The Study of Corrosion

To the great majority of people, corrosion means rust, an almost universal object of hatred. Rust is, of course, the name which has more recently been specifically reserved for the corrosion of iron, while corrosion is the destructive phenomenon which affects almost all metals. Although iron was not the first metal used by man, it has certainly been the most used, and must have been one of the first on which serious corrosion problems were encountered [1]. 

Greek philosophers viewed the physical world as matter organized in the form of bodies having length, breadth, and depth that could act and be acted upon. They also believed that these bodies made up a material continuum unpunctuated by voids. Within such a universe, they speculated about the creation and destruction of bodies, their causes, the essence they consisted of, and the purpose they existed for. Surfaces did not fit easily into these ancient pictures of the world, even those painted by the atomists, who admitted to the existence of voids. The problem of defining the boundary or limit of a body or between two adjacent bodies led Aristotle (fourth century BC) and others to deny that a surface has any substance. Given Aristotle s dominance in ancient philosophy, his view of surfaces persisted for many centuries, and may have delayed serious theoretical speculation about the nature of solid surfaces [2]. 

Perhaps the only ancient scientific account of surfaces is to be found in some passages of the great Roman philosopher Pliny the Elder (23-79 AD) who wrote at length about ferrum corrumpitur, or spoiled iron. By his time the Roman Empire had been established as the world s foremost civilization, a distinction due partly to the 

Numerous scientific and engineering discoveries have been made since then and the general understanding of corrosion mechanisms has progressed with these. Some of the discoveries that have improved the field of corrosion are listed in App. A (Historical Perspective). By the turn of the twentieth century the basic processes behind the corrosion of iron and steel were relatively well understood. One of the first modern textbooks on corrosion prevention and control was published by McGraw-Hill in 1910 [3]. The following are some excerpts that illustrate the state of knowledge when this landmark text came out. 

From the standpoint of the electrolytic theory, the explanation of the corrosion of iron is not complicated, and so far has been found in accordance with all the facts. Briefly stated, the explanation is as follows Iron has a certain solution tension, even when the iron is chemically pure and the solvent pure water. The solution tension is modified by impurities or additional substances contained in the metal and in the solvent. The effect of the slightest segregation in the metal, or even unequal stresses and strains in the surface, will throw the surface out of equilibrium, and the solution tension will be greater at some points than at others. 

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Each reactant and product appears in the Nemst equation raised to its stoichiometric power. Thermodynamic data for cell potentials have been compiled and graphed (3) as a function of pH. Such graphs are known as Pourbaix diagrams, and are valuable for the study of corrosion, electro deposition, and other phenomena in aqueous solutions.Erom the above thermodynamic analysis, the cell potential can be related to the Gibbs energy change... 

Use and Uimitations of Electrochemical Techniques A major caution must be noted as to the general, indiscriminate use of all electrochemical tests, especially the use of AC and EIS test techniques, for the study of corrosion systems. AC and EIS techniques are apphcable for the evaluation of very thin films or deposits that are uniform, constant, and stable—for example, thin-film protective coatings. Sometimes, researchers do not recognize the dynamic nature of some passive films, corrosion produc ts, or deposits from other sources nor do they even consider the possibility of a change in the surface conditions during the course of their experiment. As an example, it is note-... 

For conducting tests in pipe lines of 75-mm (3-in) diameter or larger, a spool holder as shown in Fig. 28-21, which employs the same disk-type specimens used on the standard spool holder, has been used. This frame is so designed that it may be placed in a pipe line in any position without permitting the disk specimens to touch the wall of the pipe. As with the strip-type holder, this assembly does not materially interfere with the fluid through the pipe and permits the study of corrosion effects prevailing in the pipe line. 

Apart from the application of XPS in catalysis, the study of corrosion mechanisms and corrosion products is a major area of application. Special attention must be devoted to artifacts arising from X-ray irradiation. For example, reduction of metal oxides (e. g. CuO -> CU2O) can occur, loosely bound water or hydrates can be desorbed in the spectrometer vacuum, and hydroxides can decompose. Thorough investigations are supported by other surface-analytical and/or microscopic techniques, e.g. AFM, which is becoming increasingly important. 

Ultramodern techniques are being applied to the study of corrosion thus a very recent initiative at Sandia Laboratories in America studied the corrosion of copper in air spiked with hydrogen sulphide by a form of combinatorial test, in which a protective coat of copper oxide was varied in thickness, and in parallel, the density of defects in the copper provoked by irradiation was also varied. Defects proved to be more influential than the thickness of the protective layer. This conclusion is valuable in preventing corrosion of copper conductors in advanced microcircuits. This set of experiments is typical of modern materials science, in that quite diverse themes... combinatorial methods, corrosion kinetics and irradiation damage... are simultaneously exploited. 

The study of corrosion is essentially the study of the nature of the metal reaction products (corrosion products) and of their influence on the reaction rate. It is evident that the behaviour of metals and alloys in most practical environments is highly dependent on the solubility, structure, thickness, adhesion, etc. of the solid metal compounds that form during a corrosion reaction. These may be formed naturally by reaction with their environment (during processing of the metal and/or during subsequent exposure) or as a result of some deliberate pretreatment process that is used to produce thicker films or to modify the nature of existing films. The importance of these solid reaction products is due to the fact that they frequently form a kinetic barrier that isolates the metal from its environment and thus controls the rate of the reaction the protection afforded to the metal will, of course, depend on the physical and chemical properties outlined above. 

Thus the potential-pH diagrams and the E-I diagrams may be regarded as complementary in the study of corrosion phenomena and in the solution of corrosion problems. 

In certain applications it has not always been easy to hnd suitable metallic container materials, particularly in the nuclear-energy industry, where, for certain applications, corrosion resistance of the same order as that required by the fine chemical industry has to be achieved in order to prevent contamination of the process stream. Such difflculties have stimulated the study of corrosion in fused salts and have led to a fairly high degree of understanding of corrosion reactions in these media. 

Fourier transform infrared reflection-absorption spectroscopy (FT-IFRAS) is applied to the study of corrosion protection of copper by an organic coating. Poly-N-vinyliroidazole (PVI(D) and poly-4(5)-vinylimidazole (PVI(4)) are demonstrated to be effective new polymeric anti-corrosion agents for copper at elevated temperatures. Oxidation of copper is suppressed even at 400° C. PVI(1) and PVI(4) are more effective anti-oxidants than the most commonly used corrosion inhibitors, benzotriazole and undecyllmldazole, at elevated temperatures. These new polymeric agents are water soluble and easy to treat the metal surface. 

Corrosion resistant FRP structures and also resin linings have resin or resin-rich surface layer to protect the structures from corrosive attack. Therefore, the study of corrosion behavior of resin is essentially important. 

There are many types of corrosion, as would be expected from its general definition. It has been traditional (4) to divide the study of corrosion into two areas the study of low temperature corrosion by aqueous or other solutions, controlled by electrochemical processes (wet) and the study of gaseous corrosion at high temperatures, controlled by thermodynamics and diffusion processes (dry). In addition to the obvious differences, the two areas have many phenomena in common. 

The rotating disk electrode (RDE) is an important system in electrochemistry. Axial followed by radial flow across the disk brings fresh solution to all points across the disk (Fig. 6). The surface is therefore uniformly accessible to reacting species. The RDE operates under laminar flow for Re < 1.7 X 105. Flow is turbulent above 3.5 X 105 and is transitional in between (4). Thus the system is less practical for the study of corrosion under turbulent conditions but enjoys widespread use in research electrochemistry. For the rotating disk electrode, the laminar mass transport correlation obtained in the literature is given by Levich (10) ... 

There is some reason for optimism in spite of the sub-embryonic character of our knowledge at this point. A number of studies have added information of consequence. By way of illustration, equations have been developed for unimpeded anodic reactions, among others, on the basis of electrochemical theory (IQ). Also the study of corrosion of whiskers (11) and of single crystal metals (1,9) has become somewhat more common. Studies on electrodeposition on whiskers (12) ami on dissolution and electrodeposition in copper-copper sulfate solutions (13) are sure to be pertinent. Certainly the recem Faraday Society Discussion on "Crystal Imperfections and Chemical Reactivity" (14) should prove very useful. Although it may nor. be especially pertinent to the metal-solution systems, the general trend of work and the tentative principles evolved should show the way to significant experiments in this field. 

Using the aluminum sheet substrate as the cathode of a direct current (DQ glow discharge, cathodic plasma polymerization is carried out. Dealing with metal surfaces, cathodic plasma polymerization is the most practical means to provide the best corrosion protection (see Chapter 13). A primer is applied on the surface of the plasma polymer. The thickness of the plasma polymer is roughly 50 nm on average and that of the primer layer is about 30,000 nm (30 pm). Primers used included E-coat (electrolytic deposition of paint) and spray primers, but no top coat was applied in the study of corrosion protection. 

Syrett, B. C. "The Application of Electrochemical Techniques to the Study of Corrosion of Metallic Implant Materials", In Electrochemical Techniques,lor Corrosion. Babolan, R., Ed., NACE 1977. 

The study of corrosion properties of mild steel and stainless steel in aqueous solutions has received a great general attention. Mild steel is the most common structural material and is used in a wide range of environments. It is well known that when mild steel corrodes, anodic and cathodic areas develop over the corroded surface. Conventionally, these pits are known to change in shape and move across the surface, resulting in early corrosion that is approximately... 

The power of electrochemical impedance spectroscopy (EIS) is well acknowledged and widely used in various systems [130, 131]. The history of impedance of porous, distributed electrodes in liquid electrolytes goes back to the early 1960s (see Refs. 132, 133), and some later works [134, 135]. In the context of accumulators, interpretation of electrode impedance was already long ago considered as a promising way to acquire information on the current state of the electrodes [136]. The same refers to the study of corrosion [137, 138]. Nowadays,... 

High intensity Raman signals minimize the measurement time, which is a very important factor especially for the study of corrosive melts. 

The use of expert systems [13-14] allows the solution of problems which lack reliable mathematical models and are characterized by a great amount of empirical knowledge acquired in heterogeneous ways and by a high level of abstraction. The study of corrosion phenomena has such characteristics and is a very promising field for the realization of expert systems. 

Electrochemistry is a scientific discipline with a well developed system of theories and quantitative relationships. It has many applications and uses in both fundamental and applied areas of chemistry—in the study of corrosion phenomena, for example, for the study of the mechanisms and kinetics of electrochemical reactions, as a tool for the electrosynthesis of organic and inorganic compounds, and in the solution of quantitative analytical problems. This last area will be emphasized in the next four chapters. 

Corrosion may be defined as the spontaneous deterioration of a structure or part of a structure due to the action of the total environment or individual environmental agents. For the purposes of this chapter, the structure is assumed to be metallic and the environment is assumed to be aqueous. Using this definition and the constraints noted, this chapter will outline the electrochemical techniques used to develop criteria of corrosion and those used in the study of corrosion kinetics. 

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