Electrolytes seawater

Reserve batteries have been developed for appHcations that require a long inactive shelf period foUowed by intense discharge during which high energy and power, and sometimes operation at low ambient temperature, are required. These batteries are usually classified by the mechanism of activation which is employed. There are water-activated batteries that utilize fresh or seawater electrolyte-activated batteries, some using the complete electrolyte, some only the solvent gas-activated batteries where the gas is used as either an active cathode material or part of the electrolyte and heat-activated or thermal batteries which use a soHd salt electrolyte activated by melting on appHcation of heat. 

It is possible to divide the batteries with air electrodes into three following groups primary, mechanically and electrically rechargeable. In these batteries alkaline, saline and seawater electrolytes are used (Table 2). 

Example 7.8. Calcite in Seawater Compare the composition of a CaC03(s) (calcite)-C02-H20 seawater model system, made by adding calcite to pure H2O containing the seawater electrolytes (but incipiently no Ca and no carbonates and, for simplicity, no borate) and by equilibrating this solution at 25°C and 1 atm total pressure with the atmosphere (pcoi = 3.55 X 10 atm), with the composition of a real surface seawater whose carbonate alkalinity, Ca(II) concentration, and pH have been determined as 2.4 x 10 eq liter", 1.06 X 10 M, and 8.2, respectively. Estimate the extent of oversaturation of this seawater with respect to calcite. The solubility of calcite at 25 °C is taken as "K q = [Ca/-] [CO3 ] = 5.94 X 10 , where [Ca ] and [CO37] are the concentration of total soluble Ca(II) ([Ca ] plus concentration of Ca complexes with medium ions) and of total soluble carbonate ([CO "] and concentration of carbonate complexes with medium ions), respectively. The other constants needed, Henry s law constant and the acidity constant of H2CO, are taken as ... 

The copper electrode is called the cathode and no corrosion occurs at this electrode. The electrical circuit is completed by the seawater electrolyte, which carries a charge by the movement of ions through the solution. This would be via the movement of chloride and sodium ions present in the seawater. 

The first work in RO toward water-desalting was undertaken by Prof. Charles E. Reid1 at the University of Florida in the mid-1950 s. He discovered that cellulose acetate (CA) is semipermeable to seawater electrolytes.2 The diffi-... 

The current and potential fields are uniform around the buried pipeline. Thus, the potential and current field gradients are quantitatively determined in either impiessed-cunent or sacrificial anode techniques. This assumption is not realistic since the complexity of real engineering stmc-tures and heterogeneity of the soil or seawater electrolytes preclude exact theoretical calculations. The margin of error between the fteoretical and experimental results depends on the assumptions considered to develop a particular mathematical model... 

With the development of the recirculation system in which the inflow of fresh electrolyte can be controlled, thereby maintaining the temperature and conductivity of the electrolyte, the performance of electric torpedo batteries has been improved markedly. With recirculation and flow control, a recirculation pump (see Fig. 17.2) and a voltage-sensing mechanism are added to the battery system. By this method the temperature of the battery and the conductivity of the seawater electrolyte increase. Since battery voltage increases directly with temperature and conductivity, it is possible to control the output of the battery by controlling the intake of electrolyte by means of the voltage-sensing mechanism. 

FIGURE 38.43 Schematic of concentric cylinder configuration of seawater cell. The output cylinder is comprised of porous fiberglass coated with an anti-foulant. The corrugated structure is the air cathode, which is exterior to the magnesium anode. The entire structure is open to the seawater electrolyte. Courtesy of Westinghouse Corp.)... 

Finally, if seawater electrolyte is used, precipitation of group (II) metal hydroxides, particularly Mg(OH)2, may occur on the cathode surface, due to the localized increase in pH. 

Norsk Hydro, Norway electrolytic—seawater, brine, magnesite 55... 

Ma.rine. In the presence of an electrolyte, eg, seawater, aluminum and steel form a galvanic cell and corrosion takes place at the interface. Because the aluminum superstmcture is bolted to the steel bulkhead in a lap joint, crevice corrosion is masked and may remain uimoticed until replacement is required. By using transition-joint strips cut from explosion-welded clads, the corrosion problem can be eliminated. Because the transition is metaHurgicaHy bonded, there is no crevice in which the electrolyte can act and galvanic action caimot take place. Steel corrosion is confined to external surfaces where it can be detected easily and corrected by simple wire bmshing and painting. 

Lead-silver is primarily used in seawater and strong chloride-containing electrolytes. PbAg anodes are particularly suitable for use on ships and in steel-water constructions, especially as they are relatively insensitive to mechanical stresses. The original alloy developed by Morgan [8,9] consists of 1% Ag and 6% Sb, with the remainder Pb. It is represented as alloy 1 in Table 7-2. A similar alloy developed by Applegate [10] has 2% Ag and the remainder Pb. Another alloy... 

Measuring electrodes for impressed current protection are robust reference electrodes (see Section 3.2 and Table 3-1) which are permanently exposed to seawater and remain unpolarized when a small control current is taken. The otherwise usual silver-silver chloride and calomel reference electrodes are used only for checking (see Section 16.7). All reference electrodes with electrolytes and diaphragms are unsuitable as long-term electrodes for potential-controlled rectifiers. Only metal-medium electrodes which have a sufficiently constant potential can be considered as measuring electrodes. The silver-silver chloride electrode has a potential that depends on the chloride content of the water [see Eq. (2-29)]. This potential deviation can usually be tolerated [3]. The most reliable electrodes are those of pure zinc [3]. They have a constant rest potential, are slightly polarizable and in case of film formation can be regenerated by an anodic current pulse. They last at least 5 years. 

In contrast to external protection, the anodes in internal protection are usually more heavily covered with corrosion products and oil residues because the electrolyte is stagnant and contaminated. The impression can be given that the anodes are no longer functional. Usually the surface films are porous and spongy and can be removed easily. This is achieved by spraying during tank cleaning. In their unaltered state they have in practice little effect on the current output in ballast seawater. In water low in salt, the anodes can passivate and are then inactive. 

Galvanic or impressed current anodes are used to protect these components. The anode material is determined by the electrolyte zinc and aluminum for seawater, magnesium for freshwater circuits. Platinized titanium is used for the anode material in impressed current protection. Potential-regulating systems working independently of each other should be used for the inlet and outlet feeds of heat exchangers on account of the different temperature behavior. The protection current densities depend on the material and the medium. 

Cathodic protection (CP) is an electrochemical technique of corrosion control in which the potential of a metal surface is moved in a cathodic direction to reduce the thermodynamic tendency for corrosion. CP requires that the item to be protected be in contact with an electrolyte. Only those parts of the item that are electrically coupled to the anode and to which the CP current can flow are protected. Thus, the inside of a buried pipe is not capable of cathodic protection unless a suitable anode is placed inside the pipe. The electrolyte through which the CP current flows is usually seawater or soil. Fresh waters generally have inadequate conductivity (but the interiors of galvanized hot water tanks are sometimes protected by a sacrificial magnesium anode) and the conductivity... 

Fretting in air-saturated aqueous electrolytes, such as seawater or body fluids , produces enhanced removal of material by stimulation of electrochemical reactions, increasing the reaction rates by factors of 10 to 200 compared with air, depending on the frequency. The importance of the chemical... 

In general it is wise to avoid as far as possible, the use of incompatible metallic joints in marine and offshore practice, since these are often in contact with seawater or water that contains chlorides which are effective corrosive electrolytes. It is prudent to take very considerable precautions to prevent corrosion at the design and installational stages. However, the widely diverse properties required of the materials used in such installations make it impracticable to avoid all such joints. 

The properties of platinum as an inert electrode in a variety of electrolytic processes are well known, and in cathodic protection it is utilised as a thin coating on a suitable substrate. In this way a small mass of Pt can provide a very large surface area and thus anodes of this type can be operated at high current densities in certain electrolyte solutions, such as seawater, and can be economical to use. 

This material can be used only in seawater or similar chloride-containing electrolytes. This is because the passivation of the silver at discontinuities in the platinum is dependent upon the formation of a film of silver chloride, the low solubility of which, in seawater, inhibits corrosion of the silver. This anode, consisting of Pt-lOPd on Ag, was tried as a substitute for rapidly consumed aluminium, for use as a trailing wire anode for the cathodic protection of ships hulls, and has been operated at current densities as high as 1 900 AmHowever, the use of trailing anodes has been found inconvenient with regard to ships manoeuvrability. 

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