Discuss the shielding issue of casings (cathodic protection)

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This post was last edited by blueiceqoo on 2009-7-3 16:14. I found several articles on the forum discussing the shielding effect of casings for the cathodic protection of pipelines, but I have a few questions: 1. Does the shielding effect mentioned here refer to the casing creating an environment free of electrolytes between the main pipeline and the ground, thereby preventing the formation of a circuit and achieving physical isolation? Or is it the electrical shielding effect of the sleeve, whereby the current flows to the outside of the sleeve, causing it to become an equipotential body, while the inner main pipeline remains shielded and thus not supplied with current? 2. Why can’t water get into the casing? If there is an electrolyte environment, doesn’t forming a circuit facilitate cathodic protection even more? 3. Why is the insulation between the sleeve and the main pipe poor? Finally, installing a sacrificial anode inside the casing might cause the pipe potential to drop too low, leading to over-protection and preventing the external current cathodic protection system from functioning properly (At the moment, the potentiostat is showing a pipe potential of -1500 mv, which triggers an alarm; we can only turn it off for now. After turning it off, the potential still remains around -1300 mv. Could this be caused by the use of a sacrificial anode inside the casing?) ) I really don’t understand; please help, thank you