Corrosion design aspects

Although the design aspects provide most of the corrosion protection for airplanes, corrosion inhibitors are widely used to provide additional protection when used periodically in service. Corrosion inhibitors are applied in areas such as the lobe of the fuselage and most of the aluminum parts. The corrosion inhibitors are petroleum-based compounds that either displace the water or serve as a coating. The water-displacing inhibitors are sprayed onto a structure to penetrate faying surfaces and keep water away from crevices. The application of these inhibitors must be repeated at intervals of every few years. The more viscous heavy-duty inhibitors are also sprayed, which form a much thicker film and have a lesser penetrating ability. These thicker inhibitors are applied on airplane parts that are most prone to corrosion. 

Atmospheric corrosion requires the presence of humidity to form an electrolyte (Chapter 8). Preventing accumulation of humidity in certain spots by clever design therefore greatly reduces the risk of atmospheric corrosion. This aspect is particularly important for stmetures in civil engineering and architecture that are periodically exposed to rainwater. In a similar fashion, in the chemical industry, it is important to be able to completely empty containers when they are not in use. Figure 12.1 shows how the design of a reactor can facilitate its efficient drainage. 

Considers corrosion of ceramics and refractories, failures in ceramics and refractories, and the design aspects... 

This volume on ceramics and refractories is divided into three sections. The first section, "Ceramics and Refractories," is divided into seven chapters. Apart from an introductory section. Chapter 1 mainly details the applications of ceramics and refractories. Chapter 2 is on selection of materials and it describes the two stages in selection with a case study. Chapter 3 is on new developments in the ceramic and refractory fields. Chapter 4 describes the phase equilibriums in ceramic and refractory systems and outlines the three important systems, namely, unary, binary, and ternary. Corrosion of ceramics and refractories is the subject of Chapter 5. Chapter 6 discusses failures in ceramics and refractories. Design aspects are covered in Chapter 7. 

Engineering design, then, involves many considerations (Fig. 1.7). The choice of a material must meet certain criteria on bulk and surface properties (strength and corrosion resistance, for example). But it must also be easy to fabricate it must appeal to potential consumers and it must compete economically with other alternative materials. In the next chapter we consider the economic aspects of this choice, returning in later chapters to a discussion of the other properties. 

The process engineer gets involved in many mechanical aspects of vessel design such as thickness, corrosion allowance, and internals. Here are some pitfalls to watch for along the way ... 

Nitric acid is one of the three major acids of the modem chemical industiy and has been known as a corrosive solvent for metals since alchemical times in the thirteenth centuiy. " " It is now invariably made by the catalytic oxidation of ammonia under conditions which promote the formation of NO rather than the thermodynamically more favoured products N2 or N2O (p. 423). The NO is then further oxidized to NO2 and the gases absorbed in water to yield a concentrated aqueous solution of the acid. The vast scale of production requires the optimization of all the reaction conditions and present-day operations are based on the intricate interaction of fundamental thermodynamics, modem catalyst technology, advanced reactor design, and chemical engineering aspects of process control (see Panel). Production in the USA alone now exceeds 7 million tonnes annually, of which the greater part is used to produce nitrates for fertilizers, explosives and other purposes (see Panel). 

Because systems are normally not designed for use with this type of fluid, certain aspects should be reviewed with the equipment and fluid suppliers before a decision to use such fluids can be taken. These are compatibility with filters, seals, gaskets, hoses, paints and any non-ferrous metals used in the equipment. Condensation corrosion effect on ferrous metals, fluid-mixing equipment needed, control of microbial infection together with overall maintaining and control of fluid dilution and the disposal of waste fluid must also be considered. Provided such attention is paid to these designs and operating features, the cost reductions have proved very beneficial to the overall plant cost effectiveness. 

Corrosion can, however, be a factor in another type of assessment, which is arguably more important. It could influence important decisions about the whole project, taken at an early stage in the overall design process, which are concerned with the fundamental basis of the project rather than with corrosion aspects directly. In a major project, feasibility assessments in the initial stages are used to decide between possible alternatives, later effort being concentrated on one or two preferred options. If corrosion considerations are relevant they can influence the economics of the project as a whole, and have a much larger effect than in the first type of assessment. 

A wide variety of protective coating types and systems is available for corrosion control on external and internal surfaces of structural and process plant in marine and offshore engineering. These are discussed in detail elsewhere in this text, and the purpose here is to highlight the critical importance of certain design and related operational aspects which affect both the selection and performance of protective coating systems. The following design considerations should be made ... 

Even with the velocity, turbulence and aeration of the water supplied to the equipment being within acceptable limits, if corrosion is to be avoided it is still necessary for the units themselves to be well designed. Some of the more important aspects involved are outlined below. 

In this thesis an inherent safety index for evaluating inherent safety in preliminary process design was presented. The inherent safety of a process is affected by both chemical and process engineering aspects. These have been dealt separately, since the index was divided into the Chemical Inherent Safety Index and the Process Inherent Safety Index. These two indices consist of several subindices which further depict specific safety aspects. The Chemical Inherent Safety Index describes the inherent safety of chemicals in the process. The affecting factors are the heat of the main reaction and the maximum heat of possible side reactions, flammability, explosiveness, toxicity, corrosiveness and the interaction of substances present in the process. The Process Inherent Safety Index expresses safety of the process itself. The subindices describe maximum inventory, maximum process temperature and pressure, safety of equipment and the safety of process structure. 

Mixer-settlers have been the more common type of equipment and, with the development of hydrometallurgy over the past 20 years, designs have improved considerably. To select the appropriate equipment, a clear understanding of the chemical and physical aspects of the process is required. Also the economics must be considered relative to the type of equipment to suit particular conditions of given throughput, solution and solvent type, kinetics and equilibrium, dispersion and coalescence, solvent losses, number of stages, available areas, and corrosion. 

Carbon steels. The corrosion behavior of carbon steel weldments produced by fusion welding can be due to metallurgical effects, such as preferential corrosion of the heat-affected zone (HAZ) or weld metal, or it can be associated with geometric aspects, such as stress concentration at the weld toe, or creation of crevices due to joint design. 

Selecting materials of construction is an irrqjortant aspect of designing flow systems. The process engineer, more than any other engineer, must handle corrosive as well as dangerous fluids. We will not discuss corrosion here. The interested reader can refer to Fontana and Greene [4] for further details. 

IMaterials and Scaling Issues. Two aspects of the basically simple desalination process require special attention. One is the high corrosivity of seawater, especially pronounced in the higher temperature distillation processes, which requires the use of corrosion-resistant, and therefore expensive, materials. Typical materials in use are copper—nickel alloys, stainless steel, titanium, and, at lower temperatures, fiber-reinforced polymers and special concrete compositions (39). It is noteworthy that in quest of a lower initial cost, the use of inadequate materials of constmction in many locations combined with poor operation by virtually untrained hands led to rapid deterioration and failure of plants long before their estimated design life. Adequate experience suggests by now how to avoid such failures. The other aspect is scale formation (40,41), discussed in mote detail below. 

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