Corrosion of metal

Metal corrosion is a physical-chemical interaction between a metal and a medium leading to degradation of service properties of the metal and the medium or a technical system they constitute. A chemical reaction between a metal and a medium, or their components, at their interface lies at the base of metal corrosion. 

The mechanism for metal corrosion depends, first of all, on the type of hostile medium. Gas corrosion occurs in metal contact with an active gas. A layer of solid corrosion products (scale) is formed on the metalware surface in dry oxidative gases at elevated temperatures. Metal corrosion in electrolyte solutions, even if the solution is in the form of a thin film on a metal surface, follows the reaction 

Electrochemical corrosion of metals follows the scheme indicated in Fig. 1.1 in two reactions, i.e. anodic dissolution of the metal and cathodic reduction 

These reactions are totally independent but interrelated through electric neutrality of the metal-electrolyte solution system. 

Metal corrosion is a spontaneous process accompanied by a decrease in Gibbs energy in the corrosion system. The Gibbs energy varies in metal corrosion in a similar way to in any other spontaneous chemical reaction. Thermodynamic instability of the metal in contact with an electrolyte solution is judged from the magnitude of its electrode potential. Let us consider its mechanism. 

Metallic corrosion is a major engineering and economic problem. In North America and Europe, the cost of corrosion is on the order of 3-4% of gross domestic product (GDP). Not surprisingly, there is an abundance of good books on this topic. This chapter focuses mainly on corrosion of metals in aqueous systems, as this is the most commonly encountered problem and is a natural corollary of the material of the two preceding chapters. 

We saw in Section 5.6 that the dry oxidation of metals by oxygen or air can be viewed as an electrochemical process in which the electrolyte of the cell is the developing solid oxide layer itself. If liquid water is present, diffusion of the ions and molecules involved in the electrochemical corrosion process is greatly facilitated, and consequently aqueous corrosion of metals is much more important than dry oxidation at near-ambient temperatures. Although most corrosion problems encountered in practice involve only a single metal, aqueous electrochemical corrosion can be especially severe, and its principles most clearly illustrated, in cases where two different metals are in electrical contact with one another. 

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Wlodarczyk S., Dybiec Cz., Wlodarczyk W, The application of eddy currents and other techniques for measuring corrosion of metal tanks during exploitation. Materials CCl National Conference about Science and practice in fighting against corrossion , Kule May 1994. 

Zsklarska-Smialowska Z 1986 Pitting Corrosion of Metals (Flouston, TX National Association of Corrosion Engineers)... 

Tousek J 1985 Theoretical Aspects of the Localized Corrosion of Metals (Rockport, MA TransTech) Boehni H 1987 Corrosion Mechanisms ed F Mansfeld (New York Dekker)... 

Mrowec S and Werber T 1978 Gas Corrosion of Metals (Washington, DC National Bureau of Standards) Rapp R A (ed) 1983 High Temperature Corrosion—NACE 6 (Flouston, TX NACE)... 

Vapors emitted from the materials of closed storage and exhibit cases have been a frequent source of pollution problems. Oak wood, which in the past was often used for the constmction of such cases, emits a significant amount of organic acid vapors, including formic and acetic acids, which have caused corrosion of metal objects, as well as shell and mineral specimens in natural history collections. Plywood and particle board, especially those with a urea—formaldehyde adhesive, similarly often emit appreciable amounts of corrosive vapors. Sealing of these materials has proven to be not sufficiently rehable to prevent the problem, and generally thek use for these purposes is not considered acceptable practice. 

Calcium carbonate (calcite) scale formation in hard water can be prevented by the addition of a small amount of soluble polyphosphate in a process known as threshold treatment. The polyphosphate sorbs to the face of the calcite nuclei and further growth is blocked. Polyphosphates can also inhibit the corrosion of metals by the sorption of the phosphate onto a thin calcite film that deposits onto the metal surface. When the polyphosphate is present, a protective anodic polarization results. 

Cla.riGers. Pool water may occasionally contain metallic impurities such as copper, iron, or manganese which enter the pool with the makeup water or by corrosion of metallic parts in the circulation system. These dissolved metals can discolor the water and cause stains. Chlorine oxidizes soluble Fe and to the highly insoluble Fe(OH)3 and MnO which can be removed by filtration. Water-soluble, high molecular weight polymers can be... 

Treatment with sulfuric acid and fractional distillation are the main methods used to purify bromine. It is especially important to reduce the water content to less than 30 ppm to prevent corrosion of metal transportation and storage containers. 

ACI Committee 222, Corrosion of Metal in Concrete, ACI 222R-85 (89), American Concrete Institute, Detroit, Mich., 1989, 30 pp. 

In the presence of oxygen and water the oxides of most metals are more thermodynamically stable than the elemental form of the metal. Therefore, with the exception of gold, the only metal which is thermodynamically stable in the presence of oxygen, there is always a thermodynamic driving force for corrosion of metals. Most metals, however, exhibit some tendency to passivate, ie, to form a protective oxide film on the surface which retards further corrosion. 

The thermodynamic data pertinent to the corrosion of metals in aqueous media have been systematically assembled in a form that has become known as Pourbaix diagrams (11). The data include the potential and pH dependence of metal, metal oxide, and metal hydroxide reactions and, in some cases, complex ions. The potential and pH dependence of the hydrogen and oxygen reactions are also suppHed because these are the common corrosion cathodic reactions. The Pourbaix diagram for the iron—water system is given as Figure 1. 

Chlorides, which are ubiquitous in nature, play an important role in the corrosion of metals. Chlorides and other anions also play an important role in locali2ed corrosion, ie, the breakdown of the insoluble protective reaction product films, eg, passive films, that prevent corrosion of the underlying metal. A variety of mechanisms attempting to explain the role of chloride in general and in locali2ed corrosion have been proposed (23—25). 

Inhibitors The use of various substances or inhibitors as additives to corrosive environments to decrease corrosion of metals in the environment is an important means of combating corrosion. This is generally most attractive in closed or recirculating systems in which the annual cost of inhibitor is low. However, it has also proved to be economicaUv attrac tive for many once-through systems, such as those encountered in petroleum-processing operations. Inhibitors are effective as the result of their controlling influence on the cathode- or anode-area reactions. 

Two methods of providing cathodic protection for minimizing corrosion of metals are in use today. These are the sacrificial-anode method and the impressed-emf method. Both depend upon making the metal to be protected the cathode in the electrolyte involved. 

Graphitically corroded cast irons may induce galvanic corrosion of metals to which they are coupled due to the nobility of the iron oxide and graphite surface. For example, cast iron or cast steel replacement pump impellers may corrode rapidly due to the galvanic couple established with the graphitically corroded cast iron pump casing. In this or similar situations, the entire affected component should be replaced. If just one part is replaced, it should be with a material that will resist galvanic corrosion, such as austenitic cast iron. 

Logan, H. L., The Stress Corrosion of Metals, National Bureau of Standards, Washington, D.C., John Wiley and Sons, Inc., New York, 1966. 

The discovery and use of metals at the end of the Stone Age was one of the most important steps in the development of modem technology. Most base metals are, unfortunately, not stable. In unfavorable environments they can be destroyed at variable rates by corrosion. The study of such corrosion reactions and the methods by which corrosion of metals can be fought is a task of great economic significance. 

The processes of cathodic protection can be scientifically explained far more concisely than many other protective systems. Corrosion of metals in aqueous solutions or in the soil is principally an electrolytic process controlled by an electric tension, i.e., the potential of a metal in an electrolytic solution. According to the laws of electrochemistry, the reaction tendency and the rate of reaction will decrease with reducing potential. Although these relationships have been known for more than a century and although cathodic protection has been practiced in isolated cases for a long time, it required an extended period for its technical application on a wider scale. This may have been because cathodic protection used to appear curious and strange, and the electrical engineering requirements hindered its practical application. The practice of cathodic protection is indeed more complex than its theoretical base. 

Three factors influence the rate of corrosion of metals—moisture, type of pollutant, and temperature. A study by Hudson (1) confirms these three factors. Steel samples were exposed for 1 year at 20 locations throughout the world. Samples at dry or cold locations had the lowest rate of corrosion, samples in the tropics and marine environments were intermediate, and samples in polluted industrial locations had the highest rate of corrosion. Corrosion values at an industrial site in England were 100 times higher than those found in an arid African location. 

The role of moisture in corrosion of metals and other surfaces is twofold surface wetness acts as a solvent for containments and for metals is a medium for electrolysis. The presence of sulfate and chloride ions acceler-... 

Corrosion of metals by fuel ashes only occurs where the fuel ash contains a liquid phase. Temperatures at which the first liquid will form are inversely proportional to the oxygen partial pressure. Thus, when firing fuels at high excess air ratios, fuel ash corrosion occurs at lower temperatures than when firing fuels with low excess air ratios. 

Fuel Asb Corrosion Control. A variety of methods have been used to control fuel ash corrosion of metallic members in process furnaces, utility boilers, and other combustion equipment. Among these are ... 

Chemical Reactivity - Reactivity with Water Reacts vigorously to form toxic hydrogen fluoride (hydrofluoric acid) Reactivity with Common Materials When moisture is present, causes severe corrosion of metals (except steel) and glass. If confined and wet can cause explosion. May cause fire in contact with combustible material Stability During Transport Stable Neutralizing Agents for Acids and Caustics Flush with water, rinse with sodium bicarbonate or lime solution Polymerization Not pertinent Inhibitor of Polymerization Not pertinent. 

Chemical Reactivity - Reactivity with Water Slow reaction with water to produce hydrochloric acid fumes. The reaction is more rapid with steam Reactivity with Common Materials Slow corrosion of metals but no immediate danger Stability During Transport Not pertinent Neutralizing Agents for Acids and Caustics Soda ash and water, lime Polymerization Does not occur Inhibitor of Polymerization Not pertinent. 

In addition to the formation of scale or corrosion of metal within boilers, auxiliary equipment is also susceptible to similar damage. Attempts to prevent scale formation within a boiler can lead to makeup line deposits if the treatment chemicals are improperly ehosen. Thus, the addition of normal phosphates to an unsoftened feed water ean eause a dangerous eondition by elogging the makeup line with preeipitated calcium phosphate. Deposits in the form of calcium or magnesium stearate deposits, otherwise known as "bathtub ring" can be readily seen, and are caused by the eombination of ealcium or magnesium with negative ions of soap stearates. 

Building durability, by preventing decay of wood-based materials, corrosion of metals, and spalling of masonry and concrete caused by freeze-thaw cycles... 

Corrosion of metal reinforcement bars has caused concrete to fall off the corners of cooling towers. 

Nonstoichiometric oxide phases are of great importance in semiconductor devices, in heterogeneous catalysis and in understanding photoelectric, thermoelectric, magnetic and diffusional properties of solids. They have been used in thermistors, photoelectric cells, rectifiers, transistors, phosphors, luminescent materials and computer components (ferrites, etc.). They are cmcially implicated in reactions at electrode surfaces, the performance of batteries, the tarnishing and corrosion of metals, and many other reactions of significance in catalysis. ... 

Rost-feuer, n. grate fire, -flache, /. grate surface. -fleck, m. rust spot, iron spot, -flecken-wasser, n. rust stain remover, rostfleckig, a. rust-spotted, rust-stained. Rostfrass, m. corrosion (of metals), rostfrei, a..rust-free stainless. 

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