Intergranular corrosion (IGC) testing is a critical quality control procedure for stainless steel forgings used in pressure vessels, piping systems, and chemical processing equipment. This testing evaluates the resistance of forged components to grain boundary attack caused by sensitization during thermal processing. This article provides a comprehensive technical guide covering testing standards such as ASTM A262, sample preparation requirements, test methods, acceptance criteria, and industrial applications.
Technical Overview
Intergranular corrosion is a localized form of corrosion that attacks the grain boundaries of austenitic and ferritic stainless steels while leaving the grain interiors largely unaffected. In stainless steel forgings, this phenomenon typically occurs when the material is exposed to temperatures between 425°C and 815°C during forging operations, slow cooling of heavy cross-sections, post-forging heat treatment, or subsequent welding. In this critical temperature range, chromium carbides (Cr23C6) precipitate at grain boundaries, depleting the surrounding zones of chromium to levels below the 12% threshold required for passivation, leaving these sensitized regions vulnerable to corrosive attack.
For stainless steel forgings, the risk of sensitization is particularly significant because large forging sections cool slowly through the sensitization temperature range after hot working or solution annealing. When attacked by intergranular corrosion, forgings lose cohesion between grains, resulting in grain dropping, reduced mechanical properties, and premature cracking under stress. Component failure can occur suddenly and without significant external warning signs, making IGC testing essential for safety-critical applications.
Primary testing standards for intergranular corrosion of stainless steel forgings include ASTM A262 (Practices A, B, C, E, and F), ASTM A703 for steel castings with supplementary requirements, ISO 3651-2, EN ISO 3651-2, and GB/T 4334 for Chinese market compliance. Material specifications such as ASTM A182 for forged pipe flanges, fittings, and valves frequently reference these test methods as acceptance requirements.
Testing Samples
Sample Selection from Forgings
Test specimens shall be taken from locations that are representative of the forging as a whole, typically from prolongations or sacrificial extensions of the forged component. For forgings intended for solution annealing, samples should be taken after final heat treatment to represent the delivered condition. Heavy-section forgings require samples taken from both surface and mid-radius or mid-wall locations, since the cooling rate varies significantly across the section and sensitization risk is highest at locations with slower cooling.
Specimen Heat Treatment Conditions
Depending on the specification and intended service, test coupons may be evaluated in one of two conditions: the as-delivered (solution-annealed) condition, or after a laboratory sensitizing heat treatment that simulates adverse thermal exposure. Sensitizing treatments typically involve heating specimens in the range of 620°C to 700°C for a specified hold time followed by cooling, deliberately inducing chromium carbide precipitation to assess the intrinsic resistance of the material chemistry.
Specimen Dimensions and Preparation
Specimens are prepared with a total exposed surface area of 15 to 35 cm² for weight-loss methods such as ASTM A262 Practice B, with the exact surface area measured and recorded for corrosion rate calculation. Surface preparation requirements include:
- Removal of scale, oxide, and decarburized layers by machining or grinding to expose sound metal
- Uniform surface finish to a maximum roughness of approximately 120 grit for reproducible results
- Deburring of all edges and rounded corners to eliminate crevices and edge effects
- Cleaning, degreasing, drying, and weighing to 1 mg accuracy before exposure
- Adequate specimen identification using methods that do not contaminate the test surface
Testing Items
The intergranular corrosion test program for stainless steel forgings encompasses several distinct evaluation items, each targeting specific aspects of sensitization resistance:
- Screening etch classification of grain boundary microstructure (ditched, step, or dual structures) using oxalic acid electrolytic etching
- Weight-loss corrosion rate determination after immersion exposure in boiling acid solutions, expressed in inches per month or millimeters per year
- Bend test evaluation of exposed specimens to reveal intergranular fissuring or cracking
- Metallographic cross-section examination to measure attack depth and confirm intergranular morphology
- Ultrasound or droplet examination of the bent specimen surface for micro-fissures
- Verification of solution annealing effectiveness and stabilization chemistry for grades such as 321 and 347
Testing Methods
Practice A: Oxalic Acid Etch Test
Practice A is a rapid screening method involving electrolytic etching of a polished specimen in a 10% oxalic acid solution at ambient temperature under a current density of approximately 1 A/cm² for 90 seconds. The etched microstructure is examined at 250× to 500× magnification and classified as step, dual, or ditch structure. Specimens exhibiting a step or dual structure are accepted as immune to sensitization for most grades, while a ditch structure requires confirmation through a quantitative immersion test such as Practice B or E.
Practice B: Ferric Sulfate–50% Sulfuric Acid Test
Practice B is a 120-hour boiling immersion test in a 50% sulfuric acid solution containing ferric sulfate, conducted in glass or quartz flasks fitted with cold finger condensers. This method, based on the Streicher test, measures the corrosion rate by weight loss and is suitable for evaluating chromium-depleted grain boundaries in austenitic stainless steels such as 304, 304L, 316, and 316L forgings. Acceptance typically requires a corrosion rate below the maximum value specified in the applicable material standard, commonly on the order of 0.0011 to 0.0022 in/month depending on grade.
Practice C: Nitric Acid (Huey) Test
Practice C is the most severe test, consisting of five consecutive 48-hour boiling periods in 65% nitric acid for a total exposure of 240 hours. This method detects sensitization from chromium carbide precipitation as well as sigma phase and other nitric-acid-selective attack modes. It is primarily applied to forgings intended for nitric acid service and to grades such as 304L and 316L where sigma phase formation is a concern. The average and maximum period corrosion rates must not exceed the specified limits.
Practice E: Copper–Copper Sulfate–16% Sulfuric Acid Test (Strauss Test)
Practice E exposes specimens for 24 hours to a boiling 16% sulfuric acid solution containing copper sulfate, with the specimens in contact with metallic copper turnings to accelerate the galvanic attack on sensitized boundaries. After exposure, specimens are bent 180° around a mandrel with a diameter equal to the specimen thickness. The specimen passes if no cracks or fissures attributable to intergranular attack are visible at 10× to 20× magnification on the tension surface. This method is widely specified for welded and forged austenitic components in the as-fabricated condition.
Practice F and ISO 3651-2 Methods
Practice F uses a more aggressive 50% sulfuric acid copper-copper sulfate solution for 96 hours, applied to molybdenum-bearing grades such as 316, 316L, 317, and 317L. ISO 3651-2 Method A corresponds to the Strauss-type copper-copper sulfate test for as-delivered material, while Method B includes a prior sensitizing treatment for assessment of intrinsic resistance. Both international methods conclude with a bend test and visual-metallographic evaluation identical in principle to the ASTM counterparts.
Applications
Intergranular corrosion testing of stainless steel forgings is mandatory or highly recommended across numerous safety-critical and corrosion-intensive industries:
- Petrochemical and refinery plants, where forged flanges, fittings, and nozzle forgings in ASTM A182 F304 and F316 service contact hot organic acids and chlorides
- Oil and gas processing, including subsea and topside forgings, wellhead components, and manifold hardware exposed to sour and acidic environments
- Nuclear power generation, where forged austenitic piping components and reactor internals demand demonstrated sensitization resistance under long-term thermal aging
- Chemical and fertilizer production involving nitric, sulfuric, and acetic acid media at elevated temperatures
- Pharmaceutical and food processing equipment, where sanitary forgings must maintain corrosion resistance and surface integrity for cleanability
- Pressure vessel and heat exchanger construction governed by ASME codes referencing IGC qualification of forged components
FAQ
What causes intergranular corrosion in stainless steel forgings? Intergranular corrosion results from sensitization, the precipitation of chromium-rich carbides at grain boundaries when the forging passes through the 425–815°C temperature range during slow cooling, stress relief, or welding. This depletes chromium adjacent to the boundaries below the level needed for passive film stability.
Which ASTM A262 practice should be specified for my forging? Practice A is used only for screening. Practice B suits general acceptance testing of austenitic forgings, Practice C is reserved for nitric acid service and sigma-phase detection, Practice E applies to as-delivered material with a bend acceptance test, and Practice F is intended for molybdenum-bearing grades.
How can forgings be protected against sensitization? Effective measures include full solution annealing at 1,040–1,150°C followed by rapid water quenching or accelerated cooling, specification of low-carbon grades (304L, 316L with carbon ≤ 0.030%), or titanium- or niobium-stabilized grades (321, 347) whose stabilizers tie up carbon preferentially.
What are typical acceptance criteria? Acceptance is based on corrosion rates below the grade-specific maximum in Practices B and C, or the absence of intergranular cracks after the 180° bend test in Practices E and F, supplemented by metallographic confirmation that any observed attack does not exceed permitted depth.
Is testing required on every forging lot? Requirements vary by specification, but purchase orders for corrosive service typically mandate IGC testing on a heat-treatment-lot basis, with samples representing both the thickest and thinnest sections of the forgings supplied.