Costs, corrosion control

Duplex stainless steels (ca 4% nickel, 23% chrome) have been identified as having potential appHcation to nitric acid service (75). Because they have a lower nickel and higher chromium content than typical austenitic steels, they provide the ductabdity of austenitic SS and the stress—corrosion cracking resistance of ferritic SS. The higher strength and corrosion resistance of duplex steel offer potential cost advantages as a material of constmction for absorption columns (see CORROSION AND CORROSION CONTROL). 

Fouling organisms attach themselves to the underwater portions of ships and have a severe impact on operating costs. They can increase fuel consumption and decrease ship speed by more than 20%. Warships are particularly concerned about the loss of speed and maneuverabiHty caused by fouling. Because fouling is controUed best by use of antifouHng paints, it is important that these paints be compatible with the system used for corrosion control and become a part of the total corrosion control strategy. 

Corrosivity. Corrosivity is an important factor in the economics of distillation. Corrosion rates increase rapidly with temperature, and in distillation the separation is made at boiling temperatures. The boiling temperatures may require distillation equipment of expensive materials of constmction however, some of these corrosion-resistant materials are difficult to fabricate. For some materials, eg, ceramics (qv), random packings may be specified, and this has been a classical appHcation of packings for highly corrosive services. On the other hand, the extensive surface areas of metal packings may make these more susceptible to corrosion than plates. Again, cost may be the final arbiter (see Corrosion and corrosion control). 

The industrial economy depends heavily on electrochemical processes. Electrochemical systems have inherent advantages such as ambient temperature operation, easily controlled reaction rates, and minimal environmental impact (qv). Electrosynthesis is used in a number of commercial processes. Batteries and fuel cells, used for the interconversion and storage of energy, are not limited by the Carnot efficiency of thermal devices. Corrosion, another electrochemical process, is estimated to cost hundreds of millions of dollars aimuaUy in the United States alone (see Corrosion and CORROSION control). Electrochemical systems can be described using the fundamental principles of thermodynamics, kinetics, and transport phenomena. 

The primaiy purpose of materials selection is to provide the optimum equipment for a process application in terms of materials of construction, design, and corrosion-control measures. Optimum here means that which comprises the best combination of cost, life, safety, and reliability. 

Thorough assessment of the service environment and a review of options for corrosion control must be made. In severe, humid environments it is sometimes more economical to use a relatively cheap structural material and apply additional protection, rather than use costly corrosion-resistant ones. In relatively dry environments many materials can be used without special protection, even when pollutants are present. 

Whilst careful design and informed forethought can often minimise or even prevent corrosion at little extra cost where the environmental conditions or the conditions of service are severe (as in most forms of marine and offshore engineering) reliable, secure and cost-effective corrosion control cannot be effectively achieved without considerable expense, although even in these circumstances good design can help to significantly reduce this. In... 

Corrosion Control control of the corrosion rate and form of attack of a metal of a given metal/environment system at an acceptable level and at an economic cost. 

With the added benefit of increased product purity (reduced iron contamination), near-zero operational costs speak to AP as the most underapplied corrosion control systems of the ages. 

DAF clarifiers are mainly prefabricated in stainless steel for erection cost reduction, corrosion control, better construction flexibility, and possible future upgrades, contrary to in situ constructed heavy concrete sedimentation tanks. 

Hexavalent chromate [Cr(VI)] is still used within the industry to meet critical high corrosion control and other metal surface finishing requirements. Cr(VI) is toxic and its control generates a hazardous, costly waste. 

Corrosion is the deterioration of a material by reaction with its enviromnent. Although the term is used primarily in conjunction with the deterioration of metals, the broader definition allows it to be used in conjunction with all types of materials. We will limit the description to corrosion of metals and alloys for the moment and will save the degradation of other types of materials, such as polymers, for a later section. In this section, we will see how corrosion is perhaps the clearest example of the battle between thermodynamics and kinetics for determining the likelihood of a given reaction occurring within a specified time period. We will also see how important this process is from an industrial standpoint. For example, a 1995 study showed that metallic corrosion costs the U.S. economy about 300 billion each year and that 30% of this cost could be prevented by using modem corrosion control techniques [9], It is important to understand the mechanisms of corrosion before we can attempt to control it. 

Steel is the basic material of construction for many sea-water plants. Even though it may corrode in some sections of a plant, such as a distillation unit, its low initial cost may result in lower over-all costs if good design and good corrosion-control practices are followed. 

Phosphate-based programs tend to be used under lower pH conditions (where good corrosion control is also a primary requisite) and are generally of lower cost than more modem organic programs. 

Capital Equipment Costs The initial investment made by a utility to control pollutant emissions is in the equipment comprising the flue gas treatment system. The amount of the investment in this equipment purchase is directly related to the size and complexity of the equipment itself. The vessels required in this application must be large because of the volume of flue gas that must be treated. Some equipment, such as spray dryers or absorbers, can be 20 to 50 feet in diameter, and spray towers can be over 100 feet in height. Considerable preparation must go into sizing these vessels properly in order to maximize gas-sorbent contact time and minimize scaling and other operational problems. Much of this equipment, especially in wet systems, must be constructed of costly corrosion-resistant materials. Elaborate valving systems must be set up to control flows to the system. 

It is evident from the data presented on the economics of corrosion that corrosion costs amount to about 2-4% of GNP, and about 25% of the costs are avoidable by adopting corrosion control measures. The measures taken to combat corrosion in UK, USA, Australia, China and Canada have been discussed.18 The following is a short summary of the activities in various parts of the world to combat corrosion. 

D.R. McIntyre, Evaluation of the Cost of Corrosion-Control Methods, Chemical Engineering, April 1982, 127-132. 

Permanency of paper was not questioned seriously until only recently when modern paper showed signs of excessive deterioration. In a relatively short time the technology was developed and standards were set for modern paper with specific, predictable permanency properties. Today, permanent paper is readily available and is competitively priced. Moreover, the alkaline papermaking process that is required for the manufacture of permanent paper is proving to some manufacturers to be more efficient in terms of costs, corrosion of equipment, and pollution control. 

The corrosion engineer can play a major role in system design, material selection, process or environmental control, and remediation. The focus of these efforts should not necessarily be the complete elimination or avoidance of corrosion, but rather the selection of the most cost-effective means of corrosion control and abatement. 

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