Flat Aluminum Alloy Intergranular Corrosion (IGC) Testing: Standards, Methods, and Evaluation

Intergranular corrosion (IGC) testing of flat aluminum alloy specimens is a key quality-control and materials-qualification procedure for rolled sheet, plate, and flat extruded products used in aerospace, marine, automotive, and structural engineering. This article explains the applicable standards—such as ASTM G110, ISO 11846, and GB/T 7998—along with sample preparation, immersion test procedures, metallographic evaluation, and acceptance criteria commonly applied by accredited third-party laboratories.

Technical Overview

Intergranular corrosion (IGC) is a localized form of attack that propagates along grain boundaries or the precipitate-depleted zones adjacent to them, while the grain interiors remain largely unaffected. In aluminum alloys, this type of degradation arises from electrochemical differences between grain-boundary precipitates, solute-depleted zones, and the alloy matrix. When an aggressive electrolyte is present, the boundary region behaves anodically and dissolves preferentially, producing a network of narrow fissures that can severely reduce mechanical strength and ductility with very little visible surface damage.

Susceptibility is strongly influenced by thermal history. Slow quenching after solution treatment, incorrect aging practices, prolonged service exposure at moderately elevated temperatures (sensitization), and welding thermal cycles can all drive harmful precipitation at grain boundaries. For flat (planar) products such as sheet and plate, the rolled microstructure is typically elongated and anisotropic, so intergranular attack can propagate along aligned grain boundaries and act as a precursor to exfoliation corrosion or stress corrosion cracking.

• 2xxx series (Al-Cu and Al-Cu-Mg): grain-boundary CuAl2 precipitation and copper-depleted zones, e.g., 2024, 2A12, 2014.
• 5xxx series (Al-Mg) with magnesium above approximately 3%: continuous beta-phase (Al3Mg2) films after sensitization, e.g., 5083, 5059, 5456.
• 6xxx series (Al-Mg-Si): Mg2Si precipitation control, relevant to welded and brazed sheet, e.g., 6061, 6082, 6005A.
• 7xxx series (Al-Zn-Mg-Cu): grain-boundary eta-phase (MgZn2) precipitation, e.g., 7075, 7050, 7A04.

The principal standards governing flat-specimen IGC testing include ASTM G110 (sodium chloride plus hydrogen peroxide immersion for heat-treatable aluminum alloys), ISO 11846 (immersion-based procedures for solution heat-treatable aluminum alloys), GB/T 7998 (Chinese national standard for determination of intergranular corrosion of aluminum alloys), and related screening methods such as ASTM G67 (nitric acid mass loss test, NAMLT) and ASTM G66 (ASSET test) for sensitized Al-Mg alloys.

Testing Samples

Flat Sheet and Plate Coupons

Coupons are typically cut from production sheet or plate in the form of rectangular flats, for example 50 mm x 25 mm or similar convenient dimensions agreed with the client. The test surface is the as-rolled, machined, or ground face, and specimens are identified with clear markings on non-critical edges. Both longitudinal and long-transverse orientations may be sampled so that sectioning after exposure reveals attack along different grain-boundary alignments, including assessment in the short-transverse direction where exfoliation susceptibility is greatest.

Extruded Flat Profiles and Flat Bars

Flat extrusions, flat bars, and wide profiles are sampled so that the exposed plane represents the principal service surface. Because extruded grain structures are elongated in the working direction, metallographic cross-sections are prepared both parallel and perpendicular to the extrusion direction to capture orientation-dependent attack.

Welded and Brazed Flat Panels

Flat panels containing butt or fillet welds, or brazed heat-exchanger cores, are tested to evaluate the heat-affected zone (HAZ), where thermal cycles can locally sensitize the alloy. Coupons are sectioned after exposure through the weld seam so that the weld metal, fusion line, HAZ, and base metal can be compared in a single metallographic mount.

Sensitized and Heat-Treated Specimens

For qualification and research programs, flat coupons may first receive laboratory heat treatments—controlled solutionizing, simulated slow quench rates, artificial aging, or sensitization exposures at service-relevant temperatures—before immersion testing. This allows quench-system validation, temper qualification, and comparative ranking of alloys or process parameters.

Testing Items

Typical test items delivered for flat aluminum alloy intergranular corrosion programs include the following:

• Immersion intergranular corrosion test per ASTM G110, ISO 11846, or GB/T 7998.
• Nitric acid mass loss test (NAMLT) per ASTM G67 for sensitized 5xxx series plate.
• Related exfoliation screening tests such as ASSET (ASTM G66) and EXCO (ASTM G34) where specified.
• Metallographic cross-section examination of attacked surfaces at 100x to 500x magnification.
• Measurement of maximum intergranular penetration depth (reported in micrometers or millimeters).
• Morphology classification: pitting, discontinuous intergranular attack, continuous grain-boundary network, or exfoliation-like delamination.
• Corrosion-rate or mass-loss determination where the selected method requires it.
• Grade rating and pass/fail judgment against client or specification limits.

Testing Methods

Sample Preparation

Flat specimens are degreased with suitable solvents such as acetone or ethanol, and exposed surfaces are prepared to a uniform finish—commonly abraded to a fine grit or mechanically polished—so that mill-scale oxides, lubricant residues, and handling marks do not bias the attack. Edges may be deburred, and specimens are dried and weighed to 0.1 mg when mass-loss measurement is required. A minimum solution volume-to-surface-area ratio is maintained in accordance with the governing standard so that the chemistry does not deplete during exposure.

Immersion Testing Conditions

The benchmark environment for heat-treatable 2xxx, 6xxx, and 7xxx alloys is the sodium chloride–hydrogen peroxide solution defined by ASTM G110 and referenced by GB/T 7998: nominally 57 g/L sodium chloride with the addition of hydrogen peroxide (approximately 10 mL of 30% H2O2 per liter of solution). Specimens are fully immersed at a controlled temperature, typically 30 °C to 35 °C, for exposure periods that are commonly 6 h to 24 h depending on the standard, alloy family, and product specification. ISO 11846 provides alternative immersion-based procedures (Methods A and B) whose selection depends on the alloy class and the technical question being addressed. For sensitized Al-Mg marine plate, the ASTM G67 NAMLT procedure immerses coupons in concentrated nitric acid for 24 h and evaluates the mass loss in mg/cm² as an index of grain-boundary sensitization. Freshly prepared solution, controlled bath temperature, and appropriate specimen support avoiding crevice contact are maintained throughout.

Post-Exposure Examination

After exposure, specimens are rinsed, cleaned of corrosion products in accordance with recognized cleaning practice for aluminum (for example, brief nitric acid pickling consistent with ASTM G1), dried, and re-weighed if mass loss is required. The critical evaluation is metallographic: representative sections are cut perpendicular to the exposed flat face, mounted, ground, and polished, optionally etched with Keller's reagent to reveal microstructure. Under an optical microscope, the examiner distinguishes general pitting from true intergranular attack, traces the grain-boundary fissures, and measures the maximum depth of penetration from the original surface using a calibrated microscope. Multiple fields across the section are surveyed so the reported maximum depth is statistically meaningful.

Rating and Reporting

Results are expressed as the maximum intergranular corrosion depth together with a morphological description, and where GB/T 7998 is invoked, a graded classification from no intergranular corrosion through increasingly severe penetration. Acceptance is judged against the limit fixed by the client, the material specification, or the engineering design basis—for example, a maximum allowable depth in micrometers or a required NAMLT mass-loss band. The final report documents the standard used, specimen orientation and surface condition, solution chemistry and volume, temperature and duration, photographic micrographs of representative fields, measured depth data, grade rating, and a clear statement of conformity.

Applications

Flat-specimen intergranular corrosion testing supports material selection, process qualification, and in-service integrity assurance across many industries where aluminum sheet and plate are exposed to chloride-bearing or marine environments:

• Aerospace: qualification and incoming inspection of 2024 and 7075 sheet and plate for fuselage skins, wing panels, and structural members.
• Marine and shipbuilding: sensitization assessment of 5083 and 5059 hull plate and superstructure panels after fabrication heat input or long service exposure.
• Automotive and rail transit: evaluation of 6xxx series body sheet and extruded flat sections after welding and paint-bake thermal cycles.
• Heat exchangers and HVAC: corrosion performance of brazed multi-layer flat components and clad fin stock.
• Architecture and construction: durability verification of 5xxx/6xxx curtain-wall and roofing panels in coastal atmospheres.
• Failure analysis: laboratory diagnosis of grain-boundary cracking or delamination in returned flat components.
• Research and development: ranking of new alloys, tempers, quench media, and aging schedules under standardized attack conditions.

FAQ

Why are flat aluminum alloy products tested for intergranular corrosion?

Flat rolled products have highly elongated, aligned grain structures. If grain boundaries are sensitized, corrosion can propagate along these aligned boundaries deep into the section with minimal surface evidence, reducing load-bearing capacity and serving as an initiation site for exfoliation or stress corrosion cracking. The immersion test accelerates this mechanism under controlled, reproducible conditions so susceptibility can be detected before service.

Which standard should be selected?

ASTM G110 or GB/T 7998 are the usual choices for heat-treatable 2xxx, 6xxx, and 7xxx series flat products using the sodium chloride–hydrogen peroxide immersion; ISO 11846 is selected where an ISO-based procedure is contractually required. For high-magnesium 5xxx series marine plate, ASTM G67 (NAMLT) is the recognized sensitization index. The governing product specification or the end user normally dictates the method, exposure duration, and acceptance limit.

How is the result judged as pass or fail?

Judgment is based on the metallographic evidence: a specimen passes when the maximum intergranular penetration depth is within the specified limit (often expressed as a maximum depth in micrometers, or a grade level under GB/T 7998) and the morphology shows no continuous grain-boundary network. In NAMLT testing, the mass loss in mg/cm² is compared against agreed bands indicating unsensitized, intermediately sensitized, or severely sensitized material.

How long does the complete test take?

The immersion phase is short—typically 6 h to 24 h depending on the standard—while metallographic sectioning, polishing, microscopic measurement, and reporting usually bring the standard turnaround to several working days, longer when prior sensitization heat treatments or multiple orientations are required.

Can painted or coated flat panels be tested?

Intergranular corrosion immersion tests are designed for bare metal surfaces so that the electrolyte contacts the alloy directly. Coated panels are normally tested by stripping the coating from the evaluation area, or are assessed instead through cyclic salt-spray and filiform-type tests appropriate to coated systems.

What sample information should be provided to the laboratory?

Clients should supply alloy and temper designation, product form and thickness, rolling or extrusion direction, relevant heat treatment or service history, the specified test standard and duration, and any acceptance criterion. This information ensures correct specimen orientation, sectioning planes, and a judgment basis for the final report.