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An in-line diffuse reflection spectroscopy study of the oxidation of stainless steel under boiling water reactor conditions
CorrosionScience,Vol. 38. No. 10,pp. 1763-l782, 1996 Copyright 0 1996Elsevier Science Ltd Printed in Great Britain. All rights reserved 001&938X/96 $15.00+0.00
PII: SOOlO-938X(96)00078-9
AN IN-LINE DIFFUSE REFLECTION SPECTROSCOPY STUDY OF THE OXIDATION OF STAINLESS STEEL UNDER BOILING WATER REACTOR CONDITIONS C. DEGUELDRE,*
S. O’PREYT
and W. FRANCIONIS
*Paul Scherrer Institute, Villigen-PSI CH-5235, Switzerland tQueen’s University, Belfast, Northern Ireland fTecova, Wohlen, Switzerland Abstract-A
novel cell unit was constructed to measure in-line the oxide layer build-up on a stainless steel sample by Diffuse Reflection Spectroscopy (DRS; ultraviolet, visible, near infrared) under boiling water reactor (BWR) conditions. The stainless steel samples, observed in the cell through a sapphire window, are contacted with oxidising hot water (3OO”C,9.0 MPa). Using a cold finger with the optical fibre probe, the spectroscopic investigations (2001000 nm wavelength) were performed at a fixed position from the sapphire window. The DRS spectra are the result of the coupling of both absorption (chemical) and interferometric (physical) processes. Analysis of these spectral components allows the independent determination ofthe oxide layer thickness. The build-up of the oxide layer may be directly observed and quantified, nanometre by nanometre, from 2 to 200 nm. This powerful technique may be used to study the early corrosion rates ofstainless steel under BWR conditions and should allow the development of a strategy to reduce corrosion. Copyright 0 1996 Elsevier Science Ltd Ke~~~~ords: A. stainless steel, B. in-line diffuse reflection spectroscopy,
C. reactor
conditions.
INTRODUCTION In boiling water reactors (BWR) the monitoring of the corrosion of stainless steel components is of great interest since the corrosion products are released into the reactor water. The possible activation of the corrosion products in the BWR and the fact that the release of corrosion products depends on the oxide layer build-up makes close monitoring of the surface oxidation important. Among these corrosion products 6oCo is particularly significant because about 80% .of the dose received by plant personnel results from its presence in the reactor.lm3 Many studies have been made of stainless steel corrosion under hydrothermal conditions, however no attempt has been made to measure the build-up of the sub-micrometre oxide layers formed in situ. The objective of this study is to use Diffuse Reflection Spectroscopy (DRQ4 to measure the growth rate of the oxide layer on the stainless steel under boiling water reactor conditions. Spectroscopy can be performed in a transmission or a reflection mode either by in-line or off-line investigation. The technique of Diffuse Reflection Spectroscopy has been used successfully in many fields and is often useful where traditional spectroscopic techniques fail. It has been applied for example in the study of rough pitted surfaces,5 as a non-invasive technique for blister-packaging contents identification,6 in the analysis of rust on weathering steels,’ in the in situ study of the reaction of SO2 and NO2 on pure zincite* and Manuscript received 16 January 1996; in revised form 7 March 1996. 1763
1764
C. Degueldre
et al.
in the observation of iron oxide reflectivity.’ Its main advantage lies in the fact that many substances in their natural state (e.g. powders and rough surface solids) exhibit diffuse reflection. This means that the light is scattered in all directions with the angle of the reflected light beams not always equalling the angle of the incidence beam. DRS may therefore be used as a good analytical tool in the study of the properties of thin layers or surfaces of materials in situ, since in many cases samples do not need to be specially prepared. Although it is true, that in practice diffuse reflection spectra are complex and are strongly dependent on the conditions under which they are obtained, the great advantage of DRS is its non-invasive quality which is essential in many applications.” DRS is useful as a rapid survey technique locating the main regions of absorption and providing information on the energy, width and intensity of absorption bands within a material.” The build-up of the oxide layer is a multi-step process, the first period being the formation of a transpassive oxide layer on the clean metal surface. The result is a glassy, ceramic-like oxide displaying a surface comparable to that of the metal. In the second period the surface characteristics change, there is a reaction within the oxide layer and recrystallisation takes place yielding a coarse, matt surface. Since the oxide layer formed in the first period strongly influences the subsequent corrosion process, high priority has to be given to corrosion studies of the earliest possible stages. This study focuses on the build-up of the oxide layer in the first period. The observation is performed using an optical fibre light carrier and collector to illuminate through a sapphire window the steel sample in contact with the water at about 300°C. THEORETICAL
BACKGROUND
Analytical approach
In spectroscopy, since the light intensity is of extreme importance, it must be considered that the light beam in the studied systems is not straight but cone shaped (Fig. 1). In reflection mode, the loss of intensity is dependent on the distance of the probe tip (light source and collector) from the sample surface. The probe must be placed at the minimum possible distance from the observation window. It must not, however, be in contact with it as it cannot withstand the temperatures to which the sapphire is raised. The detected intensity (Z), with perfect sample reflectivity, is given by the equation:
(1) where 1, is the incident intensity, S is the base area of the emitted light cone and So is the area detected by the probe. It is clear that only a restricted proportion of the reflected light is collected by the sensor. A reference spectrum is taken before oxidation, the small percentage of the total light reflected and detected is taken as equivalent to the incident light intensity. It is assumed that the initial polished surface is perfectly reflecting. Therefore, at the beginning, the detected light intensity must equal to the incident light intensity. Since the detected light intensity is inversely proportional to the square of the distance from the probe tip to the metal surface, it is essential that this distance is as small as possible and kept constant throughout the measurement period. The cone shaped beam is transmitted into air at the probe tip (radius rP) with an angle i,,,,. The geometry of the cell is such that the sample (object) is only visible through a small window with an external area (external halo). Refraction in the sapphire and in the water
Oxidation of stainless steel under BWR conditions
1765
20 Fig. 1. Geometric effect on the light intensity in reflection mode.
results in the angle of the cone (&) which reaches the sample being modified. The cone of light from the probe with an angle i,,, illuminates at a distance d the area of the object and causes the external halo. Therefore, a large portion of the light from the probe does not reach the sample. Instead it is reflected straight back or lost (Fig. 1). The fraction of light reaching the sample is given by the equation: AR
=
WJ
R,b = (rp + d. R rot
tanioh)* . it
(rp + d. tan &)* . TC
(2)
For the cell used (with d and rp of some mm, and ioh and i,,, of some degrees of angle) only a fraction (e.g. AR,,,) of the light from the probe passes through the window to reach the sample. The rest is lost or reflected straight back to the probe. The amount of light which does not reach the sample and is not detected by the probe is considered in the reference spectrum. This means that the recorded spectra show a smaller fraction than the predicted fraction (e.g. 1 -AR,,,) of the reflected light detected by the probe. When dealing with the transition of light from one medium to another, refraction at each interface must be considered. Meanwhile, during oxidation, the chemical properties of the surface change and the light absorption must then be taken into consideration. Transmission
through the oxide film
In the single beam mode, reference beam and reflected beam are measured separately. The intensity of the incident beam is first recorded by taking a reference spectrum over the wavelength range of interest with the unoxidized polished sample. The spectra are then analysed in comparison to the first spectrum recorded, before any oxidation. Its relative reflectivity AR,,,.,),,, is 1 (see Fig. 2(a)). Although the polished metal surface is not perfect, it is of a quality such that the light illuminating the sample is reflected to a high degree. With an oxide layer on the metal the reflected light is reduced because some of the light is absorbed in the film (see Fig. 2(b)).
1166
C. Degueldre
et al.
Initial
1 I
ewll)
= A&wlt)= 1
Wwd
~(w,o) = A&v) = 0
IO
b. Metal / oxide / water
a. Metal I water
c. Absorption in transmission water/oxide/water Fig. 2.
Reflection
and transmission
models.
Materials can display different types of behaviour in various parts of the spectrum; often being strongly absorbing in the near ultraviolet, and weakly absorbing in the near infrared. This is illustrated by magnetite (FesOa) which absorbs more in the blue than in the red hence its brown colouring. The reflection spectra were not analysed based on the Kubleka Munk equation4 because the layer is thin enough (no infinite depth) and the oxide is a weak enough absorber that the light penetrates to the metal surface where it is totally reflected. It is assumed that there is no reflection either at the water/oxide or oxide/water interfaces of the transmitted beam (ARC,.,,, = 0) or of the reflected beam (ARC,+,, = 0) and that any loss in reflection is due only to absorption within the oxide layer (Fig. 2(b)). As previously mentioned,’ the spectrum resembles the transmission spectrum of a thin slice of the same material (Fig. 2(c)). In DRS mode, twice the thickness is taken because the light passes through the oxide layer twice. Beers law is used to calculate the thickness: I
-=e
-2.@(A)..Y
&I) = absorption coefficient, dependent on the wavelength; IO = incident light intensity. I = resultant light intensity and The reflectivity or fraction of light reflected (R)is given by:
where x = thickness of oxide layer;
R = iorR%
= R '100%
Oxidation of stainless steel under BWR conditions
Considering oxide layer,
that 100% of light is reflected from the electropolished R = e-2940X
1767
sample with no (5)
the oxide layer thickness may be calculated from the reflection at each wavelength, using a realistic absorption coefficient. Reflection and interferometry by the oxide film When considering the passage of light between different materials it is necessary to take into account the reflection at each interface. The fraction of light reflected is given by the equation:
(6) The value of the refractive index (n) is relative to the two contacting materials and is given by:
where n(,i,,) is the refractive index of the medium (1) relative to vacuum for the incident light. The selected refractive index for the oxide layer is neither that for magnetite nor that for hematite, because the oxide layer is not a pure oxide. So if there is an oxide layer on the metal, part of the light does not ‘enter the layer but is instead reflected at the oxide surface (AR,,,,,, ZO). The reflections from the air/sapphire and the sapphire/water interfaces do remain constant throughout the measurement period, but the reflection from the oxide changes depending on the roughness of the layer which may increase with the thickness (ageing). When dealing with the reflection of light from a thin film on a reflecting material there is interference of the reflected light waves. This is due to partial reflection at the two interfaces. Selected wavelengths disappear from the spectrum when destructive interference occurs. Typically, the oxide layer on the metal at certain thicknesses does not show the expected red-brown but instead another colour (e.g. lilac, blue, etc.). The equation’* that determines the resulting intensity (I) at each wavelength is:
(8) where I is the wavelength of the reflected light. The path difference (6) is obtained by simple geometry. In the measurement cell the light is not a collimated beam normal to the surface but is instead a cone of light, therefore the calculation of the path difference (Fig. 3) is given by: 6 = 2 . IJ . q,,) - IH . n(,,,) IJ=JK=z ~0s qo)
Pa> Pb)
C. Degueldre
1768
et al.
Metal Fig. 3.
Interference
due to reflection
by a plane parallel
plate
ZH = ZK . cos i(,v)
(9c)
IK = 2 . x . tan Y(~)
(9d)
Equation (9a) becomes: 6 = 2 . x . rl(,,“)
1
[ cos Y(o)
cos it,.) . tan r(0) n(,, It’)
(10)
I
where i(,,., is the angle of light incidence (Fig. 3) which is some degrees of angle. / is around 1. Under these circumstances equation Since the angles are very small, cos icM., (10) can be said to be equal to: 6 = 2 . iq,,“) . x
.?7 .rz(,,“) 1 .x11
(11)
Combining equations (8) and (11) for interfering waves: 4
1 + cos
I = lo .
[
(
(12)
Equation (12) may be used to calculate the interferometric effects. The reflection spectrum provides a simple mean (peak) to determine the optical thickness of the film. The most reflected wavelength (A,,,) corresponds to twice the refractive index of the film multiplied by the film thickness: x=-
E,max 2 . qo,“)
(13)
As the oxide layer gets thicker there is a spectral shift, the reflection peak moves to higher wavelengths. The peak is related to the oxide layer thickness by equation (13).
1769
Oxidation of stainless steel under BWR conditions
Other effects Theejktofgeometry.
The effect of the cell geometry, with total and partial light intensity reflected from the object was treated earlier. Equation (2) describes the amount of light which does not pass through the window to reach the sample. The rest of the detected light which has not reached the sample is due to reflection at the air/sapphire and sapphire/water interfaces. So the total light which is detected but has not reached the sample is given by: (14) The efict of refractive index n(2). In all materials, there is a certain dependence of the refractive index with the wavelength of the incident light. This effect is supposed to be negligible in this work. The eflect of roughness. It is known that the oxide layer formation causes the relatively smooth surface of the electropolished stainless steel to become rough, that is the oxidation is more rapid in some areas than in others. This is believed to be due to pores in the metal surface, between the metal crystals, in which the oxidation is faster.’ On this rough surface, the light beam reflection passes from a direct reflection to a diffuse reflection mode. Clearly to investigate these imperfections the wavelength of the light used must be sufficiently short to be affected by this fine structure. At a certain point, because the wavelength of the light is too short the fine structure of the layer will not be recognised and the oxide layer will appear less deep than indicated by longer wavelengths. It is assumed that the effect of the roughness (p) depends linearly on both the thickness of the oxide layer and the wavelength of the incident light, and it varies between 1 and 0. The suggested equation to simulate the effect of roughness on the reflection intensity is the following:
p= 1 -a.x+b.x.h.
(15)
Combining al/processes
The equation of the spectra due to absorption and interferometry R =
only is:
[(I - LIR(,~,,J)- e-2’x’p] + dR(,,,,) .
(16)
The first term refers to light not reflected at the oxide layer surface. Light passes through the film and is subject to absorption in the oxide. Since the angle i,,, is small, the pathway is 2 . x. The second term corresponds to the fraction of light which is reflected and which is subject to the interferometric effect. The complete reflection spectra finally consists of: Light reaching the sample + Light detected which has not reached the sample The amount of light detected by the sensor which has not reached the sample is given by RG (equation (14)).
Of the light reaching the sample, some is reflected at the oxide layer surface resulting in the interferometry effect, the rest enters the oxide and is subject to absorption and reflection. The amount of light reflected at the oxide layer surface depends also on its roughness (equation (15)). The equation for the spectra becomes:
[( 1-
p +dR(,.,,$ . e-2’-Vw]+ P
.A&,.,) . +(1 -RG) IL >I1 4~ll.n(,“)~x
(17)
1770
C. Degueldre
EXPERIMENTAL
et al.
METHOD
Sample preparation and analytical background Sample coupons (2 .2 . 0.15 cm, 4.4 g) of electropolished
AISI 3 16.PI stainless steel (Fe 0.68, Cr 0.17, Ni 0.10, MO 0.02, Co 0.001) were exposed to BWR conditions in a corrosion test loop. The water of the loop was of Q-millipore quality at high temperature (e.g. of 290°C) and under a pressure of 9.0 MPa. The samples were contacted for different time periods with loop water. The loop water was produced by a Millipore unit. This water was injected after polishing and oxygen addition by bulling adequate Ar/02 mixture in the water. The loop water was monitored on-line for oxygen and locally hydrogen concentrations utilising Orbisphere units. In addition, conductivity was measured using an Endress & Hauser unit. Off-line analysis was carried out routinely by inductively coupled plasma atomic emission spectroscopy (ICP-AES, ARL, Applied Research Laboratory) and ion chromatography (IC, Dionex). The [O,] was 200 + 100 ppb and the hydrogen concentration was < 1 ppb. The conductivity was always < 0.1 ,US. cm-’ . All element concentrations measured by ICP-AES (e.g. Na, K, Ca, Mg,...) were smaller than 1 ppb and the anion concentrations measured by IC (i.e. F-, Cl-, NOs-, S04’-) were also smaller than 1 ppb. DRS spectra were obtained using a Guided Wave Model 260 Spectrum Analyser for the ultraviolet, visible and near infrared regions (200-1000nm) with an incident beam of a diameter of about 2 mm. The measurements are performed by projecting a slightly conical (emission angle 9”) light beam perpendicularly on the specimen surface and collecting all the diffuse scattered light within a 2.5 x 10e3 rad solid angle. The detection is made by the same probe which emits the light. The fibre optic spectrophotometer unit performs five distinct functions: light (source) generation, light modification (by fibre, probes and sample), monochromation (breaking the light down into its chromatic (spectral) components), detection and conversion, and input/output to the computer. A deuterium lamp provides the light source for the ultraviolet portion of the spectrum, and a tungsten-halogen lamp provides the light source for spectrophotometric applications in the visible and near infrared range. Since the light intensity is of extreme importance, it should be remembered that the light from the probe is not a straight beam, instead it is cone shaped (Fig. 1). There is a loss of intensity dependent on the distance of the probe tip from the sample surface. The probe must be placed at the minimum distance possible from the sapphire window, but not in contact with it as it cannot withstand the temperatures to which the sapphire is raised. It is clear that a restricted proportion of the reflected light is collected by the sensor (see equation (1) and Fig. 1). However, a reference spectrum is taken before oxidation, and the small percentage of the total light reflected was taken as equivalent to the incident light intensity. It was assumed that the initial polished surface was perfectly reflecting. Therefore at the beginning, the detected light intensity is equal to the incident light intensity. Since the detected light intensity is inversely proportional to the square of the distance from the probe tip to the metal surface, it is essential that this distance is made as small as possible and kept constant throughout the measurement period. The beam is cone shaped due to the fact that the light beam is transmitted down the optical fibre by a process of multi-reflection. When the light moves into air at the probe tip, radius (vp), the angle is further increased by refraction. The geometry of the cell is such that the sample is only visible through the 6 mm diameter window. With refraction in the sapphire and in the water the angle of the cone (&) which reaches the sample is between 6.5”
1771
Oxidation of stainless steel under BWR conditions
and 6.7”, depending on the temperature of the water. The cone of light from the probe (with an angle (&,J of 9”) illuminates, at a distance (6) of 20 mm, an area of diameter 8.3 mm. Therefore a large portion of the light from the probe does not reach the sample but is reflected straight back or lost (Fig. 1). The fraction of light reaching the sample is given by equation (2). When d = 20 mm, rp = 1 mm, iob = 6.5” and i,,, = 9”, only 62% of the light from the probe passes through the window to reach the sample, the rest being lost or reflected straight back to the probe. The reference spectra are saved and used each time a reflection spectrum is taken of the oxidised sample. In this way, a picture of the oxide layer build-up specific to each particular sample is obtained. In the ultraviolet region (200-300nm), spectral fluctuations are detected. This is due to the absorption behaviour of the optical fibre in this part of the spectrum. It is less accurate but was retained for completeness. Off-line/in-line strategy In a previous study the sample coupons were placed in the loop with water at 290°C at a pressure of 9.0 MPa for periods from 100,470,2024 and 4700 h. The samples were allowed to cool, were removed from the loop and were placed in a cell similar to that shown in Fig. 4,
2mm +-* 6mm 4 ?
16mm
Probe Metal _.__
I
12 mm
Air
: .
_1 .. I
Smm
Sapphire
_.._3mm
mQ - water
Oxide layer Sample Sample holder 4
20 mm Fig. 4.
In-line experimental arrangement.
C. Degueldre
1772
et al.
with water at about 15°C flowing through the cell (plastic). A problem with this approach was that no reference spectra were taken before placing the samples in the loop. Instead one electropolished sample was used as reference for all measurements. This cell was used as reference unit during the in-line measurements, because the water temperature is too low for any significant corrosion. This reference was used to check the stability of the Spectrum Analyser over a long time period. For in-line tests, the sample coupons were treated in the same way as before for the offline measurements. They were subjected to the same BWR hot water conditions but they were mounted in the cell (Fig. 4) adapted in the loop for continuous spectroscopic observation. The probe was placed in a fixed mounting perpendicular to the sample surface at a constant distance (20 mm). It was also necessary to cool the probe by a water jacket, called a cold finger. Four sample coupons were monitored, three within the circulating hot water and one, a control, in static water at room temperature (off-line). This fourth sample was needed to provide information on the stability of the probe and the spectrometer system over a long recording time. The change in the spectra, both as the temperature of the system was raised and as it was reduced to room temperature, was also recorded to study the effect of temperature changes. EXPERIMENTAL
RESULTS
AND DISCUSSION
Modelling the DRS spectra The transmission approach. The absorption coefficients of magnetite (Fe304) as reported by Muret13 and Schlegel14 are given in Table 2 and in Fig. 5. They are used to evaluate the reflection spectra, since it is assumed that magnetite is most abundant in the oxide layer.r5 However, it must be remembered that other oxides may also be present, particularly nickel oxide, which affects the results, therefore the absorption coefficients of NiO are also given. Fe0 is of little influence in the visible and infrared regions of the spectrum. NiO is only important when the incident wavelength is less than 350nm. Applying equation (5) for absorption-reflection at varying thickness (x) using the coefficients available (Table 2) the absorption component in the DRS spectra can be calculated (Fig. 6). The interferometry approach. Since AR,,.,,, is not equal to zero (equation (6)) the interference must be taken into account. The effect of interface reflection may be calculated using the experimental refractive indices reported in Table 1. Examples are given in Table 3. The maximum intensity of the reflected light is dependent on the light reflected from the oxide layer. This is given in Table 3 and equation (6) for a perfectly flat layer. The variations in the reflected light cannot be higher but may be less as a result of destructive interference.25 The reflection spectra is made up of (1 -AR o,,,oi)r due to the total reflection at the metal surface unaffected by interference, plus the interferometry factor which is given by equation (10): Table
1. Refractive
Temperature 20°C 300°C
indices (n (n,.vJ of relevant
Air
1.oo’2 1.oo”
materials
(m) vs vacuum
(v)
Sapphire
Water
Magnetite
Hematite
1.77” 1.7722
1.33” 1.10”
2.4223 2.4223
2.9923 2.992’
Oxidation
of stainless
steel under BWR conditions
1773
. \. co
-Magnetite
Muret 19731
- - - - - - Magnetite [Schlegel et al. 19781
-I;-
Hematite [Balberg et al. 19781 - Nickel Oxide [Powell et al. 1969
-Iron
Oxide [Bowen et al. 19751
0
200 250 200 350 400~450
500
550 600
650
700 750
Boa 8!3l900
950 low
Wavelength /urn Fig. 5.
Comparison
R = (1 - Ww))
of the absorption
+ AR(,,,,
coefficients
of selected oxides.
4 . n . q,,,) . x
.
x
(18)
The spectra clearly display a succession of reflection maxima (Fig. 7). The equation for the first maximum is: h maxl
=
2 . qo.“) . x
(19)
The equation for the second maximum is: ;cmax2
= q,,)
.x
(20)
When dealing with the transition of light from one medium to another, refractive indices
C. Degueldre et al.
1774 Table 2. Energy
1.24 1.31 1.38 1.46 1.55 1.65 1.77 1.91 2.07 2.26 2.48 2.76 3.10 3.54 4.14 4.96 6.20
Wavelength i (nm) 1000 950 900 850 800 750 700 650 600 550 500 450 400 350 300 250 200
Absorption coefficients of selected oxides at room temperature Magnetite
Magnetite
13 Fe304
(ILVcm-‘) 0.65 0.62 0.61 0.60 0.64 0.68 0.82 1.oo 1.19 1A2 1.56 1.91 2.39 2.90 3.26 3.75
Hematite 19
14 Fe304
(IO’cm-‘) 0.72 0.71 0.72 0.79 0.87 1 .oo 1.16 1.35 1.53 1.71 2.00 2.43 3.02 3.56 4.49 5.78 6.50
Fe203
(IO’cm-‘) 0.46 0.53 0.55 0.60 0.62 0.64 0.62 0.55 0.45 1.19 1.75 3.19 4.19 3.54 4.59 6.00 5.21
Nickel oxide NiOzO (IO’cm-‘)
0.19 0.77 5.29 5.65 5.78
Iron oxide FeO” (lO’cm_‘) 0.174 0.174 0.173 0.172 0.169 0.163 0.159 0.160 0.165 0.247 0.396 0.743 1.480 2.960 5.000
are of importance. Further, their temperature dependence must be reviewed to determine any possible effect on the reflection spectra (Table 1). Light travelling through air, then through sapphire, and then through water has the effect of lessening the diameter of the reflected cone. So the illuminated area outside the sensor detection is not as large as it would be if the medium was just air.
Coupling interferometry, absorption, roughness andgeometry effects. In this DRS study, the light reflected consists partly of light that has been directly reflected at the oxide surface and partly of light that has which has penetrated the material and is scattered. The former affects the reflection spectrum of the material, the latter the transmission spectrum. In weak absorbers, most of the reflected light has penetrated the sample, and the diffuse reflectance spectrum qualitatively resembles the transmission spectrum of a thin slice. But in our case the effects of both are apparent. The model spectra were determined on the basis of equations (2) (7) (14) (17) and:
p = 1 -x.
(1 x 10-6) * (5200 - 5h)
(21)
Equation (21) for the roughness effect (with x and 2 in nanometres) gives the best agreement with the attenuation seen in the measurements. The attenuation of the reflectivity at shorter wavelengths as the oxide thickness increases (p varies from around 1 at 1= 1000 nm to around 0 at J = 200 nm for oxide layers of about 150 nm). The coefficients of absorption used were those of Muret. i3 The refractive index for the oxide was taken as 2.8, relative to vacuum, as this value was in best agreement with the measurements. An example of full spectra calculation is shown in Fig. 8 for various oxide layer thicknesses.
Oxidation of stainless steel under BWR conditions
-x=
-
5nm
-x=
w-w
lOnm
x= 15mn
-
-
x=zonm
-
-
x525nm
-x=
Mnm
---
x=35mn
~~‘~~‘x=~"m ------x=M"m 20
-
10
r _.---
loo
----x=75m ,/..*-
_..-
#'_..",.' _r_'---_,,*
---X.100llU
_c_
--x=iMNl -
.c-
-x=2mnn
T -.
-x= -
Jnm
-x=
lO”rn
-
15nm
-
I=
-
-
x=zollm
-
-
x=25nm Mnm
-x=
---
x=
35nm
-----.r.~“m
------x=5onm -----_x775nm -I=
mm
-x=
lMN?
-x=2oonm
Fig. 6.
Reflectivity of different wavelengths at increasing oxide layer thickness using (a) Muret” and (b) Schlegel et al.‘” absorption coefficients.
Table 3.
Calculated reflection fraction at each material interface at 3OO”C,using equation (6)
Air/sapphire Sapphire/water Water/oxide
I .oo
1.77
1.77
1.77 1.10
1.10 -2.8
0.62 2.55
0.08 0.05 0.19
C. Degueldre et al.
1776
120
Fig. 7.
Reflection spectra if due to intereferometry alone equation (17) (ncO,“)=2.8).
Experimental results The cell in the loop was designed to allow the study of three samples and although the resulting spectra from each of the samples was slightly different, the overall variation for the three samples was quite small. Differences in the spectra from one sample to another were expected as the surfaces of the electropolished samples were not uniform at a nanometre level. The build-up of the oxide layer is determined by many factors, the surface relief being just one and it is not possible to make the surfaces of the three samples exactly the same. Results were obtained for the transient from room temperature to high temperature with non-oxidised stainless steel samples (Fig. 9). This shows the effect of the geometry of the cell since the refractive index of the water falls from 1.33 to 1.10 due to the temperature rise, none of the other factors affecting the reflection spectra were changed during this relatively short time (about 2 h). Samples were observed by DRS as a function of contact time in the loop (Figs 9-l 1). The build up of the oxide layer may be studied directly in-line. In the visible range of the reflection spectra it is evident how the samples appear to change colour during the observation period, turning yellow at first, then as the interferometric effects develop, the samples appear violet, then blue, eventually becoming the expected red-brown of the oxide. When comparing the recorded spectra to those calculated by the model, an excellent agreement between the theory and the results is noted. In situ investigation seems necessary since the removal of a sample from the conditions
Oxidation of stainless steel under BWR conditions
100
90 80
T
--
--
”,_._.I._....,._..- -I-
70 --
100 90 80
i
..,......“...
Wavelength
110nm
/nm
100 90 80
i
70 -60 --14Onm
Wavelength
Fig.
8.
Inm
Reflection spectra according to the model including absorption, intereferometry, geometry and roughness using equation (17) [nCO,vJ = 2.8, RG = 43%].
1778
C. Degueldre
et al.
I 140
80 .I% 60
0
200
Fig. 9.
250
300
Change
350
400
of spectra
450
500
550 600 650 Waveiengthhm
700
750
800
850
900
950
loo0 -
for transition from room to high temperature. sensor, unoxidized sample, 300 ppb OZ.)
(6&18o”C
with external
under which it corrodes may change the sample characteristics primarily due to the difference between the expansion coefficient of the oxide and the metal.16 If a metal is heated and an oxide layer grows then cooling the metal-oxide system may disturb the oxide layer (e.g. because the coefficient of expansion for the oxide is different to that of the metal). However, the difference in the expansion of the oxide layer” and the metal’s is around 0.6 permille for cooling from 300 to 25°C admittedly very small, but at this nanometre level any property altering the sample surface, however small, may have some effect.
-
ZOOY
100
---190°C 90
180°C ..-..... , , 0°C
80
.
70
F d
-l6O"C
60
-
50
-I
150°C 4O'C
-130°C
40
-l2O"C 30
IIO”C
20
100°C
10
90°C -____8O"c
II
0 w
w
8
z
zi
8
8
2
g
'. 70°C fj0 "C
Wavelenglh /nm 50°C
Fig. 10.
Change
of spectra
for transition from high to room temperature sensor, oxidized sample 300 ppb 02).
(20&5o”C
with external
Oxidation of stainless steel under BWR conditions
1779
120 -0
100
-13h 80
21h
e ti
-
29h
60 -37h 40
-45h -55h i
-63h
100 80 f$
60 40
Wavelength Inm
I20 100 80
-
I59h
-
171h
f! ti
183h 60
I._.._.“, ,95h
40
20
Fig. Il.
i
-231h
DRS spectra recorded during oxide layer build up. (25O”C,average 200 ppb 02.)
207h
C. Degueldre et al.
0
50
150
100
200
250
Time h
-+-Absorption -t-
1st Inwferometry
-&-
2nd Interferomerry
peak
I (t”o.5)
-Model Model -Model
peak
2 (In I) 3 (1’9.666)
-i 250
Time 01
Fig. 12. Oxide layer thickness as a function of time
The reflection spectra for the transition from high temperature to room temperature with oxidised samples (Fig. 10) are then of great interest. Similarly, the effect of the temperature increase on the reflection spectra of the metal is also negligible (Fig. 9). From spectral data in Fig. 11 it is possible to calculate the thickness as a function of time by three approaches (see Fig. 12). In the infrared region, for short contact times, the thickness can be evaluated purely on the basis of absorption because there is no interferometry effect for the investigated wavelength range. Interferometry peaks appear for thickness above 80 nm. The thickness can be calculated from the maximum reflected wavelength. When the first peak becomes too wide, thicknesses above 140 nm, it is possible to use the second interferometric peak. As the thickness increases, the intensity of the interferometry effects decreases until eventually there is only the effect of absorption. Results by the three different methods are comparable (Fig. 12). The trend of the plot x (thickness) vs t (time) may be tested using a logarithmic x 0: In t or a power law x 0: t”, this is done in Fig. 12. It can be seen that the power law x c( t213best fits the experimental results. Problems which were encountered:
Oxidation of stainless steel under BWR conditions
1781
- The first problem was the inconsistency of the oxygen concentration in the loop water which was around 200 ppb in the available water. - An early problem which had an effect on the first set of spectra and which was quickly overcome was the stability of the Guided Wave unit. If it was switched off between sets of measurements it could not be guaranteed that it was in the exact same state for the next set of measurements. - Water flow rate over the sample surface, which has an effect on the corrosion rate, was not measured but plans are to install a sensor for the flow rate in the cell. - The temperature inside the cell was not measurable, it was assumed as the same temperature as in the rest of the loop. The temperature during transition was measured using a thermocouple on the outside of the cell (external sensor), the readings being mathematically converted into internal temperatures. - Over long periods the sapphire window in the cell corroded, they are slowly dissolved in the loop water, reducing the transparency. In this photometric analysis constant visibility of the sample is of paramount importance. - The samples used were visibly different even though they were made of the same material and prepared in exactly the same way. This would also be the case in any practical application. The sample non-uniformity means that numerous experiments must be made to ensure statistically relevant results. In the future, it may be possible to reduce the distance of the probe from the window. This would increase the sensitivity of the set up (AR,, increases). This parameter is heavily constrained by the cell design. There is only one or two millimetres before the cold finger on the probe touches the cell wall and the cooling jacket should be efficient enough to prevent the probe from being damaged. This work shows that layer thickness determination up to 200 nm is possible. Extention to thicker layers is feasible using better geometry (d smaller) a stronger light source and a wider spectral detection range, for example up to 2000nm with a PbS detector. With increasing sensitivity, it is also possible to measure layers around 2 nm. CONCLUDING
REMARKS
We have proved that DRS is a very useful tool for detecting, in-line, the build-up of the oxide layer on a stainless steel sample under BWR conditions. Since the corrosion is dependent on the initial oxide build-up, the analytical method should allow the detection of the oxide layer in sratu nascendi. Further improvements to the experimental set-up are planned for example by the introduction of stabilised zirconia windows, by in-line analysis of the temperature, the flow rate and the oxygen concentration at the sample surface. DRS allows verification of results with independent absorption and interferometric responses, however, the results should be compared to those of other techniques such as Rutherford back scattering, sputtered neutral mass spectroscopy and/or impedance spectroscopy, in order to cross check the accuracy of the calculated thicknesses. DRS is an excellent technique for in situ investigation and the detection of the layer thickness is possible from 2 to 200 nm. Acknowledgements-E.
Schenker is thanked for operating the loop and for the fruitful discussions we had during this work, H.P. Alder is thanked for his constructive remarks and his interest in this work, R. Knecht and E. Hitz are thanked for their technical assistance.
1782
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REFERENCES 1. 2. 3. 4. 5. 6. 7. 8.
9. 10. Il. 12.
13. 14. 15. 16. 17.
C. Degueldre, J. C. Dran and E. Schenker, J. Nucl. Mat. 188,255-257 (1992). D. Lister, Nucl. Sci. Eng. 58, 239-251 (1975). D. Lister, Nucl. Sci. Eng. 59, 406426 (1976). G. Korttim, Reflectance Spectroscopy, Springer-Verlag Berlin, New York (1969). 0. Lunder, K. Nisancioglo and 0. Hunderi, Proceedings of the International Congress on Metallic Corrosion Toronto, 1, (National Research Council of Canada), 481-485 (1984). M. Dempster, B. F. MacDonald, P. J. Gemperline and N. R. Boyer, Anal. Chim. Acta, 43-51 (1995). A. Raman and B. Kuban, Nat. Ass. of Corrosion Engineers 44, 483488 (1988). J. Svensson and L. Johansson, Corrosion Control for Low Cost Reliability vol 2: Process Industries Plant Operations, pp. 662675. Nate International, Houston, USA (1993). M. Schubnell, H. R. Tschudi, P. Kuhn and H. Ries, Proceed. Acad. Sci. Chem. Sot. 105, 671-675 (1993). G. E. McGuire, M. L. Swanson, N. R. Parikh, S. Simko, P. S. Weiss, J. H. Ferris, R. J. Nemanich, D. R. Chopra and A. R. Chourasia, Anal. Chem. 67, 199R-220R (1995). R. G. J. Strens and B. J. Wood, Mineralogical Magazine 43, 347-354 (1979). M. Franon, Optical Interferometry, pp. 53-61. Academic Press, New York, London (1966). P. Muret, Solid State Commun. 14, 1119-l 122 (1974). A. Schegel, S. F. Alvarado and P. Wachter, J. Phys. C: Solid State Phys. 12, 1157-l 164 (1979). M. J. Graham, Proceedings of the International Congress on Metallic Corrosion Toronto, (National Research Council of Canada), 4, l-4 (1984). R. K&z, B. Schnyder and C. Barbero, Proceedings First International Conference on Spectroscopic Ellipsometry, (1993). Y. S. Touloukian, R. K. Kirby, R. E. Taylor and P. D. Desai, Thermal Expansion Metallic Elements and
Alloys, Thermophysical Properties of Matter 12, 275-279 (1977). 18. Y. S. Touloukian, R. K. Kirby, R. E. Taylor and T. Y. R. Lee, Thermal Expansion, Non Metallic Solids, Thermophysical Properties of Matter 13, 1138-l 139 (1977). 19. I. Balberg and H. L. Pinch, J. Mugn. Mugn. Mar. 7, 12-15 (1978). 20. R. J. Powell and W. E. Spicer, Phys. Rev. B. 2, 2182-2193 (1970). 21. H. K. Bowen, D. Adler and B. H. Auker, J. Solid State Chem. 12, 355-359 (1975). 22. Handbook of Chemistry and Physics, 51st edition, (ed. R. C. Weast), pp. E226E230. The Chemical Rubber
Co., Ohio, USA (1971). 23. E. S. Zouboulis and M. Grimsditch, J. Appl. Phys. 91, 772-776 (1991). 24. Ullmann’s Encyclopaedia of Industrial Chemistry, A20, (ed. B. Elvers, S. Hawkins, G. Schulz), p. 261. VCH Verlagsgesellschaft, Weinheim, Germany (1992). 25. H. E. Biihler and H. P. Hougardy, Atlas der Inrerferenzschichtenmerallographie. Deutsche Gesellschaft fur Metallkunde, Adenauerallee 21, D-6370 Oberursel 1, (1979).
PII: SOOlO-938X(96)00078-9
AN IN-LINE DIFFUSE REFLECTION SPECTROSCOPY STUDY OF THE OXIDATION OF STAINLESS STEEL UNDER BOILING WATER REACTOR CONDITIONS C. DEGUELDRE,*
S. O’PREYT
and W. FRANCIONIS
*Paul Scherrer Institute, Villigen-PSI CH-5235, Switzerland tQueen’s University, Belfast, Northern Ireland fTecova, Wohlen, Switzerland Abstract-A
novel cell unit was constructed to measure in-line the oxide layer build-up on a stainless steel sample by Diffuse Reflection Spectroscopy (DRS; ultraviolet, visible, near infrared) under boiling water reactor (BWR) conditions. The stainless steel samples, observed in the cell through a sapphire window, are contacted with oxidising hot water (3OO”C,9.0 MPa). Using a cold finger with the optical fibre probe, the spectroscopic investigations (2001000 nm wavelength) were performed at a fixed position from the sapphire window. The DRS spectra are the result of the coupling of both absorption (chemical) and interferometric (physical) processes. Analysis of these spectral components allows the independent determination ofthe oxide layer thickness. The build-up of the oxide layer may be directly observed and quantified, nanometre by nanometre, from 2 to 200 nm. This powerful technique may be used to study the early corrosion rates ofstainless steel under BWR conditions and should allow the development of a strategy to reduce corrosion. Copyright 0 1996 Elsevier Science Ltd Ke~~~~ords: A. stainless steel, B. in-line diffuse reflection spectroscopy,
C. reactor
conditions.
INTRODUCTION In boiling water reactors (BWR) the monitoring of the corrosion of stainless steel components is of great interest since the corrosion products are released into the reactor water. The possible activation of the corrosion products in the BWR and the fact that the release of corrosion products depends on the oxide layer build-up makes close monitoring of the surface oxidation important. Among these corrosion products 6oCo is particularly significant because about 80% .of the dose received by plant personnel results from its presence in the reactor.lm3 Many studies have been made of stainless steel corrosion under hydrothermal conditions, however no attempt has been made to measure the build-up of the sub-micrometre oxide layers formed in situ. The objective of this study is to use Diffuse Reflection Spectroscopy (DRQ4 to measure the growth rate of the oxide layer on the stainless steel under boiling water reactor conditions. Spectroscopy can be performed in a transmission or a reflection mode either by in-line or off-line investigation. The technique of Diffuse Reflection Spectroscopy has been used successfully in many fields and is often useful where traditional spectroscopic techniques fail. It has been applied for example in the study of rough pitted surfaces,5 as a non-invasive technique for blister-packaging contents identification,6 in the analysis of rust on weathering steels,’ in the in situ study of the reaction of SO2 and NO2 on pure zincite* and Manuscript received 16 January 1996; in revised form 7 March 1996. 1763
1764
C. Degueldre
et al.
in the observation of iron oxide reflectivity.’ Its main advantage lies in the fact that many substances in their natural state (e.g. powders and rough surface solids) exhibit diffuse reflection. This means that the light is scattered in all directions with the angle of the reflected light beams not always equalling the angle of the incidence beam. DRS may therefore be used as a good analytical tool in the study of the properties of thin layers or surfaces of materials in situ, since in many cases samples do not need to be specially prepared. Although it is true, that in practice diffuse reflection spectra are complex and are strongly dependent on the conditions under which they are obtained, the great advantage of DRS is its non-invasive quality which is essential in many applications.” DRS is useful as a rapid survey technique locating the main regions of absorption and providing information on the energy, width and intensity of absorption bands within a material.” The build-up of the oxide layer is a multi-step process, the first period being the formation of a transpassive oxide layer on the clean metal surface. The result is a glassy, ceramic-like oxide displaying a surface comparable to that of the metal. In the second period the surface characteristics change, there is a reaction within the oxide layer and recrystallisation takes place yielding a coarse, matt surface. Since the oxide layer formed in the first period strongly influences the subsequent corrosion process, high priority has to be given to corrosion studies of the earliest possible stages. This study focuses on the build-up of the oxide layer in the first period. The observation is performed using an optical fibre light carrier and collector to illuminate through a sapphire window the steel sample in contact with the water at about 300°C. THEORETICAL
BACKGROUND
Analytical approach
In spectroscopy, since the light intensity is of extreme importance, it must be considered that the light beam in the studied systems is not straight but cone shaped (Fig. 1). In reflection mode, the loss of intensity is dependent on the distance of the probe tip (light source and collector) from the sample surface. The probe must be placed at the minimum possible distance from the observation window. It must not, however, be in contact with it as it cannot withstand the temperatures to which the sapphire is raised. The detected intensity (Z), with perfect sample reflectivity, is given by the equation:
(1) where 1, is the incident intensity, S is the base area of the emitted light cone and So is the area detected by the probe. It is clear that only a restricted proportion of the reflected light is collected by the sensor. A reference spectrum is taken before oxidation, the small percentage of the total light reflected and detected is taken as equivalent to the incident light intensity. It is assumed that the initial polished surface is perfectly reflecting. Therefore, at the beginning, the detected light intensity must equal to the incident light intensity. Since the detected light intensity is inversely proportional to the square of the distance from the probe tip to the metal surface, it is essential that this distance is as small as possible and kept constant throughout the measurement period. The cone shaped beam is transmitted into air at the probe tip (radius rP) with an angle i,,,,. The geometry of the cell is such that the sample (object) is only visible through a small window with an external area (external halo). Refraction in the sapphire and in the water
Oxidation of stainless steel under BWR conditions
1765
20 Fig. 1. Geometric effect on the light intensity in reflection mode.
results in the angle of the cone (&) which reaches the sample being modified. The cone of light from the probe with an angle i,,, illuminates at a distance d the area of the object and causes the external halo. Therefore, a large portion of the light from the probe does not reach the sample. Instead it is reflected straight back or lost (Fig. 1). The fraction of light reaching the sample is given by the equation: AR
=
WJ
R,b = (rp + d. R rot
tanioh)* . it
(rp + d. tan &)* . TC
(2)
For the cell used (with d and rp of some mm, and ioh and i,,, of some degrees of angle) only a fraction (e.g. AR,,,) of the light from the probe passes through the window to reach the sample. The rest is lost or reflected straight back to the probe. The amount of light which does not reach the sample and is not detected by the probe is considered in the reference spectrum. This means that the recorded spectra show a smaller fraction than the predicted fraction (e.g. 1 -AR,,,) of the reflected light detected by the probe. When dealing with the transition of light from one medium to another, refraction at each interface must be considered. Meanwhile, during oxidation, the chemical properties of the surface change and the light absorption must then be taken into consideration. Transmission
through the oxide film
In the single beam mode, reference beam and reflected beam are measured separately. The intensity of the incident beam is first recorded by taking a reference spectrum over the wavelength range of interest with the unoxidized polished sample. The spectra are then analysed in comparison to the first spectrum recorded, before any oxidation. Its relative reflectivity AR,,,.,),,, is 1 (see Fig. 2(a)). Although the polished metal surface is not perfect, it is of a quality such that the light illuminating the sample is reflected to a high degree. With an oxide layer on the metal the reflected light is reduced because some of the light is absorbed in the film (see Fig. 2(b)).
1166
C. Degueldre
et al.
Initial
1 I
ewll)
= A&wlt)= 1
Wwd
~(w,o) = A&v) = 0
IO
b. Metal / oxide / water
a. Metal I water
c. Absorption in transmission water/oxide/water Fig. 2.
Reflection
and transmission
models.
Materials can display different types of behaviour in various parts of the spectrum; often being strongly absorbing in the near ultraviolet, and weakly absorbing in the near infrared. This is illustrated by magnetite (FesOa) which absorbs more in the blue than in the red hence its brown colouring. The reflection spectra were not analysed based on the Kubleka Munk equation4 because the layer is thin enough (no infinite depth) and the oxide is a weak enough absorber that the light penetrates to the metal surface where it is totally reflected. It is assumed that there is no reflection either at the water/oxide or oxide/water interfaces of the transmitted beam (ARC,.,,, = 0) or of the reflected beam (ARC,+,, = 0) and that any loss in reflection is due only to absorption within the oxide layer (Fig. 2(b)). As previously mentioned,’ the spectrum resembles the transmission spectrum of a thin slice of the same material (Fig. 2(c)). In DRS mode, twice the thickness is taken because the light passes through the oxide layer twice. Beers law is used to calculate the thickness: I
-=e
-2.@(A)..Y
&I) = absorption coefficient, dependent on the wavelength; IO = incident light intensity. I = resultant light intensity and The reflectivity or fraction of light reflected (R)is given by:
where x = thickness of oxide layer;
R = iorR%
= R '100%
Oxidation of stainless steel under BWR conditions
Considering oxide layer,
that 100% of light is reflected from the electropolished R = e-2940X
1767
sample with no (5)
the oxide layer thickness may be calculated from the reflection at each wavelength, using a realistic absorption coefficient. Reflection and interferometry by the oxide film When considering the passage of light between different materials it is necessary to take into account the reflection at each interface. The fraction of light reflected is given by the equation:
(6) The value of the refractive index (n) is relative to the two contacting materials and is given by:
where n(,i,,) is the refractive index of the medium (1) relative to vacuum for the incident light. The selected refractive index for the oxide layer is neither that for magnetite nor that for hematite, because the oxide layer is not a pure oxide. So if there is an oxide layer on the metal, part of the light does not ‘enter the layer but is instead reflected at the oxide surface (AR,,,,,, ZO). The reflections from the air/sapphire and the sapphire/water interfaces do remain constant throughout the measurement period, but the reflection from the oxide changes depending on the roughness of the layer which may increase with the thickness (ageing). When dealing with the reflection of light from a thin film on a reflecting material there is interference of the reflected light waves. This is due to partial reflection at the two interfaces. Selected wavelengths disappear from the spectrum when destructive interference occurs. Typically, the oxide layer on the metal at certain thicknesses does not show the expected red-brown but instead another colour (e.g. lilac, blue, etc.). The equation’* that determines the resulting intensity (I) at each wavelength is:
(8) where I is the wavelength of the reflected light. The path difference (6) is obtained by simple geometry. In the measurement cell the light is not a collimated beam normal to the surface but is instead a cone of light, therefore the calculation of the path difference (Fig. 3) is given by: 6 = 2 . IJ . q,,) - IH . n(,,,) IJ=JK=z ~0s qo)
Pa> Pb)
C. Degueldre
1768
et al.
Metal Fig. 3.
Interference
due to reflection
by a plane parallel
plate
ZH = ZK . cos i(,v)
(9c)
IK = 2 . x . tan Y(~)
(9d)
Equation (9a) becomes: 6 = 2 . x . rl(,,“)
1
[ cos Y(o)
cos it,.) . tan r(0) n(,, It’)
(10)
I
where i(,,., is the angle of light incidence (Fig. 3) which is some degrees of angle. / is around 1. Under these circumstances equation Since the angles are very small, cos icM., (10) can be said to be equal to: 6 = 2 . iq,,“) . x
.?7 .rz(,,“) 1 .x11
(11)
Combining equations (8) and (11) for interfering waves: 4
1 + cos
I = lo .
[
(
(12)
Equation (12) may be used to calculate the interferometric effects. The reflection spectrum provides a simple mean (peak) to determine the optical thickness of the film. The most reflected wavelength (A,,,) corresponds to twice the refractive index of the film multiplied by the film thickness: x=-
E,max 2 . qo,“)
(13)
As the oxide layer gets thicker there is a spectral shift, the reflection peak moves to higher wavelengths. The peak is related to the oxide layer thickness by equation (13).
1769
Oxidation of stainless steel under BWR conditions
Other effects Theejktofgeometry.
The effect of the cell geometry, with total and partial light intensity reflected from the object was treated earlier. Equation (2) describes the amount of light which does not pass through the window to reach the sample. The rest of the detected light which has not reached the sample is due to reflection at the air/sapphire and sapphire/water interfaces. So the total light which is detected but has not reached the sample is given by: (14) The efict of refractive index n(2). In all materials, there is a certain dependence of the refractive index with the wavelength of the incident light. This effect is supposed to be negligible in this work. The eflect of roughness. It is known that the oxide layer formation causes the relatively smooth surface of the electropolished stainless steel to become rough, that is the oxidation is more rapid in some areas than in others. This is believed to be due to pores in the metal surface, between the metal crystals, in which the oxidation is faster.’ On this rough surface, the light beam reflection passes from a direct reflection to a diffuse reflection mode. Clearly to investigate these imperfections the wavelength of the light used must be sufficiently short to be affected by this fine structure. At a certain point, because the wavelength of the light is too short the fine structure of the layer will not be recognised and the oxide layer will appear less deep than indicated by longer wavelengths. It is assumed that the effect of the roughness (p) depends linearly on both the thickness of the oxide layer and the wavelength of the incident light, and it varies between 1 and 0. The suggested equation to simulate the effect of roughness on the reflection intensity is the following:
p= 1 -a.x+b.x.h.
(15)
Combining al/processes
The equation of the spectra due to absorption and interferometry R =
only is:
[(I - LIR(,~,,J)- e-2’x’p] + dR(,,,,) .
(16)
The first term refers to light not reflected at the oxide layer surface. Light passes through the film and is subject to absorption in the oxide. Since the angle i,,, is small, the pathway is 2 . x. The second term corresponds to the fraction of light which is reflected and which is subject to the interferometric effect. The complete reflection spectra finally consists of: Light reaching the sample + Light detected which has not reached the sample The amount of light detected by the sensor which has not reached the sample is given by RG (equation (14)).
Of the light reaching the sample, some is reflected at the oxide layer surface resulting in the interferometry effect, the rest enters the oxide and is subject to absorption and reflection. The amount of light reflected at the oxide layer surface depends also on its roughness (equation (15)). The equation for the spectra becomes:
[( 1-
p +dR(,.,,$ . e-2’-Vw]+ P
.A&,.,) . +(1 -RG) IL >I1 4~ll.n(,“)~x
(17)
1770
C. Degueldre
EXPERIMENTAL
et al.
METHOD
Sample preparation and analytical background Sample coupons (2 .2 . 0.15 cm, 4.4 g) of electropolished
AISI 3 16.PI stainless steel (Fe 0.68, Cr 0.17, Ni 0.10, MO 0.02, Co 0.001) were exposed to BWR conditions in a corrosion test loop. The water of the loop was of Q-millipore quality at high temperature (e.g. of 290°C) and under a pressure of 9.0 MPa. The samples were contacted for different time periods with loop water. The loop water was produced by a Millipore unit. This water was injected after polishing and oxygen addition by bulling adequate Ar/02 mixture in the water. The loop water was monitored on-line for oxygen and locally hydrogen concentrations utilising Orbisphere units. In addition, conductivity was measured using an Endress & Hauser unit. Off-line analysis was carried out routinely by inductively coupled plasma atomic emission spectroscopy (ICP-AES, ARL, Applied Research Laboratory) and ion chromatography (IC, Dionex). The [O,] was 200 + 100 ppb and the hydrogen concentration was < 1 ppb. The conductivity was always < 0.1 ,US. cm-’ . All element concentrations measured by ICP-AES (e.g. Na, K, Ca, Mg,...) were smaller than 1 ppb and the anion concentrations measured by IC (i.e. F-, Cl-, NOs-, S04’-) were also smaller than 1 ppb. DRS spectra were obtained using a Guided Wave Model 260 Spectrum Analyser for the ultraviolet, visible and near infrared regions (200-1000nm) with an incident beam of a diameter of about 2 mm. The measurements are performed by projecting a slightly conical (emission angle 9”) light beam perpendicularly on the specimen surface and collecting all the diffuse scattered light within a 2.5 x 10e3 rad solid angle. The detection is made by the same probe which emits the light. The fibre optic spectrophotometer unit performs five distinct functions: light (source) generation, light modification (by fibre, probes and sample), monochromation (breaking the light down into its chromatic (spectral) components), detection and conversion, and input/output to the computer. A deuterium lamp provides the light source for the ultraviolet portion of the spectrum, and a tungsten-halogen lamp provides the light source for spectrophotometric applications in the visible and near infrared range. Since the light intensity is of extreme importance, it should be remembered that the light from the probe is not a straight beam, instead it is cone shaped (Fig. 1). There is a loss of intensity dependent on the distance of the probe tip from the sample surface. The probe must be placed at the minimum distance possible from the sapphire window, but not in contact with it as it cannot withstand the temperatures to which the sapphire is raised. It is clear that a restricted proportion of the reflected light is collected by the sensor (see equation (1) and Fig. 1). However, a reference spectrum is taken before oxidation, and the small percentage of the total light reflected was taken as equivalent to the incident light intensity. It was assumed that the initial polished surface was perfectly reflecting. Therefore at the beginning, the detected light intensity is equal to the incident light intensity. Since the detected light intensity is inversely proportional to the square of the distance from the probe tip to the metal surface, it is essential that this distance is made as small as possible and kept constant throughout the measurement period. The beam is cone shaped due to the fact that the light beam is transmitted down the optical fibre by a process of multi-reflection. When the light moves into air at the probe tip, radius (vp), the angle is further increased by refraction. The geometry of the cell is such that the sample is only visible through the 6 mm diameter window. With refraction in the sapphire and in the water the angle of the cone (&) which reaches the sample is between 6.5”
1771
Oxidation of stainless steel under BWR conditions
and 6.7”, depending on the temperature of the water. The cone of light from the probe (with an angle (&,J of 9”) illuminates, at a distance (6) of 20 mm, an area of diameter 8.3 mm. Therefore a large portion of the light from the probe does not reach the sample but is reflected straight back or lost (Fig. 1). The fraction of light reaching the sample is given by equation (2). When d = 20 mm, rp = 1 mm, iob = 6.5” and i,,, = 9”, only 62% of the light from the probe passes through the window to reach the sample, the rest being lost or reflected straight back to the probe. The reference spectra are saved and used each time a reflection spectrum is taken of the oxidised sample. In this way, a picture of the oxide layer build-up specific to each particular sample is obtained. In the ultraviolet region (200-300nm), spectral fluctuations are detected. This is due to the absorption behaviour of the optical fibre in this part of the spectrum. It is less accurate but was retained for completeness. Off-line/in-line strategy In a previous study the sample coupons were placed in the loop with water at 290°C at a pressure of 9.0 MPa for periods from 100,470,2024 and 4700 h. The samples were allowed to cool, were removed from the loop and were placed in a cell similar to that shown in Fig. 4,
2mm +-* 6mm 4 ?
16mm
Probe Metal _.__
I
12 mm
Air
: .
_1 .. I
Smm
Sapphire
_.._3mm
mQ - water
Oxide layer Sample Sample holder 4
20 mm Fig. 4.
In-line experimental arrangement.
C. Degueldre
1772
et al.
with water at about 15°C flowing through the cell (plastic). A problem with this approach was that no reference spectra were taken before placing the samples in the loop. Instead one electropolished sample was used as reference for all measurements. This cell was used as reference unit during the in-line measurements, because the water temperature is too low for any significant corrosion. This reference was used to check the stability of the Spectrum Analyser over a long time period. For in-line tests, the sample coupons were treated in the same way as before for the offline measurements. They were subjected to the same BWR hot water conditions but they were mounted in the cell (Fig. 4) adapted in the loop for continuous spectroscopic observation. The probe was placed in a fixed mounting perpendicular to the sample surface at a constant distance (20 mm). It was also necessary to cool the probe by a water jacket, called a cold finger. Four sample coupons were monitored, three within the circulating hot water and one, a control, in static water at room temperature (off-line). This fourth sample was needed to provide information on the stability of the probe and the spectrometer system over a long recording time. The change in the spectra, both as the temperature of the system was raised and as it was reduced to room temperature, was also recorded to study the effect of temperature changes. EXPERIMENTAL
RESULTS
AND DISCUSSION
Modelling the DRS spectra The transmission approach. The absorption coefficients of magnetite (Fe304) as reported by Muret13 and Schlegel14 are given in Table 2 and in Fig. 5. They are used to evaluate the reflection spectra, since it is assumed that magnetite is most abundant in the oxide layer.r5 However, it must be remembered that other oxides may also be present, particularly nickel oxide, which affects the results, therefore the absorption coefficients of NiO are also given. Fe0 is of little influence in the visible and infrared regions of the spectrum. NiO is only important when the incident wavelength is less than 350nm. Applying equation (5) for absorption-reflection at varying thickness (x) using the coefficients available (Table 2) the absorption component in the DRS spectra can be calculated (Fig. 6). The interferometry approach. Since AR,,.,,, is not equal to zero (equation (6)) the interference must be taken into account. The effect of interface reflection may be calculated using the experimental refractive indices reported in Table 1. Examples are given in Table 3. The maximum intensity of the reflected light is dependent on the light reflected from the oxide layer. This is given in Table 3 and equation (6) for a perfectly flat layer. The variations in the reflected light cannot be higher but may be less as a result of destructive interference.25 The reflection spectra is made up of (1 -AR o,,,oi)r due to the total reflection at the metal surface unaffected by interference, plus the interferometry factor which is given by equation (10): Table
1. Refractive
Temperature 20°C 300°C
indices (n (n,.vJ of relevant
Air
1.oo’2 1.oo”
materials
(m) vs vacuum
(v)
Sapphire
Water
Magnetite
Hematite
1.77” 1.7722
1.33” 1.10”
2.4223 2.4223
2.9923 2.992’
Oxidation
of stainless
steel under BWR conditions
1773
. \. co
-Magnetite
Muret 19731
- - - - - - Magnetite [Schlegel et al. 19781
-I;-
Hematite [Balberg et al. 19781 - Nickel Oxide [Powell et al. 1969
-Iron
Oxide [Bowen et al. 19751
0
200 250 200 350 400~450
500
550 600
650
700 750
Boa 8!3l900
950 low
Wavelength /urn Fig. 5.
Comparison
R = (1 - Ww))
of the absorption
+ AR(,,,,
coefficients
of selected oxides.
4 . n . q,,,) . x
.
x
(18)
The spectra clearly display a succession of reflection maxima (Fig. 7). The equation for the first maximum is: h maxl
=
2 . qo.“) . x
(19)
The equation for the second maximum is: ;cmax2
= q,,)
.x
(20)
When dealing with the transition of light from one medium to another, refractive indices
C. Degueldre et al.
1774 Table 2. Energy
1.24 1.31 1.38 1.46 1.55 1.65 1.77 1.91 2.07 2.26 2.48 2.76 3.10 3.54 4.14 4.96 6.20
Wavelength i (nm) 1000 950 900 850 800 750 700 650 600 550 500 450 400 350 300 250 200
Absorption coefficients of selected oxides at room temperature Magnetite
Magnetite
13 Fe304
(ILVcm-‘) 0.65 0.62 0.61 0.60 0.64 0.68 0.82 1.oo 1.19 1A2 1.56 1.91 2.39 2.90 3.26 3.75
Hematite 19
14 Fe304
(IO’cm-‘) 0.72 0.71 0.72 0.79 0.87 1 .oo 1.16 1.35 1.53 1.71 2.00 2.43 3.02 3.56 4.49 5.78 6.50
Fe203
(IO’cm-‘) 0.46 0.53 0.55 0.60 0.62 0.64 0.62 0.55 0.45 1.19 1.75 3.19 4.19 3.54 4.59 6.00 5.21
Nickel oxide NiOzO (IO’cm-‘)
0.19 0.77 5.29 5.65 5.78
Iron oxide FeO” (lO’cm_‘) 0.174 0.174 0.173 0.172 0.169 0.163 0.159 0.160 0.165 0.247 0.396 0.743 1.480 2.960 5.000
are of importance. Further, their temperature dependence must be reviewed to determine any possible effect on the reflection spectra (Table 1). Light travelling through air, then through sapphire, and then through water has the effect of lessening the diameter of the reflected cone. So the illuminated area outside the sensor detection is not as large as it would be if the medium was just air.
Coupling interferometry, absorption, roughness andgeometry effects. In this DRS study, the light reflected consists partly of light that has been directly reflected at the oxide surface and partly of light that has which has penetrated the material and is scattered. The former affects the reflection spectrum of the material, the latter the transmission spectrum. In weak absorbers, most of the reflected light has penetrated the sample, and the diffuse reflectance spectrum qualitatively resembles the transmission spectrum of a thin slice. But in our case the effects of both are apparent. The model spectra were determined on the basis of equations (2) (7) (14) (17) and:
p = 1 -x.
(1 x 10-6) * (5200 - 5h)
(21)
Equation (21) for the roughness effect (with x and 2 in nanometres) gives the best agreement with the attenuation seen in the measurements. The attenuation of the reflectivity at shorter wavelengths as the oxide thickness increases (p varies from around 1 at 1= 1000 nm to around 0 at J = 200 nm for oxide layers of about 150 nm). The coefficients of absorption used were those of Muret. i3 The refractive index for the oxide was taken as 2.8, relative to vacuum, as this value was in best agreement with the measurements. An example of full spectra calculation is shown in Fig. 8 for various oxide layer thicknesses.
Oxidation of stainless steel under BWR conditions
-x=
-
5nm
-x=
w-w
lOnm
x= 15mn
-
-
x=zonm
-
-
x525nm
-x=
Mnm
---
x=35mn
~~‘~~‘x=~"m ------x=M"m 20
-
10
r _.---
loo
----x=75m ,/..*-
_..-
#'_..",.' _r_'---_,,*
---X.100llU
_c_
--x=iMNl -
.c-
-x=2mnn
T -.
-x= -
Jnm
-x=
lO”rn
-
15nm
-
I=
-
-
x=zollm
-
-
x=25nm Mnm
-x=
---
x=
35nm
-----.r.~“m
------x=5onm -----_x775nm -I=
mm
-x=
lMN?
-x=2oonm
Fig. 6.
Reflectivity of different wavelengths at increasing oxide layer thickness using (a) Muret” and (b) Schlegel et al.‘” absorption coefficients.
Table 3.
Calculated reflection fraction at each material interface at 3OO”C,using equation (6)
Air/sapphire Sapphire/water Water/oxide
I .oo
1.77
1.77
1.77 1.10
1.10 -2.8
0.62 2.55
0.08 0.05 0.19
C. Degueldre et al.
1776
120
Fig. 7.
Reflection spectra if due to intereferometry alone equation (17) (ncO,“)=2.8).
Experimental results The cell in the loop was designed to allow the study of three samples and although the resulting spectra from each of the samples was slightly different, the overall variation for the three samples was quite small. Differences in the spectra from one sample to another were expected as the surfaces of the electropolished samples were not uniform at a nanometre level. The build-up of the oxide layer is determined by many factors, the surface relief being just one and it is not possible to make the surfaces of the three samples exactly the same. Results were obtained for the transient from room temperature to high temperature with non-oxidised stainless steel samples (Fig. 9). This shows the effect of the geometry of the cell since the refractive index of the water falls from 1.33 to 1.10 due to the temperature rise, none of the other factors affecting the reflection spectra were changed during this relatively short time (about 2 h). Samples were observed by DRS as a function of contact time in the loop (Figs 9-l 1). The build up of the oxide layer may be studied directly in-line. In the visible range of the reflection spectra it is evident how the samples appear to change colour during the observation period, turning yellow at first, then as the interferometric effects develop, the samples appear violet, then blue, eventually becoming the expected red-brown of the oxide. When comparing the recorded spectra to those calculated by the model, an excellent agreement between the theory and the results is noted. In situ investigation seems necessary since the removal of a sample from the conditions
Oxidation of stainless steel under BWR conditions
100
90 80
T
--
--
”,_._.I._....,._..- -I-
70 --
100 90 80
i
..,......“...
Wavelength
110nm
/nm
100 90 80
i
70 -60 --14Onm
Wavelength
Fig.
8.
Inm
Reflection spectra according to the model including absorption, intereferometry, geometry and roughness using equation (17) [nCO,vJ = 2.8, RG = 43%].
1778
C. Degueldre
et al.
I 140
80 .I% 60
0
200
Fig. 9.
250
300
Change
350
400
of spectra
450
500
550 600 650 Waveiengthhm
700
750
800
850
900
950
loo0 -
for transition from room to high temperature. sensor, unoxidized sample, 300 ppb OZ.)
(6&18o”C
with external
under which it corrodes may change the sample characteristics primarily due to the difference between the expansion coefficient of the oxide and the metal.16 If a metal is heated and an oxide layer grows then cooling the metal-oxide system may disturb the oxide layer (e.g. because the coefficient of expansion for the oxide is different to that of the metal). However, the difference in the expansion of the oxide layer” and the metal’s is around 0.6 permille for cooling from 300 to 25°C admittedly very small, but at this nanometre level any property altering the sample surface, however small, may have some effect.
-
ZOOY
100
---190°C 90
180°C ..-..... , , 0°C
80
.
70
F d
-l6O"C
60
-
50
-I
150°C 4O'C
-130°C
40
-l2O"C 30
IIO”C
20
100°C
10
90°C -____8O"c
II
0 w
w
8
z
zi
8
8
2
g
'. 70°C fj0 "C
Wavelenglh /nm 50°C
Fig. 10.
Change
of spectra
for transition from high to room temperature sensor, oxidized sample 300 ppb 02).
(20&5o”C
with external
Oxidation of stainless steel under BWR conditions
1779
120 -0
100
-13h 80
21h
e ti
-
29h
60 -37h 40
-45h -55h i
-63h
100 80 f$
60 40
Wavelength Inm
I20 100 80
-
I59h
-
171h
f! ti
183h 60
I._.._.“, ,95h
40
20
Fig. Il.
i
-231h
DRS spectra recorded during oxide layer build up. (25O”C,average 200 ppb 02.)
207h
C. Degueldre et al.
0
50
150
100
200
250
Time h
-+-Absorption -t-
1st Inwferometry
-&-
2nd Interferomerry
peak
I (t”o.5)
-Model Model -Model
peak
2 (In I) 3 (1’9.666)
-i 250
Time 01
Fig. 12. Oxide layer thickness as a function of time
The reflection spectra for the transition from high temperature to room temperature with oxidised samples (Fig. 10) are then of great interest. Similarly, the effect of the temperature increase on the reflection spectra of the metal is also negligible (Fig. 9). From spectral data in Fig. 11 it is possible to calculate the thickness as a function of time by three approaches (see Fig. 12). In the infrared region, for short contact times, the thickness can be evaluated purely on the basis of absorption because there is no interferometry effect for the investigated wavelength range. Interferometry peaks appear for thickness above 80 nm. The thickness can be calculated from the maximum reflected wavelength. When the first peak becomes too wide, thicknesses above 140 nm, it is possible to use the second interferometric peak. As the thickness increases, the intensity of the interferometry effects decreases until eventually there is only the effect of absorption. Results by the three different methods are comparable (Fig. 12). The trend of the plot x (thickness) vs t (time) may be tested using a logarithmic x 0: In t or a power law x 0: t”, this is done in Fig. 12. It can be seen that the power law x c( t213best fits the experimental results. Problems which were encountered:
Oxidation of stainless steel under BWR conditions
1781
- The first problem was the inconsistency of the oxygen concentration in the loop water which was around 200 ppb in the available water. - An early problem which had an effect on the first set of spectra and which was quickly overcome was the stability of the Guided Wave unit. If it was switched off between sets of measurements it could not be guaranteed that it was in the exact same state for the next set of measurements. - Water flow rate over the sample surface, which has an effect on the corrosion rate, was not measured but plans are to install a sensor for the flow rate in the cell. - The temperature inside the cell was not measurable, it was assumed as the same temperature as in the rest of the loop. The temperature during transition was measured using a thermocouple on the outside of the cell (external sensor), the readings being mathematically converted into internal temperatures. - Over long periods the sapphire window in the cell corroded, they are slowly dissolved in the loop water, reducing the transparency. In this photometric analysis constant visibility of the sample is of paramount importance. - The samples used were visibly different even though they were made of the same material and prepared in exactly the same way. This would also be the case in any practical application. The sample non-uniformity means that numerous experiments must be made to ensure statistically relevant results. In the future, it may be possible to reduce the distance of the probe from the window. This would increase the sensitivity of the set up (AR,, increases). This parameter is heavily constrained by the cell design. There is only one or two millimetres before the cold finger on the probe touches the cell wall and the cooling jacket should be efficient enough to prevent the probe from being damaged. This work shows that layer thickness determination up to 200 nm is possible. Extention to thicker layers is feasible using better geometry (d smaller) a stronger light source and a wider spectral detection range, for example up to 2000nm with a PbS detector. With increasing sensitivity, it is also possible to measure layers around 2 nm. CONCLUDING
REMARKS
We have proved that DRS is a very useful tool for detecting, in-line, the build-up of the oxide layer on a stainless steel sample under BWR conditions. Since the corrosion is dependent on the initial oxide build-up, the analytical method should allow the detection of the oxide layer in sratu nascendi. Further improvements to the experimental set-up are planned for example by the introduction of stabilised zirconia windows, by in-line analysis of the temperature, the flow rate and the oxygen concentration at the sample surface. DRS allows verification of results with independent absorption and interferometric responses, however, the results should be compared to those of other techniques such as Rutherford back scattering, sputtered neutral mass spectroscopy and/or impedance spectroscopy, in order to cross check the accuracy of the calculated thicknesses. DRS is an excellent technique for in situ investigation and the detection of the layer thickness is possible from 2 to 200 nm. Acknowledgements-E.
Schenker is thanked for operating the loop and for the fruitful discussions we had during this work, H.P. Alder is thanked for his constructive remarks and his interest in this work, R. Knecht and E. Hitz are thanked for their technical assistance.
1782
C. Degueldre et al.
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