Corrosion rates/resistance industrial atmospheres

It is also of interest to note that Wranglen considers that the decrease in the corrosion rate of steel in the atmosphere and the pitting rate in acid and neutral solution brought about by small alloying additions of copper is due to the formation of CU2S, which reduces the activity of the HS and Scions to a very low value so that they do not catalyse anodic dissolution, and a similar mechanism was put forward by Fyfe etal. to explain the corrosion resistance of copper-containing steels when exposed to industrial atmospheres. 

The addition of small amounts of nickel to iron improves its resistance to corrosion in industrial atmospheres due to the formation of a protective layer of corrosion products. Larger additions of nickel, c.g. 36% or 42%, are not quite so beneficial with respect to overall corrosion since the rust formed is powdery, loose and non-protective, leading to a linear rate of attack as measured by weight loss. Figure 3.37 of Pettibone illustrates the results obtained. 

In atmospheric exposure to industrial environments its corrosion rate is only about one-third that of zinc and the corrosion reaction is stifled by the tenacious oxide which is produced nevertheless it can frequently function as an anodic coating both for steel and for the less corrosion-resistant aluminium alloys. 

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... 

A global mean for the rate of net chemical denudation of the continental surface is about 14 mm 1000 yr-1 or 14 pm yr k In comparison to the corrosion rates of metals exposed to a range of environmental conditions, the global continental surface is less resistant to corrosion than zinc and copper, but it is considerably more resistant than iron exposed to coastal oceanic and industrial-area atmospheric conditions. 

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]. 

Lead is resistant to atmospheric exposures, particularly to industrial atmospheres in which a protective film of lead sulfate forms. Buried underground, the corrosion rate may exceed that of steel in some soils (e.g., those containing organic acids), but in soils high in sulfates the rate is low. Soluble silicates, which are components of many soils and natural waters, also act as effective corrosion inhibitors. 

A fresh lead surface slowly oxidizes into a thin, protective lead oxide (PbO) that stops further oxidation of the metal. Lead gives satisfactory resistance to corrosion in rural, marine, and industrial environments. The corrosion rate data for lead is shown as 0.5-0.7 pm/y in industrial (New York, NY), 1.2-2.2 pm/y in marine (Kure Beach, NC), and 1.05-1.85 pm/y in rural (State College, PA) [6]. Lead corrosion products in such environments, in addition to lead oxide, are sulfate, chloride, and carbonate, with lead chloride being the most soluble of all four products (see Table 1). However, lead in outdoor exposures was found to produce sulfate (PbS04) and/or carbonate (PbC03), and indoor exposures lead carboxylates. The primary atmospheric agents responsible for degradation of lead are SO2, CO2, and carboxylic acid [7]. The corrosion rate of chemical lead in Key West, Florida, and La Jolla, California is 0.58 and 0.53 pm/y (0.023 and 0.021 mpy), respectively [2]. 

In years gone by zinc coating of steel (galvanizing) was considered a satisfactory corrosion-resistant material for structures in outdoor atmospheres. The corrosion rate was only 0.5-1.0 xm/year. However, the corrosion rate has been increasing in many urban or industrial areas as a result of increasing pollution by SO2. Corrosion rates have reached 5 xm/year or more in many areas. Because of this it is often necessary to apply to galvanized steel a coat of anticorrosion paint for added protection. 

Experience has shown that for a given metal or alloy, the resistance to atmospheric corrosion may differ signihcantly from one site to another. For example, the corrosion rate of galvanised steel may vary from 1 to 100 between a semi-arid zone and the atmosphere of a coastal industrial estate [1]. For a given condition, the resistance to atmospheric corrosion may also vary from one metal to another this variation can be fairly substantial. 

Atmospheric attacks on steels have been studied on field exposed steel in industrial, rural and marine environments and found that P, Cu, Ni, Cr and Si improve the resistance to corrosion while Mn does not seem to affect it and S increases nucleation rate. The relative importance reported for marine atmosphere is P, Si, Cu (up to 0.3 %) and Cr, Ni, Cu (above 0.3 %) [52-54]. 

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