Intergranular Corrosion Test: Detecting Grain Boundary Attack

Stainless steel doesn't usually fail loudly. It fails quietly, along grain boundaries you can't see without a microscope, until a bracket cracks mid-service or a pressure vessel starts leaking. That's what makes intergranular corrosion (IGC) so dangerous for aerospace, defense, and power generation components — the damage hides until it doesn't.

The financial exposure is real. Department of Defense-contracted analysis put corrosion costs across military systems at $20.6 billion in fiscal year 2016 alone (AMPP). Globally, NACE/IMPACT estimated corrosion costs businesses $2.5 trillion annually, or 3.4% of global GDP, with up to $875 billion of that avoidable through better corrosion control.

IGC testing exists to catch sensitized material before it ever reaches service. This guide breaks down how the testing works, which ASTM practice fits your alloy, and what to do when a specimen fails.

TL;DR

• IGC testing checks whether heat treatment or welding left stainless steel open to grain-boundary attack.
• Five ASTM A262 practices (A, B, C, E, F) each use a different acid solution and evaluation method.
• Use ASTM G28 for nickel-rich chromium alloys; it does not replace A262 on 304/316/321/347.
• Specify L-grade or stabilized alloys with documented heat history to cut sensitization risk early.
• Aero-Vac Alloys & Forge supplies certified, traceable IGC-resistant grades for corrosion-critical work.

What Is an Intergranular Corrosion Test?

Intergranular corrosion is a localized attack that eats along grain boundaries instead of across a surface. It's harder to spot than general corrosion because the exterior can look fine while the internal structure is compromised.

IGC testing detects sensitization — chromium carbides precipitate at grain boundaries during improper heat treatment or welding, leaving chromium-depleted zones that corrode preferentially.

This mostly affects austenitic stainless grades: 304, 316, 321, and 347.

Test types vary by:

• Acid solution used — oxalic, nitric, ferric sulfate-sulfuric, or copper sulfate-sulfuric
• Evaluation method — qualitative (visual etch or bend test) vs. quantitative (weight-loss measurement)

Each combination targets a different failure mode, which is why picking the right practice matters more than picking the "toughest" one.

Why Intergranular Corrosion Testing Matters

Welded and heat-treated components in high-stress industries can't afford surprises. IGC testing gives you documented proof that the material will hold up.

Here's what proper testing delivers:

• Prevents catastrophic failure in pressure vessels, aircraft brackets, and reactor components
• Confirms post-weld solution annealing was effective, closing the loop on thermal processing
• Supports traceability required for ISO 9001 and AS9100 certification packages
• Reduces field failures and warranty claims by catching problems before shipment
• Qualifies new suppliers, heats, or welding procedures with documented evidence
• Provides audit-ready records for regulatory and customer reviews

One caveat: certification alone doesn't guarantee A262 or G28 testing was performed. The purchase order or drawing has to specify the standard, practice, and acceptance criteria. Without that, testing is not a contractual requirement—only a possibility.

How Intergranular Corrosion Testing Works — Step by Step

ASTM A262 breaks down into five practical stages. Skipping specimen ageing or misreading an etch structure are the two most common mistakes labs make.

Step 1 – Select the Applicable Test Practice

Match the ASTM A262 practice (A, B, C, E, or F) to the stainless grade and application, or use ASTM G28 for nickel-rich alloys. Not every A262 practice applies to every grade:

Practice	Applies to 304/316	Applies to 321/347
A (oxalic etch)	Yes	Yes
B (ferric sulfate)	Yes	No (sigma phase risk)
C (nitric acid)	Yes (304)	No
E (Strauss)	Yes	Yes
F (copper sulfate)	Molybdenum grades only	No

Step 2 – Prepare and Age the Specimen

The specimen is metallographically prepared or thermally sensitized to simulate welding or service conditions. This step matters most: a poorly aged coupon can produce a false pass.

Step 3 – Expose to Corrosive Test Solution

Immerse or electrolytically etch the sample per the practice's exact recipe. For example, Practice E (Strauss) boils the specimen in copper sulfate and sulfuric acid for at least 24 hours.

Step 4 – Evaluate the Specimen

Evaluation depends on the practice:

1. Visual/microscopic examination of the etched surface (Practice A)
2. Bend testing for fissures or cracks (Practice E)
3. Weight-loss measurement to calculate corrosion rate (Practices B, C, F)

Step 5 – Interpret and Document Results

Compare results against the practice's acceptance criteria and record everything for certification:

• Step vs. ditch structure (Practice A)
• Crack or fissure presence after bending (Practice E)
• Corrosion rate thresholds (Practices B, C, F)

That record supports material certification and traceability requirements.

Intergranular Corrosion Test — Example Case Walkthrough

A batch of welded 316 stainless steel components is flagged for qualification testing before shipment to an aerospace customer.

The process:

1. Quality selects Practice E (Strauss test) given the welded condition and grade.
2. A coupon is aged to simulate the weld heat-affected zone.
3. The specimen boils in copper sulfate-sulfuric acid for 24+ hours.
4. The specimen is bent 180 degrees and examined at 5x–20x magnification.

The mistake: post-weld solution annealing was skipped or performed at the wrong temperature. The bend test shows visible cracking: a clear fail.

The fix: the batch gets flagged, re-annealed per the governing specification, and retested. Supplier documentation is updated to reflect the corrective action, and the annealing procedure is reviewed to prevent a repeat.

Heat treatment history and traceable documentation matter as much as the test itself. A test only confirms what already happened metallurgically. It can't undo bad thermal processing.

How Aero-Vac Alloys & Forge Can Help

Every IGC test result is only as good as the material behind it. Sourcing alloys with the right carbon content and processing history reduces sensitization risk before a coupon ever hits the acid bath.

Aero-Vac Alloys & Forge, a manufacturing distributor certified to ISO 9001:2008 and AS9100, stocks corrosion-resistant grades built for this exact concern:

• 304L (AMS 5647) — low-carbon, improved weldability and sensitization resistance
• 316L (AMS 5653) — low-carbon with added molybdenum for corrosive service
• 321 (AMS 5510/5557/5559/5570/5645) — titanium-stabilized for intergranular-corrosion resistance
• 347 (AMS 5512) — stabilized grade available in sheet, strip, and plate

Beyond stock availability, Aero-Vac offers:

• Material testing—in-house or through partnered providers—for conformance verification before shipment
• In-house heat treatment plus partnered NADCAP heat treaters for volume or turnaround needs
• Just-in-time delivery for custom cuts and odd sizes
• A live metals expert to help identify the right alloy for corrosive service environments

Aero-Vac supplies aerospace and defense customers including Boeing, SpaceX, and Lockheed — industries where a failed IGC test isn't an option.

Conclusion

Intergranular corrosion testing is the difference between shipping welded, heat-treated stainless components with confidence and discovering too late that sensitization slipped through.

Picking the right ASTM A262 or G28 practice depends entirely on alloy composition and end-use application — there's no universal shortcut. No test replaces sourcing traceable, properly processed material in the first place. Getting the alloy right from the start keeps sensitization off your inspection report.

Frequently Asked Questions

What is an intergranular corrosion test?

It's a standardized method, most commonly ASTM A262, used to detect a material's susceptibility to corrosion along grain boundaries caused by sensitization.

How is the IGC test performed?

A specimen is aged if required, exposed to a specified acid solution per ASTM A262 or G28, then evaluated by visual inspection, bend testing, or weight-loss measurement depending on the practice.

What is intergranular attack?

Intergranular attack is the selective corrosion of chromium-depleted zones at grain boundaries, unlike general surface corrosion that attacks the whole exposed surface evenly.

What causes intergranular corrosion?

Sensitization from heating in the 425–870°C (800–1600°F) range, which precipitates chromium carbide at grain boundaries and depletes the chromium needed for corrosion resistance nearby.

What is an example of intergranular corrosion?

Weld decay in the heat-affected zone of welded 304 stainless steel piping, particularly when exposed to acidic or chloride-rich environments.

Which ASTM A262 practice is most commonly used?

Practice B is widely used for standard 304/316 grades, while Practice E (Strauss) is the standard choice for stabilized and low-carbon grades like 321 and 347.