The document outlines ASTM practices for detecting intergranular corrosion susceptibility in austenitic stainless steels, describing various testing methods including oxalic acid etch tests, ferric sulfate tests, nitric acid tests, and copper-sulfate tests. Each practice has specific applicability, preparation requirements, and limitations, aiming to provide a rapid assessment of material integrity after sensitizing heat treatments. The practices emphasize the importance of methodical examination to ensure accurate classifications of the etch structures and the susceptibility of materials to intergranular attack.

1. 1
Standard ASTM Practices for Detecting
Susceptibility to Intergranular Attack in
Austenitic Stainless Steels
Welding Engineering.
Date: 14.04.2006

2. 2
Contents
• What is corrosion?
• Intergranular Corrosion
• Types of ASTM Practices
• Practice A
• Practice B
• Practice C
• Practice E

3. 3
What is corrosion?
“Extractive metallurgy in reverse”
Fe2O3 Fe
Extraction
Corrosion

4. 4
What is IGC?
Cr Depleted zone
Cr23C6
Grain Boundary
Grain
Sensitization Phenomena

5. 5
Types of ASTM Practices
Type Description Used For
Practice A
Oxalic acid etch test
(rapid screening test)
Classification of etch
structures of austenitic
stainless steels.
Practice B
Ferric sulfate –
Sulfuric acid test
Detecting susceptibility to
intergranular attack in
austenitic stainless steels
Practice C Nitric acid test
Practice E
Copper – Copper
Sulfate – 16% Sulfuric
acid test
Practice F
Copper – Copper
Sulfate – 50% Sulfuric
acid test
Determining susceptibility
to intergranular attack in
austenitic stainless steels

6. 6
Practice – A (1)
Oxalic acid etch test
• Used for acceptance of material but not for rejection of
material.
• Used in connection with other evaluation tests.
• It is rapid screening test for identifying specimens that
shows free of susceptibility to rapid intergranular attack in
other hot acid tests.
• Specimens are Identified by means of their etch structures.
Scope :

7. 7
• All Austenitic Stainless Steels.
Applicable Materials :
Extra –low –carbon grades and stabilized grades
(e.g. 304L, 316L, 317L, 321 and 347) are tested after
sensitizing heat treatments.
Note :
• sensitized at 675ºC for 1 Hr.
Practice – A (2)
Applicable Tests :

8. 8
• Specimen size is 25 x 25 mm
• Sawing is preferred to shearing
Preparation of Test Specimens :
Practice – A (3)
Cutting
Polishing • Cross sectional surfaces should be polished.
• Specimens containing welds should include base plate,
weld HAZ and weld metal.
• Polishing on successively finer emery papers.
• Solution
Etching
900mL of
distilled water
100 gm of reagent grade oxalic acid
crystals (H2C2O4
.2H2O)
+
• Stirring until all crystals are dissolved

9. 9
• Etched surface is examined on a metallurgical microscope at 250X to 500X
Classifications of Etch Structure :
Practice – A (4)
• The etch structures are classified into the following types
1. Step Structure
2. Dual Structure
3. Ditch Structure
4. Isolated Ferrite
5. Interdendritic Ditches
6. End Grain Pitting I
7. End-Grain Pitting II

10. 10
Practice – A (5)
1. Step Structure 2. Dual Structure 3. Ditch Structure

11. 11
4. Isolated Ferrite 5. Interdendritic Ditches 6. End Grain Pitting - I
Practice – A (6)

12. 12
7. End Grain Pitting - II
Practice – A (7)

13. 13
Practice – A (8)
Advantages:
• Rapid method.
•Use for all austenitic stainless steel.
•It does not come on direct conclusion.
•This test depends on other practices.
Limitations:

14. 14
Practice – B (1)
Ferric sulfate – Sulfuric acid test
• Only used for cast steel.
• Only for predicting resistance to intergranular
corrosion.
• Detects susceptibility to intergranular attack
associated with the precipitation of Cr carbides not
with sigma phase.
Scope :

15. 15
304, 304L,
316, 316L,
317, 317L,
321
Applicable Materials :
Extra –low –carbon grades and Stabilized grades
(e.g. 304L, 316L, 317L and 321) are tested after sensitizing
heat treatments.
Note :
• sensitized at 675ºC for 1 Hr.
Practice – B (2)

16. 16
Practice – B (3)
Rapid Screening Test : (optional)
• It may be used before going for ferric sulfate sulfuric acid test.
• This test is done in accordance with procedures described in
PRACTICE – A.
The use of etch structure evaluation in connection with this test is specified in table below.

17. 17
Solution:
Practice – B (4)
400ml Distilled water +
236ml reagent grade sulfuric acid(95 to 98% concentration) +
25gm of reagent grade ferric sulfate solution
Apparatus:

18. 18
Practice – B (5)
•Specimen having surface area of 5 to 20cm recommended
Sample size:
• Specimen containing weld should be cut with included 13mm base
metal on either side.
Method:
•Place specimen in glass candle.
•Immerse it in boiling solution.
•No appreciable change in acid level measured.
•Process continue for 120 Hr.
Location of specimen:

19. 19
Calculation:
•It is determining by weight loss of the specimen
•Corrosion rate should be reported as millimeters of penetration per
month
Practice – B (6)
Millimeter per month = (7290 X W)/(A X t X d)
Where:
t : time of exposure, h,
A: area per cm2,
W: weight loss, g , and
d : density, g / cm2
For Cr-Ni, d = 7.9 g / cm2 & for Cr-Ni-Mo steel< d = 8.0 g / cm3

20. 20
Practice – B (7)
Advantages:
• Process accuracy is good because it is based on weight loss method.
• It does not detect susceptibility associated with sigma phase in
wrought Cr-Ni-Mo stainless ( e.g. 316, 316L, 317, 317L)
•It does not detect the susceptibility to end grain attack.
Limitations:

21. 21
Practice – C (1)
Nitric Acid Test
• The test may conduct on ‘as received material’ also.
• It is not necessarily measure the performance of material in
other corrosive environment.
• It is also used to check the effectiveness of stabilizing
elements.
• This practice may be applied to wrought products, casting
& weld metal.
Scope :

22. 22
304, 304L,
Applicable Materials :
Extra –low –carbon grades and Stabilized grades
(e.g. 304L, 316L, 317L and 321) are tested after sensitizing
heat treatments.
Note :
• sensitized at 675ºC for 1 Hr.
Practice – C (2)

23. 23
Practice – C (3)
Rapid Screening Test : (optional)
• It may be used before going for nitric acid test.
• This test is done in accordance with procedures described in
PRACTICE – A.
The use of etch structure evaluation in connection with this test is specified in table below.

24. 24
Solution:
Practice – C (4)
Reagent grade nitric acid added at the rate of 108ml per 1 liter of
distilled water.
Apparatus:

25. 25
Practice – C (5)
•Normally 100 Gm. Of specimen should be taken
Sample size:
• Specimen containing weld should be cut with included 13mm base
metal on either side.
Method:
•Use sufficient quantity of nitric acid at least 20 ml/in2.
•Bring acid on hot plate and boiling throughout test period.
•After each test rinse the specimen with water then dried and weighed.
•Conduct 5 boiling period of 48 Hr
•For every test bring new solution.
Location of specimen:

26. 26
Calculation:
•It is determining by weight loss of the specimen
•Corrosion rate should be reported as millimeters of penetration per
month
Practice – C (6)
Millimeter per month = (7290 X W)/(A X t X d)
Where:
t : time of exposure, h,
A: area per cm2,
W: weight loss, g , and
d : density, g / cm2
For Cr-Ni, d = 7.9 g / cm2 & for Cr-Ni-Mo steel< d = 8.0 g / cm3

27. 27
Practice – C (7)
Advantages:
• It detect susceptibility to intergranular attack associated primarily with
Chromium Carbide Precipitate and with sigma like precipitates.
•It is used for as received material.
•It also detect susceptibility to IGC for stabilizing elements.
• It does not detect susceptibility associated with sigma phase in
wrought Cr-Ni-Mo stainless ( e.g. 316, 316L, 317, 317L)
Limitations:

28. 28
Practice – E (1)
Copper-Copper Sulfate-Sulfuric acid test
• The test may conduct on ‘as received material’
also.
• It is also used to check the effectiveness of
stabilizing elements(e.g. Cb, Ti and so forth).
• It does not detect the susceptibility with sigma
phase.
Scope :

29. 29
201, 202, 301, 304, 304L, 304H, 316, 316L, 316H,
317, 317L, 321, 347
Applicable Materials :
Extra –low –carbon grades and Stabilized grades
(e.g. 304L, 316L, 317L and 321) are tested after sensitizing
heat treatments.
Note :
• sensitized at 675ºC for 1 Hr.
Practice – E (2)

30. 30
Practice – E (3)
Rapid Screening Test : (optional)
• It is an optional test.
• This test is done in accordance with procedures described in
PRACTICE – A.
• It may be used unless prohibited by purchaser.
• Testing period shall be minimum 15 Hr.
The use of etch structure evaluation in connection with this test is specified in table below.

31. 31
Solution:
Practice – E (4)
100gm Copper Sulfate(CuSo4.5H2O) + 700ml distilled water + add
100ml Sulfuric Acid(H2So4) and dilute to 1000ml with distilled water.
Copper addition:
Electrolytic grade copper shot or grinding may be used.
Sufficient quantity copper may used to cover full surface of the
specimen
Effective galvanic coupling may have importance.

32. 32
Practice – E (5)
•Guideline for Specimen size
given in table:
Sample size:
• The size and area of the sample generally specified in agreement
between purchaser and seller.
Location of specimen:

33. 33
•Solution should be sufficient to completely immerse the
specimen
•Each specimen can be tested in same container
•It is ideal to have all specimens of same grade
•Solution maintain in boiling condition throughout test
period.
•Time of test shall minimum 15 Hr unless longer time
agreed upon purchaser and producer.
•Fresh solution not needed if test were run 48 to 72 Hr.2
Method:
Practice – E (6)

34. 34
Bend Test:
•Taken out specimen from solution and then polish to remove scale
•Specimen may be bent through 180° and over a dia. Equal to
thickness of specimen bring bent.
•Specimen having cold worked or low ductility, it is not practically
possible to bend 180°.
•That time determine the maximum angle without causing crack and
bend another untested specimen of same configuration as specimen
to be tested.
•Specimen generally bent holding in vise and bend with hammer
Practice – E (7)

35. 35
Bent specimen examined under low magnification(5 to 20X)
Acceptable microstructure shown in fig. 11
Appearance of fissures and cracks indicate presence of
intergranular attack
Non acceptable microstructure shown in fig. 12
When evaluation is questionable metallographic examination of
bend specimen done.(See fig. 13)
Metallographic examination at magnification of 100 to 250X
Practice – E (8)
Evaluation:

36. 36
Practice – E (9)
Acceptable microstructure: Non acceptable microstructure:

37. 37
Metallographic Microstructure
Practice – E (10)

38. 38
Practice – E (11)
Advantages:
• All wrought products and weld material of austenitic SS can be
evaluated.
•We are able to measure corrosion by ductility test.
• Only predict resistance to IGC but not other form of corrosion.
Limitations: