Corrosion and surface chemistry of metals

nejron080800152.jpgThroughout history, metals have had a decisive effect on human civilization, and they largely condition the way we live today. Electric power, combustion engines, ships, cars, railways, airplanes, production of fertilizers and pharmaceuticals, precision tools, machines, high rise buildings, sanitary systems, home appliances and even computers would not exist without metals. Corrosion and wear limit the useful lifetime of this equipment, reduce their efficiency and often affect their safety. Corrosion control and surface technology are therefore of crucial importance for our technology-based society.

Metals are found in nature in their oxidized state, mostly as oxide or sulfide minerals. The elaboration of metals from minerals requires energy, and, as a consequence, if left alone metals spontaneously return to their natural state; they corrode. The rate at which this occurs varies widely among different metals and as a function of their chemical environment, the prevailing temperature and the concurrent action of mechanical forces. Furthermore, the local corrosion rate of a metallic structure may be affected by electrochemical interactions between different parts of it and by its shape. Often, corrosion rates vary with time due to changing environmental conditions or because corrosion products accumulate on the surface. Corrosion resistance therefore is not an intrinsic property of a material but depends on a given metal-environment system taken as a whole. The prediction of corrosion behavior and the implementation of efficient corrosion control measures requires a good understanding of the underlying reaction mechanisms.

In past years our knowledge about metal surfaces and corrosion mechanisms has advanced greatly, and considerable progress has been achieved in controlling corrosion. Successful examples include multi-year warranties against perforation of car body panels, oil drilling in highly corrosive marine environments or safely operated power plants. Today, at the beginning of the twenty first century, corrosion science and engineering faces many new challenges. First of all, the evident need for sustainability of future development requires that wasting of materials and energy resources due to corrosion be reduced further by developing appropriate technologies. A number of well-established methods of corrosion protection need to be replaced by more environment-friendly techniques. The development of ever smaller electronic and micromechanical devices and the emerging field of nanotechnology require new approaches to corrosion and wear control on an increasingly finer scale. On the opposite side, installations for energy production, civil engineering structures and transport systems tend to become ever larger and more complex, increasing the requirements for reliable risk analysis and lifetime prediction in relation to corrosion. Corrosion issues are also crucial for safe long-term storage of nuclear waste. Complex physical and chemical interactions between metal surfaces and biological systems govern the lifetime of medical implants and determine the extent to which microbial corrosion damages process equipment and water distribution systems. On a different level, corrosion based chemical and electrochemical shaping and surface treatment processes offer new opportunities for surface engineering and for the fabrication of both microsystems and of new types of nanostructures. To successfully tackle these and other challenges, engineers and scientists need a sound education in the basics of corrosion, electrochemistry and surface chemistry of metals.

The present book is the result of courses that the author has been teaching for many years, mostly to students of materials science and chemical engineering. The original French edition of the book was well received, requiring a second and third printing. For the present English edition the content has been enriched and updated, taking into account recent scientific and technical developments. Among these changes, more space has been given to advanced methods for the characterization of surfaces and interfaces, some sections such as those dealing with dealloying, pitting and tribocorrosion have been rewritten, and a new section on the prevention of microbially influenced corrosion has been added.
This being an introductory textbook, the understanding of principles, rather than technological detail, is emphasized throughout. A careful presentation of the electrochemistry of metals and oxide films is given, and the fundamentals and methods of surface chemistry relevant for corrosion are introduced. Whenever possible, simple quantitative models are used to promote the physical understanding of corrosion and protection phenomena. Because students often experience difficulties in bridging theoretical notions and engineering applications, the practical relevance of theoretical concepts is repeatedly stressed and an entire chapter is dedicated to the presentation of commonly used approaches to corrosion control. Thus the book provides a comprehensive introduction to the principles and methods of modern corrosion science and engineering.

The corrosion of metals

Corrosion, from the Latin corrodere, means ‘‘to chew away’’, ‘‘to attack’’. It is estimated that corrosion destroys one quarter of the world’s annual steel production, which corresponds to about 150 million tons per year, or 5 tons per second. Of course, corrosion is not limited to steel but affects all materials: metals, polymers and ceramics. It is the result of chemical and/or physical interactions between the material and its environment. Examples of the corrosion phenomena include:

• transformation of steel into rust;
• cracking of brass in the presence of ammonia;
• oxidation of an electrical contact made of copper;
• weakening of high-resistance steal by hydrogen;
• hot corrosion of a super-alloy in a gas turbine;
• swelling of PVC in contact with a solvent;
• chemical attack of a nylon tube by an oxidizing acid;
• alkaline attack on refractory bricks;
• chemical attack of mineral glass by an alkaline solution.

Metals differ from other materials by a number of favorable properties: ductility, high tensile strength, temperature resistance, electrical and thermal conductivity and ease of joining and machining. Critical elements of machines, airplanes, cars, electrical power plants, precision instruments, civil engineering structures and chemical plants are normally made of metal. Electronic components and devices also contain numerous metallic elements to provide electrical connections. Quite generally, the durability and life time of installations, machines and devices is critically dependent on their corrosion and wear resistance. Because of their unique position among engineering materials the study of the corrosion and protection of metals is an important part of materials science and engineering.

Interestingly, most metals and alloys are not thermodynamically stable in contact with the atmosphere and with water and they should spontaneously corrode. Fortunately, for most applications the rate of corrosion can be kept sufficiently small by using adequate preventive measures (choice of materials, surface treatment, electrochemical protection, etc.). In this way metallic objects can satisfactorily fulfill their function over their projected lifetime.

Some definitions

From the point of view of the construction engineer corrosion is damaging: it destroys a material or degrades its functional properties, rendering it unsuitable for the intended use. Corrosion damage is the degradation of a material or of its functional properties through a chemical reaction with the environment. Materials can also be damaged by wear, which results from rubbing between solid surfaces or from impingement of fluids or of solid particles. Wear causes a progressive loss of material from a surface by mechanical mechanisms, but chemical interactions between the material and its environment often slow or accelerate the damage. The study of corrosion and protection of metals must also include degradations that arise as a result of combined mechanical and chemical effects.

Sometimes, corrosion is a welcome, even desirable, phenomenon. For example, corrosion destroys metallic objects abandoned in nature and thus eliminates them. Corrosion reactions are also used in industrial manufacturing. A well-known example istheanodizingofaluminumwhereoneappliesananodicvoltagetothemetalina suitable electrolyte. Anodizing reinforces the natural oxide film at the surface and thus provides improved corrosion resistance and sometimes also a decorative effect. Similarly, in chemical and electrochemical polishing corrosion reactions are used to produce a smooth surface finish. We can therefore define corrosion in a general way as follows: Corrosion is an irreversible interfacial reaction of a material with its environment, resulting in the loss of material or in the dissolving of one of the constituents of the environment into the material. This definition includes both the positive and negative effects of corrosion. It also includes material damage due to the absorption of a constituent of the environment, such as hydrogen absorption into steel, which causes embrittlement and thus impairs the mechanical properties of the material.

The economic importance of the corrosion

Corrosion affects all areas of the economy, from the integrated circuit to the bridge made of reinforced concrete. The cost of corrosion has been estimated to represent 4% of the gross national product. Even for a small country like Switzerland, this number represents several billion Euros per year. These numbers include:

• direct losses: replacement of corroded materials and of equipment ruined by corrosion;
• indirect losses: cost of repair and loss of production;
• costofcorrosionprotection:useofmoreexpensivecorrosion-resistantmaterials,application of surface coatings, cathodic protection systems ;
• cost of corrosion prevention: maintenance, inspections, corrosion prevention by design.

The direct losses represent only part of the total costs of corrosion and are, in fact, often inferior to the indirect costs. If a nuclear power plant, which represents a significant capital investment, has to be stopped for repair or replacement of a corroded heat exchanger, the price of the part is insignificant relative to the cost of the lost production time. Similarly, in order to replace a corroded hot water pipe, buried in the wall of a building, the cost of the repair will usually largely exceed the price of the pipe itself.

The many different types of expenses involved make estimates of the total cost of corrosion difficult and uncertain. There is no doubt, however, that the amounts of money involved are quite elevated. Another important aspect of corrosion concerns safety. Corrosion damage can impair the safe operation of installations or machines and be at the origin of severe accidents and the loss of human life. Furthermore, corrosion is a waste of raw materials and, indirectly, of energy.

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