Corrosion corrosivity

Symbol Corrosion Corrosion Corrosion Exclamation mark No symbol... 

Aluminum and aluminum alloys are employed in many appHcations because of the abiHty to resist corrosion. Corrosion resistance is attributable to the tightly adherent, protective oxide film present on the surface of the products. This film is 5 —10 nm thick when formed in air if dismpted it begins to form immediately in most environments. The weathering characteristics of several common aluminum alloy sheet products used for architectural appHcations are shown in Eigure 30. The loss in strength as a result of atmospheric weathering and corrosion is small, and the rate decreases with time. The amount of... 

Cooling System Corrosion Corrosion can be defined as the destmction of a metal by chemical or electrochemical reaction with its environment. In cooling systems, corrosion causes two basic problems. The first and most obvious is the failure of equipment with the resultant cost of replacement and plant downtime. The second is decreased plant efficiency to loss of heat transfer, the result of heat exchanger fouling caused by the accumulation of corrosion products. 

An important function of many coatings is to protect metals, especially steel, against corrosion. Corrosion protection is required ia two different situations ia one case, the steel is protected against corrosion with iatact coating films ia the other case, the objective is to protect the steel against corrosion even when the film has been mptured. 

Corrosivity. Corrosivity is an important factor in the economics of distillation. Corrosion rates increase rapidly with temperature, and in distillation the separation is made at boiling temperatures. The boiling temperatures may require distillation equipment of expensive materials of constmction however, some of these corrosion-resistant materials are difficult to fabricate. For some materials, eg, ceramics (qv), random packings may be specified, and this has been a classical appHcation of packings for highly corrosive services. On the other hand, the extensive surface areas of metal packings may make these more susceptible to corrosion than plates. Again, cost may be the final arbiter (see Corrosion and corrosion control). 

Attack associated with nonuniformity of the aqueous environments at a surface is called concentration cell corrosion. Corrosion occurs when the environment near the metal surface differs from region to region. These differences create anodes and cathodes (regions differing in electrochemical potential). Local-action corrosion cells are established, and anodic areas lose metal by corrosion. Shielded areas are particularly susceptible to attack, as they often act as anodes (Fig. 2.1). Differences in concentration of dissolved ions such as hydrogen, oxygen, chloride, sulfate, etc. eventually develop between shielded and nearby regions. 

The state of the art in 1987 was published in Survey of standards, technical rules and regulations in the area of corrosion, corrosion testing and corrosion protection, together with the most important DIN standards and some other regulations in the DIN 219 [71]. A commentary on these standards has also appeared [72]. 

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. 

Beize,/. corrosive corrosion (Dyeing) mordant disinfectant disinfection (Med.) caustic, cauterisation (Leather) bate, drench (Metal.) pickle etching staining stain, beizempfladlich, a. sensitive to corrosion, mordanting, etc. (see beizen). beizen, r.2. corrode mordant cauterize steep bate, drench (hides) etch pickle blanch (metals) disinfect stain sauce (tobacco) ... 

Carbon dioxide dissolves in water to form a weak acid (carbonic acid), which reduces the pH of the solution and, consequently, increases its corrosivity. Corrosion caused by carbon dioxide is generally referred to as sweet corrosion, and results in pitting. The mechanism of carbon dioxide corrosion is as follows [197,198] ... 

Impedance spectroscopy This technique is essentially the extension of polarization resistance measurements into low-conductivity environments, including those listed above. The technique can also be used to monitor atmospheric corrosion, corrosion under thin films of condensed liquid and the breakdown of protective paint coatings. Additionally, the method provides mechanistic data concerning the corrosion processes, which are taking place. 

Ulanovskii, I. B., Conditions for Cathodic Protection of Stainless Steels in Crevices , Zashchita Metal, 1, 643 (1965) C.A., 64, 10751 Holmes, D. R. and Mann, G. M. W., A Critical Survey of Possible Factors Contributing to Internal Boiler Corrosion , Corrosion, 21, 370 (1965)... 

Lefrancois, P. A. and Hoyt, W. B., Chemical Thermodynamics of High Temperature Reactions in Metal Testing Corrosion , Corrosion, 19, 360t (1963)... 

Greene, N. D. and Judd, G., Relation Between Anodic Dissolution and Resistance to Pitting Corrosion , Corrosion, 21, 15 (1965)... 

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