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The breakdown of passivity on mild steel
Corrosion Science, Vol. 33, No. 2, PP. 315-316, 1992 Printed in Great Britain.
THE BREAKDOWN J. A. Department
of Materials
0010-938X/92 $5.00 + 0.00 0 1991 Pergamon Press plc
OF PASSIVITY and P. A.
DAVIES
Engineering,
University
Abstract-The voltage required for breakdown of passivity and halide concentration. A relation of the type
ON MILD STEEL BROOK
of Nottingham,
Nottingham,
on mild steel was measured
U.K.
as a function
of pH
Ebr = A + B x log ([Halide]/[OH-1) was found where A and B are constants. At constant pH and constant halide evidence that the value of Ebr increases from chloride to bromide to iodide.
concentration,
there
is
INTRODUCTION
ample evidence’ that halides, and in particular chloride, shift the value of the breakdown potential, Ebr, to more negative values. Earlier work from this laboratory2 has shown that the values of Ebr in the presence of chloride ions are related by a logarithmic relation to both the chloride concentration and the hydroxyl ion concentration. The results were explained in terms of competitive adsorption of the two ions present. Lee3 obtained similar relations between pH, chloride concentration and inhibitor concentration. In this work, the effects of bromide and iodide ions on E,_ are reported.
THERE
IS
EXPERIMENTAL
METHOD
The test anodes were annealed mild steel and the current-density described earlier.2 In order to increase the conductivity of the solutions, potassium perchlorate.
EXPERIMENTAL
RESULTS
AND
vs potential were obtained as the base electrolyte was =O.Ol M
DISCUSSION
The values of Ebr were plotted against the log ([Halide-]/[OH-1) and the results are shown in Fig. 1. The data were analysed by first obtaining the gradient, taking the points as one data set, and then calculating the intercepts taking the results for each halide separately. The values of the intercept, namely -0.0153, -0.0146 and -0.012 V for Cl-, Br- and I- respectively, are very similar to the value, -0.015 V, found previously.* However, the value of the gradient, -0.013 V, is different to that previously reported. It is possible to explain the differences in the gradients in terms of the perchlorate ions present in the solutions used here. The bulky perchlorate ions could be expected to interfere with the adsorption of both the halide ions responsible for breakdown and the hydroxyl ions producing the passivating oxide films. Lee3 has shown that, Manuscript
received
7 May 1991; in revised form 14 June 1991. 315
Short communication
316
-0.25
0.25
FIG. 1.
Breakdown
1.25
0.75
Log
1.75
( Ratio of Halide to Hydmtide )
voltage
against
ratio of halide to hydroxide.
when there are two synergistic inhibitors in a system, the relation between E,, and the concentrations of the species in solution is modified to Ebr = A + B x log ([Halide]/[OH-]
x fi [IJ) i=l
where Ii are the inhibitors used. By replacing the terms for the inhibitor concentrations with a term for the perchlorate concentration in the equation above,4 assuming that there is a simple relation between the perchlorate concentration and the ratio (Halide]/[OH-] and re-arranging, an equation is obtained of the same form as that obtained for the system in the absence of the perchlorate ion but with the pre-logarithmic term changed to include a factor for the perchlorate concentration. The results shown in Fig. 1 indicate that, at constant pH, the halide concentration required to cause breakdown increases in the order Cl- < Br- < I-. REFERENCES 1. J. M. WEST, Electrodeposition and Corrosion Processes. Van Nostrand. London 2. H. E. H. BIRD, P. A. BROOK and B. R. PEARSON, Corros. Sci. 28,81 (1988). 3. C. M. LEE, PhD Thesis, Nottingham University (1991). 4. H. P. LECKIE and H. H. UHLIG, J. electrochem. Sot. 113, 1262 (1966).
(1971).
THE BREAKDOWN J. A. Department
of Materials
0010-938X/92 $5.00 + 0.00 0 1991 Pergamon Press plc
OF PASSIVITY and P. A.
DAVIES
Engineering,
University
Abstract-The voltage required for breakdown of passivity and halide concentration. A relation of the type
ON MILD STEEL BROOK
of Nottingham,
Nottingham,
on mild steel was measured
U.K.
as a function
of pH
Ebr = A + B x log ([Halide]/[OH-1) was found where A and B are constants. At constant pH and constant halide evidence that the value of Ebr increases from chloride to bromide to iodide.
concentration,
there
is
INTRODUCTION
ample evidence’ that halides, and in particular chloride, shift the value of the breakdown potential, Ebr, to more negative values. Earlier work from this laboratory2 has shown that the values of Ebr in the presence of chloride ions are related by a logarithmic relation to both the chloride concentration and the hydroxyl ion concentration. The results were explained in terms of competitive adsorption of the two ions present. Lee3 obtained similar relations between pH, chloride concentration and inhibitor concentration. In this work, the effects of bromide and iodide ions on E,_ are reported.
THERE
IS
EXPERIMENTAL
METHOD
The test anodes were annealed mild steel and the current-density described earlier.2 In order to increase the conductivity of the solutions, potassium perchlorate.
EXPERIMENTAL
RESULTS
AND
vs potential were obtained as the base electrolyte was =O.Ol M
DISCUSSION
The values of Ebr were plotted against the log ([Halide-]/[OH-1) and the results are shown in Fig. 1. The data were analysed by first obtaining the gradient, taking the points as one data set, and then calculating the intercepts taking the results for each halide separately. The values of the intercept, namely -0.0153, -0.0146 and -0.012 V for Cl-, Br- and I- respectively, are very similar to the value, -0.015 V, found previously.* However, the value of the gradient, -0.013 V, is different to that previously reported. It is possible to explain the differences in the gradients in terms of the perchlorate ions present in the solutions used here. The bulky perchlorate ions could be expected to interfere with the adsorption of both the halide ions responsible for breakdown and the hydroxyl ions producing the passivating oxide films. Lee3 has shown that, Manuscript
received
7 May 1991; in revised form 14 June 1991. 315
Short communication
316
-0.25
0.25
FIG. 1.
Breakdown
1.25
0.75
Log
1.75
( Ratio of Halide to Hydmtide )
voltage
against
ratio of halide to hydroxide.
when there are two synergistic inhibitors in a system, the relation between E,, and the concentrations of the species in solution is modified to Ebr = A + B x log ([Halide]/[OH-]
x fi [IJ) i=l
where Ii are the inhibitors used. By replacing the terms for the inhibitor concentrations with a term for the perchlorate concentration in the equation above,4 assuming that there is a simple relation between the perchlorate concentration and the ratio (Halide]/[OH-] and re-arranging, an equation is obtained of the same form as that obtained for the system in the absence of the perchlorate ion but with the pre-logarithmic term changed to include a factor for the perchlorate concentration. The results shown in Fig. 1 indicate that, at constant pH, the halide concentration required to cause breakdown increases in the order Cl- < Br- < I-. REFERENCES 1. J. M. WEST, Electrodeposition and Corrosion Processes. Van Nostrand. London 2. H. E. H. BIRD, P. A. BROOK and B. R. PEARSON, Corros. Sci. 28,81 (1988). 3. C. M. LEE, PhD Thesis, Nottingham University (1991). 4. H. P. LECKIE and H. H. UHLIG, J. electrochem. Sot. 113, 1262 (1966).
(1971).