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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33 NACE INTERNATIONAL: VOL. 55, NO. 6 MATERIALS PERFORMANCE JUNE 2016 TABLE 1. SOIL DESCRIPTION FOR MILHAM SANDY LOAM, PORTION OF KERN COUNTY, CALIFORNIA Kern County, California, Northwestern Part 196—Milham sandy loam, 0 to 2% slopes MLRA 17 Map Unit Setting National map unit symbol: 2ss91 Elevation: 200 to 1,200 ft (61 to 366 m) Mean annual precipitation: 5 to 8 in (127 to 203 mm) Mean annual air temperature: 63 to 65 °F (17.2 to 18.3 °C) Frost-free period: 250 to 300 days Farmland classifcation: prime farmland if irrigated Typical profle Ap—0 to 4 in (0 to 102 mm): sandy loam Bk—4 to 10 in (102 to 254 mm): sandy loam Btk1—10 to 22 in (254 to 559 mm): loam Btk2—22 to 49 in (559 to 1,245 mm): clay loam 2Ck—49 to 60 in (1,245 to 1,524 mm): sandy loam Properties and qualities Slope: 0 to 2% Depth to restrictive feature: more than 80 in (2 m) Natural drainage class: well drained Runoff class: medium Capacity of the most limiting layer to transmit water (Ksat): moderately high (0.20 to 0.60 in/h [5 to 15 mm/h]) Depth to water table: more than 80 in Frequency of fooding: rare Frequency of ponding: none Calcium carbonate maximum in profle: 10% Gypsum, maximum in profle: 1% Salinity, maximum in profle: nonsaline to moderately saline (0.0 to 8.0 mmhos/cm) Sodium adsorption ratio, maximum in profle: 25.0 Available water storage in profle: high (~9.1 in) Source: USDA NRCS Web Soil Survey, National Cooperative Soil Survey, 2014. requirem ent becom es 176 mA/m 2 , and external corrosion conditions are 16 times w orse th an at th e typi cal " b a d s oi l s" threshold of 1,000 Ω-cm. If one cubic yard of this Milham sandy loam weighs 2,800 lb (1,270 kg) (as a rough estimate), then the 15% moisture level means 420 lb (191 kg) of water are present in just that one cubic yard. Some of this water is tied up in chemisorbed bonds, but much of it is freely held in the soil matrix. Every soil particle is "wetted" by that very substantial mass of water (Figure 4). 5 Studies of new pipeline right-of-way conditions using the Web Soil Survey can pro v i d e a g o o d u n d e r st a n di n g of C P requirements. Specifying a few field resis- tivity measurements to corroborate the Web Soil Sur vey understanding is also sound practice; however, much valuable knowledge can be gathered simply through use of a computer and the Internet. Study of the first several hundred feet of geology is also of great value, especially to learn how anode beds should be designed and constructed. Studies of existing pipeline corridors can be of great value. In one case study, the pipe- line operator thought that CP was needed (and being applied) in a range of ~1.5 A of protective current per km. However, exter- nal corrosion failures were occurring. Mod- eling the right-of-way for soil types and resistivities, chlorides, etc., through a Web Soil Survey review showed a more likely current requirement of 6 A per km. There are many geolog y references available online. Some of these include state-by-state databases of water well installation reports, often with driller - generated descriptions of geologic materials encountered. U.S. Geological Survey reports abound, and many states have funded stud- ies of geologic formations and groundwater aquifers. Tremendous information about the possibility of deep anode beds can be learned by interviewing local drillers and reading peer-reviewed publications. One must also judge how pipeline or facility installation has affected shallow soil and geology conditions. In most cases, the pipeline installation process increases local soil permeability for water, increases soil moisture gain, and therefore increases local soil corrosivity. When a crushed lime- ston e b a s e i s c omp a c t ed o v er n ativ e ground, oxygen no longer recharges into underlying soils. An extreme example of moisture gain is what happens under bulk storage tank bot- toms. For crude oil and refined petroleum product storage, bulk tanks are typically surrounded by a secondary containment berm . This containment fills with rain water, which soaks into shallow soils and the berm material. Any CP system designed for under-tank protection must take into account the likely water-saturated condi- tions plus local ion concentrations. Summary Soil resistivity is a key factor in deter- m i n i n g e f f e c t i v e C P c u r re n t d e n s i ty requirements. Soil survey data will help the CP design practitioner do a far better job in predicting how much CP is needed along a right-of-way or at a facility. Many factors need to be studied for external corrosion control design . Soil Publicly Available Soils Data for External Corrosion Control

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