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The corrosion behaviour of iron-chromium alloys in 0.5 M sulphuric acid
Vol. 33, No. 5, pp. 765-778, t992 Printed in Great Britain.
0010-938X/92$5.00+ 0.00 © 1992PergamonPress plc
Corrosion Science,
THE
CORROSION ALLOYS
BEHAVIOUR OF IRON-CHROMIUM IN 0.5 M SULPHURIC ACID
J. A. L. DOBBELAAR,E. C. M. HERMAN and J. H. W. DE WIT Delft University of Technology, Laboratory of Materials Sciences, Division of Corrosion Technology and Electrochemistry, Rotterdamseweg 137, 2628 AL Delft, The Netherlands Abstract--The corrosion of five different iron-chromium alloys was studied in 0.5 M H2SO4, using polarization curves and impedance measurements. The data obtained were compared with the data obtained for pure iron and chromium. It was concluded that the passive current density for the alloys with a Cr content -<9% is diffusion controlled. For alloys with a higher Cr content, 13 -< Cr% -< 25, and the pure metals this is not the case. The passive film on the lower alloyed iron-chromium alloys is a porous film just after the active-passive transition. This porosity decreases with increasing chromium content and/or potential improving the protective quality of the film.
INTRODUCTION STAINLESS STEEL is a collective n o u n for alloys which mainly consist of iron, c h r o m i u m and nickel. T h e y have the p r o p e r t y of s p o n t a n e o u s l y passivating and t h e r e b y protecting themselves f r o m the e n v i r o n m e n t . The m o s t i m p o r t a n t alloying e l e m e n t of stainless steel is c h r o m i u m which induces the f o r m a t i o n of a g o o d protective oxide film. A c c o r d i n g to the literature the alloy must contain at least 13% c h r o m i u m to f o r m a stable passive film. 1,2 T h e explanation for this critical a m o u n t of c h r o m i u m has b e e n the subject of a n u m b e r of studies. 3'4"5 T h e f o r m a t i o n of the film and the structure of the passive film itself have b e e n studied to u n d e r s t a n d the reason for the g o o d protective properties o f the alloys. 6-t3 T h e passivation m e c h a n i s m and the processes occurring in the passive region of the polarization curve are not fully understood. Most data on the passive layer of the alloys c o m e f r o m surface analytical techniques. Based on A E S and X P S m e a s u r e m e n t s it is concluded that the passive layer is enriched in c h r o m i u m . 10 T h e thickness of the passive film on F e - 1 7 C r alloys is d e t e r m i n e d to be in the o r d e r of 2 nm. Polarization curves of F e - C r alloys were r e c o r d e d by Olivier, 14 El_Basiounyl and K e d d a m . 6"7 K e d d a m also p e r f o r m e d i m p e d a n c e m e a s u r e m e n t s directed towards the active region and the first part o f the active-passive transition region. F r o m electrochemical studies F r a n k e n t h a l t5-17 concludes that two types of passive films are f o r m e d on an F e - 2 4 C r alloy: one m o n o l a y e r of which the f o r m a t i o n and reduction is reversible, and a s e c o n d multi-layer film which actually forms the oxide film. N o s t a t e m e n t was m a d e a b o u t the composition of the passive film. R e c e n t l y G e r r e t s e n s h o w e d that iron plays an i m p o r t a n t part in the f o r m a t i o n of the passive film at the end of the passive region. 11
Manuscript received 15 January 1991; in amended form 2 March 1991. 765
766
J . A . L . DOBBELAAR, E. C. M. HERMAN and J. H. W. DE WIT
The passivation processes and properties of the passive film are influenced by, for example, the state of the alloy 18 and the environment. 19 The most interesting parameter in the study of passive materials is the anodic part of the passive current density. This part of the passive current density is a direct measure for the corrosion rate. A description of the passive current density in terms of variables as the alloy composition, environment and interface would be of great importance for corrosion protection. The number of parameters in practical applications is enormous, which makes such a description difficult. In this study all environmental and surface state parameters were fixed and only the alloy composition and the potential were varied. The influence of these parameters on the passive current density was studied by recording polarization curves. To obtain more information about the processes governing the passivation and passive state other techniques have to be applied. Impedance measurements were performed because they can reveal such detailed information, since impedance data reflects the contribution of all potential dependent processes governing the passive current density. EXPERIMENTAL
METHOD
Five i r o n - c h r o m i u m alloys of the composition Fe-6Cr, Fe-9Cr, Fe-13Cr, Fe-17Cr and Fe-25Cr were used in the electrochemical study. T h e composition of the alloys, given in Table 1, was determined by averaging E P M A (Electron Probe Micro-analysis) results m e a s u r e d at various spots on the surface. For comparison of the results electrodes of pure iron and c h r o m i u m were used. The alloys were prepared by arc-melting a mixture of pure iron ( M R C , 99.99) and pure c h r o m i u m ( M R C , 99.996) u n d e r A r atmosphere. Cylinders of the alloy were m a d e ( ~ 3 m m ) , by spark-machining. These cylinders were ground. The iron, Fe-6Cr, F e - 9 C r and Fe-13Cr alloys were etched in 2 M H 2 SO 4 (Merck, 95-97% p.a.). The F e - 1 7 C r and F e - 2 5 C r alloys and the pure c h r o m i u m were etched in 3 : 1 concentrated hydrochloric acid and nitric acid (36-38%, analytical grade, Baker). After etching, the electrodes were annealed for 2 h at 850°C u n d e r A r atmosphere, followed by furnace cooling. Before e m b e d d i n g the electrodes in an epoxy resin they were covered with a thin undercoating and m e a s u r e m e n t s were performed using the electrochemical set-up as described elsewhere .20 Potentials were m e a s u r e d against a Hg/HgSOn/sat reference electrode [+660 m V ( N H E ) ] . All potentials are given with respect to this potential. Before performing electrochemical m e a s u r e m e n t s the electrodes were ground and polished down to 1/~m. The electrodes were left at open circuit for about 15 min, after which a polarization curve was recorded with a scan rate of 0.1 m V s- I from - 1 . 2 to 0.8 V (Hg/HgSO4). W h e r e impedance m e a s u r e m e n t s were performed, the electrode was polarized for two days at a starting potential (0 mV) after which the m e a s u r e m e n t was done. The electrode was then polarized at a potential cathodic (in some cases anodic) to the starting potential, left for 12-16 h at this potential to establish the stationary state and again an impedance m e a s u r e m e n t was performed. In this way, at a n u m b e r of selected potentials, impedance m e a s u r e m e n t s were performed going from the passive state to the active state.
TABLE 1.
T H E CHEMICAL COMPOSITION OF THE ALLOYS DETERMINED WITH E P M A
Fe-6Cr %Cr %Fe %Mo %W %C %N %0 %Si
6.22 +_ 0.09 94.76 + 1.24 0.01 + 0.02 0.01 + 0.01 0.27 _+ 0.01 . . 0.01 + 0.01
Fe-9Cr 9.32 92.18 0.02 0.01 0.33 . .
_+ 0.16 _+ 0.63 _+ 0.02 _+ 0.01 _+ 0.04 . . 0.03 _+ 0.03
Fe-13Cr 13.55 87.86 0.01 0.03 0.33 . . 0.01
Fe-17Cr
_+ 0.18 _+ 0.58 + 0.01 _+ 0.02 _+ 0.13
17.41 83.23 0.01 0.01 0.48
_+ 0.07 _+ 0.4 + 0.02 + 0.01 _+ 0.16
Fe-25Cr 25.88 76.06 0.02 0.04 0.31
_+ 0.35 _+ 1.09 + 0.01 + 0.03 + 0.02
. . + 0.01
0.01 _+ 0.01
0.02 + 0.01
F e - C r a l l o y s in H 2 S O 4
100
I
I
I
767
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-
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I -0.8
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i (Fe)
o
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o
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•
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,
i(Fe-13Cr)
A
i(Fe-17Cr)
•
i (Fe-25Cr)
J 0.8
p o t e n t i a l (V) The polarization curves of iron, chromium (part), Fe~SCr, Fe-9Cr, Fe-13Cr, Fe-17Cr, Fe-25Cr.
EXPERIMENTAL
RESULTS
Polarization curves
The polarization curves are shown in Fig. 1 and are similar to those reported by Olivier, 14 E1-Basiouny et al. 1 and Keddam et al. 6 Tafel slopes were obtained from the polarization curves by fitting an exponential function to a small potential range of the curve. In Table 2 the measured Tafel slopes are given, together with some other parameters which were obtained from the polarization curve. The interpretation of the value of the Tafel slope (ban) must be done with care since it cannot be used in the determination of the dissolution mechanism when, as in the case of chromium, the surface diffusion of active species plays a part in the mechanism. 21 A decrease in the passive current density (the lowest current density in the passive range) with increasing chromium content is found as displayed in Fig. 2. The passive current density obtained from the polarization curve is not a stationary passive current density. The stationary passive current density is reached after prolonged polarization and the value obtained after 24 h of polarization is also plotted in Fig. 2.
TABLE 2.
PROPERTIES OBTAINED FROM THE POLARIZATION CURVES OF THE ALLOYS
Alloy
ban ( m V )
E .... mV (Hg/HgSO4)
Fe Fe~SCr Fe-9Cr Fe-13Cr Fe-17Cr Fe-25Cr Cr
49.3 82.7 -+ 1.4 85 _+ 2 95 -+ 0.9 77 _+ 3 81 + 1 --
-957 - 9 8 9 _+ 9 - 1 0 0 5 _+ 3 - 9 8 8 _+ 10 - 1 0 1 0 _+ 5 - 1 0 2 3 _+ 5 -1125 + 5
Epass m V ( H g / H g S O 4 ) -184 -752 -794 -832 -892 -1025
112" _+ 4 _+ 35 _+ 0 . 5 t _+ 11 + 3 _+ 2
icrit ( m A ) 200 300 _+ 13 57.8 _+ 0.6 21.4 _+ 0.4 19 _+ 4 11 _+ 1 3 + 0,7
/pass ( ~ A )
1.2 2.2 11.1 20 1.7
20 _+ 1.5 x 102 _+ 0 . 6 x 102 + 0.6 _+ 0.7 _+ 0.3 10 2
* A t this p o t e n t i a l the c u r r e n t c h a n g e is the s t e e p e s t . t A c l e a r s e c o n d p e a k is f o u n d f o r F e - 9 C r at E = - 5 0 3 + 15 w i t h a c u r r e n t d e n s i t y o f i = 57 + 2, for F e - 1 3 C r at E = - 5 5 0 + 8 w i t h a c u r r e n t d e n s i t y o f i = 3.4 _+ 3.
768
J . A . L . DOBBELAAR, E. C. M. HERMANand J. H. W. DE WIT
10-3 ,--, 10-4 ,¢
10"5
.~
106
o
o
10-7
FIG. 2.
%Cr The passivecurrent densityobtainedfromthe polarizationcurves (closedcircles) and after 24 h of polarization(open circles).
Impedance data Impedance diagrams were obtained for the five alloys at various selected potentials. For all alloys three impedance diagrams are shown in Figs 3-7. The diagrams were selected in such a way to portray clearly the change of the impedance with changing potential. Also diagrams for iron (Fig. 8a and b) and chromium (Fig. 8c) are shown. Both Nyquist and Bode diagrams are displayed. The Nyquist plot clearly pictures the low frequency part of the impedance whereas in the Bode plot, specially the phase angle, shows what is happening in the high frequency region. The low frequency part. Looking at the Nyquist diagram for iron (Fig. 8a) one capacitive loop is obtained at -200 mV which is in the passive region for iron. Polarizing the metal cathodically to this potential the active-passive transition region is reached. The impedance bends over to the negative real axis with decreasing frequency which is common for diagrams obtained in the active-passive transition region (Fig. 8b). An explanation of such an impedance diagram obtained for chromium could be given in terms of a change in surface coverage. 21 The increase of surface coverage of the passive film on iron can be treated in a similar way to explain the data. This was done by Keddam et al. 19In the passive region chromium seems to behave almost as a capacitor, but data analysis showed that two time constants are present. A small deviation of the capacitance in terms of a Constant Phase Element (CPE) was found. Besides the time constant resulting from the double layer behaviour and the dissolution of chromium through the passive film, changes in the passive film can contribute to another capacitive loop in the impedance data as found for chromium a t - 8 5 0 . 21 Looking at the impedance data obtained for the alloys a capacitive loop is found in the passive region which is depicted in Figs 3(a), 4(a) to 7(a). On measuring the impedance at more cathodic potentials a straight line at low frequencies is obtained for the lower alloyed steels. The angle which these lines make with the horizontal axis is about 45 ° (Fig. 3b and 4b). When such a phenomenon is found at low frequencies in the Nyquist plots, it is interpreted as being due to a diffusion process controlling the overall reaction. The potentials at which the straight line is measured correspond with the passivation potential of iron. No influence of stirring the solution on the passive current density was found. This means that the diffusion layer does not extend into the electrolyte. In the diagrams of Fe-13Cr, Fe-17Cr and Fe-25Cr and
F e - C r alloys in H2SO4
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FIG. 3. The impedance data represented in both Nyquist and Bode plots for Fe-6Cr at the potentials given in the plots. In the Bode plot the + represents the phase angle, and the [] represents I z l . Frequencies are in Hz.
chromium such a straight line is not found. At potentials in the active-passive transition region Figs 3(c), 4(c) to 7(c) and 6(b) and 7(b) an extra time constant due the passivation process appears. The impedance bends over to the negative real axis and the time constant due to the diffusion controlled process is overruled by the passivation process. It seems that particularly in lower alloyed steels diffusion-controlled transport through the passive film is the rate determining process. Chao et al. proposed a model
770
J . A . L . DOBBELAAR, E. C. M. HERMAN and J. H. W. DE WIT
with the diffusion of vacancies through the passive film as the rate determining process for nickel. 22 The impedance is similar to the impedance expression for the Randles circuit. It was possible to obtain a good fit of the low frequency part of these impedance diagrams using the Randles circuit.
The high frequency part. From the phase angle of the impedance data of iron and the iron-chromium alloys in the passive region it is clear that two time constants are i
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*
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frequency (Hz) e: -375 mV
Fro. 5. The impedance data represented in both Nyquist and Bode plots for Fe-13Cr at the potentials given in the plots. In the Bode plot the + represents the phase angle, and the [] represents ]Z]. Frequencies are in Hz.
present. Data analysis showed that two time constants are also present in the impedance data of chromium. Since no change in surface coverage at these potentials is contributing to the impedance, another process must be taking place. A number of processes are possible candidates to explain a time constant. Film growth can occur with increasing potential as shown by Sato. 23 This potential-dependent process can contribute a time constant to the impedance. The composition of the passive film
772
J.A.L.
DOBBELAAR,
E. C. M. HERMAN and J. H. W. DE WIT
changes with potential. This was the conclusion of the potential modulated reflection spectroscopy study performed by Hara e t al. 24 An extra time constant in the impedance can also be attributed to this potential dependent process. But the structure of the passive film is also dependent on the amount of water, which can be removed by dehydration, a process which can be potential-dependent and therefore is able to yield a time constant. The oxidation state of the cations in the passive film is also dependent on the potential and a further
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.....
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10 -3 10 -2 10 -1 10 0 101 10 2 10 3 10 4 frequency (Hz) c:
-600 m V
FIG. 6. The impedance data represented in both Nyquist and Bode plots for Fe--17Cr at the potentials given in the plots. In the Bode plot the + represents the phase angle, and the [] represents Izl. Frequencies are in Hz.
F e - C r alloys in H2SO 4
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......................... -~ 30 10 -3 10 -2 10 -1 10 0 10 1 10 2 10 3 10 4
frequency (Hz) a: 0 m V
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frequency (Hz) mV
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10 -3 10 -2 10 -1 10 0 10 1 10 2 10 3 10 4
frequency (Hz) c: -600 m V
Fl6. 7. The impedance data represented in both Nyquist and Bode plots for Fe-25Cr at the potentials given in the plots. In the Bode plot the + represents the phase angle, and the [] represents IZ[. Frequencies are in Hz.
oxidation of these ions can also yield a time constant. It is clear that a detailed description of the passive film must include these processes and must also take into account the lateral surface inhomogeneity as was found from studies on Fe-25Cr with different microstructures, is If all processes mentioned contribute to the impedance measured in the frequency range of 105-10 -3, it is clear that a large overlap in time constants will occur which makes it difficult to separate them. Using the impedance analysis software package EQUIVCRT, which has been described
774
J . A . L . DOBBELAAR,E. C. M. HERMANand J. H. W. DE WIT
before, 21 an attempt was m a d e to determine the n u m b e r of time constants in the impedance diagrams. It was assumed that all processes were parallel processes and it was found that in most plots, measured at the alloys, a reasonable to good fit was obtained with three or four time constants. T w o time constants were found for iron in the passive state, but four time constants are present in the active-passive transition region. Fitting only the high
i
iron 10 4
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103
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E 'T
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i
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101
;2o
100 10-1
•
.....
u
....
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....
•
....
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....
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0
10 -3 10"2 10 -1 l0 0 101 10 2 10 3 10 4
0e+0 2e+3 4e+3 6e+3 ge*3 real part (Ohm)
frequency (Hz) a: 200 m V
iron
IOta+4- + 4- 4- +0.1
10 3
200
101 1
100
N 10° 1 10 -1 | 0 10 -3 10 -2 10 -1 100 101 102 103 104
!
-2e+2
0e+0 2e+2 real part (Ohm)
frequency (Hz) b: -$0 m V chromiumlo 7
,_. 3e+6
90 I
lO6-
+ Im
O
--,
4-
2e46
e.
105" 104-
"80 v
e'L
io 3-
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10 °
60 10 -3 10 -2 10 -1 10 0 101 10 2 10 3 10 4
frequency (Hz) c: -600 m V
Fro. 8. The impedance data represented in both Nyquist and Bode plots for Fe and Cr at the potentials given in the plots. The top two plots are impedance data measured at iron, the bottom plot is measured at chromium. In the Bode plot the + represents the phase angle, and the [] represents IZI. Frequencies are in Hz.
Fe-Cr alloys in H2SO 4
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FIG. 9. (A) (top) The value of Y0 obtained by fitting the high frequency part of the impedance data for Fe-6Cr (open squares), Fe-9Cr (closed circles) and Fe-17Cr (closed squares). (B) (bottom) the n corresponding with the Y0 obtained from the fit.
frequency part of the impedance in the frequency range of 5 x 102-104 Hz a good picture of the double layer capacitance for the various alloys and potential can be obtained. In all cases the double layer capacitance was found to be a Constant Phase Element (CPE). The CPE is defined as 1/[Y0(j~o)"] (impedance representation). Unfortunately the Y0 values obtained by fitting a Q at the high frequency data cannot be compared since their dimension depends on the value of n. In Fig. 9(a) the values obtained for the metals and different alloys are plotted against potential merely to show that a change is occurring. For the sake of clarity only the data obtained for Fe6Cr, Fe-13Cr and Fe-17Cr are shown. In Fig. 9b the corresponding n values are displayed. In all cases the change in Y0 and n is such that a larger deviation from an ideal capacitance with decreasing potential is found. This changing CPE is clear in the phase angle in the Bode diagram at high frequencies, by the compressing of the maximum when going from anodic to cathodic potentials. DISCUSSION
The passive film on the alloys and the metals studied here is considered to be amorphous. From the low frequency impedance data it can be concluded that the ion transport through the passive film is the rate controlling factor for the Fe-6Cr and Fe-9Cr alloys. The rate controlling process for the pure metals and Fe-13Cr, Fe-17Cr and Fe-25Cr alloys cannot be pinned but will certainly not be diffusion controlled. A g e n e r a l m o d e l c a n b e f o r m u l a t e d as f o l l o w s M ~,~ M "+ + n e -
metal-metal oxide interface
(1)
M "+ t r a n s p o r t
oxide
(2)
776
J.A.L.
DOBBELAAR,E.
C. M. HERMAN and J. H. W. DE WIT
Mn+ ~.~ Mn+,soi
oxide-solution interface
H 2 0 ~- Oox + 2H + + 2 e -
oxide-solution interface
0 2 - transport
oxide.
(3) (4) (5)
It is not clear which species M ~+ or 0 2 - is being transported. Calculating the impedance, 25 the electron transfer reaction at the metal-metal oxide and the metal oxide-solution interface will yield resistances. These resistances are parallel resistances and will add up to one resistance. If the transport of species through the passive film is diffusion-controlled as in the case of the lower alloyed steels, a Warburg impedance is in series with this summed resistance and will determine the low frequency range. If the transport is due to the high electric field a resistance due to the transport is in series with the summed resistance. In this case no Warburg impedance is found. These two situations are extremes and it is possible that both will occur at the same time. The amount which each transport process contributes to the total transport will determine whether a Warburg impedance is found or not. The passivation of the iron-chromium alloys occurs in two steps as can be seen in the polarization curves. Based on an earlier publication it is assumed that the first active-passive transition is due to the formation of a (mono-layer) chromium oxide passive film, while in the second transition iron contributes to the passivation, a1'26 Recently Kirchheim et al. 27 described the relation between the passive current and the Cr enrichment of the film, based on XPS measurements. The lateral distribution of Cr in the alloy and film was not taken into account, nor was the influence of incomplete surface coverage for low Cr alloys. In the lower alloyed metals the amount of chromium is too low to cover the total surface with chromium oxide and passivation will not be complete. This idea is theoretically supported by the percolation model of Sieradzki and Newman. 4 A porous passive film is formed after the first current peak in the polarization curve. The amount of porosity decreases with increasing chromium content. The distribution of chromium in the alloy is also of importance in the formation of a good protecting passive film as found from measurements on Fe-25Cr with varying microstructure. 18 It is apparent that the activation energy for various reactions in a porous layer will have a large degree of distribution. Considering an inhomogeneous surface 24 with a distribution in activation energy, a CPE can be explained. The larger the distribution in activation energy the more the CPE deviates from a true capacitance. If the processes, which are subject to a distribution of activation energy, contribute a time constant in the high frequency region, the double layer capacitance, normally obtained from this frequency region, will deviate from a true capacitor. A possible candidate process is film growth. When metal ions are generated at the metal-metal oxide interface, oxygen ions are formed at the metal oxide-solution interface. In this way a capacitance is created, which is in series with a resistance due to the electron transfer reactions generating metal ions and oxygen ions. The capacitance is not only a capacitance due to the separation of charge (metal-ions at the metal-metal oxide interface and oxygen ions at the metal oxidesolution interface), but also due to the storage of charge in the film (which is similar to a capacitance created by a change of surface coverage; see Refs 24, 25). This can explain the increasing deviation from ideal capacitive behaviour with decreasing
Fe-Cr alloys in H2SO 4
777
potential ( = increasing porosity) as shown in Fig. 9(a) and (b). The porosity of the passive film is not only a reflection of the amount of chromium but also of the distribution of the chromium in the alloy. 18The lateral inhomogeneity can be treated in a model by dividing the surface in an infinite n u m b e r of areas and considering a distribution of the c h r o m i u m content over the areas. The admittance of all the areas s u m m e d yields the total admittance. A two-dimensional distribution is then created, namely a distribution representing the potential dependent porosity in the passive film for a certain chromium content and a distribution of the local amount of chromium. It is clear that areas with a low local chromium content are the least protective places in the alloy. Unfortunately on the sole base of the impedance m e a s u r e m e n t s it is not possible to identify the time constants present. O t h e r techniques, which are able to separate the processes mentioned, are necessary to pin the various time constants to processes. It is possible that if, as shown 22 for chromium, strong coupling between processes exist the impedance cannot be represented by equivalent circuits. A more fundamental description in terms of a transfer function will be needed. Since detailed information about the processes occurring during the passivation and in the passive state is not yet available no detailed description of the passivation process of the iron chromium alloys can yet be given. CONCLUSION The passive current of i r o n - c h r o m i u m alloys decreases with increasing chromium content. The passive current versus the chromium content is shown in Fig. 2. With increasing chromium content the polarization curve changes from iron like to chromium like. The pasivation becomes m o r e and more dominated by the formation of a chromium oxide/hydroxide passive layer. Iron starts to contribute to the passivation at about - 2 0 0 inV. This contribution results in a decrease of porosity. In the potential region of about - 8 0 0 to - 2 0 0 m V (Hg/HgSO4) the amount of porosity in the passive film decreases with increasing potential and chromium content. The transport of ions through the passive film is diffusion-controlled in the lower alloyed metals. In the pure metals and higher alloyed metals no diffusion controlled transport is found. The transfer function describing the impedance diagrams could not be established. REFERENCES 1. M. S. EL-BASIOUNYand S. HARUYAMA,Corros. Sci. 16,529 (1976). 2. K. SIERADZKIand R. C. NEWMAN,J. electrochem. Soc. 133, 1979 (1986). 3. H. H. UHH6, Passivity and Breakdown o f Iron and iron base alloys, U.S.A.-Japan seminar (eds R. W. STAEHLEand H. OKADA),NACE, Houston (1975). S. Q1AN, R. C. NEWMANand K. SIERADZKI,J. electrochem. Soc. 137,435 (1990). P. BRI]ESCH, K. MILLER and H. R. ZELLER,Surf. S£i. 169, 327 (1986). M. KEDDAM,O. R. MATI'OSand H. TAKENOUTI,Electrochim. Acta 31, 1147 (1986). M. KEDDAM,O. R. MATTOSand H. TAKENOUTI,Electrochim. Acta 31, 1159 (1986). S. MISCHLER,H. J. MATHIEUand D. LANDOLT,Surf. Interface Anal. 11,182 (1988). 9. I. OLEFJORDand H. F1SHMEISTER,Corros. Sci. 15,697 (1975). 10. P. MARCUSand I. OLEEJORD, Corros. Sci. 28, 589 (1988). II. H. GERRETSEN, Passivation and breakdown of passivity of stainless steel constituents, chromium, 4. 5. 6. 7. 8.
iron-chromium, iron-chromium-molybdenum and nickel. Thesis, Technical University Delft (1990). 12. S. C. TJoyc, R. W. HOFMANand E. B. YEAGER,J. electrochem. Soc. 129, 1662 (1982).
778 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27.
J . A . L . DOBBELAAR,E. C. M. HERMANand J. H. W. DE WIT Y. KOLOa~RKIN,Electrochim. Acta 18, 593 (1973). R. OLIVIER,Thesis, Leiden University (1955). R. P. FgaNKENTnAL,J. electrochem. Soc. 114, 542 (1967). R. P. FgaNKENTnAL,J. electrochem. Soc. 116, 581 (1969). R. P. F~NKENTHAL, J. electrochem. Soc. 116, 1646 (1969). J. A. L. DOBBELAAR,E. C. M. HERMANand J. H. W. DE WIT, Corros. Sci. 33,779 (1992). M. KEDDAMand C. PALLO~A,J. electrochem. Soc. 132,781 (1985). J. A. L. DOBBELAARand J. H. W. DE WIT, to be submitted to J. Appl. Electrochem. J. A. L. DOBBELAARand J. H. W. DE WIT, J. electrochem. Soc. 137, 2038 (1990). C. Y. CHAO, L. F. LtN, D. D. MACDONALD,J. electrochem. Soc. 129, 1874 (1982). N. SATO,Passivity of Metals, Proc. Syrup. Passivity (eds R. P. FRANKENTnAEand J. KRUGER),p. 29. The Electrochem. Soc., Princeton, New Jersey (1978). N. HAga and K. SUGIMOTO,J. electrochem. Soc. 126, 1328 (1979). J. A. L. DOBBELAARand J. H. W. DE WIT, to be submitted to Electrochim. Acta. J. H. GERRETSENand J. H. W. DE WIT, Proc. 9th Eur. Corr. Congress, Utrecht, The Netherlands, FU-056 (October 1989). R. KIRCHHEIM,B. HEINE, H. FISCHMEISTER,S. HOFMAN,H. KNOTEand U. STOLZ, Corros. Sci. 29,899 (1989).
0010-938X/92$5.00+ 0.00 © 1992PergamonPress plc
Corrosion Science,
THE
CORROSION ALLOYS
BEHAVIOUR OF IRON-CHROMIUM IN 0.5 M SULPHURIC ACID
J. A. L. DOBBELAAR,E. C. M. HERMAN and J. H. W. DE WIT Delft University of Technology, Laboratory of Materials Sciences, Division of Corrosion Technology and Electrochemistry, Rotterdamseweg 137, 2628 AL Delft, The Netherlands Abstract--The corrosion of five different iron-chromium alloys was studied in 0.5 M H2SO4, using polarization curves and impedance measurements. The data obtained were compared with the data obtained for pure iron and chromium. It was concluded that the passive current density for the alloys with a Cr content -<9% is diffusion controlled. For alloys with a higher Cr content, 13 -< Cr% -< 25, and the pure metals this is not the case. The passive film on the lower alloyed iron-chromium alloys is a porous film just after the active-passive transition. This porosity decreases with increasing chromium content and/or potential improving the protective quality of the film.
INTRODUCTION STAINLESS STEEL is a collective n o u n for alloys which mainly consist of iron, c h r o m i u m and nickel. T h e y have the p r o p e r t y of s p o n t a n e o u s l y passivating and t h e r e b y protecting themselves f r o m the e n v i r o n m e n t . The m o s t i m p o r t a n t alloying e l e m e n t of stainless steel is c h r o m i u m which induces the f o r m a t i o n of a g o o d protective oxide film. A c c o r d i n g to the literature the alloy must contain at least 13% c h r o m i u m to f o r m a stable passive film. 1,2 T h e explanation for this critical a m o u n t of c h r o m i u m has b e e n the subject of a n u m b e r of studies. 3'4"5 T h e f o r m a t i o n of the film and the structure of the passive film itself have b e e n studied to u n d e r s t a n d the reason for the g o o d protective properties o f the alloys. 6-t3 T h e passivation m e c h a n i s m and the processes occurring in the passive region of the polarization curve are not fully understood. Most data on the passive layer of the alloys c o m e f r o m surface analytical techniques. Based on A E S and X P S m e a s u r e m e n t s it is concluded that the passive layer is enriched in c h r o m i u m . 10 T h e thickness of the passive film on F e - 1 7 C r alloys is d e t e r m i n e d to be in the o r d e r of 2 nm. Polarization curves of F e - C r alloys were r e c o r d e d by Olivier, 14 El_Basiounyl and K e d d a m . 6"7 K e d d a m also p e r f o r m e d i m p e d a n c e m e a s u r e m e n t s directed towards the active region and the first part o f the active-passive transition region. F r o m electrochemical studies F r a n k e n t h a l t5-17 concludes that two types of passive films are f o r m e d on an F e - 2 4 C r alloy: one m o n o l a y e r of which the f o r m a t i o n and reduction is reversible, and a s e c o n d multi-layer film which actually forms the oxide film. N o s t a t e m e n t was m a d e a b o u t the composition of the passive film. R e c e n t l y G e r r e t s e n s h o w e d that iron plays an i m p o r t a n t part in the f o r m a t i o n of the passive film at the end of the passive region. 11
Manuscript received 15 January 1991; in amended form 2 March 1991. 765
766
J . A . L . DOBBELAAR, E. C. M. HERMAN and J. H. W. DE WIT
The passivation processes and properties of the passive film are influenced by, for example, the state of the alloy 18 and the environment. 19 The most interesting parameter in the study of passive materials is the anodic part of the passive current density. This part of the passive current density is a direct measure for the corrosion rate. A description of the passive current density in terms of variables as the alloy composition, environment and interface would be of great importance for corrosion protection. The number of parameters in practical applications is enormous, which makes such a description difficult. In this study all environmental and surface state parameters were fixed and only the alloy composition and the potential were varied. The influence of these parameters on the passive current density was studied by recording polarization curves. To obtain more information about the processes governing the passivation and passive state other techniques have to be applied. Impedance measurements were performed because they can reveal such detailed information, since impedance data reflects the contribution of all potential dependent processes governing the passive current density. EXPERIMENTAL
METHOD
Five i r o n - c h r o m i u m alloys of the composition Fe-6Cr, Fe-9Cr, Fe-13Cr, Fe-17Cr and Fe-25Cr were used in the electrochemical study. T h e composition of the alloys, given in Table 1, was determined by averaging E P M A (Electron Probe Micro-analysis) results m e a s u r e d at various spots on the surface. For comparison of the results electrodes of pure iron and c h r o m i u m were used. The alloys were prepared by arc-melting a mixture of pure iron ( M R C , 99.99) and pure c h r o m i u m ( M R C , 99.996) u n d e r A r atmosphere. Cylinders of the alloy were m a d e ( ~ 3 m m ) , by spark-machining. These cylinders were ground. The iron, Fe-6Cr, F e - 9 C r and Fe-13Cr alloys were etched in 2 M H 2 SO 4 (Merck, 95-97% p.a.). The F e - 1 7 C r and F e - 2 5 C r alloys and the pure c h r o m i u m were etched in 3 : 1 concentrated hydrochloric acid and nitric acid (36-38%, analytical grade, Baker). After etching, the electrodes were annealed for 2 h at 850°C u n d e r A r atmosphere, followed by furnace cooling. Before e m b e d d i n g the electrodes in an epoxy resin they were covered with a thin undercoating and m e a s u r e m e n t s were performed using the electrochemical set-up as described elsewhere .20 Potentials were m e a s u r e d against a Hg/HgSOn/sat reference electrode [+660 m V ( N H E ) ] . All potentials are given with respect to this potential. Before performing electrochemical m e a s u r e m e n t s the electrodes were ground and polished down to 1/~m. The electrodes were left at open circuit for about 15 min, after which a polarization curve was recorded with a scan rate of 0.1 m V s- I from - 1 . 2 to 0.8 V (Hg/HgSO4). W h e r e impedance m e a s u r e m e n t s were performed, the electrode was polarized for two days at a starting potential (0 mV) after which the m e a s u r e m e n t was done. The electrode was then polarized at a potential cathodic (in some cases anodic) to the starting potential, left for 12-16 h at this potential to establish the stationary state and again an impedance m e a s u r e m e n t was performed. In this way, at a n u m b e r of selected potentials, impedance m e a s u r e m e n t s were performed going from the passive state to the active state.
TABLE 1.
T H E CHEMICAL COMPOSITION OF THE ALLOYS DETERMINED WITH E P M A
Fe-6Cr %Cr %Fe %Mo %W %C %N %0 %Si
6.22 +_ 0.09 94.76 + 1.24 0.01 + 0.02 0.01 + 0.01 0.27 _+ 0.01 . . 0.01 + 0.01
Fe-9Cr 9.32 92.18 0.02 0.01 0.33 . .
_+ 0.16 _+ 0.63 _+ 0.02 _+ 0.01 _+ 0.04 . . 0.03 _+ 0.03
Fe-13Cr 13.55 87.86 0.01 0.03 0.33 . . 0.01
Fe-17Cr
_+ 0.18 _+ 0.58 + 0.01 _+ 0.02 _+ 0.13
17.41 83.23 0.01 0.01 0.48
_+ 0.07 _+ 0.4 + 0.02 + 0.01 _+ 0.16
Fe-25Cr 25.88 76.06 0.02 0.04 0.31
_+ 0.35 _+ 1.09 + 0.01 + 0.03 + 0.02
. . + 0.01
0.01 _+ 0.01
0.02 + 0.01
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I
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J 0.8
p o t e n t i a l (V) The polarization curves of iron, chromium (part), Fe~SCr, Fe-9Cr, Fe-13Cr, Fe-17Cr, Fe-25Cr.
EXPERIMENTAL
RESULTS
Polarization curves
The polarization curves are shown in Fig. 1 and are similar to those reported by Olivier, 14 E1-Basiouny et al. 1 and Keddam et al. 6 Tafel slopes were obtained from the polarization curves by fitting an exponential function to a small potential range of the curve. In Table 2 the measured Tafel slopes are given, together with some other parameters which were obtained from the polarization curve. The interpretation of the value of the Tafel slope (ban) must be done with care since it cannot be used in the determination of the dissolution mechanism when, as in the case of chromium, the surface diffusion of active species plays a part in the mechanism. 21 A decrease in the passive current density (the lowest current density in the passive range) with increasing chromium content is found as displayed in Fig. 2. The passive current density obtained from the polarization curve is not a stationary passive current density. The stationary passive current density is reached after prolonged polarization and the value obtained after 24 h of polarization is also plotted in Fig. 2.
TABLE 2.
PROPERTIES OBTAINED FROM THE POLARIZATION CURVES OF THE ALLOYS
Alloy
ban ( m V )
E .... mV (Hg/HgSO4)
Fe Fe~SCr Fe-9Cr Fe-13Cr Fe-17Cr Fe-25Cr Cr
49.3 82.7 -+ 1.4 85 _+ 2 95 -+ 0.9 77 _+ 3 81 + 1 --
-957 - 9 8 9 _+ 9 - 1 0 0 5 _+ 3 - 9 8 8 _+ 10 - 1 0 1 0 _+ 5 - 1 0 2 3 _+ 5 -1125 + 5
Epass m V ( H g / H g S O 4 ) -184 -752 -794 -832 -892 -1025
112" _+ 4 _+ 35 _+ 0 . 5 t _+ 11 + 3 _+ 2
icrit ( m A ) 200 300 _+ 13 57.8 _+ 0.6 21.4 _+ 0.4 19 _+ 4 11 _+ 1 3 + 0,7
/pass ( ~ A )
1.2 2.2 11.1 20 1.7
20 _+ 1.5 x 102 _+ 0 . 6 x 102 + 0.6 _+ 0.7 _+ 0.3 10 2
* A t this p o t e n t i a l the c u r r e n t c h a n g e is the s t e e p e s t . t A c l e a r s e c o n d p e a k is f o u n d f o r F e - 9 C r at E = - 5 0 3 + 15 w i t h a c u r r e n t d e n s i t y o f i = 57 + 2, for F e - 1 3 C r at E = - 5 5 0 + 8 w i t h a c u r r e n t d e n s i t y o f i = 3.4 _+ 3.
768
J . A . L . DOBBELAAR, E. C. M. HERMANand J. H. W. DE WIT
10-3 ,--, 10-4 ,¢
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%Cr The passivecurrent densityobtainedfromthe polarizationcurves (closedcircles) and after 24 h of polarization(open circles).
Impedance data Impedance diagrams were obtained for the five alloys at various selected potentials. For all alloys three impedance diagrams are shown in Figs 3-7. The diagrams were selected in such a way to portray clearly the change of the impedance with changing potential. Also diagrams for iron (Fig. 8a and b) and chromium (Fig. 8c) are shown. Both Nyquist and Bode diagrams are displayed. The Nyquist plot clearly pictures the low frequency part of the impedance whereas in the Bode plot, specially the phase angle, shows what is happening in the high frequency region. The low frequency part. Looking at the Nyquist diagram for iron (Fig. 8a) one capacitive loop is obtained at -200 mV which is in the passive region for iron. Polarizing the metal cathodically to this potential the active-passive transition region is reached. The impedance bends over to the negative real axis with decreasing frequency which is common for diagrams obtained in the active-passive transition region (Fig. 8b). An explanation of such an impedance diagram obtained for chromium could be given in terms of a change in surface coverage. 21 The increase of surface coverage of the passive film on iron can be treated in a similar way to explain the data. This was done by Keddam et al. 19In the passive region chromium seems to behave almost as a capacitor, but data analysis showed that two time constants are present. A small deviation of the capacitance in terms of a Constant Phase Element (CPE) was found. Besides the time constant resulting from the double layer behaviour and the dissolution of chromium through the passive film, changes in the passive film can contribute to another capacitive loop in the impedance data as found for chromium a t - 8 5 0 . 21 Looking at the impedance data obtained for the alloys a capacitive loop is found in the passive region which is depicted in Figs 3(a), 4(a) to 7(a). On measuring the impedance at more cathodic potentials a straight line at low frequencies is obtained for the lower alloyed steels. The angle which these lines make with the horizontal axis is about 45 ° (Fig. 3b and 4b). When such a phenomenon is found at low frequencies in the Nyquist plots, it is interpreted as being due to a diffusion process controlling the overall reaction. The potentials at which the straight line is measured correspond with the passivation potential of iron. No influence of stirring the solution on the passive current density was found. This means that the diffusion layer does not extend into the electrolyte. In the diagrams of Fe-13Cr, Fe-17Cr and Fe-25Cr and
F e - C r alloys in H2SO4
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FIG. 3. The impedance data represented in both Nyquist and Bode plots for Fe-6Cr at the potentials given in the plots. In the Bode plot the + represents the phase angle, and the [] represents I z l . Frequencies are in Hz.
chromium such a straight line is not found. At potentials in the active-passive transition region Figs 3(c), 4(c) to 7(c) and 6(b) and 7(b) an extra time constant due the passivation process appears. The impedance bends over to the negative real axis and the time constant due to the diffusion controlled process is overruled by the passivation process. It seems that particularly in lower alloyed steels diffusion-controlled transport through the passive film is the rate determining process. Chao et al. proposed a model
770
J . A . L . DOBBELAAR, E. C. M. HERMAN and J. H. W. DE WIT
with the diffusion of vacancies through the passive film as the rate determining process for nickel. 22 The impedance is similar to the impedance expression for the Randles circuit. It was possible to obtain a good fit of the low frequency part of these impedance diagrams using the Randles circuit.
The high frequency part. From the phase angle of the impedance data of iron and the iron-chromium alloys in the passive region it is clear that two time constants are i
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4-+
4-
,e+4
4-4-
lw4-4-+lOm Oe+O
r
•
Oe+O
!
•
[ t
10"1 [ . . . . , .... "1 ---~ ----, .... "~ -'-', "'- 30 lO 3 lO 2 10 1 10 0 101 lO 2 ,0 3 10 4
•
,e+4 2e+4 real part (Ohm)
.~j
3e+4
frequency (Hz) b: 50 m V
104
120
10 3
I00
~
102
80 -~
~ N
101
60
3e+3 r-
g 2e+3 "
lm
++
++
b÷ 444-
. ~ le+3 "
IOta
f i
I
0e+0
•
-2e+3
i
-le+3
•
i
_
0e+0
real part (Ohm)
d
-
i
le+3
10 0 I0 -1
"40
20 10"3 10-2 10"1 100 101 102 103 104
c: -100 m V
frequency(I-Iz)
Fz6.4. The impedance data represented in both Nyquist and Bode plots for Fe-9Cr at the potentials given in the plots. In the Bode plot the + represents the phase angle, and the [] represents IZl. Frequencies are in Hz.
*
Fe-Cr alloys in H2SO 4
i
!.~3e.--
771
"lOO
1o5"I"
~2e+4 e',, ~le+4
10't ~
-
1
/
,o'1 t
++4"**"¥ *'l'g~ lOm
,o,1 ..Y
%
1
lm~
0 .0
|
1
-.
!
le+4 2e+4 3e+4 real part (Ohm)
0e+0
o
°
s
,rN.
o
~
.6o
.
-o
10 .3 10-2 10 -! 10 0 101 10 2 10 3 10 4 frequency (Hz)
a: 1 0 0 m V
10 4 ~
•4e+3
.4- .4. + t-+
10 3 1 ~
+*
~3e+3
++
.~ 2e+3 le+3
lm
lOm ++ /.4.
T Oe+O
-3e+3
,
,
-le+3
le+3
t
- 140 ] ' ~
10 2
" 120 "~ - 100
~" 101
- 80
10 0
"60
10 -1
"40
10 -3 10 -2 10 -1 10 0 10 1 10 2 10 3 10 4
real part (Ohm)
frequency (Hz) b: -225 mV
10 3
,_,4e+2" ~3e+2"
~
10m +÷
÷4,÷
4. ÷
2e+2 "
+
.4.
"~le+2" i 0e+O
.4. .4. .4. .4.
~ 0.1
"/i ~t~lm !
-2e+2-le+2 0e+0 le+2 2e+2
E t-, O N
• 180
1 !
-12o 101
,oo 10 1
'0 10 -3 10 -2 10 -1 10 0 i0 1 10 2 10 3 10 4
real part (Ohm)
frequency (Hz) e: -375 mV
Fro. 5. The impedance data represented in both Nyquist and Bode plots for Fe-13Cr at the potentials given in the plots. In the Bode plot the + represents the phase angle, and the [] represents ]Z]. Frequencies are in Hz.
present. Data analysis showed that two time constants are also present in the impedance data of chromium. Since no change in surface coverage at these potentials is contributing to the impedance, another process must be taking place. A number of processes are possible candidates to explain a time constant. Film growth can occur with increasing potential as shown by Sato. 23 This potential-dependent process can contribute a time constant to the impedance. The composition of the passive film
772
J.A.L.
DOBBELAAR,
E. C. M. HERMAN and J. H. W. DE WIT
changes with potential. This was the conclusion of the potential modulated reflection spectroscopy study performed by Hara e t al. 24 An extra time constant in the impedance can also be attributed to this potential dependent process. But the structure of the passive film is also dependent on the amount of water, which can be removed by dehydration, a process which can be potential-dependent and therefore is able to yield a time constant. The oxidation state of the cations in the passive film is also dependent on the potential and a further
10 5
2e+4 r-
104"
g
~
90
g le+4
~0~
102. ¢~
10 °
!
le+4 real part (Ohm)
0e+0
3O 101
10m~
, 0e+0
o
N
.ii.÷ ÷ ÷-I- ÷÷ ÷.I-
.e
103.
2e+4
.
0 l0 -3 10 -2 10 "1 l0 0 l0 1 10 2 l0 3 10 4 frequency (Hz)
a: 0 mV
10 3 ~
,._. 5e+2 ~_
4-
4e+2
4-
T
4.
~-
• 120 ~
.,oo
+ ÷
3e+2
+
~2e+2
lOm~
:
~ le+2 I
Oe+O
,
-le+2
l0-I ~ . 20 10-3 10-2 10-1 l0 0 101 l0 2 l0 3 10 4
,
le+2 3e+2 real part (Ohm)
frequency (Hz) b: -400 m V
104
_. 2e+3 10m + +
~
le+3
4-
t÷
. 180
-loi -9o
"~
÷÷
"120 ~
1 \ ,0° 1
e~ O
lm ÷
1 0 "1 I - - - - - n . . . . '~ . . . . , . - - - ~ - - - . n - - - ~
i i ~' Oe+O -le+3 -5e+20e+O 5e+2 le+3 real part (Ohm)
.....
30
10 -3 10 -2 10 -1 10 0 101 10 2 10 3 10 4 frequency (Hz) c:
-600 m V
FIG. 6. The impedance data represented in both Nyquist and Bode plots for Fe--17Cr at the potentials given in the plots. In the Bode plot the + represents the phase angle, and the [] represents Izl. Frequencies are in Hz.
F e - C r alloys in H2SO 4
773
]
~
8e+5 +4- lm
105'
g
+
104.
"-"
6e+5 +
] ~4e+5
10 3.
÷
I
N
: ' ~ 2e+5
10 1. :1
i
60
10 2.
Oc+O
i
!
10 0
!
0e+0 2e+5 4e+5 6e+5 8e+5 real part ( O h m )
......................... -~ 30 10 -3 10 -2 10 -1 10 0 10 1 10 2 10 3 10 4
frequency (Hz) a: 0 m V
Om!f
V
',~~ 4e+3 5e+31 ' ~3e÷3
~ '
Oe+O
!
102
,oo
101
-2e+3 Oc+O real part ( O h m )
5O i
40 - - - . n -----,I . . . . . • - - - - q - - - ~ -.,.--v ..... 20 10 -3 10 .2 10 -I 100 101 102 103 104
frequency (Hz) mV
10 6
~ le+4
~
10 5
9e+3
+÷ ÷
~, 6e+3
120
10-1
i
b: -400
e~
103
10 o
-4e+3
~
140
gO
N
le+3
10 4
•
140
t- 120
10 4
÷ 4-
lm
¢-
102 N
~ 3e+3 | ! 0e+0 -8e+3 -5e+3 -2e+3 le+3 4e+3 real part ( O h m )
l0 3 101 10 o I0 -I
-..~
.... •
..--~-..-~---~
..-~
.... ~
20
10 -3 10 -2 10 -1 10 0 10 1 10 2 10 3 10 4
frequency (Hz) c: -600 m V
Fl6. 7. The impedance data represented in both Nyquist and Bode plots for Fe-25Cr at the potentials given in the plots. In the Bode plot the + represents the phase angle, and the [] represents IZ[. Frequencies are in Hz.
oxidation of these ions can also yield a time constant. It is clear that a detailed description of the passive film must include these processes and must also take into account the lateral surface inhomogeneity as was found from studies on Fe-25Cr with different microstructures, is If all processes mentioned contribute to the impedance measured in the frequency range of 105-10 -3, it is clear that a large overlap in time constants will occur which makes it difficult to separate them. Using the impedance analysis software package EQUIVCRT, which has been described
774
J . A . L . DOBBELAAR,E. C. M. HERMANand J. H. W. DE WIT
before, 21 an attempt was m a d e to determine the n u m b e r of time constants in the impedance diagrams. It was assumed that all processes were parallel processes and it was found that in most plots, measured at the alloys, a reasonable to good fit was obtained with three or four time constants. T w o time constants were found for iron in the passive state, but four time constants are present in the active-passive transition region. Fitting only the high
i
iron 10 4
8e+3
103
.-6o
~6e+3 102 f ~ 4e+3 2e+3
E 'T
0c+0
~.4-'4- 4- ÷b+4÷÷ oo 1o.) !
i
•
"40
101
;2o
100 10-1
•
.....
u
....
,i
....
•
....
..i
....
,i
.....
"1
.....
0
10 -3 10"2 10 -1 l0 0 101 10 2 10 3 10 4
0e+0 2e+3 4e+3 6e+3 ge*3 real part (Ohm)
frequency (Hz) a: 200 m V
iron
IOta+4- + 4- 4- +0.1
10 3
200
101 1
100
N 10° 1 10 -1 | 0 10 -3 10 -2 10 -1 100 101 102 103 104
!
-2e+2
0e+0 2e+2 real part (Ohm)
frequency (Hz) b: -$0 m V chromiumlo 7
,_. 3e+6
90 I
lO6-
+ Im
O
--,
4-
2e46
e.
105" 104-
"80 v
e'L
io 3-
4-+ 4-
.~ le+6
E
N
4-
10 2"
- 70
10 I"
0.1
0e+O 0e+O
|
i
le+6 2e+6 3e+6 real part (Ohm)
10 °
60 10 -3 10 -2 10 -1 10 0 101 10 2 10 3 10 4
frequency (Hz) c: -600 m V
Fro. 8. The impedance data represented in both Nyquist and Bode plots for Fe and Cr at the potentials given in the plots. The top two plots are impedance data measured at iron, the bottom plot is measured at chromium. In the Bode plot the + represents the phase angle, and the [] represents IZI. Frequencies are in Hz.
Fe-Cr alloys in H2SO 4
775
10"1 :"o 10-3 o
i
-.•
(i'~jrD
•
•
= mm nm .m
~'=~.
" e
•
•
•
10-5 -6~0
-400 -2bO potential (mV)
6
I
• .-o :
0.9
..bGo~cJ n.-
0.8 ~
me • '~-•
0.7
.
m m'
0.5 0.4
/
""
-'"
m .
0.6
.
~= e-
-800
t -600
I I -400 -200 potential (mV)
I 0
200
FIG. 9. (A) (top) The value of Y0 obtained by fitting the high frequency part of the impedance data for Fe-6Cr (open squares), Fe-9Cr (closed circles) and Fe-17Cr (closed squares). (B) (bottom) the n corresponding with the Y0 obtained from the fit.
frequency part of the impedance in the frequency range of 5 x 102-104 Hz a good picture of the double layer capacitance for the various alloys and potential can be obtained. In all cases the double layer capacitance was found to be a Constant Phase Element (CPE). The CPE is defined as 1/[Y0(j~o)"] (impedance representation). Unfortunately the Y0 values obtained by fitting a Q at the high frequency data cannot be compared since their dimension depends on the value of n. In Fig. 9(a) the values obtained for the metals and different alloys are plotted against potential merely to show that a change is occurring. For the sake of clarity only the data obtained for Fe6Cr, Fe-13Cr and Fe-17Cr are shown. In Fig. 9b the corresponding n values are displayed. In all cases the change in Y0 and n is such that a larger deviation from an ideal capacitance with decreasing potential is found. This changing CPE is clear in the phase angle in the Bode diagram at high frequencies, by the compressing of the maximum when going from anodic to cathodic potentials. DISCUSSION
The passive film on the alloys and the metals studied here is considered to be amorphous. From the low frequency impedance data it can be concluded that the ion transport through the passive film is the rate controlling factor for the Fe-6Cr and Fe-9Cr alloys. The rate controlling process for the pure metals and Fe-13Cr, Fe-17Cr and Fe-25Cr alloys cannot be pinned but will certainly not be diffusion controlled. A g e n e r a l m o d e l c a n b e f o r m u l a t e d as f o l l o w s M ~,~ M "+ + n e -
metal-metal oxide interface
(1)
M "+ t r a n s p o r t
oxide
(2)
776
J.A.L.
DOBBELAAR,E.
C. M. HERMAN and J. H. W. DE WIT
Mn+ ~.~ Mn+,soi
oxide-solution interface
H 2 0 ~- Oox + 2H + + 2 e -
oxide-solution interface
0 2 - transport
oxide.
(3) (4) (5)
It is not clear which species M ~+ or 0 2 - is being transported. Calculating the impedance, 25 the electron transfer reaction at the metal-metal oxide and the metal oxide-solution interface will yield resistances. These resistances are parallel resistances and will add up to one resistance. If the transport of species through the passive film is diffusion-controlled as in the case of the lower alloyed steels, a Warburg impedance is in series with this summed resistance and will determine the low frequency range. If the transport is due to the high electric field a resistance due to the transport is in series with the summed resistance. In this case no Warburg impedance is found. These two situations are extremes and it is possible that both will occur at the same time. The amount which each transport process contributes to the total transport will determine whether a Warburg impedance is found or not. The passivation of the iron-chromium alloys occurs in two steps as can be seen in the polarization curves. Based on an earlier publication it is assumed that the first active-passive transition is due to the formation of a (mono-layer) chromium oxide passive film, while in the second transition iron contributes to the passivation, a1'26 Recently Kirchheim et al. 27 described the relation between the passive current and the Cr enrichment of the film, based on XPS measurements. The lateral distribution of Cr in the alloy and film was not taken into account, nor was the influence of incomplete surface coverage for low Cr alloys. In the lower alloyed metals the amount of chromium is too low to cover the total surface with chromium oxide and passivation will not be complete. This idea is theoretically supported by the percolation model of Sieradzki and Newman. 4 A porous passive film is formed after the first current peak in the polarization curve. The amount of porosity decreases with increasing chromium content. The distribution of chromium in the alloy is also of importance in the formation of a good protecting passive film as found from measurements on Fe-25Cr with varying microstructure. 18 It is apparent that the activation energy for various reactions in a porous layer will have a large degree of distribution. Considering an inhomogeneous surface 24 with a distribution in activation energy, a CPE can be explained. The larger the distribution in activation energy the more the CPE deviates from a true capacitance. If the processes, which are subject to a distribution of activation energy, contribute a time constant in the high frequency region, the double layer capacitance, normally obtained from this frequency region, will deviate from a true capacitor. A possible candidate process is film growth. When metal ions are generated at the metal-metal oxide interface, oxygen ions are formed at the metal oxide-solution interface. In this way a capacitance is created, which is in series with a resistance due to the electron transfer reactions generating metal ions and oxygen ions. The capacitance is not only a capacitance due to the separation of charge (metal-ions at the metal-metal oxide interface and oxygen ions at the metal oxidesolution interface), but also due to the storage of charge in the film (which is similar to a capacitance created by a change of surface coverage; see Refs 24, 25). This can explain the increasing deviation from ideal capacitive behaviour with decreasing
Fe-Cr alloys in H2SO 4
777
potential ( = increasing porosity) as shown in Fig. 9(a) and (b). The porosity of the passive film is not only a reflection of the amount of chromium but also of the distribution of the chromium in the alloy. 18The lateral inhomogeneity can be treated in a model by dividing the surface in an infinite n u m b e r of areas and considering a distribution of the c h r o m i u m content over the areas. The admittance of all the areas s u m m e d yields the total admittance. A two-dimensional distribution is then created, namely a distribution representing the potential dependent porosity in the passive film for a certain chromium content and a distribution of the local amount of chromium. It is clear that areas with a low local chromium content are the least protective places in the alloy. Unfortunately on the sole base of the impedance m e a s u r e m e n t s it is not possible to identify the time constants present. O t h e r techniques, which are able to separate the processes mentioned, are necessary to pin the various time constants to processes. It is possible that if, as shown 22 for chromium, strong coupling between processes exist the impedance cannot be represented by equivalent circuits. A more fundamental description in terms of a transfer function will be needed. Since detailed information about the processes occurring during the passivation and in the passive state is not yet available no detailed description of the passivation process of the iron chromium alloys can yet be given. CONCLUSION The passive current of i r o n - c h r o m i u m alloys decreases with increasing chromium content. The passive current versus the chromium content is shown in Fig. 2. With increasing chromium content the polarization curve changes from iron like to chromium like. The pasivation becomes m o r e and more dominated by the formation of a chromium oxide/hydroxide passive layer. Iron starts to contribute to the passivation at about - 2 0 0 inV. This contribution results in a decrease of porosity. In the potential region of about - 8 0 0 to - 2 0 0 m V (Hg/HgSO4) the amount of porosity in the passive film decreases with increasing potential and chromium content. The transport of ions through the passive film is diffusion-controlled in the lower alloyed metals. In the pure metals and higher alloyed metals no diffusion controlled transport is found. The transfer function describing the impedance diagrams could not be established. REFERENCES 1. M. S. EL-BASIOUNYand S. HARUYAMA,Corros. Sci. 16,529 (1976). 2. K. SIERADZKIand R. C. NEWMAN,J. electrochem. Soc. 133, 1979 (1986). 3. H. H. UHH6, Passivity and Breakdown o f Iron and iron base alloys, U.S.A.-Japan seminar (eds R. W. STAEHLEand H. OKADA),NACE, Houston (1975). S. Q1AN, R. C. NEWMANand K. SIERADZKI,J. electrochem. Soc. 137,435 (1990). P. BRI]ESCH, K. MILLER and H. R. ZELLER,Surf. S£i. 169, 327 (1986). M. KEDDAM,O. R. MATI'OSand H. TAKENOUTI,Electrochim. Acta 31, 1147 (1986). M. KEDDAM,O. R. MATTOSand H. TAKENOUTI,Electrochim. Acta 31, 1159 (1986). S. MISCHLER,H. J. MATHIEUand D. LANDOLT,Surf. Interface Anal. 11,182 (1988). 9. I. OLEFJORDand H. F1SHMEISTER,Corros. Sci. 15,697 (1975). 10. P. MARCUSand I. OLEEJORD, Corros. Sci. 28, 589 (1988). II. H. GERRETSEN, Passivation and breakdown of passivity of stainless steel constituents, chromium, 4. 5. 6. 7. 8.
iron-chromium, iron-chromium-molybdenum and nickel. Thesis, Technical University Delft (1990). 12. S. C. TJoyc, R. W. HOFMANand E. B. YEAGER,J. electrochem. Soc. 129, 1662 (1982).
778 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27.
J . A . L . DOBBELAAR,E. C. M. HERMANand J. H. W. DE WIT Y. KOLOa~RKIN,Electrochim. Acta 18, 593 (1973). R. OLIVIER,Thesis, Leiden University (1955). R. P. FgaNKENTnAL,J. electrochem. Soc. 114, 542 (1967). R. P. FgaNKENTnAL,J. electrochem. Soc. 116, 581 (1969). R. P. F~NKENTHAL, J. electrochem. Soc. 116, 1646 (1969). J. A. L. DOBBELAAR,E. C. M. HERMANand J. H. W. DE WIT, Corros. Sci. 33,779 (1992). M. KEDDAMand C. PALLO~A,J. electrochem. Soc. 132,781 (1985). J. A. L. DOBBELAARand J. H. W. DE WIT, to be submitted to J. Appl. Electrochem. J. A. L. DOBBELAARand J. H. W. DE WIT, J. electrochem. Soc. 137, 2038 (1990). C. Y. CHAO, L. F. LtN, D. D. MACDONALD,J. electrochem. Soc. 129, 1874 (1982). N. SATO,Passivity of Metals, Proc. Syrup. Passivity (eds R. P. FRANKENTnAEand J. KRUGER),p. 29. The Electrochem. Soc., Princeton, New Jersey (1978). N. HAga and K. SUGIMOTO,J. electrochem. Soc. 126, 1328 (1979). J. A. L. DOBBELAARand J. H. W. DE WIT, to be submitted to Electrochim. Acta. J. H. GERRETSENand J. H. W. DE WIT, Proc. 9th Eur. Corr. Congress, Utrecht, The Netherlands, FU-056 (October 1989). R. KIRCHHEIM,B. HEINE, H. FISCHMEISTER,S. HOFMAN,H. KNOTEand U. STOLZ, Corros. Sci. 29,899 (1989).