Intergranular corrosion (IGC) mass loss rate determination is a quantitative gravimetric test used to evaluate the susceptibility of stainless steels and nickel alloys to grain-boundary attack caused by sensitization. By measuring the mass loss of specimens exposed to boiling acidic media, the test calculates corrosion rates in g/(m²·h) or mm/a in accordance with standards such as ASTM A262, ASTM G28, ISO 3651, GB/T 4334, and JIS G 0573. This article explains the testing principle, sample requirements, test procedures, calculation formulas, acceptance criteria, and typical industrial applications of IGC mass loss rate testing.
Technical Overview
Intergranular corrosion is a localized form of corrosion in which attack proceeds along grain boundaries while the grains themselves remain largely unaffected. In austenitic stainless steels, duplex stainless steels, and nickel-chromium alloys, the most common cause is sensitization: when the material is heated in the range of approximately 425 °C to 815 °C, chromium carbides precipitate at grain boundaries, depleting the adjacent zones of chromium. When the local chromium content falls below the level required to maintain a passive film (roughly 12%), these depleted zones dissolve preferentially in oxidizing acidic media. Severe intergranular attack leads to grain dropping, loss of cohesion, and catastrophic brittle failure with little visible warning.
The mass loss rate method is the principal quantitative technique for assessing intergranular corrosion susceptibility. A weighed specimen with a known surface area is exposed to a specified boiling test solution for a defined period, after which the specimen is cleaned, dried, and reweighed. The corrosion rate is calculated from the mass loss per unit surface area per unit time and is typically expressed in g/(m²·h) or converted into a penetration rate in mm/a. Because intergranular attack and grain dropping produce disproportionately high mass loss, elevated or accelerating corrosion rates are a reliable indicator of sensitization. Where required, metallographic examination of cross sections is performed to confirm the depth and morphology of grain-boundary attack.
The method is standardized internationally. Key standards include ASTM A262 (Practices B and C for austenitic stainless steels), ASTM G28 (Method A for wrought nickel-rich, chromium-bearing alloys), ISO 3651-1 (Huey test by measurement of mass loss), GB/T 4334 (metallic materials – intergranular corrosion test for stainless steels), and JIS G 0573 (65% nitric acid test). Accredited third-party laboratories routinely perform these tests for material qualification, weld procedure qualification, and incoming material inspection.
Testing Samples
Sample Forms and Product Types
Intergranular corrosion mass loss rate testing can be performed on virtually all product forms of stainless steels and related alloys, including:
- Plates, sheets, and strips
- Bars, rods, wires, and forging blanks
- Seamless and welded pipes and tubes
- Castings and investment cast components
- Flanges, fittings, fasteners, and other pressure parts
- Welded joint specimens incorporating base metal, heat-affected zone (HAZ), and weld metal
- Nickel-chromium-iron alloys such as alloy 600 and alloy 690 tested per ASTM G28
Specimen Preparation
Specimens are machined to dimensions specified by the applicable standard, commonly rectangular coupons on the order of 30 mm × 20 mm with a total exposed surface area sufficient for reliable gravimetric measurement. Preparation requirements include:
- All surfaces ground to a uniform finish (typically 120 grit or finer) to remove scale, decarburized layer, and machining marks
- Edges deburred and corners rounded to avoid crevices and accelerated edge attack
- Welded specimens oriented so that the weld, HAZ, and base metal are all exposed to the test solution
- Where specified by the material specification, a sensitizing heat treatment (for example, 650 °C for 2 h followed by air cooling) applied before testing to simulate unfavorable service conditions
- Specimens degreased, cleaned, rinsed, and dried, then weighed on an analytical balance to the nearest 0.1 mg
- Dimensions measured accurately to calculate the total exposed surface area, including all machined surfaces
Testing Items
The core deliverable of the test is the quantitative corrosion rate derived from specimen mass loss. Typical reported testing items include:
- Mass loss after each boiling period and cumulative mass loss (in grams)
- Average corrosion rate in g/(m²·h)
- Equivalent penetration rate in mm/a or mils per year (mpy)
- Corrosion rate for each individual test period, used to detect accelerating attack (a characteristic indicator of intergranular corrosion in the Huey test)
- Visual examination for grain dropping, cracking, and edge attack after testing
- Supplementary metallographic examination of cross sections to measure the depth of intergranular penetration and confirm grain-boundary attack morphology
- Comparison of measured rates against the acceptance limits specified in the product specification or purchase order
Testing Methods
Nitric Acid Test (Huey Test)
The Huey test, specified in ASTM A262 Practice C, ISO 3651-1, GB/T 4334 (nitric acid method), and JIS G 0573, is the classical mass-loss-based intergranular corrosion test. Specimens are immersed in boiling 65% nitric acid under a reflux condenser for five consecutive 48-hour periods, for a total exposure of 240 hours. The acid is replaced with fresh solution after each period, and the specimen is weighed after each period to determine the per-period corrosion rate. The solution volume is maintained at a minimum ratio to specimen surface area, typically at least 20 mL/cm². The Huey test is particularly sensitive to chromium carbide precipitation and sigma phase, and it is widely specified for materials destined for nitric acid service and nuclear fuel reprocessing environments.
Ferric Sulfate–Sulfuric Acid Test
Specified in ASTM A262 Practice B, ASTM G28 Method A, and corresponding methods in GB/T 4334, this test immerses specimens in a boiling solution of 50% sulfuric acid containing ferric sulfate. The ferric ions provide an oxidizing environment that selectively attacks chromium-depleted grain boundaries. The standard exposure is a single 120-hour boiling period. This method is the preferred mass loss rate test for wrought nickel-rich, chromium-bearing alloys and for austenitic, ferritic, and duplex stainless steels where nitric acid testing is not required.
Complementary Copper Sulfate–Sulfuric Acid Methods
The copper–copper sulfate–sulfuric acid test (Strauss-type test), covered by ASTM A262 Practices E and F, ISO 3651-2, and GB/T 4334, is primarily evaluated by bend testing rather than mass loss. It is frequently performed alongside mass loss rate methods to provide a combined qualitative and quantitative assessment of sensitization, particularly for welded components.
Test Procedure and Calculation
The general procedure for mass loss rate determination is as follows:
- Clean, dry, and weigh the specimen to obtain the initial mass m0 (g)
- Measure the specimen and calculate the total exposed surface area A (m²)
- Expose the specimen to the boiling test solution for the specified period t (h) under reflux, maintaining the required solution volume-to-area ratio
- Remove the specimen, rinse thoroughly, remove any adherent corrosion products mechanically or chemically as permitted by the standard, rinse, and dry
- Reweigh the specimen to obtain the final mass m1 (g)
- Repeat exposure and weighing for each required period
The corrosion rate based on mass loss is calculated as: CR = (m0 − m1) / (A × t), giving the result in g/(m²·h). Where a penetration rate is required, the equivalent depth rate in mm/a is obtained from R = 8.76 × CR / ρ, where ρ is the density of the alloy in g/cm³. For the Huey test, the corrosion rate of each 48-hour period and the average of all five periods are both reported, because a steadily increasing per-period rate is strong evidence of progressive intergranular attack.
Acceptance Criteria and Evaluation
Acceptance limits are defined by the applicable material specification, product standard, or purchase agreement, since permissible corrosion rates differ between grades, product forms, and service environments. Evaluation typically combines three elements: comparison of the calculated average corrosion rate against the specified maximum limit; assessment of the trend of per-period rates for acceleration indicative of grain-boundary attack; and metallographic or visual confirmation of grain dropping or intergranular cracking. A specimen that exhibits a rate within limits but shows clear grain-boundary fissuring under examination may still be rejected, as mechanical integrity rather than mass loss alone governs fitness for service.
Applications
Intergranular corrosion mass loss rate determination is applied across many industries in which sensitized material could cause leakage or structural failure:
- Nuclear power and nuclear fuel reprocessing: qualification of nuclear-grade austenitic stainless steels for nitric acid and high-purity water service
- Chemical and petrochemical processing: reactors, distillation columns, heat exchangers, and piping handling aggressive acids and chlorinated organics
- Food, beverage, and pharmaceutical manufacturing: hygienic piping, vessels, and fittings where product purity and corrosion integrity are critical
- Pressure vessels, boilers, and power plant components fabricated to codes that require verified resistance to intergranular attack
- Welding quality control: verification of weld procedures, evaluation of the heat-affected zone, and qualification of post-weld solution annealing or stabilization treatments
- Material qualification and research: incoming material inspection at accredited third-party laboratories, alloy development, sensitization kinetics studies, and time-temperature-sensitization diagram construction
- Fabrication auditing: confirmation that low-carbon (L-grade) or stabilized (titanium- or niobium-bearing) grades have been correctly supplied and heat treated
FAQ
What is the difference between mass loss rate evaluation and bend test evaluation?
Mass loss rate evaluation is quantitative: it converts measured mass loss into a corrosion rate in g/(m²·h) or mm/a and compares it with a numerical limit. Bend test evaluation, used mainly with the copper sulfate–sulfuric acid methods, is qualitative: the specimen is bent after exposure and inspected for cracking or fissuring caused by grain-boundary attack. Mass loss methods provide numeric data suitable for specification acceptance and material comparison, while bend tests provide a direct indication of loss of ductility and cohesion.
Which standard should be selected for a given material?
ASTM A262 applies to austenitic stainless steels, with Practice C (Huey) specified for nitric acid service and Practice B (ferric sulfate–sulfuric acid) for general assessment. ASTM G28 Method A applies to wrought nickel-rich, chromium-bearing alloys. ISO 3651 and GB/T 4334 cover stainless steels with both nitric acid and sulfuric acid-based media, and JIS G 0573 provides the equivalent Japanese nitric acid test. The choice is normally dictated by the governing product specification or the purchaser's purchase order.
Why does the Huey test use five separate 48-hour periods?
The multiple-period design serves two purposes. First, replacing the acid every 48 hours prevents depletion of the oxidizing species and accumulation of corrosion products that could alter the test severity. Second, calculating a corrosion rate for each period reveals whether the attack is accelerating, which is a characteristic signature of progressive intergranular corrosion and grain dropping rather than uniform corrosion.
Can welded components be tested by the mass loss rate method?
Yes. Welded coupons are prepared so that the full cross section, including weld metal, fusion line, and heat-affected zone, is exposed to the boiling solution. This is one of the most common applications of the test, because the heat-affected zone is the region most susceptible to sensitization during welding. Where specified, a sensitizing heat treatment is applied before testing to simulate worst-case conditions.
What mass loss rate result is considered acceptable?
There is no universal limit; acceptance values are set by the material or purchase specification and vary with alloy grade, test method, and intended service. In practice, specification limits for austenitic stainless steels in the Huey test are commonly expressed as maximum average corrosion rates in g/(m²·h) or mm/a, and low-carbon or correctly solution-annealed material typically shows rates far below these limits. In addition to meeting the numeric limit, the specimen must show no significant grain dropping or intergranular cracking on examination.
How should test results be reported?
A complete report should identify the material and its heat treatment condition, the test standard and method, specimen dimensions and surface area, test solution and exposure duration, initial and final masses for each period, the calculated per-period and average corrosion rates in g/(m²·h) and, where required, mm/a, the results of visual and metallographic examination, and a clear statement of conformity with the applicable acceptance criteria. Testing performed by accredited third-party laboratories provides results suitable for certification, supplier qualification, and regulatory submission.