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11052 Cathodic Protection Behavior of API X-52 and API X-65 Steels Buried in Natural Soil

Picture for 11052 Cathodic Protection Behavior of API X-52 and API X-65 Steels Buried in Natural Soil
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Product Number: 51300-11052-SG
ISBN: 11052 2011 CP
Author: Diego Mendoza, Andres Aguilar and Ramiro Perez
Publication Date: 2011
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The goal of this work is the evaluation of the cathodic protection in API X-52 and API X-65 steel specimens buried in natural soil, using two distinct cathodic protection potentials (-850 mV and -1100 mV vs. CSE). The first protection potential is recommended by the NACE SP0169 standard and the second potential was chosen because it is considered that a pipe is overprotected if the protection potential is more negative than -1100 mV vs. CSE. Soil samples from two villages localized in Veracruz, Mexico, along a pipeline right of way, were used during the experiments in the laboratory to simulate the in-situ corrosion behavior of pipelines buried in soil. The experimental results show that the cathodic protection was less efficient in acidic soils than in neutral soils. Results further show that the least corroded steel specimens were those overprotected. An inverse relationship between corrosion potential and corrosion rate is demonstrated using a linear regression model. A mechanism involving moisture content, soil resistivity and oxygen content is proposed to explain this inverse relationship. Similarly, this mechanism can explain why the cathodic overprotection produced more positive corrosion potentials.

Key words: soil corrosion, soil moisture, soil resistivity, cathodic protection, cathodic overprotection, corrosion potential, corrosion rate, Tafel extrapolation, polarization resistance, cathodic protection standard, oxygen content, soil acidity.
The goal of this work is the evaluation of the cathodic protection in API X-52 and API X-65 steel specimens buried in natural soil, using two distinct cathodic protection potentials (-850 mV and -1100 mV vs. CSE). The first protection potential is recommended by the NACE SP0169 standard and the second potential was chosen because it is considered that a pipe is overprotected if the protection potential is more negative than -1100 mV vs. CSE. Soil samples from two villages localized in Veracruz, Mexico, along a pipeline right of way, were used during the experiments in the laboratory to simulate the in-situ corrosion behavior of pipelines buried in soil. The experimental results show that the cathodic protection was less efficient in acidic soils than in neutral soils. Results further show that the least corroded steel specimens were those overprotected. An inverse relationship between corrosion potential and corrosion rate is demonstrated using a linear regression model. A mechanism involving moisture content, soil resistivity and oxygen content is proposed to explain this inverse relationship. Similarly, this mechanism can explain why the cathodic overprotection produced more positive corrosion potentials.

Key words: soil corrosion, soil moisture, soil resistivity, cathodic protection, cathodic overprotection, corrosion potential, corrosion rate, Tafel extrapolation, polarization resistance, cathodic protection standard, oxygen content, soil acidity.
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