Coating Failure 1

[dik]

We are looking at a quality coating system over hot dipped galvanizing. The total coating system consists of hot dipped galvanised steel frames. These are coated with a suitable epoxy primer. The primer has an epoxy intermediate coat over followed by a polyurethane top coat.

The anticipated life of the frames is 20 years. The environmental exposure of the frame is that it will be submerged for several months at a time in, generally, non-saline river water. In addition, the frames will likely be subject to impact by water borne debris which may damage the coating system. It is also possible that there may be a slight salinity of the river water caused by road salt 'run-off'.

The frames will be well maintained.

For a small breach in the coating, it is my understanding that the HDG will sacrificially prevent/reduce corrosion.

Is it possible that for a larger amount of damage, where there is a significant 'gap' in the coating, that corrosion will increase due to the insulating characteristics of the coating? I think this is generally referred to as anodic corrosion.

Dik

 

[JAE]

dik - just to clarify - you have a galvanized condition now and you want to over-coat the galvanizing with a paint system?

Usually we try to use TNEMEC but there are other systems out there as well.

 

[dik]

JAE... it is one step above using a zinc rich epoxy primer, epoxy intermediate coat and polyurethane top coat.

I've not used this before and I was wondering if the added coatings could have an adverse effect on the corrsion resistance.

Dik

 

[ajk1]

I thought that epoxy does not bond well to galvanized steel.
Yes you may get accelerated corrosion at breaks in the coating; whether the zinc can mitigate that effect I suppose depends upon the width of the damaged area, as you indicate.
I have found that any liquid applied coating will not adhere to sharp corners. AISC handbook used to have (maybe still do) a good commentary on this. The corrosion rapidly starts at the corner and works itself under the coating. I have little faith in the corrosion prtoective properties of epoxy coating.
My humble advice is that you should retain a knowledgeable corrosion engineer for proper advice. I am not an expert...just giving you what knowledge I have. Hope it helps a bit.

 

[hokie66]

This is an Australian site, but gives a good discussion about duplex coating systems and some of the issues with coating galvanized surfaces. It is common practice now, particularly in marine and coastal environments, but requires knowledgeable coatings suppliers and inspectors to make it work.

http://www.ingal.com.au/IGSM/23.htm

 

[dik]

Thanks, hokie... the materials chosen are compatible with zinc. Depending on the coating material a saponification occurs that can adversely affect the bonding with the zinc coating.

I've used these coatings before with excellent results... the problem occurs because the coating may be damaged with a 'wide scratch', not a normal type of scratch.

Dik

 

[dik]

Excellent link hokie... read the article, and downloaded the manual. Thanks,

Dik

 

[Ron]

dik...saponification usually doesn't occur with the coatings you've listed.

Keep in mind that any coating breach will create a concentration of corrosion potential due to an area effect.

I would be more concerned with the interlayer bond between these coatings, particularly when subjected to wetting/drying conditions.

 

[dik]

I know... but it happens with other coating systems and many of those that have failed. Thanks, many coatings have specific maximum times for application of the next coat for this very reason...

Dik

 

[dik]

Hokie... the article notes for Brush Blasting, a distance from the surface for the nozzle to be 300-400 mnr. Do you know what 'mnr' units are?

Dik

 

[hokie66]

I'm sure that is a typo. Should be mm. Hard to see how that happened. I will send them a note.

 

[Ron]

Looks like a character recognition fubar!

 

[dik]

That's what I thought, but didn't know if there was a term for the distance of the sandblasting to be measured at the centre of the ellipse...