Atmospheric corrosion field exposures

Also nickel, often used in decorative Ni-Cr coatings, is a metal sensitive to the influence of sulphur compounds in the atmosphere. This may be illustrated by results from a field exposure giving the corrosion rate of 0.4 um/year in rural and 2.7 /jm/year in urban atmospheres (U). In other investigations as high corrosion rates as 6 yum/year has been found in industrial atmospheres (1 ). Also the life of decorative Ni-Cr coatings is substantially shorter in urban/industrial than in rural areas (13). 

The corrosion of metals exposed to the atmosphere is known to be caused by a mixture of natural and anthropogenic factors. To apportion the cause of metal corrosion, one may conduct controlled laboratory experiments or well designed field exposure experiments. 

In order to fully appreciate the reasons for carrying out the conservation method selected, it is important to understand in the first instance how the metal or alloy was manufactured. From modern theories of corrosion of metals in marine environments, it is possible to predict the mode of corrosive attack that the artefact may have experienced while being buried or laying on the bottom of the ocean floor. Any adverse effect on the rate of corrosion on exposure to the atmosphere can possibly be predicted. From this knowledge, the most efficient methods of field treatments, storage conditions and conservation can be recommended. 

Each metal behaves in a unique way with respect to atmospheric corrosion properties, and the conclusions drawn from the nickel study cannot necessarily be drawn for other metals. However, if the same or similar corrosion products are formed on a given metal when exposed to a laboratory and a natural atmospheric environment, respectively, the results surest that the same corrosion processes are operating in both exposures. Table 4 displays examples of reported laboratory tests that have generated corrosion products similar to those seen in natural field exposures [13-18]. It appears that certain combinations of two or three corrodents at concentrations below 1 ppmv, together with a proper choice of relative humidity and airflow rate, can generate the corrosion products that are formed in natural field environments. 

A useful parameter is the dry deposition velocity, which is defined as the ratio of deposition rate or surface flux per time unit of any gaseous compound and the concentration of the same compound in the atmosphere [46]. The concept of dry deposition velocity of SO2 and its relevance to atmospheric corrosion rates is well established [47]. By examining data based on both field and laboratory exposures, it can be concluded that the factors controlling dry deposition fall into aerodynamic processes and surface processes. Aerodynamic processes are connected with the actual depletion of the gaseous constituent (e.g., SO2) in the atmospheric region next to the aqueous phase and the ability of the system to mix new SO2 into this region. This ability depends on, for instance, the actual wind speed, type of wind flow, and shape of sample. Surface processes, on the other hand, are connected with the ability of the aqueous layer to accommodate SO2. This ability increases with the thickness of the aqueous layer and, hence, with the relative humidity, the pH of the solution (as discussed earlier), and the alkalinity of the solid surface. 

Access to new and more sensitive analytical techniques has resulted in substantial progress in the characterization of corrosion products formed under both laboratory and field exposure conditions. These techniques permit the determination of, e.g., thickness, chemical composition, and atomic structure of corrosion products formed at both early and later stages of exposure. When combined with environmental data, such as deposition rates of corrosion-stimulating atmospheric constituents, relative humidity, temperature, and sunshine hours, the new techniques have resulted in a more comprehensive understanding of the complex processes that govern atmospheric corrosion. In a series of papers, Graedel has summarized the corrosion mechanisms of zinc [62], aluminum [63], copper [18], iron and low-alloy steel [64], and silver [19]. It is beyond the scope of this chapter to provide... 

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. 

Accelerated tests do not precisely predict long-term corrosion behavior however, answers are needed quickly in the development of new materials. For this resison, accelerated tests are used to screen candidate alloys before conducting atmospheric exposures or other field tests. They also are used for production control of exfoliation-resistant heat treatments for the AA2XXX, AA5XXX, and AA7XXX aluminum alloys. ASTM has standardized several laboratory tests for susceptibility to exfoliation corrosion in recent years. 

It should be noted that creep- and stress-rupture-test data are obtained pnder atmospheric exposure additions in laboratories and under uniaxial loading. The stress condition existing in a vessel part under field service conditions Usually comprises stresses in three directions, a fact which complicates the application of the experimental data. In addition, the vessel material may be exposed to a corrosive atmosphere and be subject to scaling, hydrogen embritlle-menl, intergranular corrosion, and strain hardening. 

The incorporation of atmospheric species into the aqueous layer may occur through either dry or wet deposition. In dry deposition there is no involvement of any precipitation, whereas wet deposition requires, e.g., rain, dew, fog, or snow for atmospheric pollutants to deposit. Indoors or in highly polluted areas close to emission sources, dry deposition is considered to be dominating but the relative importance of wet deposition may be difficult to establish because of the incidental nature of precipitation. Controlled field studies combined with extensive laboratory exposures have been undertaken within the National Acid Precipitation Assessment Program (NAPAP) to explore the relative contribution of wet and dry deposition to increased corrosion rates of a number of metals [45]. 

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