IEC TR 62697-2:2018

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)

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.

General Information

• Status: Published
• Publication Date: 13-Feb-2018
• ICS: 29.040.10 - Insulating oils
• Technical Committee: TC 10 - Fluids for electrotechnical applications
• Current Stage: PPUB - Publication issued
• Start Date: 14-Feb-2018
• Completion Date: 12-Mar-2018

Overview

IEC TR 62697-2:2018 defines a laboratory test method for the quantitative determination of total corrosive sulfur (TCS) in unused and used insulating liquids and related solid matrices. The method converts corrosive sulfur species into metal sulfides (e.g., copper or silver sulfides), oxidizes those sulfides to sulfates, and determines the resulting sulfates by turbidity measurement or ion chromatography (IC). This Technical Report is part of the IEC 62697 series on corrosive sulfur analysis in insulating fluids.

Key topics and technical requirements

• Scope of matrices:
  • Unused and used insulating liquids (mineral insulating oils, natural esters) - quantitative TCS range 2.5 mg·kg⁻¹ to 80 mg·kg⁻¹.
  • Solid matrices in contact with liquids (e.g., insulating paper) - limits depend on sample mass; method supports quantitative or semi‑quantitative copper sulfide determination after IEC 62535 testing.
  • Insulating materials recovered from failed equipment to support failure diagnostics.
  • Qualitative assessment of metal deactivator/passivator additives in liquids.
• Analytical principle:
  • Conversion of corrosive sulfur species to metal sulfides (Cu, Ag, etc.).
  • Quantitative oxidation of sulfides to sulfates.
  • Sulfate measurement by turbidity (barium sulfate formation and turbidity monitoring) or ion chromatography (IC with conductivity detection).
• Instrumentation & reagents:
  • Turbidity monitor and ion chromatograph are specified for detection.
  • Controlled reagents, standard solutions (e.g., DBDS stock), and sample preparation steps are required.
• Performance characteristics:
  • The report includes detection limits, repeatability and reproducibility data, calibration procedures and reporting requirements.
• Limitations:
  • Not applicable for corrosion phenomena that do not produce sulfides (e.g., copper dissolution and deposition without sulfide formation).

Practical applications and users

• Transformer and reactor manufacturers: for quality control of insulating oils and ester-based fluids.
• Power utilities and asset owners: routine monitoring of in-service insulating liquids to detect corrosive sulfur risk and inform maintenance.
• Independent test laboratories: performing standardized TCS quantification for specification compliance or forensic analysis.
• Failure analysis teams: quantitative assessment of copper sulfide on paper and solid insulation to support root-cause investigations.
• Oil and additive manufacturers: evaluating formulations for corrosive sulfur content or presence of metal deactivators.

Related standards

• IEC 62535 - inspection and testing of insulating paper (method referenced for copper sulfide evaluation).
• IEC TR 62697-1 - quantitative determination of dibenzyl disulfide (DBDS).
• IEC TR 62697-3 - test method for elemental sulfur (related part of the series).

Keywords: IEC TR 62697-2:2018, total corrosive sulfur, TCS, insulating liquids, corrosive sulfur, turbidity measurement, ion chromatography, copper sulfide, transformer failure diagnostics, DBDS.

Standards Content (Sample)

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)
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized in any form
or by any means, electronic or mechanical, including photocopying and microfilm, without permission in writing from
either IEC or IEC's member National Committee in the country of the requester. If you have any questions about IEC
copyright or have an enquiry about obtaining additional rights to this publication, please contact the address below or
your local IEC member National Committee for further information.

IEC Central Office Tel.: [phone]
3, rue de Varembé [e-mail]
CH-1211 Geneva 20 www.iec.ch
Switzerland
About the IEC
The International Electrotechnical Commission (IEC) is the leading global organization that prepares and publishes
International Standards for all electrical, electronic and related technologies.

About IEC publications
The technical content of IEC publications is kept under constant review by the IEC. Please make sure that you have the
latest edition, a corrigenda or an amendment might have been published.

IEC Catalogue - webstore.iec.ch/catalogue Electropedia - www.electropedia.org
The stand-alone application for consulting the entire The world's leading online dictionary of electronic and
bibliographical information on IEC International Standards, electrical terms containing 21 000 terms and definitions in
Technical Specifications, Technical Reports and other English and French, with equivalent terms in 16 additional
documents. Available for PC, Mac OS, Android Tablets and languages. Also known as the International Electrotechnical
iPad. Vocabulary (IEV) online.

IEC publications search - webstore.iec.ch/advsearchform IEC Glossary - std.iec.ch/glossary
The advanced search enables to find IEC publications by a 67 000 electrotechnical terminology entries in English and
variety of criteria (reference number, text, technical French extracted from the Terms and Definitions clause of
committee,…). It also gives information on projects, replaced IEC publications issued since 2002. Some entries have been
and withdrawn publications. collected from earlier publications of IEC TC 37, 77, 86 and

CISPR.
IEC Just Published - webstore.iec.ch/justpublished

Stay up to date on all new IEC publications. Just Published IEC Customer Service Centre - webstore.iec.ch/csc
details all new publications released. Available online and If you wish to give us your feedback on this publication or
also once a month by email. need further assistance, please contact the Customer Service
Centre: [e-mail].
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

– 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
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
eq
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

– 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
this end and in addition to other activities, IEC publishes International Standards, Technical Specifications,
Technical Reports, Publicly Available Specifications (PAS) and Guides (hereafter referred to as “IEC
Publication(s)”). Their preparation is entrusted to technical committees; any IEC National Committee interested
in the subject dealt with may participate in this preparatory work. International, governmental and non-
governmental organizations liaising with the IEC also participate in this preparation. IEC collaborates closely
with the International Organization for Standardization (ISO) in accordance with conditions determined by
agreement between the two organizations.
2) The formal decisions or agreements of IEC on technical matters express, as nearly as possible, an international
consensus of opinion on the relevant subjects since each technical committee has representation from all
interested IEC National Committees.
3) IEC Publications have the form of recommendations for international use and are accepted by IEC National
Committees in that sense. While all reasonable efforts are made to ensure that the technical content of IEC
Publications is accurate, IEC cannot be held responsible for the way in which they are used or for any
misinterpretation by any end user.
4) In order to promote international uniformity, IEC National Committees undertake to apply IEC Publications
transparently to the maximum extent possible in their national and regional publications. Any divergence
between any IEC Publication and the corresponding national or regional publication shall be clearly indicated in
the latter.
5) IEC itself does not provide any attestation of conformity. Independent certification bodies provide conformity
assessment services and, in some areas, access to IEC marks of conformity. IEC is not responsible for any
services carried out by independent certification bodies.
6) All users should ensure that they have the latest edition of this publication.
7) No liability shall attach to IEC or its directors, employees, servants or agents including individual experts and
members of its technical committees and IEC National Committees for any personal injury, property damage or
other damage of any nature whatsoever, whether direct or indirect, or for costs (including legal fees) and
expenses arising out of the publication, use of, or reliance upon, this IEC Publication or any other IEC
Publications.
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.

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.

– 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.

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.

– 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 def
...