Oxygen concentration effect

In lower pressure boilers a variety of additional treatments may be appropriate, particularly if the steam is used in chemical process or other nonturbine appHcation. Chelants and sludge conditioners are employed to condition scale and enable the use of less pure feedwater. When the dmm pressure is less than 7 MPa (1015 psia), sodium sulfite may be added direcdy to the boiler water as an oxygen scavenger. It has minimal effect on the oxygen concentration in the system before the boiler. 

Corrosion inhibitors are substances which slow down or prevent corrosion when added to an environment in which a metal usually corrodes. Corrosion inhibitors are usually added to a system in small amounts either continuously or intermittently. The effectiveness of corrosion inhibitors is partiy dependent on the metals or alloys to be protected as well as the severity of the environment. For example, the main factors which must be considered before apphcation of a corrosion inhibitor to an aqueous system are the compatibility of the inhibitor and the metal(s), the salt concentration, the pH, the dissolved oxygen concentration, and the concentration of interfering species such as chlorides or metal cations. In addition, many inhibitors, most notably chromates, are toxic and environmental regulations limit use. Attention is now being given to the development of more environmentally compatible inhibitors (37). 

The maximum allowable oxygen concentration (MAOC), which is, in general, 2 vol % below the LOC, has to include the following considerations fluc tuatiou in oxygen concentrations due to process and breakdown conditions per time and location, as well as the requirement for protective measures or emergency measures to become effective. In addition, a concentration level for an alarm has to be set below the MAOC. 

A special form of crevice attack can occur at a waterline or at the edges of water droplets. At the water surface, a meniscus region is present where surface tension causes water to climb up the metal surface it contacts. In effect, a crevice is formed between the air-liquid and liquid-metal interface at the meniscus. Oxygen concentration is high at the meniscus due to the greater accessibility of this region to the air. The meniscus region becomes cathodic to the adjacent metal surface. Corrosion occurs just below the meniscus, and chloride, if present, is... 

The use of dispersants is highly recommended in systems containing silt, sand, oil, grease, biological material, and/or other foreign material. Not only does increased dispersion generally increase the effectiveness of chemical inhibition, it also prevents nucleation of oxygen concentration cells beneath foulants. 

Wastage was caused by classic long-term underdeposit corrosion. The combined effects of oxygen concentration cells, low flow, and contamination of system water with high chloride- and sulfate-concentration makeup waters caused corrosion. 

Figure 5.1 Effect of oxygen concentration on corrosion of mild steel in slowly moving water containing 165 ppm CaCl2 48-hour test, 25°C. [Courtesy of H. H. Uhlig, D. N. Triadis, and M. Stern, Effect of Oxygen, Chlorides, and Calcium Ion on Corrosion Inhibition of Iron by Polyphosphates, J. Electrochem. Soc. 102, p. 60 (1955). Reprinted with permission by The Electrochemical Society, Inc. ]...

The solubility of oxygen in water with a salt content up to 1 mol L is only dependent on the temperature. The oxygen concentrations in equilibrium with air amount to (in mg L- ) 0°C, 14 10°C, 11 20°C, 9 and 30°C, 7. The depth of water has no effect in the case of ships. In Hamburg harbor in summer, 7.3 mg L are measured in depths up to 7 m. The value can be much lower in polluted harbors and even fall to zero [8]. In the open sea, constant values are found at depths of up to 20 m. With increasing depth, the Oj content in oceans with low flow rates decreases [12] but hardly changes at all with depth in the North Sea [13]. 

Random and structured packings are susceptible to surface fouling due to process conditions and/or the presence of oxygen as may be related to bacterial growth. Some systems will precipitate solids or crystals from solution usually due to the temperature and concentration effects. Bravo [135] discusses air-water stripping and illus-... 

The effect of temperature is complex since there are two conflicting factors, (a) a decrease in the oxygen concentration which results in a decrease in and (b) an increase in the diffusion coefficient that increases about 3% per degree K rise in temperature. In a closed system from which oxygen cannot escape there is a linear increase in rate with temperature that corresponds with the increase in the diffusion coefficient. However, in an open system although the rate follows that for the closed system initially, the rate starts to decrease at about 70°C due to the decrease in oxygen solubility, which at that temperature becomes more significant than the increase in the diffusion coefficient see Section 2.1). 

Electrochemical corrosion of metals Since the aggressiveness of salt melts is governed by redox equilibria, and is often controlled by composition of the external atmosphere, effects analogous to electrochemical or oxygen-concentration corrosion in aqueous systems can occur in salt melts. Tomashov and Tugarinov determined cathodic polarisation curves in fused chlorides and concluded that the cathodic reactions of impurities could be represented as ... 

Fig. 8.42 Hydrogen permeation current as a function of applied potential, showing effect of oxygen concentration and applied potential (after Barth and Troiano )...

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