 9Cr-1Mo steel, approximately 19 mm x 25 mm x 100 mm. The plates were prepared by grit blasting followed by grinding with 280 grit sand paper to produce a relatively smooth finish. The HVOF torch parameters are shown in Table 2. HVOF combustion chamber pressures of 620, 720 and 340 kPa were used to generate Fe3Al coatings and designated P1, P2 and P3, respectively. The surface of the prepared plate was translated in front of the HVOF torch at a rate of 25 mm/sec with 5 mm pitch. Compressed air was applied to the front of all plates to moderate substrate heating during thermal spray deposition. The coatings were then removed from the plates by striking the side of the plate which de-bonded the HVOF Fe3Al coating from the plate. Samples for testing were then cut from the de-bonded coating. High temperature, environmental testing was carried out in various furnace atmospheres with the specific conditions given in Table 3. In each case, the atmosphere flowed through the furnace once before exiting the furnace – it was not re-circulated. Mixtures of gases were accomplished through mass flow controllers while the simulated fossil combustion gas mixture was purchased as a pre-mix compressed gas. Water was injected along with the simulated fossil combustion pre-mixed gas to achieve 20% water by volume at 1000 o C. “Wet” air was generated by bubbling air through water at room temperature. Samples were cleaned and weighed prior to exposure. The samples were exposed the various furnace atmospheres and removed at intervals and re-weighed. The samples were returned to the furnace and subjected to additional exposure. Table 1 Exposure Conditions at 1000 o C Furnace Atmosphere Gas Flow Rates, ml/ minute PO2, atm Air CO CO2 CH4 Pre-mix ** Dry Air 100 0.2 Wet Air * 100 0.2 Oxidizing 1 100 10 -10 Simulated Fossil Combustion 70 5x10 -2 Carburizing 1 100 1 10 -18 * Achieved by bubbling air through water at room temperature prior to entering the furnace. ** The composition was N2-15CO2-5O2-1SO2 + 20% H2O. The water content was achieved by injecting water vapor upstream of the hot zone in the furnace. Experimental Results and Discussion Mass Change Behavior at 1000 o C in Air The mass change behavior of free-standing coatings in both dry and wet air is shown in Fig. 1. Generally, samples P1 and P2 showed relatively small mass gains in both dry and wet air, although the mass gain of both coatings was slightly higher in wet air. Sample P3 showed significantly greater mass gain than either P1 or P2. In fact, P3 exhibited catastrophic or breakaway corrosion in wet air after a relatively short period of exposure, <1000 hrs, Fig. 1b. Figure 1. a) The mass change behavior upon being exposed to dry and wet air at 1000 o C. The anomalous behavior of P3 in wet air at 1000 o C is shown in b). The anomalous behavior of P3 was further investigated and density measurements, utilizing the Archimedes principle, indicated P3, in the as-deposited condition, contained a substantial amount of open porosity. Further support for this conclusion was obtained by optical metallography of a cross section through sample P3, Fig. 2a. Considerably more porosity is evident in P3 than in either P1 or P2, Fig. 2b & c. The surface corrosion product is completely devoid of any aluminum and consisted exclusively of iron and oxygen, suggesting iron oxide, Fig. 3. This sample also was sectioned and prepared for metallography, Fig. 4. It is evident that extensive oxidation has taken place, Fig. 4a. In the high magnification micrograph of Fig. 4b, taken near the center of the sample, it appears that the remaining 0 0.001 0.002 0.003 0.004 0.005 0.006