Corrosion prevention chemistry

The design and production of PAni-containing coating systems with commercial applicability asks for much more than just the application of PAni onto a metal surface by whatever method. It requires a full and deep understanding of corrosion prevention chemistry and physics itself, of the interfadal interactions between the PAni primer and the metal surface as weD as those between the different coating layers, and especially of the science of OM dispersion. 

In the development and fabrication of molecular-based electronics, it is essential to have a good understanding of the chemistry and electronic struemre of the electroactive polymer interface with other polymers, semi-conductors and metals. A better understanding of the CT interactions at the polymer/metal interface will also facilitate the application of conductive polymer coatings for metal passivation and corrosion prevention [268]. An overview of measurement methods and quantum chemical calculation techniques for smdying the chemical and electronic structure of conjugated... 

Figure 1.55 Schematic representation of corrosion prevention mechanism and role of redox chemistry of PANI in formation of passivating layer of Fe Oj. Reprinted from [527] with permission from Wiley.

Baboian, R., Chemistry of the Automotive Environment in Designing for Corrosion Prevention," SAE P-78, Warrendale, PA, 1978. 

Although this is a book about paints, not metallic corrosion, it becomes necessary at this point to devote some attention to the corrosion mechanisms of zinc in dry versus wet conditions. The reason for this is simple zinc-coated steel is an important material for corrosion prevention, and it is frequently painted. Accelerated tests are therefore used on painted, zinc-coated steel. In order to obtain any useful information from acceloated testing, it is necessary to understand the chemistry of zinc in dry and wet conditions. 

In 1964 Prof Ceilings was appointed professor of Inor nic Chemistry and Materials Science at the University of Twente. His main research interests were coordination chemistry and spectroscopy of transition metal compounds, corrosion and corrosion prevention, and catalysis. In 1991 he received the Cavallaro Medal of the European Federation Corrosion for his contributions to corrosion research. In 1992 he retired from his post at the University, but has remained active as supervisor of graduate students in the field of high temperature corrosion. 

Hinton BRW (1995) inHandbook of the Physics and Chemistry of the Rare Metals, Vol. 21, Chapter 140 Corrosion Prevention and Control , ed. Gschneider and L Eyre. 

Electrochemistry is a large and important area of physical chemistry. It is, however, difficult to define precisely the limits of this area, not amply on account of its size but because of its influence in so many areas in chemistry as well as in biology and physics. Many concepts, accepted universally now as fundamental to chemistry, originated in electrochemistry. On the other hand it is now realized that future developments in many fields, and we may at random quote such widely differing ones as corrosion prevention, power supply and biochemistry and cellular biology, are dependent in no small way upon the exploitation of electrochemical principles. 

Water Treatment. Water and steam chemistry must be rigorously controlled to prevent deposition of impurities and corrosion of the steam cycle. Deposition on boiler tubing walls reduces heat transfer and can lead to overheating, creep, and eventual failure. Additionally, corrosion can develop under the deposits and lead to failure. If steam is used for chemical processes or as a heat-transfer medium for food and pharmaceutical preparation there are limitations on the additives that may be used. Steam purity requirements set the allowable impurity concentrations for the rest of most cycles. Once contaminants enter the steam, there is no practical way to remove them. Thus all purification must be carried out in the boiler or preboiler part of the cycle. The principal exception is in the case of nuclear steam generators, which require very pure water. These tend to provide steam that is considerably lower in most impurities than the turbine requires. A variety of water treatments are summarized in Table 5. Although the subtieties of water treatment in steam systems are beyond the scope of this article, uses of various additives maybe summarized as follows ... 

A type of waterside maintenance chemical treatment. Any of a very wide range of chemicals that prevent or reduce tendencies of deposition, fouling, scaling, corrosion, or other unwanted phenomena to occur in a water system. Typically for smaller boiler plants, individual inhibitors are blended together to produce various multifunctional formulations specific for particular water chemistry and/or operating conditions. Larger boiler plants tend to use individual active component inhibitors. 

Whenever corrosion resistance results from the formation of layers of insoluble corrosion products on the metallic surface, the effect of high velocity may be to prevent their normal formation, to remove them after they have been formed, and/or to preclude their reformation. All metals that are protected by a film are sensitive to what is referred to as its critical velocity i.e., the velocity at which those conditions occur is referred to as the critical velocity of that chemistry/temperature/veloc-ity environmental corrosion mechanism. When the critical velocity of that specific system is exceeded, that effect allows corrosion to proceed unhindered. This occurs frequently in small-diameter tubes or pipes through which corrosive liquids may be circulated at high velocities (e.g., condenser and evaporator tubes), in the vicinity of bends in pipelines, and on propellers, agitators, and centrifugal pumps. Similar effects are associated with cavitation and mechanical erosion. 

Extraction OF metals from minerals1-7 has many points in common with corrosion control. Redox chemistry is applied to wrest metals from Nature, just as it is used to prevent her reclaiming them through corrosion. There is an additional factor to be considered, however. Most ores contain only minor amounts of the metal of interest, and, before final reduction or refining, the desired metal or its compounds must be concentrated and separated from other metals that may be present. 

Gases may also be produced from chemical and radiolytic interactions of high-level waste and the anticipated brine which may migrate to the waste canisters. Canister corrosion is expected to occur and can result in H2 and HCl generation. These reactions must be examined and quantified under the anticipated conditions of pressure, temperature and leachant chemistry to determine whether they can be accomodated or should be prevented in a full-scale HLW repository. 

Biological and medicinal aspects of silver chemistry are covered in Chapter 62 but a number of features will be outlined here. Silver salts are powerful bacteriocides and it has been known for many years that storing water in silver vessels prevents spoilage. Silver nitrate is highly corrosive and can be applied locally to remove warts or cauterize wounds. In many states in the USA, a 1% AgN03 solution is dropped into the eyes of newborn infants to prevent ophthalmia neonatorum. Silver sulfadiazine has been found effective as a topical application to prevent infections in serious bums victims. 

The relatively thick zinc hydroxide film can effectively prevent underdeposit corrosion and other problems. As a further example, in medium hardness waters, with careful control of the water chemistry, this thick Zn(OH)2 film can help offset risks of chloride-induced pitting corrosion with estuarine makeup water. 

An example of such a product is ioBio , marketed by Baltimore Aircoil Co. (BAC). It is proposed for small cooling systems. This system is, in effect, an in-line dispenser of prilled elemental iodine (U), with replaceable iodine canisters available. The equipment is placed in the makeup waterline to the tower, and in-line options include filters, water meters, and back-flow prevention devices. Claims made are that the biocidal action of the elemental iodine is not affected by changes in temperature, system load, or water chemistry. Also, the system is compatible with virtually all scale and corrosion inhibitors and does not contribute to the corrosion of metal surfaces. 

The corrosive deterioration of metal surfaces incurs a great cost to the worldwide economy. Accordingly, there have been many research efforts devoted to understanding the surface chemistry behind these reactions. As we have already seen, this has led to the development of a number of useful alloys that are sufficiently resistant to corrosion - through spontaneous formation of protective oxide layers. However, for other less resistant metals such as carbon steels, a protective layer must be postdeposited onto a metal surface in an effort to prevent corrosion. In this section. 

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