Stainless Steel Intergranular Corrosion Testing Standards: Complete Technical Guide

Intergranular corrosion (IGC) testing is an essential quality control requirement for stainless steel products destined for chemical, nuclear, and other aggressive service environments. This technical guide provides a comprehensive overview of stainless steel intergranular corrosion testing standards, including ASTM A262, ASTM G28, ISO 3651-2, and GB/T 4334, covering sample preparation, test procedures, evaluation criteria, and typical industrial applications.

Technical Overview

Intergranular corrosion is a localized form of attack that propagates along the grain boundaries of a metal while the grains themselves remain largely unaffected. In stainless steels, this phenomenon is primarily caused by sensitization, a metallurgical condition in which chromium carbides (Cr23C6) precipitate at grain boundaries when the material is exposed to temperatures between approximately 425°C and 815°C. This temperature range is commonly encountered during welding, slow cooling from solution annealing, or prolonged service at elevated temperatures.

When chromium carbides form at the grain boundaries, they deplete the surrounding matrix of chromium. If the chromium content in these depleted zones falls below approximately 12%, the passive oxide film that provides corrosion resistance can no longer be maintained locally. The affected grain boundaries become anodic relative to the grain interiors, and preferential dissolution occurs when the material is exposed to corrosive media.

The consequences of intergranular corrosion are severe. Affected materials may experience grain dropping, loss of mechanical strength, and catastrophic brittle failure with little or no external visual warning. For this reason, international testing standards have been developed to evaluate the susceptibility of stainless steels to this degradation mechanism before components enter service.

Common strategies for preventing sensitization include the use of low-carbon grades such as 304L and 316L (carbon content limited to 0.03% maximum), stabilized grades such as 321 (titanium-stabilized) and 347 (niobium-stabilized), and proper post-weld solution annealing treatments. Intergranular corrosion testing verifies the effectiveness of these measures and is frequently specified in material procurement specifications for critical applications.

Testing Samples

Intergranular corrosion testing can be performed on virtually all product forms of stainless steel. Sample selection and preparation directly influence the validity of test results, and the applicable standard typically specifies dimensions, surface conditioning, and heat treatment states to be evaluated.

Plate and Sheet Products

Flat-rolled products are the most commonly tested material form. Test coupons are usually taken from the mid-width position of the plate or sheet, at locations representing the full cross-section. Both as-received (mill-annealed) and sensitized conditions may be evaluated depending on the specification requirements.

Pipe and Tube Products

Seamless and welded pipe and tube samples are tested to verify base metal resistance and, in the case of welded products, the integrity of the weld seam and heat-affected zone. Full cross-section ring samples or longitudinal strips containing the weld are frequently used.

Bar, Rod, and Wire

Forged and rolled bar products are sampled in both longitudinal and transverse orientations to account for directional microstructural features. Samples are typically taken from mid-radius locations to represent the bulk material condition.

Forgings and Castings

Investment castings, sand castings, and forgings are tested to confirm that thermal processing and cooling practices have not induced sensitization. Cast alloys often require specific attention due to their coarser grain structure and segregation tendencies.

Welded Joints and Weldments

Weld procedure qualification frequently includes intergranular corrosion testing of cross-weld samples. These specimens encompass the base metal, heat-affected zone, and weld metal in a single coupon, providing a comprehensive assessment of the joint.

Finished Components

Fabricated components such as flanges, fittings, pressure vessel shells, and heat exchanger components may be sampled in the final heat-treated condition to verify compliance with purchase specifications before shipment.

Testing Items

Intergranular corrosion testing programs typically determine the following items:

• Detection of susceptibility to intergranular attack in the as-received condition
• Detection of susceptibility after an artificial sensitizing heat treatment (commonly 650°C for one hour, followed by water quenching)
• Measurement of average corrosion rate in boiling acid solutions, expressed in inches per month or millimeters per year
• Evaluation of bend test specimens for intergranular cracking after acid exposure
• Metallographic examination for ditch structures, grain boundary attack depth, and grain dropping
• Comparative ranking of heat treatment conditions, material heats, or welding procedures
• Verification of stabilization effectiveness in titanium- or niobium-bearing alloys
• Qualification of solution annealing and quenching practices

Testing Methods

Several standardized test methods exist, each employing a specific corrosive environment and evaluation approach. The selection depends on the alloy type, intended service environment, and specification requirements.

ASTM A262 Practice A: Oxalic Acid Etch Test

Practice A is a rapid screening test based on electrolytic etching of a polished metallographic specimen in a 10% oxalic acid solution at a current density of 1 A/cm2 for approximately 90 seconds. The resulting etch structure is classified at 250X to 500X magnification as step, dual, or ditch structure.

• Step structure: individual grain boundaries are revealed without ditches, indicating resistance to intergranular attack
• Dual structure: isolated ditch segments with steps elsewhere, requiring further testing for acceptance
• Ditch structure: continuous ditches surrounding entire grains, indicating possible susceptibility

Specimens exhibiting step structure may be accepted for certain applications without further testing, while dual or ditch structures require confirmation by one of the quantitative boiling acid practices.

ASTM A262 Practice B: Ferric Sulfate-Sulfuric Acid Test

Practice B exposes specimens to a boiling 50% sulfuric acid solution containing ferric sulfate for 120 hours. The test measures the average corrosion rate calculated from weight loss. This practice is suitable for detecting sensitization associated with chromium carbide precipitation in austenitic stainless steels and is less affected by molybdenum in the alloy than Practice C.

ASTM A262 Practice C: Nitric Acid Test (Huey Test)

Practice C, widely known as the Huey test, immerses specimens in boiling 65% nitric acid for five consecutive 48-hour periods, for a total exposure of 240 hours. The solution is refreshed after each period, and the corrosion rate is calculated for each period. This test is particularly severe and detects sensitization from chromium carbide precipitation as well as sigma phase and other chromium-rich phases. It is commonly specified for nuclear-grade materials intended for nitric acid service.

ASTM A262 Practice D: Nitric-Hydrofluoric Acid Test

Practice D uses a boiling solution of 10% nitric acid and 3% hydrofluoric acid for two 2-hour periods. It was developed specifically for molybdenum-bearing austenitic stainless steels such as 316 and 317, for which nitric acid testing alone can produce misleading results due to preferential molybdenum attack.

ASTM A262 Practice E: Copper-Copper Sulfate-Sulfuric Acid Test (Strauss Test)

Practice E, the classic Strauss test, exposes specimens to a boiling 16% sulfuric acid solution containing copper sulfate, with metallic copper contact maintained to establish a low corrosion potential. The standard exposure duration is 24 hours. After exposure, the specimen is bent through 180 degrees and examined at low magnification for intergranular cracking or fissures. Practice E is the most widely specified method for general-purpose qualification of austenitic stainless steels.

ASTM G28: Testing of Nickel-Rich Alloys

ASTM G28 covers corrosion-resistant, chromium-bearing nickel alloys such as Alloy 400-series, C-276, and 825.

• Method A: boiling ferric sulfate-50% sulfuric acid solution for 120 hours, evaluated by weight loss corrosion rate
• Method B: boiling mixed acid solution of 23% sulfuric acid, 1.2% hydrochloric acid, 1% ferric chloride, and 1% cupric chloride for 24 hours, providing faster detection of grain boundary precipitation in certain alloys

ISO 3651-2 and GB/T 4334 Equivalents

ISO 3651-2 specifies the sulfuric acid-copper sulfate test with copper contact (16% sulfuric acid) and the sulfuric acid-ferric sulfate test for austenitic stainless steels, closely corresponding to ASTM A262 Practices E and B. For the Chinese market, GB/T 4334 establishes the accepted series of methods, including the metallographic oxalic acid method, the ferric sulfate-sulfuric acid method, the 65% nitric acid method, the nitric-hydrofluoric acid method, and the sulfuric acid-copper sulfate method with copper chip contact. JIS G0575 defines the corresponding Japanese practice for the copper sulfate-sulfuric acid test. ASTM A763 provides test practices tailored to ferritic stainless steels.

Evaluation and Acceptance Criteria

Acceptance is based on one or more of the following criteria, as defined by the governing product specification:

• Maximum allowable corrosion rate, commonly 0.05 inches per month or a specification-defined equivalent in metric units
• Absence of intergranular cracks or fissures in the bend-tested specimen when bent 180 degrees around a mandrel related to specimen thickness
• Classification of oxalic acid etch structure as acceptable (step or dual, where permitted)
• Acceptable metallographic evidence of attack depth at magnifications specified by the standard

Applications

Intergranular corrosion testing is mandated or highly recommended across a wide range of industries where stainless steel components operate in corrosive media:

• Chemical and petrochemical processing: reactors, columns, piping, and heat exchangers handling acidic media
• Nuclear power: fuel reprocessing equipment, reactor internals, and primary circuit components, where Practice C nitric acid testing is frequently specified
• Oil and gas: offshore platforms, subsea components, and refinery units exposed to polythionic acid and chloride-containing environments
• Pharmaceutical and biotechnology: sanitary process equipment requiring documented corrosion integrity
• Food and beverage: processing vessels, storage tanks, and piping systems cleaned with aggressive chemical solutions
• Power generation: boiler feedwater heaters, condenser tubing, and flue gas desulfurization systems
• Pulp and paper: digesters, bleach plant washers, and liquor handling equipment
• Pressure vessel and piping fabrication: qualification of welding procedures and verification of post-weld heat treatment

FAQ

What is sensitization in stainless steel?

Sensitization is the precipitation of chromium carbides at grain boundaries, occurring when stainless steel is heated in the range of approximately 425°C to 815°C. The carbide precipitation depletes chromium from the adjacent grain boundary regions, creating zones that are vulnerable to intergranular attack in corrosive environments. Welding, improper annealing, and slow cooling are common causes.

Which ASTM A262 practice should be selected for testing?

The choice depends on the alloy and application. Practice A is a fast screening tool for austenitic grades. Practice E (Strauss test) is the most widely specified general acceptance test. Practice B is preferred when carbide sensitization alone is the concern, including for molybdenum-bearing grades. Practice C (Huey test) is specified for nitric acid service and nuclear applications. Practice D is reserved for molybdenum-bearing steels such as 316 and 317. The governing product specification or customer purchase order normally dictates the required practice.

What is the difference between the Huey test and the Strauss test?

The Huey test (Practice C) uses boiling 65% nitric acid for 240 hours in five 48-hour periods and evaluates susceptibility by weight-loss corrosion rate. It detects both chromium carbide precipitation and chromium-rich secondary phases such as sigma. The Strauss test (Practice E) uses boiling copper-copper sulfate-16% sulfuric acid for 24 hours and evaluates the bent specimen for intergranular cracking. The Strauss test operates at a lower corrosion potential and is generally less aggressive toward grain boundary chromium depletion.

Can welded components be tested for intergranular corrosion?

Yes. Cross-weld specimens containing base metal, heat-affected zone, and weld metal are routinely tested, most commonly by Practice E followed by a bend test. This approach is standard for weld procedure qualification and for verifying that welding thermal cycles have not sensitized the heat-affected zone.

What are typical pass criteria for intergranular corrosion testing?

For weight-loss methods such as Practices B, C, and D, acceptance is typically based on a maximum average corrosion rate defined by the specification, often on the order of 0.05 inches per month or less. For the Strauss bend test, the specimen must withstand a 180-degree bend without intergranular cracks or fissures visible at the specified magnification. For Practice A screening, a step structure is generally considered acceptable, while ditch structures require further quantitative testing.

Is a sensitizing heat treatment required before testing?

Not always. Many specifications require testing in the as-received (final heat-treated) condition to verify the delivered product. Others, particularly for stabilized or low-carbon grades intended for elevated temperature service, require an additional laboratory sensitizing treatment, commonly 650°C for one hour followed by water quenching, to demonstrate resistance under accelerated conditions.

How long does intergranular corrosion testing take?

Turnaround time varies by method. Practice A oxalic acid screening can be completed within a single day. Practice E requires approximately 24 hours of boiling exposure plus bend testing, typically completed in two to four working days. Practice B requires 120 hours of exposure, while Practice C requires 240 hours across five solution-change periods, often extending the total schedule to two weeks or more when metallographic evaluation and reporting are included.