Deterioration of Metals and Alloys - Corrosion

The environment has negative effects on most metals thus, when metallic archaeological objects are eventually found, they are generally in an advanced state of decay. The decay of metals and alloys caused by the chemical action of gases and/or liquids in the environment is known as corrosion. Corrosion processes are natural destructive processes that result in the waste of most metals and alloys. The ultimate result of all corrosion processes is the reversion of most metals from the metallic condition in which they are used, to the chemically combined form in which they naturally occur in the crust of the earth. Rust, the reddish-brown corrosion product that forms on 

The reaction that takes place during corrosion processes can be expressed by the general equation  

Corrosion is the process of gradual waste and degradation undergone by most metals and alloys exposed to weathering agents in the environment. The products of the process are chemical compounds in which the corroded metals are combined mainly with oxygen but also with other elements or ions, such as sulfur, carbonate, and sulfate. The composition of the corrosion products is often almost identical to that of the metalliferous ores from which the metals are extracted. 

Localized corrosion, another form of metal waste, generally occurs on small, confined areas of metallic bodies. Localized corrosion processes expand mostly by penetrating deep into the bulk of the metal, forming holes and cracks that may endanger the integrity of corroded objects (see Textbox 43). 

Almost all new metallic surfaces exposed to the environment are sooner or later coated with a layer of corrosion products metal oxides, sulfides, and carbonates, for example, are common corrosion products formed when a metal or alloy interacts with contaminants in the environment, if the layer is continuous and stable, as in uniform corrosion, it may conceal the underlying metal from further exposure and protect it from additional corrosion if it is discontinuous, or chemically unstable, however, the metal surface below the initial layer of corrosion products remains in contact with the environment. Exposed to humidity and pollutants, the corrosion process continues, penetrating deeper into the metallic bulk and eventually resulting in its total destruction. 

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FIGURE 39 Corrosion. Corrosion is the process of gradual deterioration of metals and alloys as a result of their interaction with the environment. The corrosion process is a reversal of metallurgical processes, whereby metals are recovered from the minerals in which they occur in nature (a). It is an electrolytic process, brought about by the passage of electric currents. Any metal or alloy contains sites in which there are slight local compositional differences. When such compositional differences are exposed to a humid or wet environment, extremely small electrolytic cells as the one shown in (b) are created in each cell, an electric current drives the otherwise nonspontaneous corrosion reactions. In a surface undergoing corrosion there are millions of electrolytic cells. 

Environmental corrosion and degradation of metals and alloys impose enormous losses in modem industrial societies. In Chapter 140 Hinton considers a wide variety of methods using rare-earth solutions and salts to modify advantageously the costly deterioration of metals and alloys. This topic is expanded by Ryan in Chapter 141, giving particular attention to protection against high-temperature oxidation, sulfidization and hot-salt corrosion. 

Corrosion is the deterioration of a material by reaction with its enviromnent. Although the term is used primarily in conjunction with the deterioration of metals, the broader definition allows it to be used in conjunction with all types of materials. We will limit the description to corrosion of metals and alloys for the moment and will save the degradation of other types of materials, such as polymers, for a later section. In this section, we will see how corrosion is perhaps the clearest example of the battle between thermodynamics and kinetics for determining the likelihood of a given reaction occurring within a specified time period. We will also see how important this process is from an industrial standpoint. For example, a 1995 study showed that metallic corrosion costs the U.S. economy about 300 billion each year and that 30% of this cost could be prevented by using modem corrosion control techniques [9], It is important to understand the mechanisms of corrosion before we can attempt to control it. 

Corrosion is the effect of unwanted chemical reactions on the structures and properties of metals and alloys. For engineers out in the field, corrosion means delays and production problems. Corrosion can be further defined as the deterioration of any material in contact with its surroundings. Therefore, engineers will not only have to deal with corrosion of metals but also to corrosion of plastics, wood, concrete, protective coatings, and roofing. 

Chemical plants are designed and constructed with a variety of metals, alloys, and nonmetallic materials such as plastics. The nature and mechanism of degradation of these materials differ. Corrosion of metals and alloys in aqueous media occurs by electrochemical mechanisms, whereas the degradation of a plastic is by the penetration of chemicals into its matrix. Nevertheless, the term corrosion is used liberally to describe the oxidation of metals in aqueous solutions or gas phases and the deterioration of plastics by chemical attack. 

Velocity Most metals and alloys are protected from corrosion, not by nobility [a metal s inherent resistance to enter into an electrochemical reaction with that environment, e.g., the (intrinsic) inertness of gold to (almost) everything but aqua regia], but by the formation of a protective film on the surface. In the examples of film-forming protective cases, the film has similar, but more limiting, specific assignment of that exemplaiy-type resistance to the exposed environment (not nearly so broad-based as noted in the case of gold). Velocity-accelerated corrosion is the accelerated or increased rate of deterioration or attack on a metal surface because of relative movement between a corrosive fluid and the metal surface, i.e., the instability (velocity sensitivity) of that protective film. 

Corrosion is the deterioration of a material due to interaction with its environment It is the process in which metallic atoms leave the metal or form compounds in the presence of water and gases. Metal atoms are removed from a structural element until it fails, or oxides build up inside a pipe until it is plugged. All metals and alloys are subject to corrosion. Even the noble metals, such as gold, are subject to corrosive attack in some environments. 

Corrosion resistance of stainless steel is reduced in deaerated solutions. This behavior is opposite to the behavior of iron, low-alloy steel, and most nonferrous metals in oxygenated waters. Stainless steels exhibit very low corrosion rates in oxidizing media until the solution oxidizing power becomes great enough to breach the protective oxide locally. The solution pH alone does not control attack (see Chap. 4, Underdeposit Corrosion ). The presence of chloride and other strong depassivating chemicals deteriorates corrosion resistance. 

In the case of non-metallic materials, the term corrosion invariably refers to their-deterioration from chemical causes, but a similar concept is not necessarily applicable to metals. Many authorities consider that the term metallic corrosion embraces all interactions of a metal or alloy (solid or liquid) with its environment, irrespective of whether this is deliberate and beneficial or adventitious and deleterious. Thus this definition of corrosion, which for convenience will be referred to as the transformation definition. 

On the other hand, corrosion has been defined as the undesirable deterioration of a metal or alloy, i.e. an interaction of the metal with its environment that adversely affects those properties of the metal that are to be preserved. This definition —which will be referred to as the deterioration definition —is also applicable to non-metallic materials such as glass, concrete, etc. and embodies the concept that corrosion is always deleterious. However, the restriction of the definition to undesirable chemical reactions of a metal results in anomalies which will become apparent from a consideration of the following examples. 

Corrosion is the deterioration a material undergoes as a result of its interaction with its surroundings. Although this definition is applicable to any type of material, it is usually reserved for metallic alloys. Of the 105 known chemical elements, approximately eighty are metals, and about half of these can be alloyed with other metals, giving rise to more than 40,000 different alloys. Each of the alloys will have different physical, chemical, and mechanical properties, but all of them can corrode to some extent, and in different ways. 

Corrosion of metals is defined as their spontaneous deterioration by chemical interaction with the surrounding environment. It is a two-component system involving the interaction of the metal or alloy with a medium or environment. In the absence of an environment (e.g., vacuum), corrosion will not occur. Most corrosion reactions are electrochemical in natme, and for electrochemical corrosion to occur, a cell consisting of an anode, a cathode, an electrolyte, and a pathway for electron flow between the anode and the cathode is needed. 

In this work, the potential for application of Mossbauer spectrometry to corrosion studies was demonstrated for three accelerated corrosion tests in chloride environments. This technique allowed retrieving maximum information from the inherent properties of the rust layers. With VT-MS, it was possible to identify and determine the relative iron phase abundances from which three parameters could be calculated (i) a, (ii) (A + S)/(A + L + S), and (iii) PAI. Moreover, with the physical properties retrieved from the analysis of the hyperfine parameters, it was possible to discuss prospective mechanisms of formation and therefore to contribute to the understanding of the deterioration progress. These studies showed that some corrosion product is lost and/or the conversion of metallic ions into iron oxides may be incomplete, and that the relative iron phase abundances pointed out to a nonprotective and active type of rusts. More work is required on other types and chemistry (composition and abundance of alloying elements) of steels, other environmental conditions, and different exposure times. More efforts are needed to improve the fitting models for nonstoichiometric and substituted iron oxides in rust layers. 

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