Intergranular corrosion rate determination is a specialized laboratory technique used to quantify the susceptibility of metals and alloys—most notably stainless steels, nickel-based alloys, and 5XXX series aluminum alloys—to localized attack along grain boundaries. By exposing standardized specimens to accelerated corrosive environments and measuring mass loss, attack depth, and calculated corrosion rates, accredited third-party laboratories help manufacturers verify heat treatment quality, assess weld sensitization, and qualify materials for aggressive chemical, petrochemical, and nuclear service. This article provides a comprehensive technical overview of the sample types, measurable parameters, testing methods, governing standards, and industrial applications of intergranular corrosion rate determination.
Technical Overview
Intergranular corrosion (IGC) is a localized degradation mechanism in which corrosive attack is concentrated at grain boundaries or in the immediately adjacent zones, while the grain interiors remain largely unaffected. Because the attacked boundaries lose their load-bearing capability, components can fail mechanically with little visible evidence of overall thinning, making IGC one of the most insidious forms of corrosion.
The predominant cause in austenitic stainless steels is sensitization. When the material is heated within the range of approximately 425 to 815 degrees Celsius, as occurs during welding, slow cooling from annealing, or improper stress-relief heat treatment, chromium-rich carbides of the M23C6 type precipitate at grain boundaries. This precipitation depletes chromium in the surrounding matrix to levels below the roughly 12 percent required for passivation, and the chromium-depleted zones dissolve preferentially when exposed to corrosive media. Analogous mechanisms include sigma phase precipitation in duplex and high-chromium alloys, grain-boundary segregation in ferritic steels, and precipitation of the beta phase (Mg2Al3) along grain boundaries in 5XXX series aluminum alloys.
Intergranular corrosion rate determination quantifies this susceptibility under accelerated laboratory conditions. A specimen of known surface area and initial mass is exposed to a boiling or temperature-controlled aggressive solution for a defined duration, then cleaned, dried, and reweighed. The resulting mass loss is converted into a corrosion rate, typically expressed in millimeters per year (mm/a), and is supplemented by bend testing, metallographic examination, and etch-structure classification. Principal governing standards include ASTM A262 for austenitic stainless steels, ASTM A763 for ferritic stainless steels, ASTM G28 for nickel-based alloys, ASTM G67 for aluminum alloys, ISO 3651-1 and ISO 3651-2, GB/T 4334, and JIS G 0571 through G 0575.
Testing Samples
Testing samples accepted by accredited third-party laboratories cover a wide range of alloy families and product forms. Correct material identification, thermal history documentation, and specimen orientation are essential for meaningful rate determination.
Austenitic Stainless Steels
Grades such as 304, 304L, 316, 316L, 317L, 321, 347, 904L, and six-percent-molybdenum super-austenitic alloys represent the largest testing volume. Both solution-annealed material and deliberately or inadvertently sensitized material, including heat-affected zones of weldments, are routinely evaluated.
Ferritic, Martensitic, and Duplex Stainless Steels
Ferritic grades including 409, 430, 444, and 446, martensitic grades, and duplex grades such as UNS S32205 and S32750 are tested in accordance with ASTM A763 (Practices W, X, Y, and Z) or the appropriate methods of ISO 3651-2, since their sensitization behavior differs from that of austenitic grades.
Nickel-Based Alloys
Solid-solution strengthened nickel-chromium, nickel-molybdenum, and nickel-chromium-molybdenum alloys, commonly designated under UNS numbers N06600, N06625, N10276, and N08028 families, are evaluated using ASTM G28 to detect grain-boundary carbide precipitation and secondary-phase formation.
Aluminum Alloys
Aluminum-magnesium alloys of the 5XXX series, including 5083, 5086, and 5456, are assessed for sensitization during service or artificial aging using the NAMLT test described in ASTM G67.
Product Forms and Specimen Preparation
Test pieces can be extracted from plates, sheets, strips, bars, rods, wires, seamless and welded pipes and tubes, forgings, castings, flanges, fittings, and full cross-section welded joints. Specimens are typically cut to dimensions in the range of 20 to 50 millimeters per side, with the evaluation surface representative of the product. Preparation involves degreasing, surface grinding or pickling to remove scale and embedded debris, rinsing, drying, precise measurement of exposed surface area, and weighing to an accuracy of 0.001 gram before exposure.
Testing Items
Intergranular corrosion rate determination programs report a combination of quantitative and semi-quantitative parameters, each addressing a different aspect of sensitization and boundary attack.
- Corrosion rate calculated from mass loss, expressed in millimeters per year (mm/a) or mils per year (mpy).
- Mass loss per unit area (g/m2) and mass-loss rate (g/m2·h) for each exposure period.
- Maximum and average intergranular attack depth in micrometers, measured metallographically on cross sections.
- Oxalic acid etch structure classification as step, dual, or ditch structure.
- Bend test integrity after exposure, including presence or absence of intergranular fissures and cracking.
- Degree of sensitization (DOS) derived from electrochemical reactivation charge or reactivation current ratio.
- Comparison of results against the acceptance limits stated in the applicable material specification or purchase order.
Testing Methods
Several standardized immersion and electrochemical methods are available. Method selection depends on the alloy family, the expected precipitate type, the degree of severity required, and whether a quantitative rate or a pass/fail verdict is needed.
Oxalic Acid Etch Screening Test
Described in ASTM A262 Practice A and equivalent clauses of GB/T 4334 and JIS G 0571, this rapid electrolytic method etches a polished specimen anodically in 10 percent oxalic acid at a current density of 1 A/cm2 for 90 seconds. The resulting microstructure is classified as step, dual, or ditch structure. Step and dual structures allow acceptance without further testing, while a ditch structure indicates probable susceptibility and requires a full confirmation test. The method serves as screening only and does not produce a corrosion rate.
Huey Test (Boiling Nitric Acid Method)
ASTM A262 Practice C, mirrored by GB/T 4334 and JIS G 0573, exposes specimens to boiling 65 percent nitric acid for five consecutive 48-hour periods, with fresh solution for each period, for a total of 240 hours. Mass loss after each period is measured, and the corrosion rate is calculated for every period and as an overall average. The Huey test is uniquely severe and detects both chromium-depleted zones and sigma phase, making it the preferred quantitative method for alloys intended for nitric acid service.
Streicher Test (Ferric Sulfate–Sulfuric Acid Method)
ASTM A262 Practice B and the corresponding ASTM G28 Method A for nickel alloys immerse specimens in a boiling 50 percent sulfuric acid solution containing approximately 25 g/L of ferric sulfate for 120 hours (24 hours for many nickel alloys). Ferric ions act as a depolarizer, sustaining a controlled redox potential. The method specifically quantifies chromium depletion and yields a mass-loss-based corrosion rate.
Strauss Test (Copper–Copper Sulfate–Sulfuric Acid Method)
ASTM A262 Practice E, together with ISO 3651-2 and equivalent national standards, exposes specimens in contact with metallic copper shot or turnings to a boiling solution of 16 percent sulfuric acid saturated with copper sulfate for 24 hours. The specimen is then subjected to a 180-degree bend, and the outer surface is examined for intergranular fissures. ASTM A262 Practice F, using copper-copper sulfate in 50 percent sulfuric acid, provides a more severe variant suitable for molybdenum-bearing austenitic grades such as 316 and 316L.
NAMLT Test for Aluminum Alloys
ASTM G67, the Nitric Acid Mass Loss Test, immerses 5XXX series aluminum specimens in concentrated nitric acid at 30 degrees Celsius for 24 hours. Grain-boundary precipitate is dissolved, and the resulting mass loss, expressed in milligrams per square centimeter, rates the degree of sensitization. Values exceeding the specification thresholds, commonly in the range of 15 to 25 mg/cm2 depending on alloy and temper, indicate susceptibility to intergranular attack and exfoliation.
Electrochemical Potentiokinetic Reactivation (EPR and DL-EPR)
Electrochemical techniques, including those outlined in ASTM G108 for AISI Type 304 and 304L stainless steels, polarize a polished specimen from the corrosion potential into the passive region and then reverse the scan. Reactivation current and charge are proportional to the active grain-boundary area, and the ratio of reactivation current to activation current provides the degree of sensitization. The double-loop variant (DL-EPR) offers rapid, quantitative results with minimal specimen size and is valuable for through-thickness sensitization profiling.
Corrosion Rate Calculation
The corrosion rate from immersion tests is calculated from mass loss using the standard relationship: CR (mm/a) = 8.76 × 10⁴ × W / (A × T × D), where W is the mass loss in grams, A is the exposed surface area in square centimeters, T is the exposure time in hours, and D is the material density in grams per cubic centimeter. Parallel rate expressions in mils per year or grams per square meter per hour are reported as required by the governing specification. Consistency in solution preparation, reflux condenser design, specimen support, and post-exposure cleaning directly governs result reproducibility.
Applications
Intergranular corrosion rate determination supports material qualification, fabrication control, and failure prevention across essentially every industry that deploys corrosion-resistant alloys.
- Chemical and petrochemical plants: qualification of reactor vessels, heat exchangers, columns, and piping exposed to acids, chlorides, and oxidizing media.
- Oil and gas: assessment of tubing, casings, cladding, and offshore components subject to sensitization and aggressive produced fluids.
- Nuclear power: evaluation of primary circuit stainless steel components and weld overlays where grain-boundary integrity is safety-critical.
- Pharmaceutical, food, and beverage industries: verification of hygienic equipment surfaces that must remain passive and free of boundary attack.
- Aerospace and marine engineering: control of sensitization in 5XXX aluminum ship structures, armor plate, and welded superstructures.
- Mill and fabrication quality control: acceptance testing of incoming plate, pipe, and fittings, and validation of welding procedures, post-weld heat treatment, and stabilization practice.
- Failure analysis and remaining-life assessment: characterization of service-exposed components to establish root cause and residual integrity.
FAQ
What is sensitization and how does it lead to intergranular corrosion?
Sensitization is the metallurgical condition in which chromium-rich carbides or other deleterious phases precipitate at grain boundaries after exposure to the critical temperature range of roughly 425 to 815 degrees Celsius. The precipitation locally depletes chromium, leaving the boundary zones unable to form a passive film, so they dissolve preferentially in corrosive environments and produce a measurable corrosion rate or visible intergranular cracking.
How is the appropriate test method selected?
Method selection follows the alloy family and the failure mechanism of concern. Austenitic stainless steels are screened per ASTM A262 Practice A and confirmed by Practices B, C, E, or F; ferritic grades follow ASTM A763; nickel alloys follow ASTM G28; and 5XXX aluminum alloys follow ASTM G67. When sigma phase is a concern or nitric acid service is intended, the Huey test is generally specified, whereas the Strauss test is preferred for detecting classical chromium depletion in mildly aggressive environments.
What corrosion rate is considered acceptable?
Acceptance limits are set by the governing product specification or the purchaser. For the Huey test on common austenitic grades, typical acceptance limits fall in the range of 0.025 to 0.075 mm/a, with stricter values applied to low-carbon or stabilized grades. Some specifications instead compare the specimen rate against a sensitized reference or require no intergranular fissures in the bend test. Results must always be interpreted against the specific acceptance criteria cited in the purchase documents.
How long does testing take?
Oxalic acid screening and EPR-type electrochemical tests can be completed within one to two days, including metallography. The Huey test requires approximately 240 hours of boiling exposure across five solution-change periods, so the complete workflow, including specimen preparation, exposure, cleaning, bend testing, and reporting, typically spans two to three weeks.
Can welded components and castings be evaluated?
Yes. Cross-weld specimens are prepared so that the base metal, heat-affected zone, and weld metal are all exposed and can be examined separately after testing. Castings are tested with cast-to-size coupons or specimens machined from designated locations. Evaluating the as-delivered fabrication condition is essential because welding is the most common cause of sensitization in service.
Why should testing be performed by accredited third-party laboratories?
Intergranular corrosion results are highly sensitive to solution chemistry, boiling behavior, specimen cleaning, and metallographic interpretation. Laboratories accredited to ISO/IEC 17025 with documented competence in ASTM A262, A763, G28, G67, and related ISO, GB, and JIS methods provide traceable measurements, controlled test conditions, and reports suitable for certification, dispute resolution, and regulatory submission.