Cathodic Protection
Cathodic Protection
(OP)
This has been posted in the Structural Engineering Forum also:
We have an existing steel structure that is partially submerged. Steel 'S' sections have corroded near the water line by approximately 1/8" over a period of 40 years. Sufficient material remains such that strength is not an issue at this point. We are considering the application of sacrificial zinc strips or 'hockey pucks' to minimize corrosion for the next 40 years. Is this sufficient to provide corrosion resistance and what is the farthest distance from the unprotected surface to the zinc strip is acceptable? Any other method of protection acceptable?
thanks, Dik
We have an existing steel structure that is partially submerged. Steel 'S' sections have corroded near the water line by approximately 1/8" over a period of 40 years. Sufficient material remains such that strength is not an issue at this point. We are considering the application of sacrificial zinc strips or 'hockey pucks' to minimize corrosion for the next 40 years. Is this sufficient to provide corrosion resistance and what is the farthest distance from the unprotected surface to the zinc strip is acceptable? Any other method of protection acceptable?
thanks, Dik





RE: Cathodic Protection
The issues, or hurdles that must be addressed are:
1. Waterline protection. An area wetted but not submerged will not be protected by submerged anodes. This is one of several reasons why corrosion rates are typically the greated near the "splash zone".
2. The anodes should be near "line of sight" to all surfaces needing protection.
3. You need a sufficient number of anodes to polarize the entire structure.
RE: Cathodic Protection
Is it a matter of securing aluminum or zinc anodes at a few feet o/c (2'-3'). Some of the information I downloaded has a design basis using pH and conductivity/resistivity of the water.
For the non submerged area, is it a matter of simply attaching them, or is it necessary to provide a small impressed current?
By a sufficient number, is there a manner of calculating the mass of sacrificial material; an active maintenance program can easily be established.
Is it better to use Aluminum in lieu of zinc? From a cost point of view or an electrochemical vantage?
Your assistance is appreciated, thanks, Dik
RE: Cathodic Protection
Can adding sacrificial anodes cause unexpected corrosion similar to anodic corrosion / pits at discontinuous surfaces in the epoxy coating?
Dik
RE: Cathodic Protection
1. Leave it to your retained corrosion (cathodic protection) specialist to answer.
2. Start thinking about whether it will be feasible to apply a good splash zone coating in-situ and how to attach the anodes underwater.
3. Review DNV RP B401 a commonly applied offshore CP standard.
Steve Jones
Materials & Corrosion Engineer
http://www.pdo.co.om/pdoweb/
RE: Cathodic Protection
ht
RE: Cathodic Protection
I'd planned to, but wanted information to at least ask 'intelligent' questions. Thanks for the doc info... Even if I'm not responsible for the technical info, I'm a bit of an info junkie and like to have a bit of a background on what I'm doing... Again, thanks.
TVP...
Thanks for the link... copied it and printed it off...
Dik
RE: Cathodic Protection
if you apply anodes they will protect also the area of discontinuities of the coating.
1/8 of inch in 40 ys in a splash zone is not a lot, maybe for the next 40 ys it will be enough a good coating.
S.
http://www.corrosionist.com
RE: Cathodic Protection
I'd be considering an immersion resistant coating to provide a barrier system to future corrosion.
A CO2 pellet abrasive blast removes the problem of polluting local water with the abrasive.
International have some high build coatings that will cure underwater and happily hang onto a class2.5 or better blasted metal surface.
Are there local tides you could take advantage of to paint significantly below the normal water line?
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RE: Cathodic Protection
CO2 pellet abrasive blast is a basting technique that uses 'dry ice'? I've not heard about this method. Thanks for the reply.
Based on the info from this forum, we were looking at using a high build system with aluminum 'passive' sacrificial anodes for the wetted portion. It's a fresh water system and seems that aluminum 'pucks' are most effective.
Dik