Water in atmospheric corrosion

The unfortunate action of the compound layer is observed only rarely, usually in hot water. In cooking vessels (domestic or industrial) the copper is protected satisfactorily at some sacrifice of tin, and occasional re-tinning ensures long service. In atmospheric corrosion the arrival of compounds at the surface of the coating results in some darkening and in loss of solderability. 

Materials such as metals, alloys, steels and plastics form the theme of the fourth chapter. The behavior and use of cast irons, low alloy carbon steels and their application in atmospheric corrosion, fresh waters, seawater and soils are presented. This is followed by a discussion of stainless steels, martensitic steels and duplex steels and their behavior in various media. Aluminum and its alloys and their corrosion behavior in acids, fresh water, seawater, outdoor atmospheres and soils, copper and its alloys and their corrosion resistance in various media, nickel and its alloys and their corrosion behavior in various industrial environments, titanium and its alloys and their performance in various chemical environments, cobalt alloys and their applications, corrosion behavior of lead and its alloys, magnesium and its alloys together with their corrosion behavior, zinc and its alloys, along with their corrosion behavior, zirconium, its alloys and their corrosion behavior, tin and tin plate with their applications in atmospheric corrosion are discussed. The final part of the chapter concerns refractories and ceramics and polymeric materials and their application in various corrosive media. 

This discussion of the solid phase on which the water is adsorbed has brought us to a conclusion which is important for our subsequent exeunination of the aqueous adsorbed phase. The water which is not (irreversibly) decomposed is adsorbed on an oxyhydroxide whose exact nature has only a minor effect on the adsorption phenomena. There is substantial literature on the properties of adsorbents on this type of surface, A review of this literature allows us to propose the following generalizations concerning the aqueous phase in atmospheric corrosion. 

The atmospheric region closest to Earth, the troposphere, comprises more than two thousand chemical species - either as gases or as parts of atmospheric particles. More than 99.99% by weight of all gases consist of N2, O2 and rare gases (Ne, Kr, He, and Xe). Among these, only O2 is of importance in atmospheric corrosion because of its role as an electron acceptor in cathodic reactions and its involvement in chemical transformations of the atmosphere. Other constituents of importance in atmospheric corrosion, but of lower concentration, include H2O and CO2. The important role of H2O is evident from previous sections. Its concentration varies from around 100 ppmv (parts per million per volume) to around 10 000 ppmv. CO2 is soluble in water and contributes to the acidification of the aqueous phase by forming the carbonate ion (COs ). 

The droplet cell. Fig. 2(d), has uniform current distribution and shrunken dimensions that allow resistive electrolytes to be used [5]. This approach was developed for the use of pure water as an electrolyte as a means to mimic atmospheric corrosion, but it can be used with any electrolyte. An area of a flat sample is exposed through a hole in a piece of protective tape. Electroplater s tape is a very resistant tape with good adhesion that is useful for this and other masking applications in corrosion. If the hole in the tape is made with a round punch, the same punch can be used to make circular dots from pieces of filter paper. One such dot is placed securely into the exposed hole. A small (typically 10-20 gl) droplet of soluhon is placed on the filter paper using a calibrated pipette. This wet filter paper acts as the electrolyte. A piece of woven Pt mesh is placed on top of the wet filter paper, and a reference electrode is held against the back of the Pt counterelectrode. As mentioned, the small dimensions allow the use of even very pure water. This simulates atmospheric corrosion, in which a thin water layer forms on the surface. As in atmospheric corrosion, soluble species on the sample surface and pollutant gases in the air are dissolved into the water droplet, which provides some conductivity. This technique has been used... 

The most frequent causes of heterogeneity in atmospheric corrosion are those related to water that may not be present on the surface in the form of a continuous film, but in a heterogeneous way, and those inherent to metallic materials underneath the water layer. 

In atmospheric corrosion testing, it is customary to perform tests in special climate chambers in addition to field tests. The tests are used for comparison but are also valuable for determining the behavior of anticorrosive films and coatings. The conditions used to obtain the appropriate atmospheres, constant or alternating condensed water climates, with and without the presence of such additional substances as sulfur dioxide and salt spray, and at various pH values, are specified in the standards (DIN 50 018 1978 DIN 50 021 1975 ASTM B 117-85 1985 ASTM G 87-84 1984 ASTM G 91-86 1986 ASTM G 85-85 1985). 

The vapor pressure of a small droplet is higher than that of a liquid with a flat surface. For a liquid located in a pore it is even lower. As a consequence water vapor condenses more easily on a porous surface, such as a metal covered with rust, than on a flat surface. This is of importance in atmospheric corrosion as will be discussed in Chapter 8. 

A particularly reproducible reference is a nickel surface exposed to deionized water before the measurement [87]. In the absence of further calibration, this is the point of reference. The method cannot be applied in the presence of an electrolyte solution because of the large voltages applied to the tip this would cause Faradaic reactions. Data from measurements of Volta potentials at corroding surfaces could be related to corrosion potentials of the same surface in contact with a solution because the linear correlation has been established before [88]. Nevertheless, studies at air or in the presence of ultrathin electrolyte films (i.e., under conditions frequently encountered in atmospheric corrosion) are possible. The general advantages of SKPFM, in particular the greatly enhanced spatial resolution, in comparison with SKP have been discussed in detail [88]. A critical review of applications of SKPFM focused on corrosion science with particular attention to possible artifacts and a comparison with SKP has been provided [89]. 

Oxygen, because of its ability to accept electrons and its involvement in chemical transformations of the atmosphere, is particularly important to atmospheric corrosion. Other materials present in the troposphere that play a part in atmospheric corrosion are water and carbon dioxide. Water acts as an electrolyte and carbon dioxide, which has a concentration of approximately 330 ppm and is highly soluble in water, contributes to the acidity of the aqueous layer. 

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