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QRS-1384J-1 v2.1 Appendix F
1
Appendix F Corrosion of Stainless Steels
F.1 Introduction
Stainless steels are iron-based alloys that contain a minimum of 11-13 wt.% chromium
to provide corrosion resistance and to impart their "stainless" quality (Sedriks 1996).
At this minimum chromium content a stable Cr oxide/hydroxide a passive film forms
on the surface of the steel. The different classes of stainless steel take their name from
their predominant crystal structure (ASM 1987, 2005); namely: austenitic (face-centred
cubic (fcc)), ferritic (body-centred cubic (bcc)), martensitic (body-centred tetragonal or
bcc), or duplex alloys containing and approximately equal proportion of austenite and
ferrite. Stainless steels typically exhibit good resistance to general corrosion, although
the passive film can dissolve at low pH and/or high Cl- concentration. Localised
corrosion, such as pitting and crevice corrosion can occur in the presence of Cl- and
various S-containing species, but other anions such as carbonate, sulphate, and nitrate
are inhibitors. Some classes of stainless steel, particularly the austenitics, can be
susceptible to stress corrosion cracking (SCC) in the presence of Cl-.
The possible use of stainless steel as a canister material for the disposal of HLW or SF
has been investigated in a number of national programmes (Table F.1). Generally, the
focus has been on the austenitic alloys, particular the American Iron and Steel Institute
(AISI) 300-series, although NDA RWMD have recently reviewed the properties of
duplex alloys as part of the ILW Phased Geological Repository Concept programme
(King 2009a). A major study of the properties of stainless steels was carried out in the
Belgian programme in the 1990's. Various stainless alloys were considered for both
the HLW container and as an overpack material, including ferritic AISI 430 and two
austenitic alloys (AISI 309 and 316Ti) as container materials and the austenitic and
superaustenitic alloys AISI 316, 904L, and 926 as candidate overpack materials (Druyts
and Kursten 1999, Kursten and Druyts 2000, Kursten and Van Iseghem 1999).
However, the occurrence of thiosulphate from the oxidation of pyrite in the Boom clay
and the possibility of increased Cl- levels as a result of the evaporation of dilute clay
interstitial water lead to concern over the localised corrosion of stainless steels (Druyts
and Kursten 1999, Kursten and Druyts 2000) and resulted in the adoption of an
entirely different waste package design (Kursten and Druyts 2008). A stainless alloy
(AISI 309S) has also been selected for the container for HLW in the French programme
in argillaceous clay (Féron et al. 2009), but virtually all experimental studies have been
focussed on the corrosion behaviour of the unalloyed steel overpack. In Spain, the AISI
316L alloy has been investigated, along with a large number of other materials, as a
potential canister material for the disposal of HLW/SF in a granitic geological
2
formation (Kursten et al. 2004). Stainless steels were also briefly considered for use in
the Yucca Mountain repository (AISI 316L and the related Ni alloy 825, Dunn et al.
1996), until such time that uncertainty about the severity of the environment that could
form by evaporation of drips on the waste package surface caused the programme to
select more-corrosion-resistant alloys. Finally, there is much experience in the UK with
the use of austenitic stainless steels (AISI 304, 316 and their low-carbon equivalents
304L and 316L) for the disposal of ILW in cementitious grout (Smart and Wood 2004).
This brief review will focus primarily on the 300-series austenitic alloys, but mention
will also be made of the properties of corresponding duplex alloys which offer a
number of advantages but which have not yet been widely investigated as possible
canister materials.
F.2 Stainless steels
Figure F.1 illustrates the compositional relationship between the main classes of
stainless steel (Sedriks 1996). Austenitic stainless steels contain varying quantities of
elements that stabilise the austenite phase, such as Ni, Co, C, N, Mn, and Cu. Other
elements, such as Cr, Si, Mo, V, Al, Nb, Ti, and W, stabilise δ-ferrite and form the basis
of the ferritic stainless steels.
Austenitic alloys represent the majority of the stainless steels that have been
considered for nuclear waste disposal canisters. Starting from the basic type 304
austenitic stainless steel, the addition of Mo to increase resistance to localised corrosion
produces type 316, with both available as low-C alloys (types 304L and 316L) to reduce
the possibility of sensitisation due to the precipitation of carbides (Cr23C6). An
alternative compositional strategy to prevent sensitisation is to add Ti (type 316Ti) to
precipitate C as a Ti carbide at a higher temperature than Cr23C6. Improved corrosion
resistance is offered by the so-called superaustenitic alloys (for example, types 904L
and 926, Table F.1), which contain increased Ni and Mo. High-temperature oxidation
resistance (and increased strength) is achieved by adding Cr and Ni to the basic type
304 formulation, as in types 309 and 309S. This improved high-temperature
performance is required, for instance, for the containers into which the molten HLW
will be poured.
Ferritic stainless steels contain no Ni (an austenite stabilizer) but contain varying
amounts of Cr to provide corrosion resistance. Superferritic alloys are obtained by
increasing the Cr content (up to 30 wt.%) and adding Mo. The one ferritic alloy
considered for use as a container material for HLW (i.e., not an overpack material) is
the basic ferritic type 430 alloy containing 16-18 wt.% Cr (Table F.1).
QR
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F 3
Table F.1: Compositions of Stainless Steels Considered for Use for the Disposal of ILW or HLW/SF1
UNS Number
Common name
Composition (wt.%)2
Other
Cr
Ni
C
Mn
Si
P
S
Au
sten
itic
all
oy
s
S30
400
304
18-
20
8-10
0.
08
2.0
1.0
0
.045
0.
030
S30
403
304
L
18-
20
8-12
0.
03
2.0
1.0
0
.045
0.
030
S30
900
309
22-
24
12-1
5 0.
20
2.0
1.0
0
.045
0.
030
S30
908
309S
2
2-24
12
-15
0.08
2.
0 1
.0
0.0
45
0.03
0
S31
600
316
16-
18
10-1
4 0.
08
2.0
1.0
0
.045
0.
030
Mo
2-3
S31
603
316
L
16-
18
10-1
4 0.
03
2.0
1.0
0
.045
0.
030
Mo
2-3
S31
635
316
Ti3
16
.80
10.7
0 0.
044
1.0
8 0.
40
0.0
09
0.02
8 M
o 2
.05,
Ti
0.3
N0
8904
9
04L
1
9-23
23
-28
0.02
2.
0 1
.0
0.0
45
0.03
5 M
o 4
-5, C
u 1
-2
N0
8926
9
263
20.6
0 24
.85
0.00
5 0
.92
0.30
0
.018
0.
002
Mo
6.4
0, C
u 0
.86,
N 0
.198
Fer
riti
c al
loy
s
S43
000
430
16-
18
- 0.
12
1.0
1.0
0
.040
0.
030
Du
ple
x a
llo
ys
S32
304
SA
F 2
304
21
.5-2
4.5
3
-5.5
0.
03
2.5
1.0
0
.040
0.
040
N 0
.05-
0.2,
Mo
0.0
5-0
.6
S31
803
220
5 2
1-23
4
.5-6
.5
0.03
2.
0 1
.0
0.0
30
0.02
0 N
0.0
8-0.
2, M
o 2
.5-3
.5
1 A
fter
Sed
rik
s (1
996
), e
xcep
t w
her
e n
ote
d
2 M
axim
um
un
less
oth
erw
ise
ind
ica
ted
, bal
ance
Fe.
3 C
om
po
siti
on
of
allo
y u
sed
by
Dru
yts
an
d K
urs
ten
(19
99)
4
Figure F.1: Compositional Relationship Between the Different Classes of Stainless
Steels (Sedriks 1996).
Although not formally considered as either a container material for HLW glass or as a
canister (or overpack) material for either HLW or SF, duplex stainless steels offer a
number of advantages over austenitic alloys (King 2009a). Duplex alloys contain
approximately equal proportions of austenite and δ-ferrite. As a consequence, the Ni
content (an austenite stabiliser) is lower and the Cr content (a ferrite stabiliser) is
higher than for the corresponding austenitic alloys. Table F.1 lists the composition of
two common duplex alloys, 2304 and 2205.
QRS-1384J-1 v2.1 Appendix F
5
F.3 Corrosion modes for stainless steels
F.3.1 General corrosion
Stainless steels are protected from corrosion by a Cr-based passive film, the properties
of which are discussed in detail in Appendix E: Corrosion of Nickel Alloys. Both
Cr(OH)3 (Appendix E, Figure E.1) and Cr2O3 are thermodynamically stable over a wide
range of pH and for both reducing and, up to a few 100 mV below the O2/H2O
equilibrium, oxidising redox potentials. The stability of this film, however, decreases
with increasing acidity and temperature and in the presence of aggressive anions,
particularly Cl-.
Table F.2 contains a summary of selected rates for the general corrosion of types
304/304L and 316/316L austenitic stainless steel (King 2009a). Various conclusions can
be drawn from these data, including:
• a wide range of rates have been reported, reflecting not only a variation in
environmental conditions but also of the experimental technique
• the rate decreases with increasing exposure time (due to the formation of an
ever-thickening passive film)
• the rate increases with increasing temperature
• the rate decreases with increasing pH, with corrosion rates generally
<0.1 µm/yr in alkaline pH representative of cement pore water
• the rate is higher in the presence of Cl-
• the presence of high Cl- concentrations (of the order of 10,000's mg/L) can
negate the beneficial effect of elevated pH, leading to corrosion rates that
approach 1 µm/yr in saline alkaline solution
• although the data are sparse, there is no apparent different in the rate of general
corrosion of the regular C and low C alloys or between types 304 and 316
• the rate of atmospheric corrosion is generally low, with rates typically
<0.1 µm/yr at ambient temperature
6
Table F.2: G
eneral Corrosion Rates of Types 304/304L and 316/316L Austenitic Stainless Steel in Alkaline and Near-neutral Solutions and
Under Atm
ospheric Conditions.
(a) Type 304/304L
Alloy
pH
Temperature
(oC)
[Cl-]
(µ µµµg⋅ ⋅⋅⋅g
-1)
Redox
conditions
Other
Rate (µ µµµm⋅ ⋅⋅⋅yr-1)
Reference
304
12.
8 1
0.5
30, 4
5
2
00 d
ay
s 60
day
s 0
.00
03
0.0
1 F
uji
saw
a e
t a
l. 1
999
304
13.
3 A
mb
ien
t 18
,400
A
erat
ed
28 d
ays
0.3
Mcd
on
ald
et
al.
1995
304
13
30
50
80
D
eaer
ated
0.0
6 0.
18
0.8
2 B
lack
wo
od
et
al.
2002
304
10
12.
5 1
3.5
50
D
eaer
ated
2
30 d
ay
s 0.
009
0.0
055
0
.00
63
Wad
a an
d N
ish
imu
ra
199
9
304
Am
bie
nt
90
7,00
0-43
,000
A
erat
ed
10 h
rs
10-1
30
Mo
rsy
et
al.
197
9
304
L
Am
bie
nt
25-1
00
“Fre
shw
ater
” A
erat
ed
0.
21
BS
C 2
004
304
L
Am
bie
nt
27
90
“S
altw
ater
” A
erat
ed
11
.4
5.8
2 B
SC
200
4
304
Am
bie
nt
25
50
75
Inte
rsti
tial
cla
y
wat
er
Aer
ated
0.2-
0.96
0.
22-0
.23
0.3-
0.35
C
aste
els
et a
l. 1
986
304
- A
mb
ien
t -
Aer
ated
Urb
an, 5
-15
yr
Urb
an, 5
-15
yr
Mar
ine,
5-1
5 y
r In
du
stri
al/
urb
an, 5
-15
y
<0.
03
0.02
2 0.
05-2
0.
01
Joh
nso
n a
nd
Pa
vli
k
198
2
304
- A
mb
ien
t -
Aer
ated
In
du
stri
al/
urb
an
0.03
-3
Kea
rns
et a
l. 1
984
QR
S-1
384J
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Ap
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dix
F 7
(b) Type 316/316L
Alloy
pH
Temperature
(oC)
[Cl-]
(µ µµµg⋅ ⋅⋅⋅g
-1)
Redox
conditions
Other
Rate
(µ µµµm⋅ ⋅⋅⋅yr-1)
Reference
316
13
.3
Am
bie
nt
18,4
00
Aer
ated
2
8 d
ays
0.6
Mcd
on
ald
et
al.
1995
316L
>
13
Am
bie
nt
10,0
00
Dea
era
ted
0.
1 M
Pa
H2
0.0
3 S
mar
t et
al.
20
04
316
A
mb
ien
t A
mb
ien
t 19
,00
0 A
erat
ed
8 y
rs, P
aci
fic
Oce
an
seaw
ater
4
Ale
xan
der
et
al.
1961
316L
A
mb
ien
t 30
50
-100
“F
resh
wat
er”
Aer
ated
0.0
1 0
.25
BS
C 2
004
316L
A
mb
ien
t 27
“S
altw
ate
r”
Aer
ated
1.9
4 B
SC
200
4
316
A
mb
ien
t 25
50
75
Inte
rsti
tial
cla
y
wat
er
Aer
ated
0.1-
0.24
0.
1-0.
34
0.1-
0.17
Cas
teel
s et
al.
19
86
316
-
Am
bie
nt
- A
erat
ed
Var
iou
s at
mo
sph
eres
0
.05
Dec
hem
a 1
990
8
F.3.2 Localised corrosion
Localised corrosion occurs due to breakdown of the passive film leading to either
pitting of exposed surfaces or crevice corrosion of occluded regions
(Szklarska-Smialowska 2005). Chloride ions promote film breakdown, whereas other
species, such as OH-, CO32-, SO42-, and NO3- inhibit localised corrosion. Other species
result in pitting or crevice corrosion, most notably in the current context thiosulphate
(S2O32-) (Kursten and Druyts 2000), which can form from the oxidation of pyrite
minerals in the host rock or bentonite-based sealing materials.
In a given environment, localised corrosion occurs at a characteristic electrochemical
potential (E). The characteristic potential can either be based on the value at which
film breakdown occurs (EP or ECREV for pitting and crevice corrosion, respectively) or
that at which a propagating pit or crevice re-passivates (ERP or ERCREV, respectively).
The criterion for pitting or crevice corrosion is that the corrosion potential (ECORR)
exceeds either the film breakdown or re-passivation potentials, i.e.,
ECORR > EP, ECREV (1)
or
ECORR > ERP, ERCREV (2)
respectively.
The pitting potential is a function of alloy composition, Cl- concentration (and the
concentration of other aggressive or inhibitive species), temperature, pH, and various
metallurgical parameters, such as surface finish, degree of sensitisation, etc. Figure F.2
shows the characteristic decrease in EP with the logarithm of the Cl- concentration, as
well as the decrease with increasing temperature. Figure F.3 more clearly shows the
temperature dependence of EP for austenitic alloys Type 304 and 316 and for the ferritic
Type 430 alloy. The Mo-containing Type 316 alloy exhibits the most-positive pitting
potential and, for the same value of ECORR, would be the least susceptible to pitting
corrosion. The most susceptible of the three alloys is the ferritic Type 430 alloy.
Figure F.4 shows the variation of EP with pH for the same three alloys. The
dependences shown in the figure suggest the existence of a threshold pH above which
the pitting potential rapidly increases, with that threshold increasing in the order
Type 316 < Type 430 < Type 304.
QRS-1384J-1 v2.1 Appendix F
9
Figure F.2: Dependence of the Pitting Potential for Type 304 Stainless Steel on
Chloride Concentration for Various Temperatures (Szklarska-Smialowska 2005).
As noted above, thiosulphate ions also induce pitting of stainless steels. The data in
Figure F.5 suggest a decrease in EP of 100-200 mV due to the presence of thiosulphate at
a [Cl-]:[S2O32-] ratio of 17. Kursten and Druyts (2000) also reported increased
susceptibility of both Type 316L and 904 austenitic alloys in the presence of
thiosulphate, although the latter alloy which contains approximately double the
amount of Mo as Type 316L was significantly more resistant, as indicated by a 200-
400 mV difference in EP. Interestingly, Kursten and Druyts (2000) noted that S2O32-
affected pit initiation but not pit growth.
Other anions inhibit localised corrosion. Figure F.6 shows the effect of sulphate on the
crevice corrosion of Types 304 and 316 austenitic stainless steels in chloride solutions.
A [SO42-]:[Cl-] ratio of 1.2 (on a molar basis) is sufficient to inhibit the crevice corrosion
of Type 304 and a [SO42-]:[Cl-] ratio of as little as 0.4 is sufficient for the more-resistant
Type 316 alloy.
10
Figure F.3: Dependence of the Pitting Potential on Temperature for Types 304 and
316 Austenitic and Type 430 Ferritic Stainless Steels in 3% NaCl Solution (from
Sedriks 1996).
Figure F.4: Dependence of the Pitting Potential on pH for Types 304 and 316
Austenitic and Type 430 Ferritic Stainless Steels in 3% NaCl Solution (from Sedriks
1996).
QRS-1384J-1 v2.1 Appendix F
11
Figure F.5: Effect of Thiosulphate on the Pitting Potential of Type 316L Stainless
Steel as a Function of Chloride Concentration at 80oC (Sedriks 1996).
Figure F.6: Inhibitive Effect of Sulphate on the Crevice Corrosion of Types 304 and
316 Stainless Steel in Chloride Solutions (from Sedriks 1996).
12
Figure F.7: Map of the Dependence of Crevice Corrosion and Pitting on Potential
and Chloride Concentration (from Szklarska-Smialowska 2005).
Crevice corrosion occurs under less-aggressive conditions than pitting because the
restricted mass transport of species into and out of the occluded region promotes the
development of the critical chemistry required to sustain stable pit or crevice growth.
Thus, crevice corrosion will occur at more-negative potentials and/or at lower Cl-
concentrations than pitting, as illustrated in Figure F.7 for Type 304L stainless steel.
As indicated by Equations (1) and (2), it is not the value of EP/ERP or ECREV/ERCREV that
determines the susceptibility to localized corrosion, as such, but the difference between
ECORR and the critical potential. This criterion is illustrated in Figure F.8 which shows
the dependence of the pitting and pit re-passivation potentials for Type 316L stainless
steel on Cl- concentration at 95oC and the corresponding values of ECORR in aerated and
deaerated solutions. Based on the criteria in Equations (1) and (2), therefore, pitting
would only occur in aerated solution and then only on the basis of the pit re-
passivation criterion and not on the film breakdown criterion. Localised corrosion
would not occur at all in deaerated solution.
Tables F.3 to F.8 provide data with which to assess the susceptibility to localised
corrosion of Types 304/304L and 316/316L austenitic stainless steels. These tables are
not an exhaustive review of all of the data in the literature, but do provide a significant
database for assessing localised corrosion susceptibility. Tables F.3 and F.4 summarize
pitting and pit re-passivation potentials for Types 304/304L and 316/316L,
respectively. Corresponding data for the crevice and crevice re-passivation potentials
are given in Tables F.5 and F.6 for Types 304/304L and 316L, respectively. Finally,
QRS-1384J-1 v2.1 Appendix F
13
Figure F.8: Comparison of the Pitting (EP) and Re-passivation (ERP) Potentials for
Type 316L Stainless Steel as a Function of Chloride Concentration at 95oC and the
Corresponding Values of the Corrosion Potential (ECORR) in Aerated and Deaerated
Solutions (Dunn et al. 1996).
Tables F.7 and F.8 list various measurements of the corrosion potential under various
environmental conditions for Types 304/304L and 316L, respectively. There are few
corresponding values for the other alloys in Table F.1.
There are two other commonly used methods for assessing the susceptibility to
localised corrosion of stainless steel. First, the effect of alloy composition on pitting (or
crevice corrosion) susceptibility can be compared based on the pitting resistance
equivalent number (PREN) approach. The PREN is given by
PREN = %Cr + a% Mo + b% N (3)
where the values of a and b vary for pitting and crevice corrosion.
14
Table F.3: Literature Data on Pitting of Type 304/304L Stainless Steels.
Type
Temperature
(oC)
[Cl-]
(µ µµµg⋅ ⋅⋅⋅g
-1)
Other
EP or ERP
(mVSCE)
Reference
304
80
19,0
00
Sy
nth
etic
sea
wa
ter
+50
(E
P)
Har
uk
i et
al.
199
1
304
20
34,5
00
Wit
h a
dd
itio
ns
of
Na 2
S2O
3
0 m
ol⋅
dm
-3
4 x
10-
4m
ol⋅
dm
-3
1 x
10-
3 m
ol⋅
dm
-3
2 x
10-
3 m
ol⋅
dm
-3
4 x
10-
3 m
ol⋅
dm
-3
0.0
1 m
ol⋅
dm
-3
0.1
mo
l⋅d
m-3
0.4
mo
l⋅d
m-3
1.0
mo
l⋅d
m-3
(EP)
+30
* -2
0 +
40
-130
-2
05
-195
-1
35
-95
-65
Szk
lars
ka-
Sm
ialo
wsk
a 20
05 (
Fig
7.1
5)
304
80
34,5
00
Wit
h a
dd
itio
ns
of
Na 2
S2O
3
0 m
ol⋅
dm
-3
1 x
10-
4 m
ol⋅
dm
-3
4 x
10-
4m
ol⋅
dm
-3
1 x
10-
3 m
ol⋅
dm
-3
0.0
1 m
ol⋅
dm
-3
0.1
mo
l⋅d
m-3
(EP)
-65*
-6
5 -2
55
-265
-2
60
-190
Szk
lars
ka-
Sm
ialo
wsk
a 20
05 (
Fig
7.1
5)
304
25
40
60
90
17,0
00
Wit
h a
dd
itio
n o
f 0.
1 m
ol⋅
dm
-3 N
aHC
O3, p
H 8
+4
15 (
EP)
+32
0 +
155
+65
Szk
lars
ka-
Sm
ialo
wsk
a 20
05 (
Fig
12
.2)
304
100
150
345
Sen
siti
zed
304
-5
5 (E
P)
-240
S
zkla
rsk
a-S
mia
low
ska
2005
(F
ig 1
2.5
)
QR
S-1
384J
-1 v
2.1
Ap
pen
dix
F
15
Table F.3: Literature Data on Pitting of Type 304/304L Stainless Steels (Continued).
Type
Temperature
(oC)
[Cl-]
(µ µµµg⋅ ⋅⋅⋅g
-1)
Other
EP or ERP
(mV
SCE)
Reference
304
20
40
60
80
Ra
ng
e 1
00-
20,
000
EP =
750
– 1
52 l
og
[C
l- ]
EP =
628
– 1
40 l
og
[C
l- ]
EP =
554
– 1
44 l
og
[C
l- ]
EP =
500
– 1
45 l
og
[C
l- ]
Szk
lars
ka-
Sm
ialo
wsk
a 20
05
(Fig
12.
6)
304
Am
bie
nt
34,
500
+2
32 (
EP)
Szk
lars
ka-
Sm
ialo
wsk
a 20
05
(Fig
18.
4)
304
3,
450
Wit
h a
dd
itio
n o
f 0
.1 m
ol⋅
dm
-3 N
aHC
O3
-70
(EP)
Sed
rik
s 1
996
(Tab
le 4
.3)
304
30
660
+50
µg⋅g
-1 S
O4
2- ,
2 µ
g⋅g
-1 C
u2
+
Bas
e m
etal
W
eld
HA
Z
60%
co
ld w
ork
(EP)
+39
0 +
190
+21
0
Sed
rik
s 19
96 (
Tab
le 4
.10)
304L
30
14
2,00
0 p
H 9
.3
-51
(EP)
-216
(E
RP)
Sri
dh
ar e
t a
l. 1
993
304
1
8,4
00
pH
3
pH
4
pH
5
pH
6
pH
7
pH
8
pH
9
pH
10
pH
11
pH
12
-5 (
EP)
+10
+
15
+25
+
40
+50
+
60
+70
+
80
+40
0
Sed
rik
s 1
996
(Fig
. 4.
38)
16
Table F.3: Literature Data on Pitting of Type 304/304L Stainless Steels (Concluded).
Type
Temperature
(oC)
[Cl-]
(µ µµµg⋅ ⋅⋅⋅g
-1)
Other
EP or ERP
(mV
SCE)
Reference
304
25
34,5
00
H2 a
tmo
sph
ere
0.0
76 µ
g⋅g
-1 O
2
N2 a
tmo
sph
ere
0.4
60 µ
g⋅g
-1 O
2
Ar
atm
osp
her
e 0
.057
µg⋅g
-1 O
2
O2 a
tmo
sph
ere
30.1
µg⋅g
-1 O
2
-50
(EP)
-20
+5
0 +
65
Sed
rik
s 19
96 (
Tab
le 4
.16)
304
30
18,4
00
+
60 (
EP)
Sed
rik
s 19
96 (
Fig
. 4.4
2)
304
25
17,0
00
pH
5
+20
0 (E
P)
Sed
rik
s 19
96 (
Tab
le 4
.15)
304
20
345
3,45
0 34
,50
0
Mea
n v
alu
es, d
EP/
dlo
g[C
l- ] =
127
mV
1
000
gri
t
+56
5 (E
P)
+46
0
+31
5
Lay
cock
et
al. 2
005
304
20
345
3,45
0 34
,50
0
Mea
n v
alu
es, d
EP/
dlo
g[C
l- ] =
137
mV
22
0 g
rit
+49
5 (E
P)
+38
5
+22
0
Lay
cock
et
al. 2
005
304
0
30
60
90
3,45
0
+36
0 (E
P)
+19
0
+6
0 -2
0
Par
k e
t a
l. 2
002
QR
S-1
384J
-1 v
2.1
Ap
pen
dix
F
17
Table F.4: Literature Data on Pitting of Type 316/316L Stainless Steels.
Type
Temperature
(oC)
[Cl-]
(µ µµµg⋅ ⋅⋅⋅g
-1)
Other
EP or ERP
(mV
SCE)
Reference
316
L
80
19,
000
S
yn
thet
ic s
eaw
ater
+
150
(EP)
Har
uk
i et
al.
199
1
316
L
20
34,
500
Wit
h a
dd
itio
ns
of
Na 2
S2O
3
0 m
ol⋅
dm
-3
1 x
10-
4m
ol⋅
dm
-3
2 x
10
-3 m
ol⋅
dm
-3
4 x
10
-3 m
ol⋅
dm
-3
0.01
mo
l⋅d
m-3
0.04
mo
l⋅d
m-3
0.1
mo
l⋅d
m-3
0.4
mo
l⋅d
m-3
1.0
mo
l⋅d
m-3
(EP)
+18
0*
+11
0
+12
5
+12
5
+14
0
-50
-25
+5
+5
0
Sz
kla
rsk
a-S
mia
low
ska
200
5 (F
ig 7
.15)
316
L
80
34,
500
Wit
h a
dd
itio
ns
of
Na 2
S2O
3
0 m
ol⋅
dm
-3
1 x
10-
4m
ol⋅
dm
-3
2 x
10
-3 m
ol⋅
dm
-3
4 x
10
-3 m
ol⋅
dm
-3
0.01
mo
l⋅d
m-3
0.04
mo
l⋅d
m-3
0.1
mo
l⋅d
m-3
(EP)
+55
* +
55
+8
5 0
-135
-1
15
-100
Sz
kla
rsk
a-S
mia
low
ska
200
5 (F
ig 7
.15)
316
L
1
8 3
0 4
0 5
0 6
0
19,
000
Sea
wat
er
EC
OR
R =
-15
5 m
VS
CE (
8 m
m c
rev
ice
corr
osi
on
) E
CO
RR =
-3
46 m
VS
CE (
25-3
0 m
m C
C)
EC
OR
R =
-3
93 m
VS
CE (
25-3
0 m
m C
C)
EC
OR
R =
-36
5 m
VS
CE (
10 m
m C
C, 0
.02
5 m
m p
it)
EC
OR
R =
-36
2 m
VS
CE (
40 m
m C
C, 0
.02
1 m
m p
it)
(EP/
ER
P)
+50
5/+
2
+27
4/-1
66
+15
6/-2
74
+15
5/-2
15
+28
/-1
42
Sz
kla
rsk
a-S
mia
low
ska
200
5 (T
able
13
.2)
18
Table F.4: Literature Data on Pitting of Type 316/316L Stainless Steels (Continued).
Type
Temperature
(oC)
[Cl-]
(µ µµµg⋅ ⋅⋅⋅g
-1)
Other
EP or ERP
(mV
SCE)
Reference
316
17,0
00
Wit
h a
dd
itio
n o
f 0
.1 m
ol⋅
dm
-3 N
aHC
O3
+14
0 (E
P)
Sed
rik
s 19
96 (
Tab
le 4
.3)
316
31,0
00
Ox
yg
enat
ed s
olu
tio
n, 0
.4-2
wt.
% M
n
+21
0-24
5 (
EP)
Sed
rik
s 19
96
(Ta
ble
4.2
7)
316L
40
50
60
70
21,
500
+27
5 (E
P)
+22
0
+13
5
+7
0
Sed
rik
s 19
96 (
Fig
. 4.3
6)
316L
3
0 1
42,0
00
pH
9.3
-4
8 (E
P)
-237
(E
RP)
Sri
dh
ar e
t al
. 19
93
316
18,4
00
pH
3
pH
4
pH
5
pH
6
pH
7
pH
8
pH
9
pH
10
pH
11
pH
12
+28
5 (E
P)
+28
5
+28
5
+28
5
+28
5
+29
0
+30
5
+35
0
+46
0
+58
5
Sed
rik
s 19
96 (
Fig
. 4.3
8)
316L
8
0 3
5 34
5 34
50
NaC
l so
luti
on
s o
nly
+
115
(EP)
-5
-105
S
edri
ks
1996
(F
ig. 4
.41)
316L
8
0
35
345
3450
34
,50
0
Cl-
/S
2O
32
- mix
ture
s, [
Cl-
]:[S
2O
32- ]
= 1
7
+45
(E
P)
-180
-2
15
-275
Sed
rik
s 19
96 (
Fig
. 4.4
1)
316
3
0 18
,40
0 E
ffec
t o
f te
mp
era
ture
+
230
(EP)
Sed
rik
s 19
96 (
Fig
. 4.4
2)
QR
S-1
384J
-1 v
2.1
Ap
pen
dix
F
19
Table F.4: Literature Data on Pitting of Type 316/316L Stainless Steels (Concluded).
Type
Temperature
(oC)
[Cl-]
(µ µµµg⋅ ⋅⋅⋅g
-1)
Other
EP or ERP
(mV
SCE)
Reference
316
20
345
3,
450
10,
300
3
4,4
50
120
gri
t fi
nis
h, a
era
ted
so
luti
on
+31
7 (E
P)
+2
05
+1
70
+98
La
yco
ck a
nd
New
man
199
7
316
20
3,45
0 3
4,4
50
Mea
n v
alu
es,
dE
P/
dlo
g[C
l- ] =
206
mV
10
00
gri
t +
630
(EP)
+4
15
La
yco
ck e
t al
. 200
5
316
20
345
3
450
34,
500
Mea
n v
alu
es,
dE
P/
dlo
g[C
l- ] =
220
mV
22
0 g
rit
+75
0 (E
P)
+5
00
+3
35
La
yco
ck e
t al
. 200
5
20
Table F.5: Literature Data on Crevice Corrosion of Type 304/304L Stainless Steels.
Type
Temperature (oC)
[Cl-]
(µ µµµg⋅ ⋅⋅⋅g
-1)
Other
ECREV or ERCREV
(mV
SCE)
Reference
304
17
,000
34
,500
+13
1 (E
CR
EV)
+10
S
zkla
rsk
a-S
mia
low
ska
2005
(F
ig 1
8.4)
304
L
10
20
30
40
50
60
70
80
19,0
00
Sea
wat
er, p
H 8
.2
-120
(E
RC
RE
V)
-140
-1
50
-155
-1
75
-175
-2
00
-210
Tan
i et
al.
20
08
304
80
19,0
00
Sy
nth
etic
sea
wat
er
-10
(EC
RE
V)
Har
uk
i et
al.
199
1
QR
S-1
384J
-1 v
2.1
Ap
pen
dix
F
21
Table F.6: Literature Data on Crevice Corrosion of Type 316L Stainless Steel.
Type
Temperature
(oC)
[Cl-]
(µ µµµg⋅ ⋅⋅⋅g
-1)
Other
ECREV or ERCREV
(mVSCE)
Reference
316L
10
20
30
40
50
60
70
80
19,
000
S
eaw
ate
r, p
H 8
.2
-80
(ER
CR
EV)
-95
-110
-1
20
-140
-1
55
-155
-1
65
Tan
i et
al.
200
8
316L
23
-50
6,00
0-2
4,0
00
0-8
00 µ
g⋅g
-1 S
2O
32- ,
800-
3,40
0 µ
g⋅g
-1 S
O4
2-
ER
CR
EV =
-3
01.5
– 3
.7([
Cl- ]
-15)
– 1
5.3(
[SO
42
- ]-2
.1)
–188
.7([
S2O
32
- ]-0
.4)
– 2(
T-3
6.5
) –0
.047
(([C
l- ]-1
5)([
SO
42
- ]-2
.1)
+ 3
.83(
[Cl- ]
-15)
([S
2O
32- ]
-0.4
) –0
.75
([C
l- ]-1
5)(T
-36
.5)
+ 0
.35(
[Cl-
]-15
)2
T i
n o
C, c
on
cen
tra
tio
ns
in g
/L
Sri
dh
ar e
t a
l. 2
004
316L
80
1
9,0
00
Sy
nth
etic
sea
wa
ter
+8
0 (E
CR
EV)
Har
uk
i et
al.
199
1
22
Table F.7: Literature Data on Corrosion Potential of Type 304/304L Stainless Steels.
Grade
pH
Temperature
(oC)
[Cl-]
(µ µµµg⋅ ⋅⋅⋅g
-1)
Redox
conditions
Other
ECORR
(mV
SCE)
Reference
304
L
8.2
24
30
50
60
70
80
19,0
00
Aer
ate
d
Sea
wat
er
-90
-90
-120
-1
40
-155
-2
10
Ta
ni
et a
l. 2
008
304
L
9.3
30
142
,00
0 D
eaer
ated
-488
S
rid
har
et
al. 1
993
304
5 25
17
,300
D
eaer
ated
-464
S
edri
ks
1996
(T
able
4.1
5)
304
34,5
00
Aer
ate
d
-6
3 S
zk
lars
ka-
Sm
ialo
wsk
a 2
005
(F
ig 1
8.4)
304
4.5
3,
450
Aer
ate
d
-1
4 S
rid
har
et
al. 2
004
QR
S-1
384J
-1 v
2.1
Ap
pen
dix
F
23
Table F.8: Literature Data on Corrosion Potential of Type 316L Stainless Steel.
Grade
pH
Temperature
(oC)
[Cl-]
(µ µµµg⋅ ⋅⋅⋅g
-1)
Redox conditions
Other
ECORR (mV
SCE)
Reference
316
L
9.3
30
142
,00
0 D
eaer
ated
-685
S
rid
har
et
al. 1
993
316
L
7 10
12
0.32
µg⋅g
-1 O
2
0.5
m N
a 2S
O4
-117
-1
19
-145
S
rid
har
et
al. 2
004
316
L
8.2
24
30
50
60
70
80
19,
000
A
erat
ed
Sea
wat
er
-80
-85
-105
-1
20
-140
-1
65
Tan
i et
al.
20
08
316
L
95
6
-1,0
00
Dea
erat
ed
10-
1,00
0 µ
g⋅g
-1
NO
3- ,
2 µ
g⋅g
-1 F
- ,
20-1
,000
µg⋅g
-1 S
O42
-
-739
to
-45
4 S
rid
har
et
al. 1
993
316
L
10
12.6
13
.6
22
177
,00
0 A
erat
ed
+
70
-80
-125
C
ui
and
Sag
ues
20
03
316
L
12.6
22
Aer
ated
-100
C
ui
and
Sag
ues
20
03
316
L
95
1,
000
200
,00
0 A
erat
ed
-1
25
-330
D
un
n e
t al
. 19
96
316
L
19,
000
0.45
µg⋅g
-1 O
2
2.6 µ
g⋅g
-1 O
2
7.6 µ
g⋅g
-1 O
2
34.7
µg⋅g
-1 O
2
-86
-44
-18
+4
7
Sri
dh
ar e
t al
. 200
4
24
Figure F. 9: Correlation between the pitting resistance equivalent number and
pitting potential for various austenitic, duplex, and superaustenitic stainless steels
(Malik et al. 1996).
The correlation between the PREN and the pitting potential for various austenitic,
duplex, and superaustenitic stainless steels is shown in Figure F.9. Of the alloys
considered for use as ILW or HLW/SF (Table F.1), the duplex alloy 2205 provides the
higher pitting resistance (or, at least, most-positive EP value), followed by the
superaustenitic Type 904L, and the austenitic 316L and 304L alloys.
Another method for characterizing the resistance to localised corrosion is the critical
pitting (CPT) and critical crevice (CCT) temperature. These critical temperatures are
determined in an aggressive solution using exposed and creviced samples. The actual
CPT and CCT values depend on the nature of the environment, but are typically
measured in acidified ferric solutions. Figure F.10 shows the Cl- concentration
dependence of the CPT and CCT for a number of austenitic, duplex, and
superaustenitic stainless steels and indicates the same order of corrosion resistance as
the PREN data in Figure F.9.
QRS-1384J-1 v2.1 Appendix F
25
Figure F.10: Corrosion map illustrating critical pitting and crevice corrosion
temperatures as a function of chloride concentration for various austenitic, duplex,
and super-austenitic stainless steels (ASM 2005). Critical conditions for pitting and
crevice corrosion indicated by solid and dashed lines, respectively.
F.3.3 Environmentally assisted cracking
Stainless steels exhibit different susceptibilities to environmentally assisted cracking
(EAC). By far the most common form of EAC for stainless steels is the Cl- stress
corrosion cracking (SCC) of austenitic alloys. Cracks often initiate from pits and, for
this reason, the SCC susceptibility is related to the pitting susceptibility (Figure F.11).
The susceptibility to SCC (and pitting) clearly increases with increasing temperature
and Cl- concentration and decreasing pH.
There is evidence for a threshold temperature and, possibly, Cl- concentration below
which SCC does not occur. Based on the data in Figure F.12, the threshold temperature
for SCC is of the order of 60oC and the threshold Cl- concentration is somewhat less
than 1 mg/L. Although useful as a general guide, care should be taken in the use of
such "thresholds" since they are based on industrial timescales, rather than the
extended timescales of interest for ILW/HLW/SF disposal. This caution is especially
so for any threshold Cl- concentration since evaporation will concentrate surface liquid
films.
26
Figure F.11: Ranges of pH, Chloride Concentration and Temperature for the Pitting
and SCC of 304 Stainless Steel (Jones 1992).
The microstructure and elemental composition also affect the susceptibility to SCC.
The δ-ferrite microstructure of ferritic stainless steels is inherently less susceptible to
cracking than the austenite structure; a difference which is responsible for the
precipitous increase in the time-to-failure at low Ni contents shown in Figure F.13.
Ferritic steels, however, are not immune to immune to SCC (Sedriks 1992). Figure F.14
also shows the effect of Ni content on the SCC susceptibility (in this case based on the
threshold stress intensity factor for cracking1), with the susceptibility of specific alloys
indicated. This figure clearly shows the superior SCC resistance of both the
superaustenitic Type 904L and the ferritic Type 444 alloy.
1 The threshold stress intensity factor KISCC is the value of KI, the linear-elastic fracture
mechanics stress intensity factor, below which SCC crack growth is not observed.
QRS-1384J-1 v2.1 Appendix F
27
Figure F.12: Corrosion map for the susceptibility of various austenitic and duplex
stainless steels to stress corrosion cracking in aerated chloride environments as a
function of temperature (Sedriks 1996).
The Mo content also affects SCC susceptibility (Figure F.15). The increased SCC
resistance of alloys such as Type 904L results in part from the increased resistance to
pitting which acts as a necessary precursor for crack initiation.
Both austenitic and duplex stainless steels are susceptible to SCC in sulphide- and
thiosulphate-containing environments. The severity of cracking of Types 304L and
316L have been shown to increase with decreasing pH and with increasing
thiosulphate and/or chloride concentrations (Smart 2000).
28
Figure F.13: Effect of Ni content on the susceptibility of 18-20 wt.% Cr stainless
steels in boiling magnesium chloride solution at 154oC (after Sedriks 1992).
QRS-1384J-1 v2.1 Appendix F
29
Figure F.14: Effect of Ni content on the threshold stress intensity factor for SCC for a
range of Fe-Cr-Ni alloys (after Sedriks 1992).
F.3.4 Microbiologically influenced corrosion
Like most engineering materials, stainless steels are susceptible to microbiologically
influenced corrosion (MIC) (Little et al. 1991). Apart from the inherent susceptibility of
the material(s), any assessment of the potential for MIC of stainless steel HLW/SF
canisters also needs to take into account the effect of the GDF environment on the
location and duration of microbial activity (King 2009b).
30
Figure F.14: Dependence of the threshold stress intensity factor for SCC on the Mo
content of Fe-Cr-Ni-Mo alloys in aerated 22% NaCl solution at 105oC (after Sedriks
1992).
When intimately exposed to active microbial communities and biofilms, stainless steels
are subject to various forms of MIC. In common with other materials, stainless steels
will undergo corrosion due to the formation of reduced S species (produced by the
action of sulphate-reducing bacteria) and organic acids (produced by a range of
bacteria and fungi) (Little et al. 1991). In addition to the common forms of MIC,
however, stainless steels are susceptible to other specific forms of attack, including:
QRS-1384J-1 v2.1 Appendix F
31
• localised corrosion due to the formation of thiosulphate (discussed above),
• preferential attack of welds, and
• ennoblement of ECORR.2
Various mechanisms have been proposed for the ennoblement of ECORR, including the
production of H2O2 within the biofilm and the catalysis of O2 reduction by MnO2.
Ennoblement could increase the probability of localised corrosion, especially if it is
accompanied by the formation of thiosulphate elsewhere in the biofilm.
Of crucial importance, however, is where and when microbial activity is possible in the
GDF (King 2009b). As described elsewhere in this report, if microbial activity is only
possible at locations away form the canister surface, for example, due to the use of
highly compacted bentonite or cementitious backfill, then the possible damage due to
MIC will be much reduced.
F.3.5 Galvanic corrosion
As with Ni alloys, stainless steels are relatively noble in their passive state. In any
galvanic couple with a more-active material, such as Fe, Mg, Al, Zn, stainless steel
would act as the cathode with the active material preferentially corroding. In contact
with other passive materials, the driving force for corrosion (i.e., the difference in
potential between the two passive materials) will be small and any effect minimal.
F.3.6 Anthropogenic Analogues
Stainless steels have a history of less than one hundred years. Therefore, there is
relatively little experience with these alloys compared to materials such as copper and
C-steel. Smart and Wood (2004) have reviewed a number of case histories from the
architecture, transportation, and infrastructure industries demonstrating good
corrosion resistance to atmospheric conditions and immersion in seawater for periods
exceeding 60 years. King (2009a) has reviewed a number of cases of the use of duplex
stainless steels for architectural purposes and bridge construction. These
anthropogenic analogues provide support for values for the rate of the long-term
atmospheric corrosion of stainless steels.
2 The shift in ECORR to more-positive values.
32
F.4 Corrosion behaviour of stainless steels
F.4.1 Effect of redox conditions
As for other passive materials, the evolution of redox conditions in the GDF will affect
both the rates of general and localised corrosion. Although the evidence from the rates
summarised in Table F.2 is sparse, general corrosion rates in aerated environments are
typically higher than those under deaerated conditions.
More importantly, the evolution of redox conditions will affect the probability of
localised corrosion and SCC of stainless steels. It is clear from the discussion in
Section F.3.2 that pitting and crevice corrosion are primarily of concern under aerobic
conditions when ECORR is most likely to exceed the film breakdown or re-passivation
potentials. Indeed, the evidence from Figure F.8 indicates that pitting of Type 316L
stainless steel will not occur in Cl- solutions at 95oC under anaerobic conditions.
However, as indicated in Figure F.5, the presence of thiosulphate shifts EP to more-
negative values, making pitting more likely even as the redox conditions in the GDF
shift from aerobic to anaerobic.
The probability of SCC is also linked to the evolution of redox conditions. Stress
corrosion is closely linked to conditions under which pitting occurs, so the probability
of cracking can similarly be expected to diminish as redox conditions become
anaerobic.
F.4.2 Effect of chloride
Of the groundwater and/or porewater species that may be present in the GDF,
chloride ions are the species most likely to affect the corrosion of the canister. Chloride
ions affect the stability of the passive film. Even in the absence of localised film
breakdown, Cl- ions degrade the stability of the passive film through an increase in the
solubility of Cr(III) (Pourbaix 1974), resulting in higher rates of general corrosion in
saline solutions (Table F.2).
As discussed in detail above, however, the most significant effect of Cl- is the impact on
the localised corrosion and SCC behaviour of the canister. Increasing Cl- concentration
shifts the pitting and crevice potentials to more negative values (Figures F.2, F.5, F.7,
F.8, F.10, and F.11). Increasing Cl- concentration also increases the probability of SCC
(Figures F.11 and F.12).
QRS-1384J-1 v2.1 Appendix F
33
F.4.3 Effect of temperature
Increasing temperature affects the rate of general corrosion and the probability of both
localised corrosion and SCC. Increasing temperature typically results in higher rates of
general corrosion (Table F.2), although not all of the studies in which the effect of
temperature has been studied show a consistent trend with temperature. However, the
data of Blackwood et al. (2002) in alkaline solution do exhibit a monotonic increase
with temperature and suggest an activation energy of 47 kJ/mol.
Increasing temperature shifts the pitting potential to more-negative values (Figures F.2
and F.3). Based on the data in Tables F.3 and F.4, the decrease in EP/ERP is of the order
of -3.4 to -7.0 mV/oC for Types 304 and 316L, although there is evidence that ERP for
Type 316L reaches a minimum value at ~40oC and then shifts to more-positive values
with increasing temperature. This change in the temperature dependence of the
pitting characteristics of Type 316L is also evident in Figure F.3, where the decrease in
EP with increasing temperature appears to level off at temperatures above 70oC. This
improved film stability at higher temperatures could be due to the presence of Mo in
the passive film and its inherent greater thermal stability. Based on a single report, the
temperature dependence of the crevice re-passivation for Types 304L and 316L is
somewhat smaller, decreasing by about -1.2 mV/oC (Tables F.5 and F.6).
The influence of temperature on the localised corrosion behaviour is only partly
described by the temperature dependence of EP/ERP and ECREV/ERCREV. It is also
necessary to take into account the temperature dependence of ECORR, since it is the
difference between the critical potential and ECORR that determines the probability of
localised corrosion. The corrosion potential also shifts in the active direction with
increasing temperature, but at a slower rate. Again based on a single study, the
temperature dependence of ECORR for Types 304L and 316L stainless steel in aerated
solution containing 19,000 mg/L Cl- is -2.0 mV/oC and -1.5 mv/oC, respectively
(Tables F.7 and F.8). The fact that the critical potential decreases more rapidly with
increasing temperature than the value of ECORR indicates that localised corrosion
becomes more likely with increasing temperature and is one explanation for the
observation for the critical pitting and crevice corrosion temperatures discussed earlier.
F.4.4 Effect of pH
Increasing pH promotes passivation and counteracts the effects of aggressive anions,
such as Cl-. The beneficial effects of increasing pH on the rate of general corrosion is
evident from the data in Table F.2. Under alkaline conditions (pH > 10), rates of
general corrosion are typically of the order of 0.01 µm/y, whereas rates are generally 1-
2 orders of magnitude higher at near-neutral pH.
34
Pitting is more likely to occur at lower pH (Figure F.11), which is partly explained by
the effect of pH on ECORR (which will shift to more-positive values with decreasing pH)
and partly by the effect of pH on the critical potential for localised corrosion
(Figure F.4). As noted earlier, there is evidence for a threshold pH above which EP
shifts to significantly higher values. This threshold pH is a function of the alloy, with
values of pH 10, 11, and 11.5 for Types 316, 430, and 304, respectively. The resistance
of stainless steels to localised corrosion in alkaline solutions typical of the pore water in
cement grout is confirmed by tests performed in the Nirex/NDA program. Smart
(2002) reports no pitting of 304, 304L, 316, or 316L austenitic stainless steels in cements
containing up to 10 wt.% Cl-.
The solution pH also clearly affects the probability of SCC (Figure F.11). The
environmental conditions necessary for cracking shift to higher Cl- concentrations
and/or higher temperature as the pH increases. The susceptibility of 304L and 316L
austenitic stainless steels to SCC has been determined in simulated cementitious
environments containing chloride and/or thiosulphate ions for the Nirex/NDA
program (Smart 2002). The severity of cracking increased with decreasing pH and
with increasing thiosulphate and/or chloride concentrations.
The beneficial effect of increasing pH can be expressed as a [OH-]:[Cl-] ratio, above
which the probability of localised corrosion or SCC is significantly diminished. Based
on the apparent threshold pH values for EP in Figure F.4, this "threshold" [OH-]:[Cl-]
would corresponds to 2 x 10-4 for Type 316, 2 x 10-3 for Type 430, and 6 x 10-3 for
Type 304. Provided the [OH-]:[Cl-] is above this value then localised corrosion is
unlikely. These values are consistent with the absence of pitting reported by Smart
(2002) for 304, 304L, 316, or 316L austenitic stainless steels in cements containing up to
10 wt.% Cl-, for which the [OH-]:[Cl-] would have been in the range 4 x 10-3 to 4 x 10-2
for an assumed pH range of pH 12-13.
F.4.5 Effect of sulphur species
Sulphur species have both beneficial and detrimental effects on the corrosion
behaviour of stainless steels, depending upon the oxidation state of sulphur. In the
fully oxidised +6 state, sulphate inhibits the crevice corrosion of Type 304 and 316 in
Cl- solutions (Figure F.6). A [SO42-]:[Cl-] molar ratio of 1.2 and 0.4 is sufficient to inhibit
the crevice corrosion of Types 304 and 316, respectively.
In the +4 oxidation state, thiosulphate ions increase the susceptibility to both pitting
and crevice corrosion (Figure F.5, Tables F.3, F.4, and F.6) and to SCC (Smart 2002).
Indeed, the concern over the increased susceptibility to cracking and localised
corrosion (Kursten and Druyts 2000, Kursten et al. 2004) lead to the change in the
QRS-1384J-1 v2.1 Appendix F
35
Belgian programme from stainless steel as the candidate canister (overpack) material to
a cementitious C-steel supercontainer design (Kursten and Druyts 2008).
Stainless steels are also susceptible to SCC and accelerated general corrosion in the
presence of sulphide (ASM 1987, 2005).
F.4.6 Effect of other anions and cations
In addition to the inhibitive effect of sulphate on the localised corrosion of stainless
steels in Cl- solutions, certain other anions, such as carbonate and nitrate, also act as
inhibitors (Szklarska-Smialowska 2005). Nitrate, in particular, is an effective inhibitor,
possibly due to the consumption of protons in the reduction of NO3- to either NH3 or
N2, thus raising the pit or crevice pH. Carbonate would be expected to have a similar
effect on the pH of the occluded chemistry.
Conversely, cations that can be hydrolysed to form acidic solutions (e.g., Mg2+, Ca2+)
would be expected to have an adverse impact on the localised corrosion susceptibility.
These latter species are of particular concern if present as precipitated salts on the
canister surface, since they deliquesce at low relative humidities (see Section F.4.8).
F.4.7 Effect of gamma radiation
Shoesmith and King (1999) have reviewed the available information on the effects of
radiolysis on the corrosion of stainless steels. Among the observations reported are:
• The corrosion potential in 0.018 mol⋅dm-3 NaCl increased substantially when a
radiation field (104 Gy/hr) was introduced, but no increase in general corrosion
rate or initiation of pitting was observed.
• Radiation appears to inhibit pitting, probably due to defect annealing in the
passive oxide film.
• Results on crevice corrosion ( in 10 mg/L Cl-) are ambiguous. A dose rate of 2.8
Gy/hr may be sufficient to initiate crevice corrosion, but the availability of a
large cathode may be more important.
• Crevice propagation can be maintained at 103 Gy/hr, but not at 10 Gy/hr.
Marsh et al. (1986) studied the effect of gamma radiation on the potential and localised
corrosion behaviour of Type 304L stainless steel in aerated solutions containing
300 mg/L Cl-. At a dose rate of 2 x 103 Gy/hr, the value of ECORR increased by 200-
300 mV. Although this would suggest an increased probability of localised corrosion,
the oxidising radiolysis products responsible for the positive shift in ECORR also
36
inhibited film breakdown, although they had no effect on the re-passivation of existing
pits.
Overall, and in common with other materials (Shoesmith and King 1999), no
measurable effect of irradiation is observed on stainless steels at absorbed dose rates of
less than 1 Gy/hr.
F.4.8 Effect of unsaturated conditions and atmospheric exposure
Smart (2000) has reviewed the atmospheric behaviour of stainless steels waste
containers. Corrosion can occur when salt contaminants on the surface of the canister
absorb moisture (deliquesce) from the air forming small volumes of highly
concentrated solutions. These conditions present an ideal opportunity for the
establishment of spatially separated anodic and cathodic processes and the initiation
and propagation of localised corrosion and SCC.
Recently, Tani et al. (2008) have compared the behaviour of austenitic and duplex
stainless steels exposed to humid atmospheres following contamination by salts
contained in seawater. Surface corrosion took the form of pitting at isolated salt
crystals formed by the evaporation of seawater and which subsequently deliquesced
when exposed to a humid atmosphere. These pits could serve as locations for the
initiation of SCC, an area of active research by NDA RWMD (A.J. Cook and S.B. Lyon,
unpublished work).
F.5 Lifetime predictions
No formal lifetime predictions have been made for stainless steel HLW/SF canisters.
The reluctance to adopt stainless steels for this purpose results from their sensitivity to
the effect of Cl- ions and the heat-generation from the waste. Thus, even if the Cl-
concentration in the groundwater or backfill pore water is low, it is difficult to
guarantee that evaporative concentration will not lead to higher concentrations. Given
the ubiquitous nature of Cl- in deep groundwaters and the inevitable elevated
temperature at the canister surface, it is difficult to justify the use of stainless steels in
the absence of a cementitious backfill.
In the presence of a cementitious backfill, however, stainless steels are likely to provide
containment for the duration of the alkaline phase. Thus, elevated pH (defined here as
pH > 10-11) inhibits the effects of Cl- ions on the rate of general corrosion, the initiation
of localised corrosion, and SCC. The beneficial effect of the alkaline pH will, of course,
only last as long as the pore-water pH is influenced by leaching of alkalis and
QRS-1384J-1 v2.1 Appendix F
37
portlandite/CSH gels present in cement. Prediction of the lifetime of the canister then
becomes a matter of predicting the time evolution of the pH of the cement backfill.
F.6 Critical conditions
Table F.9 discusses each of the environmental factors considered in this report and lists
a number of critical conditions for which the use of stainless steel canisters would
either not be recommended or which would require detailed investigation in order to
develop a sufficiently justifiable prediction of the long-term corrosion behaviour.
There are a number of critical conditions for stainless steel canisters in a bentonite-
backfilled or non-backfilled GDF, including:
• The presence of sulphide minerals in the host rock or backfill materials -
thiosulphate formation during the aerobic phase could induce localised
corrosion or SCC.
• Elevated temperature - localised corrosion and SCC occur at temperatures at or
slightly above ambient, depending upon the aggressiveness of the environment
and the alloy composition. In the absence of a cementitious backfill, the
threshold temperature for localised corrosion or SCC is likely to be <100oC
unless highly corrosion-resistant alloys are selected.
• Chloride ions - stainless steels are subject to localised corrosion and SCC in Cl—
containing environments, especially at elevated temperature.
• Sulphur species - thiosulphate produced by the partial oxidation of pyrite
impurities in clay-based sealing materials or the host rock could induce
localised corrosion and SCC of the canister. Sulphide present naturally in the
groundwater or produced by sulphate-reducing bacteria could lead to
accelerated general corrosion or SCC.
• External load and residual stress - austenitic and duplex stainless steels are
susceptible to SCC in the presence of Cl- and a sufficient applied or residual
tensile stress. Ferritic alloys are less susceptible, but are not immune to Cl—
induced SCC.
Many or all of these critical conditions could be obviated by the use of a cementitious
backfill. Stainless steels should provide adequate performance for the period that the
backfill pore-water pH is maintained at a value >pH 12.
38
Table F.1: List of Critical Conditions for HLW/SF Canisters M
anufactured from Stainless Steel
Parameter
Critical condition
Comment
Ho
st r
ock
P
rese
nce
of
sulp
hid
e
min
eral
s
Th
e p
rese
nce
of
sulp
hid
e m
iner
als
in
th
e h
ost
ro
ck o
r b
ack
fill
mat
eria
ls w
ou
ld b
e o
f co
nce
rn
bec
ause
of
the
po
ssib
ilit
y o
f th
iosu
lph
ate
fo
rmat
ion
du
rin
g t
he
aero
bic
ph
ase
. T
hio
sulp
hat
e
cou
ld t
hen
in
du
ce l
oca
lise
d c
orr
osi
on
or
SC
C o
f st
ain
less
ste
el c
an
iste
rs.
Red
ox
co
nd
itio
ns
No
ne
Th
e p
ass
ive
film
on
sta
inle
ss s
teel
s p
rov
ides
go
od
pro
tect
ion
ag
ain
st g
ener
al c
orr
osi
on
un
der
bo
th a
ero
bic
an
d a
nae
rob
ic c
on
dit
ion
s.
Ho
wev
er, t
he
pro
pen
sity
fo
r fi
lm b
reak
do
wn
an
d
loca
lise
d c
orr
osi
on
or
SC
C i
s g
rea
tly
en
han
ced
un
der
aer
ob
ic c
on
dit
ion
s.
Tem
per
atu
re
<10
0oC
(in
ab
sen
ce o
f
cem
enti
tio
us
ba
ckfi
ll)
Th
e p
rob
abil
ity
of
loca
lise
d c
orr
osi
on
or
SC
C i
ncr
ease
s si
gn
ific
an
tly
wit
h i
ncr
easi
ng
tem
per
atu
re.
Lo
cali
sed
co
rro
sio
n a
nd
SC
C c
an o
ccu
r at
tem
per
atu
res
at
or
slig
htl
y a
bo
ve
amb
ien
t, d
epen
din
g u
po
n t
he
agg
ress
iven
ess
of
the
env
iro
nm
ent
and
th
e a
llo
y c
om
po
siti
on
. In
the
abse
nce
of
a ce
men
titi
ou
s b
ack
fill
, th
e th
resh
old
tem
per
atu
re f
or
loca
lise
d c
orr
osi
on
or
SC
C
is l
ikel
y t
o b
e <
100
oC
un
less
hig
hly
co
rro
sio
n-r
esis
tan
t a
llo
ys
are
sele
cted
.
Gam
ma
rad
iati
on
>
1-10
Gy
/h
T
her
e is
no
ev
iden
ce f
or
adv
erse
eff
ects
of
irra
dia
tio
n a
t d
oes
rat
es <
1 G
y/
h.
Ba
ckfi
ll m
ater
ial
and
nea
r-fi
eld
mas
s
tran
spo
rt
Req
uir
es
cem
enti
tio
us
bac
kfi
ll
Giv
en t
he
ub
iqu
ito
us
nat
ure
of
Cl-
ion
s in
gro
un
dw
ate
rs a
nd
th
e h
eat
gen
erat
ion
by
th
e
HL
W/
SF
, th
e u
se o
f st
ain
less
ste
el c
an
iste
rs w
ou
ld n
ot
be
reco
mm
end
ed w
ith
ou
t th
e u
se o
f a
cem
enti
tio
us
bac
kfi
ll.
QR
S-1
384J
-1 v
2.1
Ap
pen
dix
F
39
Ch
lori
de
con
cen
tra
tio
n
Dep
end
ent
on
nat
ure
of
bac
kfi
ll a
nd
all
oy
Sta
inle
ss
stee
ls a
re s
ub
ject
to
lo
cali
sed
co
rro
sio
n a
nd
SC
C
in C
l—co
nta
inin
g
env
iro
nm
ents
, es
pec
iall
y a
t el
evat
ed t
emp
erat
ure
. G
iven
th
e u
biq
uit
ou
s n
atu
re o
f C
l- i
on
s in
gro
un
dw
ater
s an
d t
he
hea
t g
ener
ati
on
by
th
e H
LW
/S
F,
the
use
of
stai
nle
ss s
teel
can
iste
rs w
ou
ld n
ot
be
reco
mm
end
ed w
ith
ou
t th
e u
se o
f a
cem
enti
tio
us
ba
ckfi
ll.
Oth
er g
rou
nd
wa
ter
spec
ies
No
ne
Th
e p
rese
nce
of
Ca2
+ a
nd
Mg
2+ i
n t
he
gro
un
dw
ater
co
uld
lea
d t
o t
he
gen
era
tio
n o
f a
cid
ic
env
iro
nm
ents
, bu
t th
e u
se o
f st
ain
less
ste
el w
ou
ld n
ot
be
reco
mm
end
ed w
ith
ou
t a
cem
enti
tio
us
bac
kfi
ll w
hic
h s
ho
uld
ma
inta
in a
n a
lka
lin
e n
ear-
fiel
d p
H.
Su
lph
ur
spec
ies
Th
iosu
lph
ate,
sulp
hid
e
Th
iosu
lph
ate
pro
du
ced
by
th
e p
arti
al o
xid
atio
n o
f p
yri
te i
mp
uri
ties
in
cla
y-b
ased
sea
lin
g
ma
teri
als
or
the
ho
st r
ock
co
uld
in
du
ce l
oca
lise
d c
orr
osi
on
an
d S
CC
of
the
can
iste
r. S
ulp
hid
e
pre
sen
t n
atu
rall
y i
n t
he
gro
un
dw
ate
r o
r p
rod
uce
d b
y s
ulp
hat
e-re
du
cin
g b
act
eria
co
uld
lea
d t
o
acce
lera
ted
gen
eral
co
rro
sio
n o
r S
CC
.
Mic
rob
ial
acti
vit
y
Sig
nif
ican
t if
bio
film
s
form
ed
As
wit
h o
ther
ca
nis
ter
ma
teri
als
, mic
rob
ial
act
ivit
y r
emo
te f
rom
th
e ca
nis
ter
pre
sen
ts a
min
imal
thre
at
to t
he
can
iste
r li
feti
me.
Ho
wev
er, s
urf
ace
mic
rob
ial
acti
vit
y a
nd
bio
film
fo
rmat
ion
co
uld
lead
to
en
no
ble
men
t o
f E
CO
RR a
nd
th
e p
oss
ibil
ity
of
loca
lise
d c
orr
osi
on
an
d t
he
po
ssib
ilit
y o
f
pre
fere
nti
al w
eld
att
ack
.
Res
idu
al s
tres
s a
nd
exte
rnal
lo
ad
Dep
end
ent
on
nat
ure
of
bac
kfi
ll a
nd
all
oy
Au
sten
itic
an
d d
up
lex
sta
inle
ss s
teel
s ar
e su
scep
tib
le t
o S
CC
in
th
e p
rese
nce
of
Cl-
an
d a
suff
icie
nt
ap
pli
ed o
r re
sid
ua
l te
nsi
le s
tres
s. F
erri
tic
allo
ys
are
less
su
scep
tib
le, b
ut
are
no
t
imm
un
e to
Cl—
ind
uce
d S
CC
.
GD
F s
atu
rati
on
tim
e N
on
e
Sta
inle
ss s
teel
s ex
hib
it g
oo
d c
orr
osi
on
res
ista
nce
un
der
atm
osp
her
ic c
on
dit
ion
s. T
he
no
n-
un
ifo
rm w
etti
ng
of
surf
ace
salt
co
nta
min
ants
co
uld
lea
d t
o s
pat
ial
sep
arat
ion
of
ano
dic
an
d
cath
od
ic s
ites
an
d t
he
po
ssib
ilit
y o
f lo
cali
sed
co
rro
sio
n a
nd
/o
r S
CC
.
40
F.6 Advantages and disadvantages of stainless steel
as a canister material
The advantages of stainless steels as a canister material include:
• UK experience in design of a stainless steel canister for ILW.
• Extensive experience in fabrication, welding, and inspection of stainless steel
vessels.
• Excellent corrosion resistance in cementitious environments.
• Minimal impact on other barriers in terms of gas production or alteration of
bentonite.
The disadvantages of stainless steel as a canister material include:
• Stainless steel canisters are only suitable for use with a cementitious backfill,
thus limiting the flexibility of the GDF design.
• Susceptibility to localised corrosion and SCC in the presence of Cl- ions,
especially at elevated temperature.
• The need for an internal structural element, thus complicating the canister
design.
• The need to make long-term predictions of the passive corrosion behaviour
and/or localised corrosion.
• No international experience in design and licensing of a stainless steel canister
for HLW/SF.
QRS-1384J-1 v2.1 Appendix F
41
References for Appendix F
Alexander, A.L., C.R. Southwell, and B.W. Forgeson. 1961. Corrosion of metals in
tropical environments, part 5 – stainless steel. Corrosion 17, 345.
ASM. 1987. Metals Handbook, Ninth edition, Volume 13, Corrosion. American
Society for Metals International, Metals Park, OH.
ASM. 2005. ASM Handbook, Volume 13B, Corrosion: Materials. American Society for
Metals International, Metals Park, OH.
Blackwood, D.J., L.J. Gould, C.C. Naish, F.M. Porter, A.P. Rance, S.M. Sharland, N.R.
Smart, M.I. Thomas, and T. Yates. 2002. The localised corrosion of carbon steel
and stainless steel in simulated repository environments. AEAT/ERRA 0318,
Dec 2002.
BSC (Bechtel SAIC Company). 2004. Aqueous corrosion rates for waste package
materials. Prepared for US DOE, ANL-DSD-MD-000001, Oct 2004.
Casteels, F., G. Dresselaars, and H. Tas. 1986. Corrosion behaviour of container
materials for geological disposal of high level waste. Commission of the
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