revisting caustic cracking of steel vessels and pipes for alumina

REVISTING CAUSTIC CRACKING OF STEEL VESSELS AND PIPES
FOR ALUMINA PROCESSING
R.K. Singh Raman
a,b*
, Sarvesh Pal
a,1
a
Department of Mechanical and Aerospace Engineering
b
Department of Chemical Engineering
Monash University, Vic 3800, Australia
1
Current Affiliation: Department of Mechanical and Mining Engineering
The University of Queensland, Brisbane Australia 4072
*
Presenter and corresponding author: Email [email protected]
Ph + 61 3 990 53671 Fax + 61 3 990 51825
ABSTRACT
Caustic cracking tests were conducted using Bayer solutions of different chemistry at
different temperatures (that are used in Alumina industry for extraction of Alumina from
Bauxite ores). The validity of the commonly used caustic cracking susceptibility diagram
for steels exposed to plain caustic solutions has been assessed using notched and precracked specimens. The study presents first results towards the development of a model
susceptibility diagram for actual Bayer solutions, improving upon applicability of such
diagrams over the traditional plain caustic diagram.
INTRODUCTION
In the Bayer process, steel is the commonly used material for construction of reaction
vessels and pipes for different processing components for extraction of alumina from
bauxite ores, such as digesters, decomposer and precipitator (Burstein et al., 1984, Flis et
al., 2008, Le et al., 1990, Le et al., 1992, Le et al., 1993, Raman et al., 2003, Raman et al.,
2007, Shin et al., 2004, Singbeil et al., 1982, Sriram et al., 1985, Sriram et al., 1985).
Stress corrosion cracking (SCC) is the premature cracking of materials under the
synergistic action of a tensile stress and corrosive medium, neither of which would cause
cracking when acting alone. Caustic embrittlement is a form of SCC that results from
embrittlement of material exposed to caustic environment. The most fundamental and
detrimental feature of SCC is that a ductile material that would have undergone
considerable elongation before fracture may suffer embrittlement in the presence of the
corrosive environment, leading to a premature brittle fracture (i.e., without much
elongation). Shown in Figure 1 is an example of caustic embrittlement and intergranular
crack propagation over the considerable area-fraction of the fracture surface of a steel
specimen, and pure mechanical failure (as evidenced by ductile dimples over rest of the
area) due to overloading of the cross-section reduced by intergranular stress corrosion
cracking.
1
Fig.1 SEM fractograph of a caustic embrittled mild steel: showing a transition from
corrosion-assisted intergranular cracking (lower part of the micrograph) to the pure
mechanical failure (ductile dimples). Magnification: X850 (Raman, 2005).
Caustic embrittlement continues to be a concern for steels in hot caustic service, however,
low carbon steels is still the most frequently used material (Metals Handbook, 1987). Berk
et al., 1950 and Mazille et al., 1972 investigated the limits of caustic concentrations and
temperatures for caustic embrittlement. For example, steels were found to show immunity
to caustic embrittlement at 5wt% NaOH when temperatures were below 95 ºC, whereas this
temperature limit was 40 ˚C at 50wt% NaOH. Caustic cracking susceptibility (CS) diagram
(Figure 2) (Metals Handbook, 1987) is a plot of caustic concentration against temperature,
based on industrial as well as laboratory tests for a maximum of 62 days. This diagram
identifies different regions of caustic cracking susceptibility. The upper hatched area is the
severe cracking zone, whereas in the regime immediately below this area the cracking may
or may not take place. The upper hatched area was developed in the laboratories whereas
the lower curve is based on the field experience (Schmidt et al., 1951). Understanding of
caustic cracking susceptibility is less clear for the regions between the upper hatched area
and the lower curve. By developing an improved understanding of the effect of caustic
concentration and temperature on caustic cracking, it may be possible to expand the limits
for safe application of carbon steel in various steps of Bayer process (Gontijo et al., 2009).
Caustic cracking of in-service components is highly likely to be influenced by the sharp
notches and other stress-raisers that are often present in the fabricated components.
Therefore, to investigate the validity of the CS diagram it may be necessary to test
appropriately the notched and pre-cracked specimens.
Impurities/additions in caustic solutions can also changes caustic cracking susceptibility.
For example, aluminate ions (AlO2-) increase caustic cracking susceptibility of steel (Le et
al., 1989) because the incorporation of AlO2- decreases the stability of the passive film that
develop on steels in the plain NaOH solution. On the other hand, some oxidizing agents,
such as KMnO4, and NaNO3 assist the formation of γ-Fe2O3 films, which favourably shifts
the electrochemical potential and retards caustic cracking ( Humphries et al., 1967).
2
Fig.2: Caustic cracking susceptibility diagram and caustic cracking susceptibility
data generated using CNT specimens (data indicated as stars) superimposed caustic
cracking susceptibility diagram (Raman et al., 2010). The stars represent definite
cracking region. The squares are potential cracking and circles are the definite
absence of cracking
This paper establishes the need for developing a model caustic cracking susceptibility
diagram for typical Bayer solutions, using pre-cracked specimens and circumferential notch
tensile (CNT) technique. This diagram will serve as a new guideline for addressing the
ongoing concerns of caustic cracking in alumina processing industry.
EXPERIMENTAL PROCEDURES
Caustic Cracking Susceptibility Tests
Caustic cracking susceptibility tests were conducted in plain NaOH and Bayer solution at
different concentrations and temperatures that were selected on the basis of the data in
Figure 2. The test material was AS/NZ 3678-grade 250 steel having the chemical
composition (wt.%), C: 0.17, Si: 0.27, Mn: 11.19, P: 0.19, S: 0.09, Cr: 0.02, Ni: 0.01, Mo:
0.02, and the mechanical properties, yield strength: 338 MPa, ultimate tensile strength: 492
MPa and elongation: 35%. Tests were conducted using circumferential notch tensile (CNT)
specimens. The details of this technique can be referred in literature (Rihan et al., 2006, Pal,
et al. 2009). The CNT specimens were fatigue pre-cracked using rotating bending machine
before being installed in CNT testing rig. Tests were carried out using specimens with
different pre-crack depths and applied loads that resulted in different stress intensities (KI)
at the pre-cracks. A tests was either continued until the specimen failed or was terminated
after a considerable length of time for given test condition.
Fractography
The fracture surface was ultrasonically cleaned with a cleaning solution that contained 6 ml
conc. Cleaned surface was observed under JEOL 840 scanning electron microscopy (SEM)
in order to investigate the presence of the well established fractographic evidence of caustic
cracking of steel.
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RESULTS
Caustic Cracking Behaviour of Grade 250 Steel
To investigate the applicability of caustic cracking susceptibility (CS) diagram, four
concentrations at various temperatures were chosen. 10-40 wt% caustic solution will have a
good chance to caustic cracking. However, it is not possible to precisely control the precrack depth and hence stress intensity (KI). Therefore, the applied loads were in the range
of the fracture toughness and the threshold stress intensity factor for SCC (KISCC).
Investigation of Caustic Cracking in Plain Caustic Solutions
Temperatures and caustic concentrations for these tests were selected such that their
combinations would fall in the regimes of ‘definitely no cracking’, ‘definitely cracking’ and
‘may be cracking’ in the susceptibility diagram (Figure 2). The distinctive fractographic
features for caustic cracking of the pre-cracked specimens was the presence of intergranular cracking immediately adjacent to the fatigue pre-crack. Typical fractographs
showing intergranular cracking described in Figure 1.
The overall fracture surface in Figure 3a has four regions that are located an order (in
moving from edge to centre): the machined notch at the specimen edge, fatigue pre-crack,
caustic cracking and exclusively mechanical fracture. The distinctive features of these
regions are established at higher magnifications.
In the light of the features for the presence and absence of inter-granular cracking (clearly
identified in Figure 3a and b), the fractographic features of the CNT specimens tested using
the chosen combinations of caustic concentration and temperature were examined. The
results are summarized in Table 1.
Machine
Notch
Fatigue
Crack
CC Zone
Mechanical
Failure
(a)
(b)
4
(c)
Fig. 3: SEM fracture surface of CNT specimens tested in caustic solutions showing: (a)
overall surface showing different regions, (b) at higher magnification, clear inter-granular
features and (c) dimples (in the lower half) adjacent to the beach marks (in the upper half)
and the absence of inter-granular features in between.
Table 1 Caustic cracking tests in plain NaOH solutions
Caustic
concentration
(wt%)
10
20
30
40
Caustic
solution
temperature
(˚C)
Applied
stress
intensity
(MPa m1/2)
100
35.8
70
32.9
Time to failure
(h)
Evidence of intergranular
fractographic features
Caustic
cracking
susceptibility
100
39.4
Did not fail
(in 3192 h)
Did not fail
(in 6960 h)
134 h
Yes
Yes
80
35.7
408 h
Yes
Yes
80
29.5
49.7
Yes
No
Yes
65
950 h
Did not fail
(in 2064 h)
100
27
815 h
Yes
Yes
70
39.9
Did not fail
(in 2000 h)
No
No
55
32.6
Did not fail
(in 5520 h )
No
No
100
27.8
33.7
Yes
No
Yes
70
45
32.5
184 h
Did not fail
(in 6219 h)
Did not fail
(in 3192 h)
No
No
No
No
No
No
No
No
5
Investigation of Caustic Cracking in Bayer Solutions
Similar to those in plain NaOH, caustic cracking tests were conducted in different Bayer
solutions with different caustic concentrations. The results are summarised in Table 2.
Based on the test results presented in Table 1, the caustic cracking susceptibility of steel in
plain NaOH is predicted in various test conditions. This is largely consistent with the
common caustic cracking susceptibility diagram (Figure 2). However, the data in Table 2
indicate a greater susceptibility of steel in Bayer solution. For example, the steel was found
to be highly susceptible to caustic cracking even at a considerably low temperature (55 ˚C)
in 30wt% FC Bayer solution. But the cracking susceptibility diagram for steel in plain
caustic solution (Figure 1) would predict immunity under this condition. However, as the
FC concentration of the Bayer solution increased to 40wt%, the caustic cracking
susceptibility decreased drastically. The caustic cracking at this concentration was observed
only at a high temperature (100 ˚C).
Table 2 Caustic Cracking tests in Bayer solutions
Free caustic
concentration
(wt%)
10
20
30
40
Caustic
solution
temperature
(˚C)
Applied
stress
intensity
(MPa m1/2)
100
37.3
70
33.1
100
80
Time to failure
(h)
Evidence of intergranular
fractographic features
Caustic
cracking
susceptibility
Yes
Yes
31.7
2088 h
Did not fail
(in 2323 h)
241 h
Yes
Yes
27.3
2280 h
Yes
Yes
80
32.3
Yes
Yes
65
62.9
950 h
Did not fail
(in 4000 h)
100
27.3
1801 h
Yes
70
42.2
87 h
Yes
55
53.2
210.3 h
Yes
100
75.1
Yes
70
41.2
45
30
192 h
Did not fail
(in 2112 h)
Did not fail
(in 1632 h)
No
No
No
No
No
No
Yes
Yes
Yes
Yes
No
No
Figure 4a shows the fracture surface of specimen loaded in 30wt% Bayer solution at 100
º
C. It took a relatively longer time (1801 h) to fail, presumably because of the relatively
low KI employed (27.3 MPa m1/2). However, as predicated by the susceptibility diagram,
fracture surface did possess the feature for caustic cracking (i.e., inter-granular cracking).
The specimen tested at 70 ºC failed in 80 h at an applied stress intensity of 42.2 MPa m1/2,
and possessed an intergranular feature, confirming caustic cracking (Figure 4b). The CNT
6
specimens tested at 55 °C failed in 210 h at an applied stress intensity of 53.2 MPa m½. As
per caustic susceptibility diagram 30wt% NaOH at 70 °C lies in the ‘may be cracking’
zone and 55 °C is considered to be safe. However, the present results would suggest a
possibility of caustic cracking both at 70 and 55 °C.
DISCUSSION
The greater caustic cracking susceptibility in Bayer solutions than in plain NaOH can be
attributed to the role of impurities and additives in the Bayer solution. Consistent with the
very recent work on pure NaOH by the same authors ( Raman et al., 2010), caustic cracking
susceptibility in 30wt% NaOH in this study was found to be limited to 100 ºC (Table 1),
whereas in Bayer solutions the caustic cracking susceptibility was observed at much lower
temperatures (55 ºC). Sriram et al ( Sriram et al., 1985) conducted caustic cracking tests in a
Bayer solution (overall concentration 14.4% , Free caustic concentration 9% ) at 92 ºC
using notched specimens and observed that the presence of AlO2- species moves SCC
susceptibility towards a lower regime of stress intensities. Le et al ( Le et al., 1993) and
Raman ( Raman, 2005) have conducted the experiment in Bayer solutions and different
caustic aluminate solutions in conjunction with tests in plain NaOH at 100 ºC and reported
the steel to be more susceptible to caustic cracking in the Bayer solutions. Among the
impurities dissolved in the Bayer solutions, aluminate ions have the greatest damaging
influence on the susceptibility of steel ( Le et al., 1993). The detrimental effect of aluminate
ions is attributed to the formation of an amorphous film of Fe 3-xAlxO4, where X ≤ 2, which
is less stable.
(a)
(b)
7
(c)
Fig. 4: SEM fracture surface of CNT specimens tested in 30wt% Bayer solution at: (a) 100
˚C, (b) 80 ˚C, and (c) 55 ˚C, each showing inter-granular features immediately after fatigue
pre-cracked area, suggesting caustic cracking.
CONCLUSIONS
1. A caustic cracking susceptibility diagram has been developed for representative
Bayer solutions.
2. Steel is more susceptible to caustic cracking in Bayer solution than in plain caustic
solutions of similar caustic contents.
ACKNOWLEDGEMENTS
Authors record a special word of appreciation to Alcoa World Alumina for providing a
Bayer solution for testing.
REFERENCES
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(6) (1950),pp. 235
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Cracking of a Low Alloy Steel by Auger Electron Spectroscopy, Corros. Rev. 6 (1)
(1984),pp. 81-96
Flis et al., 2008 Flis, J. and Ziomek-Moroz, M., Effect of carbon on stress corrosion
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1726-1733
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Gontijo et al., 2009 Gontijo, G., Araújo, A., Prasad, S., Vasconcelos, L., Alves, J. and
Brito, R., Improving the Bayer Process productivity - An industrial case study, Miner. Eng.
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in hot bayer solutions as related to the scc phenomenon, Corros. Sci. 30 (2-3) (1990),pp.
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Le et al., 1992 Le, H. and Ghali, E., Slow strain rate and constant load tests of A 285 and A
516 steels in Bayer solutions, J. Appl. Electrochem. 22 (1992),pp. 396-403
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by slow strain rate testing, Metall. Mater. Trans. A 36 (7) (2005),pp. 1817-1823
Raman et al., 2003 Raman, R. K. S. and Muddle, B., Caustic stress corrosion cracking of a
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Technol. 19 (2003),pp. 1751-1754
Raman et al., 2010 Raman, R. K. S. and Pal, S., Investigations Using Smooth and Notched
Specimens into Validity of Caustic Cracking Susceptibility Diagram, Metall. Mater. Trans.
A 41 (9) (2010),pp. 2328-2336
Raman et al., 2007 Raman, R. K. S., Rihan, R. and Ibrahim, R., Role of imposed potentials
in threshold for caustic cracking susceptibility (KISCC): Investigations using circumferential
notch tensile (CNT) testing, Corros. Sci. 49 (12) (2007),pp. 4386-4395
Rihan et al., 2006 Rihan, R., Raman, R. K. S. and Ibrahim, R., Determination of crack
growth rate and threshold for caustic cracking (KISCC) of a cast iron using small
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425 (1-2) (2006),pp. 272-277
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Stress Corrosion Cracking in Alkaline Solutions, Corrosion 7 (1951),pp. 295-302
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Singbeil et al., 1982 Singbeil, D. and Tromans, D., Caustic Stress Corrosion Cracking of
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Sriram et al., 1985 Sriram, R. and Tromans, D., The anodic polarization behaviour of
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BRIEF BIOGRAPHY OF PRESENTER
Professor Raman Singh currently works at Monash University in the Department of
Chemical Engineering and the Department of Mechanical & Aerospace Engineering.
His interdisciplinary research expertise comprises:
•
•
•
•
•
•
•
•
•
Role of Nano-/Microstructure in Corrosion,
Stress Corrosion Cracking,
Corrosion and Corrosion-assisted Cracking of Weldments,
Failure Analysis of Metallic Industrial Components,
High Temperature Corrosion in the Industrial Environments, viz., Steamgenerators/boilers of Thermal and Nuclear Power Plants, Petroleum/Petrochemical,
Advanced Coatings and Surface Modifications for Corrosion Resistance,
Microbiologically-induced Corrosion and Cracking,
Corrosion of Magnesium Alloys,
Surface and Sub-surface Characterisation of Corrosion.
Professor Raman Singh’s primary research interest is microstructure-corrosion relationship.
He has also worked extensively on stress corrosion cracking, and corrosion and corrosionmitigation of magnesium alloys, including for the use of magnesium alloys for bio-implant
applications. Over 70% journal publications of a total of a total of his over 90 peerreviewed journal publications of the last 10 years appeared in the top 10% of the journals in
the category, Metallurgical Engineering and Metallurgy. His professional distinctions
include: editorship of a book on cracking of welds, membership of the Review Board of the
prestigious, Metallurgical and Materials Transactions A, leadership/co-chairmanships of
several international conferences and guest co-editorship of international journals, regular
keynote/invited lectures at several international conferences, over 110 peer-reviewed
international journal publications, 14 book chapters and over 90 reviewed conference
publications, and several competitive research grants.
10