Intergranular corrosion can occur in corrosion resistant alloys when the grain boundaries (2D defects in the alloy’s grain structure) have lost their corrosion-inhibiting elements. To put it simply, the boundaries of the crystals are more susceptible to corrosion than their insides.
Microscope view of a polished cross section of a material attacked by intergranular corrosion, via Wikiwand
Intergranular corrosion is a form of corrosion in which the grain boundaries are more susceptible to corrosion than the grains themselves. This form of corrosion can occur even in corrosion resistant alloys in situations where the grain boundaries have lost their corrosion-inhibiting elements through mechanisms such as the precipitation of compounds at the grain boundaries.
Intergranular corrosion can also be caused by knifeline attack, which only affects steels that contain niobium. At very high temperatures, the niobium can dissolve in the steel and under some cooling rates, niobium carbide will not precipitate. This causes the steel to behave like unstabilized steel and form chromium carbide instead of niobium carbide. This type of corrosion only affects a very thin area immediately adjacent to a weld site. To solve the problem of knifeline attack, the steel as a whole must be heated to 1950°F, causing the chromium carbide to redissolve into the steel and the niobium carbide to form.
Aluminum-based alloys (particularly those with a high copper content) are also susceptible to intergranular corrosion, especially when they are worked to a high degree or extruded. In this case, the alloy suffers from exfoliation corrosion, where debris can build up between the flat, elongated grains. The grains are then separated and undergo a leafing effect that can propagate from the edges throughout the entire material.
Sensitization occurs when carbides precipitate at grain boundaries in stainless steel or a similar alloy, making the material susceptible to intergranular corrosion. Sensitization can also be caused by the precipitation of impurities at the grain boundaries.
Intergranular corrosion can be minimized by using a high temperature heat treatment followed by water quenching to ensure that the grain boundaries do not accumulate too many impurities or other precipitates. However, this is impractical for large structures and ineffective when welding will be used on the material later. Another method used to prevent intergranular corrosion requires the use of strong carbide formers or other stabilizing elements (such as niobium or titanium). When these elements are incorporated, they preferentially form carbides and therefore reduce the carbon that is available for the formation of chromium carbides. Alternatively, the steel can be created with a lower carbon content (below .03%) to discourage the formation of chromium carbides. However, these techniques are more expensive to use and can still lead to sensitization given time. Additionally, low-carbon steels tend to have lower strengths at high temperature and may not be well-suited to some applications.
Intergranular corrosion can be difficult to detect and prevent, but can also cause extensive weakening of alloys susceptible to it. It is important to recognize when an alloy may be weakened by intergranular corrosion in order to better control its effects through the appropriate mechanism. As with most forms of corrosion, when pipe supports have been put in place to counteract it, intergranular corrosion can be well controlled and protected against.
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