Atmospheric corrodents

Although molybdenum is resistant to molten glass, except leaded, molybdenum components not coated with glass but exposed to the oxidising furnace atmosphere corrode rapidly due to volatilisation of molybdenum oxide above 370°C. To overcome this, stirrers etc. for use in glass plant are physically clad with platinum sheet in vulnerable areas. Modern plating techniques have enabled dense platinum coatings to be put onto the surface of the molybdenum and it is expected that this technique will be exploited further in the near future. 

The sulfur dioxide in industrial atmospheres corrodes aluminum alloys due to the formation of sulfurous acid. 

Atmospheric corrodents, that is, constituents capable of corroding a metal when exposed to the atmosphere, have a total concentration of less than 10 ppmv. These corrodents originate either from... 

Tab. 2 Metal sensitivities to atmospheric corrodents. H = high sensitivity, M = moderate sensitivity, L = low sensitivity, N = no sensitivity. Unfilled location implies that experimental data are missing...

Metals exposed to humid atmosphere corrode by an electrochemical mechanism due to the formation of a thin electrolyte layer on the metal surface (Chapter 3.1, this volume). This type of corrosion can be controlled by Vapor-phase Corrosion Inhibitors (VCIs), that is, volatile inhibiting substances that allow vapor-phase transport to the corroding surface (examples are amines, benzoates, imidazoles, or triazoles [3]). The vapor pressure should be sufficiently high to ensure a protective surface concentration of the inhibitor, but low enough to prevent premature depletion of... 

In the absence of moisture, iron exposed to the atmosphere corrodes at a negligible rate. For example, steel parts abandoned in the desert remain bright and tarnish-free for long periods of time. Also, the corrosion process cannot proceed without an electrolyte hence, in climates below the freezing point of water or of aqueous condensates on the metal surface, rusting is negligible. Ice is a poor electrolytic conductor. Incidence of corrosion by the atmosphere depends, however, not only on the moisture content of air, but also on the particulate matter content and gaseous impurities that favor condensation of moisture on the metal surface. 

Recognition of marked differences in corrosivity has made it convenient to divide atmospheres into types. The major types are marine, industrial, tropical, arctic, urban, and rural. There are also subdivisions, such as wet and dry tropical, with large differences in corrosivity. Also, specimens exposed to a marine atmosphere corrode at greatly differing rates depending on proximity to the ocean. At Kure Beach, North Carolina, specimens of steel located 24m (80ft) from the ocean, where salt water spray is frequent, corroded about 12 times more rapidly than similar specimens located 240 m (800 ft) from the ocean [2]. 

PVC is resistant to all of the normal atmospheric corrodents including weather and UV degradation. The primary applications are for piping and electrical conduit and wire insulation. It may be colored. 

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). 

Insoluble corrosion prodiic ts may be completely impeivious to the corroding liquid and, therefore, completely protective or they may be quite permeable and allow local or general corrosion to proceed unhindered. Films that are nonuniform or discontinuous may tend to localize corrosion in particular areas or to induce accelerated corrosion at certain points by initiating electrolytic effects of the concentration-cell type. Films may tend to retain or absorb moisture and thus, by delaying the time of drying, increase the extent of corrosion resulting from exposure to the atmosphere or to corrosive vapors. 

Sample tests in the field. These include coupons, stressed samples, electrical-resistance probes exposed to the plant corroding medium, or samples exposed to the atmosphere, to soils, or to fresh, brackish, or sahne waters. 

The confinement of the cracks to a specific area of the cooler suggests that condensate from atmospheric moisture initially formed in this area and dissolved a corrodent from the atmosphere such as ammonia, sulfur dioxide, or oxides of nitrogen. Since the previous cooler had been in service for 20 years, it is conjectured that the rapid failure of this exchanger was due principally to very high bending stresses, which may have been induced during construction of the cooler. 

How does galvanising work As Fig. 24.4 shows, the galvanising process leaves a thin layer of zinc on the surface of the steel. This acts as a barrier between the steel and the atmosphere and although the driving voltage for the corrosion of zinc is greater than that for steel (see Fig. 23.3) in fact zinc corrodes quite slowly in a normal urban atmosphere because of the barrier effect of its oxide film. The loss in thickness is typically 0.1 mm in 20 years. 

Most materials will deteriorate even when exposed to an unpoUuted atmosphere. Iron will rust, metals will corrode, and wood will rot. To prevent deterioration, protective coatings are applied. Their costs are part... 

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). 

Lead, aluminium and copper corrode initially but eventually form completely protective films". Nickel in urban atmospheres does not form a completely protective film, the corrosion/time curve being nearly parabolic". The corrosion rate of zinc appears to become linear after an initial period of decreasing corrosion rate". 

Weather conditions at the time of initial exposure of zinc and steel have a large influence on the protective nature of the initial corrosion products This can still be detected some months after initial exposure. Finally, rust on steel contains a proportion of ferrous sulphate which increases with increase in SO2 pollution of the atmosphere. The effect of this on corrosion rate is so strong that mild steel transferred from an industrial atmosphere to a rural one corrodes for some months as though it was still exposed to the industrial environment. ... 

---