Cathodic protection GA design

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1 Academic Year 2018-2019 Master of Science in Energy Engineering CORROSION AND MATERIAL PROTECTION Prof. A. Brenna Exercise #4 1) A buried coated carbon steel pipe (diameter 0.1 m, length 0.5 km) is protected from soil corrosiveness by a cathodic protection galvanic system. A magnesium alloy galvanic anode system has been adopted to protect the pipe. The expected lifetime is 25 years. Soil is aerated, with an average resistivity of 50 ·m. Protection current density (bare pipe) can be estimated 50 mA/m 2.  Which is the protection potential?  Design the cathodic protection system. Pipeline Length L 500 m Diameter  0.1 m Design life tdl 25 years Initial coating breakdown factor fi 0.001 Average yearly increase of coating breakdown factor f 0.0002 Soil parameters Soil resistivity  50 ·m Protection current density (bare pipe) ibare 50 mA/m 2 Anodic system: cylindrical anode Mg alloy with backfill Anode working potential Ea -1.55 V CSE Mg mass density  1740 kg/m 3 Anode practical consumption w 10 kg/(A·y) Utilization factor u 0.8 Initial diameter i 0.05 m Final diameter (end life) f 0.01 m Length La 0.5 m Consider the following equations:  The protection current density of a coated pipe (ic) after the time t is calculated as follows: fbarec fii   tfff if  [t] = [years]  The resistance located at the anode (Ra) can be calculated by Dwight equation:              14 2 a a a a r LlnLR Where  is soil resistivity (·m), La is the length of the anode (m), ra is the radius of the anode (m). 2  In a first approximation, the resistance located at the cathode (Rc) can be estimated as follows: i,ai,c RR 10 10 i,c f,c RR   The throwing power (Lmax) of the galvanic anode can be calculated as: f,c aprot f,c max i EE i EL    22…