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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32 JUNE 2016 MATERIALS PERFORMANCE NACE INTERNATIONAL: VOL. 55, NO. 6 s u r e d a s m i l l i m h o s p e r c e n t i m e t e r (mmhos/cm). Later, the measurement unit was converted to decisiemens per meter (dS/m). Since these units are equivalent, the numbers do not change. The Web Soil Survey was used in a par- ticular study on the western part of Kern County, California (Figure 3). There is an oilfield development in one sandy, desert- appearing area that is bordered to the east by irrigated farmland. One major soil type present is the Milham sandy loam, with 0 to 2% slope. Aerial photography shows sandy desert, irrigated farmland, and unimproved scrub- land. In the list of soil properties shown in Table 1, the soil profile describes five differ- ent layers that approximately correspond to A, B, and C horizons. Their depths go from ground surface to 60 in (1.5 m). In this Milham sandy loam soil, the soil sur vey data set indicat es that salinity i s m easured from 0 to 8.0 mmhos/cm (8 dS/m) (depending on depth), or nonsa- line to slightly saline. Average conductivity of this soil can be estimated at 4.0 mmhos/ cm (4 dS/m). That equates to a resistivity of 250 Ω-cm, a truly high-corrosivity soil! When reading the soils data, other fac- tors rapidly come into focus. This soil is found in desert conditions, but what is the moisture content? Moisture is the domi- nant factor in the corrosion circuit's elec- trolyte. Dry soils often have 10 to 20% mois- ture by weight. The available water capacity by inches of water column in the Milham soil is stated as 9.1 in (231 mm) of water within the 60-in overall column. That is a moisture content of 15%. Taking the area-specific data for Kern County into a model of cathodic protec- tion (CP) needs, a piece of mild steel in contact with the soil creates an electrical potential difference, or voltage. What if this driving voltage is 0.5 V ? For soil resis- tivity of 1,000 Ω-cm, many CP designers assign a current demand of 11 mA/m 2 for exposed steel, while others use 27.5 mA/m 2 (~2.5 times greater) for a typical CP design. If this local resistivity is actually 250 Ω-cm (conductivity of 4 mmhos/cm), what hap- pens to the design current requirement? The low end must be increased from 11 to 44 mA/m 2 . What if resistivity is 62.5 Ω-cm, which is four times lower yet? The current FIGURE 3 Web soil survey aerial overview, portion of Kern County, California. Source: USDA Web Soil Survey. FIGURE 4 Soil structure and water held in the soil matrix electrolyte. Source: University of Queensland. recent history, construction helps rainfall to get in and soak more of the soil column, and can actually lower the electrical resis- tance of soils. Compared to undisturbed soils, this increases rates of corrosion on any exposed metal. Soils also have widely varying particle sizes, from pieces as big as chunks of rock down to tiny silt and clay particles (Figure 2). 4 The amount of organic carbon/vegeta- tive matter, such as peat moss or decaying leaves and roots, may also factor into a soil's electrical prop er ties. How easily water travels downward through a soil under gravity's influence varies a great deal depending on particle size and distribu- tion. If the soil has laminar, fine-particle layers (often found eons later as shales), water moves very slowly downward. If soil is comprised of big, coarse sand particles, then water travels quickly downward. In addition, soil pH often varies signifi- cantly with depth, commonly starting as acidic near the ground surface and becom- ing more alkaline with increasing depth. Features of local conditions, such as the breakdown of vegetative matter and oxida- tion of steel or other metal, often cause the pH to become more acidic. Soil properties may var y greatly by depth but also laterally. If a river or creek has cut the area over time, some soil depos- its may be very sandy, gravelly, and coarse. Other soils nearby, deposited in quiet water conditions, will have more silts and clays. Soil temperature is yet another external corrosion factor. Pipelines with heated product will warm the soils around them. Even in cold climates, this temperature elevation will increase local biological activity, as well as corrosion rates. Conductivity/ Resistivity Estimates USDA soil science work in salinity levels and electrical conductivity were first mea- CATHODIC PROTECTION

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