appurtenance will decrease as the burial depth increases. It is unlikely that a temperature swing of more than 30 C will occur for most of the buried appurtenances.
Oxygen concentration surrounding a buried appurtenance is perhaps the most important parameter in determining its corrosion rate. As quoted from the reference by Flitton and Escalante, soils “can be divided into two broad categories: corrosion in undisturbed soils and corrosion in disturbed soils.” (Flitton, Adler, and Escalante, 2003) Corrosion of buried steel in undisturbed soils is always low, regardless of soil type and is controlled solely by availability of oxygen, whereas corrosion of steel in disturbed soils is a strong function of soil conditions. (Flitton, Adler, and Escalante, 2003) Buried appurtenances would certainly belong to the category of disturbed soils when first buried; however, after a period of a couple of years, the soil would revert back to the undisturbed state.
The concentration of oxygen surrounding an appurtenance will be controlled by several factors. When the appurtenance is initially buried in the soil, the oxygen content will be relatively high because of the direct path of air leading to the appurtenance. As time progresses and the soil surrounding the appurtenance subsides or collapses around it, the availability of oxygen will depend upon the porosity of the soil. If the water content is high, then the controlling factor may be the diffusion of oxygen from the atmosphere through the water to the appurtenance. Oxygen in the surrounding water in contact with the appurtenance will be consumed during either the oxygen reduction process, or cathodic reactions during corrosion of the appurtenance. As the corrosion product on the appurtenance thickens, the available oxygen will also be expected to decrease with time. These limiting factors usually result in a slowly decreasing corrosion rate or a constant corrosion rate over a long period of time. These rates, in turn, will determine when the time-to-perforation, or failure of the appurtenance, will occur.
Role of Galvanic Corrosion
Galvanic corrosion can play an important role in either increasing or decreasing the rate of corrosion of the appurtenance depending on whether dissimilar metals are attached to it. In a galvanic couple, the more active or anodic alloy will preferentially corrode to protect the more passive or cathodic alloy when both are electrically coupled in the presence of an electrolyte. Most of the appurtenances are constructed of dissimilar alloys with one alloy usually having the majority of surface area. An example is a first-stage regulator that has a body made from die cast zinc, and a nozzle orifice made from brass. In the absence of a protective coating, and while exposed to an electrolyte such as rain water, the zinc will act as a sacrificial anode and will have a higher corrosion rate when attached to the brass orifice. The brass orifice will essentially be protected from corrosion while in contact with the zinc body. The interface of the zinc and brass, however, will show excessive corrosion of the zinc, which may lead to failure of its threaded region over a long period of time.
Alternative Underground Propane Tank Materials, Phase II—Final Report
September 2009 Battelle and Lincoln Composites