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Study of the dissolution of lead in nitric acid by the thermometric technique
Surface
and Coatings
Technology,
29 (1986)
STUDY OF THE DISSOLUTION THERMOMETRIC TECHNIQUE S. M. SAYED Laboratory (Received
51
51
- 58
OF LEAD IN NITRIC ACID BY THE
and H. A. EL SHAYEB of Electrochemistry
and Corrosion,
National
Research
Centre,
Cairo (Egypt)
May 17,1985)
Summary The thermometric technique was used to study the dissolution of lead in HNO,. It was found that AT and the reaction number increase almost linearly with an increase in the acid concentration to a certain limit, whereafter the reaction is inhibited by a further increase owing to the formation of a protective layer of PbO,?. The mechanism of dissolution is confirmed by the effects of some addiCll ion as HCl and tives, e.g. NOz- ion, NO3 - ion as KN03 and NH4N0,, NaCl, as well as urea. These additives were found to increase or decrease the dissolution process depending on whether they or their reaction products were involved in the dissolution reaction.
1. Introduction The rate and kinetics of dissolution of lead in HNOs solutions have been studied by a number of researchers [ 1 - 71. It was found that the rate of dissolution increases with increasing acid concentration up to a certain point, whereafter the reaction is inhibited by the formation of an adherent film of Pb(NOs)* which is insoluble in HNOs [l] and, moreover, is changed to a more protective film of PbO, at higher concentrations of the acid [ 21. Lead is readily dissolved in HNO, and produces Hz which is consumed through its reaction with HNOs and provides a path of considerable decrease in free energy. The reaction displays a certain autocatalytic character and is accelerated by the corrosion products. The thermometric technique has proved to be an effective and rapid measure for the dissolution of metals [ 8 - 131 and alloys [ 141 in a variety of attacking media and in the elucidation of corrosion inhibition by surfaceactive agents and additives [B, 11,12,14 - 191. So, it was of interest to study the reaction between lead and HNO, by this technique. In order to throw more light on the mechanism of the reaction, the effect of different additives was also studied and is discussed. 0257-8972/86/$3.50
@ Elsevier
Sequoia/Printed
in the Netherlands
52
2. Experimental
details
The procedure as well as the vessel for determining the reaction between lead and HNOs have been described previously [ 111. Lead test pieces measuring 1 cm X 10 cm were used. These test pieces were abraded with different grades of emery paper and then degreased by rubbing with a cotton cloth moistened with acetone. Each experiment was carried out with 15 ml of the acid solution and with a new lead test piece. All chemicals used were of Analytical Reagent quality and were used without further purification. The experiments were started at 19 “C. The reaction number RN is defined by RN =
T,--T,
t
AT0 = t C min ’
where T, and Ti are the maximum and initial temperatures t (min) is the time taken to reach T,.
respectively
and
3. Results and discussion The curves in Fig. 1 represent the variation in the temperature of the system with time when lead test pieces are dipped in HNOs of various concentrations. The curves show a slight increase in temperature with time until the maximum measured temperature T, is attained after a period ranging from 30 to 70 min according to the acid concentration, whereafter the temperature begins to decline slowly. It is clearly seen that T, depends also on the acid content of the medium. When lower concentrations of the acid are used, i.e. 0.8 - 3.0 M, the values of T, increase slowly and the thermometric curves are characterized by the absence of an induction period. Increasing the concentration above 3.1 M HN03 causes inhibition of the dissolution process so that at 6 M acid T, drops to a value less than that obtained by the
%__, 0
20
60
40 Time,
80
IOC
min
Fig. 1. Variation in temperature trations of HN03.
with time
for the dissolution
of lead in different
concen-
53
2
Fig. 2. Variation
3
I
L
5
Concn , M
6
in both AT and RN with the molar
concentration
of HN03.
lowest concentration used. At this higher concentration an induction period is noticed. A clear picture of the reaction between lead and HNOs is obtained when one considers the variation in RN and AT with acid concentration (Fig. 2). Both relations show two straight lines which intercept at the maximum value of RN and AT. The occurrence of a maximum followed by a minimum in the AT and RN versus concentration curves indicates that the metal dissolution is governed by two opposing effects. One favours dissolution, i.e. corrosion increases with acid concentration to a certain limit. The dissolution of lead in HNOs may occur according to the reactions Pb + 2HN03 PbO + 2HN03 -
Pb(N03) Pb(NOs)
+ H2 + H,O
(1) (2)
However, a further increase in acid concentration raises the oxidizing ability of the medium, which helps in developing and sustaining a passive film of PbO* on the surface of the metal. It is known that Pb(NOs), is changed to PbOz at CHNO, > 2.5 M [20]. The stability and thickness of this passive film formed in such solutions increases with acid concentration more rapidly than the rate of its dissolution. Therefore the overall rate of lead dissolution and consequently the RN steadily decrease. Base metals are readily attacked by HNOs forming hydrogen-rich compounds, while attack of noble metals leads to the formation of oxygen-rich compounds [21]. The attack upon the latter metals displays certain “autocatalytic” features, the reaction being accelerated by the corrosion products. The reaction between lead and HNOs has features common to both types of metals. A simple mechanism which accounts for the autocatalytic character of the attack assumes the primary displacement of H+ ions from solution: H++e--H
(3)
54
As the interaction between the hydrogen atoms and HN03 represents a path with a considerable decrease in free energy, hydrogen evolution does not occur and the acid is reduced instead: H + HNO, -
H,O + NO,
(4)
The NOz formed is assumed to adsorb onto the metal surface and to take up an electron to yield N02-. This, in strong acid solutions, gives undissociated HN02 which reacts with HN03 to give two NO2 molecules: NO2 + em H+ + NO,-
NO;?-
(5)
HNO,
HN02 + HNOs -
(6)
2N02 + H,O
(7)
This means that each NO2 species will produce two NO2 molecules, and the action becomes self-stimulating and rapid, provided that the chain of events is not broken. However, when sufficient HNOz has accumulated, other reactions are actually possible, e.g. H + HNO, -
NO + H,O
(3)
or alternatively 2HN02
-
NO, + NO + H,O
At the steady state, the complete 3H+ + HNO, + 3e- -
NO + 2H,O
(9) cathodic
reaction
is given by (10)
or 2HN03
-
NO, + NO + Hz0
(11)
which balances the anodic reaction Pb -
Pb2+ + 2e-
The covering addition reduction The and this behaviour
(12)
retardation of lead dissolution is attributed to the formation of a layer of insoluble PbO, in relatively concentrated acid solutions in to the adsorption of amines (NH, and NH,OH) formed by the of HNO, and HNO*. above mechanism is in accordance with the experimental findings is supported by the effect of some additives on the thermometric of lead in 3.1 M HNO,.
3.1. Effect of additives The increase of NOs- ion content in the reaction medium was studied through the addition of KNOJ and NH,N03 (cf. Fig. 3). In both cases, corrosion promotion increases progressively with concentration, with a consequent increase in the RN. This clearly indicates that NO,- ion is directly involved in the process of lead dissolution in HN03.
55
I 02
04
06
08
10
(a)
,
O8 ‘O O204c ,MO6
C,M
ib)
Fig. 3. Effect of KN03 and NHqCI additions (a) variation in AT; (b) variation in RN.
on the dissolution
of lead in 3 M HNOS:
It is worth mentioning that the addition of NH4N03 causes a greater acceleration for lead dissolution than that of an equimolar concentration of KNOs. This indicates that the NH4+ ion contributes to the dissolution process. Provided that all the electrolytes are completely dissociated and that the K+ ions do not interfere with the dissolution reaction, the enhancement due to the NH4+ ion can be computed, e.g. at 0.7 M additive, as follows: RNNHaNO, -
RNKNO, = 0.07
“C min-’
Corrosion promotion occurs also through the addition of NaNO, to the acid solution. The values of RN are larger than those obtained in the case of KNOs at equimolar concentration. The relatively higher RN are ascribed to higher T, and lower t. This shows clearly the participation of NO?- ion or one of its direct reaction products in the rate determining step. As can be seen in accordance with the above mechanism, the dissolution process involves HN02 . The Cl- ion is considered as one of the most harmful anions that induce and promote the corrosion of metals. It seemed of interest to examine the action of the Cl- ion on the dissolution of lead, when added as HCl and NaCl (Fig. 4). On the addition of small amounts of HCl (to the base solution of 3 M HNO,), acceleration of the dissolution process is indicated by an increase in T,. This is attributed to the increase in acidity of the solution. However, in more concentrated solutions, HCl causes retardation of the reaction. This may be explained on the basis that the increase in acidity of the solution favours the formation of Pb02 which is more resistant to dissolution. This is supported by the appearance of an induction period at such a relatively high concentration of HCl, indicating the time taken for the destruction of the Pb02 layer before the initiation of the reaction. In contrast, the addition of Cl- ion in the form of NaCl causes promotion of corrosion over the whole range of concentrations studied. This is
56
02
04
(a)
06
08
01
C,M
002
00L
006
008
01
c, M
(b)
Fig. 4. Effect of HCl and NaCl additions ation in AT; (b) variation in RN.
on the dissolution
of lead in 3 M HN03
: (a) vari-
clearly understood from the ability of lead and lead compounds to dissolve in solutions of NaCl. The dissolution of lead compounds in solutions of alkali halides has wide applicability in hydrometallurgy as “brine leaching”
[W*
The curves in Fig. 5 represent the effect of increasing concentrations of urea on the thermometric behaviour of lead dissolution in 3 M HNO,. The curves obtained exhibit an increase in the maximum temperature 7’, and a consequent increase in the RN. Apart from the mention by Saleh et al. [S] of the effect of urea on copper dissolution in HNO,, this is the first time when urea, as an additive, has been reported to increase the dissolution of metal in HNO, above that in additive-free solutions. The rise in temperature, at low urea concentration, is due to the heat evolved by the reaction between urea and HN02, the latter being one of the products in the dissolution mechanism given above. This heat is added to the heat of dissolution of the metal without additive. This explanation is in accordance with that given
Fig. 5. Effect
of urea additions
on the dissolution
of lead in 3 M HN03.
57
by Saleh et al. [8] for the dissolution of copper under similar conditions. However, they found that at higher concentrations of urea retardation of attack takes place, which is not the case for lead. Over the whole range of urea additions studied acceleration of lead dissolution is always manifested. The observed corrosion promotion of lead is explained on the premise that at the steady state the rate of production of HNOz balances the rate of its destruction [23] by the autocatalytic cycle of metal dissolution. The introduction of urea breaks that cycle, reacting with HNO, formed from reaction (6) according to (NH*)CO + 2HN02 -
CO, + 2Nz + 3H,O
(13)
Two alternative explanations can be advocated for the rise in T, caused by urea addition. Firstly, there is an increase in the heat evolved in the continuous reaction between urea and HNOz on increasing the concentration of one of the reactants (urea, for the acid concentration to remain constant). Second, urea is adsorbed onto the metal surface producing a protective layer, which prevents the formation of PbO? and permits the reaction of lead with HNOs, causing the promotion of corrosion and consequently a rise in the maximum temperature. Support for the above mechanism of lead dissolution is given by the effect of a number of additives, e.g. hydrazine and SOa2-. These do not significantly affect the dissolution process over the concentration range studied (up to 0.8 M hydrazine and 1 M SOe2-). This indicates that these additives (or any of their reaction products) are not involved in the dissolution reactions and do not affect either lead or the oxides formed. References 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
I. S. Smolyaninov, Khim. Khim. Tekhnol., 14 (1971) 222. A. A. Ravdel and G. N. Gorelik, Zh. Vses. Khim. Ova., 10 (1975) 335. A. A. Ravdel and G. N. Gorelik, Zh. Prikl. Khim., 37 (1964) 65. A. A. Ravdel and G. N. Gorelik, Zh. Prikl. Khim., 37 (1964) 275. A. A. Ravdel and G. N. Gorelik, Zh. Prikl. Khim., 37 (1964) 522. A. A. Ravdel and G. N. Gorelik, Zh. Priki. Khim., 37 (1964) 527. N. Lohonyai, J. Juhasz-Kis and I. Lipovetz, Proc. 41st Meet. of the European Corrosion Federation, 1968, 1970, p. 659. R. M. Saleh, J. M. Abd El Kader, A. A. El Hosary and A. M. Shams El Din, J. Electroanal. Chem., 62 (1975) 297. V. K. Gouda, M. G. A. Khedr and A. M. Shams El Din, Corros. Sci., 7 (1967) 221. A. M. Shams El Din and M. G. A. Khedr, MetaZoberfZache, 25 (1967) 200. K. Aziz and A. M. Shams El Din, Corros. Sci., 10 (1970) 551. J. M. Abd El Kader and A. M. Shams El Din, Corros. Sci., 10 (1970) 551. A. M. Shams El Din and M. Y. Fakhr, Corros. Sci., 14 (1974) 635. A. M. Shams El Din, A. A. El Hosary and M. M. Gawish, Corros. Sci., 14 (1976) 485. R. M. Saleh and A. M. Shams El Din, Corros. Sci., 12 (1972) 689. A. A. El Hosary, R. M. Saleh and A. M. Shams El Din, Corros. Sci., 12 (1972) 897. R. M. Saleh and A. A. El Hosary, Proc. 13th Sem. Electrochem., Karakudi, India, 1972, SAEST, Karakudi, 1972, p. 5.10. I. M. Issa, A. A. El Samahy and Y. M. Temerk, U.A.R. J. Chem., 13 (1970) 121.
58 19 20 21 22 23
I. M. Issa, M. N. H. Moussa and M. A. A. Ghandour, Corros. Sci., 13 (1973) 791. N. Hora, Znd. Health, 5 (1967) 60. U. R. Evans, Trans. Faraday Sot., 40 (1944) 120. H. A. El Shayeb, M. SC. Thesis, Cairo University, 1970. U. R. Evans, The Corrosion and Oxidation of Metals, Edward Arnold, London, 1960, p. 312.
and Coatings
Technology,
29 (1986)
STUDY OF THE DISSOLUTION THERMOMETRIC TECHNIQUE S. M. SAYED Laboratory (Received
51
51
- 58
OF LEAD IN NITRIC ACID BY THE
and H. A. EL SHAYEB of Electrochemistry
and Corrosion,
National
Research
Centre,
Cairo (Egypt)
May 17,1985)
Summary The thermometric technique was used to study the dissolution of lead in HNO,. It was found that AT and the reaction number increase almost linearly with an increase in the acid concentration to a certain limit, whereafter the reaction is inhibited by a further increase owing to the formation of a protective layer of PbO,?. The mechanism of dissolution is confirmed by the effects of some addiCll ion as HCl and tives, e.g. NOz- ion, NO3 - ion as KN03 and NH4N0,, NaCl, as well as urea. These additives were found to increase or decrease the dissolution process depending on whether they or their reaction products were involved in the dissolution reaction.
1. Introduction The rate and kinetics of dissolution of lead in HNOs solutions have been studied by a number of researchers [ 1 - 71. It was found that the rate of dissolution increases with increasing acid concentration up to a certain point, whereafter the reaction is inhibited by the formation of an adherent film of Pb(NOs)* which is insoluble in HNOs [l] and, moreover, is changed to a more protective film of PbO, at higher concentrations of the acid [ 21. Lead is readily dissolved in HNO, and produces Hz which is consumed through its reaction with HNOs and provides a path of considerable decrease in free energy. The reaction displays a certain autocatalytic character and is accelerated by the corrosion products. The thermometric technique has proved to be an effective and rapid measure for the dissolution of metals [ 8 - 131 and alloys [ 141 in a variety of attacking media and in the elucidation of corrosion inhibition by surfaceactive agents and additives [B, 11,12,14 - 191. So, it was of interest to study the reaction between lead and HNO, by this technique. In order to throw more light on the mechanism of the reaction, the effect of different additives was also studied and is discussed. 0257-8972/86/$3.50
@ Elsevier
Sequoia/Printed
in the Netherlands
52
2. Experimental
details
The procedure as well as the vessel for determining the reaction between lead and HNOs have been described previously [ 111. Lead test pieces measuring 1 cm X 10 cm were used. These test pieces were abraded with different grades of emery paper and then degreased by rubbing with a cotton cloth moistened with acetone. Each experiment was carried out with 15 ml of the acid solution and with a new lead test piece. All chemicals used were of Analytical Reagent quality and were used without further purification. The experiments were started at 19 “C. The reaction number RN is defined by RN =
T,--T,
t
AT0 = t C min ’
where T, and Ti are the maximum and initial temperatures t (min) is the time taken to reach T,.
respectively
and
3. Results and discussion The curves in Fig. 1 represent the variation in the temperature of the system with time when lead test pieces are dipped in HNOs of various concentrations. The curves show a slight increase in temperature with time until the maximum measured temperature T, is attained after a period ranging from 30 to 70 min according to the acid concentration, whereafter the temperature begins to decline slowly. It is clearly seen that T, depends also on the acid content of the medium. When lower concentrations of the acid are used, i.e. 0.8 - 3.0 M, the values of T, increase slowly and the thermometric curves are characterized by the absence of an induction period. Increasing the concentration above 3.1 M HN03 causes inhibition of the dissolution process so that at 6 M acid T, drops to a value less than that obtained by the
%__, 0
20
60
40 Time,
80
IOC
min
Fig. 1. Variation in temperature trations of HN03.
with time
for the dissolution
of lead in different
concen-
53
2
Fig. 2. Variation
3
I
L
5
Concn , M
6
in both AT and RN with the molar
concentration
of HN03.
lowest concentration used. At this higher concentration an induction period is noticed. A clear picture of the reaction between lead and HNOs is obtained when one considers the variation in RN and AT with acid concentration (Fig. 2). Both relations show two straight lines which intercept at the maximum value of RN and AT. The occurrence of a maximum followed by a minimum in the AT and RN versus concentration curves indicates that the metal dissolution is governed by two opposing effects. One favours dissolution, i.e. corrosion increases with acid concentration to a certain limit. The dissolution of lead in HNOs may occur according to the reactions Pb + 2HN03 PbO + 2HN03 -
Pb(N03) Pb(NOs)
+ H2 + H,O
(1) (2)
However, a further increase in acid concentration raises the oxidizing ability of the medium, which helps in developing and sustaining a passive film of PbO* on the surface of the metal. It is known that Pb(NOs), is changed to PbOz at CHNO, > 2.5 M [20]. The stability and thickness of this passive film formed in such solutions increases with acid concentration more rapidly than the rate of its dissolution. Therefore the overall rate of lead dissolution and consequently the RN steadily decrease. Base metals are readily attacked by HNOs forming hydrogen-rich compounds, while attack of noble metals leads to the formation of oxygen-rich compounds [21]. The attack upon the latter metals displays certain “autocatalytic” features, the reaction being accelerated by the corrosion products. The reaction between lead and HNOs has features common to both types of metals. A simple mechanism which accounts for the autocatalytic character of the attack assumes the primary displacement of H+ ions from solution: H++e--H
(3)
54
As the interaction between the hydrogen atoms and HN03 represents a path with a considerable decrease in free energy, hydrogen evolution does not occur and the acid is reduced instead: H + HNO, -
H,O + NO,
(4)
The NOz formed is assumed to adsorb onto the metal surface and to take up an electron to yield N02-. This, in strong acid solutions, gives undissociated HN02 which reacts with HN03 to give two NO2 molecules: NO2 + em H+ + NO,-
NO;?-
(5)
HNO,
HN02 + HNOs -
(6)
2N02 + H,O
(7)
This means that each NO2 species will produce two NO2 molecules, and the action becomes self-stimulating and rapid, provided that the chain of events is not broken. However, when sufficient HNOz has accumulated, other reactions are actually possible, e.g. H + HNO, -
NO + H,O
(3)
or alternatively 2HN02
-
NO, + NO + H,O
At the steady state, the complete 3H+ + HNO, + 3e- -
NO + 2H,O
(9) cathodic
reaction
is given by (10)
or 2HN03
-
NO, + NO + Hz0
(11)
which balances the anodic reaction Pb -
Pb2+ + 2e-
The covering addition reduction The and this behaviour
(12)
retardation of lead dissolution is attributed to the formation of a layer of insoluble PbO, in relatively concentrated acid solutions in to the adsorption of amines (NH, and NH,OH) formed by the of HNO, and HNO*. above mechanism is in accordance with the experimental findings is supported by the effect of some additives on the thermometric of lead in 3.1 M HNO,.
3.1. Effect of additives The increase of NOs- ion content in the reaction medium was studied through the addition of KNOJ and NH,N03 (cf. Fig. 3). In both cases, corrosion promotion increases progressively with concentration, with a consequent increase in the RN. This clearly indicates that NO,- ion is directly involved in the process of lead dissolution in HN03.
55
I 02
04
06
08
10
(a)
,
O8 ‘O O204c ,MO6
C,M
ib)
Fig. 3. Effect of KN03 and NHqCI additions (a) variation in AT; (b) variation in RN.
on the dissolution
of lead in 3 M HNOS:
It is worth mentioning that the addition of NH4N03 causes a greater acceleration for lead dissolution than that of an equimolar concentration of KNOs. This indicates that the NH4+ ion contributes to the dissolution process. Provided that all the electrolytes are completely dissociated and that the K+ ions do not interfere with the dissolution reaction, the enhancement due to the NH4+ ion can be computed, e.g. at 0.7 M additive, as follows: RNNHaNO, -
RNKNO, = 0.07
“C min-’
Corrosion promotion occurs also through the addition of NaNO, to the acid solution. The values of RN are larger than those obtained in the case of KNOs at equimolar concentration. The relatively higher RN are ascribed to higher T, and lower t. This shows clearly the participation of NO?- ion or one of its direct reaction products in the rate determining step. As can be seen in accordance with the above mechanism, the dissolution process involves HN02 . The Cl- ion is considered as one of the most harmful anions that induce and promote the corrosion of metals. It seemed of interest to examine the action of the Cl- ion on the dissolution of lead, when added as HCl and NaCl (Fig. 4). On the addition of small amounts of HCl (to the base solution of 3 M HNO,), acceleration of the dissolution process is indicated by an increase in T,. This is attributed to the increase in acidity of the solution. However, in more concentrated solutions, HCl causes retardation of the reaction. This may be explained on the basis that the increase in acidity of the solution favours the formation of Pb02 which is more resistant to dissolution. This is supported by the appearance of an induction period at such a relatively high concentration of HCl, indicating the time taken for the destruction of the Pb02 layer before the initiation of the reaction. In contrast, the addition of Cl- ion in the form of NaCl causes promotion of corrosion over the whole range of concentrations studied. This is
56
02
04
(a)
06
08
01
C,M
002
00L
006
008
01
c, M
(b)
Fig. 4. Effect of HCl and NaCl additions ation in AT; (b) variation in RN.
on the dissolution
of lead in 3 M HN03
: (a) vari-
clearly understood from the ability of lead and lead compounds to dissolve in solutions of NaCl. The dissolution of lead compounds in solutions of alkali halides has wide applicability in hydrometallurgy as “brine leaching”
[W*
The curves in Fig. 5 represent the effect of increasing concentrations of urea on the thermometric behaviour of lead dissolution in 3 M HNO,. The curves obtained exhibit an increase in the maximum temperature 7’, and a consequent increase in the RN. Apart from the mention by Saleh et al. [S] of the effect of urea on copper dissolution in HNO,, this is the first time when urea, as an additive, has been reported to increase the dissolution of metal in HNO, above that in additive-free solutions. The rise in temperature, at low urea concentration, is due to the heat evolved by the reaction between urea and HN02, the latter being one of the products in the dissolution mechanism given above. This heat is added to the heat of dissolution of the metal without additive. This explanation is in accordance with that given
Fig. 5. Effect
of urea additions
on the dissolution
of lead in 3 M HN03.
57
by Saleh et al. [8] for the dissolution of copper under similar conditions. However, they found that at higher concentrations of urea retardation of attack takes place, which is not the case for lead. Over the whole range of urea additions studied acceleration of lead dissolution is always manifested. The observed corrosion promotion of lead is explained on the premise that at the steady state the rate of production of HNOz balances the rate of its destruction [23] by the autocatalytic cycle of metal dissolution. The introduction of urea breaks that cycle, reacting with HNO, formed from reaction (6) according to (NH*)CO + 2HN02 -
CO, + 2Nz + 3H,O
(13)
Two alternative explanations can be advocated for the rise in T, caused by urea addition. Firstly, there is an increase in the heat evolved in the continuous reaction between urea and HNOz on increasing the concentration of one of the reactants (urea, for the acid concentration to remain constant). Second, urea is adsorbed onto the metal surface producing a protective layer, which prevents the formation of PbO? and permits the reaction of lead with HNOs, causing the promotion of corrosion and consequently a rise in the maximum temperature. Support for the above mechanism of lead dissolution is given by the effect of a number of additives, e.g. hydrazine and SOa2-. These do not significantly affect the dissolution process over the concentration range studied (up to 0.8 M hydrazine and 1 M SOe2-). This indicates that these additives (or any of their reaction products) are not involved in the dissolution reactions and do not affect either lead or the oxides formed. References 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
I. S. Smolyaninov, Khim. Khim. Tekhnol., 14 (1971) 222. A. A. Ravdel and G. N. Gorelik, Zh. Vses. Khim. Ova., 10 (1975) 335. A. A. Ravdel and G. N. Gorelik, Zh. Prikl. Khim., 37 (1964) 65. A. A. Ravdel and G. N. Gorelik, Zh. Prikl. Khim., 37 (1964) 275. A. A. Ravdel and G. N. Gorelik, Zh. Prikl. Khim., 37 (1964) 522. A. A. Ravdel and G. N. Gorelik, Zh. Priki. Khim., 37 (1964) 527. N. Lohonyai, J. Juhasz-Kis and I. Lipovetz, Proc. 41st Meet. of the European Corrosion Federation, 1968, 1970, p. 659. R. M. Saleh, J. M. Abd El Kader, A. A. El Hosary and A. M. Shams El Din, J. Electroanal. Chem., 62 (1975) 297. V. K. Gouda, M. G. A. Khedr and A. M. Shams El Din, Corros. Sci., 7 (1967) 221. A. M. Shams El Din and M. G. A. Khedr, MetaZoberfZache, 25 (1967) 200. K. Aziz and A. M. Shams El Din, Corros. Sci., 10 (1970) 551. J. M. Abd El Kader and A. M. Shams El Din, Corros. Sci., 10 (1970) 551. A. M. Shams El Din and M. Y. Fakhr, Corros. Sci., 14 (1974) 635. A. M. Shams El Din, A. A. El Hosary and M. M. Gawish, Corros. Sci., 14 (1976) 485. R. M. Saleh and A. M. Shams El Din, Corros. Sci., 12 (1972) 689. A. A. El Hosary, R. M. Saleh and A. M. Shams El Din, Corros. Sci., 12 (1972) 897. R. M. Saleh and A. A. El Hosary, Proc. 13th Sem. Electrochem., Karakudi, India, 1972, SAEST, Karakudi, 1972, p. 5.10. I. M. Issa, A. A. El Samahy and Y. M. Temerk, U.A.R. J. Chem., 13 (1970) 121.
58 19 20 21 22 23
I. M. Issa, M. N. H. Moussa and M. A. A. Ghandour, Corros. Sci., 13 (1973) 791. N. Hora, Znd. Health, 5 (1967) 60. U. R. Evans, Trans. Faraday Sot., 40 (1944) 120. H. A. El Shayeb, M. SC. Thesis, Cairo University, 1970. U. R. Evans, The Corrosion and Oxidation of Metals, Edward Arnold, London, 1960, p. 312.