Elsevier

Acta Materialia

Volume 55, Issue 10, June 2007, Pages 3457-3466
Acta Materialia

Effect of trivalent rare earth dopants in nanocrystalline ceria coatings for high-temperature oxidation resistance

https://doi.org/10.1016/j.actamat.2007.01.043Get rights and content

Abstract

Nanocrystalline ceria (NC) and La3+-doped nanocrystalline ceria (LDN) particles synthesized by the micro-emulsion method were coated onto AISI 304 stainless steel (SS) in order to study their high-temperature oxidation resistance properties at 1243 K in dry air for 24 h. Results were compared with those for micro-ceria (MC) coatings. The coated samples showed 90% improvement in oxidation resistance compared to uncoated and MC coated steels as seen from SEM cross-sectional studies. X-ray diffraction (XRD) analysis showed the presence of chromia in both NC and 20 LDN (NC doped with 20 at.% La3+) samples which is absent in uncoated steels. From secondary ion mass spectrometry (SIMS) depth profiles, Fe, Ni depletion zones were observed in LDN-coated samples, indicating diffusion through the oxide layer. The role of oxygen vacancies in the nanoceria coatings on the formation of protective chromia layers is discussed.

Introduction

Nanoceria (NC) has been shown to possess unique properties from its large scale complement such as the shifting and broadening of Raman-allowed modes [1], lattice expansion [2], [3] and blue shift in ultraviolet absorption spectra [4]. As a result of these unique properties, NC has potential applications in UV protection, catalysis [5], [6], [7], [8], [9], and high-temperature oxidation resistance. Recently, it has been reported that small additions of lanthanides may confer even greater protection on those metals and alloys that are already well protected from corrosion by oxide films [10]. These include iron–chromium and iron–chromium–nickel stainless steels (i.e., both ferritic and austenitic alloys) and most other alloys that are dependent on chromium for their corrosion/oxidation resistance. Many high-temperature alloys rely on the formation of protective Al2O3 and Cr2O3 scales on their surfaces to resist high-temperature oxidation [10], [11], [12], [13]. However, under various isothermal and thermal cycling conditions, these protective coatings crack due to thermal stresses and grain growth. Oxide scale cracking and spalling restrict the application of such alloys as high-temperature oxidation resistant materials under demanding service conditions [10]. Addition of rare earth elements such as Ce, Y, Zr, La or their oxides improve the high-temperature oxidation resistance of alumina- and chromia-forming alloys due to the reactive-element effect (REE) [14], [15], [16], [17], [18], [19]. Due to the REE, the oxide scale growth rate decreases, with an improvement in resistance to scale spalling as a result of increased scale–alloy adhesion. Various researchers have put forward mechanisms to explain the REE. Antill and Peakall [11] indicated that the beneficial effect of the rare earth elements was primarily to improve scale plasticity for accommodating stresses due to the difference in the thermal expansion coefficients between the alloy and the oxide scale. The enhancement of oxide nucleation processes through the presence of rare earth elements was suggested by Stringer [12]. Tien and Pettit [13] reported that the application of rare earth elements provide sites for vacancy condensation in an Fe–25Cr–4Al alloy with consequent improvement of scale adhesion. A mechanism involving the pegging of the oxide scale to the alloy substrate has also been suggested [20]. Duffy and Tasker [21] supported the model of grain boundary blocking by Ce4+ ions, which associate with metal vacancies to form arrays of defect pairs along the grain boundaries. Moon and Bennett [22] concluded that the scale nucleates at the reactive-element oxide particles on the surface, blocks short-circuit diffusion paths by segregating reactive-element ions and reduces the stresses in the oxide scale by altering the microstructure.

It was first reported that ceria could be applied superficially rather than as an alloy addition and chromia growth could be slowed down in Ref. [23]. Earlier studies [24], [25] indicated that superficial coating of micrometer-sized cerium oxide particles is effective in improving the high-temperature oxidation resistance of various grades of stainless steels (SS). Various researchers have carried out preliminary investigations on the improvement of the high-temperature oxidation resistance of Ni, Cr and Ni–Cr super alloys with the application of NC coatings [26], [27]. It was also reported that NC coatings improve the high-temperature oxidation resistance of chromia-forming steels [28]. However, detailed investigations into the effects of doped and undoped ceria nanoparticles and the role of oxygen vacancies in the improvement of high-temperature oxidation resistance of SS are yet to be carried out.

The present study investigates the effects of NC coatings on the isothermal oxidation resistance of AISI 304 SS at 1243 K in dry air. We have carried out a comparative study on the oxidation kinetics of AISI 304 SS for uncoated, MC, NC and LDN (NC doped with various amounts of La3+ – 2 LDN, 20 LDN and 40 LDN, see Table 1) SS samples using detailed microstructural analysis by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD) and secondary ion mass spectrometry (SIMS).

Section snippets

Experimental method

Cerium oxide nanoparticles were synthesized by the micro-emulsion method. The micro-emulsion system consisted of sodium bis(2-ethylhexyl) sulphosuccinate (AOT) as a surfactant, water as a polar solvent and toluene as a non-polar solvent. The doped nanoparticles were synthesized using cerium nitrate and lanthanum nitrate as the precursors and ammonium hydroxide as the precipitating agent. All the chemicals were purchased from Aldrich Chemical Co. The amounts of cerium nitrate (99% purity) and

Oxidation kinetics of 304 SS coated with doped and undoped ceria

Fig. 1 shows the oxidation kinetics plots of uncoated and MC, NC, 2 LDN, 20 LDN and 40 LDN coated AISI 304 grade SS samples at 1243 K in dry air for 24 h. The NC coated sample showed improvement in high-temperature oxidation resistance over the uncoated and MC coated samples. During the initial stages of oxidation in the bare alloy, a Cr2O3 layer forms on the top surface. After the formation of Cr2O3 as the top layer, subsequent oxidation was by slow diffusion of other alloy elements through the

Role of dopants and oxygen vacancies in oxidation resistant nanoceria-based coatings

The lattice constant and oxygen vacancies increase with increasing amounts of trivalent elements such as La, Nd, etc. [41]. The lattice expansion slope for trivalent ions doped in ceria is 0.3 for La3+ ions [42]. Oxygen vacancies are created for replacement of each two Ce4+ sites by two La3+ ions to maintain electrostatic charge neutrality. The addition of La3+ ions to CeO2 results in solid solutions of the form Ce1−xLaxO2−y that have the same fluorite structure as of CeO2.

In CeO2−x, loss of

Conclusions

The high-temperature oxidation kinetics of 304 steels has been studied in the presence of NC and LDN coatings. The weight gain per unit area of NC and LDN coatings is reduced significantly (2–4 orders of magnitude decrease in kp) as compared to uncoated and MC coatings. SEM micrographs of the top oxide layer showed finer grain structure with increased porosity as the La concentration was increased in LDN coatings. XRD of the oxide scales shows a protective chromia layer in nanoceria coated

Acknowledgements

We are grateful for funding from the National Science Foundation (CMS: 0548815). S. Seal acknowledges an Alexander Von Humboldt research award at RWTH (Aachen, Germany) for allowing access to the instrumental facilities. The authors thank Professor J. Schneider and H. Koeplin (PhD student) from RWTH for help with XRD analysis. The authors also acknowledge Dr. Suresh Babu for useful discussion and suggestions.

References (45)

  • P.J. George et al.

    The effects of ion implantation upon nickel oxidation investigated by secondary ion mass spectrometry

    Mater Sci Eng A

    (1989)
  • A. Paul et al.

    Low cost rare earth elements deposition method for enhancing the oxidation resistance at high temperature of Cr2O3 and Al2O3 forming alloys

    J Alloys Compd

    (2001)
  • F.I. Wei et al.

    The development of Cr2O3 scales on iron–chromium alloys containing reactive elements

    Corros Sci

    (1989)
  • X.Q. Yu et al.

    The oxidation improvement of Fe3Al based alloy with cerium addition at temperature above 1000 °C

    Mater Sci Eng A

    (2003)
  • F. Czerwinski et al.

    Optimizing properties of CeO2 sol-gel coatings for protection of metallic substrates against high temperature oxidation

    Thin Solid Films

    (1996)
  • J.R. Lince et al.

    Interaction of O2 with the Fe0.84Cr0.16(0 0 1) surface studied by photoelectron-spectroscopy

    Surf Sci

    (1992)
  • N. Sakai et al.

    Chromium diffusion in lanthanum chromites

    Solid State Ionics

    (2000)
  • K. Sun et al.

    Xianlunxu. characterization and catalytic performances of La doped Pd/CeO2 catalysts for methanol decomposition

    Appl Catal A

    (2004)
  • P. Papaiacovou et al.

    The effect of CeO2 coatings on the oxidation behaviour of Fe–20Cr alloys in O2 at 1173 K

    Corros Sci

    (1990)
  • X. Peng et al.

    Effect of grain refinement on the resistance of 304 stainless steel to breakaway oxidation in wet air

    Acta Mater

    (2005)
  • J.E. Spanier et al.

    Size-dependent properties of CeO2−y nanoparticles as studied by Raman scattering

    Phys. Rev. B

    (2001)
  • F. Zhang et al.

    Cerium oxide nanoparticles: size-selective formation and structure analysis

    Appl Phys Lett

    (2002)
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