Materials Performance

SEP 2018

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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21 MATERIALS PERFORMANCE: VOL. 57, NO. 9 SEPTEMBER 2018 Feasibility Studies on Cathodic Protection of Deep Well External Casing Surfaces and long-line corrosion cells and because time and equipment costs a r e e c o n o m ic a l ly r e a s o n a bl e. Briefly, the method requires ap - plication of incremental amounts of cathodic current to the casing and measurement of the casing-to- soil potential at each current level. Potentials are plotted against the applied current (or more generally against the logarithm of the ap- plied current), and the current re- qu i rement is deter m i ned f rom characteristic changes in the slope of the plot. In the field, relatively large po- tential shifts due to voltage drop, which mask the shifts due to po- larization, are eliminated by mo- mentarily in terrupting the current as potentials are read. Other fac- tors that also influence the shape and possible interpretation of cur- rent-potential plots are (1) position of the reference electrode with re- spect to the ground bed and well head and (2) length of time each current increment is applied be- fore reading the potential and the magni tude of the current incre- ments. These variables are investi- gated here by comparing curves obtained using a surface reference electrode wit h curves obtained with a reference elec trode at the bottom of the casing. It is reasoned t hat t he bot tom-hole ref erence Figure 1—Schematic of field study equip- ment. RE 1 is surface reference electrode; RE 2 the bottom hole reference electrode; SW the switch; R the variable resistance; A the ammeter; G the ground bed; VTVM the vacuum tube voltmeter; and C the casing. Figure 2—Three t ypes of polarization curves. Curve A is the type most frequently encountered. Point of intercept of the exten- sions of the two straight line segments (Point 1) indicates the protective current. Protective current can be determined at Point 2. Curve B has more than one break point, uppermost break is accepted as indicative of protective current. Curve C probably represents either a diffusion lim- ited cathodic reaction, two cathodic reac- tions or an anodically controlled reaction process.

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