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The localized corrosion of Cr-Ni stainless steel
Corrosion Science, 1978, Vol. 18, pp. 53 to 59. Pergamon Press. Printed in Great Britain
THE LOCALIZED CORROSION OF Cr-Ni STAINLESS STEEL* JAROMIR TOU]EK Institute of Physical Metallurgy, Czechoslovak Academy of Sciences, 616 62 Brno, Czechoslovakia Abstract---The kinetics of localized corrosion of Cr-Ni stainless steel is characterized by the quadratic time dependence of the total current density, by the linear increase in the number of pits and by the decrease of the current density in the pits. The pits have a shape of a rotary ellipsoid. The Tafel slope of the metal dissolution in the pits is 0.50 V. The dissolution rate is the highest on the pit bottom and the lowest at the mouth of the pit. The different rate of metal dissolution is caused by the different concentration of chloride ions over the pit surface. INTRODUCTION THE CHARACTERISTICfeature o f C r - N i stainless steels is the high resistance to u n i f o r m a n d localized c o r r o s i o n a n d to c o r r o s i o n cracking. 1-4 As far as the localized c o r r o s i o n is concerned, great a t t e n t i o n has been p a i d recently to the influence o f different alloys, inclusions a n d t e m p e r a t u r e on this type o f c o r r o s i o n attack. ~-1~ T h e kinetics a n d the m e c h a n i s m o f reactions which a c c o m p a n y the f o r m a t i o n o f pits a n d the m e t a l diss o l u t i o n in the pits have been m u c h examined. ~-~s T h e present p a p e r deals with the kinetics o f localized c o r r o s i o n in a C r - N i stainless steel in 0.5M N a C l . EXPERIMENTAL METHOD A potentiostatic polarization method was used for measurements. The electrode was passivated for 5 min at the potential --180 mV (SCE), then the potential was raised above the depassivating value, and the time dependence of current density and of the number of pits was followed. The course of localized corrosion was measured in a 0.5 M NaC1 solution of pH 8.4 (this solution approximately corresponds to that one of the sea water) at 21°C. The potentials are quoted against the saturated calomel electrode. The steel had the following composition: 0.13 ~ C, 0.69~ Mn, 1.30~, Si, 0.026~, P, 0.21 ~, S, 18.58~ Cr, 9.85~ Ni. EXPERIMENTAL RESULTS A detailed analysis o f the localized c o r r o s i o n rate d e m a n d s an e x p e r i m e n t a l d e t e r m i n a t i o n o f the time, potential, a n d c o n c e n t r a t i o n d e p e n d e n c e o f the t o t a l c u r r e n t density a n d o f the n u m b e r o f pits. I n the p r e s e n t p a p e r the time a n d p o t e n t i a l dependences o f the localized c o r r o s i o n rate have been followed.
Time dependence o f localized corrosion T h e time d e p e n d e n c e o f the t o t a l current density is d e t e r m i n e d b y the general equation j = Ax (t -- t,) m q- gp, (la) *Manuscript received 25 January 1977. 53
54
JAROM|R TOU~EK
2(
200
I.'
150
u
u
E
~ I(
IOO
5
0
50
0
l
2
3 Time,
4
5
0
min
FIG. 1. Time dependence of the total current density, j, the number of pits, z, and the current density in the pits]L, E = 100 mV.
where A1, m are constants, jp is the corrosion current in the passive state, t,. is the induction period. F o r t; ~ 0, A1 > 0 a n d j p < Ax (t -- tt) m the equation (la) takes •the form j = A1 tm.
(lb)
F o r the constant m the value 1.8-2.4 has been found. In equations the value m = 2 is used. The n u m b e r of pits increases linearly with the time: z = Kt (Fig. 1). Where z is the n u m b e r of pits, K is a constant and t the time. Most pits are completely or at least partially covered with a film, probably formed by the original passive oxide. Equations (1) and z = Kt allow the derivation of the equation for pit growth if the pit shape is known. Experimentally it was found that the horizontal radius of the pit was equal to half the depth. It is possible to consider the shape of the pit as a half o f a rotational ellipsoid that arose from rotating an ellipse a r o u n d its longer axis. F o r the dimensions of the pit it holds that a = b = r and c = 2r, where r is the radius and c the depth o f the pit.
The localized corrosion of Cr-Ni stainless steel
55
Substituting the dimensions of the pit into the equation of the rotational ellipsoid volume, for the pit volume, V, is obtained: V = (4/3)rtr 3. The growth of the pit is expressed by the equation G--I t
(4/3)nr 3 = (VM/2F)fjdt,
(2)
o
in which VM is the molar volume of steel ( = 7.1 cm a) F is the Faraday constant, t is the current from one pit, (t -- t~) is the age of the pit, t~ is the time at which the pit was initiated. The current, j, at the time, t, for a pit that arose at the time t~ can be found from .} = cj (t -- ti) a,
(3)
where c], ct are constants. The number of pits, z, at the time, t~, is z = Kt~. For the time interval from t~ to t i q- dt; the increment of the number of pits dz = K dti,
(4)
and the increment of the total current density is dj, where dj = j dz = cl (t -- tl)" K dt i.
(5)
Integrating the equation (5): t
j = C l K f ( t -- ti) a dt = [clK/(o~ q- 1)] t aql.
(6)
0
F r o m equations (lb) and (6) it follows that ~ = !, so t h a t ) = c~(t -- ti) and c] = 2A]/K.
By substitution in (2) the pit radius, r, at ti --~ 0 can be calculated: r = C] 3v~.jt/z ---- Ct av'(A,/K)t2,
(7)
where Cx = 3V'(3VM/8~F) = 2.04. 10 -3 cm A-l'Ss -113. The validity of equation (7) was verified by comparing the experimentally found and calculated values of pit radii (Table 1) and further by comparing the radius of the pit at t = 1 s (determined by the extrapolation of the measured radii) with the calculated value (Fig. 2). The current densityjz for the active pit surface can be calculated from the equation Jz. = )/P, where P is the pit surface, for which it holds P = P'/2, where P ' is the surface of the rotational ellipsoid: P' = 2rca[a + (c/e) arcsin ~],
(8)
JAROM-f[R TOU~EK
56 TABLE 1.
CALCULATED AND MEASURED ]PIT RADII
E
j
z
t
r~o.
r~.,t.
(mV)
(mAcm-*)
(cm-*)
(s)
(cm. 10- s)
(cm. 108)
100 97
44.0 8.8 150.0
103 126
20.0 36.0
300 240 60 300 150
8.5 6.0
174
200 80 800 140 250
8,5 5.0 4.0 7.5 4.0
4.3
7.0 5.5
where e = e/c, e = ,v/(c ~ -- aS). O n inserting the dimensions o f the pit into (8) P
=
3.42~r a.
(9)
It can then be determined that JL = 1.12 sV' (Axlm/n V~MKt),
(10)
= B a'V' ( A d K ) a ~ / O f f ) ,
(11)
= B , V U/zt,),
(12)
where B = 1.12 aV' (F*/~ V2M) = 4.46. 102 A~las 2ia cm -2. F r o m equation (11) it can be seen that the current density in active pits decreases during the localized corrosion (see also Fig. 1). A t E -- I00 mV the current density Jz. at t = 1 s equals 5.8 A c m -a, but after 300 s only 0.5 A c m -~. It is necessary to note that equations (10-12) yield only mean values ofjL. With regard to the f o r m o f the pit, the current densityjL at its m o u t h equals only the half o f the current density on its bottom. I
I
-2 -- '41 =IO-6A cm-es-2 K = 0 . 4 5 cm-Zs -I
t.
jo
,,~
s S ~"
_~o -3 -
s,o ~ S ¢"~
~s-~-s--~---IOg r = Io03 ~
= - 3 . 5 8 ( t =Is)
V 8"rrFK
- 4 --
0
r = 2.63 x 10-4cm I i
I 2 log f
FIG. 2. The kinetics of the pit growth, r = radius of the pit in cm, t = time in s, 100 mV.
The localizedcorrosion of Cr-Ni stainless steel I
I
57
t
2--
--4
3
°~
o°'
-
~3
(Se/81ogj)=b:O.045~
_
.~
~
_, ~ |
,. 2 o~
_
I
t
I
I
50
100
150
200
ESCE, mV FIG.
3.
Potential dependence of the current density j (mA cm-2), the current density in the pitsjL ( m A c m -2) and of the number of pits z (cm-I).
Potential dependence o f the localized corrosion
The potential dependence of the current density is expressed, within the experimental errors, by the exponential function J = Jz) exp (2.303 AE/b),
(13)
in which JD is the current density at the breakdown potential Eb, b is the constant for which the value 0.045 V was found and A E = E -- E b (Fig. 3). The number of active pits shows a similar potential dependence z = ZDexp (2.303 aAE),
(14)
where a is a constant ( = 16.2 V-1) and zz) is the number of pits at the breakdown potential. A s a =A 1/b, the current density in active pits must be also potential dependent. With regard to the exponential potential dependence of the total current density and of the number of pits, it follows that JL = Jz.o exp (2.303 AE/bo).
The current densityjL, o holds for E = Eb. The constant b0 was determined graphically from Fig. 3 and by calculation, using the equation b o = 3b/(1 -- ab),
(16)
58
JAROM[R TOU~EK
which follows from equations (13), (14) and (15) and from the equation expressing the total current density as the function of the number of pits and their surfaces P j = k P J L Z.
(17)
Both methods (graphical as well as numerical) yield for the constant b0 the value 0.50 V. DISCUSSION
The potential dependence of the current density .]L indicates that the dissolution of steel in pits is not controlled by diffusion, but by electrochemical reaction. The constant bo is substantially higher than it would correspond to the active metal dissolution. It is probable that the potential in the pits is more positive than the corresponding potential region of active metal dissolution and achieves values at which the metal oxide can be formed. This oxide is formed under extreme concentration conditions, it is contaminated by the components from the solution and has a good ionic conductivity,la,~° The transport of ions through this oxide may be considered as the rate determining step in the mechanism of metal dissolution in the pits. This process is characterized by a relatively high Tafel slope b0. The existence of the pits is conditioned by a high concentration of electrolyte in them. The high electrolyte concentration is supported by the high current density, JL, which makes the ions migrate from the bulk of the solution into the pit surface. Further it is raised by the insufficient rate of salt diffusion from pits, caused partly by the geometry of the pit, e.g. by the high ratio of the pit surface to its cross section (which amounts to 3.42) and partly by the fact that the pit is partly covered with the original passive film. The concentration conditions in pits affect the time dependence of the current density, Jz, and shape of the pit. The decrease of the current density, Jz, with time and the insufficient dissolution rate of the original passive layer are connected with the metal dissolution mechanism in pits. The metal ions are transported through the oxide film21 to the phase boundary oxide/electrolyte and react with the electrolyte components in at least two ways. First of all the reaction of the metal with chlorides takes place Me + pCl' = MeCip~-p + 2e.
(18)
A competitive reaction of metal with water molecules proceeds simultaneously nMe q- mH~O = MenO,~ q- 2mH + q- 2 m e.
(19)
Besides these two electrochemical reactions the chemical reaction of oxide dissolution is going on MenOm -q- 2mH + q- npCl' = n MeClpZ(,"/")-p + mH20,
(20)
about which can be assumed to proceed slowly so that its contribution to the metal dissolution rate is small.
The localized corrosion of Cr-Ni stainless steel
59
As long as the rates of reactions (19) and (20) are the same, the thickness of the oxide film does not change and the metal dissolution in pits proceeds at a constant rate. If the reaction (19) is faster than the reaction (20) the oxide film grows more quickly than it dissolves, the thickness of the contaminated oxide film increases and the current density, Jf, decreases. In the given case the growth of the film is probably caused by the insufficient concentration o f chloride ions and by the corresponding higher water concentration at the surface of pits. In addition it is necessary to consider the high driving force of reaction (19), characteristic for alloys containing chromium and following from the low value of the chromium electrode (E ° CrrCr2Os = --0.6 V ) Y The shape of the pit is influenced first of all by the non uniform distribution of the chlorides along its surface. The diffusion process proceeds more rapidly at the m o u t h of the pit (the concentration gradient is higher here) and therefore the chloride concentration increases in the direction from the m o u t h of the pit to its bottom. The rate of reaction (18) increases in the same sense. This results in the formation o f deep pits. The potential inside the pits has no constant value. It is highest at the m o u t h o f the pit and lowest on its bottom. Since metal dissolution in pits is potential dependent, this fact could influence the shape of pits. Even if potential differences in the pit during its growth increase, pit shape remains approximately the same. This m a y be caused either by the high value of the constant b0 or by the potential difference in pits being small.
I. 2. 3. 4. 5. 6. 7. 8. 9. I0. 1I. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22.
REFERENCES G. J. BIEFER,Can. Metall. Q. 9, 537 (1973). B. E. WILDE,J. electrochem. Soc. 118, 1717 0971). R. T. NEWBERGand H. H. UHLm, J. eleetroehem. Soe. 119, 981 (1972). M. PRA~K and V. ~'iHAL, Corros. Sci. 2, 71 (1962). E. A. LIZLOVSand P. A. BOND,J. electrochem. Soc. 116, 574 (1969). A. P. BONDand E. A. LIz~ovs, J. electrochem. Soc. !15, 1130 (1968). G. S. EKLUND,J. eleetrochem. Soe. 121,467 0974). R. J. BRIGHAM,Corrosion 28, 177 (1972). R. J. BRIGHAMand E. W. TOZER, Can. Metal[. Q. 12, 171 (1973). R. J. BRIGHAMand E. W. TOZER, Corrosion 29, 33 (1973). R. J. BRIGHAMand E. W. TOZER, Corrosion 30, 161 (1974). T. SUZUKh M. YAMAHEand Y. KITAMURA,Corrosion 29, 18 (1973). Z. SKLARSKA-SMIALOWSKAand J. MANKOWSKhCorros. Sci. 12, 925 (1972). T. SUZUKXand Y. KITAMURA,Corrosion 28, 1 (1972). C. R. RAREYand A. H. ARONSON,Corrosion 28, 255 (1972). H. W. PICtuRING and R. P. FRANKENTHAL,J. eleetroehem. Soe. 119, 1297 (1972). A. A. SEYS,A. A. VAN HAtrrE and M. J. BREBERS, Werkstoffe und Korros. 25, 663 (1974). M. J. ENGELLand N. D. STOHCA,Zeit. phys. Chem. 20, 113 (1959). T. P. HOAR, Corros. Sci. 7, 341 (1967). T. P. HOAR,D. C. MEARSand G. P. ROTHWELL, Corros. Sei. 5, 279 ~10~¢~ J. TOU~EK, Werkstoffe und Korros. 25, 496 (1974). K. J. VE~'rER,£1ektrochemische Kinetik p. 607. Springer, Berlin, (1961).
THE LOCALIZED CORROSION OF Cr-Ni STAINLESS STEEL* JAROMIR TOU]EK Institute of Physical Metallurgy, Czechoslovak Academy of Sciences, 616 62 Brno, Czechoslovakia Abstract---The kinetics of localized corrosion of Cr-Ni stainless steel is characterized by the quadratic time dependence of the total current density, by the linear increase in the number of pits and by the decrease of the current density in the pits. The pits have a shape of a rotary ellipsoid. The Tafel slope of the metal dissolution in the pits is 0.50 V. The dissolution rate is the highest on the pit bottom and the lowest at the mouth of the pit. The different rate of metal dissolution is caused by the different concentration of chloride ions over the pit surface. INTRODUCTION THE CHARACTERISTICfeature o f C r - N i stainless steels is the high resistance to u n i f o r m a n d localized c o r r o s i o n a n d to c o r r o s i o n cracking. 1-4 As far as the localized c o r r o s i o n is concerned, great a t t e n t i o n has been p a i d recently to the influence o f different alloys, inclusions a n d t e m p e r a t u r e on this type o f c o r r o s i o n attack. ~-1~ T h e kinetics a n d the m e c h a n i s m o f reactions which a c c o m p a n y the f o r m a t i o n o f pits a n d the m e t a l diss o l u t i o n in the pits have been m u c h examined. ~-~s T h e present p a p e r deals with the kinetics o f localized c o r r o s i o n in a C r - N i stainless steel in 0.5M N a C l . EXPERIMENTAL METHOD A potentiostatic polarization method was used for measurements. The electrode was passivated for 5 min at the potential --180 mV (SCE), then the potential was raised above the depassivating value, and the time dependence of current density and of the number of pits was followed. The course of localized corrosion was measured in a 0.5 M NaC1 solution of pH 8.4 (this solution approximately corresponds to that one of the sea water) at 21°C. The potentials are quoted against the saturated calomel electrode. The steel had the following composition: 0.13 ~ C, 0.69~ Mn, 1.30~, Si, 0.026~, P, 0.21 ~, S, 18.58~ Cr, 9.85~ Ni. EXPERIMENTAL RESULTS A detailed analysis o f the localized c o r r o s i o n rate d e m a n d s an e x p e r i m e n t a l d e t e r m i n a t i o n o f the time, potential, a n d c o n c e n t r a t i o n d e p e n d e n c e o f the t o t a l c u r r e n t density a n d o f the n u m b e r o f pits. I n the p r e s e n t p a p e r the time a n d p o t e n t i a l dependences o f the localized c o r r o s i o n rate have been followed.
Time dependence o f localized corrosion T h e time d e p e n d e n c e o f the t o t a l current density is d e t e r m i n e d b y the general equation j = Ax (t -- t,) m q- gp, (la) *Manuscript received 25 January 1977. 53
54
JAROM|R TOU~EK
2(
200
I.'
150
u
u
E
~ I(
IOO
5
0
50
0
l
2
3 Time,
4
5
0
min
FIG. 1. Time dependence of the total current density, j, the number of pits, z, and the current density in the pits]L, E = 100 mV.
where A1, m are constants, jp is the corrosion current in the passive state, t,. is the induction period. F o r t; ~ 0, A1 > 0 a n d j p < Ax (t -- tt) m the equation (la) takes •the form j = A1 tm.
(lb)
F o r the constant m the value 1.8-2.4 has been found. In equations the value m = 2 is used. The n u m b e r of pits increases linearly with the time: z = Kt (Fig. 1). Where z is the n u m b e r of pits, K is a constant and t the time. Most pits are completely or at least partially covered with a film, probably formed by the original passive oxide. Equations (1) and z = Kt allow the derivation of the equation for pit growth if the pit shape is known. Experimentally it was found that the horizontal radius of the pit was equal to half the depth. It is possible to consider the shape of the pit as a half o f a rotational ellipsoid that arose from rotating an ellipse a r o u n d its longer axis. F o r the dimensions of the pit it holds that a = b = r and c = 2r, where r is the radius and c the depth o f the pit.
The localized corrosion of Cr-Ni stainless steel
55
Substituting the dimensions of the pit into the equation of the rotational ellipsoid volume, for the pit volume, V, is obtained: V = (4/3)rtr 3. The growth of the pit is expressed by the equation G--I t
(4/3)nr 3 = (VM/2F)fjdt,
(2)
o
in which VM is the molar volume of steel ( = 7.1 cm a) F is the Faraday constant, t is the current from one pit, (t -- t~) is the age of the pit, t~ is the time at which the pit was initiated. The current, j, at the time, t, for a pit that arose at the time t~ can be found from .} = cj (t -- ti) a,
(3)
where c], ct are constants. The number of pits, z, at the time, t~, is z = Kt~. For the time interval from t~ to t i q- dt; the increment of the number of pits dz = K dti,
(4)
and the increment of the total current density is dj, where dj = j dz = cl (t -- tl)" K dt i.
(5)
Integrating the equation (5): t
j = C l K f ( t -- ti) a dt = [clK/(o~ q- 1)] t aql.
(6)
0
F r o m equations (lb) and (6) it follows that ~ = !, so t h a t ) = c~(t -- ti) and c] = 2A]/K.
By substitution in (2) the pit radius, r, at ti --~ 0 can be calculated: r = C] 3v~.jt/z ---- Ct av'(A,/K)t2,
(7)
where Cx = 3V'(3VM/8~F) = 2.04. 10 -3 cm A-l'Ss -113. The validity of equation (7) was verified by comparing the experimentally found and calculated values of pit radii (Table 1) and further by comparing the radius of the pit at t = 1 s (determined by the extrapolation of the measured radii) with the calculated value (Fig. 2). The current densityjz for the active pit surface can be calculated from the equation Jz. = )/P, where P is the pit surface, for which it holds P = P'/2, where P ' is the surface of the rotational ellipsoid: P' = 2rca[a + (c/e) arcsin ~],
(8)
JAROM-f[R TOU~EK
56 TABLE 1.
CALCULATED AND MEASURED ]PIT RADII
E
j
z
t
r~o.
r~.,t.
(mV)
(mAcm-*)
(cm-*)
(s)
(cm. 10- s)
(cm. 108)
100 97
44.0 8.8 150.0
103 126
20.0 36.0
300 240 60 300 150
8.5 6.0
174
200 80 800 140 250
8,5 5.0 4.0 7.5 4.0
4.3
7.0 5.5
where e = e/c, e = ,v/(c ~ -- aS). O n inserting the dimensions o f the pit into (8) P
=
3.42~r a.
(9)
It can then be determined that JL = 1.12 sV' (Axlm/n V~MKt),
(10)
= B a'V' ( A d K ) a ~ / O f f ) ,
(11)
= B , V U/zt,),
(12)
where B = 1.12 aV' (F*/~ V2M) = 4.46. 102 A~las 2ia cm -2. F r o m equation (11) it can be seen that the current density in active pits decreases during the localized corrosion (see also Fig. 1). A t E -- I00 mV the current density Jz. at t = 1 s equals 5.8 A c m -a, but after 300 s only 0.5 A c m -~. It is necessary to note that equations (10-12) yield only mean values ofjL. With regard to the f o r m o f the pit, the current densityjL at its m o u t h equals only the half o f the current density on its bottom. I
I
-2 -- '41 =IO-6A cm-es-2 K = 0 . 4 5 cm-Zs -I
t.
jo
,,~
s S ~"
_~o -3 -
s,o ~ S ¢"~
~s-~-s--~---IOg r = Io03 ~
= - 3 . 5 8 ( t =Is)
V 8"rrFK
- 4 --
0
r = 2.63 x 10-4cm I i
I 2 log f
FIG. 2. The kinetics of the pit growth, r = radius of the pit in cm, t = time in s, 100 mV.
The localizedcorrosion of Cr-Ni stainless steel I
I
57
t
2--
--4
3
°~
o°'
-
~3
(Se/81ogj)=b:O.045~
_
.~
~
_, ~ |
,. 2 o~
_
I
t
I
I
50
100
150
200
ESCE, mV FIG.
3.
Potential dependence of the current density j (mA cm-2), the current density in the pitsjL ( m A c m -2) and of the number of pits z (cm-I).
Potential dependence o f the localized corrosion
The potential dependence of the current density is expressed, within the experimental errors, by the exponential function J = Jz) exp (2.303 AE/b),
(13)
in which JD is the current density at the breakdown potential Eb, b is the constant for which the value 0.045 V was found and A E = E -- E b (Fig. 3). The number of active pits shows a similar potential dependence z = ZDexp (2.303 aAE),
(14)
where a is a constant ( = 16.2 V-1) and zz) is the number of pits at the breakdown potential. A s a =A 1/b, the current density in active pits must be also potential dependent. With regard to the exponential potential dependence of the total current density and of the number of pits, it follows that JL = Jz.o exp (2.303 AE/bo).
The current densityjL, o holds for E = Eb. The constant b0 was determined graphically from Fig. 3 and by calculation, using the equation b o = 3b/(1 -- ab),
(16)
58
JAROM[R TOU~EK
which follows from equations (13), (14) and (15) and from the equation expressing the total current density as the function of the number of pits and their surfaces P j = k P J L Z.
(17)
Both methods (graphical as well as numerical) yield for the constant b0 the value 0.50 V. DISCUSSION
The potential dependence of the current density .]L indicates that the dissolution of steel in pits is not controlled by diffusion, but by electrochemical reaction. The constant bo is substantially higher than it would correspond to the active metal dissolution. It is probable that the potential in the pits is more positive than the corresponding potential region of active metal dissolution and achieves values at which the metal oxide can be formed. This oxide is formed under extreme concentration conditions, it is contaminated by the components from the solution and has a good ionic conductivity,la,~° The transport of ions through this oxide may be considered as the rate determining step in the mechanism of metal dissolution in the pits. This process is characterized by a relatively high Tafel slope b0. The existence of the pits is conditioned by a high concentration of electrolyte in them. The high electrolyte concentration is supported by the high current density, JL, which makes the ions migrate from the bulk of the solution into the pit surface. Further it is raised by the insufficient rate of salt diffusion from pits, caused partly by the geometry of the pit, e.g. by the high ratio of the pit surface to its cross section (which amounts to 3.42) and partly by the fact that the pit is partly covered with the original passive film. The concentration conditions in pits affect the time dependence of the current density, Jz, and shape of the pit. The decrease of the current density, Jz, with time and the insufficient dissolution rate of the original passive layer are connected with the metal dissolution mechanism in pits. The metal ions are transported through the oxide film21 to the phase boundary oxide/electrolyte and react with the electrolyte components in at least two ways. First of all the reaction of the metal with chlorides takes place Me + pCl' = MeCip~-p + 2e.
(18)
A competitive reaction of metal with water molecules proceeds simultaneously nMe q- mH~O = MenO,~ q- 2mH + q- 2 m e.
(19)
Besides these two electrochemical reactions the chemical reaction of oxide dissolution is going on MenOm -q- 2mH + q- npCl' = n MeClpZ(,"/")-p + mH20,
(20)
about which can be assumed to proceed slowly so that its contribution to the metal dissolution rate is small.
The localized corrosion of Cr-Ni stainless steel
59
As long as the rates of reactions (19) and (20) are the same, the thickness of the oxide film does not change and the metal dissolution in pits proceeds at a constant rate. If the reaction (19) is faster than the reaction (20) the oxide film grows more quickly than it dissolves, the thickness of the contaminated oxide film increases and the current density, Jf, decreases. In the given case the growth of the film is probably caused by the insufficient concentration o f chloride ions and by the corresponding higher water concentration at the surface of pits. In addition it is necessary to consider the high driving force of reaction (19), characteristic for alloys containing chromium and following from the low value of the chromium electrode (E ° CrrCr2Os = --0.6 V ) Y The shape of the pit is influenced first of all by the non uniform distribution of the chlorides along its surface. The diffusion process proceeds more rapidly at the m o u t h of the pit (the concentration gradient is higher here) and therefore the chloride concentration increases in the direction from the m o u t h of the pit to its bottom. The rate of reaction (18) increases in the same sense. This results in the formation o f deep pits. The potential inside the pits has no constant value. It is highest at the m o u t h o f the pit and lowest on its bottom. Since metal dissolution in pits is potential dependent, this fact could influence the shape of pits. Even if potential differences in the pit during its growth increase, pit shape remains approximately the same. This m a y be caused either by the high value of the constant b0 or by the potential difference in pits being small.
I. 2. 3. 4. 5. 6. 7. 8. 9. I0. 1I. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22.
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