Soils pitting corrosion

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.

The main problem is the external problem, mainly due to the corrosive nature of the soil. Across the pipeline route, the soil can be soft mud, hard mud, rocks, sand, sand with minerals, moisture, and other environments such as marshy lands, brackish water, seawater, and so forth. Ordinary steel with several compositions, shown in Table 8.1, shows severe corrosion, general corrosion, pitting, and so forth. One kind of serious corrosion problem that takes place is microbiological corrosion, which occurs due to presence of sulfate-reducing bacteria (SRB). They convert sulfate in soil to sulfide, which attacks steel, causing severe pits. Thus, external corrosion is taken care of by a combination of a good coating and cathodic protection. Let us now deal with internal and external corrosion in more detail. 

Soils will pit steels, which obviously affects buried pipelines. In one study of 10 carbon and low-alloy carbon steels containing Cr, Ni, Cu, and Mo and exposed to a variety of soils for 13 years, the conclusion was that factors such as soil pH, resistivity and degree of aeration have more influence on the severity of corrosion than the alloy content of the steel. In any case, protective coatings and cathodic protection are the best means of reducing corrosion in buried pipelines. 

Engineers concerned with soil corrosion and underground steel piping are aware that the maximum pit depth found on a buried structure is somehow related to the percentage of the structure inspected. Finding the deepest actual pit requires a detailed inspection of the whole structure. As the area of the structure inspected decreases, so does the probability of finding the deepest actual pit. 

In unaerated or anaerobic soils, this attack is attributed to the influence of the sulfate-reducing bacteria (SRB). The mechanism is believed to involve both direct attack of the steel by hydrogen sulfide and cathodic depolarization aided by the presence of bacteria. Even in aerated or aerobic soils, there are sufficiently large variations in aeration that the action of SRB cannot be neglected. For example, within active corrosion pits, the oxygen content becomes exceedingly low. 

High-alloy steels with >16% Cr" (e.g. 1.4301, AISI 304) Neutral waters and soils (25°C) <0.2 <-0.1 Protection against pitting and crevice corrosion... 

Stainless steels in soil can only be attacked by pitting corrosion if the pitting potential is exceeded (see Fig. 2-16). Contact with nonalloyed steel affords considerable cathodic protection at f/jj < 0.2 V. Copper materials are also very resistant and only suffer corrosion in very acid or polluted soils. Details of the behavior of these materials can be found in Refs. 3 and 14. 

For corrosion protection in soils, the anodes can be brought close to the object to be protected in the same construction pit so that practically no further excavations are needed. By connecting anodes to locally endangered objects (e.g., in the case of interference by foreign cathodic voltage cones) the interference can be overcome (see Section 9.2.3). 

Nonuniform corrosion or pitting corrosion frequently occurs on steel structures in seawater and in soil. Nonuniform and pitting corrosion easily lead to damage in tanks, pipelines, water heaters, ships, buoys and pontoons, because these structures lose their functional efficiency when their walls are perforated (see Chapter 4). 

Material Acid droplet pitting, nylon hose destruction Rubber cracking, silver tarnishing, paint blackening Corrosion, soiling, materials deterioration... 

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. 

Stainless steels have not been widely used in applications where they are buried in soil, but some applications have involved underground service. Various stainless steels from the 13% Cr to the molybdenum-bearing austenitic types were included in the comprehensive series of tests in a variety of soils reported by Romanoff . High-chloride poorly-aerated soils proved most aggressive, but even here the austenitic types proved superior to the other metals commonly used unprotected. Of special interest is the fact that though corrosion was by pitting there was little or no increase in pit depth after the first few years. 

It should be noted that it is extremely difficult to predict service lives of buried pipelines from the results of controlled trials with small specimens, whether in the laboratory or in the field. For example a study on the comparative corrosion resistances of ductile and grey iron pipes carried out jointly by European pipemakers in 1964-1973 indicated a mean pitting rate of 0 -35 mm/y for uncoated ductile iron pipe exposed in a typical heavy Essex clay of 500-900 ohm cm resistivity for 9 years. This is clearly at odds with the rate of 1 mm/y normally found on a corroded service pipe from such a soil. The discrepancy appears to be due to the use of specimens that were only a third of a pipe length each and were buried separately. It may reflect the contribution of the total surface area of the pipe as a cathode to the corrosion current at the anodic area at the pitting site. 

Patches of conductive lead sulphide can be formed on lead in the presence of sewage. This can result in the flow of a large corrosion current . Sulphate-reducing bacteria in soils can produce metal sulphides and H2S, which results in the formation of deep pits containing a black mass of lead sulphide . Other micro-organisms may also be involved in the corrosion of lead in soil . 

In tests carried out by the National Bureau of Standards in the USA specimens of copper alloys, lead, zinc and zinc alloys were buried at a number of different sites for periods varying from 11 to 14 years. The soils tested covered a pH range from 2-6 to 9-4 and resistivities ranged from 62 to 17 800 fi cm. The weight losses and maximum depths of pitting were recorded, and the results indicated that the most severe corrosion occurred in soils of poor aeration having high acid and soluble-salt contents. 

Galvanised steelwork buried in the soil in the form of service pipes or structural steelwork withstands attack better than bare steel, except when the soil is more alkaline than pH 9-4 or more acid than pH 2-6. Poorly aerated soils are corrosive to zinc, although they do not necessarily cause pitting. However, soils with fair to good aeration containing high concentrations of chlorides and sulphates may do so. Bare iron may be attacked five... 

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