Metal Science and Heat Treatment
Vol. 45, Nos. 9 – 10, 2003
UDC620.193:669.296.5
UNIFORM AND “NODULE” CORROSION OF ZIRCONIUM ALLOYS UNDER SERVICE CONDITIONS1 V. I. Perekhozhev,2 L. P. Sinel’nikov,2 A. N. Timokhin,2 S. A. Averin,2 M. V. Chernetsov,2 and V. P. Kuznetsov2 Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 10, pp. 26 – 31, October, 2003.
Results concerning uniform and nodule (local) corrosion obtained in SF NIKIÉT are reviewed. The applicability of the electrotechnical theory of high-temperature oxidation of metals to zirconium alloys is analyzed. The conditions of occurrence of nodule corrosion are determined, and the results of a study of process channels and fuel assemblies of RBMK reactors after service at nuclear power plants (NPP) are generalized.
tion or without it [1]. Figure 1 presents kinetic curves of oxidation of specimens of alloy Zr – 2.5% Nb (alloy 125) after thermomechanical treatment with and without the action of reactor radiation. The oxidation was performed at 310, 350, and 400°C in a gas medium imitating the medium of the graphite lining of RBMK. It follows from the obtained results that the growth in the test temperature causes an increase in the corrosion rate, and the effect of the reactor radiation depends on the structural state of the metal and the test temperature. The dependence of the thickness of the oxide film on the surface of the specimens of alloy Zr – 2.5% Nb on the service time of the process channels of the NPP is presented in Fig. 2. The after-service results agree well with the data of laboratory tests aimed at studying the kinetic laws of corrosion and the effect of the density of the neutron flux. Mathematical processing of the obtained results has shown that the corrosion kinetic curves possess two segments divided by what is known as a “turning point”. The first segment is satisfactorily described by an almost parabolic equation
INTRODUCTION During the almost thirty years study of corrosion (oxidation) of zirconium alloys performed at the SF NIKIÉT the institute has accumulated a considerable volume of experimental results and developed models of the process. In cooperation with other specialists in the field SF NIKIÉT researchers worked on choosing the compositions of alloys and modes of thermomechanical treatment and welding, investigated the effect of reactor radiation on the variation of the properties of the metal, created safety rules for maintenance of core regions of nuclear power plants, and performed after-service studies. In the present work we will review the results obtained at the institute on uniform and “nodule” (local) corrosion of zirconium alloys, analyze the applicability of the electrochemical theory of high-temperature oxidation of metals to zirconium alloys, and generalize the results of the studies of process channels and fuel assemblies of RBMK reactors after service at NPP. CORROSION KINETICS
x = Kp t1/2,
where x is the thickens of the oxide film, Kp is a constant of the parabolic equation, and t is the duration of the test or the service time. The second segment is commonly describable by an equation of a straight line
We studied the kinetic dependences of uniform corrosion (oxidation) of zirconium alloys in various structural states, at various temperatures, and under the action of reactor radia1
2
(1)
The material of the paper has been reported at the Russian Conference “Materials for Nuclear Engineering” (MAYaT-1) conducted in Agoi (Krasnodar Region) on September 23 – 27, 2002 [see Metalloved. Term. Obrab. Met., No. 8 (2003)]. Research and Design Institute for Power Engineering (SF NIKIÉT), Zarechny, Sverdlovsk Region, Russia.
x = Kl t + C,
(2)
where Kl is a constant (rate) of linear oxidation (corrosion) and C is a constant of the linear equation. 390 0026-0673/03/0910-0390$25.00 © 2003 Plenum Publishing Corporation
Uniform and “Nodule” Corrosion of Zirconium Alloys under Service Conditions
391
x, mm
Dm, mg/dm2
16
150
12 100
400°C
8
50
4
310°C à 0
Dm,
1000
mg/dm2
2000
3000
4000
20
40
60
80
100 t, 103 h
Fig. 2. Dependence of the thickness of the oxide film (x ) on alloy Zr – 2.5% Nb on the service time of a process channel: )) internal surface, core center; O) internal surface, core edge; &) external surface, core center; N) external surface, core edge.
20
400°C
350°C
10
xcr , mm 310°C
0
0
tîx , h
1000
2000
3000
4000
b
15
tîx , h 10
Fig. 1. Oxidation kinetics of alloy Zr – 2.5% Nb under the action of irradiation (&) and without it ()). The test temperatures are given at the curves: a) annealing at 580°C for 5 h; b ) cooling from 860°C in helium, deformation with e = 20%, annealing at 530°C for 24 h.
5
0
The dependence of the thickness of the oxide film on alloy Zr – 2.5% Nb at the “turning point” on the temperature is presented in Fig. 3. A metallographic study has shown that the “turning” of oxidation kinetic curves is caused by degradation of the protective properties of the formed oxide films. At a relatively low temperature the “turning” occurs due to the formation of a great number of macropores in the oxide film close to the metal – oxide interface; at intermediate temperatures it is caused by cracking of the external surface of the film; and at a high temperature it is again determined by the porosity of the oxide film. There is an opinion that the “after-turning” oxidation at a constant rate is caused by the transfer of the reagents through a constant-thickness oxide layer. The formation of macropores is presumed to be explainable by migration of micropores forming due to crystallization of amorphous zirconium dioxide and their coagulation into macropores on the metal – oxide interface in the field of compressive mechanical stresses arising due to the difference in the volumes of the metal and of the forming oxide. The cracking of the external surface of the oxide is associated with hardening of the sub-oxide layer of the metal by the dissolving oxygen and with the consequent growth in the tensile stresses on the surface of the oxide. At a high temperature, when the hardening action of the dissolved oxygen stops, the “turning” is again determined primarily by the processes of migration of micropores and their coagulation. Thus, we should infer that that the phenomenon of “turning” in the kinetics of oxidation of zirconium alloys is a natural process stimulated by the differ-
300
400
500
600
700
800 ttest , °C
Fig. 3. Dependence of the thickness of the oxide film (xcr is the critical thickness of the film) at the “turning” moment on the test temperature: N) zirconium iodide; ´) Zr – 1.0% Nb; )) Zr – 2.5% Nb after hardening and annealing.
ence in the volumes of the oxidizing metal and of the forming oxide, and the methods for affecting it are limited. The kinetics of the nodule (local) corrosion has not been studied well due to the absence of methods for determining it in the initial stages. It has been shown experimentally that there is an incubation period followed by a stage of local corrosion damage of the metal accompanied by formation of zirconium dioxide. As a rule, the depth of the nodule corrosion in the post-incubation stage linearly depends on the time; the duration of the incubation period and the growth rate of each “nodule” differ even under the same operational conditions [2]. The structure of the oxide film in the nodules and the linear nature of the oxidation kinetics indicate that the nodule corrosion detectable in the post-incubation period can be close in nature to uniform corrosion in the post-turning stage. MAIN CONCEPTS OF THE ELECTROCHEMICAL MODEL OF THERMAL OXIDATION OF ZIRCONIUM ALLOYS It can be assumed that the processes occurring during oxidation of a metal are similar to those occurring in a galvanic cell where the electrolyte and the external circuit is the
392
V. I. Perekhozhev et al. Oxide
Metal
Oxidizer Ri
Zr4+ O2–
O2–
E0
Re
Eex
Eex
K2
K3
Ee V
O2– K1
e
4e
Iîx Anode
Cathode
Fig. 5. Equivalent circuit of electrochemical corrosion and diagram of experiments for proving the applicability of the model to zirconium alloys: K1 ) short circuit mode; K2 ) mode of accelerated oxidation; K3 ) mode of deceleration under the action of external electric field.
Zr ® Zr4+ + 4e ja0 Dja
Djc
jc
ja
jc0
E0
E0
Equating the right-hand sides of equations (4) and (5) we can write
1 O + 2e ® O2– 2 2
E0 = iox [Re (x ) + Ri (x )] + Dja (iox ) + Djc (iox ), Fig. 4. A sketch of a galvanic cell formed during oxidation (corrosion) of zirconium and a polarization diagram illustrating the variation of electrode potentials.
forming oxide [3]. A sketch of such a cell and diagrams of polarization of the electrodes during the occurrence of electrode reactions on zirconium are presented in Fig. 4. The electromotive force E0 in the case when the electrodes preserve the equilibrium potentials j a0 and j c0 (Re /Ri p 1) is proportional to the variation of the isobaric-isothermal potential DG in the chemical reaction of oxide formation and is described by an expression E0 = j a0 – j c0 = –
DG , nF
(3)
where n is the number of charges crossing the phase boundary upon the formation of a compound molecule and F is the Faraday constant. At comparable values of the electron (Re ) and ion (Ri ) resistances of the oxide the potential on both electrodes shifts from the equilibrium position and E 0¢ = E0 – (Dja + Djc ) = E0 – Eb ,
(4)
where Eb = Dja + Djc . The electromotive force of an operating galvanic cell is equal to the sum of the voltage drops across the external and internal circuits or, in the case of deviation from the equilibrium, E ¢0 = iox (Re + Ri ).
(5)
(6)
where iox is the oxidation current characterizing the growth rate of the oxide (dx/dt) and Re (x ) and Ri (x ) are the electron and ion resistances dependent on the thickness of the oxide film x. The differential equation (6) relates the rates of the processes of diffusion transfer of the reagents and of the electrode reactions. The function x = f (t) is a solution to this equation and describes the variation of the thickness of the oxide film as a function of the oxidation time. The equivalent circuit and the diagram of the experiments for proving the applicability of the model to zirconium alloys are presented in Fig. 5. It should be noted that SF NIKIÉT specialists have developed methods and equipment for studying the parameters of a metal-oxide-oxidizer system (Re , Ri , E0 , Eb , te , ti , etc.) on specimens of zirconium alloys directly in the oxidation process under irradiation and without it. Without going into the details of the experiments we will mention some results that show the adequacy of the used model to corrosion processes in zirconium alloys (Fig. 6). Oxidation of alloy Zr – 1.0% Nb at 350°C in the “short circuit” mode considerably accelerates the corrosion rate relative to normal oxidation (Fig. 6a ). An analysis of the variation of the oxidation currents within the framework of the electrochemical model has shown that the increase in the oxsh idation rate under short circuit (i ox /iox ) is describable by the expression sh i îx DE sh 1 æ b = ç1 ç i îx t e è E 0 - E b
ö ÷. ÷ ø
(7)
It should be noted that the short circuit mode is a particular case of acceleration of the oxidation by an external electric field, when Eex = Ee = iox Re .
Uniform and “Nodule” Corrosion of Zirconium Alloys under Service Conditions
The “short circuit” experiments show that if the oxide films formed on zirconium alloys have local conductive regions or possess a high conductivity, then we should expect local corrosion damage in the former case and rapid uniform corrosion in the latter case. In addition, if the zirconium alloy contacts a metal coated with an oxide film with high conductivity and is in a conducting corrosive medium, we should also expect realization of the “short circuit” mode and an increase in the corrosion rate [4]. The process of oxidation of a zirconium alloy is inhibited by an external electric field when the negative pole of the source of the external electromotive force is connected to the alloy. In accordance with the electrochemical theory of oxidation the oxidation process can be fully inhibited if Eex = E0 . Figure 6b presents a kinetic curve of oxidation of alloy Zr – 1.0% Nb at 400°C, which shows that in the case of full inhibition (Eex = 2.5 V) the oxidation does not occur for 100 h; after the external electric field is removed, the oxidation of the alloy continues at a typical rate. Our experiments show that the electrochemical theory of high-temperature oxidation is applicable to zirconium alloys and that the total corrosion and the local corrosion of zirconium alloys may have the same nature. In addition, the developed model allows us to outline the ways for raising the corrosion resistance of zirconium alloys under service conditions and updating some stages of their production. For example, the vacuum heat treatment and the subsequent autoclaving in the production of zirconium pipes for process channels can be replaced by vacuum-free annealing in molten salts with induced inhibiting electric field.
393
Dm, mg/dm2 200
150
100
50
à 0
250
500
Dm, mg/dm2 75
Oxidation without inhibition Oxidation with inhibition
50
25
b 0
50
100
200 tîx , h
150
Fig. 6. Oxidation kinetics of alloy Zr – 1.0% Nb in molten KNO3 + NaNO2 salts: a) oxidation at 350°C under “short circuit” (&) and without it ()); b ) oxidation at 400°C without inhibition and with inhibition.
1
3
2
4
R
MECHANISM AND MATHEMATICAL MODEL OF NODULE CORROSION Ri
We based the concept of the mechanism of nodule corrosion within the electrochemical theory of high-temperature oxidation on the assumption that the oxide film formed during service has local imperfections with high electron conduction due to the presence of intermetallic and impurity inclusions in zirconium alloys, which pass into the oxide. Then, if a layer with high electron conduction is deposited during service on the surface of reactor cores structures produced from zirconium alloys, a “short circuit” mode will be realized in the water coolant near the regions of the oxide film with local imperfections, which can lead to accelerated growth of the oxide in these places, i.e., to formation of a nodule [5]. A possible scheme of formation of nodules and an equivalent circuit of the corrosion process are presented in Fig. 7. Designing the loop at equilibrium on the oxide – metal and oxide – oxidizer phase boundaries and under a “short circuit” due to local imperfections with resistance R and deposits of corrosion products of the NPP primary-coolant system with contact resistance Rc (r ), where r is the distance over the surface of the oxide film from the boundary of the inclusion, we
1000 t , h îx
750
sh
Iîx
Rc Re E0 b
à
Fig. 7. A sketch of formation of a nodule (a) and an equivalent circuit of the corrosion process (b ): 1 ) zirconium alloy; 2 ) oxide film; 3 ) intermetallic or chemical compound with resistance R; 4 ) deposition of corrosion products of the first loop with resistance Rc .
established that the current (the oxidation rate) under the “short circuit” is describable by the relation sh = iox i ox
R e + Rc + R , t e ( R e + Rc + R ) + t i ( Rc + R )
(8)
where te = Ri /(Re + Ri ), ti = Re /(Re + Ri ) are the transport sh numbers of electrons and ions and i ox and iox are the oxidation currents under the “short circuit” and normal modes, respectively.
394
V. I. Perekhozhev et al. x, mm
h, mm
7.5
600 500
5.0
400 300
100 h
2.5
200
30 h 4h
100
xun.ox.film 0
25
50
75
r, mm
100
0
Fig. 8. Thickness of oxide film (x ) in a nodule as a function of the distance from the inclusion boundary (r ) after 4, 30, and 100 h of oxidation.
Then, sh = iox /te ; (a) if R and Rc ^ Re and Ri , we have i ox (b) if R ^ Re and Ri and Rc p Re and Ri , we have sh = iox (Rc + Re )/(Rc + te Re ); i ox (c) if R and Rc p Re and Ri or Rc p Re , Ri , R, we have sh i ox @ iox . Thus, the formation and growth of nodules on zirconium alloys, even in the presence of local conducting imperfections in the oxide file, is possible only under specific conditions. The growth rate of the nodules is the highest under conditions formulated in item (a); at R and Rc close to zero the case of “short circuit” is realized over the entire surface of the zirconium alloy. Under the conditions formulated in item (c) the nodules on the surface do not grow, because the “short circuit” mode is not realized at all due to the high values of R and (or) Rc . Considering item (b) we will see that the thickness of the oxide film x and the size of the nodule can be found from the equation t
x ¢(r, t) = A ò i îx ( t ) 0
Rc ( r , t ) + R e ( t ) dt , Rc ( r , t ) + t e R e ( t )
(9)
where A is a dimensional factor. We integrated the obtained equation for the pre-turning and post-turning oxidation stages and simulated the size and depth of a nodule at te = 0.2, re = 8 ´ 10 – 3 W × mm, rc = 6 ´ 10 – 5 W × mm, kp = 0.2 mm × h – 1/2, xpr = 4 mm. The obtained dependences of the thickness of the oxide film in the nodule on the distance from the inclusion boundary are presented in Fig. 8. The developed concepts of the mechanism of nodule corrosion were used for creating methods for evaluating the susceptibility of zirconium alloys to nodule corrosion and suggesting several methods for reducing the corrosion damage of process channels and fuel rod arrays under service conditions connected with variation of Rc and R.
5
10
15
t, years
Fig. 9. Corrosion of alloy É125 in service of process channel pipes in RBMK-1000 (h is the corrosion depth): )) maximum nodule depth; O) thickness of oxide film on regions of uniform corrosion.
RESULTS OF A STUDY OF CORROSION STATE OF PROCESS CHANNELS AND FUEL ROD ARRAYS OF RBMK AFTER SERVICE. PROBLEMS AND WAYS FOR THEIR SOLUTION In about 20 years SF NIKIÉT specialists have studied about 60 process channels and 30 fuel rod arrays of RBMK at various NPP in the Soviet Union and in Russia. Figure 9 presents the results of the studies of the corrosion state of process channels of several NPP after service. The external surface of the channels, which lies in the graphite lining, experiences only uniform corrosion. After almost 20 years of service the depth of corrosion damage of the walls of the channels does not exceed 5 mm. The internal surface of process channels experiences both uniform and local (nodule) corrosion. The incubation period of the nodule corrosion is about 5.5 years. The depth of the damage caused by uniform corrosion does not exceed 50 mm after 19 years of service, and the depth of the damage caused by nodule corrosion amounts to 540 mm at a maximum yearly rate of nodule corrosion of 40 mm/year. Analyzing these results we can infer that the process channels with a wall thickness of 4.0 mm and permissible depth of corrosion damage of 0.4 mm can serve for 20 years at the detected rates of nodule corrosion. However, checking the certificates of process channels of NPP we established that the cores of RBMK have a considerable number of “variable thickness” channels meeting the tolerances stipulated by performance specifications. There are channels with an initial wall thickness of 4.2 – 3.6 mm, which means that the depth of corrosion damage in some process channels exceeds the permissible value even now. Studies of nodule corrosion after service have shown that a considerable part of oxide films in corroded places is removed by the coolant to the primary-coolant system. Computations show that at a corrosion rate of 0.04 mm/year the system might accept up to 750 g zirconium dioxide at every power-generating unit.
Uniform and “Nodule” Corrosion of Zirconium Alloys under Service Conditions
CONCLUSIONS 1. In correspondence with the laws of chemical thermodynamics, corrosion (oxidation) of zirconium alloys at enhanced temperatures is a natural process. The kinetics of the process in the initial stages is almost parabolic, but the large volume changes accompanying the formation of zirconium dioxide “turn” the kinetics, which is followed by oxidation at a constant or almost constant rate. Under certain conditions the presence of impurities and intermetallic compounds in the zirconium alloy can lead to accelerated local corrosion.
3
log (mean value of corrosion damage, mm)
Results of an after-service study of the corrosion resistance of fuel element claddings of rod arrays of RMBK reactors with spacer grids (SG) fabricated from stainless steel are presented in Fig. 10. We found out that in addition to uniform corrosion the claddings of fuel elements experience local damage caused by nodule corrosion and corrosion under spacer grids. The maximum thickness of the oxide film in some nodules at the studied burnup of 35 (MW × day)/kg attained 0.3 mm and the local reduction of the cross section of the cladding amounted to 27%, whereas the thinning due to uniform corrosion was about 1 – 2%. It should be noted that at low and intermediate burnup the nodule corrosion does not virtually affect the working capacity of the claddings, whereas at a high burnup, when the plasticity of the metal of the cladding is considerably decreased, the nodules can become places of crack nucleation, which may later lead to through damage of the cladding. Corrosion damage under spacer grids, which appears due to failure of the protective film during the vibration interaction between the cladding and the SG and due to the “short circuit” arising upon contact between the stainless steel and the metal of the cladding, is the most dangerous for the operation of the cladding. This is one more argument in favor of replacement of the material of spacer grids by zirconium alloys. The results of later studies of the corrosion resistance of the cladding metal under zirconium spacer grids show the absence of corrosion damage typical for SG fabricated from stainless steel. At the same time, a study of the condition of fuel rod arrays of RMBK with zirconium SG after service at NPP has shown that the state of the spacer grids themselves was unsatisfactory and they failed after long-term service. It was shown that the failure had been caused by the unsatisfactory corrosion resistance of the metal of the SG and by strong hydrogen charging. Just like in process channels, considerable nodule corrosion of claddings can lead to ejection of the corrosion products to the primary coolant system of the reactor. Calculations show that at a rate of nodule corrosion equal to 0.02 mm/year up to 1400 kg zirconia can arrive at the system every year at every power-generating unit.
395
Corrosion under spacer grid 2
Nodule corrosion 1
Uniform corrosion 0 – 0.5
0
0.5
1
1.5
log (burnup, MW × day/kg U) Fig. 10. Corrosion damage of fuel element claddings of RBMK with stainless spacer grid as a function of fuel burnup.
Reactor radiation commonly accelerates the corrosion, and the degree of the acceleration depends on the condition of the metal and the density of the neutron flux. 2. The electrochemical theory of high-temperature oxidation of metals can be used for describing the processes of corrosion of zirconium alloys. Nodule corrosion is a particular case arising under specific circumstances. Experiments show that corrosion of zirconium-niobium alloys can be stopped by imposing a “inhibiting” electric field. However, under the conditions of operation of zirconium alloys in reactor cores the use of this method is quite problematic. This method of corrosion protection can be recommended for replacing vacuum annealing or autoclaving in the production of process channels by vacuum-free annealing in salt melts under a superimposed “inhibiting” electric field. REFERENCES 1. V. I. Perekhozhev, V. N. Konev, M. G. Golovachev, et al., “Effect of reactor radiation on oxidation of zirconium-niobium alloys in gas media with complex composition,” VANT, Ser. FRP RM, Issue 5(33), 54 – 59 (1984). 2. A. V. Matveev, V. I. Perekhozhev, L. P. Sinel’nikov, et al., “A study of nodule corrosion of process channels of the first generating unit of the Kursk NPP,” VANT, Ser. FRP RM, Issues 1(58) and 2(59), 112 – 115 (1992). 3. V. I. Perekhozhev, Effect of Reactor Radiation on the Oxidation of Zirconium and Zirconium-Niobium Alloys in the Gas Medium of the Graphite Lining of RBMK Reactors, Author’s Abstract of Candidate’s Thesis [in Russian], Sverdlovsk (1983). 4. A. V. Matveev, V. I. Perekhozhev, A. G. Surnin, et al., “Accelerated oxidation of alloy Zr – 1% Nb in contact with stainless steel,” VANT, Ser. Materialoved. Nov. Mater., Issue 1(32), 18 – 22 (1990). 5. V. I. Perekhozhev, A. G. Surnin, and A. G. Mizyukanov, “Mechanism and mathematical model of nodule corrosion of zirconium alloys,” in: Proc. Int. Conf. of Radiation Mater., Alushta, May 22 – 28, 1990, Vol. 8 [in Russian], Kharkov (1991), pp. 144 – 150.