Corrosion rates/resistance marine atmospheres

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. 

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... 

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 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]. 

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]. 

The corrosion behavior of stainless steels in outdoor exposure tests near the shore in a marine atmosphere is also described in DIN 81249, part 4 (1997). The information given relates to a distance of 25 m from the waterline outside the splash line. Table 1-10 lists the corrosion rate and susceptibility to pitting or crevice corrosion. CrNi steels containing molybdenum are passive and have good corrosion resistance. 

Zinc is exposed to the atmosphere in the form of galvanized sheet, as in flashings on roofs as die castings, and as coatings on steel, either hot-dipped or electroplated. The general behavior of zinc metal and zinc coatings is described in the ISO tables presented earlier. Note the particularly low rates of attack on zinc as compared with steel in marine exposures where chloride deposition is important. Such excellent resistance is acquired by the hard, dense, protective products of corrosion in a chloride atmosphere. Similar results cannot be obtained in a sulfurous atmosphere where the corrosion products are soft, voluminous, and non-protective. 

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