Sometimes Intergranular corrosion is also called intercrystalline corrosion or interdendritic corrosion. In the presence of tensile stress, cracking may occur along grain boundaries.This type of corrosion is frequently called interranular stress corrosion cracking (IGSCC) or intergranular corrosion cracking.

In most cases of corrosion, the grain boundaries behave in essentially the same way as the grains themselves. The grain boundaries can undergo marked localized attack while the rest of the material remains unaffected. The alloy disintegrates and loses its mechanical properties.

This type of corrosion is due either to the presence of impurities in the boundaries, or to local enrichment or depletion of one or more alloying elements. For example, small quantities of iron in aluminium or titanium (iron has a low solubility), segregate to the grain boundaries where they can induce intergranular corrosion. Certain precipitate phases (Mg₅Al₈, Mg₂Si, MgZn₂, MnAl₆, etc.) are also known to cause or enhance intergranular attack of high strength aluminium alloys, particularly in chloride-rich media.

The exfoliation corrosion phenomenon observed in rolled aluminium alloys is usually, but not always, intergranular in nature. In this case, the corrosion products occupy a larger volume than the metal "consumed", generating a high pressure on the slivers of uncorroded metal, leading to the formation of blisters. Numerous alloy types can undergo intergranular attack, but the most important practical example is the intergranular corrosion of austenitic stainless steels, related to chromium depletion in the vicinity of the boundaries, due to the intergranular precipitation of chromium carbides (Cr₂₃C₆), during a "sensitizing" heat treatment or thermal cycle.

Exfoliation corrosion morphology inside a water pipeline

Intergranular or intercrystalline means between grains or crystals. As the name suggests, this is a form of corrosive attack that progresses preferentially along interdendritic paths (the grain bourdaries). Positive identification of this type of corrosion usually requires microstructure examination under a microscopy although sometimes it is visually recognizable as in the case of weld decay.



The photos show the microstructure of a type 304 stainless steel. The figure on the left is the normalized microstructure and the one on the right is the "sensitized" structure and is susceptible to intergranular corrosion or intergranular stress corrosion cracking.

This type of attack results from local differences in composition, such as coring commonly encountered in alloy castings. Grain boundary precipitation, notably chromium carbides in stainless steels, is a well recognized and accepted mechanism of intergranular corrosion. The precipitation of chromium carbides consumed the alloying element - chromium from a narrow band along the grain boundary and this makes the zone anodic to the unaffected grains. The chromium depleted zone becomes the preferential path for corrosion attack or crack propagation if under tensile stress.

Intergranular Corrosion occurs when a grain boundary area is preferentially attacked because of the presence of precipitates in these regions.

Grain boundaries are preferred sites for

• Segregation
• Precipitation

Two types of segregates and precipitates

• Intermetallics (intermediate constitutes): Formed from metal atoms having identifiable chemical formulae. Can either be anodic or cathodic to the metal.
• Compounds: Formed between metals and the non-metallic elements, H, C, Si, N and O. Fe₂₃C₆ and MnS in steel are cathodic to ferrite.

Any metal in which intermetallics or compounds are present at grain boundaries will be susceptible to intergranular stress corrosion cracking. Austenitic stainless steels are most susceptible to intergranular corrosion. 18-8 or type 304 stainless steel: Fe, 18%Cr, 8%Ni; When C% < 0.03%, only the austenite phase is stable. When C% > 0.03% austenite and ferrite mixed carbide (FeCr)₂₃C₆ are stable.

The proportions of carbide obtained are dependent upon the rate of cooling

• Fast cooling by water/oil quenching from > 1000oC suppresses carbide formation.
• If the material is reheated within the range 600-850oC, carbide precipitation will occur at the grain boundaries.
• The material is thus said to be sensitized and is in a dangerous condition - susceptible to Intergranular corrosion cracking
• If the material is reheated below 600oC, the rate of diffusion of Cr is too slow for carbide precipitation to occur.

12%Cr + Fe === "stainless" steels

precipitation of carbide 

(FeCr)₂₃C₆ causes Cr depletion (Cr<12%) in the metal adjacent to the precipitates.

• manufacturing
• welding
• operating

• Use low carbon (e.g. 304L, 316L) grade of stainless steels.Lower the C content to below 0.03%, so that the carbides are not stable.
• Use stabilized grades alloyed with titanium (for example type 321) or niobium (for example type 347). Titanium and niobium are strong carbide- formers. They react with the carbon to form the corresponding carbides thereby preventing chromium depletion.
• Use high temperature solution heat treatment to dissolve the precipitates. (post weld heat treatment of sensitized steel).