Intergranular corrosion (IGC) is one of the most dangerous degradation mechanisms in stainless steel, occurring when sensitized grain boundaries are preferentially attacked, leading to grain dropping, embrittlement, and sudden in-service failure. Professional intergranular corrosion testing performed to recognized standards such as ASTM A262, ASTM A763, ISO 3651, and GB/T 4334 identifies sensitization and IGC susceptibility before components enter service. This article explains the metallurgical root causes of IGC, applicable test methods, sample requirements, evaluation criteria, and typical industrial applications.
Technical Overview
Intergranular corrosion (IGC) is a localized form of corrosive attack that proceeds along the grain boundaries of a metal while the grain interiors remain largely unaffected. In stainless steels, IGC is most commonly caused by sensitization: when the material is heated within the critical temperature range of approximately 425°C to 815°C (800°F to 1500°F), chromium diffuses toward grain boundaries and combines with carbon to precipitate chromium-rich carbides, typically Cr23C6. This reaction depletes the chromium content in the narrow zone adjacent to the grain boundaries below the approximate 10.5% to 12% threshold required to maintain a stable passive film.
The chromium-depleted zones become electrochemically active (anodic) relative to the chromium-rich grain interiors. When exposed to aggressive media such as acidic chlorides, sulfuric acid, or nitric acid, these zones dissolve rapidly, causing the grains to lose cohesion and eventually fall out of the matrix, a phenomenon known as grain dropping. The consequences are severe: components may lose load-bearing capacity and ductility while showing little or no visible surface indication of damage.
Common root causes of sensitization and IGC in stainless steel include:
- Welding operations that heat the heat-affected zone (HAZ) through the sensitization temperature range
- Slow cooling from solution annealing temperatures or improper heat treatment cycles
- Excessive carbon content in the steel composition
- Inadequate stabilization with titanium or niobium, or knife-line attack in stabilized grades
- Precipitation of intermetallic phases such as sigma phase, which also depletes chromium and molybdenum at grain boundaries
Because IGC develops internally and can lead to catastrophic failure without warning, systematic defect detection through standardized corrosion testing is an essential part of material qualification, quality control, and failure analysis for stainless steel products.
Testing Samples
Austenitic Stainless Steels
Austenitic grades are the most widely tested materials for IGC susceptibility because their high carbon solubility and common welding applications make them prone to sensitization. Typical grades include 304, 304H, 304L, 316, 316L, 316Ti, 317L, 321, and 347.
Ferritic and Martensitic Stainless Steels
Ferritic grades such as 409, 430, 434, and 446 sensitize very rapidly at elevated temperatures due to low chromium diffusivity control and high carbon/nitrogen solubility issues, and they are commonly evaluated under ASTM A763. Martensitic and precipitation-hardening grades such as 17-4PH may also be assessed where service conditions demand it.
Duplex Stainless Steels
Duplex grades such as 2205 (S32205) and 2507 (S32750) are tested for IGC resistance, particularly after heat treatment or welding, which can promote harmful secondary phase precipitation along grain boundaries and phase interfaces.
Product Forms and Welded Joints
Test specimens can be taken from virtually any product form, including plates, sheets, strips, seamless and welded pipes and tubes, forgings, castings, bars, fittings, and finished components. Welded samples are frequently tested with the weld metal, fusion line, and heat-affected zone retained in the gauge section to evaluate sensitization caused by the welding thermal cycle. Specimens are typically cut to standardized dimensions (for example, 30 mm x 30 mm coupons or bend test strips), ground to a specified surface finish, degreased, dried, and weighed accurately before exposure.
Testing Items
Intergranular corrosion detection programs evaluate the following key items:
- Sensitization screening of as-received, heat-treated, or welded material
- Corrosion rate determination based on specimen weight loss over a defined exposure period
- Bend test evaluation for cracking or fissuring after acid exposure
- Metallographic classification of etch structures (step, dual, or ditch structures)
- Measurement of intergranular attack depth on cross-sections
- Assessment of weld metal and heat-affected-zone susceptibility
- Compliance verification against acceptance criteria in purchase specifications and applicable codes
Testing Methods
Several internationally standardized test methods are available, each suited to specific alloy systems and service environments. The most widely applied standards include ASTM A262 and ASTM A763 (USA), ISO 3651-1 and ISO 3651-2 (international), GB/T 4334 (China), and JIS G 0571 through G 0575 (Japan).
Oxalic Acid Etch Test (ASTM A262 Practice A)
This rapid screening method involves electrolytic etching of a polished specimen in a 10% oxalic acid solution at a current density of approximately 1 A/cm² for about 90 seconds. The etched structure is examined under a microscope and classified as a step structure (acceptable), dual structure, or ditch structure (indicating sensitization). Materials passing the screen are accepted without further testing; ditch structures require one of the quantitative acid immersion tests.
Ferric Sulfate–Sulfuric Acid Test (ASTM A262 Practice B, ASTM G28 Practice A)
Specimens are immersed for 120 hours in a boiling solution of 50% sulfuric acid saturated with ferric sulfate. The oxidizing ferric ions accelerate attack on chromium-depleted boundaries. Results are expressed as a corrosion rate calculated from weight loss, which is compared against maximum allowable rates for the specific grade.
Nitric Acid Test / Huey Test (ASTM A262 Practice C)
Specimens undergo five consecutive 48-hour boiling periods in 65% nitric acid, with weight loss measured after each period. The Huey test is the most aggressive screening for grain boundary precipitation and also detects attack associated with sigma phase and other chromium-depleted constituents. It is typically applied to grades intended for nitric acid service.
Copper–Copper Sulfate–Sulfuric Acid Test / Strauss Test (ASTM A262 Practice E, ISO 3651-2 Method A)
Specimens are placed in contact with copper chips in a boiling 16% sulfuric acid solution containing copper sulfate for 24 hours (or extended durations). After exposure, the specimen is bent 180 degrees and examined for intergranular cracking or fissuring at the bent surface and edges. This is the most common acceptance test for austenitic stainless steels in general service.
50% Sulfuric Acid–Copper Sulfate Test (ASTM A262 Practice F)
This variant uses a boiling 50% sulfuric acid solution with copper sulfate and copper contact, typically for 120 hours, and is intended for corrosion-resistant, molybdenum-bearing austenitic grades such as 316L, 317L, and higher-alloyed materials that do not respond adequately to the standard Strauss solution.
Electrochemical Reactivation Methods (EPR and DL-EPR)
Electrochemical potentiokinetic reactivation and double-loop EPR techniques quantify the degree of sensitization non-destructively by measuring the reactivation charge or current ratio during an anodic polarization scan. DL-EPR is increasingly used for rapid, quantitative assessment of duplex and austenitic stainless steels, especially in research and failure analysis work.
Post-Test Metallographic Examination
Following acid exposure or etching, cross-sectional metallography confirms the nature of the attack. Qualified laboratories measure attack depth, document grain boundary ditching and grain dropping, and, where required, use scanning electron microscopy (SEM) with energy-dispersive spectroscopy (EDS) to identify carbide or intermetallic precipitates responsible for the sensitization.
Applications
Intergranular corrosion testing is applied across industries where stainless steel components operate in corrosive environments or where structural integrity is critical:
- Petrochemical and chemical processing: reactors, pressure vessels, heat exchangers, and piping handling acidic media
- Oil and gas: offshore platforms, subsea equipment, and sour service components requiring strict material qualification
- Nuclear power: primary circuit piping and reactor internals where sensitization-induced IGC and irradiation-assisted IGC are major concerns
- Pharmaceutical, food, and beverage industries: sanitary tubing, tanks, and valves requiring corrosion-free surfaces
- Pulp and paper, fertilizer, and urea plant equipment exposed to aggressive process chemicals
- Aerospace and transportation: structural and high-temperature components
- Third-party inspection, supplier qualification, incoming material verification, and root-cause failure analysis
FAQ
What is the sensitization temperature range for stainless steel?
Sensitization typically occurs between approximately 425°C and 815°C, when chromium carbides precipitate at grain boundaries faster than chromium can diffuse back into the depleted zones. Rapid cooling through this range, or the use of low-carbon (L grades) or stabilized (titanium/niobium-bearing) alloys, suppresses sensitization.
Are low-carbon grades such as 304L and 316L immune to intergranular corrosion?
They are highly resistant under normal welding and fabrication conditions because their carbon content (typically ≤0.03%) limits carbide precipitation. However, extended exposure to sensitization temperatures, improper heat treatment, or sigma phase precipitation can still cause susceptibility, which is why testing remains necessary for critical applications.
Which test method should be selected for a given material?
The selection depends on the alloy system, intended service environment, and governing specification. The oxalic acid etch test is a rapid screen for austenitic grades; the Strauss test is the general acceptance test; the ferric sulfate test applies to many austenitic and nickel-bearing alloys; and the Huey test is reserved for nitric acid service. Molybdenum-bearing grades are often verified with Practice F or the equivalent ISO 3651-2 Method B.
How long does intergranular corrosion testing take?
The oxalic acid screen can be completed within one day. Strauss tests require 24 to 72 hours of boiling exposure plus bending and metallography. Ferric sulfate tests run 120 hours, while the Huey test requires approximately ten days of exposure across five boiling periods. Including machining, surface preparation, and reporting, most projects are completed within one to three weeks.
What does a failed bend test after Strauss exposure indicate?
Cracks or fissures observed on the bent surface or edges indicate that intergranular attack has progressed deep enough to destroy grain boundary cohesion, confirming sensitization and rejection of the material lot unless re-treatment, such as solution annealing, is permitted by the specification.
Can welded components be tested for heat-affected-zone sensitization?
Yes. Specimens are cut so that the weld metal, fusion line, and HAZ remain within the evaluated region. After acid exposure, the welded sample is bent with the root of the weld in tension, and cracking in the HAZ reveals sensitization caused by the welding thermal cycle.