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New Understanding of High Temperature Corrosion
Protection
Role of Sulfur and Reactive Metals Elucidated
Peggy Hou
An LBNL team led by Peggy Hou has developed new techniques to
determine the chemical composition of the oxide/metal interface
that develops on the surface as materials corrode. This interface
plays a critical role in blocking further corrosion; thus the
importance of understanding its characteristics. The new techniques
have been used to clarify the mechanisms by which some elements
(e.g. sulfur) weaken this interface and others (e.g. zirconium,
yttrium, etc.) strengthen it.
All metal components operating at elevated temperatures rely on
the growth of a surface oxide or "scale" for corrosion
protection. A stable and adherent oxide scale provides maximum
protection and the precise chemical composition of the interface
has a large effect on its strength. For example, at high temperatures,
sulfur, a common impurity in commercial high temperature alloys,
can diffuse (segregate) to the oxide/alloy interface, where it
weakens that interface. It is also known that the addition to
the alloy of certain reactive metals such as zirconium and yttrium
inhibits sulfur segregation, thereby maintaining the strength
(and the corrosion protection) of the oxide layer. However, the
details of this process are not well understood; the buried nature
of the oxide/metal interface makes precise chemical characterization
difficult.
The LBNL team developed a new method to study the metal/oxide
interface. After producing protective scales by heating the alloys
in dry oxygen at 900-1000 °C, the sample are transferred to
an ultra high vacuum chamber. The oxide surface is then scratched
with a diamond stylus. This causes spallation of the oxide scale,
exposing the underlying alloy surface for chemical analysis with
an Auger electron spectroscopy microprobe.
The new technique has been applied to scale formation of Fe3Al,
a prototypical high temperature intermetallic alloy. The protective
scale formed on this alloy during high temperature oxidation is
Al2O3, aluminum oxide. The team first studied commercially available
Fe3Al, which contains 28 ppm sulfur. In agreement with the current
understanding of the role of sulfur, after a few hours of high
temperature oxidation, up to 0.4 monolayers of sulfur had segregated
at the interface, and the interface strength was a relatively
low 4 J/m2. The team then reduced the sulfur content of the alloy
to less than 0.5 ppm by annealing the alloy in hydrogen before
forming the oxide scale. In this case, there was no detectable
sulfur at the interface and the interface strength was increased
to 20 J/m2. The team then studied an alloy with 25 ppm sulfur
and 0.1 wt. % zirconium, a reactive metal. The results were surprising.
As in the case of the alloy annealed in hydrogen, no sulfur was
found at the interface, confirming that reactive metals serve
to prevent sulfur from segregating to the interface to weaken
it. However, the measured interfacial strength was >1000 J/m2,
substantially higher than in either of the two other cases. Thus
the reactive metal has a strengthening effect at the interface
beyond protecting it from sulfur contamination. High sensitivity
chemical analysis of a cross section of the interface performed
by the Corrosion Science and Technology Group at ORNL (one of
LBNL's partners in the DOE Materials Science Center for Excellence
corrosion project) revealed that zirconium is enriched at the
scale/alloy interface. This implies that the reactive metal strengthens
the interface by segregating between the aluminum oxide and the
substrate. The mechanism by which the strengthening effect occurs
is unknown, and is under investigation.
These experiments constitute the first detailed observations of
sulfur and reactive metal kinetics and chemistry at growing oxide/alloy
interfaces. The results could allow the fabrication of chemically
designed interfaces that are highly resistant to high-temperature
corrosion process.
Peggy Hou, (510)486-5560; Materials Sciences Division (510)486-4755,
E. O. Lawrence Berkeley National Laboratory.
P. Y. Hou and J. L. Smialek, "The Effect
of H2-Anneal on the Adhesion of Al2O3 Scales on a Fe3Al-based
Alloy," Mater. at High Temp. 17 79-85 (2000).
P. Y. Hou, "Sulfur Segregation to Growing Al2O3/alloy Interfaces,"
J. Mater. Sci. Lett. 19, 577-8 (2000).
P. Y. Hou, "Beyond the Sulfur Effect," Oxid. Metals,
52, 337-351, Oct. 1999.
K. B. Alexander, K. Prüßner, P. Y. Hou and P. F. Tortorelli,
"Microstructure of Alumina Scales and Coatings on Zr-containing
Iron Aluminide Alloys," Microscopy of Oxidation 3,
246-255, J. B. Newcomb and J. A. Little eds., The Institute of
Metals, 1997.
Materials Sciences Division
Ernest Orlando Lawrence Berkeley
National Laboratory
One Cyclotron Road, Mail Stop 66, Berkeley, California 94720 USA