Intergranular Corrosion Product Analysis: Methods, Standards, and Industrial Applications

Intergranular corrosion (IGC) product analysis is a specialized laboratory technique used to identify the composition, morphology, and distribution of corrosion deposits along the grain boundaries of metals and alloys. By combining advanced characterization methods such as SEM-EDS, XRD, XPS, EPMA, and TEM, accredited third-party laboratories can confirm sensitization mechanisms, locate chromium-depleted zones, and identify the aggressive species responsible for grain boundary attack. This article explains the technical foundation of intergranular corrosion product analysis, common sample types, analytical items, testing methods, and typical industrial applications.

Technical Overview

Intergranular corrosion is a localized form of corrosion in which attack proceeds preferentially along grain boundaries or immediately adjacent zones while the grain interiors remain largely unattacked. In its advanced stage, grains become completely surrounded by corroded boundaries and may drop out of the surface, causing rapid loss of mechanical integrity with minimal visible weight loss. The most common metallurgical cause is sensitization: for example, in austenitic stainless steels, exposure to the temperature range of approximately 425 to 815 degrees Celsius causes chromium-rich carbides (typically M23C6) to precipitate at grain boundaries, depleting the adjacent zones of chromium below the level required to maintain a stable passive film. Similar grain boundary phenomena occur in ferritic stainless steels, duplex stainless steels, nickel-based alloys, and high-strength aluminum alloys.

Corrosion product analysis plays a decisive role in diagnosing intergranular corrosion because the deposits left behind record both the corrosion mechanism and the aggressive environment. Analyzing these products allows laboratories to confirm whether attack is truly intergranular rather than pitting, crevice corrosion, or stress corrosion cracking; to identify oxides, hydroxides, oxyhydroxides, chlorides, and sulfides formed during attack; to detect elemental segregation or depletion at grain boundaries; and to reconstruct the service or test environment responsible for the failure. The results directly support root cause analysis, material selection, welding procedure qualification, and heat treatment optimization.

Testing Samples

Failed Component Cross-Sections

Sectioned samples cut from failed components such as pipes, vessels, weldments, heat exchanger tubes, and fasteners. Cross-sections reveal the depth and morphology of grain boundary penetration and preserve corrosion products in their original position for in-situ microanalysis. Specimens are typically mounted, ground, and polished following metallographic preparation practices such as ASTM E3.

Surface Deposits and Loose Corrosion Products

Powders, flakes, scrapings, and adherent deposits collected from corroded surfaces. These samples are suitable for bulk phase identification by X-ray diffraction and for soluble anion analysis, where residual chlorides, sulfates, or polythionates can indicate the aggressive species involved in the attack.

Laboratory Sensitized Specimens

Alloy coupons subjected to accelerated intergranular corrosion tests, for example the boiling nitric acid, ferric sulfate-sulfuric acid, or oxalic acid etch practices of ASTM A262, ASTM G28, or ISO 3651. Corrosion products generated on these specimens are analyzed to study sensitization behavior, evaluate heat treatment, or qualify new alloy heats.

Extraction Replicas and Grain Boundary Precipitates

Carbon extraction replicas that lift grain boundary carbides, sigma phase, and other precipitates off the polished surface for transmission electron microscopy. These samples enable direct crystallographic and compositional identification of the precipitates that drive sensitization.

Testing Items

Typical analytical items requested in an intergranular corrosion product analysis program include:

• Morphological characterization of grain boundary attack, including ditching, grain dropping, and intergranular crack paths
• Measurement of intergranular penetration depth and attack severity rating
• Qualitative and quantitative elemental composition of corrosion products (Fe, Cr, Ni, Mo, Mn, O, Cl, S, C, and trace elements)
• Phase identification of crystalline corrosion products such as iron oxides, chromium oxides, oxyhydroxides, and metal chlorides
• Oxidation state analysis of iron and chromium species in surface films and deposits
• Detection of aggressive species residues, including chlorides, sulfides, sulfates, and polythionates
• Identification of grain boundary precipitates, including M23C6 carbides, sigma phase, and chromium nitrides
• Chromium depletion profiling across grain boundaries to quantify sensitization
• Correlation of corrosion product distribution with weld heat-affected zones or thermally exposed regions
• Classification of the corrosion mode to differentiate IGC from pitting, crevice corrosion, and intergranular stress corrosion cracking

Testing Methods

Optical Metallography

Microscopic examination of polished and etched cross-sections establishes the intergranular nature of the attack. Electrolytic oxalic acid etch screening per ASTM A262 Practice A, together with etching practices described in ASTM E407, distinguishes step, ditch, and dual structures in stainless steels and provides the first indication of sensitization severity.

Scanning Electron Microscopy with Energy Dispersive Spectroscopy (SEM-EDS)

SEM provides high-resolution imaging of grain boundary grooves, detached grains, and corrosion product morphology, while EDS performs point, line-scan, and X-ray mapping analysis of elemental distribution. This is the primary method for locating chromium-depleted regions, confirming carbide-related attack, and identifying chlorine or sulfur signals within grain boundary deposits. Quantitative EDS analysis is generally performed following guidance such as ASTM E1508.

X-Ray Diffraction (XRD)

XRD identifies the crystalline phases present in loose or scraped corrosion products, for example distinguishing magnetite, hematite, goethite, chromium oxide, and mixed spinel oxides. Identification of crystalline compounds in deposits can follow practices such as ASTM D934, which addresses X-ray diffraction analysis of deposits formed in aqueous service environments.

X-Ray Photoelectron Spectroscopy (XPS) and Auger Electron Spectroscopy (AES)

These surface-sensitive spectroscopy techniques determine the chemical states of elements within the first few nanometers of grain boundaries and passive films. XPS and AES can reveal whether chromium exists as a protective oxide or as a depleted metallic phase, and can detect chemisorbed chloride or sulfur species that participated in the initiation of intergranular attack.

Electron Probe Microanalysis (EPMA)

EPMA with wavelength dispersive spectroscopy provides high-accuracy quantitative elemental mapping and line profiles across grain boundaries. It is widely used to measure chromium concentration gradients adjacent to precipitates and to quantify molybdenum or nickel segregation effects in duplex and nickel alloys.

Transmission Electron Microscopy (TEM)

TEM analysis of extraction replicas or focused ion beam foils delivers definitive crystallographic identification of grain boundary precipitates by electron diffraction, combined with nanoscale EDS composition analysis. TEM is the reference method for confirming M23C6 carbides, sigma phase, and nanometer-scale chromium depletion profiles.

Accelerated Corrosion Testing Standards for Sample Generation

Where field samples are unavailable, specimens are sensitized and corroded under internationally recognized standards, including ASTM A262 (austenitic stainless steels), ASTM A763 (ferritic stainless steels), ASTM G28 (nickel-rich alloys), ASTM G67 and ASTM G110 (aluminum alloys), ASTM G108 (electrochemical potentiodynamic reactivation for type 304), ISO 3651-1 and ISO 3651-2, EN standards, JIS G0571 through G0573, and the GB/T 4334 series for stainless steel intergranular corrosion testing.

Applications

Intergranular corrosion product analysis is applied across industries where sensitization and grain boundary attack pose safety, reliability, or compliance risks:

• Failure analysis of stainless steel piping, vessels, and weld heat-affected zones in chemical and petrochemical plants
• Condition assessment of reactor components and recirculation piping in nuclear power generation, where intergranular stress corrosion cracking has historical significance
• Investigation of exfoliation and intergranular attack in aerospace aluminum alloys such as the 2XXX, 5XXX, and 7XXX series
• Evaluation of sensitization in refinery equipment exposed to polythionic acid during shutdowns
• Quality control of solution annealing, stabilization treatments, and welding procedures for stainless steel fabrications
• Material qualification and incoming inspection of plate, tube, forgings, and castings against ASTM, EN, ISO, JIS, and GB acceptance criteria
• Research and development of new corrosion-resistant alloys, including grain boundary engineering studies
• Support of insurance claims, litigation, and forensic engineering investigations requiring defensible laboratory evidence

FAQ

Q: What is intergranular corrosion and why does it occur?

A: Intergranular corrosion is preferential attack along grain boundaries caused by compositional differences between the boundaries and the grain interiors. In stainless steels, the classic mechanism is sensitization, in which chromium carbide precipitation at grain boundaries depletes nearby chromium and destroys local passivity. Other causes include segregation of impurities such as sulfur and phosphorus, and precipitation of detrimental phases such as sigma phase.

Q: Why is corrosion product analysis important for intergranular corrosion?

A: The corrosion products record both the mechanism and the environment of the attack. Identifying their chemistry and phases confirms that failure is intergranular, reveals the aggressive species such as chlorides or polythionates, and distinguishes sensitization-driven attack from other corrosion forms, which is essential for corrective action.

Q: Which instruments are most commonly used?

A: A typical workflow combines optical microscopy for morphology screening, SEM-EDS for imaging and elemental mapping, XRD for phase identification, XPS or AES for chemical state analysis, EPMA for quantitative grain boundary profiling, and TEM for nanoscale precipitate characterization.

Q: How much sample is required for analysis?

A: Cross-sectional analysis generally requires a section of approximately 10 to 20 millimeters square. Loose corrosion product analysis by XRD typically needs a few tens of milligrams, while surface spectroscopy can be performed directly on small fragments or on mounted metallographic sections.

Q: Can analysis distinguish intergranular corrosion from pitting or stress corrosion cracking?

A: Yes. The combination of morphological evidence, such as continuous grain boundary ditching and grain dropping, with elemental and phase analysis of deposits along the boundaries allows laboratories to confidently separate intergranular corrosion from pitting, crevice corrosion, and intergranular stress corrosion cracking.

Q: What standards govern intergranular corrosion testing?

A: Widely referenced standards include ASTM A262, ASTM A763, ASTM G28, ASTM G67, ASTM G108, ASTM G110, ISO 3651-1 and ISO 3651-2, JIS G0571 to G0573, and the GB/T 4334 series. Metallographic preparation and microanalytical quality are supported by standards such as ASTM E3, ASTM E407, and ASTM E1508.