Chemicals case study
Page 1 of 3
Surface Corrosion Studies of
Iron-Chromium Stainless Steels
Industry Sectors
Surface Science, Automotive,
Researchers from the Chalmers University of Technology and Gothenburg
Metallurgical Processing, Materials
University, Sweden, have used computational chemistry to investigate the
Manufacture
mechanism of chromium depletion from the surface of iron-chromium stainless steel alloys under humid conditions.
Organizations
Chalmers University of Technology
and Gothenburg University, Sweden
An understanding of the corrosion mechanism involved will aid the design and
manufacture of longer working-life industrial units where humid, high-temperature conditions abound, such as steel incinerators used for municipal
Key Product
waste burning and energy generation using alternative renewable fuel sources
Materials Studio’s CASTEP
such as wood, conditions that are ripe for steel chromium depletion.
Reporting in the journal Chemical Physics Letters (383 (2004) 549-554),1 the
researchers used the density functional theory (DFT) code CASTEP, operating
within the PC-based Materials Studio® modeling and simulation environment,2
to investigate the mechanism of chromium depletion from the iron-chromium
alloy surface, a process known as 'break-away' corrosion.
It is known experimentally that, at high temperatures and under humid conditions, stainless steel alloys rapidly deteriorate through chromium depletion followed by oxidation of the exposed iron (Figure 1). The computational studies
probed the mechanism of the loss of the protective Cr2O3 surface layer. It was
found that, under dry conditions, the Cr2O3 surface layer is protected with a
monolayer of Cr(VI) oxide. Under humid conditions, the Cr(VI) oxide surface
layer is oxidized and released as chromic acid, revealing that protection of this
Cr(VI) layer is the key to designing deterioration-resistant steels.
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Chemicals case study continued
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Itai Panas, Professor of Theoretical Inorganic Chemistry, Chalmers University of
Technology, said, “It all started with a graduate student showing me a stained
glass tube where he had exposed stainless steel to a flow of humid air while
keeping it at 550 AC. Auger experiments suggested that the stain was composed of Cr(VI) oxide.”
“We were then able to show that the vapour was chromic acid H2CrO4(g)
rather than CrO3(g) and we also determined the energy scale for hydrolytic
detachment by means of cluster calculations. Since then, the phenomenon
Figure 1: Coexistence of Cr(III) and
has been mapped out by several graduate students at the Competence Centre
Cr(VI) at the Cr2O3 surface. Blue cush-
for High-Temperature Corrosion at Chalmers University of Technology.”
ions (spin density) reflect non-bonding
unpaired electrons on Cr(III). Cr(VI) at
“It thus became increasingly more important to test the internal consistency
the top layer do not carry any spin density
of our qualitative understanding by additional state-of-the-art electronic
as all its electrons are paired up in the
structure calculations. CASTEP allowed us to perform the necessary surface
Cr(VI)-O bonds.
chemistry. Milestones included the characterization of a surface Cr(III) oxide
covered by a monolayer of Cr(VI) oxide, and to be able to distinguish between
the two by plotting the spin density (Fig. 2).”
Figure 2: Cr depletion of the outer part of (Fe,Cr)2O3 results in rapid Fe2O3 formation by both inwards and outwards oxide growth (Courtesy of the Competence
Centre for High Temperature Corrosion, Professor Lars-Gunnar Johansson, Chalmers
Univ. of Technol.)
Surface Corrosion Studies of Iron-Chromium Stainless Steels
Chemicals case study continued
Page 3 of 3
Panas continues, "Crucial to the effort was being able to produce a relativestability-map for the proposed intermediates. From this we were able to conclude that the activation energy for continuous evaporation of chromic acid is
not associated with any remarkable properties of the protective chromium
oxide scale but rather it is determined by the removal of the stable hydroxides
that form at its surface. Once the hydroxides are removed by water condensation, the underlying Cr(III) oxide will oxidize further into Cr(VI) which will
undergo hydrolysis leaving H2CrO4(g) and a hydroxylated surface.”
"The ease with which surface models are constructed in Materials Studio, the
impressive visualization tools, access to state-of-the-art electronic structure
software, and the seamless interfaces between the different components
makes working with problems of this degree of complexity a really pleasant
task. No less important is the fact that the non-expert can follow the reasoning owing to the powerful graphics.”
"Looking to the future, we are currently searching for possible remedies.
Introduction of SO2 into the reactor is a very promising candidate because its
adsorption, heterogeneous oxidation, and water chemistry at the Cr2O3 surface is in many ways analogous to that of surface Cr."
References
1.
Itai Panas, Jan-Erik Svensson, Henrik Asteman, Tobias J.R. Johnson, and Lars-Gunnar
Johansson, Chromic acid evaporation upon exposure of Cr2O3 (s) to H2O(g) and O2(g) mechanism from first principles, Chem. Phys. Lett.,(2004) 383, 549-554.
2.
For more information on Materials Studio, see:
http://www.accelrys.com/products/mstudio/.
For more information on CASTEP see:
http://www.accelrys.com/products/mstudio/modeling/quantumandcatalysis/castep.html
Surface Corrosion Studies of Iron-Chromium Stainless Steels
MS-CS-0004-1205