J O U R N A L O F M A T E R I A L S S C I E N C E L E T T E R S 2 2, 2 0 0 3, 1347 – 1349
Semiconducting properties of surface oxide films of pure iron, nickel and chromium metals in pure water at 288 ◦ C K . S . R A J A ∗, T . S H O J I Fracture Research Institute, Graduate School of Engineering, Tohoku University, Sendai, Japan 980-8579 E-mail:
[email protected]
Passive film formed on iron [1], nickel [2] and chromium [3] has been investigated extensively at room temperature. When the chemical composition of the oxide film deviates from stoichiometry, it behaves like a semiconductor [4]. The semiconducting behavior of passive film is considered to be important in determining the corrosion and stress corrosion cracking resistance of stainless alloys [5, 6]. As thicker films form at elevated temperatures, it is expected that the ionic mobility is much higher at higher temperatures. Not much is known about the semiconducting properties of the passive films of the above materials exposed to aqueous environments at elevated temperatures. In this study, the semiconducting properties of iron (99.9% Fe), nickel (99.5% Ni) and chromium (99.7% Cr) metal specimens were investigated in pure water at 288 ◦ C by varying the dissolved oxygen contents which simulated the boiling water reactor (BWR) environments. The electronic properties of the passive film of pure metals were compared with that of type 304 L stainless steel. The electric resistance of the passive film was measured in-situ using constant electric resistance (CER) technique, as described by Saario et al. [7]. The experimental set-up and working principles have been described in detail elsewhere [2, 3, 7]. Briefly, the electric resistance across a solid-solid contact between two specimens of similar materials exposed to the test environment is measured by passing a known value of direct current (0.75 mA and 25 mV as open circuit voltage) and monitoring the voltage drop. The surfaces of the two specimens were periodically contacted and disconnected at a desired time intervals. When the specimen surfaces were not in contact, these were exposed to the test environment and the surfaces were oxidized. When the specimens were brought into contact by accurate movement of a stepper motor—spring assembly, the contact electric resistance was measured. A pair of 2 mm diameter cylindrical specimens of identical material was used. The flat surfaces of the two specimens were polished with series of emery papers up to 1000 grit and thoroughly washed prior to the immersion into the test environment. This entire set up was introduced to an autoclave (volume 4 l) made of type 316 L stainless steel. Pure water (inlet conductivity 10 kilo ohms). When the corrosion potential decreased, the CER values decreased to lower values. Upon increasing the D.O again, there was no significant change in the CER values up to a certain potential above which increase in CER could be observed. When the D.O was maintained at 2 ppm for 12 h, the CER increased with time to higher values initially and reached a steady state. Second cycle of D.O reduction also similar trend as that of first cycle but the CER values were higher at similar corrosion potentials compared to the first cycle. The very low CER values at lower corrosion potentials could be due to predominantly Fe3 O4 type structure which is formed at lower corrosion potentials. At higher corrosion potentials the Fe3 O4 transforms to higher oxide of type Fe2 O3 . It is well documented that Fe3 O4 has higher conductivity [9–13]. However, Fe2 O3 oxide
∗ Present address: Department of Metallurgical Engineering, University of Nevada, Reno, NV 89557, USA. C 2003 Kluwer Academic Publishers 0261–8028
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Contact Electric Resistance, Ohm
potential, which is typical for a p-type semiconductor. At lower corrosion potentials the electric resistance of the surface film of the pure nickel specimens was very high, showing a reverse trend compared to that of pure iron. In p-type semiconductors, the major defect is metal cation vacancy according to the following reaction of oxide formation [4]:
1.E+05 1.E+04
Decreasing D.O-I
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Decreasing D.O-II 1.E+02 1.E+01 1.E+00
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1 3+ O2 + M2+ → OOX + V− M+M 2
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Corrosion Potential, mV(SHE)
Figure 1 Electric resistance of passive film of pure iron exposed to pure water at 288 ◦ C. Initially dissolved oxygen content was high and corrosion potential was decreased by decreasing the dissolved oxygen content (cycle 1) and again the specimen was polarized back to higher potential (cycle 2).
can exist in the form of γ -Fe2 O3 (maghemite) having Fe2.67 O4 formula or α-Fe2 O3 (hematite). As the CER values were quite high at higher corrosion potential the oxide formed could be hematite as γ -Fe2 O3 could have higher conductivity because of cation vacancies [11]. Apart from the structural changes of surface film, another reason for increased electric resistance with increased corrosion potential could be due to n-type semiconducting property of the oxide film. In n-type oxide the majority conductor is electron. According to the reaction, − 6Fe3 O4 + H2 O + O2 → 9Fe2 O3 + 2H+ (aq) + 2e (1)
1.E+04 Decreasing DO Increasing DO
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Corrosion Potential, mV(SHE)
Figure 2 Electric resistance of passive film of pure nickel exposed to pure water at 288 ◦ C. The specimens were chemically polarized from higher potential to lower potential (cycle 1) and back to higher potential by varying the dissolved oxygen content in the water.
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where V− M is cation vacancy. The equilibrium constant, K, for the above reaction is −12 3+ (3) K = (V− M )(M ) pO2 Considering the activities of oxide and M2+ ions as1 unity, the defect concentration is proportional to ( pO2 ) 4 [4]. Therefore, with increase in oxygen content (increase in corrosion potential), the electric conductivity which is proportional to the defect concentration should increase and the electric resistance should decrease. Fig. 3 shows the variation in electric resistance with the corrosion potential of the passive film of pure chromium. In this case also, a typical p-type semiconducting behavior could be observed. Compared to pure nickel, the surface film formed on pure chromium showed very high electric resistance at lower corrosion potentials. At lower corrosion potentials the conducting mechanism of chromium oxide is considered to be tunneling of electrons [3]. When two oxide covered surfaces were in contact, the tunneling could not be effective as the distance was larger. Therefore, the electric resistance was higher at lower potentials. At higher corrosion potentials the conduction mechanism changed from electron tunneling to holes migration which showed p-type conductivity. Fig. 4 shows how the electric resistance of the passive film of 304 L stainless steel varied with the corrosion potential. The results are similar to that of pure iron showing n-type semiconductivity. The electric resistance values indicate the passive film of the 304L
Contact Electric Resistance, Ohm
Contact Electric Re sistance, Ohm
formation of hematite results in loss of free electrons in order to maintain charge neutrality during the above reaction. This loss of electrons results in increased resistance of the film. Fig. 2 illustrates the electric resistance of the passive films of pure nickel specimen exposed to high temperature pure water at 288 ◦ C at different corrosion potentials. It is widely reported that NiO type oxide shows p-type semiconductivity [12]. In this study also, the surface oxide film formed on pure nickel showed a decrease in electric resistance with increase in corrosion
(2)
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Increasing DO
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Decreasing DO 1.E+04
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Corrosion Potential, mV(SHE)
Figure 3 Electric resistance of passive film of pure chromium exposed to pure water at 288 ◦ C. The specimens were chemically polarized from higher potential to lower potential (cycle 1) and back to higher potential by varying the dissolved oxygen content in the water.
contact electric resistance, ohm
1.E+04 Increasing D.O
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Corrosion potential, mV (SHE)
Figure 4 Electric resistance of passive film of type 304 L stainless steel exposed to pure water at 288 ◦ C. The specimens were chemically polarized from higher potential to lower potential (cycle 1) and back to higher potential by varying the dissolved oxygen content in the water.
stainless steel, which imparts corrosion resistance is not a pure Cr2 O3 type film. As the electric resistance was very low at lower potentials, it could be a Cr-rich spinel (Fe, Ni, Cr)3 O4 type oxide [13]. The increase in electric resistance with the increase in corrosion potential could be attributed to the oxygen vacancy defect structure. As the number of oxygen vacancies could be reduced with increase in dissolved oxygen content, less number of free electrons are required to maintain the electrical neutrality, which in turn increases the electric resistance. At higher corrosion potentials the oxide type could be (Fe, Cr)2 O3 . The resistance values at higher corrosion potential were considerably lower than that of pure iron. Relatively lower resistance as compared to α-Fe2 O3 , suggests that some Fe2+ ions could be present in the oxide layer of the stainless steel which could hop between (Cr, Fe)3+ sites increasing the conductivity. It could be noted that the electric resistance of the oxide film formed on Cr was higher than that of stainless steel at higher corrosion potentials also, which indicates that the film was not pure Cr2 O3 . Two layers of passive film have been postulated for stainless steels exposed to room temperature aqueous environments [14, 15]. The inner and outer layers were considered to be of p-type and n-type semiconductors respectively. At 288 ◦ C, the passive film was observed to be predominantly n-type semiconductor. At lower temperatures, p-type Cr2 O3 film forms preferentially as Cr has more affinity to oxygen and other ions have lower mobility. At higher temperatures, with increased ionic mobility, n-type film could form incorporating iron in
chromium oxide film [16]. The electric resistance of the passive film affects the potential drop across the passive film. Higher the conductivity of the film, lower will be the potential drop across the film, which could make the electric field assisted transportation of charged defects less effective. If the electric resistance of the film is higher, the potential drop will also be higher across the film and therefore, the flux of defects transported across the film will be higher. The stability of the passive film is considered to be affected by balance between the arrival rate of metal cation vacancies at the metal/oxide interface and rate of consumption of these vacancies to form metal ion in the oxide lattice [9]. If more cation vacancies are reaching the metal/oxide interface than they are consumed, the vacancies could condense and lead to film breakdown [9]. This kind of breakdown could be expected with the passive film having higher electric resistance. References 1. M . B U C H L E R , P . S C H M U K I , H . B O H N I , T . S T E N B E R G and T . M A N T Y L A , J. Electrochem. Soc. 145 (1998) 378. 2. M . B O J I N O V , G . F A B R I C I U S , P . K I N N U N E N , T . L A I T I N E N , K . M A K E L A , T . S A A R I O and G . S U N D H O L M , Electrochimica Acta 45 (2000) 2791. BOJINOV, G. FABRICIUS, T. LAITINEN, T . S A A R I O and G . S U N D H O L M , ibid. 44 (1998) 247. 4. B . D . C R A I G , “Fundamental Aspects of Corrosion Films in
3. M .
Corrosion Science” (Plenum Press, New York, NY, 1991) p. 56. 5. K . S . R A J A , Y . W A T A N A B E and T . S H O J I , J. Mater. Sci. Lett. 20 (2001) 965. 6. P . E . M A N N I N G and D . J . D U Q U E T T E , Corrosion Sci. 20 (1980) 597. 7. T . S A A R I O and J . P I I P P O , Materials Sci. Forum 185–188 (1995) 621. 8. K . S . R A J A and T . S H O J I , J. Mater. Sci. Lett. 21 (2002) 435. 9. E . S I K O R A and D . D . M A C D O N A L D , J. Electrochem. Soc. 147 (2000) 4087. 10. J . J . S E N K E V I C H , D . A . J O N E S and I . C H A T T E R J E E , Corrosion Sci. 42 (2000) 201. 11. H . G E R I S C H E R , ibid. 29 (1989) 191. 12. N . S A T O , Corrosion 45 (1989) 354. 13. Y . J . K I M , ibid. 56 (2000) 389. 14. N . E . H A K I K I , S . B O U D I N , B . R O N D O T and M . D A C U N H A B E L O , Corrosion Sci. 37 (1995) 1809. 15. V . V I G N A L , C . V A L O T , R . O L T R A , M . V E R N E A U and L . C O U D R E U S E , ibid. 44 (2002) 1477. 16. P . E . M A N N I N G and D . J . D U Q U E T T E , ibid. 20 (1980) 597.
Received 14 March and accepted 14 May 2003
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