Intergranular corrosion (IGC), or intergranular attack, is an insidious form of corrosion that targets the grain boundaries in metals and alloys, weakening structures from within without evident material loss visible to the naked eye. This type of corrosion is particularly dangerous because, although the metal loss is minimal, it can lead to catastrophic structural failures due to the significant reduction in the material's strength and ductility.

IGC occurs when areas adjacent to the grain boundaries become anodic and preferentially dissolve in a corrosive environment. This is caused by the presence of alloying elements, impurities, or the precipitation of secondary phases that weaken these areas. The ratio between the cathodic and anodic areas at the grain boundaries is typically high, favoring anodic dissolution and accelerating the corrosion process. Factors such as the distribution of electrochemically active phases and grain size significantly influence the corrosion rate.

One of the most common mechanisms leading to IGC is the precipitation of chromium carbide (Cr23C6) at the grain boundaries in austenitic stainless steels, such as grades 304 and 316. This phenomenon, known as sensitization, occurs when these steels are exposed to temperatures between 500 and 800°C. The precipitation of chromium carbide creates a chromium-depleted zone at the grain boundaries, reducing corrosion resistance in these areas. Chromium is essential for the corrosion resistance of stainless steel, and its depletion at the grain boundaries makes them vulnerable to preferential attack.

IGC affects not only austenitic stainless steels but also other alloys, such as nickel-copper alloys (Alloy 400), nickel-molybdenum alloys (Alloy B), and nickel-chromium alloys (Alloy 600). It can also occur in certain grades of aluminum, such as 2024 and 7075, where the precipitation of compounds at the grain boundaries facilitates corrosion.

To mitigate the risk of IGC, it is crucial to carry out appropriate heat treatments such as annealing and rapid cooling, which dissolve chromium carbides and other susceptible compounds. Adding stabilizing elements like titanium (Ti) or niobium (Nb) can also prevent the formation of chromium carbides during cooling and service use. Additionally, implementing welding techniques that minimize heat input and reducing the carbon and nitrogen content in the alloys can significantly decrease the susceptibility to IGC.

Although intergranular corrosion is not always visible, its impact on structural integrity can be devastating. Understanding the mechanisms that cause IGC and implementing appropriate preventive measures are essential to ensure the longevity and safety of metallic structures in various industries. Adopting proper manufacturing and heat treatment practices can prevent this type of corrosion and maintain the integrity of materials used in corrosive environments.