Corrosion high acid velocities increase

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. 

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. 

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. 

All of the reactions in acidic and basic solutions are generally controlled by diffusion of the reactant through the boundry layer existing on the exposed surfaces of the aggregate or bond phase. Elevated temperatures usually increase the reaction rate. Thus, elevated temperatures and high local fluid velocities tend to increase the corrosion rate of silicon carbides as the corrosion products are swept away from the active surface sites. 

In general, the corrosion rates of all alloys in the distillation units increase with an increase in temperature. NAC occurs primarily in high-velocity areas of crude distillation units in the 220-400 C (430-750 °F) temperature range. No observable corrosion damage is usually found at temperatures greater than 400 °C (750 °F) probably because of the decomposition of naphthenic acids or protection from the coke formed at the metal surface. 

High-temperature crude corrosion is a complex problem. There are at least three mechanisms (i) furnace tubes and transfer lines where corrosion is dependent on velocity and vaporization and is accelerated by naphthenic acid (ii) vacuum column where corrosion occurs at the condensing temperature, is independent of velocity, and increases with naphthenic acid concentration (iii) side-cut piping where corrosion is dependent on naphthenic acid content and inhibited somewhat by sulfur compounds. 

Acidic pH values will lead to general corrosion the other factors will generally favor localized attack. Cavitation-corrosion can be encountered in the pumps or at other locations where turbulent or high-velocity flow may occur. Stage heaters and economizers are designed to increase the feed-water temperature, which will increase the operating efficiency of the entire system, and, as the temperature... 

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