The corrosion behavior of Fe-Cr-Ni alloys in complex high temperature gaseous atmospheres

328 Pages·2017·16.38 MB·English

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The corrosion behavior of
Fe-Cr-Ni alloys in complex
high temperature gaseous
atmospheres containing the
reactants oxygen, sulphur
and carbon
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by the/an author.
Additional Information:
• A Doctoral Thesis. Submitted in partial fulfilment of the requirements
for the award of Doctor of Philosophy of Loughborough University.
Metadata Record: https://dspace.lboro.ac.uk/2134/14610
Publisher: (cid:13)c J. A. Kneeshaw
Please cite the published version.
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THE CORROSION BEHAVIOUR OF Fe-Cr-Ni ALLOYS
IN COMPLEX HIGH TEMPERATURE GASEOUS ATMOSPHERES
CONTAINING THE REACTANTS OXYGEN, SULPHUR
AND CARBON
by
JONATHAN ANDREW KNEESHAW, B.Sc., B.Eng.
A Doctoral Thesis submitted in partial fulfilment
of the requirements for the award of
Doctor of Philosophy
of the Loughborough University of Technology, U.K.,
based on research carried out at the
Commission of the European Communities Joint Research Centre,
Petten, Netherlands.
JUNE, 1987.
© by J. A. Kneeshaw, 1987.
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ABSTRACT
========
Title of Thesis: The corrosion behaviour of Fe-Cr-Ni Alloys in complex
high temperature gaseous atmospheres containing the reactants oxygen,
sulphur and carbon.
Author: J. A. Kneeshaw
A systematic in-depth study has been undertaken to establish the corro
sion mechanism of a Model 25Cr-35Ni-Fe alloy and four commercial alloys
HP40Nb, AISI314, HP40Al and Alloy 800H in low oxygen, high sulphur and
carbon containing environments typically found in coal gasification and
fluidised bed combustion processes.
A review of present knowledge of corrosion processes in purely oxidizing,
sulphidizing and carburizing environments and multiple reactant carburiz
ing/oxidizing, carburizing/sulphizing and oxidizing/sulphidizing environ
ments is given.
The experimental programme was designed to establish the role of sulphur
on the corrosion process by studying corrosion mechanisms in a sulphur
free H2-7~~0-1.5%H2o gas, a low sulphur H2-7%C0-1.5%H20-0.2%H2S gas
(pS __' § 10 bar), and a high sulphur H -7%C0-1.5%H 0-0.6%H S gas (pS
2 2 2 2
= lO bar) at 800'C. All_ 1j_hree environments had a constant partiaf
2
pressure of oxygen (po = 10 bar) and carbon activity (ac = 0.3).
2
In the sulphur-free gas the Model alloy formed a thin uniform cr o layer
2 3
which grew at a constant parabolic rate throughout the exposure period
of 0 - 5000 hours. Surface working increased the growth rate and thick
ness of the Cr o layer but created a large number of cracks and pores
2 3
which allowed carbon containing gaseous species to diffuse through the
oxide to form carbide precipitates in the alloy substrata. Alloying
additions of Si promoted the formation of an inner SiO layer which
reduced the corrosion rate by cutting off the outward diffusion of Cr,
Mn and Fe. Alloying additions of Mn promoted the formation of an addi
tional outer (Mn, Fe )Cr o layer. The 3. 5% Al content of the HP40Al was
2 4
insufficient to form a complete Al o layer. Alloy 800H was susceptible
2 3
to localised internal oxidation.
Adding a low level of sulphur (0.2% H S) to the gas increased the corro
2
sion rate of the Model alloy in the 1nitial stages. This rate gradually
slowed down before becoming parabolic after 1000 - 2000 hours. This was
due to the nucleation of sulphides in addition to oxides. The oxides and
sulphides grew side by side until the oxides overgrew the sulphides to
form a complete Cr o layer which cut off further ingress of sulphur
2 3
from the gas. The entrapped sulphides promoted localized thickening
of the oxide layer. Eventually the sulphur redistributed from the sul
phides in the scale to internal sulphide precipitates in the alloy with
the corrosion rate returning to that of the sulphur-fre,e gas for the rest
of the exposure period (5000 hours total).
In the commercial alloys the internal sulphide precipitates prevented
the inner Si0 layer becoming complete. Sulphur doped the (Mn, Fe) Cr 0
2 2 4
outer layer ana the intermediate Cr o layer formed from the spinal layer,
2 3
increasing the number of cation . vacancies and· the growth rate of the
scale. These factors caused a massive Cr depletion of the alloy substrata
after several thousand hours. The internal carbides became unstable which
led to a massive amount of internal attack and a dramatic increase
(breakaway) in the corrosion rate. Due to its thickness and the presence
of Si0 inner layer the external scale became susceptible to spallation.
2
If this occurred the oxides and sulphides nucleated on the alloy surface
-·----
-
again but insufficient Cr was available for the oxides to overgrow the
sulphides. The sulphides therefore grew to form a fast growing non
protective sulphide scale which soon led to catastrophic failure of the
alloy.
Increasing the level of sulphur in the gas to 0.6% H S caused oxides and
2
sulphides to nucleate on the surface, but in this case the sulphides
overgrew the oxides to form thick fast growing non-protective sulphide
scales on all the alloys.
Key words: High temperature corrosion, oxidation, sulphidation,
carburization, breakaway corrosion, coal gasification, fluidized bed
combustion, Fe-Cr-Ni alloys.
·--------
ACKNOWLEDGEMENTS
Mr. J. F. Norton for supervision, encouragement and enthusiasm for this
work.
Prof. I. A. Menzies for helpful suggestions and constructive advice
particularly concerning the writing of the thesis.
s. Canetoli and F. Gandrey for assistance with the corrosion experiments
and helping to assemble some of the figures.
L. Blidegn and G.D. Dickson (Laboratory Manager) for assistance with the
corrosion experiments.
Ph. Glaude (X-ray diffraction), M. Moulaert (E.P.M.A.), G. van Birgelen
(ESCA-AUGER), K. Schuster and P. Frampton (Metallography), D. Ross
(EPMA Loughborough), F. Page (SEM Loughborough) for their structural
analysis work.
J. Blom for developing and printing the photographs.
L. Brahy and D.G. Kneeshaw for typing the manuscript.
D.B. Bottomley (former senior boursier) for passing on a vast amount of
knowledge in the first year of this project.
R. Edwards (Chauffeur), and M.F. Stroosnijder (Minder) for transport,
protection and helpful discussions.
K. E. Beck, F. Frank, V. Guttmann, R.C. Hurst, E. Lang, A.S. McAllister,
·P. Tambuyser and N. Taylor for helpful discussions.
E. Bullock and R. Fordham for reading sections of the manuscript.
G. Kemeny and J.B. Marriott for assistance in dealing with administrative
difficulties.
"Mr. Micron" (P. Pex) for the markers.
E.D. Hondros (Director of J.R.C. Petten) and M. Van de Voorde (Head of
Materials Division) for permission to carry out the work within the
Petten Laboratories.
Department DG XII of the Commission of the European Communities for
financial support.
D.R. Gabe for bringing this·rather unique opportunity to my attention.
DECLARATION
===========
The original contents of this thesis are
the sole responsibility of the author.