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Breakdown of passivity in low-alloy steels
Corrosion Science, 1971, Vol. I1, pp. 403 to 410. Pergamon Press. Printed in Great Britain
BREAKDOWN
OF PASSIVITY IN LOW-ALLOY
STEELS*
L. G1ULIANbG. AGABIOand G. BOSmARA Centro Sperimentale Metallurgico, Castel Romano, Rome, Italy Abstract--Low-alloy steels can be protected in SO~- solutions through precipitation of corrosion products. Periodic breakdowns of such a "coverage passivity" result in current blips, whose shape and frequency are highly specific of the type of steel. The qualitative classification which can be drawn from this phenomenon appears to correlate quite well with the corrosion resistance of steel as determined by atmospheric exposure tests. R6sum&--Les aciers faiblement allies peuvent ~tre prot6g6s dam des solutions de SO,'- par une pr&ipitation de produits de corrosion. Les ruptures p~riodiques d'une telle couche de passivation dorment des pointes de courant, dont la forme et la fr~quence sont hautement sp~cifiques du type d'acier. La classification qualitative qui peut ~tre ~tablie d'apr~s ce ph6nom~ne est en tr~s bon accord avecla r~sistance ~ la corrosion de l'acier, d6termin~e par des essais d'exposition atmosph~rique. Zusammenfassung--Niedriglegierte St~hle k6nnen in SOl--L6sungen durch den Niederschlag von Korrosionsprodukten geschiitzt werden. Ein periodischer Zusammenbruch dieser Bedeckungspassivit~it bewirkt einzelne Stromspitzen, deren Form und Frequenz ftir den betreffenden Stahl spezifisch sind. Qualitativ stimmt diese Erscheinung iiberein mit der Neigung der Stfihle zur Bildung deckender Schichten bei der atmosph~risehen Korrosion. INTRODUCTION
ON THEbasis of apparent features and properties, various theories have been developed on the possible mechanisms of metal passivation. A clear-cut distinction, however, must be made between chemical passivity and coverage passivity. The former type of passivity implies the formation of a very thin surface layer (one to several molecular layers thick) of oxide, or other solid corrosion products, or even adsorbed 02. Such a kinetic barrier, almost irrespective of the way it is obtained--whether anodically or by the action of gaseous or dissolved O., or of any ionic oxidant---consists 9of a phase which is stable within a definite range of potentials and through which only a leakage current passes, corresponding to the very slow dissolution rate of the passive film. In many other systems, the decrease in the anodic current at certain potentials involves the formation of a substantial layer of corrosion products, having a thickness sufficient to restrain the overall ionic exchange between the metal and the electrolyte. This type of passivity may be defined as coverage passivity or, somewhat less properly, mechanical passivity? The formation of the protective layer implies a definite passage of charge and its stability depends also on physical factors, such as internal stresses, coherence, adherence, porosity and resistance to mechanical actions by the electrolyte. A simple potentiostatic investigation on such form of passivity is presented here for low-alloy steels in SO~- solutions, which shows some interesting features in the anodic behaviour with time. *Manuscript received 16 April 1970. 403
404
L. GIULIANI,G. AGAB10ann G. BOMBARA
EXPERIMENTAL Three types of low-alloy steels and a plain carbon steel have been tested. The materials are simply designated as A, B, C and D, the first one being a plain carbon steel, B and C two C u P alloys and D a Cor-Ten type (Table 1).
TABLE1. CHEMICALCOMPOSITIONor TESTEDALLOYS Steel
C
Si
bin
S
P
Cu
Cr
Ni
Others
A B C D
0-188 0-058 0-120 0-093
0-040 tr. 0-068 0.600
0-83 0-35 0-78 0-34
0.018 0-014 0.018 0-027
0.022 0-055 0.095 0-IC0
0.060 0-20-I 0-278 0-375
0.026 0-018 0.028 0"540
0.037 0"030 0-034 0-26
-AI 0.064 Nb 0-017
The cell consisted in a perspex tube with two threadedcovers. The specimen was placed at the bottom of the cell and masked with a perspex disk and a pvc gasket, so as to expose an area of 1-77 cm 2 to the solution. Steel specimens with a simply ground surface were used; they were degreased in an alcohol-ether mixture and rinsed with acetone, and tested soon after the cleaning procedure. The test solutions were 0.1, 0.5, 0.75 and 1M NazSO4 at 30 ~ 45 ~ and 60~ An Amel potentiostat and a L & N Speedomax chart recorder were used. The procedure for the test consisted irt maintaining the steel electrode potentiostatically at a potential in the "passivity" region and recording the leakage current for a period of some 15 h. The passivity range was evaluated from potentiodynamic polarization scans. During the test the solution was maintained air-saturated by continuous bubbling. RESULTS A typical potentiodynamic polarization curve for steel A in 0.1M NaoSO4 is given in Fig. I, which shows the shape and amplitude o f the active dissolution peak and the order of magnitude of the c.ds. All the steels maintained potentiostatically "passive" show the spontaneous recurrence of passivity breakdowns and repassivating current blips, and the shape and the frequency of the blips appear to be strongly dependent on the type of steel. Typical plots of current vs. time, obtained in aerated 0.1M NazSO4, at a potential of + 750 mV (NHE), are shown in Fig. 2. Comparing the current blips with the initial maximum plateau, it can be seen that the ratio of the relevant charges is a function of the type of steel. Thus steel D shows the largest pa.ssivation plateau and, under steady conditions, a frequent succession of very sharp blips of almost constant height. The charge associated with each blip is very low in comparison with the initial passivation plateau. The two Cu-alloyed steels B and C have a smaller passivation plateau and a less regular passivation-depassivation alternation; moreover, the charge involved in each blip is higher than for steel D.
Breakdown of passivity in low-alloy steels
405
f2cc
8oh
t.6
0
-400
i
Zo 9i,
F~G. 1.
I
i
I
4"9 mA/cm 2
Potentiodynamic curve for steel A in Na2SO, 0-1M at 30cC.
Steel A
Steel
C
E 9 40
0
I
2
3
_
0
_
t,
t~
~
I h 2
T 3
I
h
h
_.:4
T
, O
I
t,
! 2 h
Fla. 2. Typical potentiostatic current-time plots for the four steels in Na2SO4 0.1M at 30~
B
406
L. GIULIANI,G. AGABIOand G. ]]OMBARA
The carbon steel, A, gives the smallest passivation plateau and repassivation blips irregular both in shape and in frequency. It also shows the highest ratio of the charge involved in repassivation to that for the first passivation. In order to clarify the processes of depassivatiort and repassivation, the influence of temperature and SO~- concentration has been studied on steel D. This steel has been chosen because of the extraordinary reproducibility of the phenomenon on it: triplicate tests have given nearly identical results. A quantization has been attempted, considering the trend of the cumulative number of blips with time. Typical plots are reported in Fig. 3.
O O O O 0 "0 O
3O2
O O O 0 O O O O
O
20O
O O
25 O
AA
O
~AA AA
O
A
O
AA A~ O
100
A A
o
A o
A
A xxx•
A
A
XXXXX
X X X
A oxZ~ x • x x x A
I
I
200
400
T 600
t,
f 800
I 1000
ITli~
FIG. 3. Cumulative blips number as a function of polarization time for steel D in NacSO~ 0-75M. Temperature: x 30~ A 45~ o 60~
The initial part of the curves represents a period of rapid depassivation-repassivation alternations; only after some hours, a steady condition is reached and blips of lower height are obtained with a constant and reproducible frequency. The dependence of blip frequency, at steady state, on SO~- concentration and on temperature is reported in Figs. 4 and 5, respectively.
Breakdown of passivity in low-alloy steels
407
0-3
0-2 $,
o g =~ err
~
0-I
A
"-...
....,'~
"%.. %
...........
"..A
,,,O~o O'-" ......... I
0.1
""O
--O" ' """ f 0.5
Naz S04,
I
0.75 M
I
I
FIG.4. Blipfrequencyat steady state for steel D as a function of Na2SO4concentration. Temperature: o 30~ A 45~ g 60~ With increasing SO~- concentration the blip frequency increases up to a maximum corresponding to 0.75M. For higher concentrations the blip frequency falls to lower values. At all sulphate concentrations, the frequency increases with temperature according to art exponential law, a semilog diagram shows a definitely different slope for the 0.75M concentration. DISCUSSION Low-alloysteels represent an interestingclass of materials because of their superior corrosion resistance properties, especiallywith respect to atmospheric corrosion. Most of the relevant studies are concerned with the kinetics of rusting and thus are focused on the physieo-chemicalproperties of the rust itself. However, in developinga metallurgicalresearch on such steels, thetimelagrequired by a conventional evaluation through exposure tests can be too onerous for a compositional screening. Therefore, accelerated tests and application of electrochemical methods can be of value in characterizing behaviour during the early stages of corrosion. The results presented here show indeed a peculiarity, since the electrode system consisting of the steel and the "passivating" corrosion products on it seems to correlate with the degree of self-protection in the atmosphere. In this respect (Fig. 6) the weight
L. GIULIANI,G. AGABIO and G. BOMBARA
408
0-3
I
:
;I
.a 0.2
j iI
11"~
0-1
j/
I
30
~
1
45
60
T,
FI6.5,
I
75
~
Blip frequency at steady state for steel D as a function of temperature. NazSO~ concentration: 0 0-1M; x 0-5M; 9 0-75M; At.OM.
30
.,
.,.P
FIO. 6.
~
_~-
.....
I
I
I
I
2
3
t, y Weight-losses o f the four steels in industrial atmosphere. Steel: 0 A; t~ B; o C; zx D.
I
Breakdown of passivity in low-alloy steels
409
losses of the four steels are reported, as measured during a 48 months' exposure test in an industrial atmosphere. A possible explanation of the observed current blips can be submitted, assuming a dynamic instability, i.e. a cyclic breakdown and reconstruction, of coverage passivity. Actually, in corrosion studies of ferrous alloys, transient phenomena are often encountered, which are generally attributed to the co-existence of passive and active areas on the metal surface. According to the theory developed by Franck, 2 the existence o f such transient states is determined by the interplay of the electrical relationship i = i(E) with some chemical or physical parameter a, whose variation with time follows a law of the type: dot
dt
-- Ka.
(1)
The condition of dynamic stability of an active-passive surface is expressed by the inequality:
d~ \ d t /
< 0
(2)
Experimental examples are mainly found in pitting corrosion and in passive systems activation: they are generally attributed to relevant fluctuations of activating or passivating ions concentration. In the case under study, the coverage is thought to be due to precipitation of Fe z+ compounds (hydroxide or basic salts): as the "passive" potential is imposed, a certain amount of current flows till the solution layer adjacent to the electrode reaches a degree of over-saturation in Fe 2+ compounds sufficient for local precipitation. Fe z+ concentration is thus lowered to the solubility level and kept constant by the dissolution of the solid compounds counteracting the diffusion and convection phenomena. At some distance from the electrode, the Fe +2 ions are converted to Fe +3 ions by dissolved oxygen, and when the coverage is locally exhausted, a blip due to passage of current is observed. The role played by the temperature and SO]- concentration can be directly related to the influence of these factors on both over-saturation and saturation concentrations. According to such a scheme, the variation of ferrous concentration on the electrode surface can be expressed in a simplified form: dC - - f (C~ - - (7) dt
(3)
where Cs is the saturation concentration. For C < Cs, the electrode coverage dissolves, anodic current flows and (dC/dt) > 0; when C overcomes C,, the electrode is blocked by ferrous compounds precipitation and (dC/dt) < O.
410
L, GIULIANI,G. AGABIOand G. BOMBARA
In this case, the stability condition: d ~-Ck--~-f] 9 1
0
(4)
is fulfilled. The experimental situation, however, is one of cyclic instability: the system oscillates between all-passive and active-passive conditions with permanence times determined by the time lags of the opposing processes, that is to say, precipitation and dissolution of the coverage products. As it can be seen from this example, the fulfilment of inequality 2 is actually a necessary condition for active-passive co-existence, but not a sufficient one for maintaining a dynamic stability. Thus, an additional condition for permanence of active and passive surfaces on the same electrode must be looked for in the ratio between the kinetic constants of the opposing processes which determine the dynamic equilibrium. The phenomenological differences among the four steels in the recurrence of passivity breakdown are somewhat difficult to understand, because of the complexity of the systems involved. Beside possible variations in physico-chemical properties of the precipitating compounds, surface enrichments in alloying elements could also be of importance. Experimental evidence of such enrichments has been obtained in this laboratory, both from electrochemical measurements and from electron microprobe analyses. CONCLUSIONS
1. When low-alloy steels are maintained at a potential higher than that corresponding to the dissolution peak, a phenomenon of recurrent breakdown and reconstitution of the coverage passivity occurs with time. The shape and the frequency of the associated current blips can be useful for a qualitative appraisal of low-alloy steels self-protectiveness. 2. Although the observed instability can be elucidated by means of a theory of transient states, the differences resulted among the four types of steel are more difficult to understand, owing to compositional variations at the specimens surface, during the experiments itself. REFERENCES 1. W. J. MOLtrg, Die Bedeckungstheorie der Passivit.~t der Metalle, Verlag Chemle (1933). 2. U. FRANO:,Korrosion 13, Verlag Chemic (1960).
BREAKDOWN
OF PASSIVITY IN LOW-ALLOY
STEELS*
L. G1ULIANbG. AGABIOand G. BOSmARA Centro Sperimentale Metallurgico, Castel Romano, Rome, Italy Abstract--Low-alloy steels can be protected in SO~- solutions through precipitation of corrosion products. Periodic breakdowns of such a "coverage passivity" result in current blips, whose shape and frequency are highly specific of the type of steel. The qualitative classification which can be drawn from this phenomenon appears to correlate quite well with the corrosion resistance of steel as determined by atmospheric exposure tests. R6sum&--Les aciers faiblement allies peuvent ~tre prot6g6s dam des solutions de SO,'- par une pr&ipitation de produits de corrosion. Les ruptures p~riodiques d'une telle couche de passivation dorment des pointes de courant, dont la forme et la fr~quence sont hautement sp~cifiques du type d'acier. La classification qualitative qui peut ~tre ~tablie d'apr~s ce ph6nom~ne est en tr~s bon accord avecla r~sistance ~ la corrosion de l'acier, d6termin~e par des essais d'exposition atmosph~rique. Zusammenfassung--Niedriglegierte St~hle k6nnen in SOl--L6sungen durch den Niederschlag von Korrosionsprodukten geschiitzt werden. Ein periodischer Zusammenbruch dieser Bedeckungspassivit~it bewirkt einzelne Stromspitzen, deren Form und Frequenz ftir den betreffenden Stahl spezifisch sind. Qualitativ stimmt diese Erscheinung iiberein mit der Neigung der Stfihle zur Bildung deckender Schichten bei der atmosph~risehen Korrosion. INTRODUCTION
ON THEbasis of apparent features and properties, various theories have been developed on the possible mechanisms of metal passivation. A clear-cut distinction, however, must be made between chemical passivity and coverage passivity. The former type of passivity implies the formation of a very thin surface layer (one to several molecular layers thick) of oxide, or other solid corrosion products, or even adsorbed 02. Such a kinetic barrier, almost irrespective of the way it is obtained--whether anodically or by the action of gaseous or dissolved O., or of any ionic oxidant---consists 9of a phase which is stable within a definite range of potentials and through which only a leakage current passes, corresponding to the very slow dissolution rate of the passive film. In many other systems, the decrease in the anodic current at certain potentials involves the formation of a substantial layer of corrosion products, having a thickness sufficient to restrain the overall ionic exchange between the metal and the electrolyte. This type of passivity may be defined as coverage passivity or, somewhat less properly, mechanical passivity? The formation of the protective layer implies a definite passage of charge and its stability depends also on physical factors, such as internal stresses, coherence, adherence, porosity and resistance to mechanical actions by the electrolyte. A simple potentiostatic investigation on such form of passivity is presented here for low-alloy steels in SO~- solutions, which shows some interesting features in the anodic behaviour with time. *Manuscript received 16 April 1970. 403
404
L. GIULIANI,G. AGAB10ann G. BOMBARA
EXPERIMENTAL Three types of low-alloy steels and a plain carbon steel have been tested. The materials are simply designated as A, B, C and D, the first one being a plain carbon steel, B and C two C u P alloys and D a Cor-Ten type (Table 1).
TABLE1. CHEMICALCOMPOSITIONor TESTEDALLOYS Steel
C
Si
bin
S
P
Cu
Cr
Ni
Others
A B C D
0-188 0-058 0-120 0-093
0-040 tr. 0-068 0.600
0-83 0-35 0-78 0-34
0.018 0-014 0.018 0-027
0.022 0-055 0.095 0-IC0
0.060 0-20-I 0-278 0-375
0.026 0-018 0.028 0"540
0.037 0"030 0-034 0-26
-AI 0.064 Nb 0-017
The cell consisted in a perspex tube with two threadedcovers. The specimen was placed at the bottom of the cell and masked with a perspex disk and a pvc gasket, so as to expose an area of 1-77 cm 2 to the solution. Steel specimens with a simply ground surface were used; they were degreased in an alcohol-ether mixture and rinsed with acetone, and tested soon after the cleaning procedure. The test solutions were 0.1, 0.5, 0.75 and 1M NazSO4 at 30 ~ 45 ~ and 60~ An Amel potentiostat and a L & N Speedomax chart recorder were used. The procedure for the test consisted irt maintaining the steel electrode potentiostatically at a potential in the "passivity" region and recording the leakage current for a period of some 15 h. The passivity range was evaluated from potentiodynamic polarization scans. During the test the solution was maintained air-saturated by continuous bubbling. RESULTS A typical potentiodynamic polarization curve for steel A in 0.1M NaoSO4 is given in Fig. I, which shows the shape and amplitude o f the active dissolution peak and the order of magnitude of the c.ds. All the steels maintained potentiostatically "passive" show the spontaneous recurrence of passivity breakdowns and repassivating current blips, and the shape and the frequency of the blips appear to be strongly dependent on the type of steel. Typical plots of current vs. time, obtained in aerated 0.1M NazSO4, at a potential of + 750 mV (NHE), are shown in Fig. 2. Comparing the current blips with the initial maximum plateau, it can be seen that the ratio of the relevant charges is a function of the type of steel. Thus steel D shows the largest pa.ssivation plateau and, under steady conditions, a frequent succession of very sharp blips of almost constant height. The charge associated with each blip is very low in comparison with the initial passivation plateau. The two Cu-alloyed steels B and C have a smaller passivation plateau and a less regular passivation-depassivation alternation; moreover, the charge involved in each blip is higher than for steel D.
Breakdown of passivity in low-alloy steels
405
f2cc
8oh
t.6
0
-400
i
Zo 9i,
F~G. 1.
I
i
I
4"9 mA/cm 2
Potentiodynamic curve for steel A in Na2SO, 0-1M at 30cC.
Steel A
Steel
C
E 9 40
0
I
2
3
_
0
_
t,
t~
~
I h 2
T 3
I
h
h
_.:4
T
, O
I
t,
! 2 h
Fla. 2. Typical potentiostatic current-time plots for the four steels in Na2SO4 0.1M at 30~
B
406
L. GIULIANI,G. AGABIOand G. ]]OMBARA
The carbon steel, A, gives the smallest passivation plateau and repassivation blips irregular both in shape and in frequency. It also shows the highest ratio of the charge involved in repassivation to that for the first passivation. In order to clarify the processes of depassivatiort and repassivation, the influence of temperature and SO~- concentration has been studied on steel D. This steel has been chosen because of the extraordinary reproducibility of the phenomenon on it: triplicate tests have given nearly identical results. A quantization has been attempted, considering the trend of the cumulative number of blips with time. Typical plots are reported in Fig. 3.
O O O O 0 "0 O
3O2
O O O 0 O O O O
O
20O
O O
25 O
AA
O
~AA AA
O
A
O
AA A~ O
100
A A
o
A o
A
A xxx•
A
A
XXXXX
X X X
A oxZ~ x • x x x A
I
I
200
400
T 600
t,
f 800
I 1000
ITli~
FIG. 3. Cumulative blips number as a function of polarization time for steel D in NacSO~ 0-75M. Temperature: x 30~ A 45~ o 60~
The initial part of the curves represents a period of rapid depassivation-repassivation alternations; only after some hours, a steady condition is reached and blips of lower height are obtained with a constant and reproducible frequency. The dependence of blip frequency, at steady state, on SO~- concentration and on temperature is reported in Figs. 4 and 5, respectively.
Breakdown of passivity in low-alloy steels
407
0-3
0-2 $,
o g =~ err
~
0-I
A
"-...
....,'~
"%.. %
...........
"..A
,,,O~o O'-" ......... I
0.1
""O
--O" ' """ f 0.5
Naz S04,
I
0.75 M
I
I
FIG.4. Blipfrequencyat steady state for steel D as a function of Na2SO4concentration. Temperature: o 30~ A 45~ g 60~ With increasing SO~- concentration the blip frequency increases up to a maximum corresponding to 0.75M. For higher concentrations the blip frequency falls to lower values. At all sulphate concentrations, the frequency increases with temperature according to art exponential law, a semilog diagram shows a definitely different slope for the 0.75M concentration. DISCUSSION Low-alloysteels represent an interestingclass of materials because of their superior corrosion resistance properties, especiallywith respect to atmospheric corrosion. Most of the relevant studies are concerned with the kinetics of rusting and thus are focused on the physieo-chemicalproperties of the rust itself. However, in developinga metallurgicalresearch on such steels, thetimelagrequired by a conventional evaluation through exposure tests can be too onerous for a compositional screening. Therefore, accelerated tests and application of electrochemical methods can be of value in characterizing behaviour during the early stages of corrosion. The results presented here show indeed a peculiarity, since the electrode system consisting of the steel and the "passivating" corrosion products on it seems to correlate with the degree of self-protection in the atmosphere. In this respect (Fig. 6) the weight
L. GIULIANI,G. AGABIO and G. BOMBARA
408
0-3
I
:
;I
.a 0.2
j iI
11"~
0-1
j/
I
30
~
1
45
60
T,
FI6.5,
I
75
~
Blip frequency at steady state for steel D as a function of temperature. NazSO~ concentration: 0 0-1M; x 0-5M; 9 0-75M; At.OM.
30
.,
.,.P
FIO. 6.
~
_~-
.....
I
I
I
I
2
3
t, y Weight-losses o f the four steels in industrial atmosphere. Steel: 0 A; t~ B; o C; zx D.
I
Breakdown of passivity in low-alloy steels
409
losses of the four steels are reported, as measured during a 48 months' exposure test in an industrial atmosphere. A possible explanation of the observed current blips can be submitted, assuming a dynamic instability, i.e. a cyclic breakdown and reconstruction, of coverage passivity. Actually, in corrosion studies of ferrous alloys, transient phenomena are often encountered, which are generally attributed to the co-existence of passive and active areas on the metal surface. According to the theory developed by Franck, 2 the existence o f such transient states is determined by the interplay of the electrical relationship i = i(E) with some chemical or physical parameter a, whose variation with time follows a law of the type: dot
dt
-- Ka.
(1)
The condition of dynamic stability of an active-passive surface is expressed by the inequality:
d~ \ d t /
< 0
(2)
Experimental examples are mainly found in pitting corrosion and in passive systems activation: they are generally attributed to relevant fluctuations of activating or passivating ions concentration. In the case under study, the coverage is thought to be due to precipitation of Fe z+ compounds (hydroxide or basic salts): as the "passive" potential is imposed, a certain amount of current flows till the solution layer adjacent to the electrode reaches a degree of over-saturation in Fe 2+ compounds sufficient for local precipitation. Fe z+ concentration is thus lowered to the solubility level and kept constant by the dissolution of the solid compounds counteracting the diffusion and convection phenomena. At some distance from the electrode, the Fe +2 ions are converted to Fe +3 ions by dissolved oxygen, and when the coverage is locally exhausted, a blip due to passage of current is observed. The role played by the temperature and SO]- concentration can be directly related to the influence of these factors on both over-saturation and saturation concentrations. According to such a scheme, the variation of ferrous concentration on the electrode surface can be expressed in a simplified form: dC - - f (C~ - - (7) dt
(3)
where Cs is the saturation concentration. For C < Cs, the electrode coverage dissolves, anodic current flows and (dC/dt) > 0; when C overcomes C,, the electrode is blocked by ferrous compounds precipitation and (dC/dt) < O.
410
L, GIULIANI,G. AGABIOand G. BOMBARA
In this case, the stability condition: d ~-Ck--~-f] 9 1
0
(4)
is fulfilled. The experimental situation, however, is one of cyclic instability: the system oscillates between all-passive and active-passive conditions with permanence times determined by the time lags of the opposing processes, that is to say, precipitation and dissolution of the coverage products. As it can be seen from this example, the fulfilment of inequality 2 is actually a necessary condition for active-passive co-existence, but not a sufficient one for maintaining a dynamic stability. Thus, an additional condition for permanence of active and passive surfaces on the same electrode must be looked for in the ratio between the kinetic constants of the opposing processes which determine the dynamic equilibrium. The phenomenological differences among the four steels in the recurrence of passivity breakdown are somewhat difficult to understand, because of the complexity of the systems involved. Beside possible variations in physico-chemical properties of the precipitating compounds, surface enrichments in alloying elements could also be of importance. Experimental evidence of such enrichments has been obtained in this laboratory, both from electrochemical measurements and from electron microprobe analyses. CONCLUSIONS
1. When low-alloy steels are maintained at a potential higher than that corresponding to the dissolution peak, a phenomenon of recurrent breakdown and reconstitution of the coverage passivity occurs with time. The shape and the frequency of the associated current blips can be useful for a qualitative appraisal of low-alloy steels self-protectiveness. 2. Although the observed instability can be elucidated by means of a theory of transient states, the differences resulted among the four types of steel are more difficult to understand, owing to compositional variations at the specimens surface, during the experiments itself. REFERENCES 1. W. J. MOLtrg, Die Bedeckungstheorie der Passivit.~t der Metalle, Verlag Chemle (1933). 2. U. FRANO:,Korrosion 13, Verlag Chemic (1960).