Intergranular corrosion (IGC) is a highly localized form of material degradation that attacks the internal structure of an alloy by dissolving the material along the microscopic boundaries where crystalline grains meet. The external surface of the component may appear completely undamaged, masking severe internal structural compromise. This preferential attack can lead to a sudden loss of mechanical strength, presenting a significant risk of catastrophic failure in industrial equipment and infrastructure.
The Metallurgical Mechanism
The fundamental issue lies in the chemical and structural differences between the grain interior and the grain boundary itself. Grain boundaries are high-energy zones where the atomic arrangement is less ordered, making them susceptible to chemical reactions. When certain alloys, most notably stainless steels, are exposed to elevated temperatures, they undergo sensitization.
Sensitization causes carbon atoms to migrate to the grain boundaries where they react with chromium, forming precipitates of chromium carbide. This process consumes chromium from the metal matrix immediately adjacent to the boundary. Stainless steel requires a minimum chromium concentration, generally around 12%, to maintain its protective passive layer.
As chromium is drawn away, the narrow zone on either side of the grain boundary becomes depleted, losing the ability to form the protective oxide film. In the presence of an electrolyte, this chromium-depleted zone acts as the anode in a localized galvanic cell, while the bulk of the grain acts as the cathode. Because the anodic area is small compared to the cathodic area, the corrosion current density is concentrated at the boundary. This results in the rapid dissolution of the metal along the grain boundary network, isolating the individual grains.
Triggers and Susceptible Alloys
The primary physical trigger for IGC in stainless steels is exposure to the elevated temperature range that induces sensitization, typically spanning from \(425^\circ\text{C}\) to \(870^\circ\text{C}\). The duration the material spends within this range determines the extent of chromium depletion and susceptibility to IGC.
Welding is the most common industrial process causing this thermal exposure, leading to a localized phenomenon called “weld decay.” The heat-affected zone (HAZ) adjacent to the weld bead is heated into the sensitizing range and cooled slowly, providing the necessary conditions for carbide precipitation.
Austenitic stainless steels like Type 304 and Type 316 are the most widely recognized susceptible alloys. However, certain high-strength aluminum alloys, such as Duralumin-type alloys, can suffer IGC when specific intermetallic phases precipitate along the grain boundaries. Nickel-rich alloys, including some grades of Inconel and Incoloy, also exhibit vulnerability to intergranular attack under specific conditions.
Recognizing the Damage
Identifying intergranular corrosion is difficult because the attack is internal and often leaves the external surface visually intact. In the field, IGC may only be indicated by minor surface discoloration, slight staining, or subtle cracking parallel to the weld or heat-affected zone.
Since visual inspection is unreliable, laboratory and standardized testing are necessary to confirm the presence of IGC or the potential for sensitization. Metallographic examination involves preparing a cross-section and viewing it under a microscope to confirm the corrosion path along the grain boundaries.
For confirming susceptibility in stainless steel, standardized chemical tests are employed. The Huey test exposes the material to boiling nitric acid, which aggressively attacks the depleted zones, allowing for the quantification of material loss. The Strauss test subjects the material to a boiling solution of copper sulfate and sulfuric acid, followed by a bending test. If the material is sensitized, the bending action reveals fissures or cracks along the corroded grain boundaries.
Mitigation and Material Selection
Preventing intergranular corrosion relies on eliminating the conditions necessary for chromium carbide formation. One effective strategy is careful material selection focusing on reducing the carbon content of the alloy. Low-carbon grades, designated with an “L” (e.g., 304L or 316L), restrict carbon to a maximum of 0.03%. This low level ensures insufficient carbon is available to react with chromium, even if the material passes through the sensitizing temperature range.
Another metallurgical solution uses “stabilized” stainless steel grades, such as Type 321 or Type 347. These alloys include small additions of powerful carbide-forming elements like titanium or niobium. These stabilizing elements preferentially react with the carbon instead of chromium, forming harmless carbides distributed throughout the metal matrix. This preserves the chromium content adjacent to the grain boundaries, maintaining corrosion resistance.
For components already fabricated using susceptible materials, post-weld heat treatment known as solution annealing can be performed. This process involves heating the component to a high temperature, typically between \(1060^\circ\text{C}\) and \(1120^\circ\text{C}\), which redissolves the chromium carbides back into the metal matrix. The material is then rapidly quenched, preventing the carbides from reforming and returning chromium to the depleted zones. Controlling the operating environment by reducing corrosive agents, such as chloride ions, can also decrease the rate of attack if the material is partially sensitized.