Urban atmospheric corrosion

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 under very thin layers of electrolyte on the metal surface. This influences the amount of oxygen present on the metal surface, 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 mral, urban or marine. 

Carbonyl sulfide is overall the most abundant sulfur-beating compound ia the earth s atmosphere 430—570 parts per trillion (10 ), although it is exceeded by H2S and SO2 ia some iadustrial urban atmospheres (27). Carbonyl sulfide is beheved to origiaate from microbes, volcanoes, and the burning of vegetation, as well as from iadustrial processes. It may be the main cause of atmospheric sulfur corrosion (28). 

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. 

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". 

It is common lo consider that certain salts have a very corrosive action. This is true in the respect that the corrodibility of marine atmospheres has been shown to be greater than rural, tropical, and urban atmospheres. For example, ammonium sulfate and ammonium chloride being salts of strong acids and a weak base, that is ammonium hydroxide, hydrolyze in Water to yield the respective acids. These sails then have a corrosive action, which is due actually to the acid produced in hydrolysis. 

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 ... 

The new dress-cap substrate material should be either fiber-reinforced plastic (FRP) or Type 316 stainless steel. Being a nonmetallic insulator, FRP would eliminate any possibility of galvanic corrosion. Type 316 stainless steel demonstrates good passivity in urban atmospheres and has been successfully used in contact with copper.1... 

Corrosion of a particular metal may change in extent and mechanism by changing the environment. Although we may quote a given environment in discussing rates of corrosion, in reality environments may be a continuum. For example, the environment experienced by a car component may vary from salt spray to urban atmosphere to polluted industrial atmosphere or, at any one time, may be a "mix of environments. For an environment may not fit into a single compartment and may also vary in time and space. 

The products of atmospheric corrosion may be protective or may enhance corrosion. For example, zinc in an urban atmosphere will form a protective basic carbonate layer. However, if sulphur dioxide is present, then this layer is disrupted and corrosion proceeds. The degree of film breakdown will depend upon the concentration of sulphur dioxide in the atmosphere [12]. 

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,... 

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). 

Cramer, S. D. Carter, J. P. Covino, B. S., Jr. "Atmospheric Corrosion Resistance of Steels Prepared from the Magnetic Fraction of Urban Refuse" U.S. Bureau of Mines, RI 8477,... 

However, considering the atmospheric boundary layer profiles as discussed above, the local velocity at the rack may be considerably lower, especially for test sites in either forested or urban areas. This result emphasizes the need to measure local wind speeds at atmospheric corrosion test sites, at the test rack height. Turbulence intensity measurements might be useful as well. 

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... 

The significance of dust is mentioned above. Industrial and urban atmospheres contain more or less solid particles consisting of carbon, soot, sand, oxides, and salts, e.g. chloride and sulphate. Many of these substances attract moisture from the air some of them also attract polluting and corrosive gases. The salts cause high conductivity, and carbon particles can lead to a large number of small galvanic elements because the particles act as efficient cathodes after deposition on the surface. 

TABLE 9.1. Atmospheric Corrosive Gases in Outdoor Urban Environments [1 ]... 

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]. 

The color of the corrosion products can vary slightly, depending both on the atmosphere in which they are formed and on the alloying elements in the zinc. Marine atmospheres give slightly whiter corrosion products than rural and urban atmospheres. The corrosion products are usually darkest in urban atmospheres. 

The corrosion of zinc in urban atmospheres (Table 2.4 and Fig. 2.2) is historically about 2-6 p,m/year this corresponds to a life for zinc roofing sheet of more than 50 years, and 12-50 years for the galvanized coating on... 

Although corrosion is more severe in industrial atmospheres than in urban atmospheres, it is essentially of the same type historically, damage in industrial areas amounts to 3-15 p,m/year (Table 2.5 and Fig. 2.2). However, more severe corrosion is found near points where waste gases are emitted. Sometimes corrosion has been surprisingly low in heavily industrialized areas— the figure for Mulheim/Ruhr, for instance, was 4 p,m/year during World War II (Schikorr and Schikorr, 1943), and such low figures are likely to be usual in the future. The AWS 19-year tests (1974) showed complete protection by 225 p,m zinc (12 pum/year), but only 75-150 xm zinc was needed if it were sealed. 

The corrosion rates for tests in Hamburg harbor are worth noting. The sea is about 100 km away, but there is a significant chloride content to supplement the effects of the industrial or urban atmosphere. 

The corrosion of zinc in temperate marine atmospheres (Table 2.6) is of the same order as in urban atmospheres, about 1-7 jim/year in areas where there is much sea spray, it is of the same order as in industrial atmospheres (15 p,m/year). The AWS (1974) tests showed a corrosion rate of up to 12 (xm/year but only 4-8 xm/year when the sprayed zinc was sealed. 

Figure 8.13 and 8.14 further emphasize the role of the relative humidity. The Figure 8.13 indicates that the corrosion rate of steel increases markedly as soon as the relative humidity exceeds 80%. In order to accelerate corrosion testing, all samples had been previously subjected to a polluted urban atmosphere for a month. Figure 8.14 shows the amount of corroded material as a function of the relative humidity for steel samples that were exposed for 55 days to an atmosphere containing 0.01% SO2. In this case, corrosion becomes important already at a relative humidity above 60%. The difference with respect to the results of the previous figure is due to the higher levels of SO2 which favor the presence of sulfate on the surface. 

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. 

The rural, industrial, and marine atmospheres described earlier are very broad classifications, and do not adequately define specific exposure conditions. Other categories have been proposed, such as desert, tropical, urban, semiindustrial, and industrial-marine, but even these are not satisfactory for predictive purposes [9]. In the mid-1970s. Technical Committee 156 on Corrosion of Metals and Alloys of the International Organization for Standardization (ISO) identified atmospheric corrosion as a priority area for study. From that, a classification system was developed, and described by four standards (Table 3). The system is based on quantitative values for TOW, SO2, and Cl deposition rates, and/or mass loss of steel, aluminum, zinc, and copper samples exposed for at least one yetir. The TOW can be determined from meteorological data or measured by devices such as described in ASTM G 84, the ionic species deposition rates by ASTM G 92 and either of the Cl methods described earlier, and the mass losses determined by the procedures discussed earlier in this section. The levels of ionic species, TOW, and mass losses are placed in categories as to... 

Specific tests frequently used are (a) neutral 5 % Sodium Chloride salt spray (ASTM B 117, Test Method of Salt Spray (Fog) Testing), (b) 3.5 % Sodium Chloride by alternate immersion (ASTM G 44, Practice for Evaluating Stress Corrosion Cracking Resistance of Metals in 3.5 % Sodium Chloride Solution), and (c) exposure to various outdoor atmospheres. Guidelines for outdoor exposure are contained in ASTM G 50, Practice for Conducting Atmospheric Corrosion Tests on Metals. Generic types of atmospheres used are seacoast, industrial, urban, and rural. Sometimes specific geographical locations or local chemical conditions are important because they can produce unique results [2i],... 

The airside of the radiator, including the fins and the outside of the tubes, is exposed to the road climate. The atmospheric corrosion of radiators is influenced hy the following parameters [28] the time of wetness as corrosion of practical importance generally will take place only when the metal surface is. covered with a moisture film the t3fp>es and contents of air pollutants from urban and industrial sources splashing from the road. 

Very little information has been published on the atmospheric corrosion of zinc over long time periods (10-20 years) [27, 28]. In a long-term study in Spain, the characteristics of corrosion products formed on zinc panels (after 13-16 years) in various types of atmospheres in Spain (rural, urban, industrial, mild marine and severe marine) were studied [28], and the authors found a linearly increasing amount of zinc corrosion with time at the different test sites, except the marine atmospheres, where the zinc deviated from the usual behavior... 

---