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Fast method for determination of critical pitting temperature
Corrosion Science, Vol. 28, No. 4, pp. 423-424, 1988 Printed in Great Britain
0010-938X/88 $3.00 + 0.00 Pergamon Press pie
SHORT COMMUNICATION FAST METHOD F O R DETERMINATION OF CRITICAL P1TTING TEMPERATURE If. V U Q U A N G , P. L E G U E V E L , N. J A L L E R A T C.N.R.S., 15, rue G. Urbain, 94400 Vitry-sur-Seine, France J.C. B A V A Y UGINE, 62330 Isbergues, France For passive alloys in halogenide-containing solutions (especially chloride) at a constant temperature, the anodic polarization curves generally exhibit a steep .current increase at a certain potential value called pitting potential Ep. From a series of polarization curves recorded at different constant temperatures, the corresponding pittingpotentials Ep decrease at a temperature T c, called criticalpitting temperature 1. The present determination of the critical pitting temperature T e consists in applying a weak galvanostatic anodic polarization (50 to 200 }~A/cm2) to a prepassivated alloy sample (of 1-2 cm 2 area) and recording the alloy potential while the electrolyte temperature is increased continuously by a programmed heating. The alloy potential is shifted near the transpassive potential (which could be the pitting potential Ep) by this anodic polarization but no pitting occurs immediatly because an incubation time 1: is necessary. Indeed, under the present conditions (galvanostatic polarization) the I: values measured on the tested steels are found to depend on temperature according to the relation : 1~ : A.exp(-Ea/RT)
(1)
where A is a constant, E a the apparent activation energy of pit initiationprocess, R the gas constant and T the Kelvin temperature. A similar dependence ofT was observed in the pitting of aluminum at constant potential2. In the present experiment, the steelpotential decreases sharply at the critical pitting temperature T c during the temperature increase (Fig. 1). The reproducibility of the T c values determined by the present method is very good (_+1.3°C) when the operating conditions are kept fixed, i.e.mainly the prepaseivation conditions (here : 5 min. in 3 N nitric acid) and the heating rate co = dT/dt. The dependence ofT e on the heating rate CO obeys the relation 3, for CO values inferiorto about 4°/rain. The T c values found for nitrogen-containing stainless steels in NaCI 3 % solution show significant effects of the alloying element contents3 (Cr, Mo, N...),effectswhich are quite consistent with that found by S U U T A L A 4. In the Cl" activityrange 0,067 - 4,3 (5 - 300 g/l NaCl), T c varies according to the relation. Tc,*C = T O - k.log(CI')= 44,4 - 4,9.1og(CI') for U 4 5 N steel,T o (initial)= 22"C, dT/dt = 3*/min (Fig. 2). Interpretation based on incubation time : during an experiment where the temperature increases continously with a rate CO = dT/dt (dt = dT/co),at each temperature T and for a time interval dt, an incubation fraction dl can be defined, in taking account of the relation 1 :dI = dt/~ = dT/co.~ = (A/co).exp(-Ea/RT)dT. The total incubation I is then obtained by integration between the temperatures T o and T :
I(T)= f 1---dT=
Aexp.~EadT co
JTo ~
.[~T c
JTo
Manuscript received 13 April 1987; in amended form 21 September 1987~ 423
(2)
424
K. Vu QUANGet al.
When t h e total incubation I(T) increases up to I, the pit initiation occurs and results in a fast potential decrease. T h e corresponding temperature is t h e critical pitting t e m p e r a t u r e T c. The solution of equation 2 leads to t h e relation : AR 2
-E a
mE--~-rc exp RT---'~= 1
(3)
Equation 3 relatesT e with the differentparameters, especiallythe heating rate {o and the apparent activation energy E a which could be then derived. Further data will be given in a forthcoming paper. REFERENCES
1 .a.R.J. B R I G H A M , Matdriaux et Techniques, Nov., 49(1973) 1.b.R.J. B R I G H A M and E.W. TOZER, Corrosion 30,161,(1974) 2. S. D A L L E K , R.T. FOLEY, J. Eleetrochem. Soc.,123,12,1775(1976) 3. P. L E G U E V E L , N. JALLERAT, J.C. BAVAY, K. VII Q U A N G , A.P. Corros. Control Conf., Melbourne, 22-28 Nov. 1987~ 4. N. STJUTALA and M. K U R K E L A , Stainless steels,C~5teborg,1984
E,
V/SCE
Tcj'C
0.5
T=C •5'0
(0
7'0L
Fig.l-Variation of potential vs temperature U 5 2 N steelunder 80 p A / c m 2 galvanostatic polarization in 3 % NaCI solution.
logCcl--> FIg.2- V a r i a t i o n of cri ti ca l pit. t e m p e r a t u r e T c v s Cl ° activity in s o l u t i o n for different steels.
0010-938X/88 $3.00 + 0.00 Pergamon Press pie
SHORT COMMUNICATION FAST METHOD F O R DETERMINATION OF CRITICAL P1TTING TEMPERATURE If. V U Q U A N G , P. L E G U E V E L , N. J A L L E R A T C.N.R.S., 15, rue G. Urbain, 94400 Vitry-sur-Seine, France J.C. B A V A Y UGINE, 62330 Isbergues, France For passive alloys in halogenide-containing solutions (especially chloride) at a constant temperature, the anodic polarization curves generally exhibit a steep .current increase at a certain potential value called pitting potential Ep. From a series of polarization curves recorded at different constant temperatures, the corresponding pittingpotentials Ep decrease at a temperature T c, called criticalpitting temperature 1. The present determination of the critical pitting temperature T e consists in applying a weak galvanostatic anodic polarization (50 to 200 }~A/cm2) to a prepassivated alloy sample (of 1-2 cm 2 area) and recording the alloy potential while the electrolyte temperature is increased continuously by a programmed heating. The alloy potential is shifted near the transpassive potential (which could be the pitting potential Ep) by this anodic polarization but no pitting occurs immediatly because an incubation time 1: is necessary. Indeed, under the present conditions (galvanostatic polarization) the I: values measured on the tested steels are found to depend on temperature according to the relation : 1~ : A.exp(-Ea/RT)
(1)
where A is a constant, E a the apparent activation energy of pit initiationprocess, R the gas constant and T the Kelvin temperature. A similar dependence ofT was observed in the pitting of aluminum at constant potential2. In the present experiment, the steelpotential decreases sharply at the critical pitting temperature T c during the temperature increase (Fig. 1). The reproducibility of the T c values determined by the present method is very good (_+1.3°C) when the operating conditions are kept fixed, i.e.mainly the prepaseivation conditions (here : 5 min. in 3 N nitric acid) and the heating rate co = dT/dt. The dependence ofT e on the heating rate CO obeys the relation 3, for CO values inferiorto about 4°/rain. The T c values found for nitrogen-containing stainless steels in NaCI 3 % solution show significant effects of the alloying element contents3 (Cr, Mo, N...),effectswhich are quite consistent with that found by S U U T A L A 4. In the Cl" activityrange 0,067 - 4,3 (5 - 300 g/l NaCl), T c varies according to the relation. Tc,*C = T O - k.log(CI')= 44,4 - 4,9.1og(CI') for U 4 5 N steel,T o (initial)= 22"C, dT/dt = 3*/min (Fig. 2). Interpretation based on incubation time : during an experiment where the temperature increases continously with a rate CO = dT/dt (dt = dT/co),at each temperature T and for a time interval dt, an incubation fraction dl can be defined, in taking account of the relation 1 :dI = dt/~ = dT/co.~ = (A/co).exp(-Ea/RT)dT. The total incubation I is then obtained by integration between the temperatures T o and T :
I(T)= f 1---dT=
Aexp.~EadT co
JTo ~
.[~T c
JTo
Manuscript received 13 April 1987; in amended form 21 September 1987~ 423
(2)
424
K. Vu QUANGet al.
When t h e total incubation I(T) increases up to I, the pit initiation occurs and results in a fast potential decrease. T h e corresponding temperature is t h e critical pitting t e m p e r a t u r e T c. The solution of equation 2 leads to t h e relation : AR 2
-E a
mE--~-rc exp RT---'~= 1
(3)
Equation 3 relatesT e with the differentparameters, especiallythe heating rate {o and the apparent activation energy E a which could be then derived. Further data will be given in a forthcoming paper. REFERENCES
1 .a.R.J. B R I G H A M , Matdriaux et Techniques, Nov., 49(1973) 1.b.R.J. B R I G H A M and E.W. TOZER, Corrosion 30,161,(1974) 2. S. D A L L E K , R.T. FOLEY, J. Eleetrochem. Soc.,123,12,1775(1976) 3. P. L E G U E V E L , N. JALLERAT, J.C. BAVAY, K. VII Q U A N G , A.P. Corros. Control Conf., Melbourne, 22-28 Nov. 1987~ 4. N. STJUTALA and M. K U R K E L A , Stainless steels,C~5teborg,1984
E,
V/SCE
Tcj'C
0.5
T=C •5'0
(0
7'0L
Fig.l-Variation of potential vs temperature U 5 2 N steelunder 80 p A / c m 2 galvanostatic polarization in 3 % NaCI solution.
logCcl--> FIg.2- V a r i a t i o n of cri ti ca l pit. t e m p e r a t u r e T c v s Cl ° activity in s o l u t i o n for different steels.