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The anodic dissolution of aluminium zinc alloys in sea water
Materials Chemistry 2 (1977) 35 - 49 © CENFOR S.R.L. - Printed in Italy
THE A N O D I C D I S S O L U T I O N O F A L U M I N I U M Z I N C A L L o Y s IN SEA W A T E R P.L B O N O R A Centro S t u d i di Chimica e Chimica Fisica A p p l i c a t a alle Caratteristiche di I m p i e g o dei M a t e r i a l i - C.N.R. - Universitl di G E N O V A -Italia,
Received 15 December 1976. Summary - The electrochemical behaviour of three A1-Zn alloys (1 - 5 - 10% Zn) in 0.5 N NaC1 solutions w i t h a pH of 2 and of 8.2, under controlled fluid dynamic conditions (rotating disks at 500 - 2500 r.p.m.) at 20 - 40 60°C has been studied. Stationary potentiostatic anodic polarisation curves have been performed. S.E.M. and EDAX surface analysis have been obtained on anodically polarised specimens under coulostatic conditions. The morphology and mechanisms of anodic dissolution have been discussed as a function of anodic overpotential, charge density and zinc content of the studied alloys. Evans polarisation diagrams of the galvanic cells obtained coupling the Aluminium alloys either with Platinum or with Nickel as well as with stainless steel have been performed in sea water both under stagnant and stirred conditions.
INTRODUCTION D u r i n g the last few years we have been s t u d y i n g in o u r laboratory the a n o d i c b e h a v i o u r o f A l u m i n i u m .and its aUoys, with p a r t i c u l a r interest f o r pitting c o r r o s i o n and for the d e t e r m i n a t i o n o f the critical b r e a k d o w n potential, E RI ,2 T h e previous e x p e r i m e n t a l w o r k c o n c e r n e d passivable alloys, i.e.
36 A1-Cu, AI-Mg, A1-Si. The present paper deals with some A1-Zn alloys, on which the Aluminium oxide exherts no protective action against metal dissolution. The results herein presented regard marine corrosion, since they have been obtained in 3% NaC1 solutions and in synthetic sea water, but they might be of some interest also for cathodic protection, because these alloys are suitable as protecting anodes, and for energy storage too, because A1-Zn anodes have been employed for emergency primary cells a . All our measurements have been performed under controlled fluid-dynan)ic conditions; the effects of the temperature and of the pH have been examined too.
EXPERIMENTAL Materials and methods The alloys have been obtained using AP5 standard Aluminium (0.2% Fe, 0.15% Si, balance A1), adding different weights of Zinc (1 - 5 - I0%). The specimen, a cylinder 1.2 cm of diameter, was embedded in Scandiplast resin and a rotating disk electrode was obtained, the exposed surface of which was treated with N. 1000 emery paper. Only the specimens for micrographic analysis had a mirror-finished surface. The rotation speed has been varied between 500 and 2500 r.p.m,, and was stable within + 1%. The temperature of the solution was 20 - 40 - 60 + 0.1°C. We utilised as electrolytes 0.5M NaC1 solutions with a pH of 2 and 8.2: the choice of these pH values has been the result of assumptions we exposed in previous papers 2. The following tests have been performed: stationary corrosion potential (Ecorr.) values measurements, both in stagnant and in fluid-dynamic conditions (500-2500 rpm), at 20, 40, 60°C; stationary potentiostatic anodic potential/current curves, under the same experimental conditions. An AMEL (Milan) Metalloscan -
37
-
apparatus has been employed. The electrochemical cell with rotating working electrode has been already described4 ; dissolution at various anodic overpotential values, for periods of time corresponding to 5 or 25 coulombs discharged; micrographic SEM and EDAX surface analysis after the above mentioned controlled dissolution; Evans diagrams for the polarisation of the cells: 0,5M NaC1
Pt; Fe18Cr10Ni;
pH 8.2; 20°C
Ni
Al-Zn (1; 5; 10%)
A high precision resistance box has been connected between the electrodes; the current flowing in the external circuit has been measured by means of an AMEL null-resistance ammeter, and both electrode potentials have been recorded through two high input impedance electrometers. /,
RESULTS AND DISCUSSION We show in Figs. 1 and 2 the stationary potentiostatic anodic current-potential curves obtained on A1 1% Zn and A1 10% Zn rotating disk electrodes, under the various experimental conditions investigated. The following discussion anyway concerns also the A1 5% Zn alloys, because the curves are quite similar, the only one difference being a displacement in different potential zones, following the different Ecorr. values. All the curves present a straight line shape, and no slope variation has been caused by changing rotation rates from 500 up to 2500 r.p.m. the curves are suitable to be overlapped. No effect is to be ascrib'ed to diffusion, even if dissolution current values are very high (200 mA). The gas evolution which always follows dissolution is likely to mask any diffusion effect, providing by itself the m a s s t r a n s f e r t . No deviation from a straight line is observed even at very low overpotential
38
mA 14{] 12{]
60°C 40°C
,.2/,.~/
20°C~20°1 ,.~ ..82
~100 80 60
AI-Zn 10%
40 20
mV(ECS) Fig. 1 - Stationary potentiostatic anodic polarisation curves obtained on A l l%Zn alloy disk electrodes in 0.5 N NaCl solutions.
39
mA
121]
21
60°C pH2/
10(] 80 60
Al-Zn1%
40
20 800
I
I
600
40O
200 EmV(ECS)
Fig. 2 - Stationary potentiostatic anodic polarisation curves obtained alloy disk electrodes in 0.5 N NaCl solutions.
on A I lO%Zn
40
values. From an electrochemical point of view, the alloys m a y be considered active in the media we examined. A comparison between Ecorr. and E R values (the latter ones being extrapolated for I = 0, from the I = f(E) curves) shows that the difference' between the two series of values never exceeds + 1 0 m V (fig. 3). The Ecorr. values might vary as a function of the pH, following the anphoteric character of both A1 and Zn. It is well known that
0
T(Oc) 60 -
OI
40-
20-
nI
V~
A
AvO
I
800
z~A
900
1
1000 Ec-Er(mV)
Fig. 3 - Plotting o f Ecorr. and E R values f o r the studied aluminium alloys in 0.5 N NaCI solutions as a f u n c t i o n o f temperature: A l l % Z n : Ecorr. (a), E R ( . ) ; A 1 5 % Z n : Ecorr. ( ^ ) , E R (A); A l l O % Z n : Ecorr. (~,), E R (v).
Aluminium oxide protects the metal between pH 3 and 9, whilst Zinc is passive between pH 6 and 12. Some Ec0rr. measUrements made at 20°C on rotating (500 r.p.m.) disks in NaC1 solutions with different pH values have shown that for the Al-1%Zn alloy, Ecorr. is -- 845 mV (SCE) at pH = 8.2 which is a rather noble value; however slight pH
41 variations affect this value: at pH -- 8.7, Ecorr. is - 950 mV; at pH = 9, Ecorr. reaches - 1018 mV. The A15%Zn and A110%Zn alloys present a very active ( - 975 mV SCE) Ecorr. value, which remains unaffected between pH = 3 and pH -- 9.4. From a t h e r m o d y n a m i c point of view, The A11%Zn alloy presents a trend towards passivation, which is not kinetically confirmed, since both polarisation curves and micrographic analysis of corrosion morphology, as we shall see later, show no difference of behaviour with respect to the other two alloys. This is the reason why no difference has been noticed between the electrochemical and micrographical data obtained at pH of 2 and of 8.2 other than slight slope variations of the polarisation curves, due to the different conductivity of the solutions; the anodic dissolution mechanism seems to be the same, independently of the Zinc content. To confirm this assumption, we performed scanning electron micrographic and EDAX analysis of the surface, as a function of three parameters'. Zinc content of the alloy; anodic overpotential imposed; a m o u n t of electrical charge imposed to the alloy. The morphology of the anodic dissolution of the A1 1%Zn alloy, after a stay at three different overpotential values, potentiostatically imposed, till a charge of 5 coulombs has passed, is s h o w n in Figs. 4, 5, 6. A pitting corrosion is clearly perceivable, the size and depth of pits being decreased and their number increased, the more anodic becomes the applied overpotential. On the A1 1 0 ~ Z n alloy, the corroded surface (Fig. 7) presents a more uniform attack, with less deep and more randomly distributed pits. When a higher number of coulombs (25) is imposed, the size but not the number of active sites, i.e. the pits, increases. Aluminium is of course the anodic partner in the A1-Zn galvanic couple, and it is therefore preferentially dissolved with respect to Zinc. By increasing the applied overpotential value, the potential difference between local anodes and cathodes looses importance, and the attack is less selective. Moreover, the higher is the Zinc content, the higher and more randomly distributed is the number of cathodic sites, and the more
42
Fig. 4 - L o c a l i z e d a t t a c k on A l l % Z n
alloy: E = - 8 5 0 m V ( S . C . E . ) ; charge: 5 cou-
lombs (200 X).
Fig. 5 - L o c a l i z e d a t t a c k o n A I I % Z n l o m b s ( 2 0 0 X)..
alloy: E = - 6 0 0 m V ( S . C . E . ) ; charge: 5 cou-
43
i
Fig. 6 - Localized attack on A l l % Z n alloy: E = - 300 m V (S,C.E.); charge: 5 c o u lombs ( 2 0 0 X),
Fig. 7 - L o c a l i z e d lombs ( 2 0 0 X).
attack on AllO°loZn alloy; E = - 3 0 0 m V (S.C.E.); charge: 5 cou-
44 uniform the activity of the galvanic microcells, and the less selective the dissolution. A comparison of the EDAX distribution map of A1 in A1 1%Zn and A110%Zn alloy, under the same corrosion conditions is presented as an example in Figs. 8 and 9. Another aspect of the anodic behaviour of the A1-Zn alloys is their use as anodes in primary emergency cells in sea water. We obtained the Evans diagrams for the electrochemical cells made up with the chains previously shown. We performed all the experiments both with stagnant electrolytes and with rotating Al-alloys anodes in stirred solutions. The plottings obtained for the various cells we investigated in different fluid-dynamic conditions have shown up that the polarisation is always under cathodic control. Figs. 10 and 11 report, as an example, the Evans diagrams obtained with Pt cathodes coupled with AI-I-5-10%Zn anodes both under stagnant (Fig. 10) and dynamic (Fig. 11) conditions. The Ni and stainless steel anodes shown verY similar behaviour with slightly lower current values. The cathodic concentration polarisation gives an important contribution, since the short circuit current decreases twelve times for the 5 and 10~ Zn alloys, and even forty times for the 1% Zn alloy, by keeping the solution stagnant. From a comparison of these plottings, the only one noticeable difference between the alloy with the lowest Zinc content and the other ones appears. As we previously mentioned, a thermodynamic trend towards passivation is expressed by A1 l ~ Z n alloy, being its Ecorr. value 100 mV nobler with respect to the other alloys: the only one kinetic confirmation of it is found in the galvanic couple under stagnant conditions. As it can be in Fig. 9, the A1 1%Zn, because of its nobler Ecorr. values, hinders the couple to attain the conditions of cathodic depolarisation and the short circuit current is therefore excessively low.
CONCLUSIONS All the alloys we investigated are metallic materials which may be
45
Fig. 8 - S E M m i c r o g r a p h y a n d E D A X m a p o f A l u m i n i u m c o n c e n t r a t i o n o n an A l 1 % Z n alloy ( 1 0 0 0 X); E = - 3 O0 m V (S. C,E.); E D A X analysis: A l 96. 5 % ; Z n 3.5 %.
46
i!i....
Fig. 9 - SEM micrography and E D A X map of Aluminium concentration on an A l l O % Z n alloy ( 3 0 0 0 X); E = - 300 m V (S.C.E.); E D A X analysis: A l 5~.6010; Zn 45.8%.
47
E
c~ qua
C
,:1: o~
,l:S: ,4,.m
n_
•
i
i
eS:
r,.p
LLi
0 ILl
-4-
48
E
o~
pml
~.~
e~ N _l_
qpm ml
Wd C'~J
÷
0
c~ 0
!
c~
0
o 0
o
0
49 easily utilized as dissolving anodes both for cathodic protection and for emergency primary cells. The mechanism of dissolution involves the creation of active sites, the number and' distribution of which are depending on the electrical field applied. The least Zinc content suffices to eliminate the protective properties of Aluminium oxide. An increase of the Zinc content leads to a more uniform and less selective form of dissolution. Temperature affects in no way the mechanism of anodic dissolution of the three alloys, within the field 20-60°C. The only peculiar effect of temperature is seen in Fig. 2, and is once more related to the different thermodynamic behaviour of the A1 l%Zn alloys, since the Ecorr. and E R values are more affected by temperature variations than the corresponding values of the other alloys. Finally, we may conclude that the minimum Zinc content which allows the use of Aluminium alloys as dissolving anodes, especially under fluid-dynamic conditions, is considerably lower than the amount of 30%, which is reported by Cikovic and Karsulin s .
REFERENCES
1. 2. 3.
4. 5.
P.L. BONORA, G.P. PONZANO, M. BASSOLI - Ann. Chim.,Rome, 63, 503, 1973. P.L. BONORA, G.P. PONZANO, V. LORENZELLI -- Brit. Corros. J., 9, 108, 1973; 9, 112, 1973. C.P. W A L E S , A.C. SIMON, S. S C H U L D ~ N E R - Electrochira. Acta, 20, 895, 1975. G.P.-PONZANO,M. BASSOLI, P.L. BONORA -- Werk. u. Korr., 27, 568, 1976. N. CIKOVIC, M. KARSULIN -- Proc. 27th LS.E. Meeting, n. 153, Zurich, Sept. 1976.
THE A N O D I C D I S S O L U T I O N O F A L U M I N I U M Z I N C A L L o Y s IN SEA W A T E R P.L B O N O R A Centro S t u d i di Chimica e Chimica Fisica A p p l i c a t a alle Caratteristiche di I m p i e g o dei M a t e r i a l i - C.N.R. - Universitl di G E N O V A -Italia,
Received 15 December 1976. Summary - The electrochemical behaviour of three A1-Zn alloys (1 - 5 - 10% Zn) in 0.5 N NaC1 solutions w i t h a pH of 2 and of 8.2, under controlled fluid dynamic conditions (rotating disks at 500 - 2500 r.p.m.) at 20 - 40 60°C has been studied. Stationary potentiostatic anodic polarisation curves have been performed. S.E.M. and EDAX surface analysis have been obtained on anodically polarised specimens under coulostatic conditions. The morphology and mechanisms of anodic dissolution have been discussed as a function of anodic overpotential, charge density and zinc content of the studied alloys. Evans polarisation diagrams of the galvanic cells obtained coupling the Aluminium alloys either with Platinum or with Nickel as well as with stainless steel have been performed in sea water both under stagnant and stirred conditions.
INTRODUCTION D u r i n g the last few years we have been s t u d y i n g in o u r laboratory the a n o d i c b e h a v i o u r o f A l u m i n i u m .and its aUoys, with p a r t i c u l a r interest f o r pitting c o r r o s i o n and for the d e t e r m i n a t i o n o f the critical b r e a k d o w n potential, E RI ,2 T h e previous e x p e r i m e n t a l w o r k c o n c e r n e d passivable alloys, i.e.
36 A1-Cu, AI-Mg, A1-Si. The present paper deals with some A1-Zn alloys, on which the Aluminium oxide exherts no protective action against metal dissolution. The results herein presented regard marine corrosion, since they have been obtained in 3% NaC1 solutions and in synthetic sea water, but they might be of some interest also for cathodic protection, because these alloys are suitable as protecting anodes, and for energy storage too, because A1-Zn anodes have been employed for emergency primary cells a . All our measurements have been performed under controlled fluid-dynan)ic conditions; the effects of the temperature and of the pH have been examined too.
EXPERIMENTAL Materials and methods The alloys have been obtained using AP5 standard Aluminium (0.2% Fe, 0.15% Si, balance A1), adding different weights of Zinc (1 - 5 - I0%). The specimen, a cylinder 1.2 cm of diameter, was embedded in Scandiplast resin and a rotating disk electrode was obtained, the exposed surface of which was treated with N. 1000 emery paper. Only the specimens for micrographic analysis had a mirror-finished surface. The rotation speed has been varied between 500 and 2500 r.p.m,, and was stable within + 1%. The temperature of the solution was 20 - 40 - 60 + 0.1°C. We utilised as electrolytes 0.5M NaC1 solutions with a pH of 2 and 8.2: the choice of these pH values has been the result of assumptions we exposed in previous papers 2. The following tests have been performed: stationary corrosion potential (Ecorr.) values measurements, both in stagnant and in fluid-dynamic conditions (500-2500 rpm), at 20, 40, 60°C; stationary potentiostatic anodic potential/current curves, under the same experimental conditions. An AMEL (Milan) Metalloscan -
37
-
apparatus has been employed. The electrochemical cell with rotating working electrode has been already described4 ; dissolution at various anodic overpotential values, for periods of time corresponding to 5 or 25 coulombs discharged; micrographic SEM and EDAX surface analysis after the above mentioned controlled dissolution; Evans diagrams for the polarisation of the cells: 0,5M NaC1
Pt; Fe18Cr10Ni;
pH 8.2; 20°C
Ni
Al-Zn (1; 5; 10%)
A high precision resistance box has been connected between the electrodes; the current flowing in the external circuit has been measured by means of an AMEL null-resistance ammeter, and both electrode potentials have been recorded through two high input impedance electrometers. /,
RESULTS AND DISCUSSION We show in Figs. 1 and 2 the stationary potentiostatic anodic current-potential curves obtained on A1 1% Zn and A1 10% Zn rotating disk electrodes, under the various experimental conditions investigated. The following discussion anyway concerns also the A1 5% Zn alloys, because the curves are quite similar, the only one difference being a displacement in different potential zones, following the different Ecorr. values. All the curves present a straight line shape, and no slope variation has been caused by changing rotation rates from 500 up to 2500 r.p.m. the curves are suitable to be overlapped. No effect is to be ascrib'ed to diffusion, even if dissolution current values are very high (200 mA). The gas evolution which always follows dissolution is likely to mask any diffusion effect, providing by itself the m a s s t r a n s f e r t . No deviation from a straight line is observed even at very low overpotential
38
mA 14{] 12{]
60°C 40°C
,.2/,.~/
20°C~20°1 ,.~ ..82
~100 80 60
AI-Zn 10%
40 20
mV(ECS) Fig. 1 - Stationary potentiostatic anodic polarisation curves obtained on A l l%Zn alloy disk electrodes in 0.5 N NaCl solutions.
39
mA
121]
21
60°C pH2/
10(] 80 60
Al-Zn1%
40
20 800
I
I
600
40O
200 EmV(ECS)
Fig. 2 - Stationary potentiostatic anodic polarisation curves obtained alloy disk electrodes in 0.5 N NaCl solutions.
on A I lO%Zn
40
values. From an electrochemical point of view, the alloys m a y be considered active in the media we examined. A comparison between Ecorr. and E R values (the latter ones being extrapolated for I = 0, from the I = f(E) curves) shows that the difference' between the two series of values never exceeds + 1 0 m V (fig. 3). The Ecorr. values might vary as a function of the pH, following the anphoteric character of both A1 and Zn. It is well known that
0
T(Oc) 60 -
OI
40-
20-
nI
V~
A
AvO
I
800
z~A
900
1
1000 Ec-Er(mV)
Fig. 3 - Plotting o f Ecorr. and E R values f o r the studied aluminium alloys in 0.5 N NaCI solutions as a f u n c t i o n o f temperature: A l l % Z n : Ecorr. (a), E R ( . ) ; A 1 5 % Z n : Ecorr. ( ^ ) , E R (A); A l l O % Z n : Ecorr. (~,), E R (v).
Aluminium oxide protects the metal between pH 3 and 9, whilst Zinc is passive between pH 6 and 12. Some Ec0rr. measUrements made at 20°C on rotating (500 r.p.m.) disks in NaC1 solutions with different pH values have shown that for the Al-1%Zn alloy, Ecorr. is -- 845 mV (SCE) at pH = 8.2 which is a rather noble value; however slight pH
41 variations affect this value: at pH -- 8.7, Ecorr. is - 950 mV; at pH = 9, Ecorr. reaches - 1018 mV. The A15%Zn and A110%Zn alloys present a very active ( - 975 mV SCE) Ecorr. value, which remains unaffected between pH = 3 and pH -- 9.4. From a t h e r m o d y n a m i c point of view, The A11%Zn alloy presents a trend towards passivation, which is not kinetically confirmed, since both polarisation curves and micrographic analysis of corrosion morphology, as we shall see later, show no difference of behaviour with respect to the other two alloys. This is the reason why no difference has been noticed between the electrochemical and micrographical data obtained at pH of 2 and of 8.2 other than slight slope variations of the polarisation curves, due to the different conductivity of the solutions; the anodic dissolution mechanism seems to be the same, independently of the Zinc content. To confirm this assumption, we performed scanning electron micrographic and EDAX analysis of the surface, as a function of three parameters'. Zinc content of the alloy; anodic overpotential imposed; a m o u n t of electrical charge imposed to the alloy. The morphology of the anodic dissolution of the A1 1%Zn alloy, after a stay at three different overpotential values, potentiostatically imposed, till a charge of 5 coulombs has passed, is s h o w n in Figs. 4, 5, 6. A pitting corrosion is clearly perceivable, the size and depth of pits being decreased and their number increased, the more anodic becomes the applied overpotential. On the A1 1 0 ~ Z n alloy, the corroded surface (Fig. 7) presents a more uniform attack, with less deep and more randomly distributed pits. When a higher number of coulombs (25) is imposed, the size but not the number of active sites, i.e. the pits, increases. Aluminium is of course the anodic partner in the A1-Zn galvanic couple, and it is therefore preferentially dissolved with respect to Zinc. By increasing the applied overpotential value, the potential difference between local anodes and cathodes looses importance, and the attack is less selective. Moreover, the higher is the Zinc content, the higher and more randomly distributed is the number of cathodic sites, and the more
42
Fig. 4 - L o c a l i z e d a t t a c k on A l l % Z n
alloy: E = - 8 5 0 m V ( S . C . E . ) ; charge: 5 cou-
lombs (200 X).
Fig. 5 - L o c a l i z e d a t t a c k o n A I I % Z n l o m b s ( 2 0 0 X)..
alloy: E = - 6 0 0 m V ( S . C . E . ) ; charge: 5 cou-
43
i
Fig. 6 - Localized attack on A l l % Z n alloy: E = - 300 m V (S,C.E.); charge: 5 c o u lombs ( 2 0 0 X),
Fig. 7 - L o c a l i z e d lombs ( 2 0 0 X).
attack on AllO°loZn alloy; E = - 3 0 0 m V (S.C.E.); charge: 5 cou-
44 uniform the activity of the galvanic microcells, and the less selective the dissolution. A comparison of the EDAX distribution map of A1 in A1 1%Zn and A110%Zn alloy, under the same corrosion conditions is presented as an example in Figs. 8 and 9. Another aspect of the anodic behaviour of the A1-Zn alloys is their use as anodes in primary emergency cells in sea water. We obtained the Evans diagrams for the electrochemical cells made up with the chains previously shown. We performed all the experiments both with stagnant electrolytes and with rotating Al-alloys anodes in stirred solutions. The plottings obtained for the various cells we investigated in different fluid-dynamic conditions have shown up that the polarisation is always under cathodic control. Figs. 10 and 11 report, as an example, the Evans diagrams obtained with Pt cathodes coupled with AI-I-5-10%Zn anodes both under stagnant (Fig. 10) and dynamic (Fig. 11) conditions. The Ni and stainless steel anodes shown verY similar behaviour with slightly lower current values. The cathodic concentration polarisation gives an important contribution, since the short circuit current decreases twelve times for the 5 and 10~ Zn alloys, and even forty times for the 1% Zn alloy, by keeping the solution stagnant. From a comparison of these plottings, the only one noticeable difference between the alloy with the lowest Zinc content and the other ones appears. As we previously mentioned, a thermodynamic trend towards passivation is expressed by A1 l ~ Z n alloy, being its Ecorr. value 100 mV nobler with respect to the other alloys: the only one kinetic confirmation of it is found in the galvanic couple under stagnant conditions. As it can be in Fig. 9, the A1 1%Zn, because of its nobler Ecorr. values, hinders the couple to attain the conditions of cathodic depolarisation and the short circuit current is therefore excessively low.
CONCLUSIONS All the alloys we investigated are metallic materials which may be
45
Fig. 8 - S E M m i c r o g r a p h y a n d E D A X m a p o f A l u m i n i u m c o n c e n t r a t i o n o n an A l 1 % Z n alloy ( 1 0 0 0 X); E = - 3 O0 m V (S. C,E.); E D A X analysis: A l 96. 5 % ; Z n 3.5 %.
46
i!i....
Fig. 9 - SEM micrography and E D A X map of Aluminium concentration on an A l l O % Z n alloy ( 3 0 0 0 X); E = - 300 m V (S.C.E.); E D A X analysis: A l 5~.6010; Zn 45.8%.
47
E
c~ qua
C
,:1: o~
,l:S: ,4,.m
n_
•
i
i
eS:
r,.p
LLi
0 ILl
-4-
48
E
o~
pml
~.~
e~ N _l_
qpm ml
Wd C'~J
÷
0
c~ 0
!
c~
0
o 0
o
0
49 easily utilized as dissolving anodes both for cathodic protection and for emergency primary cells. The mechanism of dissolution involves the creation of active sites, the number and' distribution of which are depending on the electrical field applied. The least Zinc content suffices to eliminate the protective properties of Aluminium oxide. An increase of the Zinc content leads to a more uniform and less selective form of dissolution. Temperature affects in no way the mechanism of anodic dissolution of the three alloys, within the field 20-60°C. The only peculiar effect of temperature is seen in Fig. 2, and is once more related to the different thermodynamic behaviour of the A1 l%Zn alloys, since the Ecorr. and E R values are more affected by temperature variations than the corresponding values of the other alloys. Finally, we may conclude that the minimum Zinc content which allows the use of Aluminium alloys as dissolving anodes, especially under fluid-dynamic conditions, is considerably lower than the amount of 30%, which is reported by Cikovic and Karsulin s .
REFERENCES
1. 2. 3.
4. 5.
P.L. BONORA, G.P. PONZANO, M. BASSOLI - Ann. Chim.,Rome, 63, 503, 1973. P.L. BONORA, G.P. PONZANO, V. LORENZELLI -- Brit. Corros. J., 9, 108, 1973; 9, 112, 1973. C.P. W A L E S , A.C. SIMON, S. S C H U L D ~ N E R - Electrochira. Acta, 20, 895, 1975. G.P.-PONZANO,M. BASSOLI, P.L. BONORA -- Werk. u. Korr., 27, 568, 1976. N. CIKOVIC, M. KARSULIN -- Proc. 27th LS.E. Meeting, n. 153, Zurich, Sept. 1976.