Metallographic Analysis of Intergranular Corrosion in Stainless Steel

Metallographic analysis is the cornerstone laboratory technique for detecting and evaluating intergranular corrosion (IGC) in stainless steel, a localized attack that dissolves grain boundaries and can cause sudden component failure with little external warning. By combining electrolytic etching with optical microscopy, laboratories can classify microstructures as step, dual, or ditch type in accordance with ASTM A262 and related standards, providing a rapid and reliable screening method for sensitization. This article explains the metallurgical mechanism, sample types, testing items, evaluation methods, industrial applications, and frequently asked questions related to stainless steel intergranular corrosion metallographic analysis.

Technical Overview

Intergranular corrosion is a localized degradation mechanism in which corrosive attack is concentrated along the grain boundaries of a metal while the grain interiors remain largely unaffected. In stainless steels, the dominant cause is sensitization: a thermally induced change in which chromium-rich carbides, typically Cr23C6, precipitate at grain boundaries when the material passes through the critical temperature range of approximately 450 to 850 degrees Celsius (840 to 1560 degrees Fahrenheit). This cycle is routinely encountered during welding, slow cooling from hot working or annealing, improper stress relief, or extended high-temperature service exposure.

Because carbon diffuses to grain boundaries far faster than chromium can replenish them from the grain interior, a thin chromium-depleted zone forms on either side of each boundary. When the local chromium content in this zone falls below roughly 12 percent by weight, the protective passive film can no longer be maintained, and the boundary becomes electrochemically anodic relative to the grain body. In aggressive media such as sulfuric acid, nitric acid, organic acids, chlorides, or polythionic acids in refinery service, the boundary dissolves preferentially. Entire grains can eventually drop out of the surface, producing a catastrophic loss of strength and ductility with very little measurable weight loss.

Metallographic analysis addresses this risk directly. By electrolytically etching a polished cross section and examining it under an optical microscope, the analyst can determine whether grain boundaries are free of sensitization (step structure), partially attacked (dual structure), or continuously ditched (ditch structure). The technique offers several decisive advantages:

• Rapid screening: an etch test takes minutes, compared with days for boiling-acid corrosion tests.
• Direct visual evidence: attack morphology, carbide networks, and secondary phases are observed in situ.
• Standards-based classification: results map directly to acceptance criteria in ASTM A262, GB/T 4334, and ISO 3651.
• Precise localization: sensitized bands in weld heat-affected zones can be pinpointed and measured.
• Low material consumption: only a small representative sample is consumed or destroyed.

Testing Samples

Samples for intergranular corrosion metallographic analysis are selected to represent the exact metallurgical condition under evaluation. The principal sample categories are described below.

Austenitic Stainless Steels

Grades such as 304, 304L, 316, 316L, 321, 347, 310, and their cast equivalents (CF8, CF3M, CF8M) account for the majority of test volume. Samples may be taken from solution-annealed plate, pipe, or forgings to confirm an acceptable baseline structure, from deliberately sensitized material to verify heat treatment response, or from low-carbon and stabilized grades to confirm immunity to chromium carbide precipitation.

Ferritic and Martensitic Stainless Steels

Ferritic grades such as 409, 430, 436, and 444 can sensitize even more rapidly than austenitic grades because of the high diffusivity of interstitial elements in the body-centered cubic lattice. Martensitic and precipitation-hardening grades are examined for grain boundary carbides that influence both corrosion behavior and mechanical performance.

Duplex Stainless Steels

Duplex grades such as 2205, 2304, and 2507 are analyzed primarily for harmful secondary phases, including sigma, chi, and chromium nitride precipitation at phase boundaries and grain boundaries, produced by slow cooling or extended exposure in the 600 to 950 degree Celsius range. The austenite-ferrite phase balance is evaluated on the same polished sections.

Weldments and Heat-Affected Zones

Transverse cross sections through welded joints are among the most safety-critical samples, because welding inevitably drives a band of the heat-affected zone through the sensitization temperature range. Sections are cut perpendicular to the weld so that base metal, heat-affected zone, fusion line, and weld metal appear within one mounted specimen, allowing the sensitized distance from the fusion boundary to be measured accurately.

Service-Exposed and Failed Components

Heat exchanger tubes, reactor linings, column internals, and fractured parts removed from service are sectioned to characterize actual in-service attack, measure penetration depth, and distinguish intergranular corrosion from other mechanisms such as chloride stress corrosion cracking or grain boundary oxidation.

Testing Items

A complete metallographic examination for intergranular corrosion typically includes the following evaluation items.

• Etch structure classification: determination of step, dual, or ditch structure according to ASTM A262 Practice A criteria.
• Grain boundary morphology: continuity of ditching, distribution of discrete carbide particles, and attack at triple points.
• Carbide characterization: morphology and distribution of chromium-rich M23C6 precipitates at grain boundaries.
• Secondary phase detection: identification of sigma, chi, and other intermetallic phases in duplex and super-austenitic grades.
• Weld heat-affected zone evaluation: width and severity of the sensitized band relative to the fusion line.
• Penetration depth measurement: maximum intergranular attack depth on corrosion-tested or service-exposed surfaces.
• Grain size determination: performed per ASTM E112 or ISO 643, since grain size influences sensitization kinetics and ditch continuity.
• Phase balance measurement: ferrite and austenite ratio in duplex steels by point counting or image analysis.
• Photomicrographic documentation: representative micrographs at 100x, 250x, and 500x magnification with etchant, etch condition, and material state recorded.

Testing Methods

Specimen Preparation

Specimens are sectioned under abundant coolant using abrasive or precision cutting to avoid frictional heating that could alter the sensitization condition. They are mounted in hot compression mounting resin or cold-setting epoxy, ground through successive silicon carbide grit sizes to 1200 or finer, and polished with diamond suspension to a 1 micrometer or 0.25 micrometer finish. Low-deformation preparation is essential because smeared metal can mask or falsely imitate grain boundary precipitates.

Oxalic Acid Electrolytic Etch Test

The polished surface is electrolytically etched in a 10 percent oxalic acid solution at a current density of approximately 1 ampere per square centimeter for 15 to 30 seconds at room temperature, with the specimen acting as the anode. This procedure follows ASTM A262 Practice A and GB/T 4334.1. The etch attacks chromium-depleted boundaries selectively: sensitized boundaries dissolve into sharp ditches, while sound boundaries merely step. The etched structure is examined on an inverted metallographic microscope at 250x to 500x magnification.

Etch Structure Classification Criteria

• Step structure: grain boundaries appear as clean steps with no ditches; the material is unsensitized and accepted as equivalent to passing the corresponding hot-acid test for most grades.
• Dual structure: isolated ditches occur at grain boundary triple points but no grain is completely surrounded; acceptance depends on the specific grade and the applicable acceptance table.
• Ditch structure: one or more grains are completely surrounded by continuous ditches; the material is sensitized and is either rejected or subjected to a confirmatory boiling-acid test.
• End-grain attack and isolated pits are reported separately, as they influence interpretation for certain product forms such as bar and heavy plate.

Supplementary Etchants and Advanced Techniques

Glyceregia, a mixture of hydrochloric acid, nitric acid, and glycerol, reveals the general austenite and weld microstructure. Murakami's reagent, an alkaline potassium ferricyanide solution, differentiates chromium carbides from sigma phase by color response. Electrolytic sodium hydroxide etching outlines sigma phase and delta ferrite in duplex welds. Where optical evidence is ambiguous, scanning electron microscopy with energy-dispersive spectroscopy can confirm chromium depletion profiles and carbide chemistry, and the electrochemical potentiodynamic reactivation method provides a quantitative measure of the degree of sensitization.

Corroborating Hot-Acid Corrosion Tests

Where metallographic screening indicates a dual or ditch structure, confirmatory testing follows the relevant practice: ferric sulfate and 50 percent sulfuric acid immersion for 120 hours (Practice B), the 65 percent nitric acid Huey test run in five 48-hour boiling periods (Practice C), the 16 percent sulfuric acid and copper sulfate Strauss test with a bend test (Practice E), or the 50 percent sulfuric acid and copper sulfate test (Practice F). Comparable procedures are specified in GB/T 4334.2 through 4334.5, ISO 3651-2, and JIS G 0572 through G 0573.

Applications

Metallographic intergranular corrosion analysis supports quality control, fabrication, and failure investigation across virtually every industry that uses stainless steel at elevated temperatures or in corrosive media.

• Welding procedure qualification: verifying that heat input, cooling rate, and filler selection avoid sensitization of the heat-affected zone.
• Post-fabrication heat treatment verification: confirming that solution annealing and stabilization treatments produced an acceptable structure.
• Pressure vessels, piping, and heat exchangers: incoming material inspection and surveillance of components in chemical, petrochemical, and refinery service.
• Nuclear power: qualification and aging assessment of primary circuit piping, internals, and cladding where irradiation-assisted sensitization is a concern.
• Refinery equipment: screening for susceptibility to polythionic acid attack during shutdowns in hydroprocessing units.
• Food, beverage, and pharmaceutical plants: certifying sanitary piping and vessel welds that contact organic acids and chlorides.
• Failure analysis: establishing intergranular corrosion as the root cause of leakage or fracture and quantifying attack depth for fitness-for-service assessment.

FAQ

What causes intergranular corrosion in stainless steel?

The primary cause is sensitization, in which chromium carbides precipitate at grain boundaries during exposure to the 450 to 850 degree Celsius range, creating a thin chromium-depleted zone that cannot maintain the passive film. The depleted boundary then corrodes preferentially in aggressive media while the grain interiors stay protected.

What do step, dual, and ditch structures mean?

A step structure shows clean boundary steps and indicates an unsensitized, fully solution-annealed condition. A dual structure shows ditches at some triple points without any grain completely surrounded, indicating borderline sensitization. A ditch structure shows one or more grains completely encircled by attack, confirming sensitization and requiring rejection or confirmatory chemical testing.

Which standards govern metallographic intergranular corrosion testing?

The principal standards are ASTM A262 (Practices A through F), the GB/T 4334 series in China, ISO 3651-2 for the Strauss-type test, and the JIS G 0571 through G 0573 series in Japan. Supporting standards include ASTM E112 for grain size and ASTM E407 for etchants.

Can sensitized stainless steel be restored?

Yes. Re-solution annealing at approximately 1040 to 1150 degrees Celsius, followed immediately by rapid water quenching, redissolves the chromium carbides and restores a uniform chromium distribution. If re-annealing is impractical, for example on a completed welded assembly, material selection or redesign is the only remedy.

How do low-carbon and stabilized grades prevent sensitization?

Low-carbon L grades, with carbon content at or below 0.03 percent, limit the carbon available for carbide precipitation. Stabilized grades such as 321 (titanium) and 347 (niobium) tie up carbon as stable TiC or NbC precipitates, leaving insufficient carbon in solid solution to form chromium carbides at grain boundaries.

How deep can intergranular attack penetrate in service?

Penetration depends on the medium, temperature, exposure time, and degree of sensitization. Metallographic cross sectioning is the standard method for measuring maximum attack depth, and this value feeds directly into remaining-life calculations and fitness-for-service evaluations.