Coal Tar Expoxy Performance

[andymahadeo]

Hey Guys, I'm new to the forum.

Currently I'm working on a project where they have a floating bridge with pontoons across a salt water river in Guyana. We're currently using CTE to coat the pontoons (Carbon Steel) and are getting about 4-5 years out of it before the corrosion gets really bad again. Can anyone tell me past experiences they had with CTE Lifetimes in a similar environment?

 

[gr2vessels]

CTE Lifetimes? Past experience? They're now all dead of cancer or being killed at the protest rally....lol.

 

[andymahadeo]

Apparently Sherwin Williams is still willing to make CTEs though lol.

We've been using this coating for 20 odd years now, my country usually lets cancer off the hook.

Do you know of any other coatings which can yield the same price and performance and CTE though?

 

[gr2vessels]

Sorry Andy, as far as I know nothing beats CTE as cost and acceptable performance. Obviously, there are other high tech coatings from glass flake to thermal spraying, but the cost can be an issue. However, the longevity could offset the initial cost, particularly with thermal spraying.

 

[andymahadeo]

Thanks Gr2. Thermal Spraying is out of the question for my application.. told my boss about that.. he said no he wants a regular spray coating... equipment not available to do thermal spraying. i really wanna know whts a typical lifetime though.. IS 12944 i think said that 2 pack epoxies with certain paint systems can yield over 15 years life. but im not sure if that applies to CTE

 

[gr2vessels]

5 years is an outstanding performance on a pontoon across the river, most likely with suspended solids abrading constantly the coating. Obviously, the smallest crack will expand in time and undermine the lasting performance of the rest of coating. I think the surface preparation is the key thing for you. However, on site you need a ruthless paint supervisor with a whip in hands and the paint spec printed on his back.

 

[andymahadeo]

i get the suspended solids affecting the bottom but the corrosion problem is mostly occurring on the topside. The underside of the pontoons are fairly okay. maybe the application procedures needs to be a little tighter and better surface preparation.

the topside has some gunnels, manholes bolts, etc.


very cool forum btw. its nice to have some other Engineers to talk problems through. The engineers in my country are okay but not many are versed in proper corrosion mitigation because they aren't many large steel structures that arent galvanized or covered in my country. this bridge is the largest steel structure of its kind in my country. i think it was once the longest floating bridge in the world.

 

[gr2vessels]

Perhaps you have to reconsider the salt content of the water? On the top structure you will experience evaporation which in turn raise the chloride concentration in spots way above the 20000 ppm of the normal sea water

 

[CoryPad]

Perhaps the paint on top is subjected to uv radiation damage, which is common for aromatic epoxy. A uv-resistant topcoat may be necessary, e.g. urethane, siloxane, etc.

 

[andymahadeo]

These are all really good points guys. I'll take them into consideration.

 

[SJones]

What was the original paint specification: DFT, number of layers etc, and what is the specific type of failure being experienced? Are you applying exactly the same coating each time? Has the manufacturer changed the formulation without you knowing? It's no use us taking wild guesses until we know what you started with and how it failed. Once you are into an in situ maintenance scenario, paint lifetime will be degraded anyway. Get professional help on site to identify the failure and to assist with remedial work.

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/pub/8/83b/b04

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

 

[andymahadeo]

Its the Targuard Coal Tar Epoxy from Sherwin Williams being used.

Manufacturer's Specs:

DFT: 400 microns

No of Coats: 2

Mostly Pitting Corrosion is occurring and localized to the top side of the pontoon.

Manufacturer Formula Not Changed.

 

[SJones]

So how is the paint actually failing in order to allow pitting corrosion of the substrate to occur? Is it cracking, peeling, blistering?

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/pub/8/83b/b04

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

 

[TomDOT]

I'm with CoryPad. Until we have more information, UV/solar initiated degradation is the most likely, and this is typically prevented with a UV-blocking topcoat, such as a highly pigmented polyurethane. If it wasn't going right down to the waterline, acrylic latex works very well as a topcoat in atmospheric exposure.

 

[andymahadeo]

It appears to be cracking and peeling, what do you guys think?
@TomDOT you're referring to a top coat over the CTE or replacing it?

 

[Compositepro]

Your picture shows almost all of the corrosion is on corners which are exposed to chipping due to impact with tools, shackles, etc.

 

[andymahadeo]

sorry about tht.. the post was messed up here are the other pics:

http://img6.imageshack.us/img6/2725/pict1790z.jpg

http://img856.imageshack.us/img856/9586/pict1791.jpg

http://img801.imageshack.us/img801/4472/pict1784gw.jpg

 

[CoryPad]

I agree that edge/corner issues appear to be a big factor. It does look like fading/chalking exists too. You may need a multi-coat system, something like a zinc-containing primer, epoxy mid layer, and polyurethane topcoat.

 

[andymahadeo]

Thanks guys, I think they guys who were in charge of quality checking maintenance works slacked off. Those edge and corner issues seem to me like they were also influenced sloppy sanding and spraying.

anyways I'll try to source a Zinc Rich Primer and Poly Urethane topcoat and add that to the existing paint system. has anyone ever used a paint system like that though?

 

[CoryPad]

Those are standard products for marine environments.