Coating Failure
Coating Failure
(OP)
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
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






RE: Coating Failure
Usually we try to use TNEMEC but there are other systems out there as well.
RE: Coating Failure
I've not used this before and I was wondering if the added coatings could have an adverse effect on the corrsion resistance.
Dik
RE: Coating Failure
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.
RE: Coating Failure
http://www.ingal.com.au/IGSM/23.htm
RE: Coating Failure
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
RE: Coating Failure
Dik
RE: Coating Failure
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.
RE: Coating Failure
Dik
RE: Coating Failure
Dik
RE: Coating Failure
RE: Coating Failure
RE: Coating Failure