Corrosion fundamental processes

Our comprehensive understanding of materials corrosion fundamentals has advanced considerably over the decades. Modern corrosion science has made it clear that the corrosion process on metals and semiconductors consists of an anodic oxidation and a cathodic reduction both occurring across the material-aqua-solution interface. These reduction-oxidation reactions depend on the interfacial potential and hence on the electrode potential of materials. 

Corrosion is the term usually applied to the deterioration of metals by an electrochemical process. We see many examples of corrosion around us. Rust on iron, tarnish on silver, and the green patina formed on copper and brass are a few of them (Figure 19.13). Corrosion causes enormous damage to buildings, bridges, ships, and cars. The cost of metallic corrosion to the U.S. economy has been estimated to be well over 100 billion dollars a year This section discusses some of the fundamental processes that occur in corrosion and methods used to protect metals against it. 

Heitz, E., Hukovic, M., and Maier, L. H. (1970). GrundvorgSnge der Korrosion von Metallen in organischen Losungsmitteln (Fundamental processes of corrosion of metals in organic solvents). Werkst. Korros. 2/(6), 457-462 (in German). 

Because corrosion is fundamentally a surface phenomenon, those interested in the fundamental processes of corrosion have always been among the first to explore the utUity of surface analysis techniques. These techniques have had, and win continue to have, great success in illuminating many facets of corrosion phenomena. 

DeanSW. Corrosion monitoring for industrial processes. In Cramer DS, Covino BS, eds. Vol. 13A Corrosion Fundamentals, Testing, and Protection. Metals Park, Ohio ASM International, 2003 533-41. 

Figure 1 Fundamental processes in the initial stages of S02-indueed atmospheric corrosion, including coordination of HS03 with the hydroxylated metal oxide surface, replacement of surface 0H group by 803 and detachment of the 803 -coordinated metal surface center when surrounded by two or more H -bonded functional groups.

This chapter reviews some representative types of corrosion problems encountered in the various facets of the petroleum and chemical industries. The fundamental processes underlying these corrosion problems are examined. In addition, commonly used methods for corrosion prevention and control are discussed... 

G lv nic Corrosion. Galvanic corrosion is an electrochemical process with four fundamental requirements (/) an anode (magnesium), 2) a cathode (steel, brass, or graphite component), (J) direct anode to cathode electrical contact, and (4) an electrolyte bridge at the anode and cathode interface, eg, salt water bridging the adjacent surfaces of steel and magnesium components. If any one of these is lacking, the process does not occur (133,134). 

The polymer-supported catalysts are thus important conceptually in linking catalysis in solutions and catalysis on supports. The acid—base chemistry is fundamentally the same whether the catalytic groups are present in a solution or anchored to the support. The polymer-supported catalysts have replaced acid solutions in numerous processes because they minimise the corrosion, separation, and disposal problems posed by mineral acids. 

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. 

Electrochemical systems are found in a number of industrial processes. In addition to the subsequent discussions of electrosynthesis, electrochemical techniques are used to measure transport and kinetic properties of systems (see Electroanalyticaltechniques) to provide energy (see Batteries Euel cells) and to produce materials (see Electroplating). Electrochemistry can also play a destmctive role (see Corrosion and corrosion control). The fundamentals necessary to analyze most electrochemical systems have been presented. More details of the fundamentals of electrochemistry are contained in the general references. 

Almost all cooling water system deposits are waterborne. It would be impossible to list each deposit specifically, but general categorization is possible. Deposits are precipitates, transported particulate, biological materials, and a variety of contaminants such as grease, oil, process chemicals, and silt. Associated corrosion is fundamentally related to whether deposits are innately aggressive or simply serve as an occluding medium beneath which concentration cells develop. An American... 

Koszman, I., Antifoulant Additive for Steam-Cracking Process, U.S. Patent 3,531,394, Sept. 29, 1970. Hochman, R. F, Fundamentals of the Metal Dusting Reaction, Proceedings, Fourth International Congress on Metallic Corrosion, NACF (1971). 

This book is designed as a handy desk reference covering fundamental engineering principles of project planning schemes and layout, corrosion principles and materials properties of engineering importance. It is intended as a general source of typical materials property data, useful for first pass materials selection in process design problems. 

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). 

Aqueous environments will range from very thin condensed films of moisture to bulk solutions, and will include natural environments such as the atmosphere, natural waters, soils, body fluids, etc. as well as chemicals and food products. However, since environments are dealt with fully in Chapter 2, this discussion will be confined to simple chemical solutions, whose behaviour can be more readily interpreted in terms of fundamental physicochemical principles, and additional factors will have to be considered in interpreting the behaviour of metals in more complex environments. For example, iron will corrode rapidly in oxygenated water, but only very slowly when oxygen is absent however, in an anaerobic water containing sulphate-reducing bacteria, rapid corrosion occurs, and the mechanism of the process clearly involves the specific action of the bacteria see Section 2.6). 

Over the years the original Evans diagrams have been modified by various workers who have replaced the linear E-I curves by curves that provide a more fundamental representation of the electrode kinetics of the anodic and cathodic processes constituting a corrosion reaction (see Fig. 1.26). This has been possible partly by the application of electrochemical theory and partly by the development of newer experimental techniques. Thus the cathodic curve is plotted so that it shows whether activation-controlled charge transfer (equation 1.70) or mass transfer (equation 1.74) is rate determining. In addition, the potentiostat (see Section 20.2) has provided... 

No corrosion occurs in a completely dry environment. In soil, water is needed for ionisation of the oxidised state at the metal surface. Water is also needed for ionisation of soil electrolytes, thus completing the circuit for flow of a current maintaining corrosive activity. Apart from its participation in the fundamental corrosion process, water markedly influences most of the other factors relating to corrosion in soils. Its role in weathering and soil genesis has already been mentioned. 

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. 

It is generally agreed that the causes and effects of poor water chemistry, mechanical problems, boiler section corrosion, metal failure, and poor boiler plant operation are all closely interrelated. Thus, effective control over the various corrosion processes that may occur in a boiler and its auxiliary equipment is fundamental to the realization of the full life expectancy and safe operation of the plant. Corroded and wasted metal cannot be replaced easily, and the failure of a boiler in service is both potentially dangerous and expensive. 

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