ISO 4212:2023

INTERNATIONAL ISO
STANDARD 4212
First edition
2023-02
Corrosion of metals and alloys —
Method of oxalic acid etching test for
intergranular corrosion of austenitic
stainless steel
Corrosion des métaux et alliages — Méthode d'essai de gravure
à l'acide oxalique pour la corrosion intergranulaire de l'acier
inoxydable austénitique
Reference number
© ISO 2023
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ii
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 General principles . 2
5 Apparatus . 2
6 Sensitization heat treatment for test specimens. 2
7 Test specimens . 2
8 Test solution . 2
9 Test procedure .3
10 Evaluation of etching structure .4
11 Test report . 4
Bibliography . 6
iii
Foreword
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iv
Introduction
Intergranular corrosion (IGC) is a form of localized corrosion which occurs along grain boundaries. IGC
of stainless steel is attributable to grain boundary segregation and precipitates. When the formation
of chromium-rich grain boundary carbides occurs, it leads to continuous chromium depletion adjacent
to the grain boundary, and IGC occurs in certain corrosive media. This process is usually known
as sensitization. Sensitization of stainless steel can be evaluated by ISO IGC tests for austenitic
stainless steels: such as ISO 3651-1, ISO 3651-2, and ISO 12732. On the other hand, oxalic etching
tests (JIS G0571:2003 and ASTM A262-15 Practice A) cannot evaluate sensitization, however, they
can evaluate non-sensitized condition of austenitic stainless steels and suspicions of sensitization by
observing the etched surface of the stainless steels. This is because it does not etch chromium depletion
adjacent to the chromium-rich grain boundary precipitates but chromium-rich grain boundary carbides
themselves. If sensitization is suspected, IGC tests, ferric sulfate-sulfuric acid test (see ISO 3651-2),
65 % nitric acid test (see ISO 3651-1), copper sulfate-sulfuric acid test (see ISO 3651-2) or EPR test
(ISO 12732) should be conducted to evaluate sensitization of austenitic stainless steels.
The main benefits of these oxalic acid etching tests are that they are simple, easy and rapid. This
document is based on JIS G0571:2003.
v
INTERNATIONAL STANDARD ISO 4212:2023(E)
Corrosion of metals and alloys — Method of oxalic acid
etching test for intergranular corrosion of austenitic
stainless steel
1 Scope
This document specifies a method to test for intergranular corrosion (IGC) in austenitic stainless steels
using an oxalic acid etch. This method can evaluate non-sensitization of austenitic stainless steels by
observing the structure of the etched surface with a microscope after performing electrolytic etching
(hereinafter called “etching”) on austenitic stainless steel in an oxalic acid solution.
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
...