Materials Performance

JUN 2016

Materials Performance is the world's most widely circulated magazine dedicated to corrosion prevention and control. MP provides information about the latest corrosion control technologies and practical applications for every industry and environment.

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30 JUNE 2016 MATERIALS PERFORMANCE NACE INTERNATIONAL: VOL. 55, NO. 6 CATHODIC PROTECTION W External corrosion risks are always present for steel structures in contact with soils. For cathodic protection de- sign and management, it is very help- ful to learn about soil resistivity, ion content, moisture, and temperature changes with time. A U.S. Department of Agriculture detailed soil sampling program from the early 1900s through the 1990s generated descriptions and chemical analyses across much of the United States. These data are avail- able through county-by-county soil survey books, and in a Web-based format. Which soil properties affect the corrosion behavior of exposed steel? The simple answer is "all of them." Soil resistivity, ion content (especially sodium chloride [NaCl]), moisture, and temperature are all of interest. Resistivity is often field-measured at a significant cost. Measurement of ion and moisture content requires soil sampling and analysis by a qualified laboratory. Again, costs are considerable. When a pipeline is constructed, it goes into soil and shallow geology that have been in place for thousands or even mil- lions of years. Is there a way to learn about the soil properties without spending large sums of money on field surveys and soil sampling? In the United States, and some other countries, there is. A particular U.S. program related to agriculture was established to study soils throughout most of the country. Originat- ing in the 1890s and run by the U.S. Depart- ment of Agriculture (USDA) since the 1930s, Publicly Available Soils Data for External Corrosion Control Cal Chapman, Chapman Engineering, Inc., Boerne, Texas th i s pro g ra m i nv o lv e d a tre m e n d o u s amount of field investigation. Cooperation among land-grant universities, county agri- cultural ext ension agents, and federal employees generated detailed soil sampling with descriptions and chemical analyses. County-based soil survey books were cre- ated for most of the land across the United St at e s. Th e d at a are n ow onlin e and ac cessible through the Web Soil Survey 1 (websoilsur vey.nrcs.usda .gov), which is a d m i n i st e re d b y t h e U S D A's Na tu ra l Resources Conservation Service (NRCS). When using this database, the detailed properties of many soils can be reviewed. The data sets are geared toward agri cultural usage, irrigation of crops, soil mechanics, and structural/civil engineering factors. Applicability for septic system installations, use as fill material, or building foundation support can all be checked. External corro- sion risk also can be studied. What is a high-risk soil for corrosion? These soils often have significant clay con- tent, elevated soil moisture, and aggressive chemical ions present. These properties make the soil across the United States a good electrical conductor that can support the electrochemical exchanges of corrosion cells. Typically, any soil with resistivity <1,000 Ω-cm is considered highly corrosive; 2 but a soil with 10,000 Ω-cm resistivity may still be quite corrosive. More specific factors must be reviewed, including long-line effects between areas of different resistivities. USDA soil scientists did not study resis- tivity directly, but rather electrical conduc- tivity, its inverse. Measurements performed in the field were originally described as salinity. B e cau se th e studi e s' ori ginal emphasis was on agricultural production,

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