What Does Intergranular Corrosion Mean
Intergranular corrosion is the type of corrosion that attacks the boundaries of the metal crystallites, as opposed to attacking the surface of the metal. Intergranular Corrosion (IGC) can also be referred to as Intergranular Attack (IGA) under a condition known as grain boundary depletion. It can be defined as an attack along the boundaries of several grains in the metal or near the grain boundary with the largest portion of the grain remaining unaffected. Metals and alloys, like other elements, have micro-structures that can be described as grains. Metals can contain multiple grains, and these are separated by a grain boundary.
Although metal loss is minimal, IGC can cause the catastrophic failure of equipment. IGC is a common form of attack on alloys in the presence of corrosive media that results in the loss of strength and ductility. One should not mistake IGC with stress corrosion cracking (SCC). SCC requires stresses (residual or applied) to act continuously or cyclically in a corrosive environment producing cracks following an intergranular path.
Stainless steels and weld decay sensitization are the best examples of intergranular corrosion. Grain boundaries that are rich in chromium elements will precipitate lead. This makes the boundaries very vulnerable to corrosion attacks in various electrolytes. This is caused by reheating the part that has been welded, especially in multi-pass welding.
In the process of intergranular corrosion, a knife-like attack, a form of intergranular corrosion, can occur when carbon reacts with niobium, titanium or the austenitic stainless steels. Carbides form in the areas close the welded part, making it difficult for them to diffuse. This condition can be corrected by reheating the part to enable the carbides to diffuse.
How Intergranular Corrosion Is Formed
The ICG localized corrosion at grain boundaries is caused by the anodic dissolution of areas weakened by the alloying elements, second phase precipitation or regions with isolated alloying or impurity elements. The remaining part of the exposed surface typically functions as the cathode, and large cathodic areas support the anodic dissolution process.
The cathode to anode ratio is generally greater than one. It depends on factors such as the volume fraction and distribution of electrochemically active phases, the distribution of detrimental alloying and impurity elements, and grain size.
The corrosion rate is dependent on the dominant corrosion mechanism, and factors such as the diffusion of species to or from the anodic front can govern the dissolution kinetics. A significant characteristic of IGC is the development of a relatively homogeneous and uniform depth of attack. The dissolution of grain boundaries causes the dislodging of grains, often referred to as grain dropping. Grain dropping is responsible for most of the weight loss observed after IGC exposure, and corrosion rates can therefore be several orders of magnitude higher than during general corrosion.