Intergranular Corrosion Mechanism Study and Experimental Testing of Stainless Steel

Intergranular corrosion (IGC) is one of the most dangerous localized failure modes in stainless steel, occurring when sensitization depletes chromium along grain boundaries and leaves them vulnerable to preferential attack. This article explains the metallurgical mechanism of intergranular corrosion, including chromium carbide precipitation and sensitization, and describes the standardized laboratory experiments—such as ASTM A262 and DL-EPR testing—used to evaluate the susceptibility of austenitic, ferritic, and duplex stainless steels. Engineers, metallurgists, and quality-control professionals will find practical guidance on sample preparation, test selection, result interpretation, and prevention strategies.

Technical Overview

Intergranular corrosion is a localized form of attack in which corrosion proceeds preferentially along grain boundaries or in zones immediately adjacent to them, while the grain interiors remain largely unattacked. Because the attacked boundaries lose their cohesive strength, a component can suffer grain dropping, loss of load-bearing cross-section, and sudden brittle fracture with little or no visible external warning. For this reason, IGC is treated as a critical failure mechanism in pressure vessels, nuclear components, chemical plant equipment, and welded fabrications.

The dominant and most widely accepted mechanism is the chromium depletion theory. When a conventional austenitic stainless steel containing more than approximately 0.03% carbon is heated in the range of roughly 425–815°C (800–1500°F)—for example, in the heat-affected zone of a weld, during slow cooling from forming operations, or in elevated-temperature service—chromium-rich carbides of the type M23C6 nucleate preferentially at grain boundaries. Because interstitial carbon diffuses orders of magnitude faster than substitutional chromium, the growing carbides draw chromium from the immediately surrounding matrix, creating a thin depleted zone in which the local chromium content falls below the roughly 12% level required to maintain a stable passive film. In oxidizing or acidic electrolytes, this depleted band becomes anodic relative to the chromium-rich grain interior, forming a micro-galvanic cell that drives dissolution along the boundary.

Complementary mechanisms are also active in specific alloy systems: precipitation of the chromium- and molybdenum-rich sigma phase (typically 600–900°C in molybdenum-bearing grades), chromium nitride Cr2N formation in ferritic and high-nitrogen steels, molybdenum depletion adjacent to boundary precipitates, and segregation of impurity elements such as phosphorus, sulfur, and silicon to grain boundaries. Stabilized steels such as types 321 and 347 can additionally suffer knife-line attack adjacent to weld fusion lines. A complete mechanism study therefore combines controlled sensitization heat treatments, standardized chemical and electrochemical corrosion tests, and high-resolution microanalysis (SEM-EDS, TEM, AES) to map chromium concentration profiles across boundaries and verify the depletion model quantitatively.

Testing Samples

Austenitic Stainless Steels (304, 304H, 316)

Austenitic grades with standard carbon content are the classical subjects of intergranular corrosion research because their sensitization kinetics are well documented. Typical coupon dimensions for immersion testing are 30 mm × 20 mm × 3–5 mm, while oxalic acid etch specimens require a polished face of approximately 1 cm². Test conditions normally include a solution-annealed control sample, a deliberately sensitized sample (for example, 675°C for 1 hour and air cooling), and as-welded cross-sections so that the base metal, heat-affected zone, and weld metal can be compared within one specimen.

Low-Carbon and Stabilized Grades (304L, 316L, 321, 347)

Low-carbon L grades (C ≤ 0.03%) and grades stabilized with titanium or niobium are tested to verify the effectiveness of their chemistry in suppressing M23C6 precipitation. These materials are frequently subjected to extended or accelerated sensitization treatments, and stabilized grades receive additional attention for knife-line attack near weld fusion boundaries, for which the nitric acid test is particularly revealing.

Ferritic and Duplex Stainless Steels

Ferritic grades such as 430, 444, and 446 sensitize very rapidly even at low interstitial levels because chromium carbides and nitrides precipitate readily in the bcc matrix, so testing focuses on stabilized or ultra-low interstitial variants. Duplex steels such as 2205 and 2507 are inherently resistant due to their two-phase structure and nitrogen alloying, but experiments on heavily welded or thermally cycled material examine potential precipitation of sigma phase and chromium nitride, which can still produce local boundary attack.

Welded Joints and Simulated Heat-Affected Zones

Cross-weld coupons that span weld metal, fusion line, HAZ, and base metal are the most representative industrial samples. For mechanistic work, Gleeble thermo-mechanical simulation reproduces HAZ thermal cycles with peak temperatures of 600–1300°C and controlled cooling rates. Testing a matrix of simulated specimens allows construction of time-temperature-sensitization (TTS) diagrams that define safe and unsafe welding and heat-treatment windows.

Testing Items

A comprehensive intergranular corrosion mechanism study typically includes the following evaluation items:

• Degree of sensitization (DOS), quantified by electrochemical reactivation charge or current ratios
• Identification of grain-boundary precipitates (M23C6 carbides, sigma phase, Cr2N nitrides) and their morphology and distribution
• Chromium depletion depth and minimum chromium concentration at the boundary, measured by TEM-EDS or AES profiling
• Average and per-period corrosion rates after boiling acid exposure, calculated from weight loss
• Bend ductility and detection of intergranular fissures after copper–copper sulfate exposure
• Etain etch-structure classification (step, dual, or ditch structure) after oxalic acid electrolytic etching
• Time-temperature-sensitization curves for welding procedure and heat-treatment qualification

Testing Methods

Oxalic Acid Etch Test (ASTM A262 Practice A, GB/T 4334.1)

This rapid screening test electrolytically etches a polished specimen in 10% oxalic acid at a current density of 1 A/cm² for 90 seconds at room temperature. Examination at 250–500× reveals the boundary structure: a step structure indicates no harmful carbide precipitation, a dual structure shows isolated ditches without any grain completely encircled, and a ditch structure—one or more grains entirely surrounded by attacked boundaries—indicates probable sensitization that must be confirmed by a quantitative boiling-acid test.

Ferric Sulfate–50% Sulfuric Acid Test (ASTM A262 Practice B)

Coupons are exposed for 120 hours in boiling 50% sulfuric acid containing ferric sulfate, which maintains a controlled oxidizing potential that preferentially attacks chromium-depleted zones. Corrosion rate is calculated from weight loss using the relationship CR = (8.76 × 10⁴ × ΔW) / (ρ × A × t), where ΔW is mass loss in grams, ρ is density, A is exposed area, and t is time in hours. Acceptance limits are defined by the governing material specification.

Nitric Acid Huey Test (ASTM A262 Practice C)

Specimens undergo five consecutive 48-hour periods in boiling 65% nitric acid with fresh solution each period. The Huey test is uniquely sensitive not only to chromium depletion but also to sigma phase and impurity segregation, making it mandatory for nitric acid service and for stabilized grades. Corrosion rates that increase from period to period signal progressive grain-boundary attack and grain dropping.

Copper–Copper Sulfate–16% Sulfuric Acid Test (ASTM A262 Practice E)

Also known as the Strauss test, specimens are immersed for 24 hours in boiling 16% sulfuric acid with copper sulfate and metallic copper chips, which limit the cathodic reaction so that attack concentrates in sensitized boundaries. The specimen is then bent 180° around a mandrel; fissures or cracks visible at 10–20× magnification confirm intergranular corrosion. This practice is the most widely specified acceptance test for welded austenitic components.

Electrochemical Potentiodynamic Reactivation (EPR and DL-EPR)

In the single-loop EPR method (ASTM G108 for type 304), the specimen is passivated by an anodic scan in 0.5 M H2SO4 + 0.01 M KSCN at 30°C and then reactivated by reversing the scan; the reactivation charge is proportional to the sensitized boundary area. The double-loop (DL-EPR) variant measures the ratio of the reactivation peak current to the activation peak current, providing a fast, quantitative, and nearly non-destructive degree-of-sensitization index that is ideal for research, weld mapping, and constructing TTS diagrams.

Microanalysis and Corrosion Morphology Characterization

Following chemical or electrochemical exposure, attacked surfaces and cross-sections are examined by optical microscopy and SEM to document intergranular fissuring and grain dropping. SEM-EDS identifies carbide and intermetallic compositions; analytical TEM with EDS or EELS resolves chromium concentration profiles across individual boundaries on the nanometer scale; and AES depth profiling complements these measurements. Together these techniques directly verify whether the corrosion path coincides with the chromium-depleted zones predicted by the depletion theory.

Applications

Intergranular corrosion mechanism studies and qualification experiments directly support safety, reliability, and material development across many industries:

• Nuclear power: qualification of 304L/316L piping and internals, irradiation-assisted stress corrosion research, and weld procedure acceptance
• Petrochemical and chemical processing: heat exchangers, reactors, and columns handling nitric, sulfuric, and organic acids where sensitized weldments would fail rapidly
• Pharmaceutical, food, and beverage plants: certification of 316L product-contact surfaces where corrosion products are unacceptable
• Welding procedure qualification (WPS/PQR), post-fabrication heat-treatment verification, and incoming material acceptance testing
• Failure analysis and remaining-life assessment of service-exposed components and retrofit weld repairs
• Alloy research and development: quantifying the effects of carbon, nitrogen, manganese, and stabilizer chemistry, and generating TTS diagrams for welding simulation models

FAQ

What causes intergranular corrosion in stainless steel?

The primary cause is chromium depletion at grain boundaries produced by precipitation of chromium-rich M23C6 carbides when the steel is exposed to the sensitization temperature range. Sigma phase, chromium nitrides, molybdenum depletion, and impurity segregation contribute in specific alloy systems and environments.

What is the sensitization temperature range?

Sensitization occurs between approximately 425°C and 815°C, with the fastest attack typically between 600°C and 750°C. Weld heat-affected zones inevitably traverse this range, which is why welding parameters and post-weld cooling rates are decisive for avoiding sensitization.

How can intergranular corrosion be prevented?

Use low-carbon L grades with C ≤ 0.03%, specify stabilized grades 321 or 347 whose titanium or niobium ties up carbon as stable carbides, apply solution annealing at 1040–1150°C followed by rapid water quenching, control welding heat input and interpass temperature to minimize time in the critical range, and consider post-weld solution annealing where feasible.

Which test method should be selected?

Practice A is chosen for rapid screening; Practice E for general acceptance of welded austenitic parts; Practice B when chromium depletion is the controlling mechanism; Practice C when sigma phase, impurity segregation, or nitric acid service is involved; Practice F or ASTM G28 for molybdenum-bearing and nickel-bearing alloys; and DL-EPR when a quantitative degree of sensitization is required for research or weld mapping. Equivalent international standards include GB/T 4334 series, ISO 3651-2, and JIS G0571–G0575.

How long does an intergranular corrosion test take?

The oxalic acid etch test takes only minutes, the Strauss test requires 24 hours of boiling plus bend evaluation, the ferric sulfate test runs 120 hours, and the Huey test requires five 48-hour periods totaling 240 hours of exposure. Including metallography, bend examination, and reporting, a complete evaluation typically has a turnaround of one to two weeks.