Semiconducting properties of passive films on stainless steels

SEMICONDUCTING

PROPERTIES OF PASSIVE STAINLESS STEELS

FILMS

ON

AGATINO Dr PAOLA Istituto di IngegneriaChimica, University di Palermo, Viale delle Scienze, 90128 Palermo, Italy (Received 19 January

1988; in revised form

1 June 1988)

Abstract-The semiconducting properties of passive films grown on three different stainless steels have been investigated by measuring the potential dependence of the capacitance at various frequencies and in different solutions. The results of the capacitance response indicate that the passive films behave like highly doped n-type semiconductors. A strong frequency dispersion hampers the determination of the exact value of the flat band potential of the films from the Mot&Schottky plots. Donor concentrations in the range 102”-1021 crne3 have been obtained at 1 kHz. The intersection potentials at l/C?=0 show a linear dependence on the pH.

In this paper impedance measurements have beencarried out for studying the semiconducting properties of the passive films grown on three different types of stainless steels. The results have been compared with those obtained by analogous measurements on passive iron.

INTRODUCTION The electrochemical behaviour of the passive films on the stainless steels can be interpreted in terms of semiconducting properties of the films[l] and relationships have been formulated between the conductivity type and the protecting efficiency of the passive layers[ 1, 21. Capacitance and photoelectrochemical measurements have been often employed to study the passive films on several metals and alloys and the concepts of the electrochemistry of the semiconductors have been extended to these systems. Capacitance studies are useful for obtaining informations on the electronic structure of a semiconductor and by the analysis of the Mott-Schottky plots it is possible to obtain the flat band potential and the donor concentration[3]. Several capacitance measurements on passive iron have appeared[4-181 but only a few papers have been concerned with passive stainless steels[19-241. The capacitance studies have been essentially performed for studying the kinetics of the passive film growth on austenitic stainless steels in both pitting and nonpitting environments. The potential dependence of the capacitance has been examined, but the data have been generally obtained at the same potential of formation of the films and the measurements have been restricted only to one frequency. In previous works[ZS, 261 photocurrent measurements have been performed on passive films grown on stainless steels for sea water service. The stationary anodic photocurrents have suggested an n-type semiconductor behaviour of these films. The photocurrent spectra have been utilized to obtain the optical band gap of the films.

EXPERIMENTAL Three different stainless steels were investigated: a laboratory ELI (Extra Low Interstitial) ferritic stainless steel (type ITM 40), a commercial superaustenitic stainless steel (type 254 SMO) and a commercial austenitic stainless steel (type AISI 304). The experimental ferritic steel was supplied by the Istituto per la Tecnologia dei Materiali Metallici non Tradizionali de1 CNR and its preparation is described elsewhere[27]. The chemical composition of the stainless steels is given in Table 1. Electrodes were prepared from 1 cm x 1 cm x 3 mm pieces of the steels. Electrical contact was ensured by a silver paste to a copper wire enclosed in a glass tube. Iron electrodes were prepared from Fe rods (5 mm diameter) purchased from Goodfellow Metals, Cambridge (purity 99,99 % +). The sides and the back of all electrodes were sealed with epoxy resin leaving an approx. 0.2 cm2 flat surface exposed to the solution. Before each experiment the electrodes were mechanically polished with successively finer grained papers and then cleaned ultrasonically in absolute ethanol. A conventional three compartment cell was used with a coiled platinum wire counter electrode separated from the main compartment by a medium poros-

Table 1. Chemical composition (% (w/w)) of the stainless steels studied C

ITM 40 254 SMO AK1 304 gA 31:2-T

N

0.008 0.013 0.017 0.120 0.060 0.045

Cr 24.4 20.0 19.3

Ni

Si

Mn

Cu

Ti

Nb

Phase

0.06 0.33 0.72

0.48 1.40

0.7 0.1

0.28 0.05

0.22 -

Ferrite Austenite Austenite

MO

3.50 3.30 11.50 6.10 9.20 0.21

203

A.DI

204

ity glass frit. The capacitance measurements were performed with a platinum gauze counter electrode with an area of at least two orders of magnitude larger than that of the working electrode. Reference electrodes were saturated calomel or mercury/mercuric oxide electrodes, but all potentials are reported us see. All experiments were performed at room temperature. The following el&troIyte solutions were used: I NH,SO, (nH=0.36): 1 N Na,SO, buffered to pH = 6.5 and 8.4 by borate; 1 N NaOK(pH = 14). For ITM 40 and 254 SMO, a- solution 3.5% NaCl (pH=6.5) was also employed. Current vs potential plots were obtained with a PAR Model 173 potentiostat and a PAR Model 175 Universal Programmer equipped with a Houston Instruments Model 2000 XY Recorder. The electrode impedance was determined using the lock-in amplifier technique. A small amplitude ac voltage (5 mV peakto-peak) was superimposed on a linear potential sweep (5 mV s-l) and the in and out-of-phase components of the impedance were detected by means of a PAR Model 128 A lock-in amplifier. The signals were calibrated respectively with a NBN decade resistance and a Hewlett-Packard Model 444.0 B precision decade capacitor. The frequency was varied in the range 1 Hz-3 kHz. All impedance values are given as measured without any correction for the roughness factor.

RESULTS

AND

DISCUSSION

The passivity range of the electrodes was explored potentiodynamically at + S mV s- I. The passive films were obtained at high potentials but at values below oxygen evolution. The films were formed by potentiostatic polarization in one step at the desidered formation potential. To avoid any hydrogen embrittlement the stainless steels were passivated starting from the free corrosion potential. Instead the iron electrodes were cathodically treated for 1 h at the potential of -0.4 V vs hess (hydrogen electrode in the same solution) and the solutions replaced prior to the passivation. In order to compare our results with data reported in the literature[12], the film formation time for iron was limited to 1 h. For the stainless steels the film formation time was 24 h as in previous photoelectrochemical measurements[25]. IMPEDANCE

MEASUREMENTS

The equivalent parallel capacitance of the passive film*lectrolyte interface was measured as a function of frequency and applied potential. Starting from the formation potential, the electrode capacitance was measured during slow potentiodynamic sweeps (5 mVs_‘) in the potential region where faradaic processes were negligible. The capacitance measurements showed hysteresis since they were dependent of the direction (anodic or cathodic) of the potential sweep. Similar capacitance behaviour was exhibited both by the iron and by the three different stainless steels. Figure 1 is representative of the results obtained in the various electrolytes. At low potentials the capacitance was rather high. With increasing potentials the

PAOLA

201

0’

-0

2

02

0

UC/V Fig.

1. ITM 40 Capacitance-potential

in

04

06

0t

MSCW

1N curves

NGO,,

obtained

pH = X.4. at 500 Hz.

capacitance decreased, became nearly constant and then it increased again at high potentials. The potential dependence of the capacitance of the passive films on the three stainless steels can be explained on the basis that these films behave like n-type semiconductors in agreement with the results reported by several authors for passive iron[9-181. The capacitance curves can be interpreted with respect to the band structure model of the semiconducting passive film[9]. The decrease of the capacitance is attributable to the increase of the thickness of the depletion layer with increasing the band bending according to a typical Mott-Schottky behaviour. The range of constant capacity can be interpreted with the exaustion of all donors in the film which behaves like an insulator, whilst the capacitance increase is explained by the formation of an inversion layer as result of an increasing hole concentration in the valence band. This simple model proposed by Stimming and Schultze for passive iron[9] agrees quite well with the theoretical results on the capacitance of thin passive films obtained by Khan and Schmickler[28]. The capacitance values depended on the frequencv and on ihe pH. Figure 2 shows the capacitancepotential curves obtained for AISI 304 in 1 N H,SO, it different frequencies. The curves were shifted to lower values with increasing frequencies and toward lower potentials with increasing pH. Figure 3 is representative of the frequency dependence of the capacitance values obtained at constant potential. With increasing frequency the capacitance strongly decreased in the region of low frequencies and then decreased slowly to lower values for frequencies higher than 100 Hz. This behaviour was observed at every potential although the stronger decrease with increasing frequency was observed at the higher potentials and was attributed by Stimming[12] to a contribution of surface states created by OH- adsorbed at the oxide surface. A similar frequency dependence of the parallel capacitance was found by Engell and Ilschner[4] for passive iron in 1 N H,SO,. They attributed the initial decrease of the capacitance to the ionic part of the space charge which, because of the low ionic mobility, gives a contribution only at the lower frequencies. Tenth and Yeager[29] suggested that the ion movement in the semiconductors can cause frequency effects, particularly in heavily doped materials where the space charge region is very thin and the passive films

205

Passive films on stainless steels

0’ 20

lcq

I

3

2

I

I KHz

f/Hz

Fig. 3. ITM 40 in 1 N H,SO,. Frequency dependence of the capacitance values obtained at + 800 mV us see. 0

0

0.4

0.2

0.6

0.8

U,/VVSsCe

Fig. 2. AISI 304 in 1 N H,SO,. Capacitance-potential curves obtained at different frequencies. generally satisfy these requirements. The further fall of the parallel capacitance at the high frequencies can be explained by the decrease of the C, values with w2[4]. Figure 4 shows the variation of the reciprocal capacitance with logf at different applied potentials: a linear relationship between l/C and logf was found in the region of potentials where the capacitance was almost constant. Also the resistance measurements revealed a strong dependence on the frequency. As shown in Fig. 5 the equivalent parallel capacitance and resistance values were not independent of each other and a linear relationship between R and f-i holds in the same range of potentials where the plots of l/C us logfare linear. For various semiconductors[30] and for passive films on niobium[31] and tin[32], the following relationships describe the frequency dependence of the equivalent series capacitance and resistance, at cdnstant potential:

&=$+a 1ogJ; s

Cl’

2

1

I

3

4

tog f/HZ

Fig. 4. AISI 304 in 1 N H,SO,. Reciprocal capacitance l/C vs logf at different potentials. G 0; 0 + 200; A + 500 mv.

700 SOC

/

No-

(I)

0

R.=R,+b

0

f’

(2)

where C,, R,, a and b are constants. A numerical relationship exists between the coefficients a and b:

For passive niobium[31] Young explained the experimental relationships (Equations (l-3)) by assuming a model of oxide film consisting of an infinite number of parallel RC units in series, with an exponential distribution of conductivity. Van Meirhaege et ~I.[333 have shown that the theoretical value of (a/b) =9.2 found by Young is an obvious consequence of the Kramers-Kronig relations which correlate the frequency dependences of the real and imaginary parts of an electrical impedance.

100

200

f-‘/Hz-’

300

400

x IO4

Fig. 5. AISI 304 in 1 N H,S04. Variation of the resistance with j-l. The values were obtained at + 500 mV. The same relationships (Equations (1) and (2)) seem valid also for our results. Anyway the ratio of the slopes of the straight lines in Figs 4 and 5 is different from the theoretical value of 9.2. In particular, for the three stainless steels an average value of 0.9 was found whilst for passive iron the a/b ratio was only 0.1. The results reported in literature[30-321 have been found by means of impedance measurements of series RC equivalent circuits, whilst our data have been obtained by a parallel RC configuration. As shown by Di Quart0 et aI.[34] for amorphous Nb,O, anodic films, if the impedance results are expressed as parallel elements R, and C,, a different relationship holds for

A. DI PAOLA

206

the ratio of the slopes of the l/C.(f) and R,(J) relationships and the Young’s value of 9.2 is obtained as a particular case if two precise conditions are both contemporarily satisfied. Probably this fulfilment is not verified for the passive films either on iron or on the three stainless steels examined. Moreover, values different from 9.2 have been recently found also for

passive films on titanium[35] and this result was explained by the assumption that the passive layers on titanium do not represent an ideal dielectric, but are electrically unhomogeneous as probably are the passive films on iron or on the stainless steels.

MOTTSCHOTTKY

PLOTS

0.2

In Figs G3 are reported l/C* vs U, plots obtained at different frequencies. The capacitance values of the passive iron were measured during the cathodic potentiodynamic sweep[l2]. For the stainless steels, humps were sometimes observed when the direction of the sweep was cathodic, so the I /C2 data were obtained from the return sweep until the formation potential. According to Vasudev et al.[36], the humps in the capacitance-potential plots are often associated with excess charge residing in surface states. The l/C2 us U, plots were analysed by using the Mott-Schottky relationship for the expression of the space charge capacitance of a semiconductor in the depletion range[37-391: 1 C2

0.3

u, / v Fig. 7. 254

0.4

0.6

0.6

-I 0.7

0.6

vs see

SMO in 1 N H,SO,. Mott-Schottky obtained at different frequencies.

plots

(4)

where N, is the donor concentration, CJ, the electrode E the dielectric potential, U,, the flat band potential, constant of the film and the remaining symbols have the usual meaning. For all the metallic substrates examined, a straight line dependence was always observed at the lower potentials, regardless of the frequency, the pH and the electrolyte composition. According to Equation (4), the flat band potential of the passive films can be determined by extrapolating the Mott-Schottky plots

Fig. 6. ITM 40 in 1 N Na,SO,, pH=8.4. Mott-Schottky plots obtained at different frequencies.

0

-0.3-02

-0.1

0

u, /v Fig.

8. AISI

0. I “S

0.2

0.3

0.4

0.5

0.6

sm

304 in 1 N H,SO,. Mott-Schottky obtained at different frequencies.

plots

to l/C’=O, whilst the donor concentration can be calculated from the slope of the straight line. As shown in Figs 6-8, the Mott-Schottky plots of the passive films on the three stainless steels revealed a strong dispersion with the frequency and a similar behaviour was observed also for passive iron[l2]. The plots exhibited different frequency dependences according to the nature of the electrode or to the composition of the electrolyte. Dutoit et aI.[30] distinguished two diverse types of behaviour which called respectively “A-type” and “B-type”. In the “A-type” behaviour the MottSchottky plots are nearly parallel at every frequency (see Fig. 6) and the intercepts with the U, axis are shifted toward more cathodic potentials with increasing frequency. This type of behaviour allows the determination of N, but not of U,,. In the “Btype” behaviour the Mott-Schottky graphs converge to a common point of the potential axis (see Fig. 7) and it is possible to determine only the flat band potential. Figure 8 shows a third case, that we have called “(A+B)-type” behaviour, where the slopes of the Mott-Schottky plots as well as their intersections with

Passive films on stainless steels the potential axis are frequency dependent and therefore yielding no information on U, or ND. The Mott-Schottky plots converge to a common value at low frequencies (generally until 300 Hz) and become nearly parallel at higher frequencies. Table 2 reports the types of behaviour found for the stainless steels and the iron in the different solutions. An “(A + B)-type” behaviour is commonly exhibited by the passive films on the three stainless steels. It is worth noting that also Bockris and Uosaki[40] found different frequency dependence of the Mott-Schottky plots for p-Sic in acidic and alkaline solutions, but no explanation was given for this observation. Several causes for the frequency dispersion of the capacitance have been proposed. For the passive films on the stainless steels investigated possible causes could be: (i) a non-uniform distribution of donors; (ii) the contribution of surface states to the capacitance response; (iii) dielectric relaxation phenomena which occur throughout the depletion layer (“B-type” behaviour) or are confined to a surface layer small with respect to the thickness of the depletion layer (“Atype” behaviour)[30]; (iv) amorphous nature of the films[34]; (v) presence of deep donor passive states[41]. The decision in favour of a cause rather than another one seems quite difficult for systems very complicated like the passive films on the stainless steels, which also exhibit different behaviour at different pH. Several authors have reported capacitance data of passivated iron or stainless steels electrodes[424] but the possible frequency dispersion has often not be considered or disregarded, the results being commonly restricted only to the frequency of 1 kHz. To compare our results with the data reported in literature we have calculated N, and UFB values from the capacitance measurements obtained at 1 kHz. The calculation of the donor concentration from the slope of the Mott-Schottky plots requires the knowledge of the dielectric constant of the films. For passive AISI 304 in 1 NHzSO, Okamoto[l9] determined’a dielectric constant E= 15.6 and this value has been used to calculate the N, values of the passive films on the three stainless steels examined. The assumption of

207

the same E seems reasonable since almost the same values of capacitance have been measured for the films formed on the three different stainless steels. Moreover, the value 15.6 is of the same order of magnitude as the values 10 and 12 which have been used respectively for passive films on iron[12] and chromium[42], which are two of the principal constituent elements of the stainless steels. Table 3 reports the values of the donor concentrations obtained in different solutions at 1 kHz. The N, values of the passivated stainless steels are in the range 1020-1021 crK3. These values are at least one order of magnitude larger than those obtained for passive iron, which are in excellent agreement with the data reported in literature[%181. The high N, values are attributable to the highly disordered amorphous nature of the passive films. The thickness of the dielectric layer in the passive films can be estimated in the range where the capacitance is almost independent of the potential, by using the simple parallel capacitance model:

d+. For AISI 304 in 1 N H,SO+, using E = 15.6 we have obtained a value of 7.9 A which agrees quite well with ellipsometric the value 9.2 measured by an technique[19]. A comparable thickness was obtained in 1 NNaOH whilst values three times higher were determined in 1 N Na$O, at pH = 8.4. By using the same value also for 254 SMO and ITM 40, the values of thickness calculated were similar to those found for AISI 304. According to Chao et ~I.[431 the thin passive films contain many lattice defects and as shown by Sato et aZ.[17] for passive iron, the density of these defects acting as electron donors decreases with increasing the film thickness because the films tend to take a more stable structure. The lower Nn values obtained in neutral solutions confirm that the amorphieity of the passive films decreases with increasing film thickness. The extrapolation of the Mott-Schottky plots to l/C*=0 yields the potential U0 which allows the determination of the flat band potential through

Table 2. Behaviour of the Mott-Schottky

plots obtained in different solutions

Specimen

1 N H,SO., (pH = 0.36)

1 N Na,SO, (pH = 8.4)

ITM 40 254 SMO AISI 304 Fe

B B A-!-B B

AAtB A+B A

1 N NaOH (pH= 14) A+B A+B B

A+B

Table 3. Donor concentration of the passive films, calculated at 1 kHz

Specimen ITM 40 254 SMO AISI 304 Fe

1 N H,SO, (pH=0.36) 1.9 x 1.4 x 38x 214 x

10zl 102’ lo*’ 10zo

cm-3 cms3 cmP3 crne3

1 N Na,SO, (pH = 8.4) 2.6 x 10”’ cnC3 63x10+0cm-3 116 x 10zo crne3 1.2 x 10zo crne3

1 N NaOH (pH = 14) 1.2 x 1.0 x 1.9 x 2.2 x

102’ crne8 lozl err-3 102’ Cm-J lozO cm-3

A. Dr PAOLA

208 Equation

(6): lJ,=

LI,.+fi e

The frequency dispersion of the capacitance hamthe determination of U, the unless pers Mott-Schottky plots converge to the same potential value as in the case of the “B-type” behaviour. The prevailing “(A + B)-type” behaviour exhibited by the passive films on the stainless steels do not allow to obtain the exact value of Ur,, for the films formed in the most part of the solutions examined. The same uncertainty in the U, values occurs also for passive iron, as already pointed out by Cahan and Chen[8] who made impedance measurements over a relatively wide range of frequencies. Nevertheless, several authors[9-141 have reported flat band potentials of passive iron, determined for the most part only to the frequency of 1 kHz. The determination of U,, by extrapolation of the Mott-Schottky plots according to the Equation (4) is correct when the space charge layer capacitance is small in comparison with the Helmholtz layer capacitance and when the potential drop caused by the applied potential is entirely across the space charge region within the semiconductor. De Gryse et aZ.[44] and Gerischer et ~I.[451 obtained the following more general relationships between the total capacitance and the potential difference: 1 1 -_=-+ C2 C:,

2

(7)

E&,eN,

where C, is the Helmholtz capacitance. The relationship between l/C’ and Us is still linear and does not influence the slope of the line, but the intersection with the abscissa corresponds to a potential U0 equal to: u,=

kT u,,+p-T. e

.x,eN,

(8)

LLfi

The difference between the U,, values obtained according to Equations (4) and (7) can generally be neglected except for semiconductors characterized by high values of N, such as in particular the passive films on iron and on the three stainless steels. However, also using Equation (8), the determination of U,, is subjected to a certain error since the value of C, is a quantity whose value is not known exactly in most cases. 20 PF cm-’ is the value commonly assumed for the Helmholtz capacitance. Anyway, Tomkiewicz[46] for the evaluation of the flat band potential of TiO,, found that C, varied systematically with the pH, changing from 31 pFcmm2 at pH= 13 to 64 pFcmm2 at pH = 2.7. Stimming and Schultze[9] assumed c”= 20 PF cmm2 and found that the flat band potential of passive iron was about 100 mV more anodic than the value obtained according to Equation (6). In the case of the passive stainless steels, using C,= 20 pFcm-2 we have calculated a shift of several hundreds mV with respect to the values obtained by extrapolation of the l/C2 us U, plots. This apparently so high shift is a consequence of the very high donor concentration. Anyway, a correction of about only 100 mV as for passive iron can be obtained taking

Ca=SO pFcmm2 and, in a capacitance study of passive tin[32], Kapusta and Hackermann considered this high value of the Helmholtz capacitance more correct than the value of 25 ~Fcm* that they had at first assumed. Figure 9 shows that the intersections of the Mott-Schottky plots at l/C* = 0 are dependent of the pH. The shift of U, towards more negative voltages with increasing pH suggest that an acid-base equihbrium is established at the interface between the surface of the film and the solution. The U, values were found to be unaffected by the composition of the electrolyte (for ITM 40 and 254 SMO in both pitting and nonpitting environments), indicating the absence of specific adsorption. The straight lines of Fig. 9 are not parallel since the donor concentration changes with the pH (see Table 3). The flat band potential of many semiconductors varies with the pH of the solution[3] according to: U,,

= const. - 0.059 pH.

(9)

In Fig. 10 the values of U, obtained for the passive films on the four metallic substrates at different values of pH are reported. All points were determined from extrapolations at 1 kHz and represent averages at each pH. Figure 10 also includes the data obtained by Stimming for passive iron[l2]. U, rather than UFB values were plotted because of the uncertainty on the exact value of C, and hence on the correction to bring according to Equation (8). As shown in Fig. 10, the values obtained by Stimming agree well with the values calculated in this work. Linear relationships between U, and the pH were observed for every substrate. The U, values of ITM 40 and 254 SMO appear aligned on the same straight line whose slope has roughly the theoretical value of -59 mV/pH. In the case of iron the slope is a little higher whilst for AISI 304 a value of - 40 mV/pH was calculated. It is not clear whether or not the deviation of the pH dependence of the flat band potential from the theoretical value is significant. A similar deviation, which appears to increase in more acidic solution, was

”

“.

“--

0.8 Fig. 9. 254 SMO. Mott-Schottky plots obtained solutions at 1 kHz. 0 1 N NaOH; A 1 N N&30, 0 1 N H,SO,.

in dilTerent pH = 6.5;

Passive

Q

-0.2.

> .

-0.4.

9

films on stainless

-0.6. -0.6. -I t

L.

I.

0

2

4

6

I

I1

6

IO

I

12

14

PH

Fist. 10. Values of the intersection of the Mott-Schottkv pl&s U, obtained at 1 kHz, in dependence on the pH. A 254 SMO, 0 ITM 40, 0 AISI 304; 0 Fe. The full circles have been calculated from[12].

found by Kennedy and Frese (47) for polycrystalline CL- Fe,O, and was attributed to electrode instability. The nearly coincidence of the U, values obtained at pH = 14 can be explained by assuming that the passive films formed on the three stainless steels and on iron have practically the same composition. This means that for the stainless steels a selective dissolution of chromium and nickel occurs in the alkaline solution at the chosen formation potential so that the main composition of the films is supposed to be iron oxide[20]. For 254 SMO and ITM 40 the U, values obtained in 3.5 % NaCl at pH=6.5 were compared with the value of onset photocurrent potential U,, of about -0.1 V us see obtained by photocurrent measurements[25]. The U,, value is about 250mV more anodic than the U,, value obtained by capacitance measurements. For an idea1 semiconductor the flat band potential should coincide with the onset photocurrent potential. A disagreement between U,, and Ura has often been observed and has been correlated to the presence of surface states[48] which in the case of the stainless steels examined could arise by trapping of holes at the surface in nickel oxide sites[25], according to the reaction: pf+Ni2+-

OH-

=Ni3+

-OH-.

(10)

CONCLUSIONS The results of the impedance measurements seem to confirm the semiconducting nature of the passive films on the three stainless steels investigated. Cahan and Chen[8] criticized the semiconductor model of the passive films on iron, on the basis of the restricted range of linearity as well as of the frequency dependence of the Mott-Schottky plots. As shown by Di Quart0 er a[.[341 these findings are not sufficient to invalidate the use of the theory of the semiconductor electrodes for the passive films because they are observed also with crystalline semiconductors[49] and the impedance behaviour at different frequencies can be fully explained by taking into account that the

209

steels

passive films are amorphous or strongly disordered semiconductors. The highly disordered nature of the films is corroborated by the high values of donor concentration found in the different solutions. Amorphous semiconductors are characterized by a high density of states between the valence and the conduction bands and, as suggested by Peterson and Parkinson[SO], the charging of these states could lead to the strong frequency dependence and the large hysteresis observed in the impedance behaviour of the passive films examined. It is questionable if the Mott-Schottky analysis is applicable in the case of the thinner layers but Khan and Schmickler[2&] demonstrated that even for thin films the Mott-Schottky plots exhibit a linear region which, however, cannot ordinarily be used to obtain the donor concentration and the flat band potential. Anyway the sign of the Mott-Schottky plots is an indicator of the conductivity type of the semiconductor and, as expected, the positive slope indicates n-type conductivity for the passive films on the stainless steels and on iron, confirming the results of previous photoelectrochemical measurements[25]. The n-type semiconductor model justifies the potential dependence of the capacitance over the entire range of potentials studied. In particular, the capacitance increase at high potentials is explained by the formation of an inversion layer with participation of the valence band[9] without resorting to the increase of the double layer capacitance due to the adsorption of an anion such as SO:-, invoked by Okamoto for AISI 304 in 1 NH,SOJ19, 201. Also the results of photopotential measurements on various stainless steels, including AISI 304[51], confirm that the passive films behave like n-type semiconductors. This conclusion seems to contradict the results of Manning and Duquette[52], who postulated that the films formed on AISI 304 at temperatures below 220°C were p-type semiconductors, on the basis of the ratio between the apparent anodic and cathodic transfer coefficients GL,/c(, > 1. Anyway, according to Bianchi et &.[I], who first used this technique for dry oxidized AISI 304 stainless steels, not only a, > a, but also large CL,values (ie a,>0.4) are required for the p-type conductivity whilst the values of a, and a, obtained by Manning and Duquette were quite comparable and always lower than 0.4. Otherwise, Stimming and Schultze[53] showed that the reciprocal values of the transfer coefficients of oxide films with a small band gap like passive iron can change from 0 to 1 according if the electron transfer reactions take place via the conduction or the valence band. The ratio a,/a, r 1 obtained by Manning and Duquette for AISI 304 in the system [Fe(CN)J4-/[Fe(CN)J3-[52] could be then explained by an electron transfer reaction which takes place on an n-type semiconductor with participation of the valence band. Acknowledgement-This ted by the “Ministero

work has been financially supporper la Pubblica Istruzione” (Roma).

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A. DI PAOLA

210

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