ISO/DIS 3651-3

ISO/TC 156

Secretariat: SAC

Date: 2025-12-09

Determination of resistance to intergranular corrosion of stainless steels —

Part 3:
Corrosion test for low-Cr ferritic stainless steels

Détermination de la résistance à la corrosion intergranulaire des aciers inoxydables —

Partie 3: Essai de corrosion pour les aciers inoxydables ferritiques à faible teneur en chrome

DIS stage

© ISO 2025

All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting on the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address below or ISO’s member body in the country of the requester.

ISO copyright office

CP 401 • Ch. de Blandonnet 8

CH-1214 Vernier, Geneva

Phone: + 41 22 749 01 11

E-mail: copyright@iso.org

Website: www.iso.org

Published in Switzerland

Contents

Foreword iv

Introduction v

1 Scope 1

2 Normative references 1

3 Terms and definitions 1

4 Preparation of test specimens 1

4.1 Principle 1

4.2 Dimensions 1

4.3 Welded test pieces 2

4.4 Sensitization heat treatment 3

4.5 Mechanical preparation 4

4.6 Degreasing and drying 4

5 Test methods 4

5.1 Apparatus 4

5.2 Corrosive solution 4

5.3 Copper addition 4

5.4 Procedure 5

6 Evaluation by metallographic examination 5

7 Test report 5

Annex A (informative) Round-robin test results forthe low-chromium ferritic stainless steel sheets 6

Annex B (informative) Round-robin test results for welded test samplesof the low-chromium ferritic stainless steel sheets 7

Foreword

ISO (the International Organization for Standardization) is a worldwide federation of national standards bodies (ISO member bodies). The work of preparing International Standards is normally carried out through ISO technical committees. Each member body interested in a subject for which a technical committee has been established has the right to be represented on that committee. International organizations, governmental and non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.

The procedures used to develop this document and those intended for its further maintenance are described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the different types of ISO document should be noted. This document was drafted in accordance with the editorial rules of the ISO/IEC Directives, Part 2 (see www.iso.org/directives).

ISO draws attention to the possibility that the implementation of this document may involve the use of (a) patent(s). ISO takes no position concerning the evidence, validity or applicability of any claimed patent rights in respect thereof. As of the date of publication of this document, ISO [had/had not] received notice of (a) patent(s) which may be required to implement this document. However, implementers are cautioned that this may not represent the latest information, which may be obtained from the patent database available at www.iso.org/patents. ISO shall not be held responsible for identifying any or all such patent rights.

Any trade name used in this document is information given for the convenience of users and does not constitute an endorsement.

For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and expressions related to conformity assessment, as well as information about ISO's adherence to the World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT), see www.iso.org/iso/foreword.html.

This document was prepared by Technical Committee ISO/TC 156.

This edition cancels and replaces the ISO 3651-3:2017 technically revised.

The main changes are as follows:

1. Extra explanation for Sensitization heat treatment is added and related RRT results areis as Annex A.
2. 0,5% H2SO4 + 6% CuSO4 solution is added for test solution and related RRT test results are shown in Annex B.

A list of all parts in the ISO 3651 series can be found on the ISO website.

Any feedback or questions on this document should be directed to the user’s national standards body. A complete listing of these bodies can be found at www.iso.org/members.html.

Introduction

The introduction is an optional/conditional element of the text.

For rules on the drafting of the introduction, refer to the ISO/IEC Directives, Part 2:2021, Clause 13.

The term “intergranular corrosion test” denotes the corrosion test carried out by means of preferential attack of the grain boundaries.

The low-Cr [less than 16 % Cr, less than 0,3 % Ni, less than 0,3 % Ti and less than 0,3 % Nb, less than 0,3 % (Ti + Atomic weight ratio Nb)] ferritic stainless steels may be subject to such attack when they have been exposed to a temperature between about 500 °C to 1 300 °C. This heat cycle, which may provoke sensitization to intergranular corrosion, may occur during rolling or welding operation.

Low-Cr ferritic stainless steels may show high risks to uniform corrosion rates and copper deposit when tested by the methods given by ISO 3651-1 and ISO 3651-2 since the electrochemical potential difference between the matrix and the Cr depletion for low-Cr ferritic stainless steels is much less than that of medium-Cr ferritic stainless steels. These risks should be considered in selecting this test method. Application of this standard test to the other stainless steels out of the above specifications should be made based on the specific agreement between the interested parties.

Identification of patent holders: the following text shall be included if patent rights have been identified.

The International Organization for Standardization (ISO) [and/or] International Electrotechnical Commission (IEC) draw[s] attention to the fact that it is claimed that compliance with this document may involve the use of a patent.

ISO [and/or] IEC take[s] no position concerning the evidence, validity and scope of this patent right.

The holder of this patent right has assured ISO [and/or] IEC that he/she is willing to negotiate licences under reasonable and non-discriminatory terms and conditions with applicants throughout the world. In this respect, the statement of the holder of this patent right is registered with ISO [and/or] IEC. Information may be obtained from the patent database available at www.iso.org/patents.

Attention is drawn to the possibility that some of the elements of this document may be the subject of patent rights other than those in the patent database. ISO [and/or] IEC shall not be held responsible for identifying any or all such patent rights.

Determination of resistance to intergranular corrosion of stainless steels —

Part 3:
Corrosion test for low-Cr ferritic stainless steels

This document specifies the determination of the intergranular corrosion susceptibility of low-Cr [less than 16 % Cr, less than 0,3 % Ni, less than 0,3 % Ti and less than 0,3 % Nb, less than 0,3 % (Ti + Atomic weight ratio Nb)] ferritic stainless steels in the 0,5 % sulfuric acid/copper sulfate test. It also specifies the purposes which may be assigned to the test.

The method is applicable to stainless steels supplied in the form of rolled sheets and welded tubes and intended for use in a mildly oxidizing acid medium.

It is important to note that the result of the corrosion test is only strictly valid for the corrosive medium used in the test. It constitutes a basis for estimating the resistance to intergranular corrosion but cannot be used to check resistance to other forms of corrosion (general corrosion, pitting, stress corrosion, etc.). It is important for the user to adapt the specified corrosion test where steels are used. This test is, in no case, considered as an absolute criterion of the quality of the metal.

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.

ISO 8044, Corrosion of metals and alloys — Vocabulary

For the purposes of this document, the terms and definitions given in ISO 8044 apply.

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

• IEC Electropedia: available at http://www.electropedia.org/
• ISO Online browsing platform: available at http://www.iso.org/obp

Depending on the purpose of the test and agreement between supplier and purchaser, the test pieces shall be prepared as specified in 4.2, 4.3 or 4.4. The thickness of the test specimen can be the same as the original samples.

A specimen having a total surface area of 5 cm² to 20 cm² is recommended. It should include welded zone, heat-affected zone and unaffected base metal.

Welded test pieces are prepared according to Figure 1 to Figure 4. As-received heat treatment shall be specified.

Key

1 no sampling area (avoid weld defects)

2 sampling area

3 welding sample

3 no sampling area

Figure 1 — Test piece for plates and strips with welded butt joint

Key

1 welding sample

2 welding sample

3 sampling area

Figure 2 — Test piece for plates and strips with crossing welded butt joint

Key

1 sampling area

2 welding samples

Figure 3 — Longitudinal test piece for tubes with welded butt joint

Key

1 sampling area

2 welding samples

Figure 4 — Circumferential test piece for tubes with welded butt joint

In order to verify the intrinsic resistance to intergranular corrosion, it is necessary to carry out a sensitization heat treatment for low-Cr ferritic stainless steel as follows:

Step1: heating the specimen at 1 200 °C ± 10 °C for 30 min followed by water cooling;

Step2: heating the specimen at 600 °C ± 10 °C for 30 min followed by water cooling.

The sensitization heat treatment above does not simulate welding. Therefore, the IGC test results for the welded test specimens are not always consistent with those of the sensitized test specimens, as shown in Annex A.

The test pieces shall be descaled mechanically by grinding in the longitudinal plane on all surfaces and sharp edges with grade 600 or finer, iron-free abrasive paper or cloth. Over-heating of the test pieces shall be avoided.

The test piece shall be degreased using a suitable degreasing agent such as acetone or ethanol and dried before being placed into the corrosion solution.

A flask, with a capacity of 1 l with a condenser.

Heating device, to keep the solution boiling.

Metallographic microscope, magnification of ~×500.

The test solution should be prepared as follows, from 5.2.2 to 5.2.7.

Weigh 363 g of analytical reagent copper sulfate (CuSO₄·5H₂O) and add into the flask. The test pieces are covered in electrolytic grade copper filings on the bottom of the flask. The amount of copper shall be at least 50 g·l⁻¹ of the solution.

Measure 600 ml of distilled water and pour into the flask.

Measure 5,0 g of the reagent grade sulfuric acid of a density at an ambient temperature of 1,84 g·ml⁻¹. Add the acid slowly to the solution in the flask to avoid boiling by the heat evolved.

Fit the condenser to the flask and circulate cooling water.

Boil the solution until all the copper sulfate is dissolved.

Based on the above steps, the solution of 0,5 % H₂SO₄ + 24 % CuSO₄ is now prepared.

Alternatively, we may use the solution of 0,5% H₂SO₄ + 6% CuSO₄ instead to shorten the test period to 5±1 h. In this case, theother testing conditions, namelyCuSO₄concentration and testing period,are the same as using the 0,5 % H₂SO₄ + 24 % CuSO₄ solution. Comparison of testing results fromusing the solution of 0,5 % H₂SO₄ + 24 % CuSO₄for 15±3 h h and 0,5% H₂SO₄ + 6% CuSO₄ for 5h±1 hare shown in Annex B.

The corrosive solution shall be used only once.

A sufficient quantity of electrolytic grade copper with high surface area (e.g. shavings or filings) shall be used to cover all surfaces of the specimens which are embedded in the copper on the bottom of the flask.

The amount of copper shall be at least 50 g/l of solution to assure galvanic coupling between copper and test specimens.

The copper may be reused if it is cleaned in warm deionized water after each test.

The volume of acidified copper sulfate solution should be sufficient to completely immerse the specimens and provide a minimum 8 ml/cm² of the total surface area of the test specimens.

As many as three specimens being of the same grade can be tested in one flask. The different grade steels or the same grade with different processing history should not be tested in the same flask.

The test specimens shall be immersed in the test solution at ambient temperature which is then brought to a boil and maintained boiling during the test period.

Begin the test period when the test solution reaches the boiling point. The 0,5% H₂SO₄ + 24% CuSO₄solution shall be boiled for 15 h ± 3 h. If the 0,5% H₂SO₄ + 6% CuSO₄ solutionis used, the solution shall be boiled for 5 h±1 h. The test duration shall be recorded.

The test specimen is rinsed in deionized water and then dried.

Examine original surface of the test specimen under a microscope at ~×500 magnification. Grain dropping or grain boundaries corroded completely are usually indication of intergranular attack.

Cut the sample in cross-section and prepare the metallographic sample.

Examine the cross-section under a microscope. The depth of intergranular cracks that can be tolerated is subject to agreement between the supplier and purchaser. The material may be reported as being resistant to intergranular corrosion if the maximum depth of corrosion shall be not more than 5 μm.

The test report shall include the following information:

1. a reference to this document, i.e. ISO 36513;
2. steel grade including chemical composition;
3. method of preparation;
4. nature of the test piece;
5. nature of the as-delivered heat treatment and the sensitization heat treatment, when applied;
6. results of the test;
7. any incident which may have an effect on the results.
8. 
   (informative)
   
   Round-robin test results forthe low-chromium ferritic stainless steel sheets

• 1. Table A.1 shows a round-robin test of low-chromium ferritic stainless steel sheets with a thickness of 1.2 mm, containing 11.0 mass% chromium, 0.22 mass% titanium, 0.004 mass% carbon, and 0.004 mass% nitrogen. Welded test samples and sensitized test samples wereproduced according to ISO 3651-3. “No IGC”means that IGC depth wasnot more than 5μm,and “IGC”means that IGC depth was greaterthan 5 μm, according to ISO 3651-3.

Table A.1 — Round-robin test results forthe low-chromium ferritic stainless steel sheets

1. 
   (informative)
   
   Round-robin test results for welded test samplesof the low-chromium ferritic stainless steel sheets
   1. The Table B.1 shows a round-robin test of low-chromium ferritic stainless steel sheets with a thickness of 1.2 mm, containing 11.0 mass% chromium, 0.22 mass% titanium, 0.004 mass% carbon, and 0.004 mass% nitrogenfor welded test samples in 0,5% H₂SO₄+ 24% CuSO₄solution for 15 h and 0,5% H₂SO₄+ 6% CuSO₄solution for 4 h. Welded test samples were produced according to ISO 3651-3. “No IGC” means that IGC depth was not more than 5 μm, and “IGC” means that IGC depth was greater than 5 μm, according to ISO 3651-3.

Table B.1 — Round-robin test results for welded test samplesof the low-chromium ferritic stainless steel sheets

• 1. The Table B.2 shows round-robin test of low-chromium ferritic stainless steel sheets with a thickness of 1.2 mm, containing 11.0 mass% chromium, 0.22 mass% titanium, 0.004 mass% carbon, and 0.004 mass% nitrogen for sensitized test samples in 0,5% H₂SO₄ + 24% CuSO₄ solution for 15 h and 0,5% H₂SO₄ + 6% CuSO₄ solution for 4 h. Welded test samples were produced according to ISO 3651-3. “No IGC” means that IGC depth was not more than 5 μm, and “IGC” means that IGC depth was greater than 5 μm, according to ISO 3651-3.

Table B.2 — Round-robin test results for sensitized test samples of the low-chromium ferritic stainless steel sheets

• 1. The corrosion test results for 0,5% H₂SO₄ + 24% CuSO₄ for 15 h perfectly match those for 0,5% H₂SO₄ + 6% CuSO₄ for an immersion time of 4 h as shown in Table B.1 and Table B.2.