ISO 3651-3:2017

Determination of resistance to intergranular corrosion of stainless steels — Part 3: Corrosion test for low-Cr ferritic stainless steels

ISO 3651-3:2017 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.

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

General Information

• Status: Published
• Publication Date: 09-May-2017
• ICS: 77.060 - Corrosion of metals
• Technical Committee: ISO/TC 156 - Corrosion of metals and alloys
• Drafting Committee: ISO/TC 156/WG 5 - Intergranular corrosion
• Current Stage: 9092 - International Standard to be revised
• Start Date: 08-Jul-2025
• Completion Date: 19-Jul-2025

Standards Content (Sample)

INTERNATIONAL ISO
STANDARD 3651-3
First edition
2017-05
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
Reference number
©
ISO 2017
© ISO 2017, Published in Switzerland
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized otherwise in any form
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ii © ISO 2017 – All rights reserved

Contents Page
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 . 2
4.3 Welded test pieces . 4
4.4 Sensitization heat treatment . 4
4.5 Mechanical preparation . 4
4.6 Degreasing and drying . 4
5 Test methods . 4
6 Evaluation by metallographic examination. 6
7 Test report . 6
Bibliography . 7
Foreword
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This document was prepared by Technical Committee ISO/TC 156, Corrosion of metals and alloys.
A list of all parts in the ISO 3651 series can be found on the ISO website.
iv © ISO 2017 – All rights reserved

Introduction
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.
INTERNATIONAL STANDARD ISO 3651-3:2017(E)
Determination of resistance to intergranular corrosion of
stainless steels —
Part 3:
Corrosion test for low-Cr ferritic stainless steels
1 Scope
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.
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.
ISO 8044, Corrosion of metals and alloys — Basic terms and definitions
3 Terms and definitions
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
4 Preparation of test specimens
4.1 Principle
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.
4.2 Dimensions
2 2
A specimen having a total surface area of 5 cm to 20 cm is recommended. It should include welded
zone, heat-affected zone and unaffected base metal.
Dimensions in millimetres
Key
1 no sampling area (avoid weld defects)
2 sampling area
3 welding sample
4 no sampling area
Figure 1 — Test piece for plates and strips with welded butt joint
2 © ISO 2017 – All rights reserved

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
4.3 Welded test pieces
Welded test pieces are prepared according to Figures 1 to 4.
As-received heat treatment shall be specified.
4.4 Sensitization heat treatment
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.
4.5 Mechanical preparation
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.
4.6 Degreasing and drying
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.
5 Test methods
5.1 Apparatus
5.1.1 A flask, with a capacity of 1 l with a condenser.
4 © ISO 2017 – All rights reserved

5.1.2 Heating device, to keep the solution boiling.
5.1.3 Metallographic microscope, magnification of ~×500.
5.2 Corrosive solution
5.2.1 The test solution shall be prepared as follows.
5.2.2 Weigh 363 g of analytical reagent copper sulfate (CuSO ·5H O) and add into the flask. The test
4 2
pieces are covered in electrolytic grade copper filings on the bottom of the flask. The amount of copper
−1
shall be at least 50 g·l of the solution.
5.2.3 Measure 600 ml of distilled water and pour into the flask.
5.2.4 Measure 5,0 g of the reagent grade sulfuric acid of a density at an ambient temperature of
−1
1,84 g·ml . Add the acid slowly to the solution in the flask to avoid boiling by the heat evolved.
5.2.5 Fit the condenser to the flask and circulate cooling water.
5.2.6 Boil the solution until all the copper sulfate is dissolved.
5.2.7 Based on the above steps, the solution of 0,5 % H SO + 24 % CuSO is now prepared.
2 4 4
5.2.8 The corrosive solution shall be used only once.
5.3 Copper addition
5.3.1 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.
5.3.2 The amount of copper shall be at least 50 g/l of solution to assure galvanic coupling between
copper and test specimens.
5.3.3 The copper may be reused if it is cleaned in warm deionized water after each test.
5.4 Procedure
5.4.1 The volume of acidified copper sulfate solution should be sufficient to completely immerse the
specimens and provide a minimum 8 ml/cm of the total surface area of the test specimens.
5.4.2 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.
5.4.3 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.
5.4.4 Begin the test period when the test solution reaches the boiling point. The solution shall be
boiled for 15 h ± 3 h. The test duration shall be recorded.
5.4.5 The test specimen is rinsed in deionized water and then dried.
...