What is a primary cause of intergranular corrosion?

Intergranular corrosion is primarily associated with the degradation of the grain boundaries in metals, often due to the effects of improper heat treatment. When alloys, particularly stainless steels, are not heat treated correctly, it can lead to the precipitation of carbides at the grain boundaries. This creates a localized depletion of corrosion-resistant elements, such as chromium, making those areas more susceptible to corrosion.

In the case of heat treatment, if the metal is held at temperatures that allow for the diffusion of certain elements for too long or not heated to the proper temperatures for the correct duration, it can significantly affect the microstructure of the material. This can lead to a condition known as sensitization, where the grain boundaries are particularly vulnerable to corrosive attacks, especially in corrosive environments.

The other choices relate to different factors that might influence corrosion in general but do not have the same direct impact on the specific mechanism of intergranular corrosion as improper heat treatment does.