Cathodic Protection 5

[dik]

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

 

[mshimko]

Regarding cathodic protection: as long as the driving voltage of the anodes are sufficient, the anodes can "throw" current quite a distance. For salt water zinc or aluminum anodes are fine.

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.

 

[dik]

I downloaded a pile of information about this yesterday and we'll be retaining a corrosion specialist. Current corrosion is in the vicinity of the waterline for a depth of about 12'. I suspect this is due to oxygen levels, fluctuating water level, and ice.

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

 

[dik]

I forgot to add that the existing structure that is not corroded has a high build epoxy type coating.

Can adding sacrificial anodes cause unexpected corrosion similar to anodic corrosion / pits at discontinuous surfaces in the epoxy coating?

Dik

 

[SJones]

Dik,

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/

 

[TVP]

The document that Steve Jones recommended can be freely downloaded using the following link:

http://exchange.dnv.com/OGPI/OffshorePubs/ViewArea/RP-B401.pdf

 

[dik]

SJones...

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

 

[strider6]

usually the splash zone is coated to provide corrosion protection and also 'cause it will be wet/dry and cathodic protection does not work if the structures is not submerged in water.
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

 

[robsalv]

Splash zones are notorious corrosion zones. Zinc/Mg/Al blocks will provide localised protection where there is an electrolyte, but it might not penetrate to the bottom of all pits. Results could be hit and miss.


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?

- - - - - - - - - - - - - - -
"Life! No one get's out of it alive."
"The trick is to grow up without growing old..."

 

[dik]

Thanks for the reply...

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