Intergranular Corrosion Testing of Stainless Steel Welded Joints: Standards, Methods, and Evaluation

Intergranular corrosion (IGC) testing of stainless steel welded joints is a critical quality control procedure used to assess sensitization and chromium carbide precipitation in the heat-affected zone. This article provides a comprehensive technical guide covering testing standards such as ASTM A262, GB/T 4334, and ISO 3651, sample preparation requirements, testing methods, evaluation criteria, and industrial applications.

Technical Overview

Intergranular corrosion (IGC) is a localized form of corrosive attack that propagates along the grain boundaries of austenitic and other stainless steels while leaving the grain interiors largely unaffected. In welded joints, this phenomenon is of particular concern because the welding thermal cycle exposes narrow bands of the heat-affected zone (HAZ) to temperatures between approximately 450°C and 850°C, the critical sensitization range in which chromium carbides (Cr23C6) precipitate preferentially at grain boundaries.

When chromium carbides form at grain boundaries, they deplete the surrounding matrix of chromium to levels below the 10.5–12% threshold required for passivation. These chromium-depleted zones become anodic relative to the grain interiors and are preferentially dissolved in corrosive media. The result is a progressive loss of cohesion between grains, grain dropping, reduction in load-bearing cross-section, and ultimately catastrophic brittle failure with little or no visible external warning.

Intergranular corrosion testing of welded joints therefore serves three essential purposes: verifying that the welding procedure (WPS) does not sensitize the HAZ, confirming material selection such as low-carbon (L grades) or stabilized (titanium or niobium-bearing) stainless steels, and qualifying materials for service in aggressive media including nitric acid, sulfuric acid, and chloride-containing environments.

Testing Samples

Sample Types and Configurations

Test samples are selected to represent the full metallurgical cross-section of the welded joint, including weld metal, fusion line, heat-affected zone, and unaffected base metal. Typical sample configurations include:

• Butt-welded plate samples with the weld bead retained or machined flush depending on the applicable standard
• Circumferential and longitudinal welded pipe or tube segments, tested as full cross-sections or flattened strips
• Cross-sectional samples containing the complete joint, used for metallographic evaluation of the HAZ after exposure
• Repair-welded and multi-pass welded samples where repeated thermal cycles increase sensitization risk
• Base metal reference samples for comparison against welded joint performance

Sample Dimensions and Preparation

Sample dimensions follow the requirements of the specified test standard. For ASTM A262 Practice B, samples typically provide a total exposed surface area of 500 to 1,200 cm² depending on product form; for Practice C, similar minimum surface areas apply. For GB/T 4334 tests, samples are generally 80 mm in length with widths of 20 mm for plate or the full tube cross-section for piping products.

Preparation requirements include:

• Removal of all scale, slag, spatter, and surface contamination by mechanical or chemical means
• Surface grinding or pickling to ensure uniform surface condition free of embedded iron
• Measuring and recording sample dimensions and surface area to calculate corrosion rates accurately
• Cleaning, degreasing, drying, and weighing to 1 mg precision on an analytical balance before testing
• Marking samples with identification codes using a method resistant to the test environment

Testing Items

A complete intergranular corrosion evaluation of welded joints typically comprises the following test items, either individually or in combination as specified by the purchase order, material specification, or fabrication code:

• Oxalic acid electrochemical etch screening test (ASTM A262 Practice A, GB/T 4334 Method A)
• Ferric sulfate–50% sulfuric acid immersion test (ASTM A262 Practice B, GB/T 4334 Method B)
• Boiling nitric acid test, also known as the Huey test (ASTM A262 Practice C, GB/T 4334 Method C)
• Copper–copper sulfate–16% sulfuric acid test, also known as the Strauss test (ASTM A262 Practice E, GB/T 4334 Method D/E)
• Copper–copper sulfate–50% sulfuric acid test for molybdenum-bearing alloys (ASTM A262 Practice F)
• Post-exposure bend testing of welded and sensitized samples to reveal intergranular cracking
• Metallographic examination of HAZ microstructure at 100× to 500× magnification to detect ditch structures and grain boundary attack
• Corrosion rate calculation from weight loss data and comparison against acceptance limits

Testing Methods

Oxalic Acid Etch Test (ASTM A262 Practice A)

The oxalic acid etch test is a rapid electrochemical screening method in which a polished sample cross-section is anodically etched in a 10% oxalic acid solution at a current density of 1 A/cm² for 90 seconds at room temperature. The etched microstructure is examined under a microscope at 250× to 500× and classified as step, dual, or ditch structure. Samples exhibiting a step or dual structure are accepted as immune to sensitization for most grades; a ditch structure indicates probable sensitization and requires confirmatory testing by one of the immersion methods. This test is not applicable to certain molybdenum-bearing or high-chromium grades without supplementary qualification.

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

In this test, samples are immersed in a boiling solution of 50% sulfuric acid containing 25 g of ferric sulfate, maintained at approximately 120°C (boiling point of the solution), for a continuous period of 120 hours. The ferric ions act as a depolarizer, maintaining a corrosion potential at which chromium-depleted grain boundary regions dissolve preferentially. The result is expressed as a corrosion rate in inches per month or millimeters per month, calculated from the measured weight loss, exposed surface area, test duration, and material density. Typical acceptance criteria for common austenitic grades such as 304 and 316 specify a maximum corrosion rate on the order of 1.0 inch per month, with tighter limits applied to certain specifications.

Nitric Acid Huey Test (ASTM A262 Practice C)

The Huey test is the most aggressive of the standard methods and is primarily applied to materials intended for nitric acid service, including nuclear fuel reprocessing. Samples are subjected to five consecutive 48-hour boiling periods in 65% nitric acid, with the solution renewed and samples reweighed after each period. The maximum and average corrosion rates are computed from the individual period weight losses. The Huey test detects not only chromium carbide sensitization but also sigma phase and other chromium-rich precipitates, making it suitable for evaluating heat treatments as well as welding procedures. Grain dropping during the test is a clear indication of severe intergranular attack.

Copper–Copper Sulfate–Sulfuric Acid Test (ASTM A262 Practice E and F)

The Strauss test, or Practice E, exposes samples to a boiling 16% sulfuric acid solution containing metallic copper chips or copper shot, with samples in metallic contact with the copper, for 24 hours. The copper lowers the corrosion potential to a level at which only sensitized grain boundaries attack. After exposure, welded joint samples are subjected to a 180° bend test over a mandrel of appropriate diameter; the appearance of cracks or fissures on the tensile surface indicates intergranular corrosion and constitutes failure. Practice F uses a 50% sulfuric acid concentration with copper contact for 120 hours and is specified for molybdenum-bearing grades such as 316, 317, and higher-alloyed materials where the 16% acid test is insufficiently discriminating.

Chinese Standard GB/T 4334 Series

The GB/T 4334 standard series defines methods closely harmonized with their international counterparts: Method A corresponds to the 10% oxalic acid etch test (GB/T 4334.1), Method B to the ferric sulfate–50% sulfuric acid test (GB/T 4334.2), Method C to the boiling 65% nitric acid test (GB/T 4334.3), Method D to the nitric–hydrofluoric acid test for molybdenum-bearing steels (GB/T 4334.4), and Method E to the copper–copper sulfate–16% sulfuric acid test (GB/T 4334.5). For welded joints, GB/T 15260 and pressure equipment codes frequently invoke these methods together with post-test bend evaluation of the HAZ. Evaluation criteria for the copper–copper sulfate test under GB/T 4334.5 typically require no intergranular cracks visible at 10× magnification after bending the welded sample through 180°.

ISO and European Standards

ISO 3651-1 defines the general principles and evaluation of intergranular corrosion tests, while ISO 3651-2 specifies the Strauss test in 16% sulfuric acid with copper exposure and the nitric–hydrofluoric acid test for molybdenum-containing steels. European material standards such as EN 10088 reference ISO 3651 acceptance criteria. For nuclear and pressure vessel applications, additional requirements from ASME Section VIII, ASME Section III, RCC-M, and NACE guidelines may supplement the base test methods.

Evaluation and Acceptance Criteria

Evaluation of welded joint samples combines quantitative and qualitative assessments:

• Corrosion rate limits calculated from weight loss, expressed in mm/month or inch/month, compared against grade-specific acceptance values
• Bend test results, with no cracking permitted at 10× magnification after a 180° bend of the welded cross-section
• Metallographic evidence of ditch structures, grain boundary grooving, or grain dropping in the HAZ, fusion line, or weld metal
• Maximum-to-average corrosion rate ratios in the Huey test to detect localized attack accelerations
• Comparison of welded joint behavior against base metal reference samples to isolate welding-induced sensitization

Test Report Contents

A compliant test report documents the material grade, product form, welding procedure and heat treatment condition, sample dimensions and surface area, test method and standard revision, solution composition and boiling temperature, exposure duration, weight loss data, calculated corrosion rates, bend test results, metallographic findings, and a clear pass or fail conclusion against the specified acceptance criteria.

Applications

Intergranular corrosion testing of welded joints is mandated or specified across a wide range of industries where sensitization-induced failure poses safety, environmental, or economic risks:

• Chemical and petrochemical processing plants handling sulfuric acid, nitric acid, acetic acid, and organic acids at elevated temperatures
• Nuclear power generation, where Huey testing of stainless steel piping, vessel cladding, and reprocessing equipment is a regulatory requirement in many jurisdictions
• Oil and gas refineries, including delayed coker units, alkylation units, and sour water systems with welded austenitic piping
• Food, beverage, and dairy processing equipment where weld sensitization can cause product contamination and premature failure
• Pharmaceutical and biotechnology facilities requiring certified corrosion-resistant welds in sanitary piping systems
• Pulp and paper bleaching plants exposed to chloride-bearing and oxidizing media
• Pressure vessels, boilers, and heat exchangers fabricated to ASME, EN 13445, or GB 150 codes
• Welding procedure qualification (WPQR) and welder performance qualification programs verifying that heat input limits prevent sensitization
• Failure analysis investigations, where IGC testing of service-exposed samples confirms or excludes intergranular degradation as a root cause

FAQ

Why are welded joints more susceptible to intergranular corrosion than base metal?

During welding, the heat-affected zone passes through the sensitization temperature range of approximately 450–850°C, promoting chromium carbide precipitation at grain boundaries. Base metal supplied in the solution-annealed condition has dissolved carbides and uniform chromium distribution, whereas the HAZ of a non-optimized welding procedure can contain sensitized bands only a few grain diameters wide.

How can sensitization of welded joints be prevented?

Sensitization can be prevented by using low-carbon grades (304L, 316L with carbon below 0.03%), stabilized grades (321 with titanium or 347 with niobium), minimizing welding heat input, controlling interpass temperature, using narrow weld beads and faster travel speeds, and applying post-weld solution annealing at 1,050–1,150°C followed by rapid water quenching where feasible.

Which intergranular corrosion test should be selected for a given application?

Test selection depends on the material grade and service environment. The oxalic acid etch test is suitable only as a rapid screen for acceptance, not rejection. The ferric sulfate test is a general-purpose weight-loss method for most austenitic grades. The Huey test is specified for nitric acid and nuclear service. The Strauss test is the most common acceptance test for welded joints because the post-exposure bend test directly demonstrates joint integrity. The 50% sulfuric acid Practice F test is reserved for molybdenum-bearing grades.

What constitutes failure in the copper–copper sulfate test on a welded joint?

Failure is defined as the presence of intergranular cracks or fissures on the tension surface of the sample after bending through 180° around a specified mandrel, examined at 10× magnification. Cracks must be clearly intergranular in character; ductile tearing from bend test mechanics alone does not constitute failure.

How long does a typical intergranular corrosion test of a welded joint take?

The oxalic acid etch test takes under one hour. The Strauss test requires 24 hours of exposure plus bend evaluation. The ferric sulfate test requires 120 hours of boiling exposure. The Huey test is the longest, requiring five consecutive 48-hour periods totaling 240 hours, with additional time for sample preparation, weighing, and metallography. Overall turnaround including reporting is typically 3 to 12 working days depending on the method.

Can intergranular corrosion testing be performed on duplex stainless steel welded joints?

Yes. Although duplex stainless steels are generally less susceptible to classical chromium carbide sensitization, welded joints can suffer intergranular attack from sigma phase, chromium nitride precipitation, or excessive ferrite formation. Testing follows standards such as ASTM A923 for duplex grades, alongside ASTM G28 Method A or B, and the applicable ferric sulfate and nitric-hydrofluoric acid procedures where specified.