Natural, water, aqueous corrosion

Aqueous environments will range from very thin condensed films of moisture to bulk solutions, and will include natural environments such as the atmosphere, natural waters, soils, body fluids, etc. as well as chemicals and food products. However, since environments are dealt with fully in Chapter 2, this discussion will be confined to simple chemical solutions, whose behaviour can be more readily interpreted in terms of fundamental physicochemical principles, and additional factors will have to be considered in interpreting the behaviour of metals in more complex environments. For example, iron will corrode rapidly in oxygenated water, but only very slowly when oxygen is absent however, in an anaerobic water containing sulphate-reducing bacteria, rapid corrosion occurs, and the mechanism of the process clearly involves the specific action of the bacteria see Section 2.6). 

In principle, cathodic protection can be applied to all the so-called engineering metals. In practice, it is most commonly used to protect ferrous materials and predominantly carbon steel. It is possible to apply cathodic protection in most aqueous corrosive environments, although its use is largely restricted to natural near-neutral environments (soils, sands and waters, each with air access). Thus, although the general principles outlined here apply to virtually all metals in aqueous environments, it is appropriate that the emphasis, and the illustrations, relate to steel in aerated natural environments. 

Whilst cathodic protection can be used to protect most metals from aqueous corrosion, it is most commonly applied to carbon steel in natural environments (waters, soils and sands). In a cathodic protection system the sacrificial anode must be more electronegative than the structure. There is, therefore, a limited range of suitable materials available to protect carbon steel. The range is further restricted by the fact that the most electronegative metals (Li, Na and K) corrode extremely rapidly in aqueous environments. Thus, only magnesium, aluminium and zinc are viable possibilities. These metals form the basis of the three generic types of sacrificial anode. 

Aluminum pistons in an engine that bums H2 will be exposed to not only H2 but also H2O at temperatures of 80 to 120°C. Aluminum alloys can be totally immune to H2 embrittlement and H2-induced crack growth if the natural AI2O3 oxide is intact. However, there are processes that can disrupt this film, and it is known that aluminum alloys will absorb H2 when exposed to H2O vapor at 70°C. There will also be periods when the engine is cool and condensed water will be present so that aqueous corrosion could occur, but this is not expected to be any different than with an engine with cast aluminum pistons that bums gasoline. 

Corrosion of most common engineering materials at near-ambient temperatures occurs in aqueous (water-containing) environments and is electrochemical in nature. The aqueous environment is also referred to as the electrolyte, and, in the case of underground corrosion, it is moist soil. Corrosion is a common form of structure degradation that reduces both the static and cyclic strength of a pipeline. There is always the chance that pipelines could leak or rupture, and a pipeline failure can cause serious human, environmental, and financial losses [3-5]. 

For natural waters and hydrometeors, reaction (5.81) is the key formation process of hydrated electrons. However, little is known about the rate of formation of superoxides in natural aqueous solution. It is clear that different photosensitizers (and mixtures of them) provide a large variety of photocatalytic oxygen activation. The formation of hydrated electrons can explain many processes such as autoxidation and corrosion. This looks at first confusing because Q q works as a reducing species but the key oxidizing species in solution is the OH radical (a strong electron acceptor similar to the atmospheric gas phase), which is produced in a chain of electron transfer processes ... 

Since aqueous corrosion is often electrochemical in nature, the reactions that lead to loss of metal via dissolution require a cathodic counterpart, which is often either hydrogen evolution or oxygen reduction [3]. Hydrogen evolution involves the reduction of a proton (or a hydronium ion, i.e., a proton within a water molecule) to form molecular H2 ... 

Aqueous media corrosion. Natural water is widely distributed and stored in steel pipe, galvanized steel pipe, and steel tanks. Natural waters, so long as they are reasonably free from aggressive ions, such as chloride and acidic species, are noncorrosive and have been handled satisfactorily by mild steel pipes and tanks for many years. The primary impurities in these waters are calcium and magnesium salts. These salts can form a hard carbonate protective scale on the surface of steel exposed to hard water. Chemically pure, distilled water is, in fact, corrosive, and when the concentration of these salts is low, the corrosion of steel must be controlled by reducing the oxygen present in the water by chemical treatment or by cathodic protection. 

Chapters 1 to 3) contains fundamental principles governing aqueous corrosion and high-temperature corrosion and covers the main environments causing corrosion such as atmospheric, natural waters, seawater, soils, concrete, as well as microbial and biofouling environments. 

This review of the use and properties of corrosion inhibitors in aqueous solutions illustrates some common inhibitor environment interactions. A process should be analyzed carefully and some tests made before a large-scale program of corrosion inhibition is initiated. When working with natural water, special attention must be given to its composition, particularly in regard to possibilities of natural inhibition and the presence of interfering ions. 

The second approach, changing the environment, is a widely used, practical method of preventing corrosion. In aqueous systems, there are three ways to effect a change in environment to inhibit corrosion (/) form a protective film of calcium carbonate on the metal surface using the natural calcium and alkalinity in the water, (2) remove the corrosive oxygen from the water, either by mechanical or chemical deaeration, and (3) add corrosion inhibitors. 

Chemical Reactivity - Reactivity with Water Reacts slowly to form flammable hydrogen gas, which can accumulate in closed area Reactivity with Common Materials Corrosive to natural rubber, some synthetic rubbers, some greases and some lubricants Stability During Transport Stable Neutralizing Agents for Acids and Caustics Flush with 3% aqueous ammonia solution, then with water. Methyl alcohol may also be used Polymerization Not pertinent Inhibitor of Polymerization Not pertinent. 

Nature of the environment This is usually water, an aqueous solution or a two- (or more) component system in which water is one component. Inhibitors are, however, sometimes required for non-aqueous liquid systems. These include pure organic liquids (Al in chlorinated hydrocarbons) various oils and greases and liquid metals (Mg, Zr and Ti have been added to liquid Bi to prevent mild steel corrosion by the latter ). An unusual case of inhibition is the addition of NO to N2O4 to prevent the stress-corrosion cracking of Ti-6A1-4V fuel tanks when the N2O4 is pressurised... 

The need for temperature cycling should be taken into account when designing or conducting tests. The nature of the test vessel should be considered for tests in aqueous solutions at temperatures above about 60°C since soluble constituents of the test vessel material can inhibit or accelerate the corrosion process. An inhibiting effect of soluble species from glass, notably silica, on the behaviour of steel in hot water has been shown . Pure quartz or polymeric materials are often more appropriate for test vessel construction. 

The precautions generally applicable to the preparation, exposure, cleaning and assessment of metal test specimens in tests in other environments will also apply in the case of field tests in the soil, but there will be additional precautions because of the nature of this environment. Whereas in the case of aqueous, particularly sea-water, and atmospheric environments the physical and chemical characteristics will be reasonably constant over distances covering individual test sites, this will not necessarily be the case in soils, which will almost inevitably be of a less homogeneous nature. The principal factors responsible for the corrosive nature of soils are the presence of bacteria, the chemistry (pH and salt content), the redox potential, electrical resistance, stray currents and the formation of concentration cells. Several of these factors are interrelated. 

For all these reasons, the stability of the superconducting state and ways to control it are questions of prime importance. Many studies have addressed the degradation of the properties of HTSC under the influence of a variety of factors. They included more particularly the corrosion resistance of HTSC materials exposed to aqueous and nonaqueous electrolyte solutions as well as to water vapor and the vapors of other solvents. It was seen that the corrosion resistance depends strongly both on the nature (chemical composition, structure, etc.) of the HTSC materials themselves and on the nature of the aggressive medium. 

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