Atmospheric corrosion pollutants

Carbonyl sulfate and H2S are the major atmospheric corrosion pollutants. 

Sulphur oxides These (SO2 is the most frequently encountered oxide) are powerful stimulators of atmospheric corrosion, and for steel and particularly zinc the correlation between the level of SO2 pollution and corrosion rates is good However, in severe marine environments, notably in the case of zinc, the chloride contamination may have a higher correlation coefficient than SO2. 

In most districts, however, sulphur dioxide and dust particles are the main corrosive pollutants. It has been demonstrated that there is a direct relationship between sulphur dioxide in the atmosphere and the corrosion of steel exposed to it (see Fig. 3.2). In a series of tests carried out in the Sheffield area, sulphur dioxide accounted for about 50% of the variations in corrosion rate at the different sites. ... 

The atmospheric corrosion data in Table 4.34 (and also Table 13.8) is related to historic environments. Current use in the industrial areas listed with acidic pollution would show much lower corrosion rates as the corrosion of zinc in the atmosphere is essentially related to the SOj content (and the time of wetness) and in many countries the sulphurous pollution has been greatly reduced in the past 20 years. Zinc also benefits from rainwater washing to remove corrosive poultices thus, although initial corrosion rates are usually not very different on upper and lower surfaces, the latter tend —with time—to become encrusted with corrosion products and deposits and these are not always protective. 

The purity of the zinc is unimportant, within wide limits, in determining its life, which is roughly proportional to thickness under any given set of exposure conditions. In the more heavily polluted industrial areas the best results are obtained if zinc is protected by painting, and nowadays there are many suitable primers and painting schemes which can be used to give an extremely useful and long service life under atmospheric corrosion conditions. Primers in common use are calcium plumbate, metallic lead, zinc phosphate and etch primers based on polyvinyl butyral. The latter have proved particularly useful in marine environments, especially under zinc chromate primers . 

Almost all tests carried out to study the starting process of atmospheric corrosion have been performed in a surface without corrosion products however, in real conditions, the metal is covered with corrosion products after a given time and these products begin to play its role as retarders of the corrosion process in almost all cases. Corrosion products acts as a barrier for oxygen and contaminants diffusion, the free area for the occurrence of the corrosion is lower however, the formation of the surface electrolyte is enhanced. Only in very polluted areas the corrosion products accelerate the corrosion process. Water adsorption isoterms were determined to corrosion products formed in Cuban natural atmospheres[21]. Sorption properties of corrosion products (taking into account their salt content-usually hygroscopics) determine the possibilities of surface adsorption and the possibility of development of corrosion process... 

The atmospheric corrosion rate of metals depends mainly on TOW and pollutants however, if the differences in the corrosion process between outdoor and indoor conditions are taken into account, the influence of direct precipitation such as rain is very important for outdoor and negligible for indoor conditions. The acceleration effect of pollutants could change depending on wetness conditions of the surface, so the influence of the rain time and quantity should be very important in determining changes in corrosion rate. 

It has no sense to calculate TOW-ISO for coastal tropical atmospheres, because in those conditions corrosion process occurs at relative humidity lower than 80%. It has been determined that water adsorption by corrosion products is polymolecular in these conditions. As analogy, in highly polluted atmospheres, corrosion process should proceed at RH lower than 80%, so it has no sense to use TOW-ISO. 

The few reported cases concerning other metals, like zinc, aluminum, and magnesium, attest their susceptibility to corrosion due to volatile compounds in the museum environment [271]. Iron is naturally vulnerable to atmospheric corrosion whatever the pollutants, and the conservation of ferrous artifacts implicates a precise control of relative humidity, often requiring a surface protection like varnish, wax, or oil [272]. 

The most important pollutant in the atmosphere is sulfur dioxide, which causes a linear increase in the corrosion rate of zinc as a function of its concentration.93,94 Other pollutants such as NO, are not significant because of their presence in the atmosphere in trace quantities. The atmospheric corrosion rates of zinc in 1980s were found to be lower than the rates observed in 1960s and 1970s due to the decrease in the level of... 

Dissolution of steel or zinc in sulfuric or hydrochloric acid is a typical example of uniform electrochemical attack. Steel and copper alloys are more vulnerable to general corrosion than other alloys. Uniform corrosion often results from atmospheric exposure (polluted industrial environments) exposure in fresh, brackish, and salt waters or exposure in soils and chemicals. The rusting of steel, the green patina on copper, tarnishing silver and white mst on zinc on atmospheric exposure are due to uniform corrosion.14... 

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. 

Atmospheric Atmospheric corrosion due to the combined effects of rain and the deposition of salt and other pollutants will affect most equipment. Corrosion occurs while the metal surface is wet, and is strongly influenced by the composition of deposits (such as sulfates from industrial atmospheres and chlorides from marine atmospheres). External corrosion of steel and stainless steel process equipment beneath thermal insulation and fireproofing is of particular concern. 

NO in combination with SOp has a synergistic corrosion effect especially indoors on electrical contact materials, copper and steel. The influence of acid precipitation may differ for different metals and depends also on the pollution level. The atmospheric corrosion of metals due to acid deposition is in most regions mainly a local problem restricted to areas close to the pollution source. 

In the present lecture a brief review will be given of the influence of acidifying air pollutants on the atmospheric corrosion of metals based mainly on more recent results from Europe and especially from Scandinavia. 

The influence of mainly SO on the corrosion rate of several materials has been shown in numerous national exposure programs. During the last decades a number of empirical relations have been derived from measurements of atmospheric corrosion rates of the most important structural metals and from measurements of environmental factors. The results are usually presented in form of equations including pollution and meteorological parameters (5.). 

At exposure of steel in heavily polluted industrial atmosphere the corrosion rate on the upper side of steel panels exposed at 45° inclination was only 37 per cent of the total corrosion. In clean air, by contrast, the corrosion effect of rain was predominant and the upper sides of the test panels corroded faster than the undersides ( 6). The atmospheric corrosion of steel proceeds in local cells, where the sulphate nests acts as anodes. This may be the explanation why the washing effect of rain prevails in polluted atmospheres, as rain water may wash away sulphates from the nests. 

From the practical and economic point of view atmospheric corrosion is closely associated with centers of population. Three factors here coincide high pollution level, high density of population, which in turn means great use of materials. The rate of atmospheric corroion decreases sharply with increasing distance from the emission source. This may be illustrated by the corrosion of carbon steel as function of the distance from the stack of a polluting industry in Kvarntorp, see FIG.8 (26). 

The great differences of the corrosion rate in restricted geografical areas have also been demonstrated by construction of corrosion maps for cities or whole countries. The corrosion map of zinc for UK (28), the corrosion map of several metals for the Sarpsborg/Fredrikstad area in Norway ( ) and the corrosion map of steel for Madrid (30) may serve as examples. The very strong local variations of atmospheric corrosion of metals implies also the major role of dry deposition of pollutants. The wet deposition does not by far exhibit such strong variations as the corrosion rate. 

Atmospheric corrosion is thus, at least in Scandinavia, a local effect mainly caused by the country s own emissions and not affected by long-distance transport of pollutants. The situation may, however, be different in densly populated areas of e.g. Western or Central Europe, where also transport of pollutants over the national boundaries may cause appreciable corrosion damage. 

The atmospheric corrosion of metals caused by acid deposition is mainly a local problem restricted to areas close to the pollution source. 

The objective of this study was to determine whether air pollutants other than SO2 accelerate the atmospheric corrosion rate of galvanized steel. Short-term laboratory experiments were conducted in which galvanized steel panels were exposed to the following mixtures in air (1) NO2, (2) irradiated... 

The accumulation of salts within the concrete pore structure can also lead to the corrosion of reinforcing steel, the fourth form of deterioration identified above. This corrosion is accompanied by an increase in the volume of the steel, which eventually causes the concrete to crack and spall. In discussing the atmospheric corrosion of concrete reinforcements, Skoulikidis (21) notes "The increase of atmospheric pollution Intensifies the corrosion tendency of the reinforcements in the atmosphere. The cracking of the concrete was observed more frequently with an increase of the atmospheric pollution (SO2, CO2, NH3, NOx> etc.) and the acceleration of the corrosion by the formation of a more conductive environment, that also chemically attacks the metals."... 

External polythionic acid attack may occur in plants having atmospheric sulfide pollution including gas applications with water condensation. However, the problem does not usually occur on external surfaces in fired equipment because excess air causes sulfates to form, instead of sulfides. Wet sour systems containing carbon steel heat transfer surfaces may generate substantial iron sulfide. Such systems should not be flushed or drained into stainless steel piping or equipment prior to a shut-down unless appropriate precautions are taken. Several metallurgical methods address intergranular corrosion caused by sensitization. 

The droplet cell. Fig. 2(d), has uniform current distribution and shrunken dimensions that allow resistive electrolytes to be used [5]. This approach was developed for the use of pure water as an electrolyte as a means to mimic atmospheric corrosion, but it can be used with any electrolyte. An area of a flat sample is exposed through a hole in a piece of protective tape. Electroplater s tape is a very resistant tape with good adhesion that is useful for this and other masking applications in corrosion. If the hole in the tape is made with a round punch, the same punch can be used to make circular dots from pieces of filter paper. One such dot is placed securely into the exposed hole. A small (typically 10-20 gl) droplet of soluhon is placed on the filter paper using a calibrated pipette. This wet filter paper acts as the electrolyte. A piece of woven Pt mesh is placed on top of the wet filter paper, and a reference electrode is held against the back of the Pt counterelectrode. As mentioned, the small dimensions allow the use of even very pure water. This simulates atmospheric corrosion, in which a thin water layer forms on the surface. As in atmospheric corrosion, soluble species on the sample surface and pollutant gases in the air are dissolved into the water droplet, which provides some conductivity. This technique has been used... 

The pollutants include SO2, nitrogen oxides, chlorides, and phosphates. All gases in the Troposphere (Ne, Kr, He, and Xe) do not participate in atmospheric corrosion [11]. Only oxygen acts as an oxidizer in a cathodic reaction. The presence of CO2 in the electrolyte ( 300 ppm) decreases pH and increases the corrosion rate of metals. 

The evaluation and classification of atmospheric metal corrosivity help develop corrosion protection strategies and optimize material service Hfe. Two fundamental approaches are used to estimate the relationships between atmospheric corrosion rates of metals and the atmospheric variables such as pollutants types and their concentration in the atmosphere, the temperature, and the time of wetness. 

The International Standard Organization (ISO) developed a corrosivity classification system verified through exposure that has been carried worldwide. The ISO classification system is based on the assumptions that only the time of wetness and the concentration of pollutants in the atmosphere, SO2 and chlorides, control the corrosion rates of metals. Table 10.1 shows the Hst of ISO standards related to atmospheric corrosion of metals [39]. 

In a separate study, the effects of NaCl and SO2 air pollutants on the corrosion of zinc were investigated by Qu et at. [68]. Influence of NaCl deposition and SO2 on atmospheric corrosion of zinc at 90% RH and 25 °C is shown in Fig. 10.20 [68]. The corrosion rate decreases with time due to the large amounts of deposit buildup onto the zinc surface. NaCl increases the initial corrosion of zinc in air in the presence and absence of SO2. Presence of only SO2 slowly increases the initial corrosion rate. The synergistic corrosion effect was observed in the presence of both contaminants. 

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