Nickel Based Alloys

QRS-1384J-1 v2.1 Appendix E

1

Appendix E Corrosion of Nickel Alloys

E.1 Introduction

There is a wide range of Ni-based alloys that exhibit an equally wide range of corrosion

properties. Some alloys are designed for use in reducing acids, others in oxidising

acids, and others in alkalis. Nickel alloys are generally more-resistant to stress

corrosion cracking (SCC) in Cl- environments than austenitic stainless steels. Other

properties of Ni alloys include oxidation and sulphidation resistance and superior

mechanical properties in high-temperature applications. The breadth of corrosion and

mechanical properties is a result of the high solubility of other metals in Ni, resulting in

single-phase alloys. From a fabrication standpoint, Ni alloys are ductile, easily

formable, and can be welded using various techniques.

Nickel alloys have been considered as candidate HLW/SF canister materials in a

number of international programmes. AECL investigated the corrosion properties of

Hastelloy C-276 and Inconel 625 (Johnson et al. 1994). In Europe, Hastelloy C-4 has

been studied in the German (salt brine) and Belgian (Boom Clay) programmes, with

Inconel 625 and Hastelloy C-22 also considered in the latter (Kursten et al. 2004). In the

US Yucca Mountain Project, the Ni alloys considered include Alloys 625, 825, and 22

(Brossia et al. 2001).

E.2 Nickel alloys

Table E.1 lists the compositions of the various Ni alloys considered as canister

materials for the disposal of HLW/SF.

There are six major groups of Ni alloys, each characterised by the principal alloying

elements: Ni-Cu, Ni-Mo, Ni-Cr, Ni-Cr-Mo, Ni-Fe-Cr, and Ni-Fe-Cr-Mo alloys. All of

the alloys selected in the various international HLW/SF programmes belong to either

the Ni-Cr-Mo group (i.e., Alloys 625 and 22, Hastelloy C-4 and C-276) or the

Ni-Fe-Cr-Mo group (Alloy 825), primarily because of their resistance to corrosion

under both oxidising (due to the presence of Cr) and reducing (due to the presence of

Mo) conditions. Furthermore, Mo (and W in Alloy 22 and Hastelloy C-276) provides

improved resistance to localised corrosion. Silicon can improve the corrosion

performance under high-temperature oxidising conditions because of the formation of

protective Si-O films. Manganese is added as a deoxidant, but can increase the

susceptibility to localised corrosion because of the tendency for film breakdown to

occur at MnS inclusions.

2

Table E.1: Composition of Selected Nickel Alloys.*

Alloy

Alloy class

Ni

Cr

Fe

Mo

C

Si

Mn

Ti

Other

Alloy 22

Ni-Cr-Mo

56.0

22.0

3.0

13.0

0.015

0.08

- -

3.0 W

Hastelloy C-4

Ni-Cr-Mo

bal

140-18.0

3.0

14.0-17.0

0.015

0.08

1.0

0.70

2.0 Co. 0.03 S, 0.04 P

Hastelloy C-

276

Ni-Cr-Mo

bal

14.5-16.5

4.0-7.0

15.0-17.0

0.02

0.08

1.0

3.0-4.5 W

, 2.5 Co, 0.35 V, 0.03 S,

0.03 P

Alloy 625

Ni-Cr-Mo

58.0 m

in

20.0-23.0

5.0

8.0-10.0

0.10

0.5

0.5

0.4

0.4 Al, 0.015 S, 3.15-4.15 Nb

Alloy 825

Ni-Fe-Cr-Mo

36.0-46.0

19.5-23.5

24-40

2.5-3.5

0.05

0.5

1.0

0.6-

1.2

1.5-3.0 Cu, 0.2 Al, 0.03 S

* wt.% m

ax., unless stated otherwise.


3

E.3 Corrosion modes for nickel alloys

E.3.1 General corrosion

The Ni-Cr-Mo and Ni-Fe-Cr-Mo alloys are protected from corrosion by a Cr-based

passive film. Both Cr(OH)3 (Figure E.1) and Cr2O3 exhibit similar ranges of stability in

water, spanning from acidic to highly alkaline pH and potentials below the H2O/H2

equilibrium line to values within several 100 mV of the O2/H2O equilibrium. The

stability of this film is partly the result of the very low solubility of Cr(III) (Figure E.2).

However, transpassive dissolution as Cr(VI) occurs at potentials more-negative than

those of some other passive materials, e.g., titanium (Figure D.2), resulting in a

tendency towards localised corrosion (Section E.3.2).

Figure E.1: Potential-pH (Pourbaix) Diagram for the Cr-H2O System Assuming

Cr(OH)3 is the Stable Solid (Pourbaix 1974).

4

Figure E.2: Solubility of Chromium(III) Solids in Water at 25oC (Pourbaix 1974).

As a result of this passivity, the general corrosion rate in aerobic and anaerobic

environments is very low. The passive current density (which is the electrochemical

equivalent of the passive corrosion rate) decreases with increasing Cr content of the

alloy. Thus, Alloys 625 and 22 would be expected to have lower corrosion rates than

the Hastelloys (Table E.1). The mean corrosion rate of Alloy 22 in a range of simulated

concentrated Yucca Mountain pore waters at a temperature of 60oC is 5-10 nm/y (DOE

2008), based on 5- and 10-year exposure tests. The corrosion rate is independent of the

concentration and composition of the aqueous phase (except for the presence of F-),

and is moderately temperature sensitive with an activation energy of 26 kJ/mol.

Under the more-aggressive ‘Q-brines’ studied in the German programme, 3-year mean

corrosion rates ranged from 200 nm/y at 90oC to 900 nm/y at 200oC. As in the Yucca

Mountain Project, the corrosion rate was found to decrease continuously with exposure

time as the passive film thickens and becomes more protective.

The question for the use of any passive alloy is whether the alloy will maintain

passivity for the entire service life of the canister. For Ni alloys, one mechanism that

has been identified that could compromise the integrity of the passive film is the

anodic segregation of sulphur at the metal-film interface (Marcus 1995). Sulphur

accumulates at the metal-film interface as the alloy dissolves and, once it reaches a

sufficient concentration can form a non-protective Ni3S2 film resulting in an increase in

the dissolution rate. Both Cr and Mo are beneficial as they promote dissolution of the


5

sulphide film and formation of the oxide. The question is whether this Ni3S2 film is

stable or whether it spalls from the surface, permitting the Cr(III) passive film to re-

form. In the latter case, it is unlikely that there will be a significant impact on the long-

term corrosion rate. A similar effect is possible due to S species in solution.

Regardless, there are clearly advantages to specifically an alloy with low S and high Cr

and Mo contents.

E.3.2 Localised corrosion

As noted above, Ni-Cr-Mo and Ni-Fe-Cr-Mo alloys are subject to film breakdown and

transpassive dissolution. Unlike Ti alloys, for which the pitting potential is extremely

positive, Ni alloys do exhibit both pitting and crevice corrosion. However, because

crevice corrosion occurs under milder conditions (i.e., lower potentials, lower

temperature, and/or lower chloride concentration) and because there will inevitably be

occluded regions on the canister surface, crevice corrosion has received the most

attention in various national programmes.

Various approaches have been taken to the study of crevice corrosion. In the chemical

industry it is common to refer to a “Critical Crevice Temperature” above which there is

a visible indication of localised attack on a standard-type specimen in a standard,

generally highly aggressive, environment. This approach has been used in some

national programmes, but using a simulated GDF environment instead of the standard

acidic-oxidising solution usually used. Whilst this approach is useful for ranking

different alloys and, given extensive field experience, for judging whether a particular

alloy will be suitable for service in a given environment, it provides no mechanistic

insight that can be used to make predictions over very long periods of time.

In the qualification of Ni alloys for use as a HLW/SF canister material, the approach

often taken has been to determine the chemical and electrochemical conditions for the

propagation of localised corrosion. The criterion generally adopted is that the

corrosion potential (ECORR) must exceed the re-passivation potential for a creviced

sample (ERCREV). Mathematically, this criterion can be expressed as

ECORR - ERCREV > 0 (E-1)

This in itself is a conservative indicator of crevice initiation, since ERCREV is actually the

potential at which a propagating crevice ceases to grow.

Various factors affect the value of ERCREV. Chloride promotes film breakdown

(Figure E.3), whereas other anions, most notably nitrate, but also sulphate and

carbonate, inhibit both initiation and propagation of localised corrosion. Increasing

temperature leads to a decrease in ERCREV (Figure E.4). In a similar fashion to stainless

6

Figure E.3: Crevice Re-passivation Potential for Alloy 625 as a Function of Chloride

Concentration at 60oC and 95oC (Cragnolino et al. 1999)

Figure E.4: Temperature Dependence of the Crevice Re-passivation Potential of

Alloy 22 in Various Chloride Solutions (Cragnolino et al. 1999).


7

Figure E.5: Crevice Re-passivation Potential as a Function of Chloride Concentration

for Various Ni-based Alloys (Brossia et al. 2001).

steels, the resistance to crevice corrosion can be related to the Cr and Mo contents of

the alloy (Figure E.5).

An important aspect of the crevice corrosion of Ni alloys is their tendency to re-

passivate following initiation. This process, sometimes referred to as “stifling”, may

result from a number of factors, including:

o iR (potential) drop down the crevice

o mass-transport effects

o loss of critical crevice chemistry by catalysis of H+ reduction

o loss of critical crevice chemistry by the reduction of inhibiting anion, e.g., NO3-

in NO3-:Cl- mixtures

o negative shift in ECORR upon the initiation of localized corrosion

8

Figure E.6: Re-passivation of Single Propagating Pit for Alloy 825 (Dunn et al. 1996).

Figure E.6 shows the re-passivation of a single artificial pit on Alloy 825 as a function

of potential. The pit was initiated at a potential of +0.6 VSCE and the propagation

kinetics determined for various potentials. Below a potential of 0 VSCE the pit growth

slows and eventually stops. Similar effects are observed during crevice corrosion (Mon

et al. 2005).

E.3.3 Environmentally assisted cracking

Compared with austenitic stainless steels, Ni alloys are resistant to stress corrosion

cracking (SCC). Nickel alloys are much less-susceptible than austenitic stainless steels

to cracking in Cl—based groundwaters, but can be susceptible in the presence of

sulphur species and lead. Figure E.7 shows the dependence of the threshold stress

intensity factor (KISCC) in hot chloride solutions as a function of the Ni content of the

alloy. The earlier data of Speidel (1981) suggest immunity to cracking for a Ni content

>30 wt.%, whereas the later data of Roy and Flemming (1998) and McCright (1998)

suggest a benefit of increasing Ni content, but not complete immunity. On the basis of

the Speidel (1981) data, all of the alloys in Table E.1 would be immune to SCC.


9

Figure E.7: Dependence of the Susceptibility of Nickel Alloys to Stress Corrosion

Cracking as a Function of Nickel Content(Cragnolino et al. 1999). KISCC is the

threshold stress intensity factor for SCC. Measurements in 5 wt,% and 22 wt.% NaCl

solutions at temperatures of 90-120oC.

Experience from various national programmes supports the suggestion that the Ni

alloys in Table E.1 are immune to cracking in Cl—dominated GDF environments.

Hastelloy C-4 U-bend specimens did not exhibit cracking in ‘Q-brine’ between

temperatures of 90oC and 200oC (Kursten et al. 2004). Alloy 22 has proven to be

immune to cracking in a range of concentrated brines solutions at 90oC under severe

constant extension rate testing (DOE 2008). Cracking was observed in some tests, but

only at applied potentials 200-300 mV more-positive than ECORR (King et al. 2008).

Nickel alloys are susceptible to SCC and hydrogen embrittlement in certain aggressive

conditions, including: elevated temperatures (>150-200oC), pH <4, presence of H2S, and

presence of lead (ASM 1987). Of these conditions, the low pH and high temperature

will either not exist or can be avoided by appropriate GDF design. Sulphide species

may be present in the host rock and/or in the groundwater. The susceptibility of the

canister to SCC would need to be determined based on the expected interfacial

concentration of sulphide, elemental S, and thiosulphate. Similarly, if Pb were to be

used as a metal-matrix material, the concentration of Pb(II) at the canister surface due

to dissolution from a prematurely failed canister would need to be determined.

10

E.3.4 Microbiologically influenced corrosion

Although, unlike Ti alloys, it is not possible to claim that Ni alloys are immune to

microbiologically influenced corrosion (MIC) (Little et al. 1991), the susceptibility to

microbial attack appears to be limited (King 2009, Lloyd et al. 2004). Even in the

presence of large quantities of nutrients, Farmer et al. (2000)) only reported a doubling

of the general corrosion rate. This lack of susceptibility has been attributed to the wide

range of stability of the passive film (Figure E.1) (Lloyd et al. 2004).

As with other candidate canister materials, the key to predicting the long-term MIC

behaviour of a Ni alloy canister is to have an understanding of where and when the

microbial activity occurs (King 2009). Provided that microbial activity at the canister

surface can be avoided by appropriate GDF design, MIC should have a minimal effect

on the service life of the canister. However, if microbial activity at the surface cannot

be avoided and a biofilm forms on the canister, then MIC could impact the service life

because of enhanced general corrosion, localised attack, and/or SCC, especially as

induced by sulphate-reducing bacteria.

E.3.5 Galvanic corrosion

In their passive state, Ni alloys are relatively noble. In any galvanic couple with a

more-active material, such as Fe, Mg, Al, Zn, the Ni alloy would act as the cathode

with the active material preferentially corroding. In contact with other passive

materials, Ni alloys may be active or passive, but the driving force for corrosion (i.e.,

the difference in potential between the two passive materials) will be small and any

effect minimal.

E.3.6 Anthropogenic analogues

As for stainless steel and Ti alloys, there is a relatively short history of the use of Ni

alloys and, as a consequence, few analogues for the long-term performance of Ni-based

HLW/SF canister materials. Cragnolino et al. (2003), however, did study the

electrochemical characteristics of josephinite, a mineral containing a naturally

occurring Ni-Fe alloy as an analogue for Alloy 22. Although there are few similarities

between the composition and metallurgy of Alloy 22 and that of joephinite, the study

did report interesting data regarding the anodic characteristics and susceptibility to

localised corrosion of the mineral. For example, josephinite was more passive at high

pH than a cast Ni3Fe alloy, but was more susceptible to pitting corrosion.


11

E.4 Corrosion behaviour of nickel alloys

E.4.1 Effect of redox conditions

Due to the presence of Cr and Mo in the Ni-Fe-Cr-Mo and Ni-Cr-Mo series of alloys,

the alloys in Table E.1 exhibit good corrosion resistance under both aerobic and

anaerobic conditions. The Cr oxide film provides much of the thermodynamic

stability, with Mo providing additional stability under acidic reducing conditions

through the formation of MoO2. Under oxidising conditions, Cr(III) is converted to

Cr(VI) at redox potentials (EH) equivalent to those present initially in the GDF due to

the trapped atmospheric O2, but dissolution of the canister material is determined by

the corrosion potential which is many hundreds of mV more-negative than EH due to

the kinetic limitations. In the absence of localised film breakdown, therefore, Ni-Cr-Mo

alloys offer good corrosion resistance over the entire range of redox potentials expected

within the GDF.

However, due to the presence of Cl- and of occluded regions or film defects, localised

corrosion in the form of crevice corrosion or pitting is possible under aerobic

conditions. Once all of the O2 has been consumed, ECORR will drop below the re-

passivation potential and any propagating pits or crevices will cease to grow. (Stifling

of pits or crevices may occur before the consumption of O2 due to the metallurgical or

other factors described in Section E.3.2). They key issue is whether the pit or crevice

has propagated through the canister wall prior to the transition to anaerobic

conditions. This answer to this question depends on a number of factors, including the

GDF design, the time-dependence of the environmental conditions, and the alloy

selected as a canister material.

E.4.2 Effect of chloride

The Cl- concentration affects both the general and localised corrosion behaviour of Ni

alloys. The solubility of Cr(III) increases in the presence of Cl- (Pourbaix 1974),

resulting in higher a passive current density and a higher rate of passive corrosion. In

aerated solutions at temperatures of 60oC and 90oC, however, there is little dependence

of the corrosion rate of Alloy 22 on Cl- concentration over the range 70 mg/L to

178,000 mg/L, with a mean rate of ~5 nm/y. (DOE 2008). In aggressive ‘Q-brines’,

higher corrosion rates are observed, in the range of 100’s of nm/y (Kursten et al. 2004).

The effect of Cl- concentration on the initiation of localised corrosion is more apparent.

Over the range 0.01 mol/L to 1 mol/L, the crevice re-passivation potential for

Alloys 625 and 825 decrease by ~100 mV per decade change in the Cl- concentration

(Figure E.5). Selection of the appropriate material requires a knowledge of the

12

susceptibility of the alloy as a function of Cl- concentration, potential and temperature,

as well as a knowledge of how these parameters change with time at the canister

surface.

E.4.3 Effect of temperature

The most important effects of temperature are on the localised and SCC behaviour of

the alloys. There is a modest effect of temperature on the general corrosion rate of Ni

alloys, with rates increasing from 0.20 µm/y at 90oC to 0.90 µm/y at 200oC in ‘Q-brine’

(Kursten et al. 2004). The general corrosion of Alloy 22 in synthetic brine solutions

exhibits an activation energy of 25-60 kJ/mol (DOE 2008).

Increasing temperature increases the susceptibility of Ni alloys to localised corrosion.

A common method for comparing the susceptibility of different alloys is to determine a

threshold temperature in a standard solution, often an acidic ferric chloride solution.

In this aggressive environment, the critical crevice temperatures (CCT) for Hastelloy C-

276 and Alloy 22 are 55oC and 80oC, respectively (ASM 2005). In GDF-type

environments, the equivalent CCT would be significantly higher. The CCT can be

viewed as the temperature at which ECORR exceeds ERCREV in that particular

environment. The effect of temperature on ERCREV can be seen from Figure E.4.

The susceptibility of Ni alloys to SCC increases significantly with increasing

temperature. Susceptibility increases at temperatures above 150oC and particularly

above 200oC (ASM 1987).

High temperatures can also impact the microstructure of Ni alloys and, hence, the

mechanical and corrosion properties. Rebak et al. (2000) considered the effect of

thermal aging on the microstructure, mechanical properties, and corrosion resistance of

Alloy 22. Thermal aging can induce long-range ordering (LRO) and the formation of

brittle tetrahedrally close-packed (TCP) phases. Based on time-temperature-transition

(TTT) relationships, it can be shown that LRO and TCP formation will not occur over

GDF timescales, even for a maximum canister temperature of 250oC.

E.4.4 Effect of pH

The passive film on Ni-Cr-Mo and Ni-Fe-Cr-Mo alloys becomes more soluble with

decreasing pH. Many Ni alloys are, however, used successfully to handle concentrated

acid solutions (ASM 1987, 2005).

For the expected range of pH for the generic GDF designs (neutral to alkaline), Ni

alloys will exhibit good corrosion resistance. Nickel alloys would be suitable for use

with cementitious backfill of a concrete overpack, but these alloys exhibit sufficient


13

corrosion resistance without the need to condition the environment through the use of

concrete.

E.4.5 Effect of sulphur species

Nickel alloys do exhibit some susceptibility to sulphide, elemental sulphur, and

oxysulphur anions (ASM 2005). Adsorption of S on Ni inhibits passivation because of

the formation of a Ni3S2 layer (Marcus 1995). A critical S surface coverage of 0.7-

0.8 monolayers is required to suppress passivation. This surface coverage can arise

from the anodic segregation mechanism discussed in Section E.3.1 or from S species in

the environment. The relationship between the onset of S-assisted de-passivation and

the concentration or rate of supply of S species to the (canister) surface has not be

adequately determined. For Ni alloy canisters in a bentonite-backfilled GDF, it is

possible that the rate of supply of S species would be too slow to induce de-

passivation, but would need to be studied in more detail.

Enhanced crevice corrosion, increased rate of general corrosion, and a susceptibility to

pitting was observed on Hastelloy C-4 when 25 mg/L H2S was added to ‘Q-brine’

solution at 150oC (Kursten et al. 2004).

E.4.6 Effect of other anions and cations

There are few adverse effects on the corrosion behaviour of Ni alloys of anions or

cations other than Cl- or S species. Instead, a beneficial effect of sulphate, carbonate,

and, especially of nitrate, on the crevice corrosion of Ni alloys has been reported (DOE

2008).

E.4.7 Effect of gamma radiation

Nickel alloys do exhibit some sensitivity to gamma radiation at high absorbed dose

rates. An extensive series of tests was performed for the German programme in

irradiated ‘Q-brine’ solutions at a temperature of 90oC (Kursten et al. 2004). There was

no apparent effect of irradiation on the behaviour of Hastelloy C-4 at a dose rate of

1 Gy/h, but an increase in the rate of general corrosion and a susceptibility to pitting

was observed for dose rates of between 10 Gy/h and 1000 Gy/h. However, this

observation should not be interpreted as indicating a general threshold to radiation

effects in all environments, since the passive region is particularly narrow (in terms of

potential) in this aggressive environment and it does not require much additional

(radiolytic) oxidant to drive the system into the transpassive region. In less-aggressive

environments, where the passive region is wider, the threshold for the effect of

irradiation, if any, would be expected to be higher.

14

E.4.8 Effect of unsaturated conditions and atmospheric exposure

Nickel alloys provide the same excellent corrosion resistance in unsaturated conditions

as in bulk solution (ASM 1987, 2005). The major difference between the two

environments is that the rate of mass transport of aggressive species to the canister

surface could be higher in the absence of a backfill material.

E.5 Lifetime predictions

The only formal lifetime prediction for a Ni alloy HLW/SF canister material is that for

Alloy 22 canisters in the Yucca Mountain GDF (DOE 2008). Under the permanently

aerobic conditions of this unsaturated GDF, the canisters were considered to be

susceptible to a combination of: general corrosion, localised (crevice) corrosion, MIC,

and, if subject to mechanical damage from falling rocks, SCC. The extent of general

corrosion was estimated from the time-temperature history and temperature-

dependent weight-loss corrosion rates measured from 5-year exposure tests. Crevice

corrosion only affects that small fraction of the canisters for which the accompanying

drip shield has failed prior to the end of the thermal transient, and is not a significant

cause of canister failure. MIC was assumed to occur at all times, despite arguments

that the onset of microbial activity at the canister surface will be delayed for some time

because of the inhospitable near-field environment (Lloyd et al. 2004, King 2009). The

extent of MIC damage was estimated on the basis of a multiple of the general corrosion

rate of a factor of between 1 and 2, based on laboratory observations. Although SCC

was considered possible if the canister shell experienced extensive residual stress due

to rock loading, it was argued that it would not result in canister “failure” as the

tortuous, corrosion-product-filled crack would not provide a pathway for

radionuclides to exit the canister. The 20-mm-thick canisters were predicted to have

lifetimes of >500,000 years (DOE 2008).


15

E.6 Critical conditions

Table E.2 summarises the sensitivity of Ni alloys to the various environmental

conditions discussed above. There are no truly “critical” conditions for which the use

of a Ni alloy canister would not be recommended. However, there are a number of

conditions that could lead to issues, including:

o Sulphur species – Ni alloys are susceptible to de-passivation, enhanced general

corrosion, SCC, and localised corrosion in the presence of H2S, elemental S,

and/or oxysulphur species. The key question is whether the concentration or

flux of S species at the canister surface is high enough to cause adverse effects.

This, in turn, is dependent on the repository design as much as the nature of the

host rock.

o Chloride ions - Cl- will induce the localised corrosion of Ni alloys. The

threshold Cl- concentration is alloy dependent.

o Lead – Ni alloys are susceptible to SCC in the presence of lead. Lead is only

likely to be present in the system if it is used as a matrix material inside the

canister. In this event, the potential for common-mode-failure of the canisters

due to lead leaching from an initially defected canister would need to be

assessed.

16

Table E.1: List of Critical Conditions for HLW/SF Canisters M

anufactured from Nickel-based Alloys

Parameter

Critical

condition

Comment

Host rock

High sulphide

mineral content

A high sulphide mineral content in the host rock could induce de-passivation, enhanced general

corrosion, pitting, and SCC if H

2S or other S sulphur species contact the canister surface. It is not

possible to define an unacceptable sulphide mineral content since the canister susceptibility

depends on the interfacial concentration or flux of S species, which in turn is dependent on the

design of the GDF, in particular the nature of any backfill m

aterial.

Redox conditions

None

The passive film

on Ni-Fe-Cr-Mo and Ni-Cr-Mo alloys provides equal protection under aerobic and

anaerobic conditions.

Tem

perature

>250oC

>150oC

Thermal aging effects would require a sustained tem

perature of >250oC to induce m

icrostructural

changes in Alloy 22. The corresponding tem

perature and/or time for other Ni alloys is not known.

There is an increased susceptibility to SCC at temperatures greater than 150oC.

Gamma radiation

>1-10 Gy/h

In aggressive ‘Q-brine’ solutions at 90oC, an increase in the rate of general corrosion and a

susceptibility to pitting is observed at an absorbed dose rate in the range 1-10 Gy/h. In less-

aggressive environments, the threshold dose rate is likely to be significantly higher.


17

Backfill m

aterial and

near-field m

ass

transport

None

There is no apparent benefit to a cem

entitious backfill or overpack, but it is unlikely to be

detrimental to the canister perform

ance. In an un-backfilled GDF, the canister would need to be

protected from S species.

Chloride

concentration

Alloy

dependent

Under sufficiently aggressive conditions, m

ost N

i alloys are susceptible to localised corrosion in Cl-

environments. The combination of Cl- concentration, temperature, and electrochem

ical potential

determine whether localised corrosion will initiate. The threshold Cl- concentration is alloy

dependent.

Other ground water

species

None

(sensitivity to

Pb)

There are no adverse effects of other groundwater species. Species such as nitrate, sulphate, and

carbonate can inhibit the effects of Cl- on the localised corrosion of Ni alloys. N

i alloys are

susceptible to SCC in the presence of Pb which, although not a ground water species, could be

present if Pb is used as a matrix m

aterial.

Sulphur species

H2S, elemental

S,

Sulphur species can lead to de-passivation, enhanced general corrosion, SCC, and localised

corrosion of Ni alloys. T

he im

portant factor is the concentration or flux of S species at the canister

surface.

Microbial activity

Minim

al

The passive film

on Ni-Fe-Cr-Mo and Ni-Cr-Mo alloys appears to be relatively resistant to M

IC,

although some effect m

ay be observed if microbial activity occurs at the canister surface.

Residual stress and

external load

None

Ni alloys are susceptible to SCC under severe environmental conditions. It is accepted that a tensile

stress sufficient to cause SCC m

ay exist on the canister surface at some stage.

GDF saturation tim

e None

Ni alloys exhibit the same corrosion resistance under unsaturated conditions as in bulk solution.

18

E.7 Advantages and disadvantages of nickel alloys

as a canister material

The advantages of a Ni alloy canister include:

o Excellent corrosion resistance in the passive state, offering the possibility of

very long canister lifetimes.

o Resistance to SCC, except at elevated temperature or in the presence of lead or

sulphur species.

o Good industrial experience with joining and inspection of Ni alloys, aided by

the likely thin-wall design.

o Minimal impact on other barriers.

o Resistance to MIC.

o Ni containers are suitable for use with bentonite or cementitious backfill or in

un-backfilled GDF designs.

The disadvantages of a Ni alloy canister include:

o The need for an internal structural element, thus complicating the canister

design.

o The need to make long-term predictions of the passive corrosion behaviour

and/or localised corrosion.

o Susceptibility to localised corrosion in Cl- environments, but this can be

minimised through proper alloy selection.

o Little international experience in design and licensing of a Ni alloy canister.


19

References for Appendix E

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.

Brossia, S., L. Browning, D.S. Dunn, O.C. Moghissi, O. Pensado, and L. Yang. 2001.

Effect of environment on the corrosion of waste package and drip shield

materials. Center for Nuclear Waste Regulatory Analysis Report, CNWRA

2001-03.

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Nickel Based Alloys