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Corrosion of aluminium in chloride solutions containing some anions—I. In 0.01M NaCl solutions
CorrosionScience.1976.Vol. 16. pp. 637 to 643.PerBamonPress. Printedin GreatBritain
CORROSION
OF ALUMINIUM IN CHLORIDE CONTAINING SOME ANIONS--I. IN 0.01M NaC1 SOLUTIONS*
SOLUTIONS
L. A. SHALABY, K. M. EL SOBK! a n d A. A. ABDUL AZIM Laboratory of Applied Electrochemistry N.R.C., Dokki, Cairo, Egypt Abstract--A study has been made of the corrosion behaviour of A1in solutions containing0.01M NaCI alone or mixed with silicate, phosphate, dichromate, permanganate or sulphate ions covering concentration ranges of several orders of magnitude. Chloride ions become adsorbed on cracks in the pre-immersion oxide film. Silicate, phosphate and permanganate play a dual role. At small concentrations they modify the rates of nucleation and growth of the crystalline oxide layer. At higher concentrations they become incorporated in this layer in progressive amounts and hence impair its protective nature. The reduction product of dichromate improves the properties of the oxide film. At small concentrations, sulphate ions simulate chloride ions, but at higher concentrations they are adsorbed on the growing oxide nuclei hindering its growth.
INTRODUCTION
THE film formed on AI immersed in solution is duplex in nature, consisting of an inner amorphous AI2Oa and outer crystalline forms of aluminium oxides. 1-1x Layers formed on aluminium in solutions containing silicate, 4 phosphate 5 or chromate s have been separated and analysed. The aim of the present work was to compare and contrast the corrosion behaviour of AI during immersion in decinormal chloride solutions containing silicate, phosphate chromate, permanganate or sulphate anions up to 8O d. EXPERIMENTAL As-roUed 0.1 mm thick pure aluminium sheet (99.99yoAl) was cut into rectangular coupons 20 cms. Prior to use the samples were degreased for 2 h in a ternary mixture of three equal volumes of acetone, alcohol and benzene. They were then etched in a 10~NaOH for 2 rain at 50°C, rinsed with running distilled water, and with alcohol, and finally dried in filtered air. The coupons were weighed prior to immersion in 950 ml of the test solutions. After immersion the coupons were dried, stripped of the oxide film by immersion for 10 rain in a boiling solution of CrOs + I-~PO4?s After further rinsing in distilled water the coupons were dried and reweighed. A pH meter (Metrohm AG Herisau, Schweiz) was used for measuring the potentials. The electrodes used had the area I × 1 era. Before use the electrodes were prepared in the same way as described above. RESULTS Weight-loss
time curves
,.
T h e weight-loss o f A1 c o u p o n s immersed i n 10-S-lM NaCI solutions increases linearly with time over a period o f ca. 80 d, which was the longest time employed. T h e weight-loss curves o b t a i n e d for 0.01M NaCI c o n t a i n i n g 10-s-10-SM s o d i u m silicate are shown i n Fig. 1. F o r solutions o f 1 0 ~ - 1 0 - S M , the curves consist o f descending *Manuscript received 28 June 1974. 637
638
L.A.
SHALABY, K . M . EL SOBKI a n d A . A. ABDUL AZIM
and ascending linear sections which meet at the 35th day. The rate of decrease and increase in weight-loss are higher the higher the concentration of the silicate ion. The first part is not seen for solutions containing 10-eM silicate. 60
5O \
x
0 40
S•
o
.
30--
ff
_o 2 5 -
0 FIc. 1.
J 10
, 20 ,
30
I 50I, 60 40 Time, d
,g
01°
,0
The corrosion of AI in 10-2M NaCI containing different concentrations o f sodium silicate.
Figure 2 shows the results obtained for 10-2M NaCI d- 10-6-10-1M NaaPO4. At concentrations ~ 10-SM, a linear increase in weight is obtained indicating that the corrosion products are not removable by the stripping bath used. The extent and rate of increase in weight is higher for l0 -e than 10-SM phosphate. At higher concentrations the curves indicate constant corrosion rates throughout the time of the experiment. Figure 3 shows the results obtained in the presence of 10-6-10-3M K2Cr2Ov. During the first 35 d, the corrosion rate is 0.13 mg/dm2/d, irrespective of the dichromate concentration. Further changes in weight depend on concentration. The corrosion rate either increases (10-6M K2Cr20~), maintains its value (10-SM), or becomes zero for c a . l0 d and then changes its sign (10 -4 and 10-s), the rate of gain in weight being higher for the higher dichromate concentration. The weight-loss in 0.01M NaCl solutions containing 10-~-10-1M KMnO4 increases linearly with time over a period extending from 5 to 80 d. The results obtained for sulphate show that the corrosion rate which is constant over a period of two months is high at low concentration (higher than in presence of 0.01M Cl- alone) and decreases markedly with increase in SO42- ion concentration. During the period of constant corrosion rate, the following relationship (Fig. 4) holds for 0.01M NaCi or 0.01M NaC! -t- KMnO4, NaaPO4 or Na~SiOs. dW Log ~ = log K -+- n log C,
(l)
Corrosion o f alumJnium in chloride solutions conta/ning some a n J o ~ I
639
go
6(;-
4°_
j
j
JJs
>
2c
o
'c I O!- o----e~
~
,,_.
10~M o
-I¢
-4o'
--
I
o
I
~o
I
20
I
30
L
4o
Time, FIG, 2.
I
so
I
ro
60
I
so
90
d
The corrosion o f A l in 10-=M NaCI containing different concentrations o f trisodium phosphate. 80
70~ 6C-
~cE 4c-
20 10
I
o
tO
I
20
I
30
I
40
I_
I
50
Time,
60
I
70
I
80
90
lu0
d
FiG. 3, The corrosion of A.I in 10-=M NaCi containing different concentrations of potassium dichromate.
where n amounts to 0.061,0.27, 0.083 and 0.037 respectively. That a double logarithmic relationship holds between the corrosion rate and anion concentration and the fact that n is a fraction may indicate that the slow step is the adsorption of anions. 14 The corresponding straight line obtained for sulphate has a negative slope. The corrosion rates computed ~'or dichromate during the first 35 d are independent of concentration.
640
L.A.
K. M. EL SOBKI and A. A. ABDUL AZlM
SHALABY,
-0.2 -0.4
--
-0.E
--
E -0.6
--
o Pure No CL • KMnO 4 No3PO =
x No slllc6te
-I
o= ,n
g
No.
/
SO.
.e
/
.C - -
-
-I.2
-I .4 -I.6
-I.6~-0
FIG. 4.
0
1
2.O
0
I
-6
I
- 5
-4
0
I
-3 10O C
I
-2
I
-I
I
0
T h e dependence o f c o r r o s i o n rate o f AI in 10-=M N a C I upon a n i o n concentration o f the i n h i b i t o r .
Potential-log C curves
The steady-state potential E (attained after 24 h) changes with the logarithm of molar concentration, C, according to E = a -- b log C.
(2)
This relationship is shown in Fig. 5. The constant "b" amounts for a ten-fold increase of concentration to 0.07, 0.033 and 0.025 for 10-ZM NaCI or 10-~M NaCI + silicate or dichromate respectively. The corresponding straight lines obtained for NasPO4 or KMnO4 possess positive slopes. -0.1 -0.2
>
o • v x
Pure NaCt KMnO 4 Na3 P04 Na. SO= Na'Sllicafe o KzCrz0 7
-o.z~ -0.4
-~ -o.s == -0.6
-0.7 -O.~
I
I,
I
-5
-4
-3
I
I
1
-~'
-I
0
log C
FIG. 5.
The change of potential of AI in 10-2 NaCI solutions containing different concentrations (C) of inhibitor.
Corrosion of aluminium in chloride solutions containing some anions--I
641
DISCUSSION The uniform corrosion rate d W/dt may be ascribed to the absence of protective properties of the film formed on the metal surface. Alternatively the oxide layer can be divided into a very thin, continuous inner layer, which is the principal inhibitor of diffusion and hence further film growth, and an outer layer generally thicker but less continuous. The thickness of the thin, non-porous layer under unchanging conditions of temperature and chemical agents remains approximately constant, and therefore the corrosion rate. It is well-established 1-9 that the film formed on AI in water consists of a thin compact barrier film adjacent to the metal and a thicker more permeable bulk film. The former is amorphous whereas the latter consists of a crystalline hydrated oxide. The results obtained for pure chloride solution show that steady-state potentials, E, are associated with a uniform corrosion rate. The value 0.07 obtained for b in equation (2) agrees with the corresponding value reported for several metals in a variety of environments. 14 This fact leads to the conclusion that corrosion of AI in neutral chloride solutions is under anodic control, the corrosion rate being determined by the size of anodic areas (areas of breakdown of the oxide film). Pryor et al. 7 suggest that chloride ions are adsorbed on the surface and become incorporated in the film providing paths for the passage of AI a+ ions. These ions hydrolyse, leading to a lowering of pH and local anodes are thus fixed. According to Vedder and Vermilyeas the cathodic reaction occurs primarily at the metal grain boundaries where impurity segregation is results in increased electronic conductivity. In the presence of other anions, however, it is reasonable to attribute the constancy of corrosion rate to the thin continuous layer next to the metal. The magnitude of the corrosion rate is expected to depend on the solubility and composition of the bulk film. s Aiuminium reacts readily with water to form a hydrous oxide film. Growth occurs in two stages; a pseudo-boehmite film is produced initially and then this is covered with a layer of bayerite crystals. By analogy with the aging mechanism in a colloidal suspension, it is thought that the pseudo-boehmite dissolves and reprecipitates as bayerite crystals. 9 It may be suggested that the process of crystallization is influenced by the presence of certain anions such as silicate, phosphate, dichromate or permanganate. Mixed compounds serve as nucleation centres leading, probably, to the formation of small crystals and hence less porous bulk oxide layer. The results obtained for 10-4-10 -~ silicate during the 35 d indicate the gradual precipitation of a certain stable substance on the electrode surface and that the inhibiting properties of this substance are improved with time. It may be assumed that a compound of the type Na~O.AI~Oa.SiO2.H20 is formed initially at sporadic areas by a slow precipitation mechanism, z The rate of crystallization of this compound seems to be lower than the corresponding rate for bayerite. This assumption is based on the fact that the initial weight-losses amount to 6.15, 4.85 and 1.I (extrapolated to zero time) for 10-3, 10-a and 10-4 silicate respectively. The linear decrease in weightloss may be attributed to the conversion of the crystalline unstable compound into the less soluble and more compact Al~Os.SiO2.4 At the minimum, the blocking of the surface reaches its maximum efficiency. At that point the inner amorphous layer has reached its steady-state thickness and the bulk layer has attained its final porosity.
642
L . A . SHALABY, K. M. EL SOBKI and A. A. ABDUL AZIM
The fact that the minimum is markedly higher at 10-a and 10-2M silicate than at 10-4 and that the slope of the curves beyond the minimum increases with increase in concentration may indicate that the protective nature of the bulk oxide is impaired as the concentration is increased. The silicate anions seem to play a dual role. At small concentrations they improve the protective nature of bayerite by acting as nucleation centres (the corrosion rate is less than in the presence of CI- ions above). At higher concentrations, they seem to provide paths for AI a+ ions, which are, however, less conductive than those established by chloride ions. The above conclusions find support in the results obtained for phosphate additions. Concentrations as low as 10-6 or 10-e modify the rates of nucleation and growth in such a way that gain in weight is the result. Troutner 5 reported that phosphate is effective at low concentrations. The corrosion product has been characterized by X-ray diffraction as 2AI2Oa.P~Os.3H20. According to Vermilyea, 1° phosphate is a very effective inhibitor for the corrosion of AI in neutral media. It is bound to the surface whereby a mixture of variscite and boehmite is formed. The effectiveness of phosphate is immediate as compared with silicate. Curves obtained at higher concentrations are comparable with the curves obtained for silicate beyond the minima. The constancy of corrosion rate and the increase in this rate with increase in phosphate ion concentration may be explained along the same lines as for silicate. Chromate and permanganate ions are adsorbed on the metal or oxide. As migrating A1a+ ions arrive at the oxide/solution interphase, they become oxidized to 7-A1~O3 with Cr e+ or Mn ~+ becoming reduced to Cr a+ and Mn 4+ respectively. The peak in Murgulescu'sn galvanostatic time-potential curves obtained for AI in 0.01 KCI + 0.1 K2Cr~O~,KNOa or KMnO4 is highest for dichromate and lowest for permanganate. Vermilyea and Vedder1° found that 10-6M dichromate entirely prevented hydroxide formation or weight change during 10 rain at 100°C. They ascribed this to the formation of chromic oxide or hydroxide which is highly resistant to ionic diffusion corresponding to the highest peak in Murgulescu's curves. ~ In contrast to MnOa (a reduction product of KMnOt), CroOn is strongly held to the surface. The poor protection resulting from the incorporation of MnO2 in the bulk film reveals itself in the increase of corrosion rate with increase in concentration and in the low peak in Murgulescu's curves. At higher concentration the corrosion rate is higher than in NaCI solution alone. The ennoblement of potential with increase in concentration may be due to polarization of cathodic sites where reduction takes place by tunnelling of electrons through the thin natural oxide on the A1.a The break shown by the curve at 10-s dichromate indicates that this concentration is not sufficient to protect in 10-'M C1- solutions. The action of dichromate in the presence of Cl- has been ascribed e to the fact that it is more preferentially adsorbed on AlaOa than chloride ions and hence prevents their penetration. The levelling and reversal of direction shown by the curves at 10-4 and 10-aM dichromate may indicate the progressive reduction of dichromate at cathodic sites (tunnelling of electrons through thin oxide) as well as on anodic sites (reaction with emerging A1a+ ions). This assumption is supported by the observed increase in the rate of weight-gain with increase in concentration. The causes underlying the weight-gain in dichromate on the one hand and in phosphate or silicate on the other are thus different. As regards sulphate, no weight-gain is observed under any conditions and corrosion
Corrosion of aluminium in chloride solutions containing some anions--I
643
is even e n h a n c e d with dilution. Sulphate ions m a y be a d s o r b e d on the growing oxide nuclei a n d hence lower its rate o f growth, the protective n a t u r e o f the film being thus i m p r o v e d . The sulphate ions d o n o t however enter the lattice to a n y significant degree. 7 A small p o r t i o n o f SO4 "+ m a y be a d s o r b e d at cracks in the a i r - f o r m e d film a n d hence d e l a y its healing. 1~ I n this respect it m a y simulate chloride ions. A t higher concentrations, however, this effect m a y be outweighed by the modification o f the crystal size m e n t i o n e d above.
REFERENCES 1. R. K. HART, Trans. Faraday Soc. 53, 1020 (1957). 2. J. M. BRYAN,J. Soe. Chem. Ind. (Lond.) 69, 169 (1950). 3. V. H. TROUTNER, Corrosion 15, 25 (1959). 4, B. A. WILSON, Corros. Sci. 11, 527 (1971). 5. V. H. TRoLrrNER, Corrosion 15, 23 (1959). 6. M. A. HEINE and M. J. PAYOR,J. eleetrochem. Soc. 114, 1001 (1964). 7. M. A. HEINE, D. S. KEIR and M. J. PAYog, J. electroehem. Soc. 112, 24 (1965). 8. W. VEDDEgand D. A. VERMILYEA,Trans. Faraday Soc. 65, 561 (1969). 9. R. ALWlTTand L. AgCHmALD,Corro~. Sei. 13, 687 (1973). 10° D. A. VERMILYEAand W. VEDDER,Trans. Faraday Soe. 66, 2644 (1970). 11. I. G. MUROULWCU,O, RADOVlCland S. OOL^C, Corros. Sci. 4, 353 (1964). 12. A.S.T.M. Stand. Pt. 3 (B 137-45), p. 340. 13. H. P. GODARDand E. G. TOPaUeLE, Corros. Sci. 10, 135 (1970). 14. M. BRASH~R,Br. Corros. J. 2, 100 (1967). 15. N. D. TOMASHOV,Theory of Corrosion and Protection o f Metals. Macmillan, New York (1966).
CORROSION
OF ALUMINIUM IN CHLORIDE CONTAINING SOME ANIONS--I. IN 0.01M NaC1 SOLUTIONS*
SOLUTIONS
L. A. SHALABY, K. M. EL SOBK! a n d A. A. ABDUL AZIM Laboratory of Applied Electrochemistry N.R.C., Dokki, Cairo, Egypt Abstract--A study has been made of the corrosion behaviour of A1in solutions containing0.01M NaCI alone or mixed with silicate, phosphate, dichromate, permanganate or sulphate ions covering concentration ranges of several orders of magnitude. Chloride ions become adsorbed on cracks in the pre-immersion oxide film. Silicate, phosphate and permanganate play a dual role. At small concentrations they modify the rates of nucleation and growth of the crystalline oxide layer. At higher concentrations they become incorporated in this layer in progressive amounts and hence impair its protective nature. The reduction product of dichromate improves the properties of the oxide film. At small concentrations, sulphate ions simulate chloride ions, but at higher concentrations they are adsorbed on the growing oxide nuclei hindering its growth.
INTRODUCTION
THE film formed on AI immersed in solution is duplex in nature, consisting of an inner amorphous AI2Oa and outer crystalline forms of aluminium oxides. 1-1x Layers formed on aluminium in solutions containing silicate, 4 phosphate 5 or chromate s have been separated and analysed. The aim of the present work was to compare and contrast the corrosion behaviour of AI during immersion in decinormal chloride solutions containing silicate, phosphate chromate, permanganate or sulphate anions up to 8O d. EXPERIMENTAL As-roUed 0.1 mm thick pure aluminium sheet (99.99yoAl) was cut into rectangular coupons 20 cms. Prior to use the samples were degreased for 2 h in a ternary mixture of three equal volumes of acetone, alcohol and benzene. They were then etched in a 10~NaOH for 2 rain at 50°C, rinsed with running distilled water, and with alcohol, and finally dried in filtered air. The coupons were weighed prior to immersion in 950 ml of the test solutions. After immersion the coupons were dried, stripped of the oxide film by immersion for 10 rain in a boiling solution of CrOs + I-~PO4?s After further rinsing in distilled water the coupons were dried and reweighed. A pH meter (Metrohm AG Herisau, Schweiz) was used for measuring the potentials. The electrodes used had the area I × 1 era. Before use the electrodes were prepared in the same way as described above. RESULTS Weight-loss
time curves
,.
T h e weight-loss o f A1 c o u p o n s immersed i n 10-S-lM NaCI solutions increases linearly with time over a period o f ca. 80 d, which was the longest time employed. T h e weight-loss curves o b t a i n e d for 0.01M NaCI c o n t a i n i n g 10-s-10-SM s o d i u m silicate are shown i n Fig. 1. F o r solutions o f 1 0 ~ - 1 0 - S M , the curves consist o f descending *Manuscript received 28 June 1974. 637
638
L.A.
SHALABY, K . M . EL SOBKI a n d A . A. ABDUL AZIM
and ascending linear sections which meet at the 35th day. The rate of decrease and increase in weight-loss are higher the higher the concentration of the silicate ion. The first part is not seen for solutions containing 10-eM silicate. 60
5O \
x
0 40
S•
o
.
30--
ff
_o 2 5 -
0 FIc. 1.
J 10
, 20 ,
30
I 50I, 60 40 Time, d
,g
01°
,0
The corrosion of AI in 10-2M NaCI containing different concentrations o f sodium silicate.
Figure 2 shows the results obtained for 10-2M NaCI d- 10-6-10-1M NaaPO4. At concentrations ~ 10-SM, a linear increase in weight is obtained indicating that the corrosion products are not removable by the stripping bath used. The extent and rate of increase in weight is higher for l0 -e than 10-SM phosphate. At higher concentrations the curves indicate constant corrosion rates throughout the time of the experiment. Figure 3 shows the results obtained in the presence of 10-6-10-3M K2Cr2Ov. During the first 35 d, the corrosion rate is 0.13 mg/dm2/d, irrespective of the dichromate concentration. Further changes in weight depend on concentration. The corrosion rate either increases (10-6M K2Cr20~), maintains its value (10-SM), or becomes zero for c a . l0 d and then changes its sign (10 -4 and 10-s), the rate of gain in weight being higher for the higher dichromate concentration. The weight-loss in 0.01M NaCl solutions containing 10-~-10-1M KMnO4 increases linearly with time over a period extending from 5 to 80 d. The results obtained for sulphate show that the corrosion rate which is constant over a period of two months is high at low concentration (higher than in presence of 0.01M Cl- alone) and decreases markedly with increase in SO42- ion concentration. During the period of constant corrosion rate, the following relationship (Fig. 4) holds for 0.01M NaCi or 0.01M NaC! -t- KMnO4, NaaPO4 or Na~SiOs. dW Log ~ = log K -+- n log C,
(l)
Corrosion o f alumJnium in chloride solutions conta/ning some a n J o ~ I
639
go
6(;-
4°_
j
j
JJs
>
2c
o
'c I O!- o----e~
~
,,_.
10~M o
-I¢
-4o'
--
I
o
I
~o
I
20
I
30
L
4o
Time, FIG, 2.
I
so
I
ro
60
I
so
90
d
The corrosion o f A l in 10-=M NaCI containing different concentrations o f trisodium phosphate. 80
70~ 6C-
~cE 4c-
20 10
I
o
tO
I
20
I
30
I
40
I_
I
50
Time,
60
I
70
I
80
90
lu0
d
FiG. 3, The corrosion of A.I in 10-=M NaCi containing different concentrations of potassium dichromate.
where n amounts to 0.061,0.27, 0.083 and 0.037 respectively. That a double logarithmic relationship holds between the corrosion rate and anion concentration and the fact that n is a fraction may indicate that the slow step is the adsorption of anions. 14 The corresponding straight line obtained for sulphate has a negative slope. The corrosion rates computed ~'or dichromate during the first 35 d are independent of concentration.
640
L.A.
K. M. EL SOBKI and A. A. ABDUL AZlM
SHALABY,
-0.2 -0.4
--
-0.E
--
E -0.6
--
o Pure No CL • KMnO 4 No3PO =
x No slllc6te
-I
o= ,n
g
No.
/
SO.
.e
/
.C - -
-
-I.2
-I .4 -I.6
-I.6~-0
FIG. 4.
0
1
2.O
0
I
-6
I
- 5
-4
0
I
-3 10O C
I
-2
I
-I
I
0
T h e dependence o f c o r r o s i o n rate o f AI in 10-=M N a C I upon a n i o n concentration o f the i n h i b i t o r .
Potential-log C curves
The steady-state potential E (attained after 24 h) changes with the logarithm of molar concentration, C, according to E = a -- b log C.
(2)
This relationship is shown in Fig. 5. The constant "b" amounts for a ten-fold increase of concentration to 0.07, 0.033 and 0.025 for 10-ZM NaCI or 10-~M NaCI + silicate or dichromate respectively. The corresponding straight lines obtained for NasPO4 or KMnO4 possess positive slopes. -0.1 -0.2
>
o • v x
Pure NaCt KMnO 4 Na3 P04 Na. SO= Na'Sllicafe o KzCrz0 7
-o.z~ -0.4
-~ -o.s == -0.6
-0.7 -O.~
I
I,
I
-5
-4
-3
I
I
1
-~'
-I
0
log C
FIG. 5.
The change of potential of AI in 10-2 NaCI solutions containing different concentrations (C) of inhibitor.
Corrosion of aluminium in chloride solutions containing some anions--I
641
DISCUSSION The uniform corrosion rate d W/dt may be ascribed to the absence of protective properties of the film formed on the metal surface. Alternatively the oxide layer can be divided into a very thin, continuous inner layer, which is the principal inhibitor of diffusion and hence further film growth, and an outer layer generally thicker but less continuous. The thickness of the thin, non-porous layer under unchanging conditions of temperature and chemical agents remains approximately constant, and therefore the corrosion rate. It is well-established 1-9 that the film formed on AI in water consists of a thin compact barrier film adjacent to the metal and a thicker more permeable bulk film. The former is amorphous whereas the latter consists of a crystalline hydrated oxide. The results obtained for pure chloride solution show that steady-state potentials, E, are associated with a uniform corrosion rate. The value 0.07 obtained for b in equation (2) agrees with the corresponding value reported for several metals in a variety of environments. 14 This fact leads to the conclusion that corrosion of AI in neutral chloride solutions is under anodic control, the corrosion rate being determined by the size of anodic areas (areas of breakdown of the oxide film). Pryor et al. 7 suggest that chloride ions are adsorbed on the surface and become incorporated in the film providing paths for the passage of AI a+ ions. These ions hydrolyse, leading to a lowering of pH and local anodes are thus fixed. According to Vedder and Vermilyeas the cathodic reaction occurs primarily at the metal grain boundaries where impurity segregation is results in increased electronic conductivity. In the presence of other anions, however, it is reasonable to attribute the constancy of corrosion rate to the thin continuous layer next to the metal. The magnitude of the corrosion rate is expected to depend on the solubility and composition of the bulk film. s Aiuminium reacts readily with water to form a hydrous oxide film. Growth occurs in two stages; a pseudo-boehmite film is produced initially and then this is covered with a layer of bayerite crystals. By analogy with the aging mechanism in a colloidal suspension, it is thought that the pseudo-boehmite dissolves and reprecipitates as bayerite crystals. 9 It may be suggested that the process of crystallization is influenced by the presence of certain anions such as silicate, phosphate, dichromate or permanganate. Mixed compounds serve as nucleation centres leading, probably, to the formation of small crystals and hence less porous bulk oxide layer. The results obtained for 10-4-10 -~ silicate during the 35 d indicate the gradual precipitation of a certain stable substance on the electrode surface and that the inhibiting properties of this substance are improved with time. It may be assumed that a compound of the type Na~O.AI~Oa.SiO2.H20 is formed initially at sporadic areas by a slow precipitation mechanism, z The rate of crystallization of this compound seems to be lower than the corresponding rate for bayerite. This assumption is based on the fact that the initial weight-losses amount to 6.15, 4.85 and 1.I (extrapolated to zero time) for 10-3, 10-a and 10-4 silicate respectively. The linear decrease in weightloss may be attributed to the conversion of the crystalline unstable compound into the less soluble and more compact Al~Os.SiO2.4 At the minimum, the blocking of the surface reaches its maximum efficiency. At that point the inner amorphous layer has reached its steady-state thickness and the bulk layer has attained its final porosity.
642
L . A . SHALABY, K. M. EL SOBKI and A. A. ABDUL AZIM
The fact that the minimum is markedly higher at 10-a and 10-2M silicate than at 10-4 and that the slope of the curves beyond the minimum increases with increase in concentration may indicate that the protective nature of the bulk oxide is impaired as the concentration is increased. The silicate anions seem to play a dual role. At small concentrations they improve the protective nature of bayerite by acting as nucleation centres (the corrosion rate is less than in the presence of CI- ions above). At higher concentrations, they seem to provide paths for AI a+ ions, which are, however, less conductive than those established by chloride ions. The above conclusions find support in the results obtained for phosphate additions. Concentrations as low as 10-6 or 10-e modify the rates of nucleation and growth in such a way that gain in weight is the result. Troutner 5 reported that phosphate is effective at low concentrations. The corrosion product has been characterized by X-ray diffraction as 2AI2Oa.P~Os.3H20. According to Vermilyea, 1° phosphate is a very effective inhibitor for the corrosion of AI in neutral media. It is bound to the surface whereby a mixture of variscite and boehmite is formed. The effectiveness of phosphate is immediate as compared with silicate. Curves obtained at higher concentrations are comparable with the curves obtained for silicate beyond the minima. The constancy of corrosion rate and the increase in this rate with increase in phosphate ion concentration may be explained along the same lines as for silicate. Chromate and permanganate ions are adsorbed on the metal or oxide. As migrating A1a+ ions arrive at the oxide/solution interphase, they become oxidized to 7-A1~O3 with Cr e+ or Mn ~+ becoming reduced to Cr a+ and Mn 4+ respectively. The peak in Murgulescu'sn galvanostatic time-potential curves obtained for AI in 0.01 KCI + 0.1 K2Cr~O~,KNOa or KMnO4 is highest for dichromate and lowest for permanganate. Vermilyea and Vedder1° found that 10-6M dichromate entirely prevented hydroxide formation or weight change during 10 rain at 100°C. They ascribed this to the formation of chromic oxide or hydroxide which is highly resistant to ionic diffusion corresponding to the highest peak in Murgulescu's curves. ~ In contrast to MnOa (a reduction product of KMnOt), CroOn is strongly held to the surface. The poor protection resulting from the incorporation of MnO2 in the bulk film reveals itself in the increase of corrosion rate with increase in concentration and in the low peak in Murgulescu's curves. At higher concentration the corrosion rate is higher than in NaCI solution alone. The ennoblement of potential with increase in concentration may be due to polarization of cathodic sites where reduction takes place by tunnelling of electrons through the thin natural oxide on the A1.a The break shown by the curve at 10-s dichromate indicates that this concentration is not sufficient to protect in 10-'M C1- solutions. The action of dichromate in the presence of Cl- has been ascribed e to the fact that it is more preferentially adsorbed on AlaOa than chloride ions and hence prevents their penetration. The levelling and reversal of direction shown by the curves at 10-4 and 10-aM dichromate may indicate the progressive reduction of dichromate at cathodic sites (tunnelling of electrons through thin oxide) as well as on anodic sites (reaction with emerging A1a+ ions). This assumption is supported by the observed increase in the rate of weight-gain with increase in concentration. The causes underlying the weight-gain in dichromate on the one hand and in phosphate or silicate on the other are thus different. As regards sulphate, no weight-gain is observed under any conditions and corrosion
Corrosion of aluminium in chloride solutions containing some anions--I
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is even e n h a n c e d with dilution. Sulphate ions m a y be a d s o r b e d on the growing oxide nuclei a n d hence lower its rate o f growth, the protective n a t u r e o f the film being thus i m p r o v e d . The sulphate ions d o n o t however enter the lattice to a n y significant degree. 7 A small p o r t i o n o f SO4 "+ m a y be a d s o r b e d at cracks in the a i r - f o r m e d film a n d hence d e l a y its healing. 1~ I n this respect it m a y simulate chloride ions. A t higher concentrations, however, this effect m a y be outweighed by the modification o f the crystal size m e n t i o n e d above.
REFERENCES 1. R. K. HART, Trans. Faraday Soc. 53, 1020 (1957). 2. J. M. BRYAN,J. Soe. Chem. Ind. (Lond.) 69, 169 (1950). 3. V. H. TROUTNER, Corrosion 15, 25 (1959). 4, B. A. WILSON, Corros. Sci. 11, 527 (1971). 5. V. H. TRoLrrNER, Corrosion 15, 23 (1959). 6. M. A. HEINE and M. J. PAYOR,J. eleetrochem. Soc. 114, 1001 (1964). 7. M. A. HEINE, D. S. KEIR and M. J. PAYog, J. electroehem. Soc. 112, 24 (1965). 8. W. VEDDEgand D. A. VERMILYEA,Trans. Faraday Soc. 65, 561 (1969). 9. R. ALWlTTand L. AgCHmALD,Corro~. Sei. 13, 687 (1973). 10° D. A. VERMILYEAand W. VEDDER,Trans. Faraday Soe. 66, 2644 (1970). 11. I. G. MUROULWCU,O, RADOVlCland S. OOL^C, Corros. Sci. 4, 353 (1964). 12. A.S.T.M. Stand. Pt. 3 (B 137-45), p. 340. 13. H. P. GODARDand E. G. TOPaUeLE, Corros. Sci. 10, 135 (1970). 14. M. BRASH~R,Br. Corros. J. 2, 100 (1967). 15. N. D. TOMASHOV,Theory of Corrosion and Protection o f Metals. Macmillan, New York (1966).