Cast iron soils corrosion

The occurrence of graphitic corrosion is not location specific, other than that it may occur wherever gray or nodular cast iron is exposed to sufficiently aggressive aqueous environments. This includes, and is common to, subterranean cast iron pipe, especially in moist soil (Case History 17.1). Cast iron pump impellers and casings are also frequent targets of graphitic corrosion (Case Histories 17.2 through 17.5). 

Limitations on the use of cast irons are similar to those for steel, since in many environments most cast iron has poor corrosion resistance. Most grades are also susceptible to graphitization (the loss of iron, leaving a weak structure of graphite) in acidic environments below a pH of approximately 5.5. This attack occurs in soils. 

Local corrosion or pitting is more important for practical purposes than the rate of general corrosion, and may proceed 10 times or so more rapidly than this. Inasmuch as certain types of cast iron are liable to suffer graphitic corrosion, whereas steel does not, steel might theoretically be expected to show to some advantage when used for buried pipelines. In practice, however, a cast-iron pipe has to be of stouter wall than a steel pipe for equal strength, and it is doubtful whether any distinction between the rust resistance of the two materials in the soil is justified. 

Ferrous corrosion products do not have a biocidal effect however, iron corrosion tends to form a much more extensive and encapsulating matrix. This corrosion matrix, which is principally formed of iron oxides, hydroxides, and soil minerals, will encapsulate any organic materials. This usually takes the form of a negative cast (Keepax 1975), provided that the primary layer of corrosion product has been laid down prior to any extensive degradation of the organic material, then fine surface detail will be preserved in the corrosion cast. Because the iron corrosion will not inhibit degradation... 

The corrosion rate of ductile iron pipes in soils was in the range 0.62-2.5 mm/yr with an average of 1.11 mm/yr which is twice the rate of cast iron pipe. Failure of ductile iron pipe was solely due to localized pitting corrosion leading to pipe wall perforation. 

Figures 7.107 and 7.108 show the corrosive attack on samples of cast iron pipe and ductile iron pipe buried under the soil for 20 and 9 yrs, respectively. The large hole in...

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. 

Severe corrosion on ductile cast iron was observed both in the field and laboratory being attributable to acid sulfate soils containing appreciable concentration of sulfuric... 

A large percentage (57%) of mains and services (46%) is metal (steel, cast iron or copper), and corrosion is a major issue. For distribution pipe, external corrosion is of primary importance, although internal corrosion has been noted in some cases. The methods of monitoring corrosion on cathodically protected pipe are similar to those in the transmission pipeline sector, including pipe-to-soil potential and coating surveys. One difference is that in distribution systems, leak detection is an acceptable method of monitoring for these pipelines without CP (nearly 15% of the steel mains). 

Figures 5.72 and 5.73 show the corrosive attack on samples of cast iron pipe and ductile iron pipe buried under the soil for 20 and 9 years, respectively. The large hole in cast iron pipe (Fig. 5.72) and the corrosion pit and perforation in ductile iron pipe (Fig. 5.73) show the severity of soil corrosion. It is suggested that cathodic protection can reduce the extent of corrosion of iron pipes.

Corrosion in public water piping systems is responsible for large economic loss. In pipes of cast iron, steel and other metallic materials, corrosion may be prevented by use of coatings (Section 10.6) or by water treatment (addition of calcium compounds, alkalization or carbonation). Water distribution systems are further dealt with in Section 8.4, Corrosion in Soils. 

The process has been used for over 25 years and this section will document one of the older pipes that was evaluated after 20 years service. The service environment that the pipehnes experience are sewer water and corrosive soils. The process is used to restore corroded steel, cast iron and concrete pipes. Steel and cast iron pipe are susceptible to corrosion by sewer water through normal acid and salt attack, and by the galvanic... 

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. 

Gerhold, W. F., "Corrosion Behavior of Ductile Cast Iron Pipe in Soil Environment, Journal AWWA, December 1976, pp. 674-678. 

Case No. 2 Many tunnels were built with cast iron rings flanged and bolted together. One surface of the cast iron is exposed to soils and water of various corrosivity. It is... 

Are these corrosion cells common The answer is yes. Whenever a copper pipe service line is directly connected to a cast iron gas or water main, a galvanic cell is formed (Fig. 7.19) the soil is the electrolyte, the copper service line is the cathode, the iron (or steel) main is the anode, and the connecting circuit is completed by attaching the line to the main. Such cells may be relatively harmless when the anode or corroding metal occupies a much larger surface than the cathode so that the attack is spread out over a large area. 

The American Water Works Association (AWWA) has developed a more complex numerical soil corrosivity scale that is applicable to cast iron alloys. A severity ranking is generated by assigning points for different variables presented in Table 10.4 [3]. When the total... 

The assessment is directed at ferrous materials (steels, cast irons, and high-alloy stainless steels), hot-dipped galvanized steel, and copper and copper alloys. Summation of the individual ratings produces an overall corrosivity classification into one of four categories with scores less than -10 indicating a highly corrosive soil and positive values (>0), a noncorrosive environment (Table 10.7). It has been pointed out that sea or lake beds cannot be assessed using this worksheet. 

Coating of the exterior of cast iron pipe with bituminous or lower oil coatings has often been used to provide a measure of protection. More recently tape wrapping with self adherent tapes have been found to be a very cost-economical method of providing an efficient barrier between the metal and a corrosive soil Fig. 10.6. CP can then be applied provided the pipe sections are electrically joined. 

Sulfate Reducing Bacteria SRBs have been implicated in the corrosion of cast iron and steel, ferritic stainless steels, 300 series stainless steels and other highly alloyed stainless steels, copper nickel alloys, and high nickel molybdenum alloys. They are almost always present at corrosion sites because they are in soils, surface water streams and waterside deposits in general. The key s5unptom that usually indicates their involvement in the corrosion process of ferrous alloys is localized corrosion filled with black sulfide corrosion products. 

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