The effect of temperature on the passive film properties and pitting behaviour of a FeCrNi alloy

CorrosionScience,Vol. 38, No. 6, pp. 909-925,1996 Copyright 0 1996ElsevierScienceLtd Printed in Great Britain. All rights reserved OOlO-938X/96S15.00+0.00

PII: soo10_938x(96)00176x

THE EFFECT OF TEMPERATURE ON THE PASSIVE FILM PROPERTIES AND PITTING BEHAVIOUR OF A Fe-Cr-Ni ALLOY R. M. CARRANZA

and M. G. ALVAREZ

Comision National de Energia Atomica Gerencia de Desarrollo-Dpto. Materiales, Avda. de1 Libertador 8250, 1429 Buenos Aires, Argentina Ah&act--The effect of temperature (60-280°C) on the properties of the oxide films formed on Alloy 800 in 0.1 M NaCl and 0.1 M Na2SOs aqueous solutions was studied by in situ AC impedance spectroscopy and polarization in the Fe(CN;3/Fe(CN);4 redox system. The anodic behaviour under the same experimental conditions was examined by potentiodynamic polarization techniques. In both solutions the passive film was found to become more porous, and hence less protective, with increasing temperature. However, at temperatures above 1WC, the loss of film protectiveness is more pronounced in chloride solutions, in which pitting occurs. Pitting morphology was found to be strongly temperature dependent: isolated and deep pits were found up to 200°C whereas at higher temperatures a broad, shallow and more generalized type of attack was detected. No effect of temperature on the defect structure of the semiconductor oxide film was found. It was concluded that the changes in the pitting behaviour of Alloy 800 at temperatures below and above 200°C are associated to a temperature-affected variation of the protective properties of the passive film. Copyright 0 1996 Elsevier Science Ltd.

INTRODUCTION Alloy 800 is a stabilized Fe-Cr-Ni alloy widely used as a steam generator tubing material in water cooled nuclear power plants (PWR andl,PHWR). In this application tube failures caused by pitting corrosion have been reported. Passivity breakdown and pitting of All22 800 is known to occur in the presence of chloride ions under oxidizing water conditions. One of the characteristic features of the pitting corrosion behaviour of F+Cr-Ni alloys in high temperature aqueous solutions is the existence of a transition temperature (1502OO”C), below which both pitting and repassivation potentials decrease with increasing temperature, while at higher temperatures they remain almost constant or may even increase.6 Another general observation concerning the effect of temperature on pitting corrosion is the transition from a highly localized form of pitting in low temperature chloride solutions to a more generalized form of attack at elevated temperatures.7-9 It is generally recognized that the nucleation and propagation of pitting corrosion are strongly related to the characteristics of the oxide film on the metal surface. Therefore, the observed changes in the pitting behaviour of Fe-Cr-Ni alloys has been often related to changes in the composition, structure and properties of the passive film. Some

Fe-Cr-Ni

authors

have

correlated

the effect

of temperature

on the pitting

susceptibility

of

alloys with a change in the defect structure of the semiconductor anodic film.‘o3”

Manuscript received 20 April 1995: in amended form 27 September 1995. 909

910

R. M. Carranza and M. G. Alvarez

They argued that n-type films could be more susceptible to pit initiation than p-type films due to the existence of oxygen vacancies which may enhance the transport of chloride ions through the oxide lattice. On the other hand, a modification in the protective properties of the passive films,has often been suggested to be the cause of the observed changes in pitting However, there is still limited information on the influence of morphology. environmental variables, such as temperature and electrolyte composition, on the protective properties of oxide films. Wang et al.” used AC impedance measurements to study the effect of temperature (20-200°C) and chloride concentration on the protective properties of passive films formed on AISI 304 SS. They found that the charge transfer resistance decreased with increasing temperature and increasing chloride concentration, thus the protective properties of the film were adversely affected. AC impedance measurements were also used by Smialowska et al.” to study the properties of oxide films on type 304 SS on exposure to lithiated water at lOO-300°C but no clear information about film properties could be drawn by this technique due to the interference of the hydrogen reaction. In the present work, the effect of temperature on the protective properties of the passive film formed on Alloy 800 in aqueous chloride and sulphate solutions at temperatures ranging from 60 to 280°C was studied by means of in situ AC impedance measurements. The results were compared with the passivity breakdown behaviour and pitting morphology found for Alloy 800 in parallel tests run under the same experimental conditions. The influence of temperature on the semiconductive properties of the films was also examined. EXPERIMENTAL

METHOD.

The material tested was nuclear grade Alloy 800 (20.5% Cr; 32.5% Ni; 0.75% Mn; 0.44% Ti; O.O22%C; 0.01% Co; 0.07% Cu; balance Fe). Specimens 1 mm thick were prepared by rolling 14 mm diameter tubes. The rolled material was cut into rectangular 25 x 10 mm wide coupons which were annealed for 30 min at 980°C in argon and water quenched. Each coupon was abraded with silicon carbide papers up to 600 grit. A stainless steel wire was spot welded to one end of the specimens and both the wire and the point of attachment to the specimens were insulated with PTFE tubing. The exposed area of the samples was ca. 3 cm2. Before each experiment the samples were degreased with acetone and etched for 5 min at room temperature in a 2.5% (v/v) HF plus 2O%(v/v) HN03 solution. Experiments at temperatures lower than 100°C were performed in a Pyrex glass cell with a platinum counter electrode. Potentials were measured through a Luggin capillary, using a mercurous sulphate reference electrode for the experiments with Na$Od and a saturated calomel electrode for the NaCl solutions. Experiments at higher temperatures were carried out in a static 1 liter stainless steel autoclave with a Hastelloy C-4 liner. A platinized titanium spiral was used as counter electrode and potentials were measured and controlled with an external pressure-balanced Ag/AgCl/O. 1M KC1 reference electrode. Measured potential values were converted to the standard hydrogen scale at 25”C.15 Experiments were performed in 0.1 M NaCl and 0.1 M Na2S04 solutions in the temperature range from 60 to 280°C. The solutions were prepared with analytical grade reagents and double distilled water. A de-aerated condition was achieved by sparging with high purity nitrogen for at least 2 h prior to each experiment. In the Pyrex glass cell this deaeration was continued during the tests. The temperature was increased either in steps of

Passive film properties and pitting behaviour of a Fe-O-Ni

alloy

911

25°C (for AC-impedance measurements) or directly to the desired level (for polarization curves and semiconductivity measurements). Potentiodynamic polarization curves were obtained at a scan rate of 0.1 mV/s with a PAR 173 potentiostat and a PAR 175 voltage scan generator. The current and potential were continuously recorded with a Houston 2000 X-Y recorder. Corrosion morphology observations and analyses of corrosion products were conducted using scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS), respectively. AC impedance measurements were performed using a PAR 273 potentiostat with a Schlumberger-Solartron 1254 frequency response analyser in the frequency range from 10 kHz to 1 mHz with a perturbation amplitude of 10 mV. PC-386 compatible microcomputer was used for recording and analyzing the impedance data. These measurements were made in situ at gradually increasing temperatures starting from 60°C. Once the desired temperature and corrosion potential, Ecorr, were stabilized, a constant potential in the passive region (150-200 mV higher than E,,,,) was applied. Impedance measurements were started after a stationary value of the passive current was attained. They lasted about 3 h. The specimens for semiconductive properties measurements were prepared by keeping the specimen in the autoclave for 18 h at a constant potential in the passive zone at each of the temperatures tested. The autoclave was then cooled down rapidly to minimize the effect of changing temperatures on the composition and structure of the film. However, no less than 1 h was needed to cool the solution to room temperature. The semiconductive properties measurements were determined at room temperature in about 1 h after removal of the specimen from the autoclave. Anodic and cathodic polarization curves (&-200 mV from E,,,,) were measured in a deaerated solution of 0.1 M Na$O,+ 0.05 M K4 [Fe(CN)6] + 0.05 M K3 [Fe(CN)6] at a potential scan rate of 0.17 mV/s. EXPERIMENTAL

RESULTS

Anodic behaviour of alloy 800 in 0.1 M Na2S04 solution

The anodic behaviour of alloy 800 in sulphate containing solutions was described in a previous publication.‘” Figure 1 shows typical examples of the anodic polarization curves obtained. Upon increasing the potential above the corrosion potential a pseudo-active peak was observed at 200 and 280°C. At 60 and lOO”C, the specimens were in the passive state at the free corrosion potential. Further increasing the potential passive, transpassive and secondary passive regions were found before the onset of the oxygen evolution reaction. The corrosion potential, the transpassivity peak and the onset of oxygen evolution shifted toward the less noble direction with increasing temperature and the current density at any given overpotential was considerably higher than the corresponding value at a lower temperature. By microscopical examination of the specimens after polarization no localized or substantial generalized corrosion was detected. Anodic behaviour of alloy 800 in O.lM NaCl solution

Typical potentiodynamic anodic polarization curves for Alloy 800 in 0.1 M NaCl solution at various temperatures are shown in Fig. 2. All the curves showed a passive zone, where the current density increases with increasing temperature, and a pitting potential above which the current density starts to increase due to the nucleation of pits on the metallic

R. M. Carranza and M. G. Alvarez

912

1.2 0.8 0.4 0.0 -0.4 -0.8 lE-07

1 E-05

lE-06

lE-04

lE-03

1E-02

i (A/cm2) Fig. 1. Potentiodynamic anodic polarization curves for Alloy 800 in 0.1 M Na2S04 solution at different temperatures.‘6 Arrows indicate the electrochemical potential for the oxidation reaction Cr203/Cr04 = at each temperature.

surface. The abrupt decrease in the corrosion potential observed at temperatures greater than 150°C could be attributed to a change in the predominant cathodic reaction established on the passive surface of the alloy. At low temperatures reduction of traces of oxygen predominates whereas at high temperatures reduction of hydrogen ions coupled to a slightly higher anodic current density determines a mixed potential which is close to the equilibrium potential for hydrogen evolution. Similar transitions, although less abrupt, have been observed by other authors” for type 304 stainless steel in water containing low oxygen concentrations. A strong decrease of the pitting potential occurred as the temperature was increased from 60 to 175-200°C but no further substantial decrease was found at higher temperatures, Fig. 3. The above results concerning effects of temperature on th~,~~t~ing potential of Alloy 800 are in agreement with results reported by various authors ’ for many Fe-G-Ni alloys in chloride containing solutions. The morphology of the attack

0.8

+8oT

+100%

:

0.8 0.4

i

ai

r i

0.2 0.0

w

-0.2 -0.4 -OSiE-08

Fig. 2.

Potentiodynamic

1 E-07

lE-06

lE45

lE-04

lE-03

i (A/cma) anodic polarization curves for Alloy 800 in 0.1 M NaCl solution at different temperatures.

Passive film properties and pitting behaviour of a Fe-Cr-Ni alloy

0.5

P

913

’

0.4

z f 0.3 ’ $ lAE 0.2

0.1 50

100

150

200

250

300

Temperature (“C) Fig. 3.

The effect of temperature on the pitting potential of Alloy 800 in 0.1 M NaCl solution.

observed on Alloy 800 above the pitting potential was also found to change significantly with temperature. Isolated, irregularly shaped deep pits, with crystallographic facets, were found up to 150°C Figs 4(a) and (b). Although they do not contain significant amounts of corrosion products, EDS analysis indicates some chromium enrichment at the pit bottom.

Fig. 4.

Pits formed on Alloy 800 after anodic polarization in 0.1 M NaCl solution at temperatures up to 200°C. (a) 60°C; (b) 150°C; (c) 175°C; (d) 200°C. Dashes = 10 pm.

R. M. Carranza and M. G. Alvarez

Fig. 4.

(Continued)

At 175 and 200°C the corrosion attack following passivity breakdown could be characterized as isolated pits, partially or completely covered with a Cr-rich layer of corrosion products, (Figs 4(c) and (d)). At temperatures higher than 200°C a significant change in the pitting morphology took place. At 225 and 250°C corrosion extended over the metallic surface and a high density of widespread pits filled with Cr-rich corrosion products was observed, Figs 5(a) and (b). Finally, at 280°C a shallow and broad form of attack, covering large areas of the specimen surface, was detected, (Fig. 5(c)). Some areas exhibited

Passive film properties and pitting behaviour of a Fe-C-Ni

Fig. 4.

alloy

915

(Continued)

a deeper metal removal, having the appearance of widespread pits. A significant enrichment of chromium relative to nickel and iron was observed on the corrosion products. The present results on the effect of temperature on the pitting morphology are also consistent with literature data.‘-9 As noted by Cragnolino,6 the change in the appearance of the attack appears to be related to a modification in the pitting potential dependence on temperature.

Fig. 5. The morphology of the attack observed on Alloy 800 after anodic polarization in 0.1 M NaCl solution at temperatures higher than 200°C. (a) 225°C; (b) 250°C; (c) 280°C. Dashes = 10 pm.

916

R. M. Carranza and M. G. Alvarez

Fig. 5. (ContinuerI)

AS long as the typical pit morphology is preserved, the pitting potential decreases with tern kperature. If the attack becomes broad, shallow, and more generalized over the pa ssive me1tal surface, the pitting potential remains almost constant. Type .xemiconduction measurements Determination of the electronic properties of surface films by means of anodic : and sweeps in solutions containing redox couples, such as Fe+ 3/Fe’ -2,or cat1todic polarization

Passive film properties and pitting behaviour of a Fe-Cr-Ni alloy

917

1E-03

lE-04

1E-05

1E-07

1E-08

0.2

0.3

0.4

0.5

0.6

Potential(‘Vnhe)

Fig. 6. Potentiodynamic polarization curves in 0.1 M Na$04+0.05 M &[Fe(CNs)]+0.05 M K,[Fe(CN)6] solution at 25°C for films formed on Alloy 800 in 0.1 M NaCl at different temperatures.

[Fe(CN)J-4/[Fe(CN)6]-3, has been widely used’0”“‘4”9’m because their reversible potentials fall in about the middle of the passive region of several commonly used metals and alloys. According to the valence band theory the type of conductivity is determined by the ratio of the apparent anodic and cathodic transfer coefficients (a&J. Small a, values and a, 5 a, correspond to n-type films, whereas cr,10.4 and a, > a, to p-type films.” Figure 6 shows some of the typical polarization curves found for the redox K4[Fe(CN),]/K3[Fe(CN)b] system at 25°C on Alloy 800 specimens with surface films grown in 0.1 M NaCl at different temperatures. As on many other semiconductor surface films,” this redox system presents a low exchange current density (i,,) and a pronounced asymmetry of the anodic and cathodic branches on Alloy 800 film. Figure 7 shows the effect of temperature on a, and a, obtained by adjusting the Stern-Geary equation2’ to the experimental polarization curves using nonlinear least squares fit (NLLSF). Whereas a, is always lower than a, and almost independent of temperature, a, exhibited a hi&value at 60°C that decreased with increasing temperatures. It is worthwhile to remark that high a, values (a, > 0.5) such as that found at 60°C are frequently observed on p-type films and can be explained by the semiconductor theory.ZZ Concerning the effect of temperature on the exchange current density, the i, value was 6. 10d6 A/cm2 at 60°C and increased to 4. 10V5 A/cm2 at 100°C. At higher temperatures, the i, values varied between 2.7. lOA5 and 5.5. 10V5 A/cm’, without showing a definite trend with increasing temperature. The above results indicated that in the whole range of temperatures studied (60-3 1OT), Alloy 800 oxide film behaves as a p-type semiconductor. It must be noted that this study was performed ex situ and suffers of all the uncertainties applicable to these cases. A semiconductive in situ analysis by a Mott-Schottky type representation is not possible with these anodic films.”

R. M. Carranza

918

and M. G. Alvarez

oI.L_

0

100

200 Temperature

Fig. 7. The effect of temperature calculated by fitting the Stern-Geary the redox [Fe(CN)6]-4/[Fe(CN),1-’

AC impedance

300

(“C)

on apparent anodic Z~ and cathodic rc transfer coefficients equation2’ to the experimental polarization curves obtained for system on films formed on Alloy 800 in 0.1 M NaCl solution.

measurements

Figures 8(a) and (b) show the Bode diagrams obtained for Alloy 800 in 0.1 M Na2S04 solution for some of the temperatures tested. At temperatures lower than 150°C typical capacitive-like behaviour was found. At higher temperatures, two poorly defined maxima could be distinguished in the phase angle. Bode diagrams obtained for Alloy 800 in 0.1 M NaCl solution, Figs 9(a) and (b) exhibited two well defined phase angle maxima at temperatures higher than 125°C. In both solutions Bode plots presented phase angle maxima smaller than 90” and absolute values of the impedance modulus slope lower than 1. Correspondingly, Nyquist plots (not shown) showed a depressed semi-circle. Many reasons have been proposed in the literature to explain this behaviour such as surface roughness, frequency dispersion of time constants due to local inhomoE:neities in the dielectric material, porosity mass transport effects and relaxation effects. _ In order to account for these effects, non-ideal capacitors must be introduced into the equivalent circuit prop:sed to reproduce the experimental results, according to the theory presented by Jonscher. In the present case, equivalent circuits consisting of a pure resistance RQ in series with parallel circuits of ideal resistances and non-ideal capacitors (Rn - RIICnon_ideal) or with parallel circuits of ideal resistances and constant phase elements (Ra - RIICPJ$24 can be proposed. These equivalent circuits are the most simple ones that can be proposed to simulate adequately the experimental results. They do not include all the complexities involved in taking into account the exact semiconductor propertie: of the metallic oxides which could imply to add more complex circuits elements as diodes or frequency dependent resistances typical of porous electrodes with distributed resistances and capacitances in a network.24 For the proposed equivalent circuits, the following transfer functions are obtained: RLF Z(w, = Rn + 1 -+O.WRLFCLF?~~

for one phase angle maximum, and RHF -Go, = &I + 1 + (~wRHFCHF)~"~

+

RLF

1 + O.WRLFCLF)“~

Passive film properties and pitting behaviour of a Fe&L-Ni alloy

Frequency

919

(Hz)

(b) lE+05 lE+O4 lE+03 lE+02 lE+Ol

Frequency (Hz)

Fig. 8. Experimental Bode diagrams of the complex impedance obtained in situ for Alloy 800 in 0.1 M Na2S04 at different temperatures. (a) Variation of the phase angle with frequency; (b) variation of the impedance modulus frequency.

for two phase angle maxima, where i is the imaginary unit (sqr - l), o is the angular frequency, Rn is the ohmic component of the complex impedance _&, at very high frequencies, C denotes ideal capacitors, /J is the dispersion parameter indicating the deviation of the model from pure R-C circuits2* and subscripts LF and HF correspond to the parameters fitted at low and high frequencies, respectively. High frequency parameters correspond to the time constants associated with the phase angle maxima at high frequencies that can be distinguished at temperatures higher than 125150°C. Equations (1) and (2) were adjusted to the experimental impedance spectra using NLLSF. Figure 10 shows the good agreement obtained between fitted and experimental results in a Bode representation. Figure 11 shows the effect of temperature on the parameters calculated by fitting eqns (1) and (2) for both solutions. In 0.1 M Na2S04 solution, (Fig. 1l(a)) the Cr_~value increases by an order of magnitude in going from 25 to 28O”C, whereas RLF decreases by two orders of magnitude in the same temperature range. Since in this electrolyte the high frequency parameters cannot be determined with certainty due to poor definition of the phase angle maxima, they are not shown in Fig. 1l(a).

920

R. M. Carranza

and M. G. Alvarez

Frequency (Hz)

@I

lE+07

E

lE+OO L “““” -

“““” -

Frequency

“‘,“. “““I

“,I”- ’ -J

(Hz)

Fig. 9. Experimental Bode diagrams of the complex impedance obtained in situ for Alloy 800 in 0.1 M NaCl solution at different temperatures. (a) Variation of the phase angle with frequency; (b) variation of the impedance modulus with frequency.

In O.lM NaCl solution Ci_r was strongly affected by temperature (Fig. 1l(b)). Crr increases with temperature up to 150°C in a way similar to that found in sulphate solutions. In going from 150 to 225°C the C rF value increases abruptly by two orders of magnitude. A further temperature increase causes almost no change in CLF. Meanwhile, RL~ diminishes with increasing temperature, attaining at 280°C a value two orders of magnitude lower than the value at 60°C as is the case for sulphate. Similar Cr.~ and RLF values and trends were observed for AISI 304 in chloride solutions up to 200”C.‘3 The high frequency parameter RHF seemed to be almost independent of temperature, while CHF exhibited a behaviour similar to that found for Crr. The values of the fitted dispersion parameter ,&J were between 0.8 and 1 at all temperatures, thus supporting the physical validity of the proposed equivalent circuits. Rather low fi~r values (0.6-0.8) were found at temperatures equal to and higher than 225°C.

Passive film properties and pitting behaviour of a Fe-Cr-Ni alloy

921

Frequency (Hz)

Fig. 10. The fitting of eqn (2) to the experimental complex impedance results obtained in situ for Alloy 800 in 0.1 M NaCl solution at 175°C using NLLSF. Symbols: experimental data; lines: eqn. (2); 0 phase angle; 0 modulus.

DISCUSSION The above results, concerning the effects of temperature on both the pitting potential and pitting morphology of Alloy 800, show that the pitting corrosion behaviour of this alloy in high temperature chloride solutions is very different below and above 175-200°C. Below these temperatures the pitting potential decreases with increasing temperature and isolated and deep pits are found on the metallic surface after passivity breakdown. On the other hand, the almost constant value of the pitting potential at temperatures higher than 200°C corresponds with a gradual change of the corrosion morphology to a shallow and more generalized type of attack. The evaluation of the semiconductive properties of the passive film by means of anodic and cathodic polarization in the redox system show that the film formed on Alloy 800 in chloride containing solutions is of the p-type over the entire temperature range tested (60310°C). Stellwag” also reported p-type conductivity for Alloy 800 films formed in alkalinized water at 300°C and in air at 350°C. He also found a decrease in pitting susceptibility with pre-oxidation time that was not reflected in the shape of the polarization curves for the redox system. From the present results, it can be concluded that the type of conductivity does not control the susceptibility of the oxide film to local breakdown. However, Fig. 7 shows that although there is no change in the electronic properties of the film, the p-type conductivity of the film decreases as the formation temperature increases. This tendency towards n-type conductivity could indicate a relative increase of oxygen vacancies in the oxide lattice. Therefore, chloride ions penetration into the film at high temperature” cannot be discarded completely but it cannot be considered the controlling factor in pit initiation at high temperatures. The most striking characteristic of the experimental impedance spectra is the appearance of two peaks in the phase angle at temperatures above 100°C. It could be related to the well known duplex structure of passive films formed on Fe-Cr-Ni alloys in high temperature The outer, crystalline, part of the oxide is reported to be Fe-rich aqueous s01utions.‘6’20’30 while the inner, amorphous, protective layer is Cr-rich. Increases in film thickness were found to occur via thickening of the Cr-rich layer.”

R. M. Carranza

c

and M. G. Alvarez

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/ ---.. .LCI 250 Temperature

, E-05

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(“C)

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11. The effect of icmperaturc

experm~ental

complex

on the parameters caiculared by Wing eqns (1) and (I) to the impedance data &tamed 1~ ~iru for .411oy 800 using NLLSF (a) 0.1 M Na2SOJ solution; (b) 0.1 M NaCl solution.

Concerning the high frequency parameters, it was not possible to obtain confident values of them in sulphate solutions. In chloride solutions. where CHF and RHO can be reasonably fitted at almost all temperatures, the meaning of these parameters cannot be easily associated to the outer, non-protective, Fe-rich layer. Even if the constancy of RHF could be related to a constant outer-film thickness, no clear interpretation was found for the as well as for the low values of flHF obtained at high variation of CHF with temperature temperature. Moreover, the Helmholtz double-layer capacity, C”, was not considered in the equivalent circuit. All these facts obscure the interpretation of the high frequency relaxation phenomenon. A more detailed and laborious analysis of impedance responses is needed to construct a more realistic equivalent circuit representing quantitatively all the parameters which are characteristics of this semiconductor film. associated to the The low frequency parameters CLF and RLF are undoubtedly protective, inner layer of the passive film. It has often been assumed that the porosity of stainless steel oxides increases with increasing temperature, rendering the film less protective. loJL3’ The increase of film porosity would lead to an increase of film capacity and to a decrease of film resistance. Figure 11 shows that the temperature-dependence of the low frequency parameters CLF and R LF of the passive films formed on Alloy 800 in chloride

Passive film properties and pitting behaviour of a Fe-C-Ni

alloy

923

and sulphate solutions are consistent with an increase of the film porosity. The higher the temperature is, the higher the CLF and the lower the R LF values are. Film thinning is another phenomenon that could also lead to an increase in film capacity and to a less protective film. This possibility is not supported by the experimental evidence that films formed on Fe-CrNi alloys becomes thicker at increasing temperatures.‘4’33’34 Exposure to high temperature water of Alloy 800 led to the formation of an oxide film which was typically 100-300 nm thick. 16.X.35 An increase of the density of charge carriers may also give rise to larger capacity and smaller resistance values. It would also lead to increases of the exchange current density of the redox couple with increasing temperature, which are not found in our experiments, (Fig. 6). The AC impedance results show that the passive films formed on Alloy 800 in chloride solutions become more porous, and hence less protective, with increasing temperature up to 225°C (Fig. 11(a)). At higher temperatures, no further increase in film porosity was detected. The pronounced rise of film capacity at temperatures between 175 and 225°C indicates that the loss of film protectiveness is more pronounced in this temperature range than at lower temperatures. This effect may be caused by chloride ions, since similar behaviour was not observed in sulphate-containing solutions (Fig. 1l(b)). At these high temperatures, the contribution of a process such as chloride incorporation into the film cannot be excluded.“-zz It should be noted that the decrease of the protective properties of the passive film at temperatures above 200°C is coincident with the already described change in the morphology of the attack. As is clearly shown by AC impedance results, the change in the pitting behaviour of Alloy 800 in chloride solutions with temperature is at least partially associated with a temperature-dependent modification of the porosity of the passive film, resulting in a decrease of its resistance to breakdown as temperature is increased. A thick and porous film, with a large number of local imperfections, would facilitate the initiation of pits at far greater number of sites, followed by a broad and shallow spread of the attack. From the AC impedance results, it follows that also in sulphate-containing solutions, in which no pitting occurs, the passive film becomes more porous with increasing temperature. As indicated by differences in the C LF value at temperatures higher than 150°C (Fig. 1l), the film formed in the presence of a non-aggressive anion, such as sulphate, is more protective than the film formed at the same temperature in chloride solutions. CONCLUSIONS The pitting potential of Alloy 800 in chloride solutions decreases with increasing temperature up to 200°C but at higher temperatures it remains almost constant. The pitting morphology is also strongly dependent on temperature; isolated and deep pits are found up to 200°C whereas a transition to a broad, shallow and more generalized form of attack is detected at higher temperatures. The change in pitting morphology is not related to a change in the defect structure of the surface oxide, since the passive film formed on Alloy 800 in chloride solutions is a p-type semiconductor in the whole temperature range from 60 to 3 10°C. The low frequency parameters obtained from AC impedance diagrams can be associated to the protective characteristics of the oxide film. The temperature-dependence of these parameters clearly shows that the change in the pitting behaviour of Alloy 800 in chloride

R. M. Carranza

924

and M. G. Alvarez

solutions with temperature is at least partially related to an increase of film porosity, resulting in a decrease of its resistance to breakdown as temperature is increased. The differences in AC impedance results obtained in sulphate and chloride solutions suggest that chloride ions are responsible for the strong deterioration of the film protective properties observed at high temperatures at potentials below the pitting potential. work was done m the frame of the Cooperation Agreement between the Comision National de Energia Atomica (ARGENTINA) and the Kernforschungszentrum Karlsruhe (GERMANY) and partially supported by the Programa Latinoamericano de Degradation de Materiales (OEA-CNEA) and by the Consejo National de Investigaciones Cientificas y Tecnicas (CONICET). AcknonYedgenzent.s--This

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