Test methods for quantitative determination of corrosive sulfur compounds in unused and used insulating liquids - Part 2: Test method for quantitative determination of total corrosive sulfur (TCS)

IEC TR 62697-2:2018(E) specifies a test method for the quantitative determination of total corrosive sulfur (TCS) in unused and used insulating liquids and solid matrices through the conversion of corrosive sulfur species to metal (copper, silver etc.) sulfides. The sulfides formed are quantitatively converted to sulfates; sulfates are determined through turbidity measurement or with ion chromatography.
The method is applicable with the following matrices:
a) Unused and used insulating liquids, for example mineral insulating oils and natural esters, which allow the determination of corrosive sulfur compounds over concentrations ranging between 2,5 mg kg-1 to 80 mg kg-1 TCS.
b) Solid matrices that come in contact with the insulating liquid, for example insulating papers in electrical equipment. The quantification limits for these matrices depend on the amount of matrix used during the determination. The method can be used for the quantitative or semi-quantitative determination of copper sulfide on paper after the test according to IEC 62535. The method can provide unambiguous quantitative assessment of copper sulfide present on paper rather than qualitative results obtained with the SEM-EDX examination stipulated in case of doubts in the interpretation of results obtained from the inspection of paper according to IEC 62535:2008, 6.3.
c) Paper and other solid insulating material/s obtained from failed transformers, reactors and other electrical equipment to assist in failure diagnostics.
d) Metal deactivator or passivators additives present in insulating liquids (qualitative assessment).
However, the method is not applicable for assessing corrosion phenomena for example the dissolution of copper in insulating liquids and deposition on solid matrices, which do not lead to sulfide formation.

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Status
Published
Publication Date
13-Feb-2018
Current Stage
PPUB - Publication issued
Start Date
14-Feb-2018
Completion Date
14-Feb-2018
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IEC TR 62697-2
Edition 1.0 2018-02
TECHNICAL
REPORT
Test methods for quantitative determination of corrosive sulfur compounds in
unused and used insulating liquids –

Part 2: Test method for quantitative determination of total corrosive sulfur (TCS)

IEC TR 62697-2:2018-02(en)
---------------------- Page: 1 ----------------------
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---------------------- Page: 2 ----------------------
IEC TR 62697-2
Edition1.0 2018-02
TECHNICAL
REPORT
Test methods for quantitative determination of corrosive sulfur compounds in
unused and used insulating liquids –

Part 2: Test method for quantitative determination of total corrosive sulfur (TCS)

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
ICS 29.040.10 ISBN 978-2-8322-5362-5

Warning! Make sure that you obtained this publication from an authorized distributor.

® Registered trademark of the International Electrotechnical Commission
---------------------- Page: 3 ----------------------
– 2 – IEC TR 62697-2:2018 © IEC 2018
CONTENTS

FOREWORD ........................................................................................................................... 4

INTRODUCTION ..................................................................................................................... 6

1 Scope .............................................................................................................................. 8

2 Normative references ...................................................................................................... 8

3 Terms and definitions ...................................................................................................... 8

4 Sampling ......................................................................................................................... 9

5 Procedure ........................................................................................................................ 9

5.1 Principle ................................................................................................................. 9

5.2 Significance and use ............................................................................................... 9

5.3 Interferences........................................................................................................... 9

5.3.1 General ........................................................................................................... 9

5.3.2 Interferences in turbidity measurements ........................................................ 10

5.3.3 Interferences in ion chromatography measurements ...................................... 10

5.4 Apparatus ............................................................................................................. 10

5.4.1 Balance ......................................................................................................... 10

5.4.2 Hot plate with stirrer ...................................................................................... 10

5.4.3 Heating block ................................................................................................. 10

5.4.4 Turbidity monitor ............................................................................................ 10

5.4.5 Ion chromatograph ......................................................................................... 10

5.4.6 Data system .................................................................................................. 10

5.5 Reagents and materials ........................................................................................ 10

5.5.1 Purity of reagents .......................................................................................... 10

5.5.2 Gases ............................................................................................................ 10

5.5.3 Solvents ........................................................................................................ 10

5.6 Standard materials ................................................................................................ 11

5.6.1 Dibenzyl disulfide (DBDS) .............................................................................. 11

5.6.2 Blank oil ........................................................................................................ 11

5.6.3 Copper powder .............................................................................................. 11

5.6.4 Potassium nitrate ........................................................................................... 11

5.6.5 Hydrochloric acid ........................................................................................... 11

5.6.6 Barium chloride ............................................................................................. 11

5.7 Standard solutions ................................................................................................ 11

5.7.1 Stock solution ................................................................................................ 11

5.7.2 Internal standard (IS) solution ........................................................................ 11

6 Instrument set-up .......................................................................................................... 12

6.1 Turbidity monitor ................................................................................................... 12

6.2 Ion chromatograph ................................................................................................ 12

6.2.1 General ......................................................................................................... 12

6.2.2 Column .......................................................................................................... 12

6.2.3 Mobile phase ................................................................................................. 12

6.2.4 Injector .......................................................................................................... 12

6.2.5 Suppressor .................................................................................................... 12

6.2.6 Conductivity detector ..................................................................................... 12

6.3 Corrosive chemistry .............................................................................................. 13

6.3.1 Reaction of corrosive sulfur compounds with copper – formation of

Cu S ............................................................................................................. 13

---------------------- Page: 4 ----------------------
IEC TR 62697-2:2018 © IEC 2018 – 3 –

6.3.2 Conversion of cuprous sulfide into cupric sulfate ........................................... 13

6.3.3 Quantification of cupric sulfate in solution ...................................................... 14

6.3.4 Turbidity measurement .................................................................................. 14

6.3.5 Ion chromatography measurement ................................................................. 14

6.4 Calibration ............................................................................................................ 14

6.4.1 General ......................................................................................................... 14

6.4.2 Calibration procedure .................................................................................... 14

6.4.3 Quantification of total corrosive sulfur (TCS) .................................................. 15

6.4.4 Quantification of total corrosive sulfur and DBDS equivalents (DBDS ) ....... 15

6.5 Results ................................................................................................................. 15

7 Precision data ............................................................................................................... 15

7.1 Detection limit ....................................................................................................... 15

7.2 Repeatability ......................................................................................................... 16

7.3 Reproducibility ...................................................................................................... 16

8 Report ........................................................................................................................... 16

Annex A (informative) Figures with typical chromatograms and results ................................ 17

Annex B (informative) RRT results for quantitative determination of corrosive sulfur ............ 18

Bibliography .......................................................................................................................... 18

Figure A.1 – Ion exchange chromatography detection DBDS as sulfate ................................ 17

Figure A.2 – Photograph of turbidity meter cuvette containing BaSO suspension ................ 17

Table 1 – Repeatability limit .................................................................................................. 16

Table 2 – Reproducibility limit ............................................................................................... 16

Table B.1 – Turbidity and ion chromatography ...................................................................... 18

Table B.2 – RRT results for quantitative determination of DBDS (GC-ECD or GC-AED)

– Sample number 1 to 7 ........................................................................................................ 19

Table B.3 – RRT results for potentially corrosive sulfur according to IEC 62535 –

Sample number 1 to 7 ........................................................................................................... 19

---------------------- Page: 5 ----------------------
– 4 – IEC TR 62697-2:2018 © IEC 2018
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
TEST METHODS FOR QUANTITATIVE DETERMINATION OF CORROSIVE
SULFUR COMPOUNDS IN UNUSED AND USED INSULATING LIQUIDS –
Part 2: Test method for quantitative determination
of total corrosive sulfur (TCS)
FOREWORD

1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising

all national electrotechnical committees (IEC National Committees). The object of IEC is to promote

international co-operation on all questions concerning standardization in the electrical and electronic fields. To

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8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is

indispensable for the correct application of this publication.

9) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of

patent rights. IEC shall not be held responsible for identifying any or all such patent rights.

The main task of IEC technical committees is to prepare International Standards. However, a

technical committee may propose the publication of a technical report when it has collected

data of a different kind from that which is normally published as an International Standard, for

example "state of the art".

IEC TR 62697-2, which is a Technical Report, has been prepared by IEC technical committee

10: Fluids for electrotechnical applications.
The text of this Technical Report is based on the following documents:
Draft TR Report on voting
10/1013/DTR 10/1027/RVDTR

Full information on the voting for the approval of this Technical Report can be found in the

report on voting indicated in the above table.
---------------------- Page: 6 ----------------------
IEC TR 62697-2:2018 © IEC 2018 – 5 –

This document has been drafted in accordance with the ISO/IEC Directives, Part 2.

A list of all parts in the IEC 62697 series, published under the general title Test methods for

quantitative determination of corrosive sulfur compounds in unused and used insulating

liquids, can be found on the IEC website.

The committee has decided that the contents of this document will remain unchanged until the

stability date indicated on the IEC website under "http://webstore.iec.ch" in the data related to

the specific document. At this date, the document will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
A bilingual version of this publication may be issued at a later date.
---------------------- Page: 7 ----------------------
– 6 – IEC TR 62697-2:2018 © IEC 2018
INTRODUCTION

During the IEC technical committee 10 plenary meeting in 2007, it was decided to set up a

working group with the aim of developing a standard on “quantitative determination of

corrosive sulfur compounds in insulating fluids”.
TC 10 decided to divide the overall task into three parts:

• Part 1: Test method for quantitative determination of dibenzyl disulfide (DBDS);

• Part 2: Test method for quantitative determination of total corrosive sulfur (TCS);

• Part 3: Test method for quantitative determination of elemental sulfur.

Part 1 was published in 2012, however the work for the preparation of Part 2 and Part 3 took

longer than anticipated. During the TC 10 plenary meeting in 2015, in order to finalize the

important work achieved, a proposal was made to complete the work and publish Part 2 and

Part 3 as Technical Reports.

Sulfur can be present in insulating liquids in various forms, including elemental sulfur,

inorganic sulfur compounds and organic sulfur compounds. Hundreds of diverse sulfur

species comprised of different isomers and homologous have been identified in petroleum

products. To simplify quantification, sulfur species are expressed as the total sulfur (TS).

Total sulfur concentration in insulating liquids depends on the origin of the base oils, refining

processes and the degree of refining and formulation including addition of additives to the

base oils. Base oils include mineral based paraffinic and naphthenic oils, synthetic iso-

paraffins obtained through gas to liquid conversion process (GTL-Fischer-Tropsch), esters,

poly olefins, poly alkylene glycols, etc. To improve characteristics of insulating liquids,

additives are sometimes added. Additives can be comprised of electrostatic discharge

depressants, metal deactivators, metal passivators, phenolic and sulfur containing

antioxidants.

Certain sulfur compounds present in the insulating liquids exhibit antioxidant and metal

deactivating properties without being corrosive, whereas other sulfur compounds have been

known to react with metal surfaces. Specifically, sulfur compounds such as mercaptans are

very corrosive to metallic components of electrical devices and lead to the formation of metal

sulfides. Presence of these corrosive sulfur species has been linked to failures of electrical

equipment used in generation, transmission and distribution of electrical energy for several

decades. Therefore, IEC 60296 states that corrosive sulfur compounds shall not be present in

unused and used insulating liquids.

Serious detrimental impact of corrosive sulfur has also been linked to the presence of a

specific highly corrosive sulfur compound, DBDS. This compound has been found in certain

mineral insulating oils [1], [15], [16], [17] ; presence of this compound has been shown to

result in copper sulfide formation on the surfaces of copper conductors under normal

operating conditions of transformers [2]. A specific standard test method for quantitative

determination of this corrosive compound has been developed (see IEC 62697-1).

However, current standard test methods for the detection of corrosive sulfur species ([11],

and [13]) and potentially corrosive sulfur in used and unused insulating oil (IEC 62535) are

empirical and yield qualitative results based on visual and subjective perception of colour

profiles.

Several field examinations of transformers and other electrical equipment filled with insulating

liquids have shown that copper sulfide formation is related to corrosive sulfur compounds.

Stability and the reactivity of different classes of sulfur species (elemental sulfur, aliphatic and

aromatic mercaptans, sulfides, disulfides, thiophens) which could be present in the insulating

liquids have been examined. Corrosivity of nine compounds sulfur containing organic

___________
Numbers in square brackets refer to the Bibliography.
---------------------- Page: 8 ----------------------
IEC TR 62697-2:2018 © IEC 2018 – 7 –
compounds including dodecylmercaptan, hexadecylmercaptan, benzylmercaptan,

butyldisulfide dibenzylsulfide, phenylvinylsulfur, phenyldisulfide, dibenzyldisulfide, and

dibenzothiophene, was evaluated under conditions which simulated hermetically sealed or

free breathing type transformers. Corrosivity was assessed quantitatively through conversion

of copper sulfide to copper sulfate which was determined through turbidity measurement. The

data obtained was used for the ranking of compounds according to their corrosivity towards

copper. Corrosivity was found to vary with temperature, for example at temperatures between

80 °C to 120 °C, mercaptans were found to be the most corrosive compounds, while at

temperatures between 150 °C to 180 °C, the highest corrosivity was exhibited by disulfides

[18].

Furthermore, methods for corrosive sulfur and potentially corrosive sulfur in insulating liquids

([8] and [11]) can be used only for mineral insulating oils that do not contain metal passivator

additives. In the presence of such additives, methods can yield negative results even when

corrosive sulfur compounds are present in the insulating liquids – thus providing a false

negative test result [11]. On the other hand, the test method when used with aged insulating

oils (e.g. those with relative high acidity), may give ambiguous results and lead to a false

positive corrosive sulfur test result. In such cases, further analysis of insulating liquids is

stipulated, for example IEC 62535 specifies that if there are doubts in the interpretation of the

results from the inspection of paper, the composition of precipitate should be analysed by

other methods (e.g. by SEM-EDX).

To overcome limitations of standard test methods for corrosive sulfur, a working group within

IEC TC 10 was set up to prepare test methods which will yield the unambiguous quantitative

results for corrosive sulfur compounds in unused and used insulating liquids. This test method

is described in this part of IEC 62697.
WARNING – Health and safety

This part of IEC 62697 does not purport to address all the safety problems associated with its

use. It is the responsibility of the user of this document to establish appropriate health and

safety practices and determine the applicability of regulatory limitations prior to use.

The insulating liquids which are the subject of this document should be handled with due

regard to personal hygiene. Direct contact with eyes may cause slight irritation. In the case of

eye contact, irrigation with copious quantities of clean running water should be carried out and

medical advice sought.

Some of the tests specified in this document involve the use of processes that could lead to a

hazardous situation. Attention is drawn to the relevant standard for guidance.
WARNING – Environment

This document involves mineral insulating oils, natural ester insulating liquids, chemicals and

used sample containers. The disposal of these items should be carried out in accordance with

current national legislation with regard to the impact on the environment. Every precaution

should be taken to prevent the release of chemicals used during the test into the environment.

---------------------- Page: 9 ----------------------
– 8 – IEC TR 62697-2:2018 © IEC 2018
TEST METHODS FOR QUANTITATIVE DETERMINATION OF CORROSIVE
SULFUR COMPOUNDS IN UNUSED AND USED INSULATING LIQUIDS –
Part 2: Test method for quantitative determination
of total corrosive sulfur (TCS)
1 Scope

This part of IEC 62697 specifies a test method for the quantitative determination of total

corrosive sulfur (TCS) in unused and used insulating liquids and solid matrices through the

conversion of corrosive sulfur species to metal (copper, silver etc.) sulfides. The sulfides

formed are quantitatively converted to sulfates; sulfates are determined through turbidity

measurement or with ion chromatography. The method is applicable with the following

matrices:

a) Unused and used insulating liquids, for example mineral insulating oils and natural esters,

which allow the determination of corrosive sulfur compounds over concentrations ranging

-1 -1
between 2,5 mg kg to 80 mg kg TCS.

b) Solid matrices that come in contact with the insulating liquid, for example insulating

papers in electrical equipment. The quantification limits for these matrices depend on the

amount of matrix used during the determination. The method can be used for the

quantitative or semi-quantitative determination of copper sulfide on paper after the test

according to IEC 62535. The method can provide unambiguous quantitative assessment of

copper sulfide present on paper rather than qualitative results obtained with the SEM-EDX

examination stipulated in case of doubts in the interpretation of results obtained from the

inspection of paper according to IEC 62535:2008, 6.3.

c) Paper and other solid insulating material/s obtained from failed transformers, reactors and

other electrical equipment to assist in failure diagnostics.

d) Metal deactivator or passivators additives present in insulating liquids (qualitative

assessment).

However, the method is not applicable for assessing corrosion phenomena for example the

dissolution of copper in insulating liquids and deposition on solid matrices, which do not lead

to sulfide formation.
2 Normative references

The following documents are referred to in the text in such a way that some or all of their

content constitutes requirements of this document. For dated references, only the edition

cited applies. For undated references, the latest edition of the referenced document (including

any amendments) applies.

IEC 62697-1, Test methods for quantitative determination of corrosive sulfur compounds in

unused and used insulating liquids – Part 1: Test method for quantitative determination of

dibenzyldisulfide (DBDS)
3 Terms and definitions

For the purposes of this document, the terms and definitions given in IEC 62697-1 and the

following apply.

ISO and IEC maintain terminological databases for use in standardization at the following

addresses:
---------------------- Page: 10 ----------------------
IEC TR 62697-2:2018 © IEC 2018 – 9 –
• IEC Electropedia: available at http://www.electropedia.org/
• ISO Online browsing platform: available at http://www.iso.org/obp
3.1
ion chromatography

chromatography technique that separates ions based on their affinity for the immobilized ion

exchange sites on the ion exchanger followed by quantitation of ions through conductivity

measurement
3.2
turbidity measurement

measurement that involves monitoring of transmitted light intensity through a liquid due to the

presence of non-transparent particles in the liquid
4 Sampling

Samples should be taken, following the procedure given in IEC 60475. A representative

portion should be taken after thorough mixing. The specific sampling technique can affect the

accuracy of this test method. Precautions should be taken to prevent cross-contamination

during sampling.
5 Procedure
5.1 Principle

The TCS test involves the conversion of corrosive sulfur in insulating liquid, into metal sulfide

for example copper sulfide (Cu S) (see Equation 1). The reaction between copper and oil is

accomplished preferably in vials with argon headspace, to prevent oxidation of copper with air.

This reaction is accomplished at 150 °C.
Cu + Corrosive sulfur Cu S (1)
Cu S is then oxidized to cupric sulfate (CuSO ) (2)
2 4
Cu S + Oxidizer CuSO (2)
2 4

Sulfate thus obtained is dissolved in particle free deionized water and quantified through

turbidity measurement after addition of a precipitating agent. The sulfate concentration in

solution can also be determined with ion chromatography.
5.2 Significance and use
This test method describes the determination of TCS in insulating liquids.

TCS is the sum of all elemental and chemically bound sulfur in an insulating liquid that r

...

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