Cast iron pitting

MAZAL,P.-DVOftACEK,J.- KOLAft,D. AET Utilisation for Pitting Observation of Grey Cast Iron with Heat Treated Sur ce. Surface Modif. Technologies XI, will be published 1998. 

When the layer of graphite and corrosion products is impervious to the solution, corrosion wdl cease or slow down. If the layer is porous, corrosion will progress by galvanic behavior between graphite and iron. The rate of this attack will be approximately that for the maximum penetration of steel by pitting. The layer of graphite formed may also be effective in reducing the g vanic action between cast iron and more noble alloys such as bronze used for valve trim and impellers in pumps. 

The operator took about one to two minutes to open the valve halfway very soon afterward, there was a loud bang as a 6-in. cast-iron valve on a branch (unused and blanked) failed as a result of water hammer. The operator was able to climb out of the pit, but later died from his burns, which covered 65% of his body [17J. Figure 9-11 explains the mechanism. 

Blowdown sumps should be constructed from brick and/or concrete and the blowdown lines should drain under gravity. Where the blowdown lines enter the sump they should turn to discharge downwards and the bottom of the sump should be protected below this area with a cast iron tray to prevent erosion. The drain or overflow from the pit should be at such a level to produce a weir effect, thus holding water for dilution. 

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. 

Very rapid and highly localised pitting is sometimes observed on components exposed to very turbulent flow conditions leading to cavitation in the stream. In general, these conditions appear to induce corrosion rather than erosion on cast iron surfaces, in contradistinction to what usually happens with other metals, apparently because the erosive component of the liquid flow scours away corrosion-stifling films and allows the development of very active electrochemical cells on the exposed metal surfaces . 

A characteristic of the corrosion on buried ferrous metals is that the attack is usually mostly in the form of pitting, especially with the cast irons. This raises a problem in measuring the extent of corrosion in burial trials. Usually both the weight loss, measuring the average loss of section, and the deepest pit, measuring the maximum loss of section, are reported. For assessing the severity of the attack on buried pipes, the second parameter is clearly the most important. 

Nearly simultaneous wl th the earliest improvements in the manufacture of east-iron by ptea B of pit-coal,1 were, others of, an equally important kind, for converting the cast-iron, into malleable or bar-iron. Hitherto this had been effected. by means. of wopd charooal in what are, termed refiners, aq ft. stiff practised In the, South of. France but owing to the scarcity, of wood foal in. this country, the, charcoal was mixed up with coke, for the refining operation the iron produced from. this was hard and of an inferior quality, and much time was required for the conversion of a ton of cast into malle-... 

Cast Iron. The iron phase in cast iron is readily attacked by sea water, as is the case for mild steel. If the layer of graphite left with the corrosion product is dense and compact, further corrosion tends to be stifled. If the layer is porous, corrosion may be accelerated by the galvanic action between the graphite and the iron beneath. The attack then approaches a rate similar to that found for the pitting of mild steel. 

The properties of the interface metal/solution. Cast iron corrodes because of exposure of its graphite to the surface (graphitic corrosion), which is cathodic to both low-alloy and mild steels. The trim of a valve must always maintain dimensional accuracy and be free of pitting and hence it should stay cathodic to the valve body. Hence, in aggressive media, valve bodies are frequently chosen of steel rather than cast iron. Because of increased anodic polarization, low-alloy steel (Cr and Ni as noble components) is cathodic to normal steel in most natural media. Accordingly, steel bolts and nuts coupled to underground mild steel pipes, or a weld rod used for steel plates on the hull of a ship, should always be of a low-nickel, low chromium steel or from a similar composition to that of the steel pipe.7... 

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. 

A sudden rush of liquids from equipment into the drainage system would cause compression of entrapped gases in the sewer ahead of the stream and a vacuum behind the stream. Such pulsation drains all the traps and permits permeation of odors and sewer gases. The breather system, by proper installation of vents, is always installed according to code. In order to prevent the back rush of liquids, a ball check is installed ahead of a condenser. Both check valve and subsequent trap should be accessible for cleanout. The bell of the trap extends down from the perforated or gridded floor screen and covers a portion of the drain pipe extending above the bottom of the drain pit. More commonly the floor drains, drain screen, bell, and outside case of the pit are cast iron. 

Water meter pit (or vault)- The box /cast iron bonnet and concrete rings that contains the water meter. 

Phosphates and silicate corrosion inhibitors have been used with or without pH control, to reduce the metal release and to prolong the service life of distribution systems or domestic installations. When the concentration is limited, the inhibitors may not avert localized corrosion such as pitting or the corrosion of galvanized steel, steel, cast iron, copper, or lead, sufficiently to extend the life of the system beyond 75-100 years. Corrosion inhibitors are useful when concerns about water quality deterioration have to be resolved. Unfortunately, there is no simple solution for balancing water quality, health risks, system reliability, and environmental impact. 

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.

In hard or moderately hard waters, galvanised steel hot-water tanks, with galvanised circulating pipes and cast-iron boilers, usually give trouble-free service, but failure by pitting occurs occasionally. Sometimes this is due to extraneous causes, such as rubbish left inside the tank when it is installed. Iron filings left in the bottom of the tank or deposits of inert material are liable to interfere with the formation of the protective scale by the water, and can lead to failure. 

Cast iron is initially anodic to low-alloy steels and not far different in potential from mild steel. As cast iron corrodes, however, especially if graphitic corrosion takes place, exposed graphite on the surface shifts the potential in the noble direction. After some time, therefore, depending on the environment, cast iron may achieve a potential cathodic to both low-alloy steels and mild steel. This behavior is important in designing valves, for example. The trim of valve seats must maintain dimensional accuracy and be free of pits consequently, the trim must always be chosen cathodic to the valve body making up the major internal area of the valve. For this reason, valve bodies of steel are often preferred to cast iron for aqueous media of high electrical conductivity. 

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