CP test station location is design process, Normally for a sacrificial anode system for onshore. the test station is located for every anode location(say 1km) based on you design philosophy and need for monitoring.For Impressed Current, the conservative standard for Potential Test Post is...
First to solve the problem is by Design.
First since there is the presence of CO2 then calculate the corrosion rate using De warrd and Miller Equation. Then if the corrosion rate is very high you can reduce it by applying the Inhibitor Availability Equation asuming that inhibitor is available...