How can intergranular corrosion be identified in the laboratory?

La corrosion intergranulaire dans le secteur du BTP est une forme d’attaque localisée qui progresse le long des joints de grains, souvent sans rouille superficielle visible. Elle affecte en particulier les aciers inoxydables sensibilisés après un cycle thermique inadapté, une soudure mal maîtrisée ou un maintien prolongé dans une plage de température favorable à la précipitation de carbures de chrome.

Cette précipitation provoque une déchromisation locale au voisinage des joints de grains et diminue la résistance à la corrosion de la matrice. Le risque est majeur : la pièce peut perdre sa cohésion interne et rompre brutalement, sans déformation préalable significative.

En laboratoire, l’enjeu n’est pas seulement de constater l’endommagement, mais de distinguer une attaque intergranulaire d’autres mécanismes comme la corrosion par piqûres, la fissuration sous contrainte, la corrosion galvanique ou un défaut métallurgique non corrosif.

In the field, intergranular corrosion should be suspected when a stainless steel part or a welded mechanical assembly shows an unexplained drop in mechanical performance, cracking near welds, flaking, localized embrittlement or sudden failure in service. Heat-affected zones, weld beads and parts that have undergone poorly controlled heat treatment are particularly vulnerable.

In some cases, the external appearance remains relatively unremarkable even though the attack is already advanced in depth. For a quality or maintenance manager, any mismatch between the surface appearance and the level of mechanical failure should trigger a prompt expert assessment.

L’identification de la corrosion intergranulaire repose d’abord sur l’observation de la microstructure. Le microscope optique met en évidence la continuité de l’attaque le long des joints de grains après préparation métallographique.

Le MEB-EDX apporte un niveau de résolution supérieur pour examiner la morphologie de l’endommagement, la présence de dépôts, de produits de corrosion ou d’hétérogénéités locales. En cas de rupture, la fractographie permet de qualifier le mode de rupture et de rechercher une propagation intergranulaire.

La comparaison des duretés et des structures entre zones défaillantes et non défaillantes aide à relier l’attaque à une transformation métallurgique locale.

The root cause is identified by comparing the analytical results with the manufacturing and service history. Intergranular corrosion is often associated with thermal exposure that has favored chromium carbide precipitation in austenitic stainless steels, poorly controlled welding heat input, unsuitable cooling, the choice of a non-stabilized grade or non-compliant heat treatment. The laboratory therefore examines the heat-affected zones, the distribution of the attack, the condition of the grain boundaries and the consistency between the actual composition and the expected specifications.

This multi-scale analysis makes it possible to determine whether the degradation is linked to the material, the process or the environment.

The expert assessment approach is based on a structured analytical investigation: visual inspection, targeted sampling, metallographic preparation, microscopic observation, sensitization tests, chemical analysis and, if necessary, fractography.

The laboratory compares sound and failed areas in order to correlate microstructure, hardness, composition and attack morphology. This approach makes it possible to determine whether the origin is linked to poor heat treatment, an overly sensitized heat-affected zone, a non-compliant grade, surface contamination or an aggressive environment.

The objective is twofold: to characterize the degradation mode and to provide actionable findings for bringing the material, welding process or service conditions back into compliance.

Confirmation cannot rely on a simple visual inspection. The laboratory performs oriented micrographic cross-sections, applies suitable preparations and looks for grain boundary attack.

The examination may be supplemented by a comparison between sound and affected areas, hardness measurements and elemental analysis to verify the grade and rule out other causes. When the failure resulted in fracture, fractographic analysis makes it possible to look for signs of brittle or corrosion-assisted propagation.

The laboratory complements these observations with composition analysis to verify the conformity of the metal grade and detect any aggravating elements.

Techniques such as ICP, elemental analysis C/S, N/O, H, or XPS may be used as needed to characterize the chemistry of the material, the surface or the deposits. For stainless steels sensitive to sensitization, a stainless steel sensitization test such as the Strauss test (ASTM A262) can be used to assess the material’s susceptibility to intergranular attack under standardized conditions.

Depending on the case, electrochemical tests or searches for oxidizing agents and contaminants make it possible to link the damage to actual service conditions.

The expert assessment must also rule out misdiagnoses. A localized attack can be mistaken for pitting corrosion, crevice corrosion, stress corrosion cracking, wear-corrosion or a surface defect. Cross-checking micrographs, fracture observations, semi-quantitative deposit analysis and the search for corrosive agents makes it possible to reach a conclusion.

This distinction is essential to define the right corrective actions: changing the grade, adjusting welding, redoing the heat treatment, controlling contamination or modifying operating conditions.

A poorly diagnosed intergranular corrosion exposes you to unsuitable decisions, unnecessary replacements, or, on the contrary, keeping a weakened structure in service.

A specialized laboratory has the means to examine the part at several scales, from metallographic observation to surface chemistry and fractography.

This combination of tools makes it possible to produce a diagnosis that can be used by quality managers, claims experts, maintenance engineers, and manufacturers of welded mechanical structures. The findings can then guide concrete actions: have it assessed, verify the grade, check the welding process, assess corrosion sensitivity, compare sound and degraded areas, identify contaminants and secure recommissioning.