Intergranular Corrosion Tests for Austenitic and Ferritic Fittings

Intergranular attack (IGA) can occur when chromium carbides or nitrides precipitate at grain boundaries, depleting the surrounding matrix of chromium and leaving the material susceptible to localized corrosion. For stainless steel fittings used in corrosive service, verifying resistance to IGA is often a procurement requirement. But the right test is not a single universal procedure. The alloy family determines which standard is relevant, and confusing the two will produce misleading results.

Why Alloy Family Determines the Test Standard

ASTM A262 contains practices for detecting susceptibility to intergranular attack in austenitic stainless steels. ASTM A763 contains practices for detecting susceptibility to intergranular attack in ferritic stainless steels. These two standards are not universal substitutes. The microstructure and precipitation behavior of austenitic and ferritic grades are different, so each standard is designed around the relevant metallurgy. A buyer who applies A262 to a ferritic fitting or A763 to an austenitic fitting is not getting meaningful data.

ASTM A262 for Austenitic Stainless Steels

ASTM A262 covers five practices:

• Practice A – Oxalic Acid Etch Test: A rapid screening method that classifies etch structures as acceptable or suspect by simple electrolytic etching. It applies only to grades listed in the individual hot acid tests. Acceptable specimens can be eliminated from further hot acid immersion testing.
• Practice B – Ferric Sulfate-Sulfuric Acid Test: A weight-loss-based immersion test for detecting susceptibility to IGA.
• Practice C – Nitric Acid Test: A weight-loss-based immersion test under boiling nitric acid conditions.
• Practice E – Copper-Copper Sulfate-Sulfuric Acid Test: A weight-loss-based immersion test using copper and copper sulfate in sulfuric acid.
• Practice F – Copper-Copper Sulfate-50% Sulfuric Acid Test: Specifically for molybdenum-bearing cast austenitic stainless steels.

The standard also describes specimen preparation, apparatus setup, testing procedures, calculations, and report contents. Etch structure classifications include step, dual, ditch, isolated ferrite, interdendritic ditches, and end-grain pitting types I and II.

ASTM A763 for Ferritic Stainless Steels

ASTM A763 covers four practices:

• Practice W – Oxalic Acid Etch Test: A rapid screening test for stabilized ferritic stainless steels, used to identify specimens not susceptible to IGA associated with chromium carbide precipitation. It is appropriate only for alloys listed in the standard's Table 1.
• Practice X – Ferric Sulfate-Sulfuric Acid Test: A weight-loss-based immersion test.
• Practice Y – Copper-Copper Sulfate-50% Sulfuric Acid Test: An immersion test for detecting IGA.
• Practice Z – Copper-Copper Sulfate-16% Sulfuric Acid Test: A similar immersion test at a lower acid concentration.

For Practices X and Y, specimens are examined under a binocular microscope at 40x magnification. The standard also covers bend test evaluations and can be used to evaluate the effect of heat treatment or fusion welding on susceptibility to IGA. Note that some stabilized ferritic stainless steels may show high rates in Practice X due to metallurgical factors not associated with chromium carbide or nitride precipitation. This must be considered in selecting the test method.

What Buyers Must Specify in RFQs

A statement like "test per ASTM A262" is incomplete. The purchaser must specify the exact practice, specimen condition, any sensitization treatment, and the acceptance criterion. This is not optional; the standards provide methods, but pass/fail limits depend on the application and the product specification.

When writing an RFQ for stainless steel fittings, include:

• The correct standard and practice letter (e.g., ASTM A262 Practice E for austenitic, or ASTM A763 Practice Y for ferritic).
• Material grade, product form, and heat treatment condition.
• Specimen condition: as-welded, post-weld heat treated, or solution annealed.
• Sensitization treatment details, if required, including time and temperature.
• Acceptance criteria: maximum corrosion rate, minimum bend angle, or etch structure classification for screening tests.
• Whether third-party witnessing or review is required.

The same level of detail should be entered into your purchase order to keep suppliers accountable. When reviewing our product range, you can see the grades and standards we support at /en/products. If you are uncertain which practice applies to your project, send your technical requirements through /en/inquiry and we can discuss what to specify.

Procurement Checks and Inspection Points

During procurement and inspection, verify the following on the test report:

• The report references the correct standard and practice letter.
• Specimen identification matches the production lot and heat number.
• The specimen condition and any sensitization treatment are stated.
• For screening tests (Practice A or W), the report classifies each specimen as acceptable or suspect. If suspect, the subsequent hot acid test should be identified.
• For immersion tests, weight-loss results and acceptance criteria are clearly reported.
• For Practice X and Y in A763, the report notes the 40x microscope examination and any bend test results.

The report must come from a competent laboratory and be signed or stamped by an authorized person. A certificate of compliance alone is not sufficient evidence that intergranular corrosion testing was performed. Always ask for the full test report or have it available for audit.

Common Risks and How to Avoid Them

• Using the wrong standard: Applying A262 to ferritic stainless steels is a frequent mistake. Check the alloy family first.
• Treating screening tests as acceptance tests: Oxalic acid etch tests are rapid classifiers, not final pass/fail tests. If a specimen is suspect, the hot acid immersion test must be performed.
• Not specifying the acceptance criterion: Without a defined limit, the test result is open to interpretation and disputes.
• Ignoring weld or heat treatment effects: A test specimen must represent the actual service condition, including any post-weld heat treatment.
• Overlooking Practice X caveats: For stabilized ferritic alloys, high corrosion rates in Practice X may be caused by metallurgical factors other than chromium carbide or nitride precipitation. Investigate before rejecting a lot.

How to Apply This in Your Next Procurement

Intergranular corrosion testing is a critical quality gate for stainless steel fittings. Start by identifying whether your material is austenitic or ferritic, then select the corresponding ASTM standard and practice. Specify the specimen condition, sensitization treatment, and acceptance criteria in your RFQ and purchase order. During inspection, check that the report includes all required information and that the practice was applied correctly.

For more information on our products and the standards we manufacture to, visit /en/products. To discuss your specific testing requirements, submit an inquiry at /en/inquiry.

Sources

• https://store.astm.org/a0262-15r21.html
• https://store.astm.org/a0763-15r25.html