review of the corrosion performance of selected metals as canister materials for uk spent fuel and...

8/10/2019 Review of the Corrosion Performance of Selected Metals as Canister Materials for UK Spent Fuel and or HLW Appe

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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.% chromiumto provide corrosion resistance and to impart their "stainless" quality (Sedriks 1996).At this minimum chromium content a stable Cr oxide/hydroxide a passive film formson the surface of the steel. The different classes of stainless steel take their name fromtheir predominant crystal structure (ASM 1987, 2005); namely: austenitic (face-centredcubic (fcc)), ferritic (body-centred cubic (bcc)), martensitic (body-centred tetragonal orbcc), or duplex alloys containing and approximately equal proportion of austenite andferrite. Stainless steels typically exhibit good resistance to general corrosion, althoughthe 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 - andvarious S-containing species, but other anions such as carbonate, sulphate, and nitrateare inhibitors. Some classes of stainless steel, particularly the austenitics, can besusceptible 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 SFhas been investigated in a number of national programmes (Table F.1). Generally, thefocus has been on the austenitic alloys, particular the American Iron and Steel Institute(AISI) 300-series, although NDA RWMD have recently reviewed the properties ofduplex 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 theBelgian programme in the 1990's. Various stainless alloys were considered for boththe HLW container and as an overpack material, including ferritic AISI 430 and twoaustenitic alloys (AISI 309 and 316Ti) as container materials and the austenitic andsuperaustenitic alloys AISI 316, 904L, and 926 as candidate overpack materials (Druytsand Kursten 1999, Kursten and Druyts 2000, Kursten and Van Iseghem 1999).However, the occurrence of thiosulphate from the oxidation of pyrite in the Boom clayand the possibility of increased Cl - levels as a result of the evaporation of dilute clayinterstitial 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 anentirely 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 programmein argillaceous clay (Fron et al. 2009), but virtually all experimental studies have beenfocussed on the corrosion behaviour of the unalloyed steel overpack. In Spain, the AISI316L alloy has been investigated, along with a large number of other materials, as apotential canister material for the disposal of HLW/SF in a granitic geological


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3/44

Q R S - 1 3 8 4 J - 1 v 2 . 1

A p p e n d i x F 3

T a b l e F . 1 : C o m p o s i t i o n s o f S t a i n l e s s S t e e l s C o n s i d e r e d f o r U s e f o r t h e D i s p o s a l o f I L W o r H L W / S F 1

U N S N u m b e r

C o m m o n n a m e

C o m p o s i t i o n ( w t . % ) 2

O t h e r

C r

N i

C

M n

S i

P

S

A u s t e n i t i c a l l o y s

S 3 0 4 0 0

3 0 4

1 8 - 2 0

8 - 1 0

0 . 0 8

2 . 0

1 . 0

0 . 0 4 5

0 . 0 3 0

S 3 0 4 0 3

3 0 4 L

1 8 - 2 0

8 - 1 2

0 . 0 3

2 . 0

1 . 0

0 . 0 4 5

0 . 0 3 0

S 3 0 9 0 0

3 0 9

2 2 - 2

1 2 - 1 5

0 . 2 0

2 . 0

1 . 0

0 . 0 4 5

0 . 0 3 0

S 3 0 9 0 8

3 0 9 S

2 2 - 2 4

1 2 - 1 5

0 . 0 8

2 . 0

1 . 0

0 . 0 4 5

0 . 0 3 0

S 3 1 6 0 0

3 1 6

1 6 - 1 8

1 0 - 1 4

0 . 0 8

2 . 0

1 . 0

0 . 0 4 5

0 . 0 3 0

M

o 2 - 3

S 3 1 6 0 3

3 1 6 L

1 6 - 1 8

1 0 - 1 4

0 . 0 3

2 . 0

1 . 0

0 . 0 4 5

0 . 0 3 0

M

o 2 - 3

S 3 1 6 3 5

3 1 6 T i 3

1 6 . 8

0

1 0 . 7

0

0 . 0 4

1 . 0 8

0 . 4 0

0 . 0 0 9

0 . 0 2 8

M o 2 . 0

5 , T i 0 . 3

N 0 8 9 0 4

9 0 4 L

1 9 - 2 3

2 3 - 2 8

0 . 0 2

2 . 0

1 . 0

0 . 0 4 5

0 . 0 3 5

M o 4 - 5 , C u 1 - 2

N 0 8 9 2 6

9 2 6 3

2 0 . 6

0

2 4 . 8

5

0 . 0 0 5

0 . 9 2

0 . 3 0

0 . 0 1 8

0 . 0 0 2

M o 6 . 4 0 , C u 0 . 8 6 , N

0 . 1 9 8

F e r r i t i c a l l o y s

S 4 3 0 0 0

4 3 0

1 6 - 1 8

-

0 . 1 2

1 . 0

1 . 0

0 . 0 4 0

0 . 0 3 0

D u p l e x a l l o y s

S 3 2 3 0 4

S A F 2 3 0 4

2 1 . 5 - 2 4 . 5

3 - 5 . 5

0 . 0 3

2 . 5

1 . 0

0 . 0 4 0

0 . 0 4 0

N 0 . 0 5 - 0 . 2 , M o 0 . 0 5 - 0 . 6

S 3 1 8 0 3

2 2 0 5

2 1 - 2 3

4 . 5 - 6 . 5

0 . 0 3

2 . 0

1 . 0

0 . 0 3 0

0 . 0 2 0

N 0 . 0 8 - 0 . 2 , M o 2 . 5 - 3 . 5

1 A f t e r S e d r i k s ( 1 9 9 6 ) , e x c e p t w h e r e n o t e d

2 M a x i m u m u n l e s s o t h e r w i s e i n d i c a t e d , b a l a n c e F e .

3 C o m p o s i t i o n o f a l l o y u s e d b y D r u y t s a n d K u r s t e n ( 1 9 9 9 )


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4

Figure F.1: Compositional Relationship Between the Different Classes of StainlessSteels (Sedriks 1996).

Although not formally considered as either a container material for HLW glass or as acanister (or overpack) material for either HLW or SF, duplex stainless steels offer anumber of advantages over austenitic alloys (King 2009a). Duplex alloys containapproximately equal proportions of austenite and -ferrite. As a consequence, the Nicontent (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 oftwo common duplex alloys, 2304 and 2205.


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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 propertiesof which are discussed in detail in Appendix E: Corrosion of Nickel Alloys. BothCr(OH) 3 (Appendix E, Figure E.1) and Cr 2O3 are thermodynamically stable over a widerange of pH and for both reducing and, up to a few 100 mV below the O 2/H 2Oequilibrium, oxidising redox potentials. The stability of this film, however, decreaseswith 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 types304/304L 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 inenvironmental conditions but also of the experimental technique

the rate decreases with increasing exposure time (due to the formation of anever-thickening passive film)

the rate increases with increasing temperature

the rate decreases with increasing pH, with corrosion rates generally


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6 T a b l e F . 2 : G e n e r a l C o r r o s i o n R a t e s o f T y p e s 3 0 4 / 3 0 4 L a n d 3 1 6 / 3 1 6 L A u s t e n i t i c S t a i n l e s s S t e e l i n A l k a l i n e a n d N e a r - n e u t r a l S o l u t i o n s a n d

U n d e r A t m o s p h e r i c C o n d i t i o n s .

( a ) T y p e 3 0 4 / 3 0 4 L

A l l o y

p H

T e m p e r a t u r e

( o C )

[ C l - ]

( g g - 1 )

R e d o x

c o n d i t i o n s

O t h e r

R a t e ( m y r - 1 )

R e f e r e n c e

3 0 4

1 2 . 8

1 0 . 5

3 0 , 4 5

2 0 0 d a y s

6 0 d a y s

0 . 0 0 0 3

0 . 0 1

F u j i s a w a e t a l . 1

9 9 9

3 0 4

1 3 . 3

A m

b i e n t

1 8 , 4

0 0

A e r a t e d

2 8 d a y s

0 . 3

M c d o n a l d e t a l . 1

9 9 5

3 0 4

1 3

3 0 5 0 8 0

D e a e r a t e d

0 . 0 6

0 . 1 8

0 . 8 2

B l a c k w o o d e t a l . 2

0 0 2

3 0 4

1 0 1 2 . 5

1 3 . 5

5 0

D e a e r a t e d

2 3 0 d a y s

0 . 0 0 9

0 . 0 0 5 5

0 . 0 0 6 3

W a d a a n d N i s h i m u r a

1 9 9 9

3 0

A m b i e n t

9 0

7 , 0 0 0 - 4 3 , 0

0 0

A e r a t e d

1 0 h r s

1 0 - 1 3 0

M o r s y e t a l . 1

9 7 9

3 0 4 L

A m b i e n t

2 5 - 1 0 0

F r e s h w a t e r

A e r a t e d

0 . 2 1

B S C 2 0 0 4

3 0 4 L

A m b i e n t

2 7 9 0

S a l t w a t e r

A e r a t e d

1 1 . 4

5 . 8 2

B S C 2 0 0 4

3 0 4

A m b i e n t

2 5 5 0 7 5

I n t e r s t i t i a l c l a y

w a t e r

A e r a t e d

0 . 2 - 0 . 9 6

0 . 2 2 - 0 . 2

3

0 . 3 - 0 . 3 5

C a s t e e l s e t a l . 1

9 8 6

3 0 4

-

A m

b i e n t

-

A e r a t e d

U r b a n , 5 - 1 5 y r

U r b a n , 5 - 1 5 y r

M a r i n e , 5 - 1 5 y r

I n d u s t r i a l / u r b a n , 5 - 1 5 y

< 0 . 0

3

0 . 0 2 2

0 . 0 5 - 2

0 . 0 1

J o h n s o n a n d P a v l i k

1 9 8 2

3 0 4

-

A m

b i e n t

-

A e r a t e d

I n d u s t r i a l / u r b a n

0 . 0 3 - 3

K e a r n s e t a l . 1

9 8 4


7/44

Q R S - 1 3 8 4 J - 1 v 2 . 1

A p p e n d i x F 7

( b ) T y p e 3 1 6 / 3 1 6 L

A l l o y

p H


( o C )

[ C l - ]

( g g - 1 )

R e d o x

c o n d i t i o n s

O t h e r

R a t e

( m y r - 1 )

R e f e r e n c e

3 1 6

1 3 . 3

A m b i e n t

1 8 , 4

0 0

A e r a t e d

2 8 d a y s

0 . 6

M c d o n a l d e t a l .

1 9 9 5

3 1 6 L

> 1 3

A m b i e n t

1 0 , 0

0 0

D e a e r a t e d

0 . 1 M P a H

2

0 . 0 3

S m a r t e t a l . 2 0 0 4

3 1 6

A m b i e n t

A m b i e n t

1 9 , 0

0 0

A e r a t e d

8 y r s , P a c i f i c O c e a n

s e a w a t e r

4

A l e x a n d e r e t a l .

1 9 6 1

3 1 6 L

A m b i e n t

3 0 5 0 - 1 0 0

F r e s h w a t e r

A e r a t e d

0 . 0 1

0 . 2 5

B S C 2 0 0 4

3 1 6 L

A m b i e n t

2 7

S a l t w a t e r

A e r a t e d

1 . 9 4

B S C 2 0 0 4

3 1 6

A m b i e n t

2 5 5 0 7 5

I n t e r s t i t i a l c l a y

w a t e r

A e r a t e d

0 . 1 - 0 . 2 4

0 . 1 - 0 . 3 4

0 . 1 - 0 . 1 7

C a s t e e l s e t a l .

1 9 8 6

3 1 6

-

A m b i e n t

-

A e r a t e d

V a r i o u s

a t m o s p h e r e s

0 . 0 5

D e c h e m a 1 9 9 0


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8

F.3.2 Localised corrosion

Localised corrosion occurs due to breakdown of the passive film leading to eitherpitting of exposed surfaces or crevice corrosion of occluded regions(Szklarska-Smialowska 2005). Chloride ions promote film breakdown, whereas otherspecies, such as OH -, CO32-, SO42-, and NO 3- inhibit localised corrosion. Other speciesresult in pitting or crevice corrosion, most notably in the current context thiosulphate(S2O32-) (Kursten and Druyts 2000), which can form from the oxidation of pyriteminerals in the host rock or bentonite-based sealing materials.

In a given environment, localised corrosion occurs at a characteristic electrochemicalpotential (E). The characteristic potential can either be based on the value at which

film breakdown occurs (E P or ECREV for pitting and crevice corrosion, respectively) orthat at which a propagating pit or crevice re-passivates (E RP or ERCREV, respectively).The criterion for pitting or crevice corrosion is that the corrosion potential (E CORR)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 theconcentration of other aggressive or inhibitive species), temperature, pH, and variousmetallurgical parameters, such as surface finish, degree of sensitisation, etc. Figure F.2shows the characteristic decrease in E P with the logarithm of the Cl - concentration, aswell as the decrease with increasing temperature. Figure F.3 more clearly shows thetemperature dependence of E P for austenitic alloys Type 304 and 316 and for the ferriticType 430 alloy. The Mo-containing Type 316 alloy exhibits the most-positive pittingpotential and, for the same value of E CORR, 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 E P with pH for the same three alloys. Thedependences shown in the figure suggest the existence of a threshold pH above whichthe pitting potential rapidly increases, with that threshold increasing in the orderType 316 < Type 430 < Type 304.


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9

Figure F.2: Dependence of the Pitting Potential for Type 304 Stainless Steel onChloride Concentration for Various Temperatures (Szklarska-Smialowska 2005).

As noted above, thiosulphate ions also induce pitting of stainless steels. The data inFigure F.5 suggest a decrease in E P of 100-200 mV due to the presence of thiosulphate ata [Cl -]:[S2O32-] ratio of 17. Kursten and Druyts (2000) also reported increasedsusceptibility of both Type 316L and 904 austenitic alloys in the presence ofthiosulphate, although the latter alloy which contains approximately double theamount of Mo as Type 316L was significantly more resistant, as indicated by a 200-400 mV difference in E P. Interestingly, Kursten and Druyts (2000) noted that S 2O32- affected pit initiation but not pit growth.

Other anions inhibit localised corrosion. Figure F.6 shows the effect of sulphate on thecrevice 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 corrosionof Type 304 and a [SO 42-]:[Cl-] ratio of as little as 0.4 is sufficient for the more-resistantType 316 alloy.


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10

Figure F.3: Dependence of the Pitting Potential on Temperature for Types 304 and316 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 316Austenitic and Type 430 Ferritic Stainless Steels in 3% NaCl Solution (from Sedriks

1996).


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11

Figure F.5: Effect of Thiosulphate on the Pitting Potential of Type 316L StainlessSteel as a Function of Chloride Concentration at 80 oC (Sedriks 1996).

Figure F.6: Inhibitive Effect of Sulphate on the Crevice Corrosion of Types 304 and316 Stainless Steel in Chloride Solutions (from Sedriks 1996).


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12

Figure F.7: Map of the Dependence of Crevice Corrosion and Pitting on Potentialand Chloride Concentration (from Szklarska-Smialowska 2005).

Crevice corrosion occurs under less-aggressive conditions than pitting because therestricted mass transport of species into and out of the occluded region promotes thedevelopment 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 E P/E RP or E CREV/E RCREV thatdetermines 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 showsthe dependence of the pitting and pit re-passivation potentials for Type 316L stainlesssteel on Cl - concentration at 95 oC and the corresponding values of E CORR in aerated anddeaerated solutions. Based on the criteria in Equations (1) and (2), therefore, pittingwould 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 corrosionwould not occur at all in deaerated solution.

Tables F.3 to F.8 provide data with which to assess the susceptibility to localisedcorrosion 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 significantdatabase for assessing localised corrosion susceptibility. Tables F.3 and F.4 summarizepitting and pit re-passivation potentials for Types 304/304L and 316/316L,respectively. Corresponding data for the crevice and crevice re-passivation potentialsare given in Tables F.5 and F.6 for Types 304/304L and 316L, respectively. Finally,


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13

Figure F.8: Comparison of the Pitting (E P) and Re-passivation (E RP) Potentials forType 316L Stainless Steel as a Function of Chloride Concentration at 95 oC and the

Corresponding Values of the Corrosion Potential (E CORR ) in Aerated and DeaeratedSolutions (Dunn et al. 1996).

Tables F.7 and F.8 list various measurements of the corrosion potential under variousenvironmental conditions for Types 304/304L and 316L, respectively. There are fewcorresponding values for the other alloys in Table F.1.

There are two other commonly used methods for assessing the susceptibility tolocalised corrosion of stainless steel. First, the effect of alloy composition on pitting (orcrevice corrosion) susceptibility can be compared based on the pitting resistanceequivalent 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.


14/44

1 4

T a b l e F . 3 : L i t e r a t u r e D a t a o n P i t t i n g o f T y p e 3 0 4 / 3 0 4 L S t a i n l e s s S t e e l s .

T y p e


( o C )

[ C l - ]

( g g - 1 )

O t h e r

E P o r E R

P

( m V S C E )

R e f e r e n c e

3 0 4

8 0

1 9 , 0

0 0

S y n t h e t i c s e a w a t e r

+ 5 0 ( E

P )

H a r u k i e t a l . 1 9 9 1

3 0 4

2 0

3 4 , 5

0 0

W i t h a d d i t i o n s o f N a 2 S 2 O

3

0 m o l d m - 3

4 x 1 0 - 4 m o l d m - 3

1 x 1 0 - 3

m o l d m - 3

2 x 1 0 - 3

m o l d m - 3

4 x 1 0 - 3

m o l d m - 3

0 . 0 1

m o l d m - 3

0 . 1 m o l d m - 3

0 . 4 m o l d m - 3

1 . 0 m o l d m - 3

( E P )

+ 3 0 *

- 2 0

+ 4 0

- 1 3 0

- 2 0 5

- 1 9 5

- 1 3 5 - 9

5 - 6 5

S z k l a r s k a - S m i a l o w s k a 2 0 0 5 ( F i g 7 . 1 5 )

3 0 4

8 0

3 4 , 5

0 0


3

0 m o l d m - 3

1 x 1 0 - 4

m o l d m - 3

4 x 1 0 - 4 m o l d m - 3

1 x 1 0 - 3

m o l d m - 3

0 . 0 1

m o l d m - 3

0 . 1 m o l d m - 3

( E P )

- 6 5 * - 6

5 - 2 5 5

- 2 6 5

- 2 6 0

- 1 9 0

S z k l a r s k a - S m i a l o w s k a 2 0 0 5 ( F i g 7 . 1 5 )

3 0 4

2 5 4 0 6 0 9 0

1 7 , 0

0 0

W i t h a d d i t i o n o f 0 . 1 m o l d m - 3

N a H C O

3 , p H 8

+ 4 1 5 ( E

P )

+ 3 2 0

+ 1 5 5

+ 6 5

S z k l a r s k a - S m i a l o w s k a 2 0 0 5 ( F i g 1 2 . 2

)

3 0 4

1 0 0

1 5 0

3 4 5

S e n s i t i z e d 3 0 4

- 5 5 ( E

P )

- 2 4 0


)


15/44

Q R S - 1 3 8 4 J - 1 v 2 . 1

A p p e n d i x F 1 5

T a b l e F . 3 : L i t e r a t u r e D a t a o n P i t t i n g o f T y p e 3 0 4 / 3 0 4 L S t a i n l e s s S t e e l s ( C o n t i n u e d ) .

T y p e


( o C )

[ C l - ]

( g g -

1 )

O t h e r

E P o r E R

P

( m V S C E )

R e f e r e n c e

3 0 4

2 0 4 0 6 0 8 0

R a n g e 1 0 0 -

2 0 , 0

0 0

E P = 7 5 0 1 5 2 l o g [ C l - ]

E P = 6 2 8 1 4 0 l o g [ C l - ]

E P = 5 5 4 1 4 4 l o g [ C l - ]

E P = 5 0 0 1 4 5 l o g [ C l - ]

S z k l a r s k a - S m i a l o w s k a 2 0 0 5

( F i g 1 2 . 6

)

3 0 4

A m b i e n t

3 4 , 5

0 0

+ 2 3 2 ( E

P )


( F i g 1 8 . 4

)

3 0 4

3 , 4 5 0

W i t h a d d i t i o n

o f 0 . 1 m o l d m - 3

N a H C O

3

- 7 0 ( E

P )

S e d r i k s 1 9 9 6 ( T a b l e 4 . 3 )

3 0 4

3 0

6 6 0

+ 5 0 g g - 1

S O

4 2 - , 2 g g -

1 C u 2

+

B a s e m e t a l

W e l d H A Z

6 0 % c o l d w o r k

( E P )

+ 3 9 0

+ 1 9 0

+ 2 1 0

S e d r i k s 1 9 9 6 ( T a b l e 4 . 1 0 )

3 0 4 L

3 0

1 4 2 , 0 0 0

p H 9 . 3

- 5 1 ( E

P )

- 2 1 6 ( E

R P )

S r i d h a r e t a l . 1 9 9 3

3 0 4

1 8 , 4

0 0

p H 3

p H 4

p H 5

p H 6

p H 7

p H 8

p H 9

p H 1 0

p H 1 1

p H 1 2

- 5 ( E

P )

+ 1 0

+ 1 5

+ 2 5

+ 4 0

+ 5 0

+ 6 0

+ 7 0

+ 8 0

+ 4 0 0

S e d r i k s 1 9 9 6 ( F i g . 4 . 3

8 )


16/44

1 6

T a b l e F . 3 : L i t e r a t u r e D a t a o n P i t t i n g o f T y p e 3 0 4 / 3 0 4 L S t a i n l e s s S t e e l s ( C o n c l u d e d ) .

T y p e


( o C )

[ C l - ]

( g g - 1 )

O t h e r

E P o r E R

P

( m V S C E )

R e f e r e n c e

3 0 4

2 5

3 4 , 5

0 0

H 2 a t m o s p h e r e 0 . 0 7 6 g g -

1 O 2

N 2 a t m o s p h e r e 0 . 4 6 0 g g -

1 O 2

A r a t m o s p h e r e 0 . 0 5 7 g g -

1 O 2

O 2 a

t m o s p h e r e 3 0 . 1

g g -

1 O 2

- 5 0 ( E

P )

- 2 0

+ 5 0

+ 6 5

S e d r i k s 1 9 9 6 ( T a b l e 4 . 1 6 )

3 0 4

3 0

1 8 , 4

0 0

+ 6 0 ( E

P )

S e d r i k s 1 9 9 6 ( F i g . 4 . 4

2 )

3 0 4

2 5

1 7 , 0

0 0

p H 5

+ 2 0 0 ( E

P )

S e d r i k s 1 9 9 6 ( T a b l e 4 . 1 5 )

3 0 4

2 0

3 4 5

3 , 4 5 0

3 4 , 5

0 0

M e a n v a l u e s , d

E P / d l o g [ C l - ] = 1 2 7 m V

1 0 0 0 g r i t

+ 5 6 5 ( E

P )

+ 4 6 0

+ 3 1 5

L a y c o c k

e t a l . 2

0 0 5

3 0 4

2 0

3 4 5

3 , 4 5 0

3 4 , 5

0 0


E P / d l o g [ C l - ] = 1 3 7 m V

2 2 0 g r i t

+ 4 9 5 ( E

P )

+ 3 8 5

+ 2 2 0

L a y c o c k

e t a l . 2

0 0 5

3 0 4

0 3 0 6 0 9 0

3 , 4 5 0

+ 3 6 0 ( E

P )

+ 1 9 0

+ 6 0

- 2 0

P a r k e t a l . 2 0 0 2


17/44

Q R S - 1 3 8 4 J - 1 v 2 . 1


T a b l e F . 4 : L i t e r a t u r e D a t a o n P i t t i n g o f T y p e 3 1 6 / 3 1 6 L S t a i n l e s s S t e e l s .

T y p e


( o C )

[ C l - ]

( g g -

1 )

O t h e r

E P o r E R

P

( m V S C E )

R e f e r e n c e

3 1 6 L

8 0

1 9 , 0

0 0


+ 1 5 0 ( E

P )

H a r u k i e t a l . 1 9 9 1

3 1 6 L

2 0

3 4 , 5

0 0


3

0 m o l d m - 3

1 x 1 0 - 4 m o l d m - 3

2

x 1 0 - 3

m o l d m - 3

4

x 1 0 - 3 m

o l d m - 3

0 . 0 1 m o l d m - 3

0 . 0 4 m o l d m - 3

0 . 1 m o l d m - 3

0 . 4 m o l d m - 3

1 . 0 m o l d m - 3

( E P )

+ 1 8 0 *

+ 1 1 0

+ 1 2 5

+ 1 2 5

+ 1 4 0

- 5 0

- 2 5

+ 5 + 5 0


( F i g 7 . 1 5 )

3 1 6 L

8 0

3 4 , 5

0 0


3

0 m o l d m - 3

1 x 1 0 - 4 m o l d m - 3

2

x 1 0 - 3

m o l d m - 3

4

x 1 0 - 3

m o l d m - 3

0 . 0 1 m o l d m - 3

0 . 0 4 m o l d m - 3

0 . 1 m o l d m - 3

( E P )

+ 5 5 *

+ 5 5

+ 8 5 0 - 1

3 5 - 1 1 5

- 1 0 0


( F i g 7 . 1 5 )

3 1 6 L

1 8 3 0 4 0 5 0 6 0

1 9 , 0

0 0

S e a w a t e r

E C O R R = - 1 5 5 m V

S C E

( 8 m m c r e v i c e c o r r o s i o n )

E C O R R = - 3 4 6 m V

S C E

( 2 5 - 3 0 m m C C )

E C O R R = - 3 9 3 m V

S C E

( 2 5 - 3 0 m m C C )

E C O R R = - 3 6 5 m V

S C E

( 1 0 m m C C , 0 . 0

2 5 m m p i t )

E C O R R = - 3 6 2 m V

S C E

( 4 0 m m C C , 0 . 0

2 1 m m p i t )

( E P

/ E R P

)

+ 5 0 5 / + 2

+ 2 7 4 / - 1 6 6

+ 1 5 6 / - 2 7 4

+ 1 5 5 / - 2 1 5

+ 2 8 / - 1 4 2


( T a b l e 1 3 . 2

)


18/44

1 8

T a b l e F . 4 : L i t e r a t u r e D a t a o n P i t t i n g o f T y p e 3 1 6 / 3 1 6 L S t a i n l e s s S t e e l s ( C o n t i n u e d ) .

T y p e


( o C )

[ C l - ]

( g g -

1 )

O t h e r

E P o r E R

P

( m V S C E )

R e f e r e n c e

3 1 6

1 7 , 0

0 0

W i t h a d d i t i o n o f 0 . 1 m o l d m - 3

N a H C O

3

+ 1 4 0 ( E

P )

S e d r i k s 1 9 9 6 ( T a b l e 4 . 3 )

3 1 6

3 1 , 0

0 0

O x y g e n a t e d s o l u t i o n , 0 . 4 - 2 w t . % M n

+ 2 1 0 - 2 4 5 ( E

P )

S e d r i k s 1 9 9 6 ( T a b l e 4 . 2 7 )

3 1 6 L

4 0 5 0 6 0 7 0

2 1 , 5

0 0

+ 2 7 5 ( E

P )

+ 2 2 0

+ 1 3 5

+ 7 0

S e d r i k s 1 9 9 6 ( F i g . 4 . 3

6 )

3 1 6 L

3 0

1 4 2 , 0 0 0

p H 9 . 3

- 4 8 ( E

P )

- 2 3 7 ( E

R P )

S r i d h a r e t a l . 1 9 9 3

3 1 6

1 8 , 4

0 0

p H 3

p H 4

p H 5

p H 6

p H 7

p H 8

p H 9

p H 1 0

p H 1 1

p H 1 2

+ 2 8 5 ( E

P )

+ 2 8 5

+ 2 8 5

+ 2 8 5

+ 2 8 5

+ 2 9 0

+ 3 0 5

+ 3 5 0

+ 4 6 0

+ 5 8 5

S e d r i k s 1 9 9 6 ( F i g . 4 . 3

8 )

3 1 6 L

8 0

3 5 3 4 5

3 4 5 0

N a C l s o l u t i o n s o n l y

+ 1 1 5 ( E

P )

- 5 - 1 0 5

S e d r i k s 1 9 9 6 ( F i g . 4 . 4

1 )

3 1 6 L

8 0

3 5 3 4 5

3 4 5 0

3 4 , 5

0 0

C l - / S 2 O

3 2 - m i x t u r e s , [ C l - ] : [ S 2 O 3 2 - ] = 1 7

+ 4 5 ( E

P )

- 1 8 0

- 2 1 5

- 2 7 5

S e d r i k s 1 9 9 6 ( F i g . 4 . 4

1 )

3 1 6

3 0

1 8 , 4

0 0

E f f e c t o f t e m p e r a t u r e

+ 2 3 0 ( E

P )

S e d r i k s 1 9 9 6 ( F i g . 4 . 4

2 )


19/44

Q R S - 1 3 8 4 J - 1 v 2 . 1


T a b l e F . 4 : L i t e r a t u r e D a t a o n P i t t i n g o f T y p e 3 1 6 / 3 1 6 L S t a i n l e s s S t e e l s ( C o n c l u d e d ) .

T y p e


( o C )

[ C l - ]

( g g - 1 )

O t h e r

E P o r E R

P

( m V S C E )

R e f e r e n c e

3 1 6

2 0

3 4 5

3 , 4 5 0

1 0 , 3

0 0

3 4 , 4

5 0

1 2 0 g r i t f i n i s h , a e r a t e d s o l u t i o n

+ 3 1 7 ( E

P )

+ 2 0 5

+ 1 7 0

+ 9 8

L a y c o c k a n d N e w m a n 1 9 9 7

3 1 6

2 0

3 , 4 5 0

3 4 , 4

5 0


E P / d l o g [ C l - ] = 2 0 6 m V

1 0 0 0 g r i t

+ 6 3 0 ( E

P )

+ 4 1 5

L a y c o c k e t a l . 2

0 0 5

3 1 6

2 0

3 4 5

3 4 5 0

3 4 , 5

0 0


E P / d l o g [ C l - ] = 2 2 0 m V

2 2 0 g r i t

+ 7 5 0 ( E

P )

+ 5 0 0

+ 3 3 5

L a y c o c k e t a l . 2

0 0 5


20/44

2 0

T a b l e F . 5 : L i t e r a t u r e D a t a o n C r e v i c e C o r r o s i o n o f T y p e 3 0 4 / 3 0 4 L S t a i n l e s s S t e e l s .

T y p e

T e m p e r a t u r e ( o C )

[ C l - ]

( g g - 1 )

O t h e r

E C R E V o r E R

C R E V

( m V S C E )

R e f e r e n c e

3 0 4

1 7 , 0

0 0

3 4 , 5

0 0

+ 1 3 1 ( E

C R E V

)

+ 1 0


)

3 0 4 L

1 0 2 0 3 0 4 0 5 0 6 0 7 0 8 0

1 9 , 0

0 0

S e a w a t e r , p

H 8 . 2

- 1 2 0 ( E

R C R E V

)

- 1 4 0

- 1 5 0

- 1 5 5

- 1 7 5

- 1 7 5

- 2 0 0

- 2 1 0

T a n i e t a l . 2 0

0 8

3 0 4

8 0

1 9 , 0

0 0


- 1 0 ( E

C R E V

)

H a r u k i e t a l . 1 9 9 1


21/44

Q R S - 1 3 8 4 J - 1 v 2 . 1


T a b l e F . 6 : L i t e r a t u r e D a t a o n C r e v i c e C o r r o s i o n o f T y p e 3 1 6 L S t a i n l e s s S t e e l .

T y p e


( o C )

[ C l - ]

( g g - 1 )

O t h e r

E C R E V o r E R C R E V

( m V S C E )

R e f e r e n c e

3 1 6 L

1 0 2 0 3 0 4 0 5 0 6 0 7 0 8 0

1 9 , 0

0 0

S e a w a t e r , p

H 8 . 2

- 8 0 ( E

R C R E V

)

- 9 5

- 1 1 0

- 1 2 0

- 1 4 0

- 1 5 5

- 1 5 5

- 1 6 5

T a n i e t a l . 2 0 0 8

3 1 6 L

2 3 - 5 0

6 , 0 0 0 -

2 4 , 0

0 0

0 - 8 0 0 g g -

1 S 2 O 3 2 - ,

8 0 0 - 3 , 4 0 0 g g - 1 S O

4 2 -

E R C R E V = - 3 0 1 . 5 3 . 7 ( [ C l - ] - 1 5 ) 1 5 . 3

( [ S O

4 2 - ] - 2 . 1 )

1 8 8 . 7

( [ S 2 O 3 2 - ] - 0 . 4 ) 2 ( T - 3 6 . 5

)

0 . 0

4 7 ( ( [ C l - ] - 1 5 ) ( [ S O

4 2 - ] - 2 . 1 )

+ 3 . 8 3 ( [ C l - ] - 1 5 ) ( [ S

2 O 3 2 - ] - 0 . 4 )

0 . 7

5 ( [ C l - ] - 1 5 ) ( T - 3 6 . 5

) + 0 . 3 5 ( [ C l - ] - 1 5 ) 2

T i n o C , c o n c e n t r a t i o n s i n g / L

S r i d h a r e t a l . 2

0 0 4

3 1 6 L

8 0

1 9 , 0

0 0


+ 8 0 ( E

C R E V

)

H a r u k i e t a l . 1

9 9 1


22/44

2 2

T a b l e F . 7 : L i t e r a t u r e D a t a o n C o r r o s i o n P o t e n t i a l o f T y p e 3 0 4 / 3 0 4 L S t a i n l e s s S t e e l s .

G r a d e p H


( o C )

[ C l - ]

( g g -

1 )

R e d o x

c o n d i t i o n s

O t h e r

E C O R R

( m V S C E )

R e f e r e n c e

3 0 4 L

8 . 2

2 4 3 0 5 0 6 0 7 0 8 0

1 9 , 0

0 0

A e r a t e d

S e a w a t e r

- 9 0

- 9 0

- 1 2 0

- 1 4 0

- 1 5 5

- 2 1 0

T a n i e t a l . 2

0 0 8

3 0 4 L

9 . 3

3 0

1 4 2 , 0 0 0

D e a e r a t e d

- 4 8 8

S r i d h a r e t a l . 1 9 9 3

3 0 4

5

2 5

1 7 , 3

0 0

D e a e r a t e d

- 4 6 4

S e d r i k s 1 9 9 6 ( T a b l e 4 . 1 5 )

3 0 4

3 4 , 5

0 0

A e r a t e d

- 6 3


( F i g 1 8 . 4

)

3 0 4

4 . 5

3 , 4 5 0

A e r a t e d

- 1 4

S r i d h a r e t a l . 2 0 0 4


23/44

Q R S - 1 3 8 4 J - 1 v 2 . 1


T a b l e F . 8 : L i t e r a t u r e D a t a o n C o r r o s i o n P o t e n t i a l o f T y p e 3 1 6 L S t a i n l e s s S t e e l .

G r a d e

p H


( o C )

[ C l - ]

( g g -

1 )

R e d o x c o n d i t i o n s

O t h e r

E C O R R

( m V S C E )

R e f e r e n c e

3 1 6 L

9 . 3

3 0

1 4 2 , 0 0 0

D e a e r a t e d

- 6 8 5


9 9 3

3 1 6 L

7 1 0 1 2

0 . 3

2 g g -

1 O 2

0 . 5 m N a 2 S O 4

- 1 1 7

- 1 1 9

- 1 4 5


0 0 4

3 1 6 L

8 . 2

2 4 3 0 5 0 6 0 7 0 8 0

1 9 , 0

0 0

A e r a t e d

S e a w a t e r

- 8 0

- 8 5

- 1 0 5

- 1 2 0

- 1 4 0

- 1 6 5

T a n i e t a l . 2 0 0 8

3 1 6 L

9 5

6 - 1 , 0 0 0

D e a e r a t e d

1 0 - 1 , 0

0 0 g g - 1

N O

3 - , 2

g g - 1 F

- ,

2 0 - 1 , 0

0 0 g g - 1 S O

4 2 -

- 7 3 9 t o - 4 5 4


9 9 3

3 1 6 L

1 0 1 2 . 6

1 3 . 6

2 2

1 7 7 , 0 0 0

A e r a t e d

+ 7 0

- 8 0

- 1 2 5

C u i a n d S a g u e s 2 0 0 3

3 1 6 L

1 2 . 6

2 2

A e r a t e d

- 1 0 0

C u i a n d S a g u e s 2 0 0 3

3 1 6 L

9 5

1 , 0 0 0

2 0 0 , 0 0 0

A e r a t e d

- 1 2 5

- 3 3 0

D u n n e t a l . 1

9 9 6

3 1 6 L

1 9 , 0

0 0

0 . 4

5 g g -

1 O 2

2 . 6

g g -

1 O 2

7 . 6

g g -

1 O 2

3 4 . 7 g g -

1 O 2

- 8 6

- 4 4

- 1 8

+ 4 7


0 0 4


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24

Figure F. 9: Correlation between the pitting resistance equivalent number andpitting 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 alloysconsidered for use as ILW or HLW/SF (Table F.1), the duplex alloy 2205 provides thehigher pitting resistance (or, at least, most-positive E P value), followed by thesuperaustenitic Type 904L, and the austenitic 316L and 304L alloys.

Another method for characterizing the resistance to localised corrosion is the criticalpitting (CPT) and critical crevice (CCT) temperature. These critical temperatures aredetermined in an aggressive solution using exposed and creviced samples. The actualCPT and CCT values depend on the nature of the environment, but are typicallymeasured in acidified ferric solutions. Figure F.10 shows the Cl - concentrationdependence of the CPT and CCT for a number of austenitic, duplex, andsuperaustenitic stainless steels and indicates the same order of corrosion resistance asthe PREN data in Figure F.9.


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25

Figure F.10: Corrosion map illustrating critical pitting and crevice corrosiontemperatures 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 - stresscorrosion cracking (SCC) of austenitic alloys. Cracks often initiate from pits and, forthis reason, the SCC susceptibility is related to the pitting susceptibility (Figure F.11).The susceptibility to SCC (and pitting) clearly increases with increasing temperatureand Cl - concentration and decreasing pH.

There is evidence for a threshold temperature and, possibly, Cl - concentration belowwhich SCC does not occur. Based on the data in Figure F.12, the threshold temperaturefor SCC is of the order of 60 oC and the threshold Cl - concentration is somewhat lessthan 1 mg/L. Although useful as a general guide, care should be taken in the use ofsuch "thresholds" since they are based on industrial timescales, rather than theextended timescales of interest for ILW/HLW/SF disposal. This caution is especiallyso for any threshold Cl - concentration since evaporation will concentrate surface liquidfilms.


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26

Figure F.11: Ranges of pH, Chloride Concentration and Temperature for the Pittingand SCC of 304 Stainless Steel (Jones 1992).

The microstructure and elemental composition also affect the susceptibility to SCC.The -ferrite microstructure of ferritic stainless steels is inherently less susceptible tocracking than the austenite structure; a difference which is responsible for theprecipitous 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.14also shows the effect of Ni content on the SCC susceptibility (in this case based on thethreshold stress intensity factor for cracking 1), with the susceptibility of specific alloysindicated. This figure clearly shows the superior SCC resistance of both thesuperaustenitic Type 904L and the ferritic Type 444 alloy.

1 The threshold stress intensity factor K ISCC is the value of K I, the linear-elastic fracturemechanics stress intensity factor, below which SCC crack growth is not observed.


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27

Figure F.12: Corrosion map for the susceptibility of various austenitic and duplexstainless 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 SCCresistance of alloys such as Type 904L results in part from the increased resistance topitting which acts as a necessary precursor for crack initiation.

Both austenitic and duplex stainless steels are susceptible to SCC in sulphide- andthiosulphate-containing environments. The severity of cracking of Types 304L and

316L have been shown to increase with decreasing pH and with increasingthiosulphate and/or chloride concentrations (Smart 2000).


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28

Figure F.13: Effect of Ni content on the susceptibility of 18-20 wt.% Cr stainlesssteels in boiling magnesium chloride solution at 154 oC (after Sedriks 1992).


29/44


29

Figure F.14: Effect of Ni content on the threshold stress intensity factor for SCC for arange of Fe-Cr-Ni alloys (after Sedriks 1992).

F.3.4 Microbiologically influenced corrosion

Like most engineering materials, stainless steels are susceptible to microbiologicallyinfluenced 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/SFcanisters also needs to take into account the effect of the GDF environment on thelocation and duration of microbial activity (King 2009b).


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30

Figure F.14: Dependence of the threshold stress intensity factor for SCC on the Mocontent of Fe-Cr-Ni-Mo alloys in aerated 22% NaCl solution at 105 oC (after Sedriks

1992).

When intimately exposed to active microbial communities and biofilms, stainless steelsare subject to various forms of MIC. In common with other materials, stainless steelswill undergo corrosion due to the formation of reduced S species (produced by theaction of sulphate-reducing bacteria) and organic acids (produced by a range ofbacteria 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:


31/44


31

localised corrosion due to the formation of thiosulphate (discussed above),

preferential attack of welds, and

ennoblement of E CORR.2

Various mechanisms have been proposed for the ennoblement of E CORR, including theproduction of H 2O2 within the biofilm and the catalysis of O 2 reduction by MnO 2.Ennoblement could increase the probability of localised corrosion, especially if it isaccompanied by the formation of thiosulphate elsewhere in the biofilm.

Of crucial importance, however, is where and when microbial activity is possible in theGDF (King 2009b). As described elsewhere in this report, if microbial activity is onlypossible at locations away form the canister surface, for example, due to the use ofhighly compacted bentonite or cementitious backfill, then the possible damage due toMIC will be much reduced.

F.3.5 Galvanic corrosion

As with Ni alloys, stainless steels are relatively noble in their passive state. In anygalvanic couple with a more-active material, such as Fe, Mg, Al, Zn, stainless steelwould act as the cathode with the active material preferentially corroding. In contactwith other passive materials, the driving force for corrosion (i.e., the difference inpotential between the two passive materials) will be small and any effect minimal.

F.3.6 Anthropogenic AnaloguesStainless steels have a history of less than one hundred years. Therefore, there isrelatively little experience with these alloys compared to materials such as copper andC-steel. Smart and Wood (2004) have reviewed a number of case histories from thearchitecture, transportation, and infrastructure industries demonstrating goodcorrosion resistance to atmospheric conditions and immersion in seawater for periodsexceeding 60 years. King (2009a) has reviewed a number of cases of the use of duplexstainless steels for architectural purposes and bridge construction. Theseanthropogenic analogues provide support for values for the rate of the long-term

atmospheric corrosion of stainless steels.

2 The shift in E CORR to more-positive values.


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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 affectboth the rates of general and localised corrosion. Although the evidence from the ratessummarised in Table F.2 is sparse, general corrosion rates in aerated environments aretypically higher than those under deaerated conditions.

More importantly, the evolution of redox conditions will affect the probability oflocalised corrosion and SCC of stainless steels. It is clear from the discussion inSection F.3.2 that pitting and crevice corrosion are primarily of concern under aerobicconditions when E CORR is most likely to exceed the film breakdown or re-passivationpotentials. Indeed, the evidence from Figure F.8 indicates that pitting of Type 316L

stainless steel will not occur in Cl- solutions at 95

oC under anaerobic conditions.

However, as indicated in Figure F.5, the presence of thiosulphate shifts E P to more-negative values, making pitting more likely even as the redox conditions in the GDFshift from aerobic to anaerobic.

The probability of SCC is also linked to the evolution of redox conditions. Stresscorrosion is closely linked to conditions under which pitting occurs, so the probabilityof cracking can similarly be expected to diminish as redox conditions becomeanaerobic.

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. Chlorideions affect the stability of the passive film. Even in the absence of localised filmbreakdown, Cl - ions degrade the stability of the passive film through an increase in thesolubility of Cr(III) (Pourbaix 1974), resulting in higher rates of general corrosion insaline solutions (Table F.2).

As discussed in detail above, however, the most significant effect of Cl - is the impact onthe localised corrosion and SCC behaviour of the canister. Increasing Cl - concentrationshifts 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).


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33

F.4.3 Effect of temperature

Increasing temperature affects the rate of general corrosion and the probability of bothlocalised corrosion and SCC. Increasing temperature typically results in higher rates ofgeneral 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, thedata of Blackwood et al. (2002) in alkaline solution do exhibit a monotonic increasewith temperature and suggest an activation energy of 47 kJ/mol.

Increasing temperature shifts the pitting potential to more-negative values (Figures F.2and F.3). Based on the data in Tables F.3 and F.4, the decrease in E P/E RP is of the orderof -3.4 to -7.0 mV/ oC for Types 304 and 316L, although there is evidence that E RP forType 316L reaches a minimum value at ~40 oC and then shifts to more-positive valueswith increasing temperature. This change in the temperature dependence of thepitting characteristics of Type 316L is also evident in Figure F.3, where the decrease inEP with increasing temperature appears to level off at temperatures above 70 oC. Thisimproved film stability at higher temperatures could be due to the presence of Mo inthe passive film and its inherent greater thermal stability. Based on a single report, thetemperature dependence of the crevice re-passivation for Types 304L and 316L issomewhat 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 partlydescribed by the temperature dependence of E P/E RP and E CREV/E RCREV. It is alsonecessary to take into account the temperature dependence of E CORR, since it is thedifference between the critical potential and E CORR that determines the probability oflocalised corrosion. The corrosion potential also shifts in the active direction withincreasing temperature, but at a slower rate. Again based on a single study, thetemperature dependence of E CORR for Types 304L and 316L stainless steel in aeratedsolution 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 withincreasing temperature than the value of E CORR indicates that localised corrosionbecomes more likely with increasing temperature and is one explanation for theobservation 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 isevident from the data in Table F.2. Under alkaline conditions (pH > 10), rates ofgeneral corrosion are typically of the order of 0.01 m/y, whereas rates are generally 1-2 orders of magnitude higher at near-neutral pH.


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34

Pitting is more likely to occur at lower pH (Figure F.11), which is partly explained bythe effect of pH on E CORR (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 E P 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 resistanceof stainless steels to localised corrosion in alkaline solutions typical of the pore water incement 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 cementscontaining up to 10 wt.% Cl -.

The solution pH also clearly affects the probability of SCC (Figure F.11). Theenvironmental conditions necessary for cracking shift to higher Cl - concentrationsand/or higher temperature as the pH increases. The susceptibility of 304L and 316Laustenitic stainless steels to SCC has been determined in simulated cementitiousenvironments containing chloride and/or thiosulphate ions for the Nirex/NDAprogram (Smart 2002). The severity of cracking increased with decreasing pH andwith increasing thiosulphate and/or chloride concentrations.

The beneficial effect of increasing pH can be expressed as a [OH -]:[Cl-] ratio, abovewhich the probability of localised corrosion or SCC is significantly diminished. Basedon the apparent threshold pH values for E P 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 forType 304. Provided the [OH -]:[Cl-] is above this value then localised corrosion isunlikely. 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 to10 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 corrosionbehaviour of stainless steels, depending upon the oxidation state of sulphur. In thefully oxidised +6 state, sulphate inhibits the crevice corrosion of Type 304 and 316 in

Cl- solutions (Figure F.6). A [SO 42-]:[Cl-] molar ratio of 1.2 and 0.4 is sufficient to inhibitthe crevice corrosion of Types 304 and 316, respectively.

In the +4 oxidation state, thiosulphate ions increase the susceptibility to both pittingand 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 localisedcorrosion (Kursten and Druyts 2000, Kursten et al. 2004) lead to the change in the


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35

Belgian programme from stainless steel as the candidate canister (overpack) material toa cementitious C-steel supercontainer design (Kursten and Druyts 2008).

Stainless steels are also susceptible to SCC and accelerated general corrosion in thepresence 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 stainlesssteels in Cl - solutions, certain other anions, such as carbonate and nitrate, also act asinhibitors (Szklarska-Smialowska 2005). Nitrate, in particular, is an effective inhibitor,possibly due to the consumption of protons in the reduction of NO 3- to either NH 3 orN 2, thus raising the pit or crevice pH. Carbonate would be expected to have a similareffect on the pH of the occluded chemistry.

Conversely, cations that can be hydrolysed to form acidic solutions (e.g., Mg 2+, 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 thecanister 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 ofradiolysis on the corrosion of stainless steels. Among the observations reported are:

The corrosion potential in 0.018 mol dm -3 NaCl increased substantially when aradiation field (10 4 Gy/hr) was introduced, but no increase in general corrosionrate or initiation of pitting was observed.

Radiation appears to inhibit pitting, probably due to defect annealing in thepassive oxide film.

Results on crevice corrosion ( in 10 mg/L Cl -) are ambiguous. A dose rate of 2.8Gy/hr may be sufficient to initiate crevice corrosion, but the availability of alarge cathode may be more important.

Crevice propagation can be maintained at 10 3 Gy/hr, but not at 10 Gy/hr.

Marsh et al. (1986) studied the effect of gamma radiation on the potential and localisedcorrosion behaviour of Type 304L stainless steel in aerated solutions containing300 mg/L Cl -. At a dose rate of 2 x 10 3 Gy/hr, the value of E CORR 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 E CORR also


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36

inhibited film breakdown, although they had no effect on the re-passivation of existingpits.

Overall, and in common with other materials (Shoesmith and King 1999), nomeasurable 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 atmosphericexposure

Smart (2000) has reviewed the atmospheric behaviour of stainless steels wastecontainers. Corrosion can occur when salt contaminants on the surface of the canisterabsorb moisture (deliquesce) from the air forming small volumes of highlyconcentrated solutions. These conditions present an ideal opportunity for theestablishment of spatially separated anodic and cathodic processes and the initiationand propagation of localised corrosion and SCC.

Recently, Tani et al. (2008) have compared the behaviour of austenitic and duplexstainless steels exposed to humid atmospheres following contamination by saltscontained in seawater. Surface corrosion took the form of pitting at isolated saltcrystals formed by the evaporation of seawater and which subsequently deliquescedwhen exposed to a humid atmosphere. These pits could serve as locations for theinitiation 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 tothe 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 toguarantee that evaporative concentration will not lead to higher concentrations. Giventhe ubiquitous nature of Cl - in deep groundwaters and the inevitable elevatedtemperature at the canister surface, it is difficult to justify the use of stainless steels inthe absence of a cementitious backfill.

In the presence of a cementitious backfill, however, stainless steels are likely to providecontainment for the duration of the alkaline phase. Thus, elevated pH (defined here aspH > 10-11) inhibits the effects of Cl - ions on the rate of general corrosion, the initiationof 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


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37

portlandite/CSH gels present in cement. Prediction of the lifetime of the canister thenbecomes 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 listsa number of critical conditions for which the use of stainless steel canisters wouldeither not be recommended or which would require detailed investigation in order todevelop 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 orslightly above ambient, depending upon the aggressiveness of the environmentand the alloy composition. In the absence of a cementitious backfill, thethreshold temperature for localised corrosion or SCC is likely to be pH 12.


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3 8

T a b l e F . 1 : L i s t o f C r i t i c a l C o n d i t i o n s f o r H L W / S F C a n i s t e r s M a n u f a c t u r e d f r o m S t a i n l e s s S t e e l

P a r a m e t e r

C r i t i c a l c o n d i t i o n

C o m m e n t

H o s t r o c k

P r e s e n c e o f s u l p h i d e

m i n e r a l s

T h e p r e s e n c e o f s u l p h i d e m i n e r a l s i n t h e h o s t r o c k o r b a c k f i l l m a t e r i a l s w o u l d b e o f c o n c e r n

b e c a u s e o f t h e p o s s i b i l i t y o f t h i o s u l p h a t e f o r m a t i o n d u r i n g t h e a e r o b i c p h a s e . T h i o s u l p h a t e

c o u l d t h e n i n d u c e l o c a l i s e d c o r r o s i o n o r S C C

o f s t a i n l e s s s t e e l c a n i s t e r s .

R e d o x c o n d i t i o n s

N o n e

T h e p a s s i v e f i l m o n s t a i n l e s s s t e e l s p r o v i d e s g o o d p r o t e c t i o n a g a i n s t g e n e r a l c o r r o s i o n u n d e r

b o t h a e r o b i c a n d a n a e r o b i c c o n d i t i o n s . H o w e v e r , t

h e p r o p e n s i t y f o r f i l m b r e a k d o w n a n d

l o c a l i s e d c o r r o s i o n o r S C C i s g r e a t l y e n h a n c e d u n d e r a e r o b i c c o n d i t i o n s .


< 1 0 0 o C ( i n a b s e n c e o f

c e m e n t i t i o u s b a c k f i l l )

T h e p r o b a b i l i t y o f l o c a l i s e d c o r r o s i o n o r S C C

i n c r e a s e s s i g n i f i c a n t l y w i t h i n c r e a s i n g

t e m p e r a t u r e .

L o c a l i s e d c o r r o s i o n a n d S C C c a n o c c u r a t t e m p e r a t u r e s a t o r s l i g h t l y a b o v e

a m b i e n t , d e p e n d i n g u p o n t h e a g g r e s s i v e n e s s o f t h e e n v i r o n m e n t a n d t h e a l l o y c o m p o s i t i o n . I n

t h e a b s e n c e o f a c e m e n t i t i o u s b a c k f i l l , t h e t h r e s h o l d t e m p e r a t u r e f o r l o c a l i s e d c o r r o s i o n o r S C C

i s l i k e l y t o b e < 1 0 0 o C u n l e s s h i g h l y c o r r o s i o n - r e s i s t a n t a l l o y s a r e s e l e c t e d .

G a m m a r a d i a t i o n

> 1 - 1 0 G y / h

T h e r e i s n o e v i d e n c e f o r a d v e r s e e f f e c t s o f i r r a d i a t i o n a t d o e s r a t e s < 1 G y / h .

B a c k f i l l m a t e r i a l a n d

n e a r - f i e l d m a s s

t r a n s p o r t

R e q u i r e s

c e m e n t i t i o u s b a c k f i l l

G i v e n t h e u b i q u i t o u s n a t u r e o f C l - i o n s i n g r o u n d w a t e r s a n d t h e h e a t g e n e r a t i o n b y t h e

H L W / S F , t h e u s e o f s t a i n l e s s s t e e l c a n i s t e r s w o u l d n o t b e r e c o m m e n d e d w i t h o u t t h e u s e o f a

c e m e n t i t i o u s b a c k f i l l .


39/44

Q R S - 1 3 8 4 J - 1 v 2 . 1


C h l o r i d e

c o n c e n t r a t i o n

D e p e n d e n t o n n a t u r e

o f b a c k f i l l a n d a l l o y

S t a i n l e s s s t e e l s a r e s u b j e c t t o l o c a l i s e d c o r r o s i o n a n d S C C i n C l c o n t a i n i n g e n v i r o n m e n t s ,

e s p e c i a l l y a t e l e v a t e d t e m p e r a t u r e .

G i v e n t h e u b i q u i t o u s n a t u r e o f C l - i o n s

i n g r o u n d w a t e r s

a n d t h e h e a t g e n e r a t i o n b y t h e H L W / S F , t h e u s e o f s t a i n l e s s s t e e l c a n i s t e r s w o u l d n o t b e

r e c o m m e n d e d w i t h o u t t h e u s e o f a c e m e n t i t i o u s b a c k f i l l .

O t h e r g r o u n d w a t e r

s p e c i e s

N o n e

T h e p r e s e n c e o f C a 2

+ a n d M g 2

+ i n t h e g r o u n d w a t e r c o u l d l e a d t o t h e g e n e r a t i o n o f a c i d i c

e n v i r o n m e n t s , b u t t h e u s e o f s t a i n l e s s s t e e l w

o u l d n o t b e r e c o m m e n d e d w i t h o u t a c e m e n t i t i o u s

b a c k f i l l w h i c h s h o u l d m a i n t a i n a n a l k a l i n e n e a r - f i e l d p H .

S u l p h u r s p e c i e s

T h i o s u l p h a t e ,

s u l p h i d e

T h i o s u l p h a t e p r o d u c e d b y t h e p a r t i a l o x i d a t i o n o f p y r i t e i m p u r i t i e s i n c l a y - b a s e d s e a l i n g

m a t e r i a l s o r t h e h o s t r o c k c o u l d i n d u c e l o c a l i s e d c o r r o s i o n a n d S C C o f t h e c a n i s t e r . S u l p h i d e

p r e s e n t n a t u r a l l y i n t h e g r o u n d w a t e r o r p r o d u c e d b y s u l p h a t e - r e d u c i n g b a c t e r i a c o u l d l e a d t o

a c c e l e r a t e d

g e n e r a l c o r r o s i o n o r S C C .

M i c r o b i a l a c t i v i t y

S i g n i f i c a n t i f b i o f i l m s

f o r m e d

A s w i t h o t h e r c a n i s t e r m a t e r i a l s , m

i c r o b i a l a c t i v i t y r e m o t e f r o m t h e c a n i s t e r p r e s e n t s a m i n i m a l

t h r e a t t o t h e c a n i s t e r l i f e t i m e . H o w e v e r , s u r

f a c e m i c r o b i a l a c t i v i t y a n d b i o f i l m

f o r m a t i o n c o u l d

l e a d t o e n n o b l e m e n t o f E

C O R R a n d t h e p o s s i b i l i t y o f l o c a l i s e d c o r r o s i o n a n d t h e

p o s s i b i l i t y o f

p r e f e r e n t i a l w e l d a t t a c k .

R e s i d u a l s t r e s s a n d

e x t e r n a l l o a d

D e p e n d e n t o n n a t u r e

o f b a c k f i l l a n d a l l o y

A u s t e n i t i c a n d d u p l e x s t a i n l e s s s t e e l s a r e s u s c e p t i b l e t o S C C i n t h e p r e s e n c e o f C l - a n d a

s u f f i c i e n t a p p l i e d o r r e s i d u a l t e n s i l e s t r e s s . F e r r i t i c a l l o y s a r e l e s s s u s c e p t i b l e , b u t a r e n o t

i m m u n e t o C l i n d u c e d S C C .

G D F s a t u r a t i o n t i m e

N o n e

S t a i n l e s s s t e e l s e x h i b i t g o o d c o r r o s i o n r e s i s t a n c e u n d e r a t m o s p h e r i c c o n d i t i o n s . T h e n o n -

u n i f o r m w e t t i n g o f s u r f a c e s a l t c o n t a m i n a n t s c o u l d l e a d t o s p a t i a l s e p a r a t i o n o f a n o d i c a n d

c a t h o d i c s i t e s a n d t h e p o s s i b i l i t y o f l o c a l i s e d c o r r o s i o n a n d / o r S C C .

8/10/2019 Review of the Corrosion Performance of Selected Metals

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