Cast iron atmospheric corrosion

With some important exceptions, gray-iron castings generally have corrosion resistance similar to that of carbon steel. They do resist atmospheric corrosion as well as attack by natural or neutral waters and neutral soils. However, dilute acids and acid-salt solutions will attack this material. 

Mild steel, also low-alloy irons and steels 0 3 0 3 < 400 1 < 750 Wronglit, cast Good Good 67 6.7 Higli strengths obtainable by alloying, also improved atmospheric corrosion resistance. See ASTM specifications for particular grade... 

Because cast iron components are normally very heavy in section, the relatively low rates of attack associated with atmospheric corrosion do not constitute a problem and little work has been carried out on the phenomenon. A summary of some of the data available is given in Table 3.42. The most extensive work in this field was initiated by the A.S.T.M. in 1958 and some of the results produced by these studies are quoted in Table 3.43. It will be noted that there is a marked fall in corrosion rate with time for all the metals tested. 

Although the Ni-Resist irons will not remain rust-free when exposed to the atmosphere their corrosion resistance is much better than that of plain cast iron or mild steel. The results of a 7.5 year exposure trial carried out in a... 

When cast iron is exposed to high temperatures under oxidising conditions, oxidation of the metal results, with the formation of a surface scale. In addition, the dimensions of the component become distorted. Although such dimensional changes can occur also in inert atmospheres or in vacuum, the evidence available suggests that this growth is frequently associated with oxidation, and accordingly it is appropriate to consider it as an aspect of the corrosion of the iron. 

The fact that steel and cast iron suffered by far the greatest corrosion under supposedly the mildest conditions indicated an extraneous influence. This could only have been atmospheric corrosion the evaporator was shut down and the samples were exposed to the air far oftener when the early sludge runs were made—runs which sometimes lasted only a day or two each. 

Even though the test spools were exposed to the atmosphere after 508, 593, and 1093 hours, corrosion rates for steel and cast iron were acceptably low, especially for samples immersed in the liquid. 

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. 

Thick or so-called hard chromium coatings are used directly on steel, cast iron and light alloys to improve resistance to wear and corrosion in hydraulic cylinders, pump shafts, bearings, dies etc. Thermodynamically, chromium is less noble than iron, but under atmospheric and many other conditions it is practically more noble due to the strong tendency to passivation. 

In other respects, corrosion in soils resembles atmospheric corrosion in that observed rates, although usually higher than in the atmosphere, vary to a marked degree with the type of soil. A metal may perform satisfactorily in some parts of the country, but not elsewhere, because of specific differences in soil composition, pH, moisture content, and so on. For example, a cast iron water pipe may last 50 years in New England soil, but only 20 years in the more corrosive soil of southern California. 

While the corrosion rate of bare steel tends to decrease with time, the difference in corrosivity of different atmospheres toward unalloyed cast irons or steels can be quite dramatic. The relative corrosivity for open-hearth steel in atmospheres ranging from a desert to the spray zone on an ocean beach is shown in Table 9.5. Similar ranges in corrosivity were determined by the ISO 9223 corrosion rates for steel (Table 9.1). In a few cases, the corrosion rates of ferrous metals have been reported as increasing with time, and careful analysis of the exposure conditions generally reveals that an accumulation of contaminating corrosive agents has occurred, thus changing the severity of the exposure. 

Zamak alloys have the strong resistance to atmospheric corrosion arni weathering that has been associated for centuries with rolled zinc and zinc-coated iron. This corrosion resistance was confirmed by 10 years of test data compiled by NJZ s research department and the appearance of die cast test bars after 20 years of exposure at the several ASTM exposure sites. 

Gray cast iron and malleable iron are resistant to atmospheric corrosion. They show a higher resistance than low alloy high strength steels. They are, however, sensitive to attack by H2S released by sulfate-reducing bacteria. 

All structural metals corrode to some extent in natural environments (e.g., the atmosphere, soil, or waters). Bronze, brass, most stainless steels, zinc, and pure aluminum corrode so slowly in service conditions that long service life is expected without protective coatings. Corrosion of structural grades of cast iron and steel, the 400 series stainless steels, and some aluminum alloys, however, proceeds rapidly unless the metal is protected against corrosion. As described in Chapter 1, corrosion of metals is of particular concern because annual losses in the United States attributed to corrosion amount to hundreds of billions of dollars. 

Copper is added to cast irons in special cases. Copper additions of 0.25 to 1% increase the resistance of cast iron to dilute acetic, sulfuric, and hydrochloric (HCl) acids as well as acid mine water. Small additions of copper are also made to cast irons to enhance atmospheric corrosion resistance. Additions of up to 10% are made to some high nickel-chromium cast irons to increase corrosion resistance. 

It has been known for more than 50 years that sihcon- and magnesium-containing casting alloys have an excellent resistance to atmospheric corrosion. The first testing results over 10 years in industrial atmosphere in the United States, published in 1946, showed that the pitting depth of samples of A-S5 (5% silicon) and A-S12 (alloy 44100, containing 1% iron) does not exceed 175 p.m [14]. These results were confirmed by subsequent tests, also in industrial atmosphere over 13 years, that showed that the pitting depth does not exceed 200 p,m on the alloys A-S13 (alloy 44100) and A-G5 (alloy 51300) [15]. 

The impetus for further developments was the recognition of the economic significance of corrosion phenomenon during the 19th century that led the British Association for the Advancement of Science to sponsor corrosion testing projects such as the corrosion of cast and wrought iron in river and seawater atmospheres in 1837. Early academic interest in corrosion phenomenon (up to the First World War) was followed by industrial interest due to the occurrence of equipment failures. An example of this is the corrosion-related failure of condenser tubes as reported by the Institute of Metals and the British Non-ferrous Metals Research Association in 1911. This initiative led to the development of new corrosion-resistant alloys, and the corrosion related failure of condenser tubes in the Second World War was an insignificant problem. 

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