Underground corrosion can manifest itself in a number of ways, from uniform thinning of a tank or pipe wall to very localized pitting corrosion. The type of corrosion will depend on the particular metal and the nature of the soil environment. Microbiological and galvanic corrosion (or dissimilar metal corrosion) can also play a role.

The Materials

Underground structures and pipelines are built using many different types of material, both metallic and non-metallic. From a corrosion perspective, we are primarily interested in metals such as cast iron, steel, copper, and galvanized steel (i.e., zinc coated steel).

The Environment

Underground corrosion is concerned with structures buried underground or within soil, for example, storage tanks and pipelines. The corrosivity of a specific soil depends on many factors. Two of the key factors are moisture content and degree of aeration. Other factors such as pH, permeability, chloride and sulfate content, and the presence of micro-organisms also play an important role.

Corrosion Assessment

There are many steps involved in determining the extent and cause of corrosion. The key steps in most investigations are: a site inspection; sample collection; review of important documents, plans and specifications; soil testing to determine corrosivity; and examination of the material that corroded. Some of the tools we use include: aerial photographs; published soil surveys; and laboratory methods to determine the soilís chemical and physical characteristics. In addition, we can perform a thorough examination of the corroded material. This typically involves examining cut sections; reviewing images from the Scanning Electron Microscope (SEM); creating and examining metallurgical mounts; and using Energy Dispersive Spectroscopy (EDS) to identify elements that may have contributed to the corrosion.