Intergranular Corrosion Testing of Stainless Steel Equipment: Standards, Methods, and Acceptance Cri

Intergranular corrosion (IGC) testing determines the resistance of stainless steel equipment to grain-boundary attack caused by sensitization during welding, heat treatment, or elevated-temperature service. Accreded laboratories perform IGC testing to ASTM A262, ASTM A763, and ISO 3651-2 to verify material quality, confirm proper solution annealing, and prevent sudden in-service failures of pressure vessels, piping, and process equipment. This guide explains the mechanisms of sensitization, sample requirements, standard test practices, evaluation criteria, and typical industrial applications.

Technical Overview

Intergranular corrosion (IGC) is a localized form of corrosion in which attack proceeds preferentially along the grain boundaries of a metal, while the grain interiors remain largely unattacked. In stainless steels, this phenomenon is almost always associated with sensitization: the precipitation of chromium-rich carbides (typically Cr23C6) at grain boundaries when the material is exposed to the critical temperature range of approximately 425 °C to 815 °C (800 °F to 1500 °F). Because carbon diffuses to grain boundaries much faster than chromium replenishes them from the grain interior, a thin zone adjacent to each boundary becomes depleted of chromium to below the approximately 12% level required to maintain a stable passive film. These chromium-depleted zones corrode rapidly in aggressive media.

Sensitization commonly occurs during fabrication of stainless steel equipment. Welding is the most frequent cause, since the heat-affected zone (HAZ) on either side of a weld passes through the sensitizing temperature range during heating and cooling. Other causes include slow cooling from hot working or solution annealing, improper stress-relief heat treatment, and prolonged service exposure at elevated temperatures. The consequences of undetected IGC are severe: progressive grain dropping, loss of load-bearing cross section, intergranular cracking under stress, and catastrophic leakage or rupture with little visible external warning.

Intergranular corrosion testing is therefore a mandatory quality control and qualification step for stainless steel equipment in demanding industries. The most widely applied standards include ASTM A262 (austenitic stainless steels), ASTM A763 (ferritic stainless steels), EN ISO 3651-1 and 3651-2, JIS G 0571–0575, and GB/T 4334. These standards define both rapid screening techniques and quantitative immersion tests that measure susceptibility to grain-boundary attack.

Testing Samples

Sample Forms and Product Types

Intergranular corrosion testing can be performed on virtually all product forms used in equipment fabrication. Typical sample submissions include:

• Plate, sheet, and strip in the delivery heat-treatment condition
• Seamless and welded pipe and tubing, including ring sections containing the weld seam
• Forgings, castings, fittings, flanges, and bar stock
• Cross-weld specimens covering weld metal, heat-affected zone, and base metal
• Production test coupons welded to the same procedure specification (WPS) as the actual equipment
• Wire and consumable electrode stock for welding qualification

Specimen Preparation Requirements

Specimen geometry and surface preparation directly affect test validity. General requirements include:

• Coupons with accurately measured surface area (typically 25 mm to 50 mm coupons for immersion tests)
• Surfaces ground to a uniform finish, commonly 120 grit, and degreased before testing
• Weight measurement to 0.001 g accuracy before and after exposure for weight-loss-based practices
• Identification markings applied in a way that does not contaminate the test surface
• Replicate specimens (usually a minimum of two or three per material condition) to ensure representative results

Sensitizing Heat Treatment

For stabilized grades (Type 321, Type 347) and low-carbon grades (304L, 316L), product specifications or the purchaser may require an accelerating sensitizing treatment, commonly 650 °C for 1 to 2 hours followed by air cooling, prior to testing. This laboratory sensitization demonstrates whether the material can withstand fabrication thermal cycles without becoming susceptible, and it is a key acceptance element in ASTM A262 and ISO 3651-2 procurement specifications.

Testing Items

A comprehensive intergranular corrosion test program for stainless steel equipment typically includes the following evaluation items:

• Determination of susceptibility to intergranular attack in the as-received (mill-annealed) condition
• Corrosion rate measurement by weight loss after boiling acid exposure, calculated in mpy, mm/year, or μm/48 h
• Bend test evaluation after acid exposure to detect intergranular fissuring
• Metallurgical structure classification of oxalic acid etch specimens (step, dual, or ditch structures)
• Weld and heat-affected zone (HAZ) susceptibility assessment for welded joints
• Verification of the effectiveness of solution annealing heat treatment
• Qualification of low-carbon (L grades) and stabilized (Ti/Nb-bearing) grades against sensitization
• Electrochemical reactivation (EPR/DL-EPR) measurement for quantitative sensitization degree per ASTM G108
• Failure analysis of in-service components exhibiting suspected grain-boundary attack

Testing Methods

The test method is selected based on the steel grade, metallurgical condition, and the service environment of the equipment. ASTM A262 defines five practices for austenitic stainless steels; parallel methods exist in ISO 3651-2 and other national standards.

Oxalic Acid Etch Test (ASTM A262 Practice A)

Practice A is a rapid screening test in which a polished specimen is electrolytically etched at 1 A/cm² for 90 seconds in a 10% oxalic acid solution at room temperature. The etched microstructure is examined under a microscope at approximately 250× and classified as a step structure (grain boundaries cleanly delineated, no carbide precipitation), a dual structure (isolated ditch formation at some boundaries), or a ditch structure (continuous ditches surrounding entire grains). Step and dual structures are considered acceptable; a ditch structure indicates susceptibility and requires confirmation by a full immersion test (Practice B, C, E, or F). Practice A is a screening test only and cannot be used alone for acceptance.

Ferric Sulfate–Sulfuric Acid Test (ASTM A262 Practice B)

Practice B, often called the Streicher test, immerses weighed specimens for 120 hours in a boiling solution of 50% sulfuric acid containing 25 g/L ferric sulfate. The oxidizing ferric ions suppress general corrosion while attacking chromium-depleted zones. Corrosion rate is calculated from weight loss, surface area, exposure time, and density, and compared against the maximum rate permitted by the applicable product specification. Practice B detects susceptibility associated with chromium carbide precipitation and is suitable for most common austenitic grades.

Nitric Acid Test (ASTM A262 Practice C)

Practice C, known as the Huey test, exposes specimens to five consecutive 48-hour boiling periods in 65% nitric acid, with the solution renewed and the specimen reweighed after each period. Because nitric acid preferentially attacks grain-boundary segregates such as sigma phase, chromium carbides, and molybdenum-rich phases in addition to chromium-depleted zones, Practice C is the most discriminating test and is specified for equipment intended for nitric acid service and for high-purity applications. The highest or average corrosion rate among the five periods, as defined by the specification, is compared against the acceptance limit.

Copper–Copper Sulfate–Sulfuric Acid Tests (ASTM A262 Practices E and F)

Practice E, the Strauss test, immerses specimens in a boiling 16% sulfuric acid solution with copper sulfate, with the specimen in continuous contact with metallic copper shot or chips. After a 24-hour exposure (72 hours under earlier editions), the specimen is bent 180° around a mandrel appropriate to its thickness and the tension surface is examined at low magnification for cracks and fissures. Cracking indicates intergranular attack and constitutes failure. Practice F uses a more aggressive boiling 50% sulfuric acid–copper sulfate solution for 120 hours with copper contact and is evaluated by weight-loss corrosion rate; it provides improved sensitivity for molybdenum-bearing grades such as Type 316 and Type 317.

ISO 3651-2 and Other International Methods

EN ISO 3651-2 specifies sulfuric acid–copper sulfate test methods with metallic copper contact for austenitic and ferritic-austenitic steels, including a 16% sulfuric acid Strauss-type method evaluated by bend testing and higher-concentration sulfuric acid methods evaluated by corrosion rate. Acceptance under ISO 3651-2 is typically a maximum corrosion rate limit, commonly 1.2 μm/48 h for the weight-loss methods, or absence of cracking in the bend test. Japanese JIS G 0571–0575 and Chinese GB/T 4334 series provide technically equivalent procedures, allowing globally consistent material qualification. For ferritic stainless steels, ASTM A763 provides dedicated practices covering oxalic acid etching and ferric sulfate–sulfuric acid immersion.

Electrochemical Techniques

Electrochemical potentiodynamic reactivation (EPR and double-loop DL-EPR, covered by ASTM G108) quantifies the degree of sensitization by measuring the charge associated with grain-boundary activation and repassivation. These techniques are fast, require small specimens, and are valuable for research, weld procedure development, and remaining-life assessment of operating equipment.

Method Selection and Acceptance Criteria

Selection depends on grade and service. As general guidance:

• Use Practice A for rapid screening of annealed material prior to a confirmatory immersion test
• Use Practice B for general-purpose qualification of austenitic stainless steels
• Use Practice C for nitric acid service and applications where sigma phase is of concern
• Use Practice E for carbon steel-contact service environments and rapid pass/fail evaluation of weldments
• Use Practice F for molybdenum-bearing grades requiring greater test sensitivity

Acceptance is based on the criteria defined in the governing material specification: a corrosion rate below the specified maximum, or a post-exposure bend specimen free of intergranular fissures at 10× to 20× magnification, or an acceptable Practice A structure combined with a qualifying immersion test result.

Applications

Intergranular corrosion testing is specified wherever sensitized stainless steel could fail in aggressive media. Principal application areas include:

• Chemical and petrochemical plants: reactors, distillation columns, heat exchangers, and process piping
• Oil and gas: offshore platforms, LNG facilities, sour service piping, and refinery equipment
• Pharmaceutical and biotechnology: sanitary tubing, clean-in-place systems, and purified water (WFI) distribution loops
• Food and beverage processing: tanks, kettles, and hygienic weldments exposed to organic acids
• Nuclear power: primary circuit components and reactor internals with strict material qualification requirements
• Pulp and paper, desalination, and pollution control: scrubbers, digesters, and chloride-bearing environments
• Supply chain quality assurance: incoming material inspection, weld procedure qualification, and supplier certification
• Failure analysis and fitness-for-service: diagnosing in-service cracking and setting re-inspection intervals

Beyond new construction acceptance, IGC testing supports retrofit and repair decisions, validates post-weld heat treatments, and provides documented evidence of metallurgical integrity for regulatory audits and pressure vessel code compliance.

FAQ

What causes intergranular corrosion in stainless steel equipment?

IGC is caused by sensitization, the precipitation of chromium carbides at grain boundaries when stainless steel passes through approximately 425 °C to 815 °C. This leaves a thin chromium-depleted zone along each boundary that cannot maintain passivity and corrodes preferentially. Welding, slow cooling, improper heat treatment, and long-term elevated-temperature service are the usual triggers.

Which ASTM A262 practice should be selected?

The choice depends on the steel grade and service environment. Practice A is a screening etch test; Practice B suits general qualification of austenitic grades; Practice C is reserved for nitric acid and sigma-phase-sensitive applications; Practice E provides a rapid bend-test pass/fail result; and Practice F offers higher sensitivity for molybdenum-bearing grades such as 316 and 317. The governing product specification or end-user standard normally dictates the practice.

Can the oxalic acid etch test be used alone for acceptance?

No. Practice A is a screening test only. Material showing a step or dual structure may be accepted without further testing, but any ditch structure must be verified by a full immersion practice (B, C, E, or F) before an acceptance decision is made.

How are welded components tested for sensitization?

Cross-weld specimens spanning weld metal, HAZ, and base metal are exposed in the selected acid solution. After exposure, the specimen is bent so that the weld and HAZ are placed in tension, and the surface is inspected for intergranular cracking. Alternatively, sectioned coupons are examined metallographically for grain-boundary ditching in the HAZ.

How long does intergranular corrosion testing take?

Test durations vary by practice: Practice A requires minutes of etching plus microscopy; Practice E involves a 24-hour boil plus bending; Practices B and F involve 120-hour exposures; and Practice C requires five successive 48-hour periods (10 days). Including machining, conditioning, sensitizing treatment where required, and reporting, a complete program typically takes one to two weeks.

How can sensitization be prevented in equipment fabrication?

Effective preventive measures include selecting low-carbon grades (304L, 316L with C ≤ 0.03%) or stabilized grades (321, 347), solution annealing at approximately 1050 °C to 1150 °C followed by rapid water quenching, minimizing weld heat input and interpass temperature, using narrow-gap and low-energy welding techniques, and avoiding unnecessary post-weld heating through the sensitizing range.