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An electrochemical investigation of corrosive wear of electroplated nickel
wear, 30 (1974) 3 11-3 19 :(?: Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands
AN ELECTROCHEMICAL INVESTIGATION ELECTROPLATED NICKEL
OF CORROSIVE
311
WEAR OF
E. BROSZEIT, F. J. HESS and E. WAGNER l~stitut fiir Werkstof~unde, Tec~nische Hochschule Dur~stadt (Germany) (Received May 13, 1974; in final form June 20, 1974)
SUMMARY
Electrochemical studies have been chemical component in a corrosive wear results demonstrate that current, weight coefficient strongly depend on the applied
carried out to show the influence of the mechanism of electroplated nickel. The loss of the worn specimens and friction potential.
1. INTRODUCTION
Wear is a very complex phenomenon and is caused by the interaction of two surfaces in contact under load and subjected to relative motion. The attack on the metallic surfaces of a wear system is threefold: mechanical, thermal and chemical. Depending on the conditions of the environment the chemical component is of minor or major influence on the behavior of a wear system. Only under high vacuum conditions or in inert gas atmospheres can the chemical influence be neglected, and then attack on the contacting surfaces of a wear system is caused only by mechanical and thermal parameters. In most cases, however, where a reactive gaseous, liquid or solid environment (e.g. air or freon, lubricant or water or molybdenum disul~de) is present in the contact zone, the wear process is influenced or even controlled by metal~nvironment reactions. The influence of a reactive environment can have both a decreasing or increasing effect on the damage of a wear system. For example a positive influence is the action of certain lubricant additives such as metal-dithiophosphates. Under localized heavy load conditions (high local temperatures) a reaction of the dithiophosphates with the metal surface takes place resulting in the formation of an iron phosphorus coating. Thus danger offurther local metal to metal contact, local welding and adhesive wear is prevented by a temperature controlled corrosion process; a nonmetallic coating separating the metals of the rubbing partners. An example of the increasing effect of the chemical parameter on wear damage in metal couples is cylinder wear in heavy diesel engines burning sulfur containing fuel. Investigations have demonstrated1 that with a 1% sulfur content, wear increases by 4007; and with 1.3% sulfur content by 700% compared with wear with a sulfur free fuel. Despite many examples showing that much observed wear damage is the
312
E. BROSZEIT,
F. J. HESS, E. WAGNER
result of mechanical-chemical interaction at surfaces in contact under load and subjected to relative tangential slip, few papers describing the phenomenon and the mechanism’ have been published. This paper explores the possibility of investigating the influence of an agressive environment on wear by electro-chemical methods. 2. EXPERIMENTAL
The electrolytic cell used for the tests is shown in Fig. 1. The cylindrical specimen, 1 (20 mm diameter, 70 mm length), was rotated at 800 r.p.m. by a speed-controlled electric motor. The counterspecimen, 4, a sintered Al,O,-plate (10 x 10 x 30 mm) was held in an arm which also served as a means of applying the load. Small horizontal displacements of the loading arm were used to measure
Fig. I. Electrolytic Fig. 2. Current
cell for conducting
and friction
coefficient
wear experiments. US. applied
potential
under potentiokinetic
conditions.
CORROSlVE
WEAR OF NICKEL
313
friction forces by strain gauges. The electrolyte, 6, (H,SO,, pH 1) was continuously pumped through the perspex trough, 5, (contents 300 ml) from a 10 1 container and was in contact with open air. All but a 15 mm broad strip of the rotating specimen was stopped off with Scotch tape, 3. Tests were carried out at room temperature for test times of 2 h and 2.5 h. The rotating specimen, 1, constituted the working electrode. The counter electrode, 7, consisted of a platinum net fixed in the perspex trough at a radial distance from the specimen of about 30 mm, forming a half circle around the rotating cylinder. A saturated calomel electrode enabled the measurement of the potential close to the surface of the worn part of the cylindrical specimen. The presented potential values are based on the Normal Hydrogen Electrode (NHE). During the tests the resulting current, the friction coefficient and the applied potential were continuously registered on a 3-pen-recorder. After each test wear was determined gravimetrically with a mg-range balance. The coatings were produced by electroplating in a Watts bath. The composition of the nickel bath and the plating conditions are given in Table I. TABLE I COMPOSITION
OF THE WATTS BATH AND PLATING CONDITIONS
Watts-bath: 240 g/l nickel sulfate NiS04. 7H20 45 g/l nickel chloride NiCl,-6H,O 30 g/l boric acid H3B03 pH: 4.7...4.9 plating conditions: temperature: 50°C cathodic current density: 5 A/dm2 exposition time: 2iXl min (thickness of the coating: 200 pm)
3. RESULTS
3.1. Potentiokinetic conditions Initially, tests were carried out under potentiokinetic conditions with a continuously applied voltage change of 1000 mV/h, starting from &n= - 500 mV. Figure 2 shows current-voltage curves and p-value curves for the mechanical conditions nonrotating-unloaded, rotating-unloaded and rotating-loaded with different applied normal (N) loads, ranging from iV= 200 to N = 1200 g. The results indicate: (a) An increase in the current peak Jr, of the rotating specimen (n=800 r.p.m., N=O) in the anodically polarized region, compared to the nonrotating specimen (a = 0, N = 0). (b) Due to abrasive wear of the loaded specimen a further increase in the current peak J, and a broadening of the peak in the anodically polarized region is observed. The course of the value FA (area of the active region in VA units, Fig. 2, last row, left side) demonstrates this.
314
E. BROSZEIT,
F. J. HESS, E. WAGNER
(c) A dependence of the friction coefficient on applied potential and load. Figure 3 summarizes the results, showing the course of the different electrochemical values versus applied load. This figure also shows that the weight loss of the cylindrical specimen after the 2.5 h tests is far less dependent on applied load than maximum current ( JP). Under potentiokinetic conditions, however, the evidence of weight loss values is limited and tests run under these conditions can only be regarded as preliminary tests for further examination under potentiostatic conditions.
B n=O.N=O
0
200
400
F,= Area of the active reglcm Fig. 3. Electrochemical
values and weight
600
800
N Igl
SpEcinmalter test loss us. applied
load, potentiok
inetic conditions.
3.2. Potentiostatic conditions Tests under potentiostatic conditions were carried out with different applied constant voltages over a range of potential -200 to +2000 mV. During these experiments current and friction force were continuously recorded with a 2-channelstrip chart recorder. The test time was 2 h. After each test the weight loss of the cylindrical specimen was determined gravimetrically (balance 200 g maximum weight, 1 mg reading). Figure 4 summarizes the results of a series of tests with non-rotating and unloaded specimens. This graph demonstrates the course of the current at the beginning (JB) and at the end (JE) of the tests; the course of the current between beginning and end of the experiments is non-linear. For example, at ti= 800 mV
CORROSIVE WEAR OF NICKEL
315
Fig. 4. Currents and weight losses of non-rotating, unloaded specimens as function of applied potential. potentiostatic conditions.
(see Fig. 4) the test starts with high current values (about 0.8 mA) and after some minutes the current has decreased to an asymptotical value of about 0.15 mA. Figure 4 also includes the weight loss values of the purely electroche~~ally stressed specimens. From the AGk-& curve the weight loss is also a function of the applied potential. Figure 5 shows the influence of specimen rotation under unloaded conditions. While the course of the current is similar to that of non-rotating specimens in Fig. 4, the AGk-s curve shows increased weight losses in the active (200.. .400 mV) and in the transpassive region (beyond 1400 mV) as a result of specimen rotation. Figure 6 shows the influence of a normal load of N = 100 g on corrosion current, weight loss of the specimens and friction value. Abrasion causes a strong increase of passivation current and weight loss (at E= 400 mV). In the passive region, however, mechanical stress has no influence on corrosion current and weight loss (see also Fig. 7). The coefficient of friction in the active and in the transpassive region is higher than in the passive region and correlates with corrosion current and weight loss (see Figs. 6 and 7). Besides similar variation of current, weight loss and p-value as a function of the applied potential there is also a similar variation in the timedependent behavior of current p-value. The graphs on the right side of Fig. ‘7clearly verify that even short time changes of the current correspond to similar tendencies of the friction coefficient. Figures 8 and 9 give an insight into the time and load dependent behavior of the mechanically and electrochemically stressed nickel coating. Figure 8 shows changes in the corrosion currents at the beginning (JB) and at the end of the tests as a function of different applied loads in the passive region at a potential of
E. BROSZEIT,
F. J. HESS, E. WAGNER
Ni-coohg
-200 0
500
am
600 2000
opplodpotentiif ImV1Fig. 5. Currents and weight potentiostatic conditions.
losses of rotated,
unloaded
specimens
as function
of applied
potential,
E=800 mV. The current values at the start of the tests increase with increasing normal load while the current values at the end of the tests are almost independent of load. Even at very high loads in the range of N= 2000 to 3000 g the nickel coating passivates under the described conditions. In the first row of the small graphs typical examples for the current change during test time are presented. The second row gives typical examples of current-time relations for loaded wear systems in the active potential region (E= 350 and 400 mV) and in the transpassive zone (E= 1600 mV). 4. DISCUSSION
The results show clearly that under corrosive wear conditions friction and wear are considerably influenced by the electrochemical aggressivity of the “lubricat-
317 Ni-ding
PDprkd pdmthl
P h1V1
-
Fig. 6. Current, weight losses and friction coefkient N=lcKIg.
vs. applied potential, potentiostatic conditions, load
318
E. BROSZEIT,
N=ZWg.
c=ELXmV
N=3WOg.
F. J. HESS, E. WAGNER
r=BWmV
roil lm xl 0
~ LxNwml
03050
N=lEg;
r=UJhV
0 tire t lmlrdNdOOg.
r=SOmV
NdKig,
c=lKIlnN
Fig. 8. Current at beginning and end of tests KS. applied as function of test time at different loads and potentials. Fig. 9. Weight
losses and p-values
US.applied
Nlgl -
-
*___ -
load, potential
load at different
r*monN
/
.._.I
cs.zmny
p&
c= 800 mV. Courses
of current
potentials.
ing” liquid and the applied mechanical conditions (relative movement and normal load). Figure 2 shows that there is an increase in the passivating current JP and of the current in the transpassive region when the cylindrical nickel coated specimen is rotated under unloaded conditions. The increase in current is an effect of the stirring of the electrolyte which disrupts the boundary layer. In the case of mechanical stressing of the specimen surface with normal loads ranging from N = 200 to 800 g the peak of the passivating current J, increases with increasing load. Together with increasing peak height an increase of the potential for achieving passivity is observed. Thus the passive region (between active-passive transition, sP and transpassive, an) decreases with increasing normal load. Under potentiostatic conditions current and weight loss of the specimens strongly depend on applied potential and load. Concerning current values it must be
CORROSIVE
WEAR OF NICKEL
319
noted that there is also an influence of test time. Current changes during test time depend on applied voltage and load. For example under non-rotating and unloaded conditions (Fig. 4) maximum passivating current JP decreases with test time, while under loaded conditions (Fig. 6) maximum current for example at s=400 mV increases during the course of the test. In the passive region, however, e.g. at E= 800 mV (Fig. 8) the current always starts at a high level initially and decreases during the first 15 to 20 min. At the end of the test currents are very low and independent of the applied load. These high currents are evidently needed to regain passivity after the initial disruption of the first passive coating. As can be seen from Fig. 9 a change of the chemical aggressivity of the “lubricating” liquid by application of different potentials leads to pronounced changes in the behavior of the Ni-Al@-H2S0, wear system. Compared to noncorrosive reacting deionized water only under the potentiostatic conditions in the passive region of E= 800 mV the same weight loss values are reached. Tests under potentiostatic conditions in the active region, for example at E= 200 mV result in low weight losses of the nickel coated specimens when loads up to N= 1000 g are applied. At higher loads an increase in weight loss is observed. For example at N= 1500 g the interaction of abrasion and corrosion leads to a weight loss of AG= 1012 mg. In the transpassive region for example at E= 1600 mV the wear system is very sensitive to normal load. Friction values are also different at different potentials. In the passive region (E= 800 mV) mean values of p= 0.1. _.0.2 are observed in the range of normal loads from N = 100.. .3000 g while mean friction coefficients of ,u= 0.3 have been measured in deionized water, when loads between N = 500 and N = 2000 g are applied. The forming of a passive coating in the passive region (~=800 mV) seems to be responsible for the observed results. REFERENCES 1 C. C. Moore and W. L. Kent, S.A.E. Trans., 61 (1953) 244-251. 2 M. P. Sherwin, D. E. Taylor and R. B. Waterhouse, Corros. Sci., 11 (1971) 419429.
AN ELECTROCHEMICAL INVESTIGATION ELECTROPLATED NICKEL
OF CORROSIVE
311
WEAR OF
E. BROSZEIT, F. J. HESS and E. WAGNER l~stitut fiir Werkstof~unde, Tec~nische Hochschule Dur~stadt (Germany) (Received May 13, 1974; in final form June 20, 1974)
SUMMARY
Electrochemical studies have been chemical component in a corrosive wear results demonstrate that current, weight coefficient strongly depend on the applied
carried out to show the influence of the mechanism of electroplated nickel. The loss of the worn specimens and friction potential.
1. INTRODUCTION
Wear is a very complex phenomenon and is caused by the interaction of two surfaces in contact under load and subjected to relative motion. The attack on the metallic surfaces of a wear system is threefold: mechanical, thermal and chemical. Depending on the conditions of the environment the chemical component is of minor or major influence on the behavior of a wear system. Only under high vacuum conditions or in inert gas atmospheres can the chemical influence be neglected, and then attack on the contacting surfaces of a wear system is caused only by mechanical and thermal parameters. In most cases, however, where a reactive gaseous, liquid or solid environment (e.g. air or freon, lubricant or water or molybdenum disul~de) is present in the contact zone, the wear process is influenced or even controlled by metal~nvironment reactions. The influence of a reactive environment can have both a decreasing or increasing effect on the damage of a wear system. For example a positive influence is the action of certain lubricant additives such as metal-dithiophosphates. Under localized heavy load conditions (high local temperatures) a reaction of the dithiophosphates with the metal surface takes place resulting in the formation of an iron phosphorus coating. Thus danger offurther local metal to metal contact, local welding and adhesive wear is prevented by a temperature controlled corrosion process; a nonmetallic coating separating the metals of the rubbing partners. An example of the increasing effect of the chemical parameter on wear damage in metal couples is cylinder wear in heavy diesel engines burning sulfur containing fuel. Investigations have demonstrated1 that with a 1% sulfur content, wear increases by 4007; and with 1.3% sulfur content by 700% compared with wear with a sulfur free fuel. Despite many examples showing that much observed wear damage is the
312
E. BROSZEIT,
F. J. HESS, E. WAGNER
result of mechanical-chemical interaction at surfaces in contact under load and subjected to relative tangential slip, few papers describing the phenomenon and the mechanism’ have been published. This paper explores the possibility of investigating the influence of an agressive environment on wear by electro-chemical methods. 2. EXPERIMENTAL
The electrolytic cell used for the tests is shown in Fig. 1. The cylindrical specimen, 1 (20 mm diameter, 70 mm length), was rotated at 800 r.p.m. by a speed-controlled electric motor. The counterspecimen, 4, a sintered Al,O,-plate (10 x 10 x 30 mm) was held in an arm which also served as a means of applying the load. Small horizontal displacements of the loading arm were used to measure
Fig. I. Electrolytic Fig. 2. Current
cell for conducting
and friction
coefficient
wear experiments. US. applied
potential
under potentiokinetic
conditions.
CORROSlVE
WEAR OF NICKEL
313
friction forces by strain gauges. The electrolyte, 6, (H,SO,, pH 1) was continuously pumped through the perspex trough, 5, (contents 300 ml) from a 10 1 container and was in contact with open air. All but a 15 mm broad strip of the rotating specimen was stopped off with Scotch tape, 3. Tests were carried out at room temperature for test times of 2 h and 2.5 h. The rotating specimen, 1, constituted the working electrode. The counter electrode, 7, consisted of a platinum net fixed in the perspex trough at a radial distance from the specimen of about 30 mm, forming a half circle around the rotating cylinder. A saturated calomel electrode enabled the measurement of the potential close to the surface of the worn part of the cylindrical specimen. The presented potential values are based on the Normal Hydrogen Electrode (NHE). During the tests the resulting current, the friction coefficient and the applied potential were continuously registered on a 3-pen-recorder. After each test wear was determined gravimetrically with a mg-range balance. The coatings were produced by electroplating in a Watts bath. The composition of the nickel bath and the plating conditions are given in Table I. TABLE I COMPOSITION
OF THE WATTS BATH AND PLATING CONDITIONS
Watts-bath: 240 g/l nickel sulfate NiS04. 7H20 45 g/l nickel chloride NiCl,-6H,O 30 g/l boric acid H3B03 pH: 4.7...4.9 plating conditions: temperature: 50°C cathodic current density: 5 A/dm2 exposition time: 2iXl min (thickness of the coating: 200 pm)
3. RESULTS
3.1. Potentiokinetic conditions Initially, tests were carried out under potentiokinetic conditions with a continuously applied voltage change of 1000 mV/h, starting from &n= - 500 mV. Figure 2 shows current-voltage curves and p-value curves for the mechanical conditions nonrotating-unloaded, rotating-unloaded and rotating-loaded with different applied normal (N) loads, ranging from iV= 200 to N = 1200 g. The results indicate: (a) An increase in the current peak Jr, of the rotating specimen (n=800 r.p.m., N=O) in the anodically polarized region, compared to the nonrotating specimen (a = 0, N = 0). (b) Due to abrasive wear of the loaded specimen a further increase in the current peak J, and a broadening of the peak in the anodically polarized region is observed. The course of the value FA (area of the active region in VA units, Fig. 2, last row, left side) demonstrates this.
314
E. BROSZEIT,
F. J. HESS, E. WAGNER
(c) A dependence of the friction coefficient on applied potential and load. Figure 3 summarizes the results, showing the course of the different electrochemical values versus applied load. This figure also shows that the weight loss of the cylindrical specimen after the 2.5 h tests is far less dependent on applied load than maximum current ( JP). Under potentiokinetic conditions, however, the evidence of weight loss values is limited and tests run under these conditions can only be regarded as preliminary tests for further examination under potentiostatic conditions.
B n=O.N=O
0
200
400
F,= Area of the active reglcm Fig. 3. Electrochemical
values and weight
600
800
N Igl
SpEcinmalter test loss us. applied
load, potentiok
inetic conditions.
3.2. Potentiostatic conditions Tests under potentiostatic conditions were carried out with different applied constant voltages over a range of potential -200 to +2000 mV. During these experiments current and friction force were continuously recorded with a 2-channelstrip chart recorder. The test time was 2 h. After each test the weight loss of the cylindrical specimen was determined gravimetrically (balance 200 g maximum weight, 1 mg reading). Figure 4 summarizes the results of a series of tests with non-rotating and unloaded specimens. This graph demonstrates the course of the current at the beginning (JB) and at the end (JE) of the tests; the course of the current between beginning and end of the experiments is non-linear. For example, at ti= 800 mV
CORROSIVE WEAR OF NICKEL
315
Fig. 4. Currents and weight losses of non-rotating, unloaded specimens as function of applied potential. potentiostatic conditions.
(see Fig. 4) the test starts with high current values (about 0.8 mA) and after some minutes the current has decreased to an asymptotical value of about 0.15 mA. Figure 4 also includes the weight loss values of the purely electroche~~ally stressed specimens. From the AGk-& curve the weight loss is also a function of the applied potential. Figure 5 shows the influence of specimen rotation under unloaded conditions. While the course of the current is similar to that of non-rotating specimens in Fig. 4, the AGk-s curve shows increased weight losses in the active (200.. .400 mV) and in the transpassive region (beyond 1400 mV) as a result of specimen rotation. Figure 6 shows the influence of a normal load of N = 100 g on corrosion current, weight loss of the specimens and friction value. Abrasion causes a strong increase of passivation current and weight loss (at E= 400 mV). In the passive region, however, mechanical stress has no influence on corrosion current and weight loss (see also Fig. 7). The coefficient of friction in the active and in the transpassive region is higher than in the passive region and correlates with corrosion current and weight loss (see Figs. 6 and 7). Besides similar variation of current, weight loss and p-value as a function of the applied potential there is also a similar variation in the timedependent behavior of current p-value. The graphs on the right side of Fig. ‘7clearly verify that even short time changes of the current correspond to similar tendencies of the friction coefficient. Figures 8 and 9 give an insight into the time and load dependent behavior of the mechanically and electrochemically stressed nickel coating. Figure 8 shows changes in the corrosion currents at the beginning (JB) and at the end of the tests as a function of different applied loads in the passive region at a potential of
E. BROSZEIT,
F. J. HESS, E. WAGNER
Ni-coohg
-200 0
500
am
600 2000
opplodpotentiif ImV1Fig. 5. Currents and weight potentiostatic conditions.
losses of rotated,
unloaded
specimens
as function
of applied
potential,
E=800 mV. The current values at the start of the tests increase with increasing normal load while the current values at the end of the tests are almost independent of load. Even at very high loads in the range of N= 2000 to 3000 g the nickel coating passivates under the described conditions. In the first row of the small graphs typical examples for the current change during test time are presented. The second row gives typical examples of current-time relations for loaded wear systems in the active potential region (E= 350 and 400 mV) and in the transpassive zone (E= 1600 mV). 4. DISCUSSION
The results show clearly that under corrosive wear conditions friction and wear are considerably influenced by the electrochemical aggressivity of the “lubricat-
317 Ni-ding
PDprkd pdmthl
P h1V1
-
Fig. 6. Current, weight losses and friction coefkient N=lcKIg.
vs. applied potential, potentiostatic conditions, load
318
E. BROSZEIT,
N=ZWg.
c=ELXmV
N=3WOg.
F. J. HESS, E. WAGNER
r=BWmV
roil lm xl 0
~ LxNwml
03050
N=lEg;
r=UJhV
0 tire t lmlrdNdOOg.
r=SOmV
NdKig,
c=lKIlnN
Fig. 8. Current at beginning and end of tests KS. applied as function of test time at different loads and potentials. Fig. 9. Weight
losses and p-values
US.applied
Nlgl -
-
*___ -
load, potential
load at different
r*monN
/
.._.I
cs.zmny
p&
c= 800 mV. Courses
of current
potentials.
ing” liquid and the applied mechanical conditions (relative movement and normal load). Figure 2 shows that there is an increase in the passivating current JP and of the current in the transpassive region when the cylindrical nickel coated specimen is rotated under unloaded conditions. The increase in current is an effect of the stirring of the electrolyte which disrupts the boundary layer. In the case of mechanical stressing of the specimen surface with normal loads ranging from N = 200 to 800 g the peak of the passivating current J, increases with increasing load. Together with increasing peak height an increase of the potential for achieving passivity is observed. Thus the passive region (between active-passive transition, sP and transpassive, an) decreases with increasing normal load. Under potentiostatic conditions current and weight loss of the specimens strongly depend on applied potential and load. Concerning current values it must be
CORROSIVE
WEAR OF NICKEL
319
noted that there is also an influence of test time. Current changes during test time depend on applied voltage and load. For example under non-rotating and unloaded conditions (Fig. 4) maximum passivating current JP decreases with test time, while under loaded conditions (Fig. 6) maximum current for example at s=400 mV increases during the course of the test. In the passive region, however, e.g. at E= 800 mV (Fig. 8) the current always starts at a high level initially and decreases during the first 15 to 20 min. At the end of the test currents are very low and independent of the applied load. These high currents are evidently needed to regain passivity after the initial disruption of the first passive coating. As can be seen from Fig. 9 a change of the chemical aggressivity of the “lubricating” liquid by application of different potentials leads to pronounced changes in the behavior of the Ni-Al@-H2S0, wear system. Compared to noncorrosive reacting deionized water only under the potentiostatic conditions in the passive region of E= 800 mV the same weight loss values are reached. Tests under potentiostatic conditions in the active region, for example at E= 200 mV result in low weight losses of the nickel coated specimens when loads up to N= 1000 g are applied. At higher loads an increase in weight loss is observed. For example at N= 1500 g the interaction of abrasion and corrosion leads to a weight loss of AG= 1012 mg. In the transpassive region for example at E= 1600 mV the wear system is very sensitive to normal load. Friction values are also different at different potentials. In the passive region (E= 800 mV) mean values of p= 0.1. _.0.2 are observed in the range of normal loads from N = 100.. .3000 g while mean friction coefficients of ,u= 0.3 have been measured in deionized water, when loads between N = 500 and N = 2000 g are applied. The forming of a passive coating in the passive region (~=800 mV) seems to be responsible for the observed results. REFERENCES 1 C. C. Moore and W. L. Kent, S.A.E. Trans., 61 (1953) 244-251. 2 M. P. Sherwin, D. E. Taylor and R. B. Waterhouse, Corros. Sci., 11 (1971) 419429.