Painting and anodes???

[Greigster]

Hope you can offer some badly needed advice

During a discussion it was mentioned that you always paint the underside of anodes, the side adjacent to the stucture. Do you have any specifications, codes or litreture to prove why this is done, the advantages / disadvantages etc..

We are being asked why we paint the underside of anodes and I am having great difficulty in producing any justification as to why we do this.
It maybe coded somewhere or a general rule of thumb!

Do you then adjust the cathodic protection calc to remove the painted surface area?

Can you help with any information on this topic.

Best Regards

Greig

 

[mshimko]

I've never heard of painting the underside of scrificial anodes, and I've been involved with CP for about 20 years.

Never specified; no rule-of-thumb. In fact, the primary guidance is one contained in both cathodic protection and painting references - DO NOT PAINT ANODES.

 

[Metalguy]

The only reason I can think of would be to remove the anode area (protection-wise) from the circuit. That would possibly give wider protection coverage and make the anodes last longer. But the anodes would have to be pretty close to whatever's being protected.

 

[SJones]

Generally, it would only be pipeline bracelet anodes that would have the side in contact with the pipe coating coated in order to prevent corrosion from both sides and disintegration of the anode.

Steve Jones
Materials & Corrosion Engineer

http://www.pdo.co.om/pdo/

 

[Greigster]

The anodes in question are flush fitting. NORSOK M-503 calls for flush fitting and bracelet anodes to be coated on the side facing the mounting surface and for it to be a minimum of 100mic epoxy.

We are fabricating subsea structures which are painted with 350 to 450mic of epoxy highbuild and also have flush fitting and stand off type anodes.

We currently paint the steel underneath the anode and don't apply any coating at all to any of the stand off anodes, but coat the underside of the flush fits.

Do you guys think this is acceptable? Opinions please.

 

[mshimko]

I looked at what references I have available after reading your (greigster) second posting and it's mention of NORSOK 503. I found the item you refer to in Section 7.2.

WHY is this a requirement?

I would imagine it's because coating damage is likely under the anode, and this damaged area will then contribute to the load on the anode, shortening the life of the anode.

However, I can't imagine this decrease being significant, especially if a good quality durable coating is used to protect the structure (A BIG assumption, I admit).

Bottom line(s):
(1) If coating the underside of flush-mounted anodes is the current standard practice, I wouldn't change.
(2) If current procedures do not coat the undersides, I wouldn't change.
(3) I doubt a cost-benefit analysis would support coating the undersides, but that's just an opinion off the top of my head.

 

[WayneAlexander]

Anodes attached on vessels have their straps placed 1/4" from the "hull" side of the anode surface, and 1" to 2" from the outer anode surface (depending on the thickness). This is because the anode consumption is from the outside surface areas in this application. If it were to consume from underneath the anode, once the strap is exposed, nothing holds the anode material to the core and it can fall off. A coating would prevent the bottom surface from oxidation and corrosion. In other applications, the core is place in the center of the anode to allow consumption from all sides, and a coating shouldn't be placed on the back-side.

 

[AndrewLindsay]

The reason you coat the underside of flush mount anodes is to remove the low resistance path to the structure. This would then result in hot spot cathodic protection, which is generally only really a problem with ICCP systems.

 

[AndyKayll]

I have come across localised paint blistering around bare anodes.
It could be nothing to do with proximity (can't find any data) but this may explain (or rather NOT explain) why you do it.

Never Assume, you make an Ass out of U and Me

 

[mshimko]

For Andy - are you refering to sacrificial anodes or ICCP anodes? If sacrifical, the blistering should not be from the anode, as hull coatings SHOULD be formulated and tested to be resistent to cathodic disbondment when subjected to the potentials of common anodes. US Navy paint specs typically require these coatings to be tested with a Mg anode (-1.5 volts), whereas in servce the coatings would be used with zinc or auminim anodes (-1.0 volts, approx). If an ICCP system is used, there is an 8-ft dielectric shield around the anode, but I have occasionally seen blistering near the edge of dielectric shields.

 

[AndyKayll]

They were sacrificial anodes, and I hope the new ones still are
I have no knowlage of how the existing paint scheme was applied so the fault could lie there, it just seemed odd that the blistering was localised around the Anodes.
This is even more puzzling as the old Anodes were Zinc but the water was brackish so they would (and had) quickly become useless due to the surface becoming non conductive.

Never Assume, you make an Ass out of U and Me

 

[geogr]

Hi, I ve been in the field of newbuildings for more than a decade, it is first time I listen to something like this, so for sure it is not "rule of the thumb", technically wise again I cant think of any reason why doing this. I would think of it more as a disadvantage reducing the abilities of the anode.

 

[seatrendgokhan]

Greigster

Could it be that, you heard it as "the hull structure which would stay underneath of anode, has to be painted" ???

I assume it is so, and the reason is so simple, we paint the area which the anode will be fitted upon it, beacuse after welding the anode to the hull, then it will be impossible to apply the paint undertneath the anode.

Hope this is the answer you need.

GOKHAN
Naval Arc.& Marine Eng.

 

[ssreese1500]

Interesting topic. I guess we are talking about sacrificial anodes. I have never heard of painting an anode, and as a matter of fact most are marked "do not paint" Generally the hull of the vessel is painted before welding the anode tabs on to make sure the steel under the anode is coated, and then the tabs are touched up with paint. I dont think that painting the bottom of the anode itself is going to do any good at all.

Two things I read in this that I need to dispute. Most anodes are manufactured with the bar running through it in such away that the corroding anode will not "fall off" until it is almost gone, probably 20% or less left of the anode.

Secondly, to say that you would do something to increase the life of the anode is not what you want to achieve. Increasing the life of the anode means that it isnt giving maximum protection to the structure it is supposed to be protecting.


If you want to get more info on this, I would get a hold of the guys at Wilson Walton. Google them and it will give you the website with contact info.