Corrosion Science, Vol. 26, No. 10, pp. 769-780, 1986 0010-938X/86 $3.00 + 0.00
Printed in Great Britain © 1986 Pergamon Journals Ltd.
T H E R O L E O F A L L O Y E D T U N G S T E N O N T H E
C O N D U C T I V I T Y O F S T A I N L E S S S T E E L P A S S I V E L A Y E R S A. IRHZO, Y. SEGUI,* N. Bui and F. DABOSI
Laboratoire de M6tallurgie Physique, U A du CNRS 445, E.N.S.C.T., Toulouse, France Abstract--Electrical measurements were carried out on passive films grown on W-containing stainless steels, with mercury as the second electrode. The measured charging and discharging currents showed the effect of W on the polarity inversion, leading to the assumption that W increased the ion density in the films. These ions were considered to be tungstate WO 2-, on the basis of their time constant and of previous surface analysis data. The frequency responses of the conductance and capacitance of the passive films further indicated that tungstate ions reduce the mobile positive charge density in the films. The beneficial effect of W in improving the passivity of stainless steels is interpreted in terms of the interaction with the positive ions and of the tungstate inhibitive effect.
I N T R O D U C T I O N
THE ADDITION of tungsten, initially intended to increase the abrasion resistance of 20Cr-25Ni-MoCu steels, has proved to be beneficial to corrosion resistance) -5 Tungstate dissolved in CI- solutions has the same effect as alloying with tungsten in the amelioration of the stainless steel passivity. 6 Tungsten, either from tungstate adsorption or from alloyed tungsten, is present in the passive layer in the 6+
oxidation state. 7 One of the proposed mechanisms is the direct reaction of W with H20 to form WO3 which interacts with other metallic oxides to increase the stability of the passive oxide layer and to strengthen the metal-oxide interface bond. 7
To have a better insight into the role played by tungsten, investigations on the electrical properties of the passive film have been undertaken. A survey of the literature pointed out a lack of studies on the influence of tungsten on the conduction mechanisms in the passive film. In this study, results obtained from electrical measurements on metal-film-metal structures and an analysis of possible conduction mechanisms are reported.
E X P E R I M E N T A L M E T H O D
The chemical composition of the steels studied are given in Table 1. Passive films were grown on cross sections of 1.5 mm diameter stainless steel rods. Control of the passive film growth was achieved by the polarization technique, using a Tacussel electrochemical arrangement composed of a potentiostat, a current record and a high impedance electronic millivoltmeter. The electrolyte was 3 M HNO 3. Passive films were cleaned in running distilled water then in ethyl alcohol and dried in a reduced pressure desiccator.
Electrical measurements were carried out with the passive film sandwiched between two metallic electrodes. The lower electrode was the steel substrate itself. Best results were obtained when the upper electrode was a mercury drop. Vacuum deposited gold or aluminium did not give satisfactory results. A step voltage was applied across this metal-film-metal structure. The voltage was regulated by a stabilized power source (HP 6827 A). The current was measured by a Keithley 612 electrometer, whose analog output allowed the recording of the 1-t curves. For measurement of the conductance and the capacitance
* Present address: Laboratoire de G6nie Electrique, LA du CNRS No. 304, Universit6 Paul Sabatier, Toulouse, France.
Manuscript received 19 July 1985; in amended form 26 March 1986.
769
770
FIG. 1.
A . IRHZO,
Y.
SEGUI,N. Bul and F. DABOSI E mV/5.SE
~700 / ~ ~
, 0o ///
B 6 4 II
B6
I B61
- 5OO
I I I I
10"t 10"s 10-4 1O'S [A/cr~
Anodic polarization curves of studied steels in 3 M HNO3, with potential scan rate o f l V h - : .
of the passive film, an a.c. bridge (General Radio 1659) was used, in the frequency range of 100 Hz to 100 KHz.
EXPERIMENTAL RESULTS AND D I S C U S S I O N Formation of the passive films
T h e anodic polarization~curves of the steels studied in 3 M H N O 3 at 20°C, with 1 V h -1 potential scan rate, are shown in Fig. 1. T h e s e stainless steels exhibit c o m p l e t e passivity in this acid solution, and the corrosion potential increases slightly when the W content in the alloy is increased. Figure 2 shows the current d e n s i t y - t i m e curves o b t a i n e d by polarizing the electrode at - 5 0 m V vs saturated sulphate electrode (SSE) in 3 M HNO3. A steady-state passive current was o b t a i n e d after a b o u t I h. In preliminary tests, electrical m e a s u r e m e n t s were carried out with passive films grown during 1,2 or 4 h at - 5 0 m V ( S S E ) . Dielectric b r e a k d o w n s were o b s e r v e d w h e n voltages o v e r 0.75 and 1.4 V were applied, respectively, to I h and 2 h passive films. H o w e v e r , the 4 h passive films could withstand voltages up to 1.7 V. This
TABLE 1. CHEMICAL COMPOSITION OF THE STUDIED STEELS*
Composition (wt %)
Steel C Mn Si Ni Cr Mo Cu W
B6 0.013 1.25 0.79 25.07 20.14 4.29 1.25
B6P 0.015 1.17 0.92 24.98 20.45 4.70 2.41 2.04
B64 0.015 1.00 0.87 25.18 20.70 4.60 2.29 3.90
* Heat treated at 1150°C for 15 rain and water quenched.
The role of alloyed tungsten on the conductivity of stainless steel 771
Io"
|Alcrn ~
10 3O lmn
FIG. 2. Current density-time curves of studied steels polarized at - 5 0 mV(SSE).
voltage is large enough to allow the plotting of meaningful current-voltage curves.
All subsequent results were obtained on 4 h passive films.
Measurements of transient currents
Figures 3 and 4 show the charging and discharging currents as a function of time, when 0.9 and 1.2 V were applied through passive films on B6 steel. The charging
FIG. 3.
log j
- 7
-8
-9
0 0
& & & •
0 0 0
O 0 0
• " " - , . c h a r g i n g
O
O
O d i s c h a r g i n g
o
o%
0
I I_
Charging and discharging current density as a function of time when a potential of
0.9 V is applied through the B6 steel passive film.
772 A. IRnzo, Y. SEGUI, N. Btrl and F. DABOSI
FIG. 4.
-8
-9 'logJ
7
0 I 2 3
Ilog
Charging and discharging current density as a function of time when a potential of 1.2 V is applied through the B6 steel passive film.
currents seem to follow a power law of the type J oc t-". T h e discharging current
differs greatly from the charging current. This behaviour suggests that the charges
involved in this process were not of a dipole-type. F o r a dipole-type film, the charging
and discharging currents would be of the same amplitude in the time domain
considered for this experiment. T h e observed characteristics can be related to a space
charge-type p h e n o m e n o n . This statement is supported by o t h e r experimental evi-
dence: when the applied voltage was 1.2 V, an inversion of the discharging current
was observed after a delay time of 10 s. T o explain this behaviour, one can assume
that the increase in the polarization voltage leads either to an increase in the n u m b e r
of charges available in the passive film or to an accumulation of charge in a smaller
region. T h e result is that the charge distribution p(x) is modified and that the charge
gradient is increased. T o clarify this idea, the negative charges which can be attracted
by the anode (Fig. 5) can be considered. If the electrode does not neutralize these
charges completely, a space charge will be established. Figure 5 represents schematic
distributions of the charge density p and of the field E vs the distance x, calculated
from the Poisson equation BE(x, t)/~x = p(x, t)/eeo. It can be seen that f r o m x = 0
to x = z, the negative charges are u n d e r a negative field and from z to r0, the field is
positive. U p o n discharging, the charges will then move in opposite directions and will
give rise to opposite currents in the circuit. T h e sign of the overall current will depend
on the algebraic sum of the elementary currents. It should be n o t e d that the zero field
position during charging is time-dependent, because at t = +At, the p(x) distribution
will change, so that E (x, + At) will differ from E (x, 0) and consequently z ( + At) will
differ from z(0). A numerical resolution of the fundamental equations has been used
to describe the p h e n o m e n o n and to allow us to verify that the discharging current can
have a polarity inversion as a function of time.
The role of alloyed tungsten on the conductivity of stainless steel 773
I®®® I
.,IO®®®
i ® ®®®
lp(x) f i~E(x)
J
0 r° I
::r I I I
FIG. 5. Schematic representation of the charge distribution in the film under an applied voltage V and of the variation of charge density and of the field E as a function of the distance
x between the electrodes.
Concerning the B64 steel, Fig. 6 shows that the current polarity inversion appears already at an applied voltage of 0.6 V. Assuming that the thickness of the passive film on this W-containing steel is not less than that for the B6 passive film, and considering that the applied field (V/d) is not higher, charge accumulation takes place more readily in the B64 than in the B6 passive film. Whence we infer that the addition of
-iO
40,5
-11
-I1,5
FIG. 6.
log j
0 0 0 0
U 0 0 0
O 0 0 o
o o o o o °
charging
° o odischarging "
V
V
V
• Iogt
~ ~ ~ •
Charging and discharging current density as a function of time when a potential of
0.6 V is applied through the B64 steel passive film.
774 A. IRHZO, Y. SEGOI, N. BUI and F. DABOSI
log d
o
-10.25
:10,5
:I0,75
0 0
0 0
0
0 0
0
0
0
0 o charging
o o
o
o o
o
o o
0 o
o
o o
o 0
o discharging
: 1 1 o
FIG. 7.
0 0
o 0 0
0
Charging and discharging current density as a function of time when a potential of 0.9 V is applied through the B64 steel passive film.
tungsten to these stainless steels either enhances the ion production in the film or else activates the transport of existing ions in the film, leading to an increase in the charge gradient under the same applied field. Figure 7 also shows the non-linear behaviour of the passive film on B64 steel when the applied voltage is 0.9 V.
Electrical characteristics as a function of frequency
The conductance G and the capacitance C of the passive film were measured at various frequencies and under different voltages. Figures 8 and 9 show the plots of log C vs log to and log (G/to) vs log to. The linear relations indicate that the passive films obey the universal law in to,-1, as do most solid materials with the exponent in the range 0 < n < 1.9 The slopes of the log Cvs log to lines are - 0 . 9 6 , - 0 . 8 and - 0 . 7 6 , respectively for B6, B6P and B64 steels. It has been shown by other authors that the slope values approach 1 when ion transport is the main conduction mechanism. 9 For example, in a study of SiO2 doped with SnO2 or Sb20 5 ,t0 the slope was 0.95, the same value found for the B6 steel in this study.
The above experimental results suggest that the passive films did not behave as
conventional dielectrics having few mobile charges (thus having a flat frequency
response for C), but rather as an ionic conductor. In that case, a direct current
polarization will lead to ion migration and an important capacity variation.
The role of alloyed tungsten on the conductivity of stainless steel 775
FIG. 8.
log C
t0-9
The frequency dependence of the capacitance C of the passive films developed on B6, B6P and B64 steels.
FIG. 9.
log (~- }
10-s
%.
~6 I " ' ~ \
10-'° \ \
\ \
\
The frequency dependence of the conductance G expressed as GIoo (dielectric loss)
of the passive films developed on B6 and B64 steels.
776 A. IRHZO, Y. SEGUI, N. Bu[ and F. DABOSI
0,5
•
I | I
-O,S 0 0,5 4 "v
FIc. 10. Variation of the capacitance C, relative to the value Co measured at - 1 V, as a function of the d.c. applied voltage on B6 and B64 passive films, at the frequency of 1 KHz.
Figures 10 and 11 show large variations in the capacitance as a function of the d.c.
applied voltage, respectively, at 1 Hz and 10 Hz, for B6 and B64 steels. To explain this behaviour, the measured capacitance can be considered as the sum of a volume capacitance Cv in series with a surface capacitance Cs which is related to the deserted region where ions are repulsed by the electrode (Fig. 12). When the mercury electrode is positive, i.e. in the positive voltage domain, it can be seen in Figs 10 and 11 that the capacitance is decreased. It can be assumed that the positive ions were displaced, creating a surface capacitance, so that the overall capacitance was lowered. In the negative voltage domain close to - 1 V, the capacitance remains nearly constant, verifying that the passive film contains positive ions and that no surface capacitance is developed in this case. Figures 10 and 11 show that tungsten additions to the steels have resulted in an increase of the capacitance value for any applied voltage. This feature may indicate a diminution of the positive charge density and/or a partial blocking of the positive ions by tungsten species in the film.
Additional experimental evidence to confirm the above result is obtained by plotting 1/C 2 vs V, the slope of which is inversely proportional to the charge density, according to the following relation: 11
- - V +
C 2 q e e o N i qeeoNi
where ~b n is the metal-film barrier height, e the permittivity of the film, N i the ion
density. In Fig. 13 linearity is obtained over a large potential range. The slopes of
these straight lines increase when the amount of tungsten in the steel is increased. It
The role of alloyed tungsten on the conductivity of stainless steel 777
c_
Co
1
B64
8 6 ~
0 5 •
-0,5 0 0,5 ~ " I
V ~
FIG. 11. Relative variation of the capacitance C as a function of the d.c. applied voltage on B6 and B64 passive films, at the frequency of 10 KHz.
follows from this that the ionic character of the film, as reasoned out from the analysis of the log C vs log to curves, is less p r e d o m i n a n t as the W content is increased. T h e decrease in the n u m b e r of positive charges in the passive film can be considered as a result of their neutralization by negative charges afforded by tungsten species.
F r o m the analysis of the transient currents, the addition of W to the steels studied led to either the direct creation of new ions or to the activation of preexisting ions. By the m e a s u r e m e n t of frequency response, alloying with tungsten led to a decrease in the n u m b e r of positive charges in the passive films. These two results seem to be contradictory at first glance. In the transient current studies, the charges considered have a time constant of the o r d e r of 10 2 to 10 3 s. T h e n the equivalent frequency can be evaluated, by the toz = 1 relation, to be in the range of 10 -3 to 10 -4 Hz. In our
F[6. 12.
+V
T
w +Cs
• @ • :1~ Cv
• ®
• •
&
Schematic representation of volume capacitance Cv in series with surface capaci-
tance Cs when positive ions are repulsed by the positive electrode.
778 A. IRHZO, Y.
S E G U I ,N. But and F. DABOSI
30
20
IG
_- "2- ----
I !
-0,5
oB6
I I
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