Depends on many factors like criticality, possible outage scenarios, and of course, length of the piping.

It seems to me okay to limit the coated area.

DHURJATI SEN
Kolkata, India

Thanks, Dhurjati,

To make sure I understand your thinking, when you say it seems okay to "limit the coated area", do you mean that it seems okay to to take credit for the coated area, i.e. one could reduce the area used for the calculation and not use the entire area, if it is coated?

As much as possible, do it right the first time...

The bare area is determined using a notional 'coating breakdown factor'. Try consulting a design standard such as ISO 15589-1.

Steve Jones
Corrosion Management Consultant

www.linkedin.com/in/drstevejones

All answers are personal opinions only and are in no way connected with any employer.

Whether cathodic is "required" is more of a question of whether the pipeline is "regulated". You have to research the applicable regulatory codes to determine whether your specific application is a regulated line, and if so, if that regulation requires cathodic protection.

If cathodic protection is required, then normally a new pipeline will be assumed to have an "effective coating area (aka coating efficiency)". New pipeline that is jeeped, with coated fittings/valves/etc. and installed properly should be 99+% (99.9+% maybe even) coated. Therefore, the bare area will be quite small. The higher the bare area, the more anodes that you'll need to polarize the pipeline. You need to run tests to actually determine what the coating efficiency is.

SO

"When a buried pipe is externally coated, is cathodic protection (sacrificial anode) still required?" - Required implies "Must have".It's not required, but is often recommended, but depends on how aggressive the soil is, whether you have inspection systems set up to detect coating breakdown or corrosion, what the material is of the pipe and how long is the design life?

"and if it is, is it okay to limit the coated area used for calculation of the weight to the "estimated" bare pipe or all the pipe?" - As said by others, this is all part of the calculation used to determine how big an anode you need. CP current starts off low and increases over time usually as the coating slowly breaks down and damage appears.

Sacrificial anodes are usually designed for 15 to 20 years before needing replacement and are needed on a spacing of <500m. Hence for long pipelines an impressed current system is normally specified.

Ask a CP engineer / consultant as if you get it wrong you can either under or over protect the system.

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