Natural Atmospheric Exposures

The amount of data available in the literature involving long exposure times in hot humidity of metal adherends other than aluminum is also quite limited. Brockmann ) has compared the rate of degradation on steel joints exposed to 95% RH conditions for four months to the same time exposure in 40°C water or the natural climate of North Germany. The environments were decreasing in severity on the joints in the order 40°C >95% RH > the natural atmosphere. Exposure of titanium joints to 95-100% RH was used by Mahoon(i36) to discriminate the relative durability imparted by varying surface pretreatments. 

Alloys having varying degrees of corrosion resistance have been developed in response to various environmental needs. At the lower end of the alloying scale are the low alloy steels. These are kon-base alloys containing from 0.5—3.0 wt % Ni, Cr, Mo, or Cu and controlled amounts of P, N, and S. The exact composition varies with the manufacturer. The corrosion resistance of the alloy is based on the protective nature of the surface film, which in turn is based on the physical and chemical properties of the oxide film. As a rule, this alloying reduces the rate of corrosion by 50% over the fkst few years of atmosphere exposure. Low alloy steels have been used outdoors with protection. 

The test specimens were suspended in five different test tanks. One tank had 1.0 N NaOH, two had 0.25N NaOH and two had 3.5% (by wt.) NaCl solution. One 0.25N NaOH and one 3.5% NaCl solution tank were purged of oxygen with a nitrogen atmosphere (oxygen concentration of less than 3%), the complimentary pair of tanks had natural air exposure (oxygen concentration 20%). A summary of the initial status of each tank is given in Table I. 

The exposure to ionizing radiation from natural sources is continuous and unavoidable. For most individuals, this exposure exceeds that from all human-made sources combined (UNSCEAR 2000a). The two main contributors to natural radiation exposures are high-energy cosmic ray particles incident on the earth s atmosphere and radioactive nuclides that originate in the earth s crust and are present everywhere in the environment, including the human body itself. 

When a piece of jewelry is fabricated, solder is the glue that holds together the individual pieces. After the object is assembled, a surface color may be desired. This surface color, or patina, may develop naturally upon exposure to the atmosphere or may be produced as the result of an anticipated chemical change. Usually, the patina will make the piece of jewelry more attractive than it was originally. 

Media considerations. SCC tests can be divided into those conducted in natural environments, such as atmospheric exposure tests and seawater immersion tests, and those which are conducted under laboratory conditions or other fabricating operations. The principal disadvantage of atmospheric exposure tests is the comparatively long time required for their completion however, they are reliable since they can reflect the projected use. There is a standard practice for evaluating stress-corrosion cracking resistance of metals and alloys by alternate immersion in a solution of NaCl 3.5%, pH 6.5. For spray testing, ASTM B-117, 2003 states the relevant conditions for conducting the test. (ASTM G44)4... 

CO2 concentration. The concentration of carbon dioxide in the atmosphere may vary from 0.03% in rural environments to more than 0.1% in urban environments. Comparatively high concentrations can be reached under specific exposure conditions, such as inside motor vehicle tunnels. As the CO2 content in the air increases, the carbonation rate increases. Accelerated tests carried out in the laboratory to compare the resistance to carbonation in different types of concrete show that, indicatively, one week of exposure to an atmosphere containing 4% CO2 will cause the same penetration of carbonation as a year of exposure to a normal atmosphere [8]. Some researchers suggest that with a high concentration of CO2 the porosity of carbonated concrete is higher than that obtained by exposure to a natural atmosphere, particularly if the concrete has been made with blended cement or has a high cement content However, this is controversial, since it was shown that even 100 % CO2 under increased pressure, produced the same microstructure as natural carbonation [9]. 

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. 

Media Considerations SCC tests are conducted in (i) natural atmospheres (ii) seawater immersion, and (iii) laboratory or other fabrication conditions. Atmospheric exposure tests take a long exposure time, but are reliable as they... 

Other environmental related tests include ASTM D-904, Standard Practice for Exposure of Adhesive Specimens to Artificial (Carbon-Arc Type) and Natural Light ASTM D-1828, Standard Practice for Atmospheric Exposure of Adhesive-Bonded Joints and Structures ASTM D-1879, Standard Practice for Exposure of Adhesive Specimens to High Energy Radiation and ASTM D-3310, Standard Test Methods for Determining Corrosivity of Adhesive Materials. 

From this discussion, it follows that characterisation of the physical and chemical environment of the reactive sites of a particle on a molecular level will be required to better understand its fate in the atmosphere. Exposure experiments should be continued, not only using simplified model systems, but also using natural combustion related aerosols of various particle size and matrix composition. Filter exposures of particulate matter should yield more detailed information on sampling artifacts. Exposure of aerosol particles to gaseous compounds should be tried in a static or... 

Lead is resistant to atmospheric exposures, particularly to industrial atmospheres in which a protective film of lead sulfate forms. Buried underground, the corrosion rate may exceed that of steel in some soils (e.g., those containing organic acids), but in soils high in sulfates the rate is low. Soluble silicates, which are components of many soils and natural waters, also act as effective corrosion inhibitors. 

Upon exposure in the natural atmosphere many corrosion cells are formed on the surface of a sacrificial metal coating, thereby accelerating the corrosion rate. During this period corrosion products are gradually formed and converted to a stable layer. This period may last for several months, after which the corrosion rate becomes constant. These corrosion products form the second barrier... 

Sprowls, D. O., Sitmmerson, T. J., and Loftin, F. E., Exfoliation Corrosion Testing of 7075 and 7178 Aluminum AUoys-Interim Report on Atmospheric Exposure Tests, Corrosion in Natural Environments, ASTM STP 558, ASTM International, West Conshohocken, PA, 1974, pp. 99-113. 

In a normal atmospheric exposure, a Iresh lead surface will slowly be oxidized into a thin, protective lead oxide, which halts further oxidation of the metal. The rate of formation of lead oxide is determined by the absorption of oxygen and water vapor into the lead. Such factors as industrial and marine pollution, humidity, temperature, and rainfall profoundly affect the aggressiveness of the atmosphere, and most metals suffer accordingly. However, the protective films formed of lead are so effective that corrosion is insignificant in most natural atmospheres. The extent of this protection is demonstrated by the survival of lead roofing and auxiliary products after hundreds of years of atmospheric exposure that may continue for a much longer time if these films are not damaged [2]. 

When products or materials are exposed to the natural environment, they are considered field or service tests. Included in this category are atmospheric exposure, mobil test racks, fleet tests, and field surveys (Table 8). 

Atmospheric exposure consists of placing specimens such as coupons, parts, components, on racks at stationary test sites [53]. The example in Fig. 7 shows bumpers imder test at the marine splash and spray facility of the LaQue Center for Corrosion Technology at Wrightsville Beach, North Carolina. The nature of the site varies depending on geographic area and specific location within an area. Factors such as environmental chemistry (chlorides, pollutants. 

Electronics are exposed to a wide range of outdoor and indoor environments generally considered as atmospheric exposure. The corrosion behavior is determined by the actual environment, which can be as benign as a simple low humidity, purified atmosphere, indoor location, to the aggressive environment existing at a pulp and paper mill or on an automobile, which is subjected to road salt splash and spray. The electronics design as well as the nature of the environment are important because failures in printed circuit boards, integrated circuits, and other components have been known to occur even in extremely low levels of moisture and contaminants. Electronics components are mostly indoor or sheltered from direct exposure to liquid splash, spray, rain, snow, etc., and therefore the environment is considered atmospheric exposure. 

In NO2 atmospheres, the corrosion of zinc is very low, in concordance with another studies published in literature [19, 32, 38, 43]. Only a very thin layer of corrosion products was detected. XPS analysis has detected a greater amount of Zn(OH)2 than in uncontaminated atmospheres. Nitrites, nitrates or any other nitrogen compound are not found in the corrosion products. This may be due either to the fact that they have not yet formed due to the incipient nature of the process, that they have been leached out due to their low concentration and high solubility or because, as noted by Friel [52], zinc nitrates are highly unstable in acid conditions such as those commonly found in atmospheric exposure. 

It is difficult to determine a relationship between the results of accelerated tests and of the natural exposure of certain durations. However, many attempts are published in which a safe life cycle is estimated from the results of accelerated tests. For example, it has been proposed that exposure of concrete elements to pure CO2 over a period of 36 days with a ratio of concentration 3000 times higher may give the same carbonation of concrete as 300 years of service in natural atmosphere (Levy 1992). In the tests carried out by Sisomphon and Franke (2007) it has been derived on the basis of the second Fick s law that the carbonation in natural exposure with concentration CO2 of 0.03% is approximately 10 times slower than in the accelerated tests at concentration CO2 of 3%. 

---