ATMOSPHERIC-INDUCED STRESS CORROSION CRACKING OF AUSTENITIC STAINLESS STEELS UNDER LIMITED CHLORIDE...

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International Corrosion Congress 2011 Paper ### -

ATMOSPHERIC-INDUCED STRESS CORROSION CRACKING OF AUSTENITIC STAINLESS

STEELS UNDER LIMITED CHLORIDE SUPPLY

Anthony Cook1, Nicholas Stevens1, Jonathan Duff1, Ayuba Mishelia, Tat Suen Leung1, Stuart Lyon1, James Marrow2, Wayne Ganther3 and Ivan Cole3

1The University of Manchester, UK, 2The University of Oxford, UK,

3CSIRO, Melbourne

SUMMARY: The occurrence of atmospheric-induced chloride stress corrosion cracking in types 304L

and 316L stainless steels contacted with sea-salt and magnesium chloride deposits has been studied under

controlled conditions of relative humidity (ca. 30%) and temperature (80°C). The time to cracking and

extent of cracking are shown, at least semi-quantitatively, for specimens contacted with sea-salt deposits

to be related to the deposit size prior to environmental exposure with each of these quantities appearing to

decrease with decreasing deposit size. For specimens contacted with MgCl2 these quantities correlate with

the nominal deposition density decreasing with decreasing deposition density. Specimens of these

stainless steels, U-bends and pieces taken from an intermediate level waste container, were also exposed

to a marine environment in sheltered and open exposures. Pitting and oxide build up was observed on

these specimens after ca. 5 months with greater pitting occurring on the specimens exposed under the

shelter. No cracking was observed in the atmospheric exposures.

Keywords: Austenitic Stainless Steels, Atmospheric-Induced Chloride Stress Corrosion Cracking

(AISCC), Chloride deposition density. Aerosol deposition

1. INTRODUCTION

The UK concept for the disposal of intermediate level nuclear waste (ILW) involves interim storage of cement-encapsulated

wasteforms within 500L drums fabricated from 304L and 316L austenitic stainless steels (ASS) for an as yet unspecified

period in above ground vaults, probably several decades, followed by transfer to underground disposal facilities where they

will housed for many more years prior to burial in cement back-filled repositories [1]. UK ILW containers are currently located

in both coastal and rural areas [2] and clearly during above ground storage, and for that matter during below ground storage

prior to burial, the outer surfaces of drums housed close to marine environments will very probably be exposed to sea-salt

aerosol. The choice of container materials was based mainly on their excellent resistance towards both corrosion in

atmospheric conditions and the alkaline conditions encountered during exposure to cementitious environments [1]. Such

materials are, however, susceptible to localized corrosion and associated phenomena such as low temperature stress corrosion

cracking (SCC) under chloride-containing deposits [3-9]; a phenomenon we will refer to here as atmospheric-induced chloride

SCC (AISCC). With regards to AISCC perhaps the most famous study is that of, Shoji and Ohnaka [3] who reported the

susceptibility of these alloys to at both ambient temperature and above on exposure to wetted deposits of synthetic sea-water

and various other chloride salts over a range of relative humidity (RH). It is considered that the conditions necessary for

AISCC in waste containers are unlikely to develop during storage [1] but is nonetheless is deemed to be a possibility. In terms

of the development of AISCC the relevant variables are many including the material stress state, temperature, environmental

RH, the composition of salt-deposits and, it seems, their deposition density. Clearly, knowledge of the parameter space, with

regards to these quantities, over which AISCC may occur in 304L and 316L ASS is of great importance to the UK nuclear

industry.

An oft expressed criticism of laboratory work concerning AISCC is that it provides no real indication of the likelihood of

AISCC developing during container storage as it is perceived that investigations are usually performed under vastly more

aggressive conditions than are ever likely to be encountered in reality, i.e. the chloride deposition density, temperature and

tensile stresses used in many studies far exceed those likely to be met in practice. To be fair, however, with the exception of

working at realistic temperatures, tailoring experiments to the needs of the nuclear industry is difficult, as information

regarding the stress state of the drums and the environmental conditions in waste stores is limited. In most laboratory

investigations U-bend specimens are employed to provide the stress, magnesium chloride often employed as a surrogate for

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sea-salt and the chloride salt deposition density used varies considerably. For example in experiments involving MgCl2, Shoji

and Ohnaka [3] employed a deposition density of ca. 2500 μg cm-2

as Cl-, Prosek et al [8] used ca. 10 times that amount

whereas Fairweather et al [10] used much lower levels, albeit as a nominally uniform coverage, of either 1 or 10 μg cm-2

. In a

recent study to ascertain the effect of this parameter on the occurrence of AISCC under magnesium chloride salt-deposits on

U-bend specimens taken from the walls of an ILW waste container (316L), Albores-Silva et al [11] observed an apparent

threshold deposition density below which the phenomenon was not observed. No AISCC was seen below 10 μg cm-2

at 50°C

and 25 μg cm-2

at 30°C in tests that were conducted at 30% RH over a 12 week period; at 60% RH higher thresholds were

found. Phan [12] found that U-bend samples subtending magnesium chloride films at a nominal deposition density greater than

50 μg cm-2

displayed cracking at 40°C and 40% RH within 2800 hours (ca. 17 weeks) whereas below this value only pitting

was observed over a similar timescale; the extent of cracking was seen to decrease with increasing RH. Under cylindrical sea-

salt layers (ca. 3mm diameter) deposited from 5 μl of synthetic sea-water small cracks developed within 2500 hours at 40°C

and 40% RH; once again the extent of cracking decreased with increasing humidity. Such results are clearly encouraging as if

they can be established beyond doubt they may well provide the basis for a mitigation strategy based on the restricting the

amount of adhered chloride and controlling the environmental RH. However, all these tests were conducted over very short

timescales in comparison to those likely to be encountered during interim storage. Consequently the question as to whether a

threshold really exists or that these results simply reflect an increase in the time to cracking as the chloride deposition density

is reduced is still to be answered. Given that AISCC takes some time to initiate in 316L under relatively mild conditions,

ca.3500 hours at 40°C and 30% RH at a deposition density of 350 μg cm-2

as Cl- [13], experiments to answer this question

could take some time It would appear that the best way forward would be the development of some kind of accelerated test

method based on a sound mechanistic understanding of AISCC to generate some sort of predictive capability.

It is our contention that thresholds related to the salt deposition density may not prove an altogether reliable means of

predicting the tendency towards AISCC. The parameter simply represents the average amount of salt deposited per unit area of

the specimen and gives no real indication of a specimen‟s wetted area. In the laboratory the quantity can easily be related to

deposit size and the way in which the salt is distributed on any particular specimen via optical or electron microscopy but it is

difficult, however, to envisage its usefulness in the field unless correlation maybe made to the size and distribution of aerosol

deposits since many different combinations would give rise to the same value. Perhaps then the more relevant quantities are the

composition and dimensions of individual deposits and their proximity to others; i.e. are they, under the environmental RH and

temperature, of sufficient size to allow the development of the slow state of stable localized corrosion necessary for the

transition to AISCC and are they close enough to allow any cracks that do initiate to propagate and coalesce? Unless such

conditions are indeed realised it is unlikely that container integrity will be threatened. The behaviour of deposits on wetting

may also be important. For example is it possible that deposits that could not support localized corrosion if wetted individually

lie close enough to each other to merge on wetting to form a single wetted deposit that could? Furthermore would such a

deposit revert to the original array of its components on drying or remain as one large salt deposit. As a first step in addressing

such issues we have embarked on a systematic series of experiments to determine the way in which sea-salt segregates on the

surfaces of these alloys, the time to and extent of cracking under aerosol type deposits over a range of temperatures and deposit

size. Herein we present some recent results regarding the effects of deposit size prior to wetting on AISCC susceptibility at

80°C and 30% RH and, in addition, report on observations from outdoor exposures tests.

2. EXPERIMENTAL DETAILS

2.1 Materials

Type 304L and 316L austenitic stainless steels obtained from Goodfellows Metals Cambridge as 0.914 mm thick sheet was

used in this work. An analysis of their compositions is given in Table 1:

Table 1. Steel Composition (wt%)

C Si Mn P S Ni Cr Mo Al Cu Ni Ti Va Fe

304L 0.025 0.51 1.28 0.028 0.003 7.79 17.87 0.43 0.01 0.24 0.02 0.02 0.09 Balance

316L 0.022 0.47 1.32 0.032 0.003 9.67 16.43 2.18 0.01 0.45 0.03 0.01 0.09 Balance

U-bend specimens were prepared according to ASTM G30-97 [14] with bending radius 9 mm from specimens of length

120mm and width 15mm. Prior to fabrication the metal strips were abraded to 1200 grit with silicon carbide paper and

thoroughly degreased ultrasonically in acetone and then ethanol to remove surface contamination.

2.2 Laboratory exposures

2.2.1 Salt deposition and exposure methods

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Prior to exposure deposits of sea-salt or MgCl2 were applied to the domed outer surface of the U-bends from evaporation at

room temperature of droplets applied via a fine mist of salt solution delivered from a spray bottle; one spray from a distance of

ca. 5 cm was applied to each specimen. Six different solutions, including diluted synthetic sea water solutions, were used for

spray deposition and hence six deposition regimes were investigated for both materials; the deposition regime identifier,

solution identifier and solution composition for each are provided in table 2. Diluting the sea water was intended to produce

salt crystallites of varying size and spacing to produce discontinuous moisture layers that allow the effect of deposit size on

AISCC to be examined. The ultimate goal of the work was to develop a procedure to artificially generate deposits of a similar

size to those produced as a consequence of natural aerosol deposition, hence producing the same type of moisture layers that

occur in practice. The nominal weight of salt delivered via spray deposition was estimated by weighing the total amount of dry

salt accumulated on coupons of 304L or 316L from a known number of spray deposits. Exposure of chloride laden U-bend

specimens at 80°C and 60°C was performed by placing specimens in a air-tight container containing a reservoir of saturated

MgCl2 to maintain the required environmental RH, nominally 30 and 26.5%, respectively, within ovens. Exposure of U-bend

specimens and the tensile specimen at 40°C was carried out in a Binder KBF 240 environmental chamber programmed to

maintain a RH of 33%.

Table 2: Deposition regimes employed in laboratory exposure tests

Deposition regime Deposition Solution Solution composition

1 A sea water

2 B sea water diluted x10

3 C sea water diluted x100

4 D 0.3 mol dm-3

MgCl2

5 E 0.03 mol dm-3

MgCl2

6 F 0.003 mol dm-3

MgCl2

2.2.1 Optical and electron microscopy of laboratory exposed specimens

Imaging of salt deposits on the U-bend specimens was performed using an Olympus SZH Stereo Zoom Microscope. During

imaging specimens were removed from the environment for no more than 30 minutes. Dry sea-salt deposits on coupons of

304L were imaged and analysed using a Zeiss Evo 60 Environmental SEM (EP-SEM) operating in extended pressure mode

fitted with a hot/cold stage which allowed wetting of the samples from 0% humidity to 100% humidity. Deposit composition

was measured by Energy Dispersive X-Ray Spectroscopy (EDX) with the Ka peaks being used for elemental detection and

concentration.

2.2.2 Parameter definitions

To aid quantification of our results, namely the average maximum crack length, crack frequency and failure frequency. These

are defined are defined as follows:

2.3 Field exposures

2.3.1 Exposure and specimens

A range of specimens were exposed at the Flinders exposure site in Southern Victoria, Australia. The flinders sight is a severe

marine sight with full details of the sight may be found elsewhere [15]. Specimens were exposed under a shelter (which

prevents rain from impacting on the specimens) and in fully open conditions. Two types of specimens were used – those cut

from a nuclear storage container defined as flat barrel pieces (dimensions 50 x 100mm), barrel piece with weld (dimensions

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100 x 100 mm) and barrel with rib (dimensions 100 x 100 mm) and U-bends of 304L and 316 L – dimensions as described in

section 2.2.

2.3.2 Microclimate measurement

A weather station was installed at the site with temperature (Vaisala sensors), RH (Vaisala sensors), Global Radiation, Rainfall

and an instrumented stainless steel plates. The instrumented plate was equipped with surface temperature and wetness sensors.

The surface temperature was measured using thermistors physically attached to the plate surface while wetness was measured

using a grid made of two gold electrodes arranged in an interdigitated grid. The backing of the grid is made of aluminum oxide

with good thermal conductivity, so that the grid follows the temperature of the plate. The grid wetness is determined from the

conductivity of the grid according to the system proposed by Norberg [16]. Further details of the sensors can be found

elsewhere [17].

2.3.3 SEM analysis and mapping

Scanning electron microscopy (SEM) imaging and mapping, andenergy-dispersive spectroscopy (EDS) were performed using

a Philips XL30 FESEM with a LinkISIS X-ray analysis system (Oxford Instruments). EDS was carried out with a beam current

of 5 kV. All samples were mounted using conductive carbon tape.

2.3.4 Optical Examination.

Specimens were examined using a bifocal optical microscope. Pit depths were measured by the standard variable focus

technique in which the distance to the focal plane of the specimen surface is compared with the distance to the focal plane at

the bottom of the pit. These measurements could only be undertaken on flat specimens. For each plate a minimum of 5 pits

were measured.

3. RESULTS AND DISCUSSION

3.1 Laboratory exposures

3.1.1 Wetting of dried synthetic sea-salt deposits

Figure 2 presents an SEM image of the various deposits formed on drying of a droplet on sea-salt on a coupon of 316L ASS

together with a series of elemental maps. It may be inferred from these maps that the larger crystalline particles are NaCl

crystals. MgCl2 forms around and in between these deposits in a seemingly amorphous state. It would also appear that any

calcium present exists as CaSO4 rather than CaCl2.

60 μm

MgNa

O

Cl

SCa

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Figure 2: SEM image of dried synthetic sea-salt together with elemental maps

On re-wetting the deposit by varying the environmental RH from 0 to 55% in increments of 5% with the ESEM contrast begins

to be lost in the areas around the NaCl crystals at an RH of ca. 35%, i.e. in the areas where the MgCl2 has deposited, see the

SEM micrographs of figure 3. This suggests that at the RH employed in our experiments corrosion in the presence of sea-salt

deposits will occur under wetted MgCl2.films.

A B

Figure 3: SEM micrographs of a dry sea-salt deposit (A) the same deposit wetted at 35%RH (B)

3.2.1 U-bend exposures

Figure 4 shows examples of typical dry salt deposits formed on U-bend specimens of 316L prior to their exposure. The optical

micrographs of figure 4A → C demonstrate clearly that the size of the sea-salt deposits formed on drying decreases with

decreasing sea-salt concentration in the solution from which they were applied. Deposits produced via contact with solution A

are on average several mm in diameter, those produced with solution B are on the order of a millimetre and contact with

solution C produces sub millimetre deposits. Spray deposits from the various MgCl2solutions, figure 4D → F, are too small to

be seen via conventional optical microscopy.

A B C

D E F

Figure 4: U-bend specimens with dry sea-salt (A → C) and magnesium chloride (D → F) deposits; the

letters on each of the photographs correspond to the identifier of the solution from which the salt was

spray-deposited (details of the composition of solutions A → F are provided in the experimental section

and table 2).

Table 3 summarises the results of a 1000 hour test in which the variously salt loaded specimens of 304L and 316L were

exposed to a temperature of 80°C at a RH set by nominally saturated MgCl2. According to the literature [18] the RH above this

solution at this temperature should be ca. 26%; in practice it was found to be ca. 32% indicating our MgCl2 solution is,

although clearly saturated at room temperature, not fully saturated at 80°C. The data in table 3 shows qualitatively that the

extent of cracking, for specimens laden with the sea-salt deposits, decreases with decreasing deposit size. For example the

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failure frequency of specimens with deposits applied from solutions A → C decrease from 1 for an average deposit size of a

few mm to 0.4 as the average deposit size is reduced to less than 1 mm with 304L and from 1 to 0 with 316L. For specimens

contacted with MgCl2 deposits this parameter decreases from 0.6 to 0.2 for 304L and from 0.4 to 0.2 for 316L for deposits

applied from solutions D → F. One may also conclude from an inspection of this data that sea-salt is more aggressive than

MgCl2 in terms of the occurrence of AISCC; such a conclusion would probably be an error, however, since the latter material

seems to deposit more evenly over the steel surface and hence it is likely that this apparent greater severity of AISCC in

specimens contacted with sea-salt is simply due to it producing clusters of more concentrated , in terms of the amount of

MgCl2 per unit area, wetted areas than those that develop with pure MgCl2. The concentration in moles per unit volume is, of

course, determined by the RH.

Table 3: Summary of observations after exposure of salt-loaded U-bends at 32% RH and 80°C at 1000 hours

Material Deposition regime Average nominal salt deposition

density (μg cm-2)

No. of Specimens

exposed

No. of

Failures

Cracked

but not

failed

Failure

frequency

304L 1 370 5 5 0 1

304L 2 35 5 4 1 0.8

304L 3 4 5 2 3 0.4

304L 4 39 5 3 2 0.6

304L 5 3.5 5 2 3 0.4

304L 6 0.41 5 1 3 0.2

316L 1 363 5 5 0 1

316L 2 34 8 6 2 0.75

316L 3 6 2 0 1 0

316L 4 33 5 2 1 0.4

316L 5 3.9 5 1 3 0.2

316L 6 0.41 5 1 4 0.2

Figures 5 and 6 graphically illustrates the crack frequency, failure frequency and average maximum crack length as a function

of exposure time for the test summarised in table 3. In compiling this data any specimens within any particular deposition

regime that had failed was assigned an average maximum crack length of 15 mm, the width of the specimen.

Brief inspection of of the data in figure 5 reveals that the specimens of 316L with salt layers derived via contact with solution

A and B, i.e. those with the largest average pre-exposure deposits display very similar profiles with regards to the time

dependence of the average maximum crack length; 316L with deposits produced via contact with the latter solution do

however display a longer induction time, somewhere between 7 and 14 days as opposed to sometime during the first seven

days. Once AISCC has initiated the initial rate of change in average maximum crack length is roughly similar at ca. 1.4 x 10-9

ms-1

. Specimens with deposits formed via contact with solution C (sub mm deposits) display the longest induction time (21

days) and a much lower initial rate of change in average crack length (1.78 x 10-10

ms-1

). The crack frequency reaches its

maximum value of 1 within 14 days for specimens with deposits applied from solution A and 21 days for those with deposits

applied via solution B. The parameter reaches a maximum of 0.5 for specimens with deposits applied from solution C. The

failure frequency reaches 1 only for those specimens with deposits derived from solution A. With regards to the behaviour of

316L contacted with MgCl2 the picture is a little different. The crack frequency reveals that 40% of the specimens with

deposits produced via contact with solution D (deposition density ca. 33 μg cm-2

) display cracking within seven days with all

specimens in the group having undergone complete failure by day 14. The initial rate of change in average maximum crack

length is ca. 1.2 x 10-9

ms-1

similar to that found in the presence of the largest sea water deposits. Specimens with deposits

derived from contact with solutions E (deposition density ca. 3.9 μg cm-2

) and F (deposition density ca. 0.41 μg cm-2

) display

the same induction time (21 days) with the former specimens reaching crack and failure frequencies of 0.4 of 0.2 and the latter

specimens 0.2 and 0 by the end of the testing period. By comparison with data produced with 304L, see later, it seems

surprising that the induction time does not progressively decrease with decreasing amounts of adhered chloride. This may be

due to small cracks being masked by corrosion product, the irreproducibility of the spray technique or the stochastic nature of

localized corrosion. Taken as a whole however the results do suggest that a decrease in the size of surface chloride deposits

does lead to a decrease in the severity of cracking in 316L contacted with both sea-salt and MgCl2. A caveat should be inserted

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here as we have not yet demonstrated conclusively that a discontinuous surface layer is formed on wetting our array of salt

particles and that this discontinuity increases as the salt particles decrease in size. Given however that the Cl- concentration of

any moisture layer that forms on our specimens will be determined solely by the RH then if a continuous layer does form on

decreasing the amount of adhered salt it would simply be a thinner layer with the same Cl- concentration. It would then seem

likely that a similar propensity towards cracking would be observed regardless of the deposition density. Consequently, we are

cautiously confident that changes in the severity of cracking are related to the changes in deposit size.

An examination of the data pertaining to the behaviour of 304L towards the different various exposure regimes presented

graphically in figure 6 reveals similar trends to those observed with 316L. Specimens with deposits from the sea salt solutions

(A & B) develop cracks within seven days and display identical trends in crack frequency. It should be noted however that the

average maximum crack length at the seven day point is highest for specimens with deposits from solution A (4 as opposed to

2 mm) suggesting that the induction time is indeed a function of pre-exposure deposit size. Failure frequency of specimens

with deposits from solution A reach the maximum value in 14 days whereas none of those specimens with deposits from

solution B or C undergo complete failure. The initial rate of change in the average maximum crack length also correlates with

the strength of the solution used to deposit the sea-salt, decreasing from 2.3 x 10-9

ms-1

for specimens with deposits from

solution A to 4.6 x 10-10

for those with deposits from solution C. Specimens contacted with deposits from the MgCl2 solutions

(D → F) show a decrease in induction time with decreasing amounts of surface chloride, within 7 days, between 7 and 14 days,

and between 14 and 21 days for specimens with deposits from solution D, E and F, respectively. These specimens do however

display a similar value for the initial rate of change in average maximum crack growth rate at ca. 2 x 10-9

ms-1

. An interesting

feature in the average crack growth rate data is the spike seen in the data corresponding to the deposition regime involving

solution F at day 28 – this observation is related to fact that at this point in time only one specimen exposed under such

conditions had developed cracks; when the rest developed cracks they were smaller and hence produced a lower average value.

Crack frequency and failure frequency reaches its maximum value of 1 for specimens with deposits from solution D after 35

days and after 42 days for those with deposits from solution E. It reaches a maximum of 0.8 after 42 days for specimens with

deposits from solution F. As has been observed by others the extent of cracking under MgCl2 deposits seen here correlates with

the nominal deposition density. We believe however that the effect is more likely to be related to average deposit size and

increasing discontinuity in the moisture layer. Clearly demonstrating this belief will, for both alloys, require electron

microscopy to determine the average crystallite size

Similar exposures at 60 and 40°C are ongoing and have been running for 42 and 28 days respectively. No evidence of cracking

has yet been observed. Specimens in both data sets have displayed pits from ca. day14. These investigations will be reported

on in due course.

0 10 20 30 40 50

0

2

4

6

8

10

12

14

16

A

v m

axim

um

cra

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gth

/ m

m

A

B

C

Time / Days

Sea-salt deposits

0.0

0.2

0.4

0.6

0.8

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req

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MgCl2 deposits

Time / Days

0.0

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0.4

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0.8

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req

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ncy

0.0

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0.4

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0.8

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E

F

Fa

ilure

Fre

qu

en

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Figure 5: Average maximum crack length, crack frequency and failure frequency of 316L stainless steel in contact with wetted

sea-salt and magnesium chloride deposits at 30% RH and 80°C as a function of exposure time; the letters in the figure legends

refer to the solution from which the salt was spray-deposited (details of the composition of solutions A → F are provided in

table 2).

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0 10 20 30 40 50

0

2

4

6

8

10

12

14

16

A

v m

axim

um

cra

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/ m

m

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Sea-salt deposits

Time / Days

0.0

0.2

0.4

0.6

0.8

1.0 A

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req

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/ m

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MgCl2 deposits

Time / Days

0.0

0.2

0.4

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req

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0.0

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0.8

1.0 D

E

F

Fa

ilure

Fre

qu

en

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Figure 6: Average maximum crack length, crack frequency and failure frequency of 304L stainless steel in contact with wetted

sea-salt and magnesium chloride deposits at 32% RH and 80°C as a function of exposure time; the letters in the figure legends

refer to the solution from which the salt was spray-deposited (details of the composition of solutions A → F are provided in

table 2).

3.2 Field Exposures

After 5 months of exposure all specimens showed evidence of pitting (an example is shown in a U-bend specimen in Figure 7).

For the 316 plate and the Flat Barrel specimens the optical focusing measurement could be used to make an estimate of pit

depth as given in Table 4. After 11 months a rust layer covered the specimens and thus the pits.

After 11 months an SEM examination of the surface of the flat barrel specimen NW2 was undertaken. SEM examination of

Openly exposed 316 plates should the formation on a continuous film which had two constituents – a metal rich phase which

always contained Fe, O, Si, Mo.Cr and sometimes also contained Na, S and Al and Si and a oxygen rich phase which contained

O, Fe, Na, Cl, Ni, S, Cr Mg and C. In addition occasional NaCl crystals were observed on top of this film (as can be seen in

Figure 8).

Table 4: Average and maximum pit depth after 5 months exposure

Specimen Type No Exp Av Max

316 plate S648 Openly exposed 2.5 4

316 plate S643 Openly Exposed 3.2 6

316 plate S641 Openly Exposed 3.8 6

316 plate S642 Sheltered 33 48

Flat Barrel NW1 Sheltered 15 27

Flat Barrel NW2 Sheltered 16 27

Flat Barrel NW3 Open 16 17

Flat Barrel NW4 Open 17 18

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The wetting of the gold grid wetness sensor placed on the sheltered exposure rack was also analysed for the period from

23/6/2010 to 25/11/2010. Wetting first occurred on at 25/6/2010, at 5.15 am when the ambient RH was 75.7 % and the surface

RH was 79.4 %. There after surface remains wet except for isolated drying incidence listed in table 5 below. The maximum

wetness reading is 284 (arbitrary units ) which corresponds to a fully wet sensor so that normalised minimum wetness is the

raw minimized wetness value divided by the maximum wetness multiplied by 100. Drying events are recorded if the

normalised wetness falls 50% and is defined as partial if normalised wetness is between 50 and 31% or full if it is less than

31%. The minimum surface RH is the RH when wetness is at a minimum. Although wetness does vary with RH at a certain

humidity it drops sharply - this is defined as surface RH rapid decline.

Table 5: Conditions in which the wetness sensor under the shelter commences to dry.

Date /time Minimum Wetness

(Raw)

Minimum

Wetness

Normalized

Minimum

RH

Minimum

Surface RH

Surface RH

Rapid Delinee

2/8/2010-10.45 P 132 46 64 56 56

16/8/2010-

15.15

P 124 44 56 50 53

14/9/2010-

15.15

f 65 23 44 22 26

4/11/2010-

12.15

f 85 30 41 22 29

16/11/2010-

12.45

f 29 10 47 34 36

17/11/2010-

18.00

f 17 6 54 24 31

19/11/2010-

12.30

f 46 16 46 26 36

23/11/2010-

12.45

F 46 16 31 26 33

The field analysis indicates that pitting occurs relatively quickly on all specimens with pit growth being faster in specimens

sheltered from rain compared to those openly exposed. The analysis of the data from the wetness sensor is consistent with the

initial formation of a moisture layer, wetting being promoted by the hygroscopic wetting of NaCl but the fact that “full” drying

of the sensor only occurs when the surface RH falls below 36% (or lower) implies that significant MgCl salts build up on the

surface. SEM analysis of the surface product also indicates the presence of Mg, Na and Cl indicating that MgCl and NaCl may

build up on the specimen surfaces.

Figure 7: U-bend specimen (304L) after 5 months in the sheltered Exposure

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Figure 8: Salt crystals sitting on top of Metal rich film in a flat barrell

specimen after 11 months of sheltered exposure

4. CONCLUSIONS

The induction time and severity of AISCC in U-bend specimens of 316L and 304L ASS contacted with sea-salt deposits and

exposed to moist air at 80°C and 32% RH are functions of the pre-exposure deposit size. Both parameters decrease with

decreasing deposit size. Such observations suggests that knowledge of size and particle distribution over a surface would

provide a better indication of the likelihood the development of AISCC than deposition density in cases where clusters of

discrete particles form, such as during natural aerosol deposition. That is, it is likely that there exists a critical deposition size

below which AISCC cannot initiate rather than a critical deposition density.

Salt deposits applied to 304L and 316L from spray deposition of MgCl2 solutions produces particles too small to be viewed via

conventional optical microscopy so the effect of deposit size on induction time and other parameters has not been quantified;

the induction time and severity of cracking does scale with the amount of adhered chloride however. Whether the observed

decrease in induction time and the severity of cracking as the amount of surface chloride is reduced are related to a change in

the continuity of the electrolyte layer as the deposit size changes remains to be seen.

Initial results from the field exposures indicate that pitting occurs within 5 months of exposure and after 11 months of exposure

only pits, no evidence of AISCC, have been found in the U-bend specimens. Wetness sensors and SEM examination indicate

that both MgCl2 and NaCl build up on the surfaces.

5. ACKNOWLEDGEMENTS

This work was performed as part of the DIAMOND („Decommissioning, Immobilization and Management of Nuclear Wastes

for Disposal‟) consortium project. We are therefore grateful to EPSRC for the funding of Tony Cook under grant number

EP/F055412/1.

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2011

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