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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31 NACE INTERNATIONAL: VOL. 55, NO. 6 MATERIALS PERFORMANCE JUNE 2016 c o n d u c t i v i t y m e a s u r e m e n t s s h o w e d whether the content of NaCl or similar ions was harmful to plant growth. NaCl often occurs naturally because the land lay under an ocean or shallow sea eons ago. This is true for much of the American West and for m a ny d e s e r t a n d s e m i - d e s e r t l o c a l e s around the world. Measurements of soil conductivity (or resistivity) are extremely valuable when evaluating external corrosion risk. Gener- ally, soil resistivity is figured as a bulk property of the soil. The USDA NRCS data typically involve the first 5 to 8 ft (1.5 to 2.4 m) of the soil column , which coincides nicely with pipeline burial depths. The Web Soil Sur vey descriptions also often give conductivity estimates within smaller depth intervals. Chloride ions and sodium ions found in plain old table salt are bad for plants and soil quality. Chlorides stunt plant growth and even prevent seeds from germinating. Sodium reduces plant uptake of vital cal- cium, magnesium, and other mineral ions. It also destroys clay aggregation properties, thereby "clogging up" fine-grained soils so wat er no longer p ercolat es ef fectively through the soil column. This latter prop- erty is called the sodium adsorption ratio (SAR). Any SAR number >10 for soil or irri- gation water indicates excessive sodium ion concentration. Soil Structure What does a typical soil column look like? Soil scientists describe the top-most surface soil segment as the O horizon , speaking to its organics-rich nature (Figure 1). 3 Below this O portion is the A horizon, which is very heavily affected by plant roots and many f lourishing subsurface organ- isms. Rich American farmland of the Mid- west and Great Plains may have the A hori- zon more than 10 ft (3 m) below grade. In other areas, it may not reach more than a few inches. O and A horizons are often comprised of good quality soil that is dark brown to black, the color of top soil, and ideal for planting. Shallow soils tend to exchange air with the atmosphere, so more oxygen is present SOIL HORIZONS CHANGE WITH DEPTH Horizon Depth Description O 0 to 2 in (0 to 51 mm) Rich in organisms, roots, vegetative debris (top soil from A) A 2 to 10 in (51 to 254 mm) Rich in roots and organic material B 10 to 30 in (254 to 762 mm) Less roots, less air recharge, more CO 2 , and some methane (CH 4 ) C 30 to 48 in (762 to 1,219 mm) More like geology below, very little organic activity, low oxygen Source: USDA NRCS Web Soil Survey, National Cooperative Soil Survey, 2014. FIGURE 2 Soil grain types and sizes. FIGURE 1 Soil layers by horizon. Source: U.S. Department of Agriculture Natural Resources Conservation Service. there than in deeper soil. Many organisms grow in the shallow soil root zone much more effectively than in deeper soils, so the highest organic content is usually at shal- low depths. The B horizon has much less root vol- ume and fewer active organisms, less air recharge from the atmosphere, more car- bon dioxide (CO 2 ) accumulation from plant transpiration, and frequently some meth- ane (CH 4 ). The CH 4 load is generated by anaerobic microbes. When sulfate ion con- tent is elevated and oxygen content is depleted, sulfate-reducing bacteria (SRB) become a significant corrosion factor for exposed steel. The C horizon, found at even greater depth, is much more like the geol- ogy below it than the horizon above. It has very little organic activity, low oxygen con- tent, and not much in common with shal- lower soils. All of these horizons change with other factors, like excess moisture that moves through soil during prolonged rainfall/wet climate periods, or substantially dry soils, that can even suffer from desiccation crack- ing during extended drought conditions. Constructing a pipeline or other facility can change soil properties. By digging a ditch and breaking up soils for the first time in

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