Atmospheric corrosion zinc

One of the few impressed current zinc systems in the United Kingdom at the time of going to press is shown in Figure 7.10. There has been some concern about the rise in resistance seen on some systems. This may be due to a build up of corrosion products between the zinc and the concrete, or to treatment of the zinc after application to protect it from atmospheric corrosion. Zinc, of course, is not inert and is consumed by corrosion from the atmosphere and water impingement. The anodic reaction also consumes the zinc and gives rise to the formation of oxides and sulphates at the anode/concrete interface which may increase the electrical resistance between anode and cathode. 

Zinc diffusion is used for protection against atmospheric corrosion. Aluminum diffusion is used to improve the oxidation resistance of low-carbon steels. 

BS2569 Sprayed Metal Coatings. Part 1 Protection of Iron and Steel by Aluminum and Zinc Against Atmospheric Corrosion. ... 

However, the object of this section is to outline the principles which govern atmospheric corrosion, and the emphasis is placed on metals whose atmospheric corrosion is of economic importance. These include iron and steel, zinc, copper, lead, aluminium and chromium. 

However, in this section emphasis is placed upon damp and wet atmospheric corrosion which are characterised by the presence of a thin, invisible film of electrolyte solution on the metal surface (damp type) or by visible deposits of dew, rain, sea-spray, etc. (wet type). In these categories may be placed the rusting of iron and steel (both types involved), white rusting of zinc (wet type) and the formation of patinae on copper and its alloys (both types). 

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. 

Table 4.34 Atmospheric corrosion of zinc in various part of the UK...

Table 4.35 Further examples of atmospheric corrosion rates for zinc ...

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. 

Schikorr, G. Atmospheric Corrosion Resistance of Zinc, English edition 1965, ZDA, London, 4, (1964)... 

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 . 

The reason for the use of zinc as a power-impressed rather than a sacrificial anode is that the high concrete resistivity limits the current output, and a higher driving voltage than that provided by the e.m.f. between zinc and steel in concrete is used to provide the necessary current output. No cementitious overlay is required, although it may be advisable to paint the top surface of the sprayed zinc to prevent atmospheric corrosion of the zinc anode. 

Zinc diffusion sherardisingY " is mainly used for protection of ferrous metals against atmospheric corrosion. It has, in some respects, properties related to other types of zinc coating such as galvanising, but owing to the small dimensional change involved, it is of particular value for the treatment of machined parts, bolts, nuts, etc. 

The corrosion resistance of zinc is discussed in Section 4.7, and it is only necessary here to say that zinc is protected against further attack by a film of corrosion products. It is remarkably resistant to atmospheric corrosion except perhaps in the most heavily contaminated industrial areas, and even there its use as a protective coating is still a sound practical and economic proposition. The value of zinc coatings as a basis for painting under very aggressive conditions has been clearly demonstrated. 

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. 

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 relative ranking of the degree of corrosivity of different atmospheres for zinc and steel is given in Table 4.75. The corrosivity of the atmosphere in one location or another varies as much as a factor of 100 for zinc and 500 for steel. The corrosion rates of zinc in most cases are ten times lower than those of steel. The lower corrosion rates of zinc are profitably used for the protection of steel by galvanizing steel with zinc. The plot of the corrosion rate ratio of steel to zinc as a function of corrosion rate of the steel in various atmospheres (data from Table 4.75) gives a linear plot with R2 = 0.495. 

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... 

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... 

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... 

Materials such as metals, alloys, steels and plastics form the theme of the fourth chapter. The behavior and use of cast irons, low alloy carbon steels and their application in atmospheric corrosion, fresh waters, seawater and soils are presented. This is followed by a discussion of stainless steels, martensitic steels and duplex steels and their behavior in various media. Aluminum and its alloys and their corrosion behavior in acids, fresh water, seawater, outdoor atmospheres and soils, copper and its alloys and their corrosion resistance in various media, nickel and its alloys and their corrosion behavior in various industrial environments, titanium and its alloys and their performance in various chemical environments, cobalt alloys and their applications, corrosion behavior of lead and its alloys, magnesium and its alloys together with their corrosion behavior, zinc and its alloys, along with their corrosion behavior, zirconium, its alloys and their corrosion behavior, tin and tin plate with their applications in atmospheric corrosion are discussed. The final part of the chapter concerns refractories and ceramics and polymeric materials and their application in various corrosive media. 

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]. 

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. 

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