ASTM practice for IGC testing.ppt

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Standard ASTM Practices for Detecting Susceptibility to Intergranular Attack in

Austenitic Stainless Steels

Welding Engineering.

Date: 14.04.2006


Contents

• What is corrosion?

• Intergranular Corrosion

• Types of ASTM Practices

• Practice A

• Practice B

• Practice C

• Practice E


What is corrosion?

“Extractive metallurgy in reverse”

Fe2O3 FeExtraction

Corrosion


What is IGC?

Cr Depleted zone

Cr23C6

Grain Boundary

Grain

Sensitization Phenomena


Types of ASTM Practices

Type Description Used For

Practice AOxalic acid etch test

(rapid screening test)

Classification of etch structures of austenitic stainless steels.

Practice BFerric sulfate – Sulfuric acid test

Detecting susceptibility to intergranular attack in austenitic stainless steels

Practice C Nitric acid test

Practice ECopper – Copper Sulfate – 16% Sulfuric acid test

Practice FCopper – Copper Sulfate – 50% Sulfuric acid test

Determining susceptibility to intergranular attack in austenitic stainless steels


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 :


• 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 :


• 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


• 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


Practice – A (5)

1. Step Structure 2. Dual Structure 3. Ditch Structure


4. Isolated Ferrite 5. Interdendritic Ditches 6. End Grain Pitting - I

Practice – A (6)


7. End Grain Pitting - II

Practice – A (7)


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:


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 :


304, 304L,

316, 316L,

317, 317L,

321


Extra –low –carbon grades and Stabilized grades

(e.g. 304L, 316L, 317L and 321) are tested after sensitizing

heat treatments.

Note :


Practice – B (2)


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.


Solution:

Practice – B (4)

400ml Distilled water +

236ml reagent grade sulfuric acid(95 to 98% concentration) +

25gm of reagent grade ferric sulfate solution

Apparatus:


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:


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


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:


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 :


304, 304L,




heat treatments.

Note :


Practice – C (2)


Practice – C (3)

Rapid Screening Test : (optional)

• It may be used before going for nitric acid test.


PRACTICE – A.



Solution:

Practice – C (4)

Reagent grade nitric acid added at the rate of 108ml per 1 liter of distilled water.

Apparatus:


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.



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


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:


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 :


201, 202, 301, 304, 304L, 304H, 316, 316L, 316H,

317, 317L, 321, 347




heat treatments.

Note :


Practice – E (2)


Practice – E (3)Rapid Screening Test : (optional)

• It is an optional test.


PRACTICE – A.

• It may be used unless prohibited by purchaser.

• Testing period shall be minimum 15 Hr.



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.


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.



•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)


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)


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:


Practice – E (9)

Acceptable microstructure: Non acceptable microstructure:


Metallographic Microstructure

Practice – E (10)


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:

Documents

ASTM practice for IGC testing.ppt