Corrosion basic thermodynamics

The present section illustrates how calculations from basic thermodynamic data can lead to open-circuit cell potential in any condition of temperature and pressure. Chemical power sources, with the exception of fuel cells, are all based on the corrosion of a metal connected to the negative terminal. The aluminum-air power source, that owes its energy to the corrosion of aluminum in caustic, was chosen for this example because of the relative simple chemistry. 

The subject is also closely related to fuel-ash corrosion which in most cases is caused by a layer of fused salts such as sulphates and chlorides Attention has been focused on the electrochemistry of this type of corrosion and the relevant thermodynamic data summarised in the form of diagrams . Fluxing and descaling reactions also resemble in some respects reactions occurring during the corrosion of metals in fused salts. A review of some of the more basic concepts underlying corrosion by fused salts (such as acid-base concepts and corrosion diagrams) has appeared. ... 

In contrast with the Pb-HjO system, it can be seen in Figure 4.13 that in the presence of SO " the corrosion zone in the region of low pH no longer exists, owing to the thermodynamic stability of PbSO. The Pb-HjO-COj system has been expressed in a similar pH/potential diagram in which account has been taken of insoluble carbonates and basic carbonates of lead. 

Corrosion is a mixed-electrode process in which parts of the surface act as cathodes, reducing oxygen to water, and other parts act as anodes, with metal dissolution the main reaction. As is well known, iron and ferrous alloys do not dissolve readily even though thermodynamically they would be expected to, The reason is that in the range of mixed potentials normally encountered, iron in neutral or slightly acidic or basic solutions passivates, that is it forms a layer of oxide or oxyhydroxide that inhibits further corrosion. 

NaCl, interact with the sulphur and vanadium oxides emitted from the combustion of technical grade hydrocarbons and the salt spray to form Na2S04 and NaVCh. These corrosive agents function in two modes, either the acidic mode in which for example, the sulphate has a high SO3 thermodynamic activity, of in the basic mode when the S03 partial pressure is low in the combustion products. The mechanism of corrosion is similar to the hot corrosion of materials by gases with the added effects due to the penetration of the oxide coating by the molten salt. 

Tower, Stephen. All About Electrochemistry. Available online. URL http //www.cheml.com/acad/webtext/elchem/. Accessed May 28, 2009. Part of a virtual chemistry textbook, this excellent resource explains the basics of electrochemistry, which is important in understanding how fuel cells work. Discussions include galvanic cells and electrodes, cell potentials and thermodynamics, the Nernst equation and its applications, batteries and fuel cells, electrochemical corrosion, and electrolytic cells and electrolysis. 

The sour natural gas sulfur recovery industry covers virtually the entire gamut of chemistry. From the sour gas reservoir to the Claus plant end product problems are encountered in thermodynamics, kinetics, corrosion, catalysis, redox, rheology and the environment - plus all the rest In reviewing recent developments in such a wide ranging field it is only possible to select examples. It is hoped, however, that these highlights will serve to illustrate the dynamism of the industry in recent years and the progress it has made in developing a new source of one of the world s most basic and essential elements in an environmentally acceptable manner. 

Chapter 1 provides an introduction to some of the basic terms and concepts of electrochemistry and corrosion and provides a detailed overview of the remainder of the book. Chapter 2 provides an overview of the important thermodynamic and kinetic parameters of relevance to corrosion electrochemistry. Chapter 3 focuses on what might be viewed as an aberration from normal dissolution kinetics passivity. This aberration—or peculiar condition, as Faraday referred to it— is critical to the use of stainless steels, aluminum alloys, and all the so-called corrosion resistant alloys (CRAs). 

The broadest definition of corrosion is the degradation of a material s properties or mass over time because of the effect of the environment. We can think of this in simpler terms by recognizing this process as the tendency for a material to return to its most thermodynamically stable state. For most metallic materials, this means the formation of oxides or sulfides, or other basic metallic compounds generally considered to be ores. For polymeric materials, the end result could be a variety of simple organic compounds. Only in vacuums or under inert atmospheres can corrosion processes be expected to halt entirely. In most cases, these processes are slow enough to afford useful and practical equipment life. 

Corrosion is an electrochemical process and corrosion processes follow the basic laws of thermodynamics. Under controlled conditions, corrosion can be measured, repeated, and predicted. However, because corrosion takes place on an atomic level, corrosion can take place in an accelerated localized fashion, appear as uniform visible attack, or result in subsurface microscopical damage. Normal service environments can rapidly complicate these processes and mechanisms with such variables as pH, temperature, stress, surface finish, flow rates, etc. With the wide range of variables that can come into play, it should not be surprising that corrosion appears to be unpredictable at times. 

The basic types of thermodynamic diagrams useful in interpreting the results of high-temperature corrosion experiments have been discussed. While no attempt at a complete discussion of this subject has been made, it is hoped this chapter will facilitate a clearer understanding of the high-temperature corrosion phenomena to be described in the remainder of this book. 

High temperature corrosion and oxidation will be covered in a separate chapter. However, some basics of the thermodynamics will be addressed here. Consider a metal M reacting at high temperature with O2 ... 

Exact corrosion kinetics must be modeled by solving the second law of Pick for the geometry of the case at hand. However, in some cases a net effect may be calculated from simple thermodynamics, as for closed system conditions in active corrosion [8], For the case of diffusion through scales it has been demonstrated that quasi-steady-state modeling is often a good approximation for an exact solution, at least for conditions tD/x > 2 [9] (where t = time, D = diffusivity, X = layer thickness). Some basic solutions for situations with instant singular corrosion can also be found in the literature [10]. 

An overview of the different concepts involved in electrochemistry, spanning (torn basic theory (thermodynamics, transport and electrode kinetics, etc.) to the main applications (batteries corrosion, electrosynthesis and sensors). In addition, it covers the research methods used in electrochemistry, as well as giving an insight into organic electrochemistry. For master s degree level and engineering schools. A dozen practical experiments are described in brief, with a few problems to solve (corrections provided on the website). 

As mentioned in the introductory chapter, metals were originally combined with oxygen, sulfur, and other non-metallic components to form minerals. From the viewpoint of thermodynamics, the state of compounds has lower energy and is more stable than that of metals themselves. It means that we generally need to add energy in order to produce metals. Since we could consider corrosion to be a reverse reaction of metal production, it is a very natural chemical reaction that occurs spontaneously in nature (Fig. 1). At this point, we already know that corrosion is basically an oxidation reaction. Therefore, corrosion (as a redox reaction) can be analyzed from the viewpoint of electrochemistry. Here we can see the fundamental aspect of electrochemistry as it relates to corrosion. 

Although the basic organization of the book is unchanged from the previous edition, there is in this edition a separate chapter on Pourbaix diagrams, very useful tools that indicate the thermodynamic potential-pH domains of corrosion, passivity, and immunity to corrosion. A consideration of the relevant Pourbaix diagrams can be a useful starting point in many corrosion studies and investigations. As always in corrosion, as well as in this book, there is the dual importance of thermodynamics (In which direction does the reaction go Chapters 3 and 4) and kinetics (How fast does it go Chapter 5). 

This chapter examined gas-solid kinetic processes. We saw how to apply the basic tools we learned in calculating thermodynamic driving forces (Chapter 2), reaction rates (Chapter 3), and mass diffusion (Chapter 4) to understand and model a number of important gas-solid kinetic processes including adsorption/desorption, active gas corrosion, chemical vapor deposition, and passive oxidation. The main points introduced in this chapter include ... 

Kinetics, chemical, thermodynamic, and physical principles will all be operating in high-temperature service test environments, requiring each investigator to have an adequate huniliarily of basic mechanisms and corrosion phenomena. A brief introduction to these aspects of service testing is presented here. 

Resistance of ceramics to corrosion is due to one of three basic behaviors immunity, passivation, or kinetically limited corrosion. When a ceramic is thermodynamically incapable of spontaneous reaction with its environment it is referred to as having immunity. When the necessary thermodynamic data are available this type of corrosion resistance may be predicted by calculation. Metals, except for the precious metals such as gold, do not exhibit immunity. 

---