Stainless Steel Intergranular Corrosion Method C Test (Boiling 65% Nitric Acid Huey Test)

The stainless steel intergranular corrosion Method C test, widely known as the Huey test or boiling 65% nitric acid test, is the most severe standard laboratory method for evaluating the susceptibility of stainless steels to grain boundary attack. Conducted in accordance with ASTM A262 Practice C, GB/T 4334 Method C, ISO 3651-1, and JIS G 0573, the test exposes specimens to five consecutive 48-hour boiling periods and quantifies sensitization through weight-loss corrosion rates, bend testing, and metallographic examination. This article provides a complete technical guide to the principle, sample requirements, test procedure, calculation, acceptance criteria, and industrial applications of Method C testing.

Technical Overview

Intergranular corrosion (IGC) is a localized degradation mechanism in which the grain boundaries of a metal are preferentially attacked while the grain interiors remain largely unaffected. In stainless steels, the dominant cause is sensitization: when the material is heated within the 450 to 850 °C range, during welding, slow cooling from solution annealing, or improper heat treatment, chromium carbides precipitate at grain boundaries and deplete the adjacent zones of chromium to below the approximately 12% level required for passivity. These chromium-depleted corridors become anodic and dissolve rapidly in corrosive media.

Method C evaluates this susceptibility by immersing specimens in boiling 65% nitric acid, an oxidizing medium that aggressively attacks chromium-depleted grain boundaries, chromium-rich phases, and certain segregated solutes. Because the test is more severe than other standardized intergranular corrosion methods, it detects not only chromium carbide sensitization but also sigma phase and grain boundary segregation of elements such as silicon, phosphorus, and molybdenum. The quantitative result is the corrosion rate derived from specimen mass loss over five 48-hour boiling periods.

• Most aggressive of the standard IGC screening tests for austenitic stainless steels
• Detects sensitization from carbide precipitation as well as sigma phase and solute segregation
• Quantitative evaluation through weight-loss corrosion rate measurement
• Standardized in ASTM A262 Practice C, GB/T 4334 Method C, ISO 3651-1, and JIS G 0573

Testing Samples

Austenitic Stainless Steels

Method C is primarily applied to austenitic grades, including AISI 304, 304L, 316, 316L, 321 (titanium-stabilized), 347 (niobium-stabilized), 310S, and their European designation equivalents such as 1.4301, 1.4307, 1.4404, 1.4541, and 1.4550. These grades are the principal materials of construction for nitric acid service, which is the environment this test simulates most directly.

Ferritic and Duplex Stainless Steels

Ferritic grades and austenitic-ferritic duplex steels can also be evaluated where the product specification requires it, provided the higher general corrosion and different sensitization mechanisms of these materials are considered during interpretation. Stabilized and ultra-low-carbon variants are frequently tested to verify the effectiveness of their chemical composition and stabilization treatments.

Specimen Preparation Requirements

Specimens are typically cut to approximately 30 mm by 20 mm, with the thickness representing the actual product section, unless otherwise agreed. All surfaces are ground to a uniform finish, sharp edges are removed, and oxide scale, grease, and embedded iron are eliminated by pickling, passivation, and degreasing as required. The exact dimensions are measured to calculate the total exposed surface area, and each specimen is marked and weighed on an analytical balance, typically to the nearest 0.1 mg, before testing. For welded components, the specimen should include the weld metal, heat-affected zone, and base metal so that the entire joint is evaluated.

Testing Items

A complete Method C test program delivers the following results:

• Corrosion rate for each of the five 48-hour boiling periods, expressed in g/(m²·h) or mm/a, with mil-per-year equivalents where required
• Average corrosion rate over the full 240-hour exposure
• Maximum single-period corrosion rate and the rate trend across successive periods
• Supplementary 180° bend test after exposure, with visual and low-magnification examination for intergranular cracking
• Metallographic examination of cross sections to measure the depth and morphology of grain boundary attack
• Final acceptance decision against the limit agreed in the product specification or purchase order

Testing Methods

Test Solution and Apparatus

The test medium is nitric acid of 65% ± 0.5% mass fraction, prepared from reagent-grade concentrated acid and distilled or deionized water. Testing is conducted in a glass vessel, commonly a 1 L flask, fitted with an all-glass reflux condenser that maintains the acid at continuous boiling (approximately 118 to 121 °C) without change in concentration. Specimens are supported on glass hooks or cradles and fully immersed, with the solution volume kept at the minimum ratio to specimen surface area prescribed by the applicable standard.

Test Procedure

• Measure and record specimen dimensions, calculate the total surface area, degrease, dry, and weigh each specimen.
• Introduce fresh 65% nitric acid into the vessel, bring it to boiling, and immerse the specimens; maintain uninterrupted refluxing for 48 hours.
• Remove the specimens, rinse in water and alcohol, mechanically clean corrosion products without attacking the base metal, dry, and reweigh.
• Repeat the cycle with completely fresh acid for a total of five consecutive 48-hour periods, giving a cumulative exposure of 240 hours.
• On completion, perform the optional bend test and metallographic examination to confirm the nature and depth of the attack.

Calculation and Evaluation of Results

The corrosion rate for each period is calculated from the mass loss using the relationship v = m / (S × t), where m is the mass loss in grams, S is the total surface area in square metres, and t is the test duration in hours, giving the rate in g/(m²·h).

The depth-rate equivalent in millimetres per year is obtained from d = 8.76 × v / ρ, where ρ is the density of the steel in g/cm³ (approximately 7.9 for austenitic grades); the same result may be expressed in mils per year for specification compliance.

Acceptance limits are defined by the product specification or by agreement between purchaser and supplier. Many commercial specifications permit average rates on the order of 0.5 to 1.0 mm/a, whereas demanding applications such as nuclear fuel reprocessing impose far stricter limits, frequently expressed as micrometres of penetration per 48-hour period together with a maximum-to-minimum period ratio criterion. A distinctly rising corrosion rate in the later periods is a characteristic indication of sigma phase or progressive grain boundary attack and is treated as a cause for rejection even when the average remains low.

Applications

• Qualification of stainless steels for nitric acid production, fertilizer plants, and chemical process equipment where the Huey environment directly represents service conditions
• Material selection and certification for nuclear fuel reprocessing plants, where extremely low intergranular attack is mandatory
• Verification of solution annealing, quenching, and stabilization heat treatments at steel mills and forging shops
• Welding procedure qualification and evaluation of sensitization in the heat-affected zone of welded joints
• Incoming inspection and failure analysis of components suspected of sensitization-related cracking
• Research and development, including time-temperature-sensitization diagrams and the development of new low-interstitial and stabilized alloys

FAQ

What is the difference between Method C and the other intergranular corrosion methods?

Method A is a rapid 10% oxalic acid etch used only for screening classification. Method B, the ferric sulfate and 50% sulfuric acid test, is a milder immersion test that does not reveal sigma phase. Method E, the copper-copper sulfate and 16% sulfuric acid test, evaluates mainly carbide sensitization through a bend test. Method C, the boiling 65% nitric acid test, is the most severe of the group and additionally detects sigma phase and grain boundary segregation, making it both a quality-control and a materials-selection tool for nitric acid service.

Why are five consecutive 48-hour periods specified?

Repeated exposure with fresh acid in each period ensures that the oxidizing power of the solution remains constant and allows progressive intergranular penetration to accumulate. The period-by-period rate profile is diagnostically important: continuously increasing rates in later cycles are a strong indicator of sigma phase or severe sensitization that a single exposure might understate.

What can cause a high corrosion rate in Method C besides carbide sensitization?

Sigma or chi phase precipitation, grain boundary segregation of silicon, phosphorus, boron, or molybdenum, end-grain attack on machined surfaces, residual oxide scale, and contamination by embedded iron or chlorides can all elevate the measured rate. A metallographic cross section is therefore recommended whenever the weight-loss result is borderline or unexpectedly high.

Is the test suitable for molybdenum-bearing grades such as 316 and 316L?

Yes, the test is regularly applied to these grades, but interpretation requires care because nitric acid can attack molybdenum-rich phases and produce elevated rates unrelated to carbide sensitization. Where the service environment does not involve nitric acid, Method B or Method E is often specified as more appropriate.

How should Method C results be reported?

A complete report identifies the steel grade, product form, heat number, heat treatment condition, and specimen location, together with the surface area, the mass loss and calculated corrosion rate for each of the five periods, the average and maximum rates, the results of any bend or metallographic examination, and a clear statement of conformity with the agreed acceptance criterion.