Intergranular corrosion rate determination is an essential quality control and material qualification test for assessing the sensitization resistance of stainless steels exposed to aggressive service environments. This technical guide explains the metallurgical mechanism of grain boundary attack, the principal testing standards such as ASTM A262 and ISO 3651, laboratory procedures for quantitative corrosion rate measurement, and how results are interpreted across chemical, nuclear, and oil and gas industries.
Technical Overview
Intergranular corrosion (IGC) is a localized form of degradation in which attack proceeds preferentially along the grain boundaries of a metal while the grain interiors remain largely unaffected. In austenitic stainless steels, this phenomenon is driven by sensitization: when the material is heated within the critical range of approximately 425 to 815 degrees Celsius, such as during welding, slow cooling from forming operations, or improper heat treatment, chromium carbides precipitate at the grain boundaries. This precipitation depletes the adjacent zones of chromium to levels below the approximate 12 percent required to maintain a stable passive film. The chromium-depleted regions become anodic relative to the grain bodies, and in specific electrolytes they dissolve preferentially, which can lead to grain dropping, loss of load-bearing cross section, and catastrophic brittle fracture with little visible surface evidence.
Intergranular corrosion rate determination quantifies this susceptibility through accelerated immersion testing in standardized boiling acid media, followed by measurement of mass loss per unit area and time. The results provide a numerical corrosion rate, typically expressed in grams per square meter per hour or converted to millimeters per year, that allows engineers to compare heats of material, verify heat treatment quality, and qualify alloys for corrosive service. The governing standards include ASTM A262 (Practices A, B, C, E, and F), ASTM G28, ISO 3651-1 and ISO 3651-2, GB/T 4334, and the JIS G 0571 to G 0575 series, each defining specific test solutions, exposure durations, and acceptance evaluation procedures.
Testing Samples
Plate, Sheet, and Strip Coupons
Rectangular coupons, commonly on the order of 30 by 20 millimeters or full-thickness sections, are cut from representative material. Surfaces are ground to a uniform fine finish, degreased, dimensioned, and weighed to 0.1 milligram precision before exposure. Both as-received and sensitized (heat-treated) conditions may be evaluated depending on the specification requirements.
Pipe and Tube Specimens
Ring segments or longitudinal strips are prepared from seamless and welded tubular products. For welded pipe, specimens are oriented so that the weld metal, heat-affected zone, and base metal are all exposed to the test solution, allowing evaluation of sensitization caused by the welding thermal cycle.
Bar, Forging, and Casting Samples
Samples are removed in transverse and longitudinal orientations in accordance with the product specification. Castings may be tested in the as-cast or solution-annealed condition, and large forgings often require sampling at both surface and mid-radius locations to capture variations in thermal history.
Weldments and Heat-Affected Zones
Cross-section specimens spanning weld metal, heat-affected zone, and adjacent base metal are tested in the as-welded condition or after a deliberate sensitizing heat treatment. These samples are critical for qualifying welding procedures, filler metals, and post-weld heat treatments.
Finished Components and Lot Samples
Coupons representing a specific heat-treatment lot or production batch may be tested alongside finished components. Where destructive sampling of the component is not possible, representative witness coupons processed through the identical thermal cycle are substituted.
Testing Items
Testing programs for intergranular corrosion rate determination typically include the following measurable outputs:
- Corrosion rate by mass loss, expressed in grams per square meter per hour (g/m²·h)
- Corrosion rate converted to millimeters per year (mm/y) or mils per year (mpy) using material density
- Depth of intergranular penetration measured metallographically in micrometers
- Bend test results after acid exposure, documenting the presence or absence of fissures and cracking
- Etch structure classification by oxalic acid screening, categorized as step, dual, or ditch structure
- Evidence of grain dropping or grain boundary ditching on the exposed surface
- Comparison of rates across successive test periods to detect accelerating attack, as required by the Huey test protocol
Testing Methods
Oxalic Acid Etch Test (ASTM A262 Practice A)
The specimen is anodically etched at 1 ampere per square centimeter for 90 seconds in a 10 percent oxalic acid solution at room temperature, then examined microscopically at 250 to 500 times magnification. Step structures indicate the material is free of sensitization and acceptable without further testing; ditch structures indicate probable susceptibility and require quantitative confirmation by a boiling acid test. This rapid screening method significantly reduces testing cost for large production lots.
Ferric Sulfate–Sulfuric Acid Test (ASTM A262 Practice B / ASTM G28 Method A)
Samples are immersed for 120 hours in a boiling solution of 50 percent sulfuric acid saturated with ferric sulfate. The oxidizing ferric ions maintain a corrosion potential at which chromium-depleted grain boundary regions dissolve preferentially. The corrosion rate is calculated from mass loss, providing a quantitative measure of sensitization suitable for austenitic grades containing molybdenum, including type 316 and 317.
Nitric Acid Test (ASTM A262 Practice C, Huey Test)
Specimens are exposed to boiling 65 percent nitric acid in five consecutive 48-hour periods, with weighing after each period. The test detects sensitization as well as sigma phase and other chromium-rich precipitates that are resistant to other practices. Because nitric service itself can cause intergranular attack, the Huey test is specified for equipment in nitric acid plants and nuclear fuel reprocessing facilities. Acceptance requires that the rate in each period not increase progressively, which would indicate advancing grain boundary attack.
Copper–Copper Sulfate–16% Sulfuric Acid Test (ASTM A262 Practice E, Strauss Test)
The specimen is placed in contact with metallic copper chips in boiling 16 percent sulfuric acid with copper sulfate addition for a minimum of 24 hours. The metallic copper establishes a low corrosion potential that selectively attacks sensitized grain boundaries. Evaluation is performed by bending the specimen 180 degrees around a suitable mandrel and examining the tension surface at low magnification for cracks and fissures.
Copper–Copper Sulfate–50% Sulfuric Acid Test (ASTM A262 Practice F)
This 120-hour test in boiling 50 percent sulfuric acid with copper sulfate and copper contact is applied to highly alloyed austenitic stainless steels and nickel-bearing grades for which the milder Strauss solution may not be sufficiently aggressive to reveal sensitization.
Corrosion Rate Calculation
After exposure, specimens are cleaned of corrosion products, dried, and reweighed. The corrosion rate is determined from the following relationship:
- Corrosion rate (g/m²·h) = (m1 − m2) / (A × t), where m1 and m2 are the masses in grams before and after testing, A is the total exposed surface area in square meters, and t is the exposure time in hours
- Corrosion rate (mm/y) = 8.76 × (m1 − m2) / (A × t × ρ), where ρ is the material density in grams per cubic centimeter, approximately 7.9 to 8.0 for austenitic stainless steel
Replicate specimens are tested whenever possible, and reagent volume-to-specimen-area ratios are strictly controlled per the standard, typically 20 milliliters of solution per square centimeter of exposed surface, to ensure valid and reproducible results.
Applications
Intergranular corrosion rate determination supports material selection, fabrication quality assurance, and failure prevention across a wide range of industries:
- Chemical and petrochemical processing: qualification of reactors, heat exchangers, piping, and vessels exposed to organic and inorganic acids
- Nuclear power and fuel reprocessing: evaluation of austenitic components for nitric acid service using the Huey test
- Oil and gas, LNG, and refining: verification of sensitization resistance in welded piping and pressure components after field heat treatment
- Food, beverage, and pharmaceutical industries: certification of sanitary tubing and fittings where product purity and grain boundary integrity are critical
- Material qualification: acceptance testing of low-carbon L grades and stabilized grades such as 321 and 347 to confirm carbide precipitation control
- Failure analysis: diagnosing whether in-service cracking of stainless components is attributable to welding-induced sensitization or improper annealing
FAQ
What causes intergranular corrosion in stainless steel?
It is caused primarily by chromium carbide precipitation at grain boundaries when the steel is heated in the sensitization range of roughly 425 to 815 degrees Celsius, which locally depletes chromium and destroys passivity in the grain boundary region.
How can intergranular corrosion be prevented?
Prevention measures include using low-carbon L grades with carbon below 0.03 percent, using titanium- or niobium-stabilized grades, applying proper solution annealing followed by rapid cooling, and controlling weld heat input and cooling rates.
Which test method should be selected?
Selection depends on the alloy and intended service. Practice A is a rapid screen, Practice B provides a quantitative rate for most austenitic grades, Practice C is specified for nitric acid service and detects sigma phase, and Practice E is widely applied as a general acceptance bend test for welded components.
What are typical acceptance criteria?
Criteria are defined by the material specification. Common limits include a maximum mass-loss rate on the order of 1.0 to 1.2 grams per square meter per hour for Practice B on standard austenitic grades, no progressive rate increase across test periods for Practice C, and no cracking after the bend test for Practice E.
How long does a complete test take?
Screening by oxalic acid etch takes under an hour, the Strauss bend test requires 24 hours of exposure plus metallography, and quantitative rate tests such as Practice B, C, and F require 120 hours of boiling exposure in addition to specimen preparation, cleaning, and weighing.
Can stabilized or low-carbon grades still fail the test?
Yes. Extended thermal exposure, multiple repair welds, improper stabilizing heat treatment, or sigma phase formation in heavily alloyed grades can still produce sensitization or grain boundary precipitation severe enough to cause failing corrosion rates or bend test cracks.