Rural atmospheric corrosion

Chromates are very effective inhibitors of the corrosion of magnesium alloys by saline and other waters, and many treatments have been developed by means of which substantial hlms containing slightly soluble chromate are formed in the metal surface. Except on parts which are to be exposed only to a rural atmosphere, chromate treatment must be supplemented by paint, for which it provides a good base. 

The superficial characteristics of atmospheric corrosion products of steel depend on the type of atmosphere where the sample has been exposed. The way of adsorption of the corrosion products obtained in the coastal atmosphere is polymolecular due to a higher content of salts. This makes easier the presence of water in the metal-corrosion products interface and determines a high corrosion rate. The adsorption of water of a corrosion product formed in a rural zone obeys a Langmuir isotherm, i.e. a monomolecular adsorption takes place. It causes a lower corrosion rate. 

Atmospheric corrosion of lead involves exposure to industrial, rural and marine environments. The mode of corrosion in the three environments is different. The rural environment consists of humidity, airflow and rainfall, which may be considered to be innocuous. The marine environment consists of chloride entrained in air and could... 

The corrosion rates of various metals in industrial, marine and rural atmospheres are given in Table 4.76. Zinc has higher corrosion resistance than cadmium and iron in all the three atmospheres. Zinc has higher corrosion resistance than copper in industrial... 

Other factors of importance in atmospheric corrosion of zinc are (i) the distance from the ground (ii) orientation of the samples (iii) wind or rain shielding (iv) distance to the local contaminant sources (v) wind, radiation (vi) condensation and drying rate (vii) amount of contaminants and nature of corrosion products and (viii) seasonal variation of factors also should be considered. This shows the complexity of the problem of determining the atmospheric corrosion rates to a high degree of certainty. This uncertainty is exemplified by the observed corrosion rate of 0.6-3.8 pm/yr at 26 sites in rural area in Spain.95 The corrosion rate of 8.5 pm/yr observed on the zinc coating in an under-vehicle situation is comparable to severe marine atmospheric conditions.96... 

Corrosion products such as the oxides, hydroxides, carbonates, sulfates, basic sulfates hydroxy carbonates, hydroxy chlorides are formed in the various environments (marine, urban, rural, industrial) and the initially loosely bound products may become adherent in the course of time. Corrosion can occur in the pores of the corrosion product layers. The low corrosion rate observed in atmospheric corrosion, R has been expressed as ... 

It is obvious from the data that galvanic corrosion of zinc in rural atmosphere can be five times the rate in corresponding atmospheric corrosion and three times that in marine atmospheres. Mild steel appears to be the most efficient cathode among the metals studied. More detailed discussions are given in the literature.92... 

Zinc belongs to the materials that exerts a strong dependence of the corrosion rate on the concentration of sulphur pollutants. In several investigations the corrosion rate in urban atmospheres was found to be 2-6 times higher than in rural atmospheres (5,... 

It is a well known fact that formation of the green patina takes a substantially shorter time in urban than in rural atmosphere, where often very long time elapses before the surface is covered by patina or in very pure atmospheres the surface remains covered by a black oxide layer. Also the corrosion rate in rural atmosphere is usually lower (<1 um/year) than in urban or industrial atmospheres (1-3 /Lim/year) (8 . 

Atmospheric corrosion results from a metal s ambient-temperature reaction, with the earth s atmosphere as the corrosive environment. Atmospheric corrosion is electrochemical in nature, but differs from corrosion in aqueous solutions in that the electrochemical reactions occur imder veiy thin layers of electrolyte on the metal surface. This influences the amount of oxygen present on the metal smface, since diffusion of oxygen from the atmosphere/electrolyte solution interface to the solution/metal interface is rapid. Atmospheric corrosion rates of metals are strongly influenced by moisture, temperature and presence of contaminants (e.g., NaCl, SO2,. ..). Hence, significantly different resistances to atmospheric corrosion are observed depending on the geographical location, whether rural, urban or marine. 

In wet atmospheres, nickel initially forms NiO and (NiOH)2 [35,36]. Nickel sulfates are present as corrosion products on the surface in outdoor exposures [37].Jouenefatmospheric corrosion of nickel in industrial, urban, and rural atmospheres. Nickel corrodes through a pitting corrosion process. The highest corrosion rates were observed in industrial areas. The corrosion products were mainly sulfates, chlorides, and n ligible amounts of nitrates surrounded by carbonate species. The pitting corrosion process occurs in two steps on nickel surfaces exposed to an outdoor atmosphere, as shown in Fig. 10.9 [38]. 

The susceptibility of zinc to sulfur dioxide, a common poUutant found to cause atmospheric corrosion, was studied by Veleva et al. [67]. Galvanized steel and zinc plates were subjected to a humid tropical environment, inducing atmospheric corrosion during 2 years in rural and urban atmospheres. Runoffsamples were taken from each of the plates and analyzed to compare the results. Runoff samples taken from the galvanized steel... 

Magnesium is thermodynamically one of the less noble metals, and it can protect most other metals when used as sacrificial anodes (see Section 10.4). In the atmosphere the metal is covered by an oxide film. Therefore it resists rural atmospheres but is subject to pitting in marine atmospheres. Magnesium alloys are also liable to SCC and erosion corrosion, and are attacked by most acids. Mg alloys are used in automobile engines, aircraft, missiles and various movable and portable equipment, in all cases primarily because of their low density (1.76 g/cm ). 

A metal that resists corrosion in one atmosphere may lack effective corrosion resistance elsewhere hence, relative corrosion behavior of metals changes with location for example, galvanized iron performs well in rural atmospheres, but is relatively less resistant to industrial atmospheres. On the other hand, lead performs in an industrial atmosphere at least as well as, or better than, elsewhere because a protective film of lead sulfate forms on the surface. 

Nickel is quite resistant to marine atmospheres, but is sensitive to sulfuric acid of industrial atmospheres (Table 9.2), forming a surface tarnish composed of basic nickel sulfate. Corrosion in the industrial atmosphere of New York City is about 30 times higher than in the marine atmosphere of La Jolla, California, and about 20 times higher than in the rural atmosphere of State College, Pennsylvania (Table 9.2). 

Zinc coatings are relatively resistant to rural atmospheres and also to marine atmospheres, except when seawater spray comes into direct contact with the surface. Table 14.1 lists the ranges of typical atmospheric corrosion rates in each of the three types of atmospheres, rural, marine, and urban/ industrial [18]. 

Atmospheric corrosion is an electrochemical process with the electrolyte being a thin layer of moisture on the metal surface. The composition of the electrolyte depends on the deposition rates of the air pollutants and varies with the wetting conditions. The factors influencing the corrosivity of atmospheres are gases in the atmosphere, critical humidity and dust content. Two rural environments can differ widely in average yearly rainfall and temperature and can have different corrosive... 

Oh, S.J., Cook, D.C., Townsend, H.E. Atmospheric corrosion of different steels in marine rural industrial environments. Corro. Sci. 41, 1687-1702 (1999)... 

WS is used to protect structures from atmospheric corrosion in specific environments. Rust layers on MS are not protective and are permeable to air and moisture. Protective impervious rust coatings are supposed to form on WS with time and due to high initial cost and sometimes unfavourable environments WS cannot be used. Studies on atmospheric influences on steels in rural, industrial and marine environments have indicated that P, Cu, Ni, Cr and Si improve the resistance to corrosion while Mn does not. In an industrial atmosphere the weathering process is rapid than rural atmosphere, whereas in coastal environment. 

Several reviewers have given typical corrosion rates for these atmospheres (Table 2.1). Vazquez and Damborenea (1991) in particular point out the subjective nature of these descriptions and the different associated numerical values of corrosion rates such materials are used in this book because often that is the only description initially available for a location. In general terms, Scandinavia and the Iberian peninsula have purer air than central and northwestern Europe North America aligns more with the less polluted parts of Europe. This distribution is reflected in the use of the subjective terms thus corrosion in an industrial atmosphere in Finland may be little worse than in some rural atmospheres in England. 

Truly rural atmospheres (Table 2.3) give very low corrosion rates on zinc, but at some country locations the air can be polluted by acid compounds brought by the prevailing wind from polluted areas more than 100 km away. The adverse effect is particularly felt if there are early morning mists, which can deposit highly acidic moisture, especially on structures near ground level. Caution is advised in considering zinc-coated steel if pH measurements of such deposits are less than S. 

According to Anderson (1955), corrosion of wire fences in rural atmospheres is a little greater than normal in the shade and at points where the fence is covered in grass, presumably because moisture is retained longer at these points. Possibly, however, the metabolism of the grass or leaves has an effect (see below). 

Considerable corrosion of zinc in rural atmospheres was observed in the following special cases ... 

In the humid tropical atmosphere of Singapore, zinc is corroded as little as in a European rural atmosphere in tropical marine atmospheres on the African coast, the corrosion decreases rapidly a short distance away from the coast (Fig. 2.41). The relationship between corrosion and airborne seawater salts is shown in Fig. 2.42 corrosion is very severe in regions where there is surf from tropical seawater, low in magnesium salts. 

Rural atmosphere < urban atmosphere < industrial atmosphere We also observe that the average corrosion rate (given by the slope of the curves) is not constant. It is highest at the start of the experiments, then it decreases with time, finally reaching a constant value after a period of several years. The initial period, corresponding to a non-steady-state corrosion rate, shortens with increasing degree of atmospheric pollution. 

Figure 8.16 Corroded mass of a steel sample (a) exposed to a rural atmosphere (b) to an nrban atmosphere and to (c) an industrial atmosphere for a period of several years. The dashed lines indicate the time required to reach the steady-state corrosion rate [14],...

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