Objects, buried

The data for the average decrease in metal thickness in 4 years and the linear corrosion rate are given in Table 4-2. In addition, extrapolations of the rate for 50 and 100 years are given, which are of interest for the corrosion likelihood of objects buried in earth. It can be seen from the results that film formation occurs in class I soil. In class II soils, the corrosion rate decreases with time only slightly. In class III soils, the decrease with time is still fairly insignificant. 

In particular, the study of plumes of molecules released in water from a submerged source has been the focus of substantial research. The migration of the molecules from within or from the surface of a submerged, buried munition is similar to the case of munitions buried in the ground. Following that release there is little similarity. Almost no work has been done to date to examine the processes that affect the molecules released from an object buried in the seabed. This may form a fruitful area for research. 

There has been almost no research for objects buried in the seafloor13 that parallels the work of Phelan and Jenkins and their colleagues on land. Therefore, we can only surmise that processes similar to those they describe are at work. Clearly, as diagrammed in Figure 4.4, there will be diffusion and convection in the liquid phase. For buried objects, there must still be partitioning. 

As mentioned in Section 4.1.1, the extensive work of Phelan and Webb has no parallel for objects buried in the seafloor.16 Whether searching rice paddies or oceans, if a munition has been dropped from the surface, or lain in the water for very long, it is likely to have become at least partially buried in the seafloor. The understanding of the processes at work in these conditions is rudimentary at best. 

Presence of underground utilities or metal objects buried within the treatment area may preclude the use of Lasagna or any other DC electrically based processes. Metal objects can short the electrical path and corrode very quickly, causing hot spots. A survey of the area during the design phase is important. It is also best to have all building and footers removed from the site to allow complete access to the subsurface. 

Chirp-Acoustic-Profiler This system is a development of a sub-bottom-profiler, and can - due to the use of external hydrophone arrays - achieve resolutions of 8 -10 cm. The data is being displayed in real time and gives a multi-colour representation of the layering of the sediments on the seabed. Objects buried into the sedimentation layer will resolve with the same accuracy so that size and shape can be evaluated. The datasets are available in a variety of formats so that compatibility with other data sources can be achieved... 

Corrosion likelihood describes the expected corrosion rates or the expected extent of corrosion effects over a planned useful life [14]. Accurate predictions of corrosion rates are not possible, due to the incomplete knowledge of the parameters of the system and, most of all, to the stochastic nature of local corrosion. Figure 4-3 gives schematic information on the different states of corrosion of extended objects (e.g., buried pipelines) according to the concepts in Ref. 15. The arrows represent the current densities of the anode and cathode partial reactions at a particular instant. It must be assumed that two narrowly separated arrows interchange with each other periodically in such a way that they exist at both fracture locations for the same amount of time. The result is a continuous corrosion attack along the surface. 

The sum can only be obtained with buried objects and provides information on anodic damage through cell formation as in Fig. 4-3d. More detailed considerations can provide information on whether preferential anodic or cathodic regions are formed and how active they are [3,14]. 

In soil, anodes are connected by cables to the object to be protected. The cable must be low resistance in order not to reduce the current delivery. Therefore with long lines, the cable cross-section must be proportionately large. A cable with NYM sheathing with 2.5 mm Cu is mostly sufficient. Occasionally stronger cables and special insulation are required, e.g., NYY 4 mm Cu. Power supply cable buried in soil should have a noticeably light color. For use in seawater, occasionally temperature, oil and seawater-resistant cable is demanded, e.g., HOVRN. ... 

In the Verband Deutscher Elektrotechniker (VDE) regulations [1,4], no demands are made on the accuracy of the measured or calculated voltage drops in a rail network. An inaccuracy of 10% and, in difficult cases, up to 20%, should be permitted. A calculation of the annual mean values is required. If the necessary equipment is not available, a calculation is permitted over a shorter period (e.g., an average day). Voltage drops in the rail network only indicate the trend of the interference of buried installations. Assessment of the risk of corrosion of an installation can only be made by measuring the object/soil potential. A change in potential of 0.1 V can be taken as an indication of an inadmissible corrosion risk [5]. 

The use of electrochemical protection in the chemical industry started about 20 years ago, which is somewhat recent, compared with its use for buried pipelines 40 years ago. Adoption was slow because the internal protection has to be tailored to the individual plant, which is not the case with the external protection of buried objects. Interest in internal protection came from the increasing need for greater safety for operating plants, increased demands for corrosion resistance, and larger plant components. While questions of its economy cannot generally be answered (see Section 22.6), the costs of electrochemical protection are generally less than the cost of equivalent and reliable coatings or corrosion-resistant materials. 

Electromagnetic (EM) Conductivity Measures the electrical conductivity of materials in microohms over a range of depths determined by the spacing and orientation of the transmitter and receiver coils, and the nature of the earth materials. Delineates areas of soil and groundwater contamination and the depth to bedrock or buried objects. Surveys to depths of SO to 100 ft are possible. Power lines, underground cables, transformers and other electrical sources severely distort the measurements. Low resistivities of surficial materials makes interpretation difficult. The top layers act as a shunt to the introduction of energy info lower layers. Capabilities for defining the variation of resistivity with depth are limited. In cases where the desired result is to map a contaminated plume in a sand layer beneath a surficial clayey soil in an area of cultural interference, or where chemicals have been spilled on the surface, or where clay soils are present it is probably not worth the effort to conduct the survey. 

If the positive potential changes are very small and confined to a few points on a small unprotected structure, it may be practicable to reduce the potential at these points by installing reactive anodes. The anodes will probably be most effective if they can be buried between the two structures. In some circumstances a similar screen of zinc, aluminium or steel may be installed between the structures. The screen must be electrically connected to the unprotected structure since it is installed with the object of providing an electrolytic path to earth for the interaction current. 

Most corrosion processes in copper and copper alloys generally start at the surface layer of the metal or alloy. When exposed to the atmosphere at ambient temperature, the surface reacts with oxygen, water, carbon dioxide, and air pollutants in buried objects the surface layer reacts with the components of the soil and with soil pollutants. In either case it gradually acquires a more or less thick patina under which the metallic core of an object may remain substantially unchanged. At particular sites, however, the corrosion processes may penetrate beyond the surface, and buried objects in particular may become severely corroded. At times, only extremely small remains of the original metal or alloy may be left underneath the corrosion layers. Very small amounts of active ions in the soil, such as chloride and nitrate under moist conditions, for example, may result, first in the corrosion of the surface layer and eventually, of the entire object. The process usually starts when surface atoms of the metal react with, say, chloride ions in the groundwater and form compounds of copper and chlorine, mainly cuprous chloride, cupric chloride, and/or hydrated cupric chloride. 

FIGURE 79 Bone carving. A seventh-century b.c.e. decorative carving on bone that was inlayed in wood, Tel Malhata, Israel. Bone has been crafted into practical and decorative objects since the dawn of time. Bone carvings hold great detail, and the surface polish that can be achieved is high. Many bone-made objects have survived, partly because it was widely used, but also because buried bone is generally well preserved in many types of soil. 

Radar Radar Buried or distant objects Toland (2000)... 

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