Stainless steels soils, 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. 

As in the case of corrosion at the insulating connection due to different potentials caused by cathodic protection of the pipeline, there is a danger if the insulating connection is fitted between two sections of a pipeline with different materials, e.g., mild and stainless steel. The difference between the external pipe/soil potential is changed by cell currents so that the difference between the internal pipe/ medium potential has the same value, i.e., both potential differences become equal. If the latter is lower than the former for the case of free corrosion, the part of the pipe with the material that has the more positive rest potential in the soil is polarized anodically on the inner surface. The danger increases with external cathodic protection in the part of the pipeline made of mild steel. 

Light, sandy, well-drained soil of high electrical resistivity is low in corrosivity and coated steel or bare stainless steels can be employed. It is unlikely that the whole pipe run would be in the same type of soil. In heavier or damp soils, or where the quality of back filling cannot be guaranteed, there are two major corrosion risks. Steel, copper alloys and most stainless steels are susceptible to sulfide attack brought about by the action of sulfate-reducing bacteria in the soil. SRB are ubiquitous but thrive particularly well in the anaerobic conditions which persist in compacted soil, especially clay. The mechanism of corrosion where SRB are involved is described in Section... 

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. 

The corrosivity of soils also depends upon the oxidation-reduction potential as classified by Booth et al.15 The classification scheme of the corrosivity of soils is given in Table 4.4b. Macrogalvanic cells are formed in underground pipelines due to foreign structure the combination of new and old pipe dissimilar metals (stainless steel and carbon steel) differential aeration dissimilar soils and stray currents. All these lead to localized corrosion of underground pipelines. 

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. 

The hot brine found in crystallization plants can be quite corrosive. Inconel, Monel, and stainless steel are used in most of the major equipment. Nonmetallic piping, including FRP, is common, within its serviceable temperature range, and Schedule 80 carbon steel pipe is often used in pump lines. The soil in a salt-processing plant can itself become corrosive as a result of spillage. Underground carbon steel lines therefore are protected by sacrificial anodes of magnesium or zinc or by application of direct electrical current. 

Gerhold, W. F., Escalante, E., and Sanderson, B. T., "The Corrosion Behavior of Selected Stainless Steels in Soil Environments, NBSIR 81-2228(NBS), American Iron and Steel Institute, Washington, DC, February 1981. 

In another program the corrosion resistance of aluminum, copper and copper clad stainless steel has been studied in four different soils for three years [4]. 

Coating and cathodic protection should be considered for stainless steel in soil and water. Stainless steel may be used in most atmospheric exposures and may also be used as hardware for connection to steel. Stainless steel should not be used for complex structures with overlapping bolted connections in soil or fluid exposures. Bolted connections of this type in soil or fluid exposures can experience very rapid crevice corrosion. 

In soil, stainless steel is fairly resistant to uniform corrosion, which occurs over the entire surface however, it may be subject to pitting corrosion. Stainless steel is most often used in situations where contamination of the material carried in the pipe is the prime concern. However, as pitting of the buried structures might occur, where soil conditions surrounding the pipe vary, it would be prudent to install stainless steel pipe with a uniform, well-installed backfill where differential oxygen corrosion cells will not occur. Coatings and cathodic protection of buried pipehnes in corrosive soils should be considered. In noncorrosive soils, coatings for stainless steel are recommended. 

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. 

Stainless steels are rarely used in soil applications, as their corrosion performance in soil is generally poor and not better than bare steel. Localized corrosion attack is a particularly serious concern. The presence of chloride ions and concentration cells developed on the surface of these alloys tends to induce localized corrosion damage. 

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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