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
RE: Coating Failure
RE: Coating Failure
For water immersion condition for HDG steel we normally use two coats of high build epoxy with dry film 150 microns for each layer. I would not use polyurethane top coat for immersion condition.
h ttp://nz.l inkedin.co m/pub/jona than-smith /39/25/589
RE: Coating Failure
Dry film thickness is stipulated elsewhere and as recommended by the manufacturer.
There will be a program of inspection of the coating and maintenance of same.
These are from my General Notes... a compilation of 30+ years of 'mistakes'... <G>.
SYSTEM 05: HOT DIPPED GALVANISING
BASE METAL - STEEL
CLEANING - TO SSPC 1 AND SSPC 6
HDG - TO CSA C164, THICKNESS TO Z275
SYSTEM 07: QUALITY PRIMER WITH TOPCOAT (INTERIOR)
BASE METAL - STEEL
CLEANING - TO SSPC 1 AND SSPC 6
PRIMER - DEVOE 'DEVGUARD 4160 - MULTIPURPOSE TANK AND STRUCTURAL PRIMER'
TOPCOAT - DEVOE 'SPEEDENAMEL 4318 - QD GLOSS ENAMEL'
SYSTEM 08: QUALITY PRIMER WITH TOPCOAT (EXTERIOR)
BASE METAL - STEEL
CLEANING - TO SSPC 1 AND SSPC 6
PRIMER - DEVOE 'DEVGUARD 4160 - MULTIPURPOSE TANK AND STRUCTURAL PRIMER'
TOPCOAT - DEVOE 'DTM URETHANE-ALKYD GLOSS ENAMEL'
SYSTEM 09: EPOXY WITH ZINC RICH PRIMER
BASE METAL - STEEL
CLEANING - TO SSPC 1 AND SSPC 10
PRIMER - DEVOE 'CATHACOAT 302HB - REINF INORGANIC ZINC PRIMER
TOPCOAT - DEVOE 'BARRUST 235 - MULTIPURPOSE EPOXY COATING'
SYSTEM 10: EPOXY COATING OVER HDG
BASE METAL - HDG STEEL
CLEANING - TO SSPC 1
WASH - POLYCLUTCH 97-687 BY PPG OR APPD EQ
PRIMER - AMERLOCK 400 TWO COMPONENT EPOXY BY PPG OR APPD EQ
TOPCOAT - AMERCOAT 450H TWO COMPONENT ALIPHATIC POLYURETHANE TOPCOAT BY PPG
SYSTEM 11: EPOXY COATING OVER HDG (ALTERNATIVE)
BASE METAL - HDG STEEL
CLEANING - TO SSPC 1
PRIMER - DEVOE DEVRAN 201 EPOXY
TOPCOAT - DEVOE DEVTHANE 379 URETHANE
SYSTEM 12: THREE PART EPOXY COATING OVER HDG
BASE METAL - HDG STEEL
CLEANING - TO SSPC 1
PRIMER - DEVOE DEVRAN 201 EPOXY
SUBCOAT - DEVOE 'BARRUST 235 - MULTIPURPOSE EPOXY COATING'
TOPCOAT - DEVOE 'DEVTHANE 379H - ALIPHATIC URETHANE GLOSS ENAMEL'
My concern is that a 'wide' breach in the coating system may contribute to 'increased' anodic corrosion, because of the 'concentration' of exposed metal.
Dik
RE: Coating Failure
Dik
RE: Coating Failure
Steve Jones
Materials & Corrosion Engineer
http://www.linkedin.com/pub/8/83b/b04
RE: Coating Failure
Dik