Article
Stainless steel that has been poorly heat treated or held too long at certain temperatures can corrode along its grain boundaries while the surface looks perfect. Intergranular corrosion (IGC) testing checks for this hidden weakness before the material goes into service.
What causes intergranular corrosion?
When austenitic stainless steel is exposed to roughly 425–815 °C, chromium carbides can form at grain boundaries. The zones next to them lose chromium and become vulnerable. The material is then called sensitised. Correct solution annealing and quenching prevent this.
ASTM A262 practices
| Practice | Method | Purpose |
|---|---|---|
| A | Oxalic acid etch, microscopic exam | Fast screening; acceptable structures pass without further testing |
| B | Ferric sulphate–sulphuric acid, 120 h | Weight loss test |
| C | Boiling nitric acid (Huey test), 5 × 48 h | Weight loss; used for nitric acid service |
| E | Copper–copper sulphate–sulphuric acid (Strauss), then 180° bend | Most commonly specified; checks for fissures after bending |
| F | Copper–copper sulphate–50% sulphuric acid | Weight loss for cast 316 grades |
Practice E – the common choice
In Practice E, specimens are boiled in an acidified copper sulphate solution with copper shot, usually for 15 or 24 hours, then bent through 180°. The bent surface is examined under low magnification. If there are no intergranular fissures, the material passes. Many project specifications for 304L, 316L and stabilised grades call for "IGC test per ASTM A262 Practice E".
Sensitising heat treatment
For L grades and stabilised grades, the specimen is often given a sensitising heat treatment (for example 1 hour at 675 °C) before testing. This checks that the material will resist IGC even after welding. Confirm whether your specification requires it.
How to order
- State "IGC test per ASTM A262 Practice E" (or the practice required).
- Specify whether sensitising heat treatment is required.
- Ask for the test report to be referenced on the EN 10204 3.1 certificate, by heat number.
Frequently asked questions
Which ASTM A262 practice is most commonly specified?
Practice E, the copper–copper sulphate–sulphuric acid test followed by a bend test.
What does a pass in Practice E mean?
No intergranular fissures are seen on the bent specimen under low magnification.
Why do L grades still get IGC tested?
To confirm correct heat treatment and resistance to sensitisation, often after a sensitising heat treatment.
Key takeaways
- IGC testing detects sensitised stainless steel.
- Practice E is the usual choice for pipes and fittings.
- Specify practice, sensitising treatment and reporting by heat.
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