Materials stray currents

Beyond the simple resistance of a material of construction to dissolution in a given chemical, many other properties enter into consideration when makiug an appropriate or optimum MOC selection for a given environmental exposure. These factors include the influence of velocity, impurities or contaminants, pH, stress, crevices, bimetallic couples, levels of nuclear, UV, or IB radiation, microorganisms, temperature heat flux, stray currents, properties associatea with original production of the material and its subsequent fabrication as an item of equipment, as well as other physical ana mechanical properties of the MOC, the Proverbial Siebert Changes in the Phase of the Moon, and so forth. 

The adjustment of a protection station or of a complete protection system where there is stray current interference is made much easier by potential control. Potential control can be indispensable for electrochemical protection if the protection potential range is very small (see Sections 2.4 and 21.4). This saves anode material and reduces running costs. 

Aluminum-sheathed cables should not be connected to other cables because aluminum has the most negative rest potential of all applicable cable sheathing materials. Every defect in the protective sheath is therefore anodically endangered (see Fig. 2-5). The very high surface ratio SJS leads to rapid destruction of the aluminum sheathing according to Eq. (2-44). Aluminum can also suffer cathodic corrosion (see Fig. 2-11). The cathodic protection of aluminum is therefore a problem. Care must be taken that the protection criterion of Eq. (2-48) with the data in Section 2.4 is fulfilled (see also Table 13-1). Aluminum-sheathed cables are used only in exceptional cases. They should not be laid in stray current areas or in soils with a high concentration of salt. 

Stray currents follow paths other than their intended circuit. They leave their principal path because of poor electrical connections or poor insulation around the intended conductive material. The escaped current then will pass through the soil, water or any... 

The active layer contains the semiconductor. The active layer is typically patterned to avoid leakage currents between transistors, avoid the formation of unintentional parasitic transistors and MOS capacitors, and to avoid unintentional parasitic paths in ungated areas. When the transistor threshold voltage is such that the semiconductor is depleted it is possible in many cases to avoid patterning the active layer. There are also circuit geometries and layout topographies which suppress stray currents and allow the use of unpatterned semiconductor material if that is desirable. These are discussed in further detail in Chapter 5. 

In practice, these processes take place when steel is subject to acid corrosion, is cathodically polarized, is coupled with less noble materials, is interfered by stray current or has undergone special production or finishing processes (such as pickling or galvanising). 

General corrosion is the most common form of corrosion. This can be uniform (even), quasi-uniform, or uneven. General corrosion accounts for the greatest loss of metal or material. Electrochemical general corrosion in aqueous media can include galvanic or bimetallic corrosion, atmospheric corrosion, stray current dissolution, and biological corrosion (Table 1.1). 

It is important to avoid, if possible, galvanic couplings between different materials (reinforcing bars/pipes/copper grounding networks) and between the same material in different environments, and to interpose impermeable layers between concrete and soil in order to avoid contact with groundwater and the flow of stray currents (Figure 12.52). 

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