Role of Crevice Corrosion
Crevice corrosion can occur when a metal is in close contact with another surface (metal or nonmetal) in the presence of an electrolyte. The main reason corrosion occurs in these cases is that the concentration of oxygen in the crevice will be generally less than the concentration in the solution that is outside of the crevice, often referred to as the bulk solution. This differential in oxygen concentration results in a difference in electrochemical potential between the metal surfaces within the crevice and outside of the crevice. The inside surfaces tend to become anodic or electrochemically more active with respect to metal surfaces outside the crevice resulting in accelerated corrosion attack in the crevice. For similar reasons, the pH within the crevice will decrease, forming an aggressive acid solution.
The various areas that form a crevice in the various appurtenances include threaded areas, heads of screws or fasteners, and mating surfaces between gaskets.
Role of Microbiologically Influenced Corrosion (MIC)
Corrosion processes accelerated by the presence of certain types of bacteria most often occurs in anaerobic conditions. These conditions will most often be found beneath pipelines in soil or beneath buried appurtenances. The increase in corrosion rates due to MIC can be significant. Anecdotal evidence indicates that pipelines can be perforated on their bottom sides whereas their top sides show much less corrosion damage (Davis, 1999). The presence of these types of bacteria will be controlled almost exclusively by the type of soil surrounding the appurtenance because some soils are better hosts than others for these types of bacteria.
Estimate of Corrosion Rates of Appurtenances in Soils
The following paragraphs summarize the expected corrosion performance of the materials of construction of the appurtenances if they were buried in soils.
For underground applications, the wrought and die-cast aluminum alloys listed in Table 12 are most frequently used for buried pipeline applications. While early applications used uncoated pipes that often last many years, it is now accepted that buried aluminum should be coated because the risk of pitting could not be eliminated, even in high-resistivity soils. It is not common practice to cathodically protect buried aluminum using sacrificial anodes but impressed current protection could be used.
Alternative Underground Propane Tank Materials, Phase II—Final Report
September 2009 Battelle and Lincoln Composites