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1962 G. A. Oweimreen et al.: The Depletion of Dibenzyl Disulfide from a Mineral Transformer Insulating Oil

1070-9878/12/$25.00 2012 IEEE

The Depletion of Dibenzyl Disulfide

from a Mineral Transformer Insulating Oil

G. A. Oweimreen, A. M. Y. Jaber, A. M. AbulkibashKing Fahd University of Petroleum and MineralsDhahran, 312 61

Saudi Arabia

and N. A. MehannaAl-Bassam Group of Companies

Industrial Support Services Laboratories

Dammam, 31463

Saudi Arabia

ABSTRACTThis study examines the kinetics of the net depletion of dibenzyl disulfide (DBDS) from

its solution in mineral insulating oil due to copper corrosion at elevated temperaturesin power transformers. The concentration of DBDS is determined using liquid-liquid

extraction followed by GC detection. Measurements, at 24 h intervals, up to 168 h,

show that the depletion of DBDS from its solutions in mineral insulating oil at 150 C

and initial concentrations of 3.00, 6.00, 9.00, 12.00 and 15.00 mg L1 is very slight

compared to identical measurements in the presence of copper strips in the mineral oil.

The analysis of the kinetic data reflects the complexity of the corrosion process and its

occurrence at a concentration as low as 1 mg L1 which is much lower than what has

been previously believed. Measurements on solutions at initial DBDS concentrations of

150 mg L1

indicate that even when the insulation oil matrix is in contact with copper

strips the corrosion does not occur to any measurable extent below 100 C and starts to

speed up appreciably only beyond 125 C. From data at 100, 125 and 150 C an energy

of activation equal to 123 kJ mol1

is estimated.

Index Terms - Copper, corrosion sulfur, dibenzyl disulfide, mineral insulating oil,

power transformers.

1 INTRODUCTION

THERE is a great interest in detecting and studying theeffects of copper corrosion in transformers [1-8]. The

corrosiveness of mineral insulating oils to copper windings

wrapped in insulating Kraft tape in transformers and in turn

the failure of the transformers has been attributed by the above

mentioned studies to the presence of dibenzyl disulfide

(DBDS) in them. As a result there is a good deal of interest in

the determination of the concentration of DBDS in mineraltransformer oils and in particular the determination of a

threshold concentration above which DBDS becomes

corrosive to the copper windings in transformers [9-11].

Maina et al [5, 10] found that copper corrosion is visibly

observed at 150 C and a DBDS concentration above 10 mg

L1 and that the commercially available non corrosive DBDS-

free naphtenic oil became corrosive at 150 C when spiked

with DBDS concentrations in excess of 50 mg L1. Rather

than follow the copper corrosion visibly this study focuses on

measuring the change in the concentration of DBDS over

time. It finds that the depletion in DBDS concentration and in

turn the corrosion of copper associated with such depletion

continues to occur down to a DBDS concentration as low as 1

mg L1. This takes the so called threshold concentration

needed for copper corrosion to a much lower limit than has

been previously believed.

Most of the failures reported in transformers generally

occur when they are operated at elevated temperatures.

Amimoto et al [12] attribute the formation of a layer of copper

sulfide on copper strips placed in mineral insulating oils to the

presence of DBDS in these oils. They measured the deposition

rate of copper sulfide at 120, 130 and 140 C and found that it

doubled for each 10 C rise in temperature. Clearly elevated

temperatures enhance the rate of corrosion of the copper

windings in transformers which in turn depends on the rate of

depletion of DBDS from the mineral insulating oil.Manuscript received on 24 February 2012, in final form 21 April 2012.

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IEEE Transactions on Dielectrics and Electrical Insulation Vol. 19, No. 6; December 2012 1963

On heating insulating oils containing copper strips

Toyama et al [11] observed that the increase in the mass of

the copper strips due to the formation of copper sulfide is

accompanied by the formation of bibenzyl (BiBZ) and

dibenzyl sulfide (DBS) as by products. Based on their

findings they proposed a mechanism that leads to the overall

reaction given in equation (1).

4Cu (s) + (C6H5CH2)2S2 (in oil) 2 Cu2S (s) +C6H5CH2CH2C6H5 (in oil) (1)

This reaction involves the formation of the complex

(C6H5CH2)2S2-Cu2 that breaks down into Cu2S and the

radicals C6H5CH2 and C6H5CH2S

which in turn recombine

to produce C6H5CH2CH2C6H5(bibenzyl; BiBZ),

(C6H5CH2)2S (dibenzyl sulfide; DBS) and DBDS. Finally,

DBS reacts with copper, to form more Cu2S and BiBZ.

Toyama et al [4] measured the amount of Cu2S formed by

weighing the copper strips before and after their incubation

in the transformer oil. In addition to being a delicate

procedure liable to introduce somewhat large errors this

procedure does not take into account the loss in weight dueto the not well understood transformation of copper into ions

that are miscible with the mineral oil matrix and the

detection of copper in the Kraft tapes even in the absence of

copper sulfide deposits [1].

In this study the reaction rate is followed from

measurement of the variation of the DBDS concentration

with time. Based on the above mentioned mechanism [11]

the reaction steps involving DBDS are its consumption via

the formation of the intermediate complex (C6H5CH2)2S2-

Cu2(s) according to equation (2).

2 Cu(s) + (C6H5CH2)2S2(in oil)(C6H5CH2)2S2-Cu2(s) + Cu2S (s)C6H5CH2

(in oil)

+ C6H5CH2S(in oil) (2)

and its formation via the recombination of the radicals

according to equation (3).

C6H5CH2 (in oil) + C6H5CH2S

(in oil)

C6H5CH2CH2C6H5 (in oil)

+ (C6H5CH2)2S (in oil)

+ (C6H5CH2)2S (in oil) (3)

If only reaction (2) occurs one may expect the rate at

which DBDS is consumed to be first order in DBDS since

the number of sites on the surface of the copper strip is very

large relative to the low concentrations of DBDS used.

According to the mechanism above, forwarded on the basis

of experimental evidence by Toyama et al [11], a fraction

of the DBDS consumed in reaction (2) would be converted

back to DBDS via the recombination-of-radicals in reaction

(3). Such back reaction is initially slow but is likely to pick

up speed if the concentrations of the radicals increase. This

and the conversion of many sites on the copper strip into

copper sulfide tend to slow down the rate at which DBDS is

consumed and in turn give rise to a lower apparent ordern

and/or rate constant k for the depletion of DBDS. For

solutions with different initial DBDS concentrations the

rate at which radicals form and recombine would vary and

in turn influence, in a complex manner, either or both of the

order n and the rate constant k for the overall DBDS

depletion reaction.In a recent study Kato et al [13] found that the formation

of Cu2S is likely to increase with increase in either the

DBDS concentration or temperature. They defined the

incubation period as the time of the inflection point of

[the] formation rate [of Cu2S] which they found to

increase as the concentration of DBDS increases.

The measurements made in this study were analyzed

assuming a rate equation of the form,

nkdtd ]DBDS[/]DBDS[ (4)

where [DBDS] is the concentration of DBDS in mg L

1, t isthe time in hours (henceforth written h) and is the rate of

depletion of DBDS in mg L1 h

1.

2 EXPERIMENTAL PROCEDUREThe mineral insulating oil used in this study was obtained

from Gulf Chemicals and Industrial Oils Co.(Dammam,

Saudi Arabia). It is free from DBDS and the metal

passivator 1, 2. 3-Benzotriazol or its derivatives thus it

conforms to the ASTM D 3487-00 (reapproved 2006). To

minimize any effect that oxygen dissolved in the mineral oil

may have all samples from it were kept in a headspace oven

at a temperature of 70 C, shaken for 30 minutes thenpurged with argon for about one minute. The determination

of oxygen content by the ASTM D 3612 showed that this

purging process lowers the oxygen content of the oil matrix

from about 16,000 mg L1 to less than 2,000 mg L

1.

During the experiments the vials were isolated from

atmospheric oxygen using tightly screwed aluminum caps.

The depletions over time of DBDS from solutions in

mineral oil were monitored in two sets of identical samples

under the same experimental conditions except that in one

set the samples contained identical copper strips wrapped

with Kraft insulating tape (to mimic to some degree the

transformer conditions) while in the other set the copper

strips were not used.The objective from the two sets of experiments is the

differentiation between the depletion of DBDS due to its

action solely as an antioxidant and its depletion due to both

its antioxidant action and its corrosive behavior. For the

same DBDS initial concentration and the same time interval

at which the temperature of the sample was elevated

subtracting the DBDS depleted in the absence of the copper

strip from that depleted in the presence of the copper strip

allows the determination of the DBDS depleted solely due

to its corrosive action.

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Mineral oil samples (15 mL each) containing different

initial DBDS concentrations (3.00, 6.00, 9.00, 12.00 and

15.00 mg L1) were sealed in 20 mL glass headspace

vials. Henceforth in this paper these concentrations will

be referred to as 3, 6, 9, 12 and 15 mg L1. The vials were

incubated at 150 C for 24, 48, 72, 96, 120, 144 and 168 h

thus seven vials were required for each of the five initial

concentrations. At the end of each incubation period the

mineral oil sample was analyzed to determine the DBDSconcentration in it.

The analytical procedure which is described in detail

elsewhere [14] involves dissolution of the mineral oil

sample containing DBDS in an n-hexane-acetonitrile

solution, extraction of the DBDS from the acetonitrile

layer and its analysis by Gas Chromatography using

electron capture detection (ECD).

The method which has been verified to be highly

accurate and precise down to a DBDS concentration of 1

mg L-1 exceeds the lower concentration limit of 5 mg L -1

quoted in the IEC 62697 standard method [15]. The

results of this set of measurements are presented in Figure

1 and show a slight decrease in DBDS concentration overtime.

Figure 1. Linear least squares fits showing the depletion over time of

DBDS due solely to its antioxidant action in mineral oil matrices at 150

C and initial DBDS concentrations of 3, 6, 9, 12 and 15 mg L1.

As already mentioned another set of experiments was

carried out using the same conditions, but this time a

copper strip wrapped with Kraft insulating tape was

immersed in each mineral oil sample. The copper strips in

the different samples were identical. The values in Table 1

were obtained from the two sets of measurements and give

the net values that the DBDS concentrations would reach

after different incubation times if the DBDS depletion was

solely due to the corrosion process. All DBDS

concentrations

considered from this point onwards refer to these net values. The

estimated relative percent errors in the values in Table 1 range

from as high as 2 % for the measurements starting with an initial

concentration of 3 mg L-1 to as low as 0.6 % for themeasurements starting with an initial concentration of 15 mg L-1.

Figure 2. The Variation of the concentration of DBDS from solutions in

mineral oil at 150 oC and initial DBDS concentrations of 15 mg L1 in the

absence and presence of copper strips wrapped with Kraft insulation tape.

Figure 2 illustrates the depletions of DBDS from mineral oil

solutions with initial DBDS concentrations of 15 mg L-1 at

150 C with and without copper strips wrapped in insulating

Kraft tape in them. The results in Table 1 are graphically

presented, arbitrarily, as quadratic least squares fits of

[DBDS], the concentrations of DBDS in mg L1, versus time t

in h in Figure 3. The coefficients and correlation coefficients

of these quadratic fits are given in Table 2.

Except for the effect of temperature study, outlined in the next

paragraph, the results and discussion section is dedicated to

the analysis of the data given in Table 1and the results of the

quadratic fits in Table 2.

Table 1. The concentrations of DBDS in matrices of a mineral oil kept at

150 C in contact with copper strips wrapped with Kraft insulation tape

after different incubation times in h for samples of different initial DBDS

concentrations in mg L -1.

Initial DBDS

concentration

(mg L1)

DBDS concentration* in mg L-1 afterth

24 48 72 96 120 144 168

3.00 2.83 2.45 2.07 1.82 1.52 1.22 0.88

6.00 4.84 4.38 3.66 2.95 2.23 2.17 1.74

9.00 7.87 5.91 5.34 4.52 3.99 3.20 2.75

12.00 10.13 8.36 6.18 5.12 4.32 3.27 2.39

15.00 12.63 10.11 8.03 6.58 5.12 3.60 2.80

* After correcting for the depletion of DBDS in the absence of copper strips.

t(h)

t(h)

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Figure 3. Quadratic least squares fits for the variation of the concentration

of DBDS with time, due solely to the corrosion process, from solutions in

mineral oil at 150 C with initial DBDS concentrations of 3, 6, 9, 12 and

15 mg L1 and in contact with copper strips wrapped with Kraft insulation

tape.

To study the effect of temperature on the corrosive action

of DBDS its depletion from solutions of it with an initial

concentration at 150 mg L1 in contact with copper strips

wrapped in insulating Kraft tape was measured over

incubation periods of 24, 48, 72, 96 and 120 h at 50, 75,100, 125 and 150 C. The results of these measurements are

given in Table 3. They show that the corrosion process does

not occur or is extremely slow below 100 C.

A visual examination of the copper strips suggests that

copper sulfide may not start to form below 100 C even

after longer periods of time. Figures 4a and 4b contrast

copper strips recovered and examined by the IEC-62535

method after incubations in DBDS solutions in mineral oil

at initial concentrations of 150 mg L-1 at 50 and 150 C

respectively.

Temperature :

50 C

[DBDS] = 150 mg L-1

Time / h 24 48 72 96 120

IEC 62535

(4a)

Temperature :

150 C

[DBDS] = 150 mg L-1

Time / h 24 48 72 96 120

IEC 62535

(4b)

Figure 4. Copper strips tested using the IEC-62535 method after different

incubation times in a solution containing 150 mg DBDS per liter of insulation

mineral oil at (a) 50 C and (b) 150 C.

3 RESULTS

For solutions of DBDS, in mineral oil that is in contact

with a copper strip wrapped in insulating Kraft tape, started

with an initial concentration of 3 mg L1, a linear least

squares fit of [DBDS] versus incubation time t data in

Table 1 gave: 0.9984 indicating that the reaction is of zero

Table 3. The concentrations of DBDS, initially at 150 mg L 1 in

matrices of a mineral oil kept at 150 C in contact with copper stripswrapped with Kraft insulation tape, after different incubation times in

h.

Temperature / C

DBDS concentration* in mg L-1 afterth

24 48 72 96 120

50 150.0 149.6 149.6 150.0 149.1

75 149.6 149.6 148.4 148.0 148.0

100 149.6 149.6 148.4 148.0 148.0

125 148.0 145.7 142.9 140.1 136.2

150 129.6 82.2 53.1 47.1 27.9

* After correcting for the depletion of DBDS in the absence of copper strips.

Table 2. The coefficients bo, b1 and b2 and the correlation coefficients (r) in

the relation [DBDS] = bo + b1 t + b2 t2

, obtained from quadratic least

squares fits of the variation of [DBDS] for solutions with different [DBDS]o

values incubated at 150 C while in contact with copper strips wrapped with

Kraft insulation tape for times t.

[DBDS]o

/ mg L1bo b1( h

1) b2(h2) r

3 3.057700 - 0.0128266 - 4.877049 107 0.9984

6 5.966698 - 0.0399757 8.854157 10

5

0.9954

9 9.032776 - 0.0623648 1.523949 104 0.9945

12 12.134709 - 0.0923936 2.092388 104 0.9982

15 15.107721 - 0.1130118 2.364207 104 0.9995

t(h)

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order with a rate constant equal to 0.0129 mg L1 h

1. The

[DBDS] versus incubation time, t, data for the other four

solutions in Table 1 were highly curved as shown in Figure

3. The plots shown in Figure 5 are for ln {[DBDS] /

[DBDS] o} (obtained from the data for the five initial

concentrations given in Table 1) versus t, where [DBDS]o is

the initial concentration of DBDS. Intercepts (bo), slopes

(b1) and correlation coefficients (r) obtained from linear

least squares fits of ln {[DBDS] / [DBDS] o} versus t aregiven in Table 4. The somewhat low correlation coefficient

for the system with an initial DBDS concentration of 3 mg

L1 is in line with the likelihood that for this initial

concentration the reaction is of zero order. It is therefore

tempting to state that for an initial DBDS concentration of

3 mg L1 zero order kinetics is apparently followed with a

rate constant of 0.0129 mg L1 h

1 while at the higher

initial DBDS concentrations first order kinetics are

apparently followed with rate constants that average 0.009

0.002 h1.

Figure 5. Linear least squares sits for ln { [DBDS] / [DBDS]0 } versus

incubation time t in h for solutions of DBDS at initial concentrations of 3, 6,

9, 12 and 15 mg L1 in mineral oil at 150 C and in contact with copper strips

wrapped with Kraft insulation tape.

In spite of the high correlation coefficients in Table 4, Figure

6, which arbitrarily uses quadratic fits of ln{[DBDS] /

[DBDS] o} versus t, shows that the points depicted in Figure 5

fall on different slightly curved lines, that are mostly convex

upwards, with the points of the systems of initial DBDS

concentrations of 15 and 12 mg L1 lowest and close to each

other, above them are the points for the systems of

initialDBDS concentrations of 9 and 6 mg L1 which are

also somewhat close together, and above these are thepoints for the system with an initial DBDS concentration of

3 mg L1. These 3 sets of lines/pairs-of-lines are not

parallel which indicates that the slopes change and in turn

the rate changes with both the initial concentrations and the

time that elapses.

Figure 6. Quadratic least squares fits for ln { [DBDS] / [DBDS] 0 } versus

incubation time t in h for solutions of DBDS at initial concentrations of 3, 6,

9, 12 and 15 mg L1 in mineral oil at 150 C and in contact with copper strips


From the results of the quadratic fits given in Table 2

[DBDS] at time t, and in turn ln [DBDS] at time t, may be

calculated from,

221]DBDS[ tbtbb (5)

and at that same time , and in turn ln , may be calculated

from,

tbb

dt

DBDSd

221

][ (6)

Using this approach the values given in Table 5 were

obtained. Equation (4) may be written in the form,

]DBDS[lnlnln nk (7)

Table 4. The intercepts (bo), slopes (b1) and correlation coefficients (r)

from linear least squares fits of ln {[DBDS] / [DBDS] o} versus incubationtimes tin h for DBDS solutions at different initial concentration ([DBDS]o)

in mineral oil at 150 C and in contact with a copper strip wrapped with

Kraft insulation tape.

[DBDS]o / mg L1 intercept k/ h1 =slope / h1 R

3 0.109 0.00715 0.9834

6 0.00230 0.00742 0.9935

9 0.00760 0.00704 0.9961

12 0.0530 0.00952 0.9967

15 0.0743 0.01009 0.9960

t(h)

t(h)

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Table 5. ln [DBDS] and ln values at different times in h for systems of solutions of DBDS in mineral oil in contact with copper wrapped in Kraft insulation

tape and starting with initial DBDS concentrations of 3, 6, 9, 12 and 15 mg L1.

Time / h 3.00 mg L1 6.00 mg L1 9.00 mg L1 12.00 mg L1 15.00 mg L1

ln [DBDS] ln ln [DBDS] ln ln [DBDS] ln ln [DBDS] ln ln [DBDS] ln

0 1.11766 4.35623 1.78619 3.21948 2.20086 2.77475 2.49607 2.38170 2.71521 2.18026

24 1.01145 4.35441 1.62103 3.33188 2.03128 2.89952 2.30636 2.49678 2.52825 2.28608

48 0.89237 4.35259 1.44736 3.45854 1.85479 3.04209 2.10192 2.62684 2.32511 2.40445

72 0.75690 4.35077 1.26623 3.60359 1.67383 3.20843 1.88207 2.77638 2.10370 2.53872

96 0.59985 4.34896 1.08012 3.77332 1.49296 3.40807 1.64736 2.95229 1.86213 2.69387

120 0.41309 4.34715 0.89389 3.97786 1.32002 3.65777 1.40131 3.16590 1.59954 2.87758

144 0.18279 4.34534 0.71598 4.23528 1.16699 3.99134 1.15335 3.43788 1.31813 3.10281

168 0.11758 4.34354 0.55949 4.58285 1.04966 4.49541 0.92352 3.81266 1.02674 3.39399

thus if the reaction has an order a double logarithmic plot

of ln versus ln [DBDS] would be a straight line of slope

equal to the ordern and an intercept with the ln axis equal

to ln k. If a straight line is not obtained the reaction does

not have a clear cut order with respect to DBDS. This

method cannot of course be used to accurately determine k

because of the error associated with the extrapolation to ln

[DBDS] = 0.

This procedure can be applied in two different ways [16]. The

first approach involves several runs at different initial (t = 0)

concentrations (as has been done in this study) and the initial

rates are the initial slopes. The order of the reaction is then

obtained from a double logarithmic plot of ln versus ln

[DBDS]. Because this procedure avoids possible complications

from products and/or back reactions the order obtained from it is

referred to as the true order or order with respect to

concentration, nc. A linear least squares fit of ln versus ln

[DBDS]o at t = 0 (i.e., the data in the first row in Table 5) gave a

slope (i.e., an nc value) of 1.36 (first row in Table 6).

In this paper we extend this approach to cover presumed

initial concentrations for other times tduring the reaction.

If no complications arise from products and/or back

reactions, as the reaction proceeds, similar plots of ln

versus ln [DBDS] when concentrations at other times tare

considered to be initial concentrations will be expected to

be linear and give slopes (now referred to as nc) having

the same value as nc. The results of linear least squares

fits for ln versus ln [DBDS] where the concentrations

after 24, 48 and 72 h (rows two, three and four in Table 5)

are considered initial concentrations gave very good

correlation coefficients (in excess of 0.994) and nc values

close to nc and averaging 1.35 (Table 6). This finding

points to the absence of interference from products for up

to about 72 h. Of these fits the plot for t = 72 h is

selected for Figure 7a because it shows the onset of a

departure from linearity at the point corresponding to the

run for 9 mg L1 at t = 0. The situation is more drastic

when the concentrations after 96, 120 and 144 h (rows

five, six and seven in Table 5 and Figures 7b-7d) were

taken as initial concentration. The data for the systems with

Figure 7. ln Versus ln [DBDS] values after (a) 72 h, (b) 96 h, (c) 120 h

and (d) 144 h during runs at initial concentrations of 3, 6, 9, 12 and 15 mg L1.

concentrations of 15 and 12 mg L1

at t= 0 fell on lines of

slopes about 1.2, 1.5 and 2.0 when the concentrations after 96,

120 and 144 h respectively were assumed to be initial

concentrations. On the other hand the data systems with

concentrations of 9, 6 and 3 mg L1 at t= 0 fall on lines of

lower slopes of about 1.1, 0.8 and 0.4 when the concentrations

after 96, 120 and 144 h respectively were assumed to be initial

concentrations. Clearly after 72 h the reaction products start

to influence the rate of DBDS depletion and such influence

depends on the initial concentration of DBDS. Additionally

there seems to be a change in the kinetic behavior associated

with the system where the DBDS concentration at t= 0 is in

the vicinity of 9 mg L1. No pattern can be discerned from the

results for t = 168 h (row 8 in Table 5); they are far too

scattered as is reflected by the very poor correlation

coefficient given in Table 6.

(b) t= 96 (h)(a) t= 72 (h)

(c) t= 120 (h) (d) t= 144 (h)

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Table 6 Results from linear least squares fits for ln versus ln [DBDS] at the

same times tin h during runs with different initial DBDS concentrations (i.e.,

rows in Table 5) in mineral oil at 150 C in contact with a copper strip


Time / h Intercept Slope= cn (or 'cn )a

Correlation

coefficient (r)

0 5.74891 1.35829 0.9941

24 5.65431 1.35843 0.9953

48 5.51114 1.35372 0.9966

72 5.35166 1.34812 0.9962

96 5.16784 1.29634 0.9876

120 4.94466 1.18999 0.9491

144 4.67284 0.937033 0.8137

168 4.42633 0.43675 0.4238

a only the slope in the first row is equal to cn .

The second procedure [16] involves considering a single

run and measuring slopes at the different times and theircorresponding reactant concentrations. Again the logarithms

of the rates are plotted against the logarithms of the

corresponding reactant concentrations. The slope is also the

order but since now time is varying it is referred to as the

order with respect to time, nt. Such plots for runs at initial

concentrations of 3 (), 6 (), 9 (), 12 () and 15 () mg

L1 are shown in Figure 8. The points for each of the

concentrations in Figure 8 correspond to the data in the pairs

of columns under these concentrations in Table 5, The plot for

an initial concentration of 3 mg L1 gave a slope very close to

zero which is indicative of zero order and the correlation

coefficient of the linear fit is 0.9844. The results for runs at

initial concentrations of 6, 9, 12 and 15 mg L

1

are curved andconvex upwards indicating that the order increases as the

reaction progresses and [DBDS] decreases. Because the plots

hint at an appreciable change in slope in the vicinity of 96 h

three different linear least squares fits were obtained; one

covering all the times used, one covering the times up to 96 h

and one for 120, 144 and 168 h. The results of these fits are

given in Table 7. Figure 9 is provided as a typical

representation of fits from zero to 96 h and between 120 and

168 h (row 3 of Table 7) for an initial concentration of 9 mg

L1. Similar fits are obtainable for the other initial

concentrations in Table 7.

Figure 8. ln versus ln [DBDS] at different times for runs with initial

DBDS concentrations of 3(), 6(), 9(), 12 () and 15 () mg L1 .

Figure 9. Linear least squares fits for ln versus ln [DBDS] for the run at

an initial DBDS concentration of 9 mg L1. The lower line is for 120, 144

and 168 h after the start of the reaction while the upper line is for times up to

72 h after the start of the reaction. The data for these fits are in row 3 of Table

7.

Table 7. Slopes 1b , intercepts b and correlation coefficients r from linear least squares fits of ln versus ][ln DBDSat different times t in h,during runs with the same initial concentration (i.e., columns in Table 5) for systems of solutions of DBDS in mineral oil in contact with copper wrapped with

Kraft insulation tape.

[DBDS]o / mg L1

All data up to 168 h* Data up to 96 h* Data for 120, 144 and 168 h

1b ** b r 1b ** b r 1b ** b r

3 0.010 4.3436 0.9841 0.014 4.3403 0.9969 0.007 4.3443 0.9970

6 1.054 5.0074 0.9804 0.782 4.6032 0.9983 1.801 5.5674 0.9925

9 1.360 5.6087 0.9580 0.889 4.7126 0.9967 3.053 7.6470 0.9814

12 0.869 4.4670 0.9857 0.671 4.0466 0.9991 1.350 5.0377 0.9936

15 0.703 4.0440 0.9951 0.601 3.8081 0.9997 0.902 4.3109 0.9980

* At 24 h intervals. ** The slope 1b is equal to the order with respect to time, tn .

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4 CONCLUSION

This study shows that at operational temperatures below

100 C, copper conductors inside transformer units will not

react to form copper sulfide even when the DBDS

concentration in the mineral oil matrix is 150 mg L-1. It also

shows that in the absence of a copper strip the depletion rate

of DBDS from its solution in the mineral transformer oil at a

temperature as high as 150 C is small but increases

appreciably in the presence of a copper strip wrapped with

insulating Kraft tape even at an initial DBDS concentration as

low as 3 mg L1. That the DBDS concentration in this system

dropped to 0.88 mg L1

after 168 h is indicative that some

degree of corrosion is liable to occur at any DBDS

concentrations and the concept that there is a threshold

concentration below which corrosion does not occur needs to

be reconsidered. All discussions henceforth are for systems of

DBDS in mineral oil at 150 C and in contact with copper.

For the run at an initial DBDS concentration of 3 mg L1

the plot of [DBDS] versus time was highly linear indicating

zero order in DBDS with a rate constant of 0.0129 mg L1

h1. Similar plots for the higher initial DBDS concentrations

were nonlinear. Anticipating that the depletion rate may be

first order in DBDS linear least squares fits were obtained

(Table 4 and Figure 5) for ln {[DBDS] / [DBDS]o} versus

incubation times t in h for all runs. These fits had reasonably

high correlation coefficients yielding the apparent first order

rate constants given in Table 4. The runs at initial DBDS

concentrations of 3, 6 and 9 mg L1 yielded k values

averaging 0.0073 h1 while the runs at initial DBDS

concentrations of 12 and 15 mg L1 yielded k values

averaging 0.0098 h1. The strong overall evidence that the

run at an initial DBDS concentration of 3 mg L1 is of zero

order and the lack of firm evidence that the jump from

0.0073 h1 to 0.0098 h1 is outside the precision of the

experiment leads us to assume an apparent k value in therange from 0.0070 to 0.0085 h

1. A close examination of

Figure 5 shows that the fits for ln {[DBDS] / [DBDS]o}

versus incubation times t are somewhat curved. That this is

the case is seen in the improvement in the correlation

coefficients when quadratic least squares fits were obtained

for ln {[DBDS] / [DBDS]o} versus incubation times t

(Figure 6) for the same systems. This and the reaction

mechanism for the depletion of DBDS proposed by Toyama

et al [4] which involves the formation of DBDS via a back

reaction suggest that the kinetics are more complex than that

of a simple first order reaction.

To investigate the kinetics of these systems further

quadratic least squares fits of [DBDS] versus t (Figure 3 andTable 2) were used, as described in the results and

discussion section, to obtain Table 5 which gives ln and ln

[DBDS] at different times during runs at different initial

concentrations. Again as described in the result and

discussion section the values in rows in Table 5 were used to

obtain the orders nc and nc with respect to concentration

(Table 6 and Figures 7a 7d) and the values in columns

were used to obtain the orders nt with respect to time

(Table 7 and Figures 7 and 8). Table 6 shows that nc = 1.36

and that the nc values for 24, 48 and 72 h are practically

constant and averaging 1.35. Beyond 72 h nc decreases

with time until it reaches the value of 0.44 at t = 168 h.

Associated with this decrease are corresponding decreases in

the correlation coefficient r which are, as Figures 7a to 7d

clearly show, due to a change in the order and possibly rate

coefficient value after 72 h for initial DBDS concentrations

of 9, 12 and 15 mg L1. Table 7 shows that nt is zero for

runs where the initial DBDS concentration is 3 mg L1,

which is in harmony with the zero order deduced from thelinearity of the [DBDS] versus t plot. For the remaining

initial concentrations different nt values are obtained in the

range 0 to 96 h and the range from 120 to 168 h. In the 0 to

96 h range nt is at its highest value of 0.9 for the run at an

initial DBDS concentration of 9 mg L1. In the range

covering t values of 120, 144 and 168 h nt is at its highest

value of 1.8 for the run at an initial DBDS concentration of

9 mg L1. Figure 9 illustrates the different nt values in these

time ranges for the run at an initial DBDS concentration of 9

mg L1

.

Clearly the kinetics for the depletion of DBDS in mineral

insulating oil in contact with copper are quite complex and

the order varies with both the initial concentration of DBDSas well as the duration of the reaction. The lower values of

nt relative nc and, after 72 h, the lower values ofnc relative

to nc indicate that as the reaction proceeds the rate of

depletion of DBDS falls of less rapidly than is expected

from the true order. This finding is in agreement with the

regeneration of DBDS via the recombination of radicals in

reaction (3) suggested in the mechanism forwarded by

Toyama et al [4]. It may also be attributed to different ways

of forming the intermediate complex (C6H5CH2)2S2-Cu2. For

example this complex may form via the interaction of DBDS

with either the copper surface or the copper ions detected in

the mineral oil [1]. The finding of copper ions in the oil

matrix is to this date unexplained but one may conjecturethat it happens via dissolution within cages formed by

polar molecules in the mineral oil matrix.

Assuming first order kinetics the results for initial DBDS

concentrations of 150 mg L1 at 100, 125, and 150 C mg L

1

given in Table 3 yielded, respectively, k values of 1.344

104, 8.556 104 and 0.0151 h1. From the slope of a linear

fit of ln kversus 1/T the energy of activation for the depletion

of DBDS at 150 C was estimated to be 123 kJ mol1.

ACKNOWLEDGMENT

The authors thank King Fahd University of Petroleum and

Minerals, The Industrial Support Services Laboratories andGulf Chemical and Industrial Oils for their support of this

research project.

REFERENCES[1] L. Lewand, The Role of Corrosive Sulfur in Transformers and

Transformer Oil, 69th Annual Intl. Conf. Doble Clients, Boston, MA,

Insulating Materials Session, Copyright Doble Engineering Company, pp.

1-15, 2002.

[2] L. Lewand and R. Scott, Destruction of Dibenzyl Disulfide in

Transformer Oil, 75th Annual International Conf. Doble Clients, Boston,

MA, Insulating Materials Session, Copyright Doble Engineering

Company, pp. 1-20, 2008.

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[3] V. Tumiatti, R. Maina, F. Scatiggio, M. Pompili and R. Bartnikas,

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Bartnikas,Corrosive Sulfur in Insulating Oils: Its Detection and

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23, pp. 508-510, 2008.

[5] R. Maina, V. Tumiatti and R. Bartnikas, Corrosive Sulfur Effects in

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Insul., Vol. 16, pp. 1655-1663, 2009.[6] F. Scatiggio, V. Tumiatti, R. Maina, M. Pompili and R. Bartnikas,

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Transformers and Shunt Reactors, IEEE Trans. Power Delivery, Vol.

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[7] F. Scatiggio, M. Pompili and R. Bartnikas, Oils with Presence of

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Conf. (EIC), Montreal, Canada, pp. 478-481, 2009.

[8] V. Tumiatti, M. Pompili and R. Bartnikas, Dielectric Loss

Characteristics of Copper-Contaminated Transformer Oils, IEEE

Trans. Power Delivery, Vol. 25, pp. 1673-1677, 2010.

[9] CIGRE WG A2-32, Copper Sulfide in Transformer Insulation, Final

report brochure 378, 2009.

[10] R. Maina, F. Scatiggio, S. Kapila, V. Tumiatti, M. Tumiatti and M.

Pompili, Dibenzyl Disulfide (DBDS) as Corrosive Sulfur

Contaminant in Used and Unused Mineral Insulating Oils, presented

at the CIGRE Task Force A2-32.10, Paris, France, 2007.[11] S. Toyama, J. Tanimura, N. Yamada and E. Nagao, Highly Sensitive

Detection Method of Dibenzyl Disulfide and the Elucidation of the

Mechanism of Copper Sulfide Generation in Insulating Oil, IEEE

Trans. Dielectr. Electr. Insul., Vol. 16, pp. 509-515, 2009.

[12] T. Amimoto, E. Nagao, J. Tanimura, S. Toyama, Y. Fujita, H. Kawarai

and N. Yamada, Identification of Affecting Factors of Copper Sulfide

Deposition on Insulating Paper in Oil, IEEE Trans. Dielectr. Electr.

Insul., Vol. 16, pp. 265-272, 2009.

[13 ] F. Kato, T. Amimoto, E. Nagao, N. Hosokawa, S. Toyama and J.

Tanimura, Effect of DBDS Concentration and Heating Duration on

Copper Sulfide Formation in Oil-immersed Transformer Insulation,

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[14]. A. Jaber, N. Mehanna and A. Abulkibash Simultaneous Liquid-Liquid

Extraction of Dibenzyl Disulfide, 2,6-di-tert-butyl-p-cresol and 1,2,3-

benzotriazole from Power Transformer Oil Prior to GC and HPLC

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Corrosive Sulfur Compounds in Used and Unused Insulating Liquids

Part 1: Test Method for Quantitative Determination of Dibenzyl

disulfide (DBDS), IEC, ed. 01, 2010.

[16] K. Laidler, Chemical Kinetics, 3rd. ed., Longman, NewYork, USA,

pp. 1821, 1987.

G. A. Oweimreen was born in Jordan on

September 25, 1942. He received the B.Sc. (1970)

and M.Sc. (1972) degrees from the American

University in Cairo, and the Ph.D. (1977) degree

from Georgetown University in Washington, D.C.,

U.S.A. After a year of postdoctoral research at the

University of Athens, Georgia he joined the

chemistry faculty at King Fahd University of

Petroleum and Minerals. He has broad teaching

and research interests in physical chemistry with

emphasis on solution thermodynamics of non-electrolyte solutions.

A. M. Abulkibash is Jorddanian. He received the

Ph.D. degree from Exeter University, U.K., in 1979

and joined King Fahd University of Petroleum and

Minerals in 1980. His research interests include

electro-analytical techniques especially polarized

electrodes, corrosion inhibition, electroplating and

applications of nanotechnology for the removal of

heavy metals from aqueous solutions.

A. M. Y. Jaber was born in Palestine in 1943. Hereceived the B.Sc. degree from Alexandria

University, Egypt, in 1967 and the Ph.D. degree from

the University of Wales Institute of Science and

Technology (currently Cardiff University), U.K. in

1977. Currently, Dr. Jaber is a Professor of

Analytical Chemistry at the Faculty of Pharmacy,

Philadelphia University-Jordan. His research interests

include electrochemical, spectroscopic and separation

methods of analysis in the pharmaceutical, environmental, petroleum and

petrochemical area.

N. A. Mehanna was born in Saudi Arabia on 27

November in 1973. He received the B.Sc. degree in

Syria in 1996 and the M.S. and PhD. degrees inanalytical chemistry from King Fahd University of

Petroleum and Minerals, KFUPM, in 2007 and 2011,

respectively. He joined Industrial Support Services

Laboratories, Al-Bassam Group of Companies, in 2001

and has been engaged in the research and testing of

Lubricating and insulating oils.

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