Cadmium atmosphere

Y. Noack, M. Lefloch, D. Robin, Environmental impact of a cadmium atmospheric pollution at Marseille (South France) , Proceedings of the Journal de Physique IV (Xllth International Conference on Heavy Mateals in the Environment), France, 2003, pp. 961-964. 

Chemical Gas Detection. Spectral identification of gases in industrial processing and atmospheric contamination is becoming an important tool for process control and monitoring of air quaUty. The present optical method uses the ftir (Fourier transform infrared) interference spectrometer having high resolution (<1 cm ) capabiUty and excellent sensitivity (few ppb) with the use of cooled MCT (mercury—cadmium—teUuride) (2) detectors. 

Tin—cadmium coatings are particularly resistant to marine atmospheres and have appHcations in the aviation industry. 

Hazards of Production. In most zinc mines, zinc is present as the sulfide and coexists with other minerals, especiaHy lead, copper, and cadmium. Therefore, the escape of zinc from mines and mills is accompanied by these other often more toxic materials. Mining and concentrating, usuaHy by flotations, does not present any unusual hazards to personnel. Atmospheric poHution is of Httle consequence at mine sites, but considerable effort is required to flocculate and settle fine ore particles, which would find their way into receiving waters. 

Cadmium Silicates. Cadmium orthosihcate [15857-59-2] Cd2SiO, (mp 1246°C d = 5.83 g/ inL) and cadmium metasihcate [13477-19-5] CdSiO, are both prepared by direct reaction of CdO and Si02 at 390°C under 30.4 MPa (300 atm) or at 900°C and atmospheric pressure ia steam. The materials are phosphors whea activated with Mn (IT) ioa and are both fluorescent and phosphorescent. 

The reproducibility of test results between labs using the neutral salt spray tests has not been consistent, but the repeatability, within one lab, is better, and the test has value in comparing variations in coating systems. Correlation of hours of exposure in the salt spray test to actual performance of the plated part in service, even in marine atmospheres, is not consistent and usually avoided. A classic example is that cadmium deposits outlast zinc deposits on steel in salt spray tests and clean marine atmospheres, yet zinc outlasts cadmium when exposed to real, industrial atmospheres, because of the presence of sulfur-bearing corrodents in industrial environments. An important variable in salt spray testing is the position of the surface to be tested. Whereas the surface of test panels is specified to be 15—30° from the vertical (40), when salt spray testing chromated zinc-plated specimens, this range has appeared excessive (41). 

Zinc (76ppm of the earth s crust) is about as abundant as rubidium (78 ppm) and slightly more abundant than copper (68 ppm). Cadmium (0.16 ppm) is similar to antimony (0.2 ppm) it is twice as abundant as mercury (0.08 ppm), which is itself as abundant as silver (0.08 ppm) and close to selenium (0.05 ppm). These elements are chalcophiles (p. 648) and so, in the reducing atmosphere prevailing when the earth s crust solidified, they separated out in the sulfide phase, and their most important ores are therefore sulfides. Subsequently, as rocks were weathered, zinc was leached out to be precipitated as carbonate, silicate or phosphate. 

For some non-ferrous metals (copper, lead, nickel) the attack by sulphuric acid is probably direct with the formation of sulphates. Lead sulphate is barely soluble and gives good protection. Nickel and copper sulphates are deliquescent but are gradually converted (if not leached away) into insoluble basic sulphates, e.g. Cu Cu(OH)2)3SO4, and the metals are thus protected after a period of active corrosion. For zinc and cadmium the sulphur acids probably act by dissolution of the protective basic carbonate film. This reforms, consuming metal in the process, redissolves, and so on. Zinc and cadmium sulphates are formed in polluted winter conditions whereas in the purer atmospheres of the summer the corrosion products include considerable amounts of oxide and basic carbonate. ... 

The excellent resistance of zinc to corrosion under natural conditions is largely responsible for the many and varied applications of the metal. In fact nearly half the world consumption of zinc is in the form of coatings for the prevention of corrosion of steel fabrications exposed to the atmosphere and to water. For its varied applications zinc is obtainable in a number of grades. Ordinary commercial (G.O.B.) zinc contains up to about I -5% total of lead, cadmium and iron. Electrolytic zinc has a minimum zinc content of 99-95% and contains small amounts of the same impurities. Special high-purity zinc has a minimum of 99-99% zinc. Even purer zincs are commercially available. 

Table 13.4 Corrosion rates of zinc and cadmium coatings in various atmospheres...

In conclusion, relative cost and relative behaviour towards different conditions of exposure lead to the use of zinc on parts on which thick films can be tolerated and for general industrial use, and of cadmium for fine-tolerance special applications, such as aircraft and instrument parts, required to withstand conditions include humid and marine atmospheres. 

The relative susceptibility of metals to atmospheric corrosion varies widely with the type of contaminant, e.g. zinc and cadmium, two metals that are used for the protection of steel in exposed environments, are both rapidly attacked by organic acidson the other hand, aluminium alloys resist attack by organic acids but may be rapidly corroded by chlorides, especially at crevices or areas of contact. 

In 1988 a method for the formation of CdS particles in Langmuir-Blodged (LB) matrix was suggested (Smotkin et al. 1988). LB film of cadmium arachidate was exposed to an atmosphere of H2S. During the reaction, the head groups of arachidic acid were proto-nated, and CdS was produced according to the following reaction ... 

Apart from transistor-like devices, single-electron junctions can also be useful for sensor applications. The simplest one might be the monitoring of H2S. Since the formation of CdS nanogranules takes place when an initial cadmium arachidate layer is exposed to this gas, we can expect the appearance of single-electron conductivity only when it is present in the atmosphere. 

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