Other Corrosion Considerations

Pump selection is made on the flow rate and head required, together with other process considerations, such as corrosion or the presence of solids in the fluid. 

Of course, the engineer should specify items of equipment that have been designed for the proper Class, Group and Division (see Table I) as specified in the National Electrical Code (NEC). However, careful consideration must be given to the environment in which this equipment will operate. Caustics, acids and other corrosive agents can ruin electrical contact surfaces, and destroy exposed components in a short time. 

Other important considerations must be taken into account when formulating LADDs. Because of the corrosiveness of typical LADDs, ingredients must be added which protect both the machine itself and fine items such as china and silverware. Generally, silicate is added for this purpose, but its inherent alkalinity in aqueous solutions is not always desired. In Table 9.12 several patents are listed which describe other anticorrosion agents. 

Corrosion considerations are critical in many applications. For instance, the chemical industry uses very aggressive environments for synthesis, and purity is usually extremely important. Therefore, the reactors and vessels must be essentially inert and resist the aggressive attack of the process environment. Corrosion resistant alloys have wide usage in this industry. Microelectronics is another area in which corrosion is important even though the environment is typically not very aggressive. In this case, the size of the structures is such that the smallest amount of corrosion could cause a failure, for example, a short in a submicron-sized conductor line. Other corrosive environments and industries in which corrosion is critical include oil and gas recovery and processing, marine and seawater applications, underground... 

However, there is no obvious provision to include corrosion resistance considerations in Ashby s model. The main reason for this may be that unlike other design considerations such as fatigue, designing with corrosion in mind is far from being a straightforward process since each form of corrosion is essentially a separate failure mode that needs... 

Finally, the amount of oxygen that can be tolerated by liquid Pb and Pb-Bi systems is dictated by the fact that the upper limit of oxygen is given by the fonnalion of the liquid metal oxide (to be avoided in order to avoid risks of clogging) and the lower limit is driven by corrosion considerations. In the case of liquid Na the lower limit of oxygen content is driven not mainly by corrosion concern but also by other technological factors. 

The optimum choice of a steel for a particular application should be made in the light of expressions such as Eq. (5.2), which reflects the corrosivity of the environment as a function of the metallurgical composition and structure. But other practical considerations such as availability of the materials, maintainability, and economical requirements inevitably dictate the use of an alloy out of its safe envelope, in which case the application of coatings, cathodic protection, and/or some other protection scheme, appropriate for the operating conditions, have to be considered. Another important consideration is the accidental damage that can locally modify the pattern of stresses imposed on a metallic component or can destroy some of the protective barriers. 

Factors that influence corrosion consideration in material selection are distinct from those that interact in a more complex fashion. For example, application influences selection because the type of process, and the variables during operation etc., will deflne whether a material can be used for the intended purpose or not. On the other hand mechanical and metallurgical properties are not uniquely defined for all environments. For example, if the material is to be used at low temperature then embrittlement can be a serious problem. 

These considerations have a direct influence on corrosion consideration in material selection. However, when there is discrepancy amongst sections of this book, or between this and other disciplines regarding selection of materials, other priorities, such as client preference, in-house experience, and specific industry standards, should also be observed. 

Transfer lines are usually assembled by soldering with a torch, and unless considerable care is exercised, it is quite likely that oxidation, soldering flux, or carbon deposits from the flame torch will greatly deteriorate the joint surfaces. To ensure low emissivity, 10% aluminum persulfate solution has been used to remove the oxide and other corrosion from copper tubing. 

For many practically relevant material/environment combinations, thennodynamic stability is not provided, since E > E. Hence, a key consideration is how fast the corrosion reaction proceeds. As for other electrochemical reactions, a variety of factors can influence the rate detennining step. In the most straightforward case the reaction is activation energy controlled i.e. the ion transfer tlrrough the surface Helmholtz double layer involving migration and the adjustment of the hydration sphere to electron uptake or donation is rate detennining. The transition state is... 

Many reaction schemes have been proposed (161,162). All reaction schemes ate designed such that reaction steps having positive A. " values are operated at high (625—725°C) temperatures, whereas reaction steps having negative AA values are operated at low (about 225°C) temperatures. The purpose is to lower the free energy change, ie, the work requirement, and increase the thermal requirement, for improved efficiency. Other considerations, such as reaction kinetics, corrosion, cost of materials, and side reactions must also be taken into account. 

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