Corrosion acid velocities

Materials of Construction. Resistance of alloys to concentrated sulfuric acid corrosion iacreases with increasing chromium, molybdenum, copper, and siUcon content. The corrosiveness of sulfuric acid solutions is highly dependent on concentration, temperature, acid velocity, and acid impurities. An excellent summary is available (114). Good general discussions of materials of constmction used ia modem sulfuric acid plants may be found ia References 115 and 116. More detailed discussions are also available (117—121). For nickel-containing alloys Reference 122 is appropriate. An excellent compilation of the relatively scarce Hterature data on corrosion of alloys ia Hquid sulfur trioxide and oleum may be found ia Reference 122. 

A brick lining has the secondary effect of protecting the lead from abrasion by contained slurries or suspended matter. The protective surface film of insoluble salts can be thinned or removed by such abrasion, and so the resistance to acids seriously affected. Figure 12-7 shows graphically how the velocity of a 20% sulfuric acid solution at 77°F, without any entrained solids, passing over the face of a lead lining can cause increasing corrosion as velocity increases. 

A special feature of plate exchangers that requires some design attention is the possible acceleration of corrosion by the flow of the acid. Keeping the acid velocity low, at a level determined by the manufacturer, protects against such corrosion but reduces the acid-side film coefficient. The cooling water velocity should be sufficiently high to... 

Temperature, acid concentration, and acid velocity are three main factors that influence the corrosion rate of acid plant equipment. 

Corrosion rates generally increase with increasing acid velocity. An example is carbon steel which requires low temperatures (<38 °C) and low velocities of 0.3-0.9 m/s (DuPont, 2012) to ensure low corrosion rates in sulfuric acid service. 

An insoluble iron sulfate (FeS04) passive fihn forms on the surface of the carbon steel when in contact with sulfuric acid which minimizes corrosion (Eq. 30.1). High acid velocities break down the passive film exposing fresh metal to acid increasing corrosion. 

In recent years alkylations have been accompHshed with acidic zeoHte catalysts, most nobably ZSM-5. A ZSM-5 ethylbenzene process was commercialized joiatiy by Mobil Co. and Badger America ia 1976 (24). The vapor-phase reaction occurs at temperatures above 370°C over a fixed bed of catalyst at 1.4—2.8 MPa (200—400 psi) with high ethylene space velocities. A typical molar ethylene to benzene ratio is about 1—1.2. The conversion to ethylbenzene is quantitative. The principal advantages of zeoHte-based routes are easy recovery of products, elimination of corrosive or environmentally unacceptable by-products, high product yields and selectivities, and high process heat recovery (25,26). 

Chlorosulfuric acid attacks brass, bronze, lead, and most other nonferrous metals. From a corrosion standpoint, carbon steel and cast Hon are acceptable below 35°C provided color and Hon content is not a concern. Stainless steels (300-series) and certain aluminum alloys are acceptable materials of constmction, as is HasteUoy. Glass, glass-lined steel, or Teflon-lined piping and equipment are the preferred materials at elevated temperatures and/or high velocities or where trace Hon contamination is a problem, such as in the synthetic detergent industry. 

Access of oxygen to steel surfaces during corrosion influences the wastage process in nonoxidizing acids. Fluid velocity can influence the amount of oxygen reaching the metal surface and, therefore, the corrosion rate. In deaerated acid solutions, steel corrosion rate is constant with fluid velocity. If dissolved oxygen is present, however, the corrosion rate is proportional to fluid velocity. 

Organic acids—except formic, oxalic, and some chlorine-containing acids—do not appreciably attack aluminum near room temperature. In most acids, the corrosion rate increases slightly with flow velocity. 

Corrosion was caused by carbonic acid. A film of condensed moisture and dissolved carbon dioxide formed the acid. The erosion was caused by high-velocity movement of air across the tubes. Attack occurred intermittently. Deepest metal loss was 33% of the 0.040 in. (0.10 cm) wall thickness. 

Several boilers discharging into a single-core chimney are to be avoided. At times of low load the efflux velocity will be very low, which, in turn, will allow the chimney to cool. This may then drop to dewpoint temperatures and, where sulfur is present in the fuel, acid will form. If the chimney is unprotected steel, it will suffer rapid corrosion. Even worse, as the boilers increase their load the efflux velocity will increase and start to discharge the acid droplets, which quickly fall out and cause damage to surrounding property. 

The austenitic irons are superior to ordinary cast iron in their resistance to corrosion by a wide range of concentrations of hydrochloric acid at room temperature (Table 3.50). However, for practical uses where such factors as velocity, aeration and elevated temperatures have to be considered, the austenitic irons are mostly used in environments where the hydrochloric acid concentration is less than 0- 5%. Such environments occur in process streams encountered in the production and handling of chlorinated hydrocarbons, organic chlorides and chlorinated rubbers. 

Sulphuric acid is frequently made, stored and conveyed in lead. The corrosion resistance is excellent (see Figure 4.15) provided that the sulphate film is not broken in non-passivating conditions. Rupture of the film may be caused by erosion by high velocity liquids and gases containing acid spray. 

Domestic heating coil internal corrosion. Where naturally soft or lean city water is supplied and the Langelier Saturation Index (LSI) is below -1.0, acid corrosion takes place as a result of the acidic nature of the water. This water often has a high dissolved gas content, which additionally leads to pinhole corrosion. Where water velocities are too high (say, over 6 ft/s 1.8 m/s) the protective oxide layer is stripped off and erosion corrosion takes place. 

Water also increases the corrosive nature of the acid in the reaction section. Metal losses in the turbulent high velocity sections of reactors can be quite rapid in units circulating high water content acids. 

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