Stainless Steel Intergranular Corrosion Mass Loss Rate Testing: Principles, Standards, and Procedure

The stainless steel intergranular corrosion mass loss rate test is a quantitative laboratory method used to evaluate the susceptibility of stainless steels to grain boundary attack after sensitizing thermal exposure or welding. By measuring the mass lost per unit surface area per unit time during immersion in boiling acid media, the test delivers objective corrosion-rate data for material qualification, heat treatment validation, and failure prevention. This article explains the testing principle, sample preparation, standard methods, calculation formulas, acceptance criteria, and typical industrial applications of the mass loss rate experiment.

Technical Overview

Intergranular corrosion (IGC) is a localized form of attack that propagates along or immediately adjacent to the grain boundaries of a metal while the grain interiors remain largely unaffected. In stainless steels, the dominant mechanism is sensitization: when the material is heated within the critical temperature range of approximately 450 to 850 degrees Celsius, chromium combines with carbon to precipitate chromium-rich carbides (typically Cr23C6) at the grain boundaries. This depletes the chromium content in the narrow boundary zone below the approximately 12 percent level required to maintain a stable passive film, so the depleted zones become anodic relative to the grain interiors and dissolve preferentially in corrosive media.

The mass loss rate experiment quantifies this susceptibility gravimetrically. A weighed test specimen with a known exposed surface area is exposed to a boiling aggressive solution for a defined period, cleaned, and reweighed. The mass loss per unit area per unit time, together with the converted corrosion penetration rate, provides a numerical index of sensitization. Because severely attacked grains may detach entirely from the surface (a phenomenon known as grain dropping), the measured mass loss integrates both the direct dissolution of boundary material and the loss of whole grains, making the metric highly sensitive to intergranular attack.

The mass loss rate approach is codified in several internationally recognized standards, and the most commonly applied methods include:

• ASTM A262 Practice B and GB/T 4334.2: ferric sulfate–50 % sulfuric acid test for austenitic stainless steels.
• ASTM A262 Practice C and GB/T 4334.3: boiling 65 % nitric acid (Huey) test.
• ASTM G28 Method A: ferric sulfate–sulfuric acid test for wrought nickel-rich alloys and stainless steels.
• ISO 3651-2 and JIS G0574: ferric sulfate–sulfuric acid procedures aligned with ASTM A262 Practice B.
• ASTM A262 Practices D and E and GB/T 4334.4/4334.5: nitric–hydrofluoric acid and copper sulfate–sulfuric acid procedures for special grade families and pass/fail screening.

Testing Samples

Sample Dimensions and Surface Area

Test specimens are machined or sheared from representative material and are prepared so that the entire exposed surface can be accurately measured before exposure. Depending on the governing standard, the total exposed surface area is typically maintained within a range of roughly 15 to 35 square centimeters for the ferric sulfate test, and the exact area is calculated from measured dimensions, including hole walls and all wetted faces. A precise surface area is essential because the mass loss rate is normalized to it.

Material States to Be Evaluated

Specimens may be tested in one or more metallurgical conditions depending on the purpose of the evaluation:

• As-received or mill-annealed condition, for routine product acceptance.
• Sensitized condition, produced by a laboratory heat treatment such as holding at approximately 650 to 675 degrees Celsius followed by air cooling, to assess intrinsic sensitization tendency.
• Post-weld or simulated heat-affected-zone condition, for welding procedure qualification.
• Solution-annealed reference condition, used as a baseline for comparative ratio criteria in the nitric–hydrofluoric acid method.

Surface Preparation and Initial Weighing

All scale, grease, and foreign matter must be removed before testing. Typical preparation includes solvent degreasing, mechanical grinding or pickling to remove oxide layers, thorough rinsing in water and alcohol, and drying in warm air or a desiccator. Each specimen is then weighed on an analytical balance with a readability of 0.001 g or better, and the initial mass, dimensions, and calculated surface area are recorded. Specimens are usually identified by stamping or engraving on a non-critical edge, or by separate tagging when stamping is prohibited.

Testing Items

The core deliverable of the experiment is the quantitative mass loss rate and its derived corrosion rate. The principal measured and calculated items are:

• Initial specimen mass (m0), final specimen mass (m1), and absolute mass loss (m0 minus m1) in grams.
• Total exposed surface area (S) in square meters or square centimeters.
• Exposure duration (t) in hours at sustained boiling.
• Mass loss rate R, calculated as R = (m0 minus m1) / (S multiplied by t), expressed in g/(m2·h).
• Converted corrosion penetration rate, CR = 8.76 multiplied by 10 to the fourth power, multiplied by (m0 minus m1), divided by (S, t, and the density of the alloy), expressed in mm/a when consistent units are used.
• Visual and metallographic examination results, including evidence of grain dropping, ditch structures, and intergranular cracking after bending.
• Verification records of solution concentration, solution volume-to-area ratio, boiling temperature, and solution renewal schedule.

Testing Methods

Ferric Sulfate–Sulfuric Acid Method (ASTM A262 Practice B / GB/T 4334.2)

The specimen is immersed in a boiling solution of 50 % by mass sulfuric acid containing dissolved ferric sulfate, commonly prepared at a ratio of 25 grams of ferric sulfate per 600 milliliters of acid solution, in a flask fitted with a reflux condenser to maintain constant concentration. Exposure is continuous for 120 hours, with the solution maintained at boiling and a specified minimum solution volume provided per unit specimen surface area. The ferric ions act as a depolarizer, sustaining a corrosion potential at which chromium-depleted boundaries dissolve rapidly. The result is reported as a mass loss rate in g/(m2·h) or as an equivalent corrosion rate, and it is the primary quantitative acceptance method for austenitic grades such as AISI 304, 304L, 316, and 316L.

Boiling Nitric Acid Method (Huey Test, ASTM A262 Practice C / GB/T 4334.3)

The specimen is exposed to boiling 65 % by mass nitric acid in five consecutive 48-hour periods, for a cumulative 240 hours, with the solution renewed after each period. Mass loss is determined after every period, which allows detection of accelerating attack associated with grain boundary segregation of sigma phase or other second phases in addition to carbide sensitization. Results are commonly reported as the average corrosion rate in mils per month or as a mass loss rate, and the Huey test is particularly relevant for alloys intended for nitric acid service.

Nitric–Hydrofluoric Acid Method (ASTM A262 Practice D / GB/T 4334.4)

This short-duration method uses a 10 % nitric acid plus 3 % hydrofluoric acid solution at a controlled elevated temperature for molybdenum-bearing stainless steels such as AISI 316 and 317. Because the medium attacks the matrix appreciably, the evaluation is comparative: the mass loss rate of a sensitized specimen is compared with that of a solution-annealed reference specimen of the same heat, and a rate ratio above an agreed limit, commonly around 1.5, indicates unacceptable sensitization.

General Test Procedure

• Measure all specimen dimensions, calculate the total exposed surface area, and record it.
• Degrease, clean, rinse, and dry the specimen, then weigh it to 0.001 g to obtain m0.
• Prepare the test solution at the specified concentration and charge it into a flask equipped with an efficient reflux condenser.
• Suspend the specimen in the boiling solution using a glass cradle or holder, ensuring full immersion, no galvanic contact with other metals, and the required solution volume-to-area ratio.
• Maintain continuous boiling for the specified duration, verifying reflux and solution level periodically.
• Remove the specimen, rinse thoroughly, remove adherent corrosion products by gentle mechanical or chemical cleaning without attacking the base metal, rinse, dry, and reweigh to obtain m1.
• Calculate the mass loss rate and converted corrosion rate, and perform visual and metallographic examination for intergranular attack and grain dropping.
• Issue a test report documenting the material, heat number, metallurgical condition, solution preparation, test duration, mass data, calculated rates, and metallographic findings.

Equipment and Apparatus

A typical setup comprises a borosilicate glass boiling flask with an Allihn-type reflux condenser, a heating mantle or hot plate capable of sustained boiling, a calibrated thermometer, glass specimen cradles, an analytical balance with 0.001 g readability, a desiccator, and metallographic facilities for post-test cross-sectional examination.

Applications

The intergranular corrosion mass loss rate test is applied across the full life cycle of stainless steel components in demanding industries:

• Pressure vessels, boilers, and piping in the petrochemical and chemical processing industries, where sensitized weld zones face hot acid service.
• Nuclear power plant components and heat exchanger tubing, for which quantitative sensitization data support integrity assessments and life extension.
• Mill product acceptance testing of plate, sheet, bar, pipe, and tube against ASTM, GB/T, ISO, and JIS specifications before shipment.
• Welding procedure qualification and post-weld heat treatment validation, verifying that fabrication thermal cycles have not sensitized the heat-affected zone.
• Research and development of low-carbon grades (304L, 316L), stabilized grades (321, 347), and modern high-alloy steels, where mass loss rates quantify the benefit of titanium or niobium stabilization and controlled carbon levels.
• Failure analysis, distinguishing intergranular corrosion from pitting, stress corrosion cracking, and general corrosion in service failures.
• Procurement quality control and third-party inspection, providing documented, reproducible evidence of intergranular corrosion resistance.

FAQ

What causes intergranular corrosion in stainless steel? Intergranular corrosion is caused mainly by sensitization: chromium carbide precipitation at grain boundaries during heating in the 450 to 850 degrees Celsius range, which locally depletes chromium and destroys passivity in the boundary zone. Segregation of impurities or precipitation of sigma phase can also produce grain boundary attack in certain media.

How is the mass loss rate calculated? The mass loss rate equals the specimen mass loss divided by the product of its exposed surface area and exposure time, expressed in g/(m2·h). Multiplying this rate by 8.76 multiplied by 10 to the fourth power and dividing by the alloy density yields the penetration rate in mm/a.

What results are considered acceptable? Acceptance limits are set by the governing product specification or purchase order. For the ferric sulfate–sulfuric acid test, many specifications for low-carbon and stabilized austenitic grades limit the mass loss rate to values of the order of 1 to 2 g/(m2·h), while the nitric–hydrofluoric method uses a sensitized-to-annealed rate ratio, commonly limited to about 1.5. Any evidence of grain dropping under metallographic examination is normally grounds for rejection regardless of the numerical rate.

Why choose a mass loss rate test instead of a screening etch test? The oxalic acid etch test (ASTM A262 Practice A / GB/T 4334.1) is a rapid metallographic screen that classifies etch structures but cannot produce a numerical corrosion rate. The mass loss rate test provides quantitative, reproducible, and contractually enforceable data, and it serves as the confirmatory method when a screening test indicates a doubtful or unacceptable structure.

Which method applies to molybdenum-bearing stainless steels? For molybdenum-bearing grades such as 316 and 317, the nitric–hydrofluoric acid method is generally specified because molybdenum influences behavior in nitric acid media; results are interpreted through the comparative rate ratio rather than an absolute limit.

How long does a typical test take? The ferric sulfate–sulfuric acid test requires a continuous 120-hour boiling exposure, while the Huey test requires five 48-hour periods, or 240 hours total, plus specimen preparation and metallographic evaluation. Total turnaround, including machining, sensitizing heat treatment, and reporting, is usually scheduled over one to two weeks.