Corrosion of metal and alloys - Determination of resistance of magnesium alloys to stress corrosion cracking (ISO 20728:2018)

This international Standard specifies a method for the determination of resistance to stress corrosion cracking (SCC) of magnesium alloys. This International Standard covers the method of sampling, the types of specimens, the loading procedure, the type of environment and the Interpretation of results.
This International Standard is aimed at the determination of the resistance to SCC as a function of the chemical composition, the method of manufacture and heat treatment of magnesium alloys. This International Standard applies to cast and wrought magnesium alloys in the form of castings, semi-finished products, parts and weldments.
Since most natural and many artificial environments contain chlorides, this International Standard can be used to compare the performance of products employed in environments containing chlorides providing that the failure mechanism is not changed. However, the results of this test should not be considered as an absolute criterion for the quality of alloys.

Korrosion von Metall und Legierungen - Bestimmung der Beständigkeit von Magnesiumlegierungen gegen Spannungsrisskorrosion (ISO 20728:2018)

Corrosion des métaux et alliages - Détermination de la résistance des alliages de magnésium à la fissuration par corrosion sous contrainte (ISO 20728:2018)

Le présent document spécifie une méthode pour la détermination de la résistance à la corrosion sous contrainte (CSC) d'alliages de magnésium destinés à être utilisés dans des applications structurales (tels les trains avant de véhicule en magnésium, les carters de boîte de vitesses et d'embrayage, les pièces de colonne de direction, les actionneurs de boîte de vitesses, les couvercles et logements de soupapes, les brides et lames de collecteur d'admission, les dispositifs électroniques, les outils électriques et les appareils médicaux). Cette méthode permet de déterminer la résistance à la corrosion sous contrainte en fonction de la composition chimique, du procédé de fabrication et du traitement thermique des alliages de magnésium.
Le présent document s'applique aux alliages de magnésium coulés et corroyés se présentant sous forme de produits moulés, produits semi-finis, pièces et assemblages soudés; il couvre la méthode d'échantillonnage, les types d'éprouvettes, le mode opératoire de mise en charge, le type de milieu et l'interprétation des résultats.
Le présent document permet d'évaluer la performance relative de matériaux et de produits dans des milieux contenant des chlorures ou des sulfates, à condition que le mécanisme de rupture ne soit pas modifié, mais il ne qualifie pas un matériau ou un produit pour son application en service.

Korozija kovin in zlitin - Ugotavljanje odpornosti magnezijevih zlitin proti pokanju zaradi napetostne korozije (ISO 20728:2018)

Ta mednarodni standard določa metodo za ugotavljanje odpornosti magnezijevih zlitin proti pokanju zaradi napetostne korozije (SCC). Ta mednarodni standard zajema metodo vzorčenja, vrste vzorcev, postopek nalaganja, vrsto okolja in razlago rezultatov. Ta mednarodni standard je namenjen ugotavljanju odpornosti proti pokanju zaradi napetostne korozije kot funkcije kemijske sestave, načina proizvodnje in toplotne obdelave magnezijevih zlitin. Ta mednarodni standard se uporablja za ulite in gnetene magnezijeve zlitine v obliki ulitkov, polizdelkov, delov in zvarov. Večina naravnih okolij in številna umetna okolja vsebujejo kloride, zato se lahko ta mednarodni standard uporablja za primerjavo zmogljivosti izdelkov, ki se uporabljajo v okoljih z vsebnostjo kloridov, pri čemer mora mehanizem okvare ostati enak. Vendar se rezultati tega preskusa naj ne bi upoštevali kot absolutno merilo za kakovost zlitin.

General Information

Status
Published
Public Enquiry End Date
14-Jul-2017
Publication Date
11-Nov-2018
Current Stage
6060 - National Implementation/Publication (Adopted Project)
Start Date
22-Oct-2018
Due Date
27-Dec-2018
Completion Date
12-Nov-2018

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SLOVENSKI STANDARD
SIST EN ISO 20728:2018
01-december-2018

Korozija kovin in zlitin - Ugotavljanje odpornosti magnezijevih zlitin proti pokanju

zaradi napetostne korozije (ISO 20728:2018)

Corrosion of metal and alloys - Determination of resistance of magnesium alloys to

stress corrosion cracking (ISO 20728:2018)
Korrosion von Metall und Legierungen - Bestimmung der Beständigkeit von
Magnesiumlegierungen gegen Spannungsrisskorrosion (ISO 20728:2018)

Corrosion des métaux et alliages - Détermination de la résistance des alliages de

magnésium à la fissuration par corrosion sous contrainte (ISO 20728:2018)
Ta slovenski standard je istoveten z: EN ISO 20728:2018
ICS:
77.060 Korozija kovin Corrosion of metals
77.120.20 Magnezij in magnezijeve Magnesium and magnesium
zlitine alloys
SIST EN ISO 20728:2018 en

2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

---------------------- Page: 1 ----------------------
SIST EN ISO 20728:2018
---------------------- Page: 2 ----------------------
SIST EN ISO 20728:2018
EN ISO 20728
EUROPEAN STANDARD
NORME EUROPÉENNE
October 2018
EUROPÄISCHE NORM
ICS 77.060
English Version
Corrosion of metal and alloys - Determination of resistance
of magnesium alloys to stress corrosion cracking (ISO
20728:2018)

Corrosion des métaux et alliages - Détermination de la Korrosion von Metall und Legierungen - Bestimmung

résistance des alliages de magnésium à la fissuration der Beständigkeit von Magnesiumlegierungen gegen

par corrosion sous contrainte (ISO 20728:2018) Spannungsrisskorrosion (ISO 20728:2018)

This European Standard was approved by CEN on 6 September 2018.

CEN members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this

European Standard the status of a national standard without any alteration. Up-to-date lists and bibliographical references

concerning such national standards may be obtained on application to the CEN-CENELEC Management Centre or to any CEN

member.

This European Standard exists in three official versions (English, French, German). A version in any other language made by

translation under the responsibility of a CEN member into its own language and notified to the CEN-CENELEC Management

Centre has the same status as the official versions.

CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia,

Finland, Former Yugoslav Republic of Macedonia, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania,

Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland,

Turkey and United Kingdom.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION
EUROPÄISCHES KOMITEE FÜR NORMUNG
CEN-CENELEC Management Centre: Rue de la Science 23, B-1040 Brussels

© 2018 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN ISO 20728:2018 E

worldwide for CEN national Members.
---------------------- Page: 3 ----------------------
SIST EN ISO 20728:2018
EN ISO 20728:2018 (E)
Contents Page

European foreword ....................................................................................................................................................... 3

---------------------- Page: 4 ----------------------
SIST EN ISO 20728:2018
EN ISO 20728:2018 (E)
European foreword

This document (EN ISO 20728:2018) has been prepared by Technical Committee ISO/TC 156

"Corrosion of metals and alloys" in collaboration with Technical Committee CEN/TC 262 “Metallic and

other inorganic coatings, including for corrosion protection and corrosion testing of metals and alloys”

the secretariat of which is held by BSI.

This European Standard shall be given the status of a national standard, either by publication of an

identical text or by endorsement, at the latest by April 2019, and conflicting national standards shall be

withdrawn at the latest by April 2019.

Attention is drawn to the possibility that some of the elements of this document may be the subject of

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

According to the CEN-CENELEC Internal Regulations, the national standards organizations of the

following countries are bound to implement this European Standard: Austria, Belgium, Bulgaria,

Croatia, Cyprus, Czech Republic, Denmark, Estonia, Finland, Former Yugoslav Republic of Macedonia,

France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta,

Netherlands, Norway, Poland, Portugal, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland,

Turkey and the United Kingdom.
Endorsement notice

The text of ISO 20728:2018 has been approved by CEN as EN ISO 20728:2018 without any modification.

---------------------- Page: 5 ----------------------
SIST EN ISO 20728:2018
---------------------- Page: 6 ----------------------
SIST EN ISO 20728:2018
INTERNATIONAL ISO
STANDARD 20728
First edition
2018-09
Corrosion of metal and alloys —
Determination of resistance of
magnesium alloys to stress corrosion
cracking
Corrosion des métaux et alliages — Détermination de la résistance
des alliages de magnésium à la fissuration par corrosion sous
contrainte
Reference number
ISO 20728:2018(E)
ISO 2018
---------------------- Page: 7 ----------------------
SIST EN ISO 20728:2018
ISO 20728:2018(E)
COPYRIGHT PROTECTED DOCUMENT
© ISO 2018

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
Fax: +41 22 749 09 47
Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland
ii © ISO 2018 – All rights reserved
---------------------- Page: 8 ----------------------
SIST EN ISO 20728:2018
ISO 20728:2018(E)
Contents Page

Foreword ........................................................................................................................................................................................................................................iv

1 Scope ................................................................................................................................................................................................................................. 1

2 Normative references ...................................................................................................................................................................................... 1

3 Terms and definitions ..................................................................................................................................................................................... 1

4 General principles ............................................................................................................................................................................................... 2

5 Apparatus and materials.............................................................................................................................................................................. 2

5.1 Loading apparatus ............................................................................................................................................................................... 2

5.2 Construction materials for the test set-up ...................................................................................................................... 2

5.3 Specimen holders ................................................................................................................................................................................. 2

5.4 Apparatus for alternate immersion in solutions ....................................................................................................... 3

6 Sampling ........................................................................................................................................................................................................................ 3

7 Specimens .................................................................................................................................................................................................................... 5

7.1 Type and sizes.......................................................................................................................................................................................... 5

7.2 Surface preparation ............................................................................................................................................................................ 5

7.3 Specimen identification ................................................................................................................................................................... 6

7.4 Precautions ................................................................................................................................................................................................ 6

8 Test environments .............................................................................................................................................................................................. 6

9 Loading ........................................................................................................................................................................................................................... 7

10 Procedure..................................................................................................................................................................................................................... 7

11 Assessment of results ...................................................................................................................................................................................... 8

12 Expression of results ........................................................................................................................................................................................ 9

13 Test report ................................................................................................................................................................................................................... 9

Annex A (informative) Assessment of grain size and orientation .......................................................................................11

Bibliography .............................................................................................................................................................................................................................12

© ISO 2018 – All rights reserved iii
---------------------- Page: 9 ----------------------
SIST EN ISO 20728:2018
ISO 20728:2018(E)
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 documents 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).

Attention is drawn to the possibility that some of the elements of this document may be the subject of

patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of

any patent rights identified during the development of the document will be in the Introduction and/or

on the ISO list of patent declarations received (see www .iso .org/patents).

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, Corrosion of metals and alloys.

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.
iv © ISO 2018 – All rights reserved
---------------------- Page: 10 ----------------------
SIST EN ISO 20728:2018
INTERNATIONAL STANDARD ISO 20728:2018(E)
Corrosion of metal and alloys — Determination of
resistance of magnesium alloys to stress corrosion
cracking

WARNING — This document calls for the use of substances and/or procedures that can

be injurious to health if adequate safety measures are not taken. This document does not

address any health hazards, safety or environmental matters associated with its use. It is

the responsibility of the user of this document to establish appropriate health, safety and

environmentally acceptable practices.
1 Scope

This document specifies a method for the determination of resistance to stress corrosion cracking

(SCC) of magnesium alloys intended for use in structural applications (such as magnesium front end,

gearbox and clutch housing units, steering column parts, shift actuators, valve covers and housings,

brackets and intake manifold blades, electronic devices, power tools and medical equipment). The

method allows determination of the resistance to SCC as a function of the chemical composition, the

method of manufacture and heat treatment of magnesium alloys.

The document is applicable to cast and wrought magnesium alloys in the form of castings, semi-finished

products, parts and weldments and covers the method of sampling, the types of specimens, the loading

procedure, the type of environment and the interpretation of results.

The document allows assessment of the relative performance of materials and products in environments

containing chlorides or sulphates, provided that the failure mechanism is not changed, but does not

qualify a material or product for service application.
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 7539-1, Corrosion of metals and alloys — Stress corrosion testing — Part 1: General guidance on testing

procedures

ISO 7539-4, Corrosion of metals and alloys — Stress corrosion testing — Part 4: Preparation and use of

uniaxially loaded tension specimens

ISO 7539-7:2005, Corrosion of metals and alloys — Stress corrosion testing — Part 7: Method for slow

strain rate testing
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 and ISO 7539-1 apply.

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

— ISO Online browsing platform: available at https: //www .iso .org/obp
— IEC Electropedia: available at http: //www .electropedia .org/
© ISO 2018 – All rights reserved 1
---------------------- Page: 11 ----------------------
SIST EN ISO 20728:2018
ISO 20728:2018(E)
4 General principles

4.1 Stress corrosion cracking of magnesium alloys is sensitive to microstructural orientation with

respect to the stress axis. Accordingly, in testing SCC resistance, it is necessary to consider the manner

in which the specimens are prepared from cast or wrought alloy so that orientation dependent SCC

resistance can be assessed.

4.2 The corrosion of magnesium is associated with hydrogen evolution and generation of often very

soluble corrosion products. For that reason, testing in stagnant conditions is preferred during continuous

immersion as stirring can cause secondary effects, e.g. removal of corrosion product.

4.3 Two methods of immersion in the solution are proposed:
— alternate immersion;
— continuous immersion.

4.4 Tests may be conducted under constant load, constant strain or by the slow strain rate technique

with evaluation criteria for stress corrosion cracking resistance appropriate to the chosen loading method.

4.5 The method of loading, the value of stresses, corrosive environment and criteria of evaluation

should be agreed between the interested parties according to the purpose of the testing.

5 Apparatus and materials
5.1 Loading apparatus

Tensile stresses in the specimens are produced with yokes, stressing screws, springs, lever devices and

special testing machines.
5.2 Construction materials for the test set-up

5.2.1 If in contact with the corrosive environment, the construction materials for the test set-up shall

not be affected by the corrodent to such an extent that they can cause contamination of the solution and

change its corrosiveness.

5.2.2 Use of inert plastics or glass is recommended for the corrosion cell where feasible.

5.2.3 Metallic components in contact with the solution shall be made from an appropriate corrosion

resistant material, or protected with a suitable corrosion-resistant coating, sufficient to avoid galvanic

coupling.
5.3 Specimen holders

5.3.1 The specimen holders shall be designed to electrically insulate the specimens from each other

and from any bare metal parts. When this is not possible, as in the case of certain stressing bolts or

jigs, the bare metal contacting the specimen shall be isolated from the corrodent by a suitable coating.

Protective coatings shall be of a type that will not leach inhibiting or accelerating ions or protective oils or

leave any residue, e.g. vapour, on the non-coated portions of the specimen holder. In particular, coatings

containing chromates or releasing any other contaminants should be avoided. All samples holders should

be degrea
...

SLOVENSKI STANDARD
oSIST prEN ISO 20728:2017
01-julij-2017

Korozija kovin in zlitin - Ugotavljanje odpornosti magnezijevih zlitin proti pokanju

zaradi napetostne korozije (ISO/DIS 20728:2017)

Corrosion of metal and alloys - Determination of resistance of magnesium alloys to

stress corrosion cracking (ISO/DIS 20728:2017)
Korrosion von Metall und Legierungen - Bestimmung der Beständigkeit von
Magnesiumlegierungen gegen Spannungsrisskorrosion (ISO/DIS 20728:2017)
Corrosion des métaux et alliages - (ISO/DIS 20728:2017)
Ta slovenski standard je istoveten z: prEN ISO 20728
ICS:
77.060 Korozija kovin Corrosion of metals
oSIST prEN ISO 20728:2017 en

2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

---------------------- Page: 1 ----------------------
oSIST prEN ISO 20728:2017
---------------------- Page: 2 ----------------------
oSIST prEN ISO 20728:2017
DRAFT INTERNATIONAL STANDARD
ISO/DIS 20728
ISO/TC 156 Secretariat: SAC
Voting begins on: Voting terminates on:
2017-04-26 2017-07-18
Corrosion of metal and alloys — Determination of
resistance of magnesium alloys to stress corrosion
cracking
Corrosion des métaux et alliages —
ICS: 77.060
THIS DOCUMENT IS A DRAFT CIRCULATED
This document is circulated as received from the committee secretariat.
FOR COMMENT AND APPROVAL. IT IS
THEREFORE SUBJECT TO CHANGE AND MAY
NOT BE REFERRED TO AS AN INTERNATIONAL
STANDARD UNTIL PUBLISHED AS SUCH.
IN ADDITION TO THEIR EVALUATION AS
ISO/CEN PARALLEL PROCESSING
BEING ACCEPTABLE FOR INDUSTRIAL,
TECHNOLOGICAL, COMMERCIAL AND
USER PURPOSES, DRAFT INTERNATIONAL
STANDARDS MAY ON OCCASION HAVE TO
BE CONSIDERED IN THE LIGHT OF THEIR
POTENTIAL TO BECOME STANDARDS TO
WHICH REFERENCE MAY BE MADE IN
Reference number
NATIONAL REGULATIONS.
ISO/DIS 20728:2017(E)
RECIPIENTS OF THIS DRAFT ARE INVITED
TO SUBMIT, WITH THEIR COMMENTS,
NOTIFICATION OF ANY RELEVANT PATENT
RIGHTS OF WHICH THEY ARE AWARE AND TO
PROVIDE SUPPORTING DOCUMENTATION. ISO 2017
---------------------- Page: 3 ----------------------
oSIST prEN ISO 20728:2017
ISO/DIS 20728:2017(E)
COPYRIGHT PROTECTED DOCUMENT
© 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

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
Ch. de Blandonnet 8 • CP 401
CH-1214 Vernier, Geneva, Switzerland
Tel. +41 22 749 01 11
Fax +41 22 749 09 47
copyright@iso.org
www.iso.org
ii © ISO 2017 – All rights reserved
---------------------- Page: 4 ----------------------
oSIST prEN ISO 20728:2017
ISO/DIS 20728:2017(E)
Contents Page

Foreword ........................................................................................................................................................................................................................................iv

1 Scope ................................................................................................................................................................................................................................. 1

2 Normative references ...................................................................................................................................................................................... 1

3 Terms and definitions ..................................................................................................................................................................................... 2

4 General principles ............................................................................................................................................................................................... 2

5 Apparatus and materials.............................................................................................................................................................................. 2

5.1 Loading apparatus ............................................................................................................................................................................... 2

5.2 Construction materials for the test set-up ...................................................................................................................... 2

5.3 Specimen holders ................................................................................................................................................................................. 2

5.4 Apparatus for alternate immersion in solutions ....................................................................................................... 3

6 Sampling ........................................................................................................................................................................................................................ 3

7 Specimens .................................................................................................................................................................................................................... 5

7.1 Type and sizes.......................................................................................................................................................................................... 5

7.2 Surface preparation ............................................................................................................................................................................ 5

7.3 Specimen identification ................................................................................................................................................................... 6

7.4 Precautions ................................................................................................................................................................................................ 6

8 Test environments .............................................................................................................................................................................................. 6

9 Loading ........................................................................................................................................................................................................................... 7

10 Procedure..................................................................................................................................................................................................................... 7

11 Assessment of results ...................................................................................................................................................................................... 8

12 Expression of results ........................................................................................................................................................................................ 9

13 Test report ................................................................................................................................................................................................................... 9

Annex A (informative) Assessment of grain size and orientation .......................................................................................10

© ISO 2017 – All rights reserved iii
---------------------- Page: 5 ----------------------
oSIST prEN ISO 20728:2017
ISO/DIS 20728:2017(E)
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.

International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.

The main task of technical committees is to prepare International Standards. Draft International

Standards adopted by the technical committees are circulated to the member bodies for voting.

Publication as an International Standard requires approval by at least 75 % of the member bodies

casting a vote.

Attention is drawn to the possibility that some of the elements of this document may be the subject of

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

ISO 20728 was prepared by Technical Committee ISO/TC 156, Corrosion of metals and alloys.

This second/third/... edition cancels and replaces the first/second/... edition (), [clause(s) / subclause(s)

/ table(s) / figure(s) / annex(es)] of which [has / have] been technically revised.

iv © ISO 2017 – All rights reserved
---------------------- Page: 6 ----------------------
oSIST prEN ISO 20728:2017
DRAFT INTERNATIONAL STANDARD ISO/DIS 20728:2017(E)
Corrosion of metal and alloys — Determination of
resistance of magnesium alloys to stress corrosion
cracking

WARNING — This International Standard calls for the use of substances and/or procedures

that may be injurious to health if adequate safety measures are not taken. This International

Standard does not address any health hazards, safety or environmental matters associated with

its use. It is the responsibility of the user of this International Standard to establish appropriate

health, safety and environmentally acceptable practices and take suitable actions for any

national and international regulations. Compliance with this International Standard does not in

itself confer immunity from legal obligations.
1 Scope

This standard specifies a method for the determination of resistance to stress corrosion cracking (SCC)

of magnesium alloys intended for use in structural applications (such as magnesium front end, gearbox

and clutch housing units, steering column parts, shift actuators, valve covers and housings, brackets

and intake manifold blades, electronic devices, power tools and medical equipment). The method

allows determination of the resistance to SCC as a function of the chemical composition, the method of

manufacture and heat treatment of magnesium alloys.

The standard is applicable to cast and wrought magnesium alloys in the form of castings, semi-finished

products, parts and weldments and covers the method of sampling, the types of specimens, the loading

procedure, the type of environment and the interpretation of results.

The standard allows assessment of the relative performance of materials and products in environments

containing chlorides or sulphates, provided that the failure mechanism is not changed, but does not

qualify a material or product for service application.
2 Normative references

The following documents, in whole or in part, are normatively referenced in 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 7539-1, Corrosion of metals and alloys — Stress corrosion testing — Part 1: General guidance on testing

procedures

ISO 7539-2, Corrosion of metals and alloys — Stress corrosion testing — Part 2: Preparation and use of

bent-beam specimens

ISO 7539-3, Corrosion of metals and alloys — Stress corrosion testing — Part 3: Preparation and use of

U-bend specimens

ISO 7539-4, Corrosion of metals and alloys — Stress corrosion testing — Part 4: Preparation and use of

uniaxially loaded tension specimens

ISO 7539-5, Corrosion of metals and alloys — Stress corrosion testing — Part 5: Preparation and use of

C-ring specimens

ISO 7539-6, Corrosion of metals and alloys — Stress corrosion testing — Part 6: Preparation and use of

precracked specimens for tests under constant load or constant displacement

ISO 7539-7:2005, Corrosion of metals and alloys — Stress corrosion testing — Part 7: Method for slow

strain rate testing
© ISO 2017 – All rights reserved 1
---------------------- Page: 7 ----------------------
oSIST prEN ISO 20728:2017
ISO/DIS 20728:2017(E)

ASTM D 1384-96, Standard Test Method for Corrosion Test for Engine Coolants in Glassware

3 Terms a nd definiti ons

For the purposes of this document the terms and definitions given in ISO 8044 and in ISO 7539-1 apply.

4 General principles

4.1 Stress corrosion cracking of magnesium alloys is sensitive to microstructural orientation with

respect to the stress axis. Accordingly, in testing SCC resistance, it is necessary to consider the manner

in which the specimens are prepared from cast or wrought alloy so that orientation dependent SCC

resistance may be assessed.

4.2 The corrosion of magnesium is associated with hydrogen evolution and generation of often very

soluble corrosion products. For that reason, testing in stagnant conditions is preferred during continuous

immersion as stirring may cause secondary effects, e.g. removal of corrosion product.

4.3 Two methods of immersion in the solution are proposed:
— alternate immersion,
— continuous immersion.

4.4 Tests may be conducted under constant load, constant strain or by the slow strain rate technique

with evaluation criteria for stress corrosion cracking resistance appropriate to the chosen loading method.

4.5 The method of loading, the value of stresses, corrosive environment and criteria of evaluation

should be agreed between the interested parties according to the purpose of the testing.

5 Apparatus and materials
5.1 Loading apparatus

Tensile stresses in the specimens are produced with yokes, stressing screws, springs, lever devices and

special testing machines.
5.2 Construction materials for the test set-up

5.2.1 If in contact with the corrosive environment, the construction materials for the test set-up shall

not be affected by the corrodent to such an extent that they can cause contamination of the solution and

change its corrosiveness.

5.2.2 Use of inert plastics or glass is recommended for the corrosion cell where feasible.

5.2.3 Metallic components in contact with the solution shall be made from an appropriate corrosion

resistant material, or protected with a suitable corrosion-resistant coating, sufficient to avoid galvanic

coupling.
5.3 Specimen holders

5.3.1 The specimen holders shall be designed to electrically insulate the specimens from each other

and from any bare metal parts. When this is not possible, as in the case of certain stressing bolts or

jigs, the bare metal contacting the specimen shall be isolated from the corrodent by a suitable coating.

2 © ISO 2017 – All rights reserved
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oSIST prEN ISO 20728:2017
ISO/DIS 20728:2017(E)

Protective coatings shall be of a type that will not leach inhibiting or accelerating ions or protective oils or

leave any residue, e.g. vapour, on the non-coated portions of the specimen holder. In particular, coatings

containing chromates or releasing any other contaminants should be avoided. All samples holders should

be degreased after coating.

5.3.2 The equipment required for slow strain rate testing is a device that permits a selection of strain

rates whilst being powerful enough to cope with the loads generated. Strain rates that have been used

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most frequently in testing initially plain specimens are in the range 10 s to 10 s .

5.4 Apparatus for alternate immersion in solutions

5.4.1 Any suitable mechanism may be used to accomplish the immersion portion of the cycle

provided that:
a) it achieves the specified rate of immersion and removal; and
b) the apparatus is constructed of suitable inert materials.
The usual methods of alternate immersion are

a) specimens are placed on a movable rack that is periodically lowered into a stationary tank

containing the solution;

b) specimens are placed on a corrosion wheel arrangement which rotates every 10 min through 60°

and thereby passes the specimens through a stationary tank of solution;

c) specimens are placed in a stationary tray open to the atmosphere and the solution is moved by air

pressure, by a non-metallic pump, or by gravity drain from the reservoir to the tray.

5.4.2 The rate of immersion and removal of the specimens from t
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