Capped mound deposits

In a review of various marine disposal options, Kester et al.(1983) suggested that the best strategy for disposing of contaminated sediments is to isolate them in a permanently reducing environment. Disposal in capped mound deposits above the prevailing sea-floor, disposal in subaqueous depressions and capping deposits... 

Confined deposition (mounded deposits or capped borrow pits)... 

Tubercles are mounds of corrosion product and deposit that cap localized regions of metal loss. Tubercles can choke pipes, leading to diminished flow and increased pumping costs (Fig. 3.1). Tubercles form on steel and cast iron when surfaces are exposed to oxygenated waters. Soft waters with high bicarbonate alkalinity stimulate tubercle formation, as do high concentrations of sulfate, chloride, and other aggressive anions. 

Stainless steels attacked by sulfate reducers show well-defined pits containing relatively little deposit and corrosion product. On freshly corroded surfaces, however, black metal sulfides are present within pits. Rust stains may surround pits or form streaks running in the direction of gravity or flow from attack sites. Carbon steel pits are usually capped with voluminous, brown friable rust mounds, sometimes containing black iron sulfide plugs fFig. 6.10). 

A number of questions on the composition of the carbon mounds remain unanswered. It is not clear whether the carbon cap contains iron with the concentration increasing with depth from the surface, or whether the iron present (Figure 11) has been deposited by the sputtering process. The outer surface of the unetched deposit usually consists of 98-100 At% carbon. The composition of the material under the cap is also uncertain the validity of the observed carbon profile requires confirmation. 

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